A Trigger Identification Service for Defending Reactive Jammers in WSN by n.rajbharath

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									IEEE TRANSACTION ON MOBILE COMPUTING                                                                                                   1




    A Trigger Identification Service for Defending
             Reactive Jammers in WSN
                                  Ying Xuan, Yilin Shen, Nam P. Nguyen, My T. Thai

     Abstract—During the last decade, Reactive Jamming Attack has emerged as a greatest security threat to wireless sensor
     networks, due to its mass destruction to legitimate sensor communications and difficulty to be disclosed and defended.
     Considering the specific characteristics of reactive jammer nodes, a new scheme to deactivate them by efficiently identifying all
     trigger nodes, whose transmissions invoke the jammer nodes, has been proposed and developed. Such a trigger-identification
     procedure can work as an application-layer service and benefit many existing reactive-jamming defending schemes. In this paper,
     on the one hand, we leverage several optimization problems to provide a complete trigger-identification service framework for
     unreliable wireless sensor networks. On the other hand, we provide an improved algorithm with regard to two sophisticated
     jamming models, in order to enhance its robustness for various network scenarios. Theoretical analysis and simulation results
     are included to validate the performance of this framework.

     Index Terms—Reactive Jamming, Jamming Detection, Trigger Identification, Error-tolerant Nonadaptive Group Testing, Opti-
     mization, NP-Hardness.

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1    I NTRODUCTION                                                        On the other hand, various network diversities are in-
            the last decade, the security of wireless sensor           vestigated to provide mitigation solutions [6]. Spreading
S   INCE
     networks (WSNs) has attracted numerous attentions,
due to its wide applications in various monitoring systems
                                                                       spectrum [12][5][8] making use of multiple frequency
                                                                       bands and MAC channels, Multi-path routing benefiting
and invulnerability toward sophisticated wireless attacks.             from multiple pre-selected routing paths [6] are two good
Among these attacks, jamming attack where a jammer                     examples of them. However, in this method, the capability
node disrupts the message delivery of its neighboring                  of jammers are assumed to be limited and powerless to
sensor nodes with interference signals, has become the most            catch the legitimate traffic from the camouflage of these
critical threat to WSNs. Thanks to the efforts of researchers          diversities. However, due to the silent behavior of reactive
toward this issue, as summarized in [12], various effi-                 jammers, they have more powers to destruct these mitiga-
cient defense strategies have been proposed and developed.             tion methods. To this end, other solutions are in great need.
However, a reactive variant of this attack, where jammer               A mapping service of jammed area has been presented in
nodes stay quite until an ongoing legitimate transmission              [11], which detects the jammed areas and suggests that
(even has a single bit) is sensed over the channel, emerged            routing paths evade these areas. This works for proactive
recently and called for stronger defending system and more             jamming, since all the jammed nodes are having low PDR
efficient detection schemes.                                            and thus incapable for reliable message delay. However, in
   Existing countermeasures against Reactive Jamming at-               the case of reactive jamming, as we will show later, this
tacks consist of jamming (signal) detection and jamming                is not always the case. Only a proportion of these jammed
mitigation.                                                            nodes, named as trigger nodes, whose transmissions wake
   On the one hand, detection of interference signals from             up the reactive jammers, are required to be blocked to avoid
jammer nodes is non-trivial due to the discrimination be-              the jamming effects.
tween normal noises and adversarial signals over unstable                 In this paper, we present an application-layer real-time
wireless channels. Numerous attempts to this end monitored             trigger-identification service for reactive-jamming in wire-
critical communication related objects, such as Receiver               less sensor networks, which promptly provides the list of
Signal Strength (RSS), Carrier Sensing Time (CST), Packet              trigger-nodes using a lightweight decentralized algorithm,
Delivery Ratio (PDR), compared the results with specific                without introducing neither new hardware devices, nor
thresholds, which were established from basic statisti-                significant message overhead at each sensor node.
cal methods and multi-modal strategies [9][12]. By such                   This service exhibits great potentials to be developed
schemes, jamming signals could be discovered, however,                 as reactive jamming defending schemes. As an example,
how to locate and catch the jammer nodes based on these                by excluding the set of trigger nodes from the routing
signals is much more complicated and has not been settled.             paths, the reactive jammers will have to stay idle since
                                                                       transmissions can be sensed. Even though the jammers
• Y. Xuan, Y. Shen, Nam P. Nguyen and My T. Thai are with the          move around and detect new sensor signals, the list of
  Department of Computer Information Science and Engineering.          trigger nodes will be quickly updated, so are the routing
  E-mail: {yxuan, yshen, nanguyen, mythai}@cise.ufl.edu
                                                                       tables. As another example, without prior knowledge of
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the number of jammers, the radius of jamming signals and        we will first illustrate our framework solution toward the
specific jamming behavior types, it is quite hard to locate      basic attacker model, and then validate its performance
the reactive jammers even the jammed areas are detected         toward multiple advanced attacker models theoretically and
(e.g. by [11]). However, with the trigger nodes localized,      experimentally.
the possible locations of reactive jammers are significantly
narrowed down.                                                  2.2.1 Basic Attacker Model
   Although the benefits of this trigger-identification service
are exciting, its hardness is also obvious, which dues          Conventional reactive jammers [12] are defined as mali-
to the efficiency requirements of identifying the set of         cious devices, which keep idle until they sense any ongoing
trigger nodes out of a much large set of victim nodes, that     legitimate transmissions and then emit jamming signals
are affected jamming signals from reactive jammers with         (packet or bit) to disrupt the sensed signal (called jammer
possibly various sophisticated behaviors. To address these      wake-up period), instead of the whole channel, which
problem, a novel randomized error-tolerant group testing        means once the sensor transmission finishes, the jamming
scheme as well as minimum disk cover for polygons are           attacks will be stopped (called jammer sleep period). Three
proposed and leveraged.                                         concepts are introduced to complete this model.
   The basic idea of our solution is to first identify the set       Jamming range R. Similar to the sensors, the jammers
of victim nodes by investigating corresponding links’ PDR       are equipped with omnidirectional antennas with uniform
and RSS, then these victim nodes are grouped into multiple      power strength on each direction. The jammed area can
testing teams. Once the group testing schedule is made at       be regarded as a circle centered at the jammer node, with
the base station and routed to all the victim nodes, they       a radius R, where R is assumed greater than rs , for
then locally conducts the test to identify each of them as      simulating a powerful and efficient jammer node. All the
a trigger or non-trigger. The identification results can be      sensors within this range will be jammed during the jammer
stored locally for reactive routing schemes or delivered to     wake-up period. The value of R can be approximated based
the base station for jamming localization process.              on the positions of the boundary sensors (whose neighbors
   In the remainder of this paper, we first present the          are jammed but themselves not), and then further refined.
problem definition in Section 2, where the network model,            Triggering range r. On sensing an ongoing transmis-
victim model and attacker models are included. Then we          sion, the decision whether or not to launch a jamming signal
introduce three kernel techniques for our scheme, Random-       depends on the power of the sensor signal Ps , the arrived
ized Error-Tolerant Non-adaptive Group Testing, Clique-         signal power at the jammer Pa with distance r from the
independent Set and Minimum Disk Cover in a Simple              sensor, and the power of the background noise Pn .
Polygon in Section 3. The core of this paper: trigger iden-         According to the traditional signal propagation model,
tification procedure and its error-tolerant extension toward     the jammer will regard the arrived signal as a sensor
sophisticated jammer behaviors are presented respectively       transmission as long as the Signal-Noise-Ratio is higher
                                                                                                         Pa
in Section 4 and 5. A series of simulation results for evalu-   than some threshold, i.e., SN R = Pn > θ where Pa =
ating the system performance and validating the theoretical     Ps
                                                                 rξ
                                                                     · Y with θ and ξ called jamming decision threshold
results are included in Section 6. We also present some         and path-loss factor, Y as a log-normally random variable.
                                                                                  θ·Pn 1
related works in Section 7 and summarize the whole paper        Therefore, r ≥ ( Ps ·Y ) ξ is a range within which the sensor
in Section 8.                                                   transmission will definitely trigger the jamming attack,
                                                                named as triggering range. As will be shown later, this
2   P ROBLEM M ODELS            AND   N OTATIONS                range r is bounded by R from above, and rs from below,
2.1 Network Model                                               where the distances from either bounds are decided by the
We consider a wireless sensor network consisting of n           jamming decision threshold θ. For simplicity, we assume
sensor nodes and one base station (larger networks with         triggering range is the same for each sensor.
multiple base stations can be split into small ones to              Jammer distance. Any two jammer nodes are assumed
satisfy the model). Each sensor node is equipped with           not to be too close to each other, i.e., the distance between
omnidirectional antennas, m radios for in total k channels      jammer J1 and J2 is δ(J1 , J2 ) > R. The motivations
throughout the network, where k > m. For simplicity, the        behind this assumptions are three-fold: 1) the deployment
power strength in each direction is assumed to be uniform,      of jammers should maximize the jammed areas with a
so the transmission range of each sensor can be abstracted      limited number of jammers, therefore large overlapping
as a constant rs and the whole network as a unit disk graph     between jammed areas of different jammers lowers down
(UDG) G = (V, E), where any node pair i, j is connected         the attack efficiency; 2) δ(J1 , J2 ) should be greater than R,
iff the Euclidean distance between i, j: δ(i, j) ≤ rs . We      since the transmission signals from one jammer should not
leave asymmetric powers and polygonal transmission area         interfere the signal reception at the other jammer, otherwise,
for further study.                                              the sensed sensor signals mixed with the jamming signals
                                                                from the other jammer will not invoke this jammer; 3) the
2.2 Attacker Model                                              communications between jammers are impractical, which
We consider both a basic attacker model and several ad-         will expose the jammers to anomaly detections at the
vanced attacker models in this paper. In the next sections,     network authority.
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                                                                    We assume that the detection of jammed signals can be
                                                                 100% correctly completed via comparing the SNR, PDR
                                                                 and RSS, as shown in [9] in this work. Although this
          V1       0950      victim     30                       detection problem is also quite challenging, it is orthogonal
                                                                 to the service framework proposed in this paper. We will dig
                                                                 into this problem in our future work, where various real-
                                                                 time applications embedded with this service framework
Fig. 1.   sensor periodical status report message
                                                                 will be developed.

2.2.2 Advanced Attacker Model                                    3    T HREE K ERNEL T ECHNIQUES
Although the basic reactive jamming model is quite energy-       In this section, we mention three kernel techniques that
efficient, the attackers may alter their behaviors to evade       we resort to in the proposed protocol. Most existing anti-
the detection, for which two advanced reactive jamming           jamming works consider only proactive jammers, while re-
models: probabilistic attack and asymmetric response time        active jammers can bring up larger damage due to efficient
delay are considered in this paper. In the first one, the jam-    attack and hardness to detect. To this end, we embed a
mer responds each sensed transmission with a probability η       group testing process, i.e., the randomized error-tolerant
independently. In the second one, the jammer delays each         group testing by means of our designed random (d, z)-
of its jamming signals with an independently randomized          disjunct matrix, to the routing update scheme, which avoids
time interval.                                                   unnecessarily large isolated areas as [11] does. Moreover,
   We do not specify the possible changes of jamming range       most existing topology-based solutions [24][25] can only
R as an advanced model, since the trigger set in this case       handle the single-jammer case, since lacking of knowledge
will not change, though the victim set varies. Further, we       over the jamming range and inevitable overlapping of the
do not theoretically analyze the effects of various jamming      jammed areas bring ups the analytical difficulties. Regard-
decision threshold θ in this paper version, but we evaluate      ing these issues, we resort to a minimum disk cover problem
all these above factors in the simulation section. Jammer        in within simple polygon problem and a clique-independent
mobilities are out of the scope of this paper, which assumes     set problem.
that the jammers are static during our trigger-identification
phase. This is quite reasonable, since the time length of this   3.1 Error-tolerant        Randomized         Non-Adaptive
phase is short, as to be shown later.                            Group Testing
                                                                 Group Testing was proposed since WWII to speed up
2.3   Sensor Model                                               the identification of affected blood samples from a large
Besides monitoring the assigned network field and gen-            sample population. This scheme has been developed with
erating alarms in case of special events (e.g., fire, high        a complete theoretical system and widely applied to med-
temperature), each sensor periodically sends a status report     ical testing and molecular biology during the past several
message to the base station, which includes a header and         decades [1]. Notice that the nature of our work is to identify
a main message body containing the monitored results,            all triggers out of a large pool of victim nodes, so this
battery usage, and other related content. As shown in            technique intuitively matches our problem.
Fig.1, the header is designated for anti-jamming purpose,           The key idea of group testing is to test items in multiple
which is 4-tuple: Sensor ID as the ID of the sensor              designated groups, instead of testing them one by one. A
node, Time Stamp as the sending out time indicating the          sketch of the traditional group testing can be find in the
sequence number, as well as a Label referring to the node’s      Appendix.
current jamming status and TTL as the time-to-live field
which is initialized as the 2D where D is the diameter of        3.1.1 Traditional Non-adaptive Group Testing
this network.                                                    The key idea of group testing is to test items in multiple
   According to the jamming status, all the sensor nodes         designated groups, instead of testing them one by one.
can be categorized into four classes: trigger nodes T N ,        The traditional method of grouping items is based on
victim nodes V N , boundary nodes BN and unaffected              a designated 0-1 matrix Mt×n where the matrix rows
node U N . Trigger nodes refer to the sensor nodes whose         represent the testing group and each column refers to an
signals awake the jammers, i.e. within a distance less than r    item, as Fig. 2 shows. M [i, j] = 1 implies that the j th item
from a jammer. Victim nodes are those within a distance R        appears in the ith testing group, and 0 otherwise. Therefore,
from an activated jammer and disturbed by the jamming            the number of rows of the matrix denotes the number of
signals. Since R > r, T N ⊆ V N . Other than these               groups tested in parallel and each entry of the result vector
disturbed sensors, U N and BN are the unaffected sensors         V refers to the test outcome of the corresponding group
while the latter ones have at least one neighbor in V N ,        (row), where 1 denotes positive outcome and 0 denotes
hence BN ⊆ U N , and V N ∩ U N = ∅. The Label field of            negative outcome.
each sensor indicates the smallest class it belongs to. The         Given that there are at most d < n positive items
relationships among these classes are shown in Fig. 3.           among in total n ones, all the d positive items can be
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                                                               
             0   0   0    0   1    1   1    1               0                    lies in the real test scenarios, the error probability of each
            0   0   1    1   0    0   1    1             0                   test is unknown and asymmetric, hence it is impossible to
                                                               
            0   1   0    1   0    1   0    1    testing  1     
   M =                                          =⇒ V =                       evaluate z before knowing the number of pools.
            1   1   1    1   0    0   0    0             1     
            1   1   0    0   1    1   0    0             1     
             1   0   1    0   1    0   1    0               1
                                                                                    We only show the performance of this new construction,
                                                                                 namely, ETG algorithm in this section. For the review
                                                                                 purpose, we include the details of the construction and
Fig. 2. Binary testing matrix M and testing outcome vector V . Assumed           analysis in the Appendix.
that item 1 (1st column) and item 2 (2nd column) are positive, then only the
first two groups return negative outcomes, because they do not contain these
                                                                                    Theorem 3.1: The ETG algorithm produces a (d, z)-
two positive items. On the contrary, all the other four groups return positive   disjunct matrix with probability p′ where p′ can be arbi-
outcomes.
                                                                                 trarily approaching 1.
                                                                                    • The worst-case number of rows of this matrix
                                                                                       is bounded by 3.78(d + 1)2 log n + 3.78(d +
efficiently and correctly identified on condition that the                               1) log( 1−p′ ) − 3.78(d + 1) + 5.44(d + 1)(z − 1), much
                                                                                                 2
testing matrix M is d-disjunct: any single column is not                                                            2
                                                                                       smaller than 4.28d2 log 1−p′ +4.28d2 log n+9.84dz +
contained by the union of any other d columns. Owing
                                                                                       3.92z 2 ln 2n−1 .
                                                                                                   1−p′
to this property, each negative item will appear in at least
                                                                                    • Assume z ≤ γt, the worst-case number of rows
one row (group) where all the positive items do not show                                                          2
                                                                                                                    −2τ (d+1)    ′

up, therefore, by filtering all the items appearing in groups                           becomes t = τ ln n(d+1)−γ(d+1))2 ln(1−p ) where τ =
                                                                                                               (τ
with negative outcomes, all the left ones are positive. Al-                            (d/(d + 1))d and asymptotically t = O(d2 log n).
though providing such simple decoding method, d-disjunct                            Theorem 3.2: The time complexity of the ETG algorithm
matrix is non-trivial to construct [1][2] which may involve                      is O(d2 n log n), smaller than O(n2 log n), provided that
                                                                                       √
with complicated computations with high overhead, e.g.,                          d < n.
calculation of irreducible polynomials on Galois Field. In
order to alleviate this testing overhead, we advanced the                        3.2 Minimum Disk Cover in a Simple Polygon
deterministic d-disjunct matrix used in [7] to randomized
                                                                                 Given a simple polygon with a set of vertices inside, the
error-tolerant d-disjunct matrix, i.e., a matrix with less rows
                                                                                 problem of finding a minimum number of variable-radii
but remains d-disjunct w.h.p. Moreover, by introducing this
                                                                                 disks that not only cover all the given vertices, but also are
matrix, our identification is able to handle test errors under
                                                                                 all within the polygon, can be efficiently solved.
sophisticated jamming environments.
                                                                                    The latest results due to the near-linear algorithm pro-
   In order to handle errors in the testing outcomes, the                        posed recently by [26], which investigates the medial axis
error-tolerant non-adaptive group testing has been devel-                        and voronoi diagram of the given polygon, and provides
oped using (d, z)-disjunct matrix, where in any d + 1                            the optimal solution using O(ϖ + κ(log ϖ + log 6 κ)) time
columns, each column has a 1 in at least z rows where                            and O(ϖ + κ log log κ) space, where the number of edges
all the other d columns are 0. Therefore, a (d, 1)-disjunct                      of the polygon is ϖ and nodes within it as κ. We employ
matrix is exactly d-disjunct. Straightforwardly, the d posi-                     this algorithm to estimate the jamming range R.
tive items can still be correctly identified, in the presence
of at most z − 1 test errors. In the literature, numerous
deterministic designs for (d, z)-disjunct matrix have been                       3.3 Clique-Independent Set
provided (summarized in [1]), however, these constructions                       Cliques-Independent Set is the problem to find a set of max-
often suffer from high computational complexity, thus are                        imum number of pairwise vertex-disjoint maximal cliques,
not efficient for practical use and distributed implementa-                       which is referred to as a maximum clique-independent set
tion. On the other hand, to our best knowledge, the only                         (MCIS) [4]. Since this problem serves as the abstracted
randomized construction for (d, z)-disjunct matrix dues to                       model of the grouping phase of our identification, its
Cheng’s work via q-nary matrix [20], which results in a                          hardness is of great interest in this scope. To our best
(d, z)-disjunct matrix of size t1 × n with probability p′ ,                      knowledge, it has already been proved to be NP-hard for
where t1 is                                                                      cocomparability, planar, line and total graphs, however its
               2                                         2n − 1                  hardness on UDG is still an open issue. We prove that this
4.28d2 log         +4.28d2 log n+9.84dz+3.92z 2 ln                               problem is NP-complete and include the detailed proof in
            1 − p′                                       1 − p′
                                                                                 the appendix.
   with time complexity O(n2 log n). Compared with this                             There have been numerous polynomial exact algorithms
work, we advance a classic randomized construction for                           for solving this problem on graphs with specific topology,
d-disjunct matrix, namely, random incidence construction                         e.g., Helly circular-arc graph and strongly chordal graph
[1][2], to generate (d, z)-disjunct matrix which can not only                    [4], but none of these algorithms gives the solution on
generate comparably smaller t × n matrix, but also handle                        UDG. In this paper, we employ the scanning disk approach
the case where z is not known beforehand, instead, only the                      in [3] to find all maximal cliques on UDG, and then find all
error probability of each test is bounded by some constant                       the MCIS using a greedy algorithm. In fact, by abstracting
γ. Although z can be quite loosely upperbounded by γt,                           this problem as a Set Packing problem, we can obtain a
                                                                                 √
yet t is not an input. The motivation of this construction                         n-approximation algorithm, however, it exhibits worse
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performance than the greedy algorithm proposed in our                       value of the Label field (Initially trigger ”TN”). In detail,
trigger identification procedure.                                            if a node v hears jamming signals, it will not try to send
                                                                            out messages but keep its label as victim. If v cannot sense
4     T RIGGER I DENTIFICATION P ROCEDURE                                   jamming signals, its report will be routed to the base station
                                                                            as usual, however, if it does not receive ACK from its
We propose a decentralized trigger-identification procedure.                 neighbor on the next hop of the route within a timeout
It is lightweight in that all the calculations occur at the                 period, it tries for 2 more retransmissions. If no ACKs are
base station, and the transmission overhead as well as the                  received, it is quite possible that that neighbor is a victim
time complexity is low and theoretically guaranteed. No                     node, then v updates Label tuple as boundary ”BN” in its
extra hardware is introduced into the scheme, except for the                status report. Another outgoing link from v with the most
simple status report messages sent by each sensor, and the                  available capacity is taken to forward this message. If the
geographic locations of all sensors maintained at the base                  status report is successfully delivered to the base station
station. Three main steps of this procedure are as follows:                 with Label = TN, the corresponding node is regarded as
   1) Anomaly Detection – the base station detects potential                unaffected. All the messages are queued in the buffer of the
       reactive jamming attacks, each boundary node tries to                intermediate nodes and forwarded in an FCFS manner. The
       report their identities to the base station.                         TTL value is reduced by 1 per hop for each message, and
   2) Jammer Property Estimation – The base station cal-                    the message will be dropped once its TTL = 0, to avoid
       culates the estimated jammed area and jamming range                  self-loops.
       R based on the locations of boundary nodes.                             The base station waits for the status report from each
   3) Trigger Detection –                                                   node in each period of length P. If no reports have been
         • the base station makes a short testing schedule                  received from a node v with a maximum delay time, then
           message Z which will be broadcasted to all the                   v will be regarded as victim. The maximum delay time is
           boundary nodes.                                                  related with graph diameter and will be specified later. If
         • boundary nodes keep broadcasting Z to all the                    the aggregate report amount is less than ψ, the base station
           victim nodes within the estimated jammed area                    starts to create the testing schedule for the trigger nodes,
           for a period Q.                                                  based on which the routing tables will be updated locally.
         • all the victim nodes locally execute the testing
           procedure based on Z and a global uniform                        4.2 Jammer Property Estimation
           clock, identify themselves as trigger or non-
                                                                            We estimate the jamming range as R and the jammed areas
           trigger.
                                                                            as simple polygons, based on the locations of the boundary
                                                                            and victim nodes.
                                                                               In the sparse-jammer case where the distribution of
                                                                            jammers is relatively sparse and there is at least one jammer
                                                                            whose jammed area does not overlap with the others, like
                                                                            J2 in Fig. 3. By denoting the set of boundary nodes for
                                                                            the ith jammed area as BNi , the coordinate of this jammer
                                                                            can be estimated as
                                                                                                      ∑BNi        ∑BNi
                                                                                                         k=1 Xk            Yk
                                                                                       (XJ , YJ ) = (            , k=1        )
                                                                                                        |BNi |      |BNk |
                                                                            where (Xk , Yk ) is the coordinate of a node k is the jammed
                                                                            area BNi and then further the jamming range R can be
                                                                            estimated as
Fig. 3.     Nodes in grey and blue are victim nodes around jammer nodes,                            √
where blue nodes are also trigger nodes, which invoke the jammer nodes.         R = min { max ( (Xk − XJ )2 + (Yk − XJ )2 )}
Nodes surrounding the jammed are are boundary nodes, while the others are            ∀BNi k∈BNi
unaffected nodes.
                                                                            since we assume all the jammers have the same range.
                                                                               Otherwise in the dense-jammer case, as shown in Fig.
                                                                            4, we need to first estimate the jammed areas, which are
4.1    Anomaly Detection                                                    simple polygons (unnecessarily convex) containing all the
Each sensor periodically sends a status report message to                   boundary and victim nodes. This process consists of three
the base station. However, once the jammers are activated                   steps: (1) discovery of convex hulls of the boundary and
by message transmissions,the base station will not receive                  victim nodes, where no unaffected nodes are included in
these reports from some sensors. By comparing the ratio of                  the generate convex polygons. (2) for each boundary node
received reports to a predefined threshold ψ, the base station               v not on the hull, choose two nodes on the hull and connect
can thus decide if a jamming attack is happening in the                     v to them in such a way that the internal angle at this
networks. When generating the status report message, each                   reflex vertex is the smallest, hence the polygon is modified
sensor can locally obtain its jamming-status and decide the                 by replacing an edge (dotted one in Fig. 4) by the two
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                                                                                     TABLE 1
                           Reflex
                                                                    Message Containing Trigger Detection Schedule
                           Vertex

                                                                             Time Slot   Channel           Node List
                                                                                0          f1          v1 , v3 , · · · , vn
                                             R
                                                                                0          f2      v1 , v2 , v4 , · · · , vn−1
                                                                                            .
                                    Reflex                                                  .
                                    Vertex                                      0           .                  ···
                                                                                0         fm          v2 , v5 , · · · , vn
                                                                                1          f1        v2 , v4 , · · · , vn−2
                                                                                .           .
Fig. 4. Estimated R and Jammed Area                                             .
                                                                                .
                                                                                            .
                                                                                            .                 ···



new ones. The resulted polygon is the estimated jammed
area. (3) execute the near-linear algorithm [26] to find the
optimal variable-radii disk cover of all the victim nodes,
but constrained in the polygon, and return the largest disk
radius as R.

4.3 Trigger Detection                                            Fig. 5. Interference Teams
Since the jammer behavior is reactive, in order to find
all the trigger nodes, a straightforward way is that each
sensor broadcasts one by one, and monitors if the jammers        nodes in one testing team invokes a jammer node, its
are invoked by sensing the jamming signals. However,             jamming area will not reach the victim nodes in another
this individual detection is quite time-consuming and all        testing team. Therefore, by trying broadcasting from victim
the victim nodes thus have to be isolated for a long             nodes in each testing team and monitoring the jamming
detection period, or even returns wrong detection result in      signals, we can conclude if any members in this team are
the presence of mobile jammers. In this case, the network        triggers. In addition, all the tests in different testing teams
throughput would be dramatically decreased. Therefore, to        can be executed simultaneously since they will not interfere
promptly and accurately find out these triggers from a large      each other. Fig. 5 provides an example for this. 3 maxi-
pool of victim nodes, emerges as the most challenging part       mal cliques C1 = {v1 , v2 , v3 , v4 }, C2 = {v3 , v4 , v5 , v6 },
of the proposed protocol, for which the idea of group testing    C3 = {v5 , v7 , v8 , v9 } can be found within 3 jammed areas.
is applied.                                                      Assume these three cliques are respectively the three teams
   In this section, we only consider a basic attack model        we test at the same time. If v4 in the middle team keeps
where the jammers deterministically and immediately              broadcasting all the time and J2 is awaken frequently, no
broadcasts jamming signals once it senses the sensor signal.     matter the trigger v2 in the leftmost team is broadcasting
Therefore as long as at least one of the broadcasting victim     or not, v3 will always hear the jamming signals, so these
nodes is a trigger, some jamming signals will be sensed, and     two teams interfere each other. In addition, node-disjoint
vice versa. The performance of this protocol toward sophis-      groups do not necessarily interference-free, as the leftmost
ticated attacker models with probabilistic attack strategies     and rightmost teams show.
will be validated in the next section.                              Second-level, within each testing team, victims are fur-
   All the following is the testing schedule over all the        ther divided into multiple testing groups. This is completed
victim nodes, which is designed at the base station based on     by constructing a randomized (d, 1)-disjunct matrix, as
the set of boundary nodes and the global topology, stored        mentioned in Section 3.1, mapping each sensor node to
as a message (illustrated in Table 1) and broadcasted to         a matrix column, and make each matrix row as a testing
all the boundary nodes. After receiving this message, each       group (sensors corresponding to the columns with 1s in
boundary node broadcasts this message one time using             this row are chosen). Apparently tests within one group
simple flooding method to its nearby jammed area. All             will possibly interfere that of another, so each group will
the victim nodes execute the testing schedule and indicate       be assigned with a different frequency channel.
themselves as non-triggers or triggers. Since all the sensor        The duration of the overall testing process is t time
nodes are equipped with a global uniform clock, and no           slots, where the length of each slot is L. Both t and L
message transmissions to the base station are required           are predefined, yet the former depends on the total number
during the detection, the mechanism is easy to implement         of victims and estimated number of trigger nodes, and
and practical for applications.                                  the latter depends on the transmission rate of the channel.
   As shown in Table 1, for each time slot, m sets of victim     Specifically, at the beginning of each time slot, all the
sensors will be tested. The selection of these sets involves     sensors designated to test in this slot broadcast a τ -bit test
a two-level grouping procedure.                                  packet on the assigned channel to their 1-hop neighbors.
   First-level, the whole set of victims are divided into sev-   Till the end of this slot, these sensors keeps detecting
eral interference-free testing teams. Here by interference-      possible jamming signals. Each sensors will label itself as a
free we mean that if the transmissions from the victim           trigger unless in at least one slot of its testing, no jamming
IEEE TRANSACTION ON MOBILE COMPUTING                                                                                                                   7



signal is sensed, in which case, the label is converted to a
                                                                        V0                                        V0
non-trigger.
                                                                                             V5                                         V5
                                                                              V1                                         V1
   The correctness of this trigger test procedure is theo-                              V4                                         V4
                                                                                                       V7                                         V7
retically straightforward. Given that all the testing teams              V2                                         V2
are interference-free, then the testing with different teams                       V3                                         V3
                                                                                                  V6                                         V6
can be executed simultaneously. Given that we have an
upperbound d on the number of trigger nodes and each
testing group follow the (d, 1)-disjunct matrix, which guar-
antees that each non-trigger node will be included in at        Fig. 6. clique C1 = V1 V2 V3 V4 is chosen by CIS, but its CC ′ covers
least one group, which does not contain any trigger node,       boundary node V0 , then clique C2 = V4 V5 V6 V7 replaces C1 in the testing
                                                                team for the first round. Clique V1 V2 V3 are left for the next round.
so each non-trigger node will not hear jamming signals in
at least one time slot, but the trigger nodes will since the
jammers are activated once they broadcast the test packets.
                                                                   input : Induced Subgraph G′ = (W, E ′ )
Therefore, two critical issues need to be addressed to ensure      output: The set C of maximum number of disjoint maximal cliques.
this correctness: how to partition the victim set into             Find out the set S of all maximal (not disjoint) cliques by using Gupta’s
maximal interference-free testing teams and estimate the           MCE algorithm [3];
                                                                   while S ̸= ∅ do
number of trigger nodes d, as follows. Though these two                 Choose clique C ∈ S which intersects with the minimum number of
involve geometric analysis over the global topology, since              other cliques in S;
                                                                        C ← C ∪ {C};
it only takes the information of boundary and victim nodes              Remove all the maximal cliques intersecting with C;
as inputs, and is calculated at the base station, no message            S ← S \ {C};
                                                                   end
complexity is introduced.
                                                                                   Algorithm 1: CIS discovery
                                                                   Local Refinement. Each clique we select is expected to
                                                                represent the jammed area poisoned by the same jammer,
                                                                and this area should not cover the boundary nodes. How-
4.3.1   Discovery of Interference-free Testing Teams            ever, we did not take this into account when discovering
                                                                the CIS, and need to locally update it. Specially, for
                                                                each clique, we find its circumscribed circle CC and the
As stated above, two disjoint sets of victim nodes are          concentric circle CC ′ with radius R of CC. In the case that
interference-free testing teams iff the transmission within     CC ′ covers any boundary nodes, we locally select another
one set will not invoke a jammer node, whose jamming            clique by adding/removing nodes from this clique, to see
signals will interfere the communications within the other      if the problem can be solve. If not, we keep this clique as
set. Although we have estimated the jamming range R, it is      it is, otherwise, we update it. This is illustrated in Fig. 6.
still quite challenging to find these interference-free teams       Team Detection. The cliques in CIS can also interfere
without knowing the accurate locations of the jammers.          each other, e.g. the clique V1 V2 V3 V4 and V5 V7 V8 V9 in Fig.
Notice that it is possible to discover the set of victim        5. This is because the signals from V4 will wake J2 , who
nodes within the same jammed area, i.e. with a distance         will try to block these signals with noises and affect V5 by
R from the same jammer node. Any two nodes within               the way. But if any two cliques C1 and C2 are not con-
the same jammed area should be at most 2R far from              nected by any single edge, then they are straightforwardly
each other, i.e. if we induce a new graph G′ = (V ′ , E ′ )     interference-free, since the shortest distance between any
with all these victim nodes as the vertex set V ′ and           node in C1 and C2 is larger than 2R. But the farthest
E ′ = {(u, v)|δ(u, v) ≤ 2R}, the nodes jammed by the            jammer waken by and from C1 is r < R distance away,
same jammer should form a clique. The maximum number            whose jamming range can only reach another R distance
of vertex-disjoint maximal cliques (i.e. clique-independent     further, which is thus away from C2 . Therefore, the cliques
set (CIS) ) of this kind provides an upperbound of possible     in the obtained CIS of this kind are selected as testing
jammers within the estimated jammed area, where each            teams. While the others are left for the next time slot.
maximal clique is likely to correspond to the nodes jammed         In addition, in the worst case, any single maximal clique
by the same jammer.                                             C has at most 12 interfering cliques in the CIS, as the
                                                                shadowed ones in Fig. 7. Therefore, at most 13 testing
   The solution consists of three steps: CIS discovery
                                                                teams are required to cover all these cliques. If the number
on the induced graph from the remaining victim with-
                                                                of channels k given is larger than 13, then a frequency-
out test schedules, boundary-based local refinement and
                                                                division is available, i.e. these interfering cliques can still
interference-free team detection. We iterate three steps to
                                                                become simultaneous testing teams, on the condition each
decide the schedule for every victim node.
                                                                team can only use min{⌈ 13 ⌉, m} of the given channels,
                                                                                              k

   CIS discovery. We first employ Gupta’s MCE algorithm          where m is the number of radios per sensor. Otherwise, we
[3] to find all the maximal cliques, then use a greedy           have to use time-divisions, i.e. they have to be tested in
algorithm, as shown in Alg. 1 to get the CIS.                   different time slots.
IEEE TRANSACTION ON MOBILE COMPUTING                                                                                                8



                                                                       4.4 Analysis of Time and Message Complexity
                                                    J1       J2        Time complexity: By time complexity we mean the
                                                         J
                                                                       identification delay counted since the attack happens till
                                                                       all the nodes successfully identify themselves as trigger




                                               r
                                           R
                                               J3                 J4
                                                                       or non-trigger. Therefore, the complexity break downs into
                                                    J5       J6
                                                                       four parts: (1) the detection of jamming signals at local
                                                                       links Td ; (2) the routing of sensor report to the base station
                                                                       from each sensor node, and the testing schedule to each
                                                                       victim node from the base station, aggregated as Tr ; (3)
Fig. 7.   Maximum # Interfering        Fig. 8.     Maximum # Jammers
Cliques                                invoked by one team             the calculation of CIS and R at the base station Tc ; (4) the
                                                                       testing at each jammed area Tt .
                                                                          The local jamming signal detection involves the statisti-
4.3.2     Estimation of Trigger Upperbound                             cal properties of PDR, RSS and SNR, which is orthogonal
                                                                       to our work. We regard Td as O(1) since it is an entirely
Before bounding the trigger quantity from above, the trig-             local operation and independent with the network scale.
gering range r should be estimated. As mentioned in the                   The routing time overhead is quite complicated, since
attacker model, r depends not only on the power of both                congestions need to be considered. For simplicity, we
sensors and jammers, but also the jamming threshold θ and              consider that all the 1-hop transmission takes O(1) time and
path-loss factor ξ:                                                    bound Tr using the diameter D of the graph. As mentioned
                                  Pn · θ ξ 1
                                                                       earlier, the base station waits at most O(2D) for the reports,
                         r≥(             )                             so that is the upperbound of the one-way routing. As to the
                                  Ps · Y
                                                                       other way, we also bound it using O(2D) to match any
since the real time Pn and Ps are not given, we estimate r             collision and retransmission cases.
based on the SNR cutoff θ′ of the network setting. In fact,               The calculation of CIS resorts to the algorithm in
the transmission range of each sensor rs is a maximum                  [3], which finds O(l∆) maximal cliques on UDG within
radius to guarantee                                                    O(l∆2 ) time, where l = |E| and ∆ refers to the maxi-
                            Pa   Ps · Y                                mum degree. We used a greedy algorithm to find a MCIS
                 SN R =        =         ≥ θ′
                            Pn         ξ
                                 Pn · rs                               from these O(l∆) cliques with O(l3 ∆3 Q) time: O(l∆)-
                                                                       time for each clique to check the overlapping with other
Therefore, we can estimate r as
                                                                       cliques, O(l∆)-time to find a clique overlapping with
                                 θ ξ
                          r ≈ rs (   )
                                       1                               minimum other cliques, and Q denotes the number of
                                 θ ′                                   testing teams. Notice that in practice, sensor networks are
          ′                                                            not quite dense, so the number of edges l and maximum
where θ and ξ are parts of the network input, while θ is
assumed as a constant, which indicates the aggressiveness              degree ∆ are actually limited to small values. On the
of the jammer. For this estimation, θ can be first set as 10db,         other hand, the time complexity of estimating R is up to
which is the normally lower bound of SNR in wireless                   O( n∆ + n(log n∆ + log6 n) using the minimum disk cover
                                                                            2            2
transmission, and then adaptively adjusted to polish the               algorithm as mentioned.
service quality.                                                          The testing delay Tt depends on the number of testing
   With estimated r, since all the trigger nodes in the same           rounds and the length of each round. Since the reactive
team should be within a 2r distance from each other,                   jamming signal disappears as soon as these sensed 1-
by finding another induced graph G′′ = (Wi , E ′′ ) from                hop transmission finishes, each round length is then O(1).
the victim nodes Wi in team i, with E ′′ = {(u, v) ∈                   The number of testing rounds is however complicated and
E ′′ if δ(u, v) ≤ 2r}, the size of the maximal clique                  bounded by Theorem 4.1.
indicates the upperbound of the trigger nodes, thus can be                Lemma 4.1: Based on the ETG algorithm, the number
an estimate over d.                                                    of tests to identify d trigger nodes from |W | victim nodes
   As mentioned above, all the parallel testing teams se-              is upperbounded by t(|W |, d) = O(d2 ⌈ln |W |⌉) w.h.p.
                                                                                                               i
lected are interference-free, therefore we roughly regard                 Theorem 4.1: (Main) The total number of testing rounds
each team to be the jammed area of one jammer. As a                    is upper bounded by
deeper investigation, the number of jammers that can be                               Q      13 min{d2 ⌈ln |Wi |⌉, |Wi |}
invoked by the nodes in the same team (six 3-clique within                        O(max{              i
                                                                                                                          })
                                                                                        i=1              m
the red circles) can be up to 6, since the minimum distance                                     ∑6
between two jammers is greater than R and r ≤ R, as                    w.h.p, with di = min{ s=1 |cs (Gi )|, |Wi |} and cs (Gi ) is
shown in Fig. 8. Therefore on the induced graph, the largest           the sth largest clique over an induced unit disk subgraph
6 cliques form the possible trigger set. However, since                Gi = (Wi , Ei , 2r) in the testing team i.
the jammer distribution cannot be that dense for the sake                    Proof: First, from Lemma 4.1, at most t(|W |,d) =
                                                                                                                             m
                                                                       di ⌈ln |W |⌉
                                                                         2
of energy-conserving, the former estimate over d is large                   m       testing rounds are needed to identify all nodes
enough.                                                                in testing team i. Second, the set of testing teams that can
IEEE TRANSACTION ON MOBILE COMPUTING                                                                                                 9



be tested in parallel is 13, as mentioned earlier. Combining                                   TABLE 2
with the worst-case upperbound of triggers in each team,                                       Notations
the upperbound on round is derived.                                          Notation                   Content
  If the jamming range R is assumed known beforehand,                         T+         The number of false positive outcomes
similar to [7], the whole time complexity is thus                             T−         The number of false negative outcomes
                                                                              u(i)        The number of trigger nodes in test i
                       Q    13d2 ⌈ln |Wi |⌉, |Wi |}                           x(i)      The reaction time of jammer toward test i
                O(max{         i
                                                    )                         g(i)                The outcome of test i
                      i=1             m
and asymptotically bounded by O(n2 log n). It is asymp-
totically smaller than that of [7]:                                   Since our scheme is robust and accurate in the steps
                                                                   of grouping, generating disjunct matrix and decoding the
          ∑
        ∆(H)
                                       d2 log2 |Wj |
                                        j    2                     testing results, the only possible test errors arise from the
     O(         max⌈(2 + o(1))                             ,/m⌉)
          i=1
                 j                  log2 (dj log2 |Wj |)
                                       2
                                                                   generation of testing outcomes. Nevertheless, by using the
                                                                   error-tolerant disjunct matrix and relaxing the identification
where ∆(H) refers to the maximum degree of the induced             procedures to asynchronous manner, our scheme will pro-
graph H (in this new solution, maximum degree is not               vide small false rates in these cases. Some notations can be
involved). By taking the calculation overhead for R into           found in Table 2. In this section, the terms test and group,
account, the overall time complexity is asymptotically             the terms column and nodes are interchangeable.
O(n2 log n + n log6 n), which is O(n log6 n) for n ≥ 4.
Message Complexity: On the one hand, the broadcasting
of testing schedule Z from the base station to all the             5.1 Upperbound on the Expected Value of z
victim nodes costs O(n) messages in the worst case. On
the other hand, the overhead of routing reports toward the         First, we investigate the properties of both jamming be-
base station depends on the routing scheme used and the            haviors and obtain the expected number of error tests in
network topology as well as capacity. The upperbound is            both cases through the following analysis. Since in practice,
straightforward obtained in a line graph with the base sta-        it is not trivial to establish accurate jamming models, we
tion at one end, whose message complexity is O( n(n−1) ).          derive an upperbound of the error probability which does
                                                     2
   With regard to the message overhead of the testing              not require the beforehand knowledge of the objective
                                                                   jamming models, which is therefore feasible for real-time
process. Considering that there are approximately |Wi |d+1         identifications. Since it is a relaxed bound, it could be
victim nodes in each testing group of team Wi (mentioned
                                                                   further strengthened via learning the jamming history.
in the construction of randomized (d, z)-disjunct matrix in
Appendix), the overhead of each testing group in a testing
                                                                   5.1.1 Probabilistic Jamming Response
round is |Wi | 1-hop testing message broadcasted by all
           d+1
victim nodes in each group of team Wi . Therefore, the             A clever jammer can choose not to respond to some sensed
over message complexity is                                         ongoing transmissions, in order to evade the detection.
                                                                   Assume that each ongoing transmission has an independent
             Q
             ∑        Q                                            probability η to be responded. In our construction algorithm
       O(n +2
               |Wi | max{di ⌈ln |Wi |⌉, |Wi |}m)                   ETG, where each matrix entry is IID and has a probability
                              i=1
                     i=1
                                                                   p to be 1, therefore for any single test i with i ∈ [1, t]:
which is O(n2 log n).                                                                           ( )
                                                                                                  d x
                                                                                Pr[u(i) = x] =       p (1 − p)d−x              (1)
                                                                                                  x
5  A DVANCED S OLUTIONS TOWARD S O -                               Hence for each test i, the event that it contains no trigger
PHISTICATED ATTACK M ODELS                                         nodes but returns a positive result, has a probability at most:
In this section, we consider two sophisticated attacker mod-                     Pr[g(i) = 0 & u(i) ≥ 1]
els: probabilistic attack and variant response time delay,                       ∑d          ( )
                                                                                              d x
where the jammers rely each sensed transmission with dif-                      =    (1 − η)x    p (1 − p)d−x
                                                                                              x
ferent probabilities, instead of deterministically, or delay the                 x=1
jamming signals with a random time interval, instead of im-                    = [(1 − η)p + 1 − p]d − (1 − p)d
mediately. This may mismatch with the original definition                       = (1 − ηp)d − (1 − p)d < (1 − η)p
of reactive jamming, which targets at transmission signals,
instead of nodes or channels. However, clever jammers can          Meanwhile, the event that it contains at least one trigger
possibly change their strategies to evade possible sensed          but returns a negative result, has a probability:
detections. Also, a common sense indicates that as long                              Pr[g(i) = 1 & u(i) = 0] = 0                    (2)
as an activity is sensed by the jammer, it is quite possible
that some other activities are following this. So delaying         Since in practical η ≥ 1 , we therefore have the expected
                                                                                          2
the response time still guarantees the attack efficiency, but       number of false positive and negative tests is respectively
minimize the risk of being caught by reactive detections.          at most pt/2 and 0.
IEEE TRANSACTION ON MOBILE COMPUTING                                                                                                  10



5.1.2    Variant Reaction Time                                             Therefore, the expected number of false positive tests is at
                                                                           most
                                                                                                 ∑t
                                                                                                                                 2
The introduction of group testing techniques aims to de-                           T+    ≤           (1 − (1 − p)d )(1 − p)d (β)
                                                                                                 i=1
                                                                                                                                 β
crease the identification latency to the minimum, there-
fore, if the jammer would not respond intermediately after                                        ∑
                                                                                                  t
                                                                                         ≤ 2        (1 − (1 − p)d )(1 − p)d
sensing the ongoing transmissions, but instead wait for a
                                                                                                  i=1
randomized time delay, the test outcomes would be messed
                                                                                         ≤ 2(1 − (1 − p)d )(1 − p)d t
up. Since it is expensive to synchronize the tests among
sensors, we use a predefined testing length as L, thus the
test outcome of test i ∈ [1, t] is generated within time                      (2) For event F n(i), following the similar arguments
interval [(⌈ m ⌉ − 1)L, ⌈ m ⌉L]. There are two possible error
             i            i
                                                                           above, we have an upperbound of the probability for F n(i)
events regarding any test i.                                               (assume that any delays larger than l at test i will interfere
                                                                           the tests j following i where j ∈ [max(i%m, i − m − β −
  •   F p(i): test i is negative, but some jamming signals                 1), i − m]):
      are delayed from previous tests and interfere this test,                                      ∫ +∞
      where we have a false positive event;                                         (1 − (1 − p)d )      P(w)dw
  •   F n(i): test i is positive, but the jammer activated in                                       l                               
      this test delayed its jamming signals to some subse-                                  ∑∫ ( i−j +1)L
                                                                                                    m

      quent tests, meanwhile, no delayed jamming signals                            · 1 −                 P(w)dw(1 − (1 − p)d )
                                                                                             j     ( i−j −1)L
                                                                                                      m
      from previous tests exists, where we have a false
      negative event.                                                          ≤ (1 − (1 − p)d )(1 − 2(1 − (1 − p)d ))(β − l)/β
                                                                               ≤ (1 − (1 − p)d )(1 − 2(1 − (1 − p)d ))
    Since the jammers in this paper are assumed to block                   So the expected number of false negative tests is at most
communications only on the channels where transmissions
are sensed, for the following analysis, we claim that the                         T − ≤ (1 − (1 − p)d )(1 − 2(1 − (1 − p)d ))t       (4)
interferences can only happen between any two tests i, j
                                                                           Therefore, we could use a union bound and obtain a worst-
with i ≡ j(mod m). Denote the delay of jamming signals
                                                                           case error rate of each test:
as a random variable X = {x(1), x(2), x(3), · · · x(t)}
                                                                                          p
where x(i) is the delay for possible jamming signals arisen                      γ =        + 2(1 − (1 − p)d )(1 − p)d
from test i. (1) For event F p(i), consider the test i − m,                               2
                                                                                          +(1 − (1 − p)d )(1 − 2(1 − (1 − p)d ))
in order to have its jamming signals delayed to test i, we
have a bound on x(i − m) ∈ (0, 2L). Similarly, in order to                           = (10τ − 8τ 2 − τ −d − 1)/2
have the signals of any test j delayed to i, we have x(j) ∈
                                                                           where τ = (d/(d + 1))d . Intuitively, we can have an
[( i−j − 1)L, ( i−j + 1)L]. Further assume the probability
    m             m                                                        upperbound on the number of error tests as z = γt =
density function of X is P(i) = Pr[X = x(i)]. Consider
                                                                           (10τ −8τ 2 −τ −d −1)/2, and take it as an input to construct
all the tests prior to i, which are i%m, 1+i%m, · · · , i−m,
                                                                           the (d, z)-disjunct matrix. However, notice that z depends
we then have the probability for F p(i):
                                                                           on t, i.e., the number of rows of the constructed matrix, we
                                                                           therefore derive another bound of t related to γ, as shown
                ∑
                i−m     ∫   ( i−j +1)L
                               m
                                                                           by Corollary B.1 in the appendix.
  (1 − p)   d
                                         P(w)dw(1 − (1 − p)d )       (3)
                j=i%m   ( i−j −1)L
                           m
                                                                           5.2 Error-tolerant Asynchronous Testing within
To simplify this expression, we assume that X/L follows a                  each testing team
uniform distribution within the range [0, β] with a small β,               By applying the derived worst-cast number of error tests
which is reasonable and efficient for attackers in practice.                into the ETG construction, we can obtain the following
Since the nature of jamming attacks lies in adapting the                   algorithm where tests are conducted in an asynchronous
attack frequency due to the sensed transmissions, too large                manner to enhance the efficiency.
delay does not make sense to tackle the ongoing trans-                        As shown in Algorithm 2, after all the groups are
missions. Under a uniform distribution, the probability of                 decided, conduct group testing on them in m pipelines,
F p(i) becomes:                                                            where in each pipeline any detected jamming signals will
                                                   ∑
                                                   i−m
                                                                 2
                                                                           end the current test and trigger the next tests while groups
          (1 − (1 − p)d )(1 − p)d                                          receiving no jamming signals will be required to resend
                                                                 β
                                           j=max i%m,i−m−β−1               triggering messages and wait till the predefined round time
                                             i        2                    has passed. These changes over the original algorithm,
      = (1 − (1 − p)d )(1 − p)d (⌈             ⌉ − 1)                      especially the asynchronous testing are located in each
                                             m        β
IEEE TRANSACTION ON MOBILE COMPUTING                                                                                                              11



testing team, thus will not introduce significant overheads,                            6.2 Benefits for Jamming-resistent Routing
however, the resulted error rates are limited to a quite low                           JAM[11] proposed a jamming-resistent routing scheme,
level.                                                                                 where all the detected jammed areas will be evaded and
    input : n victim nodes in a testing team                                           packets will not pass through the jammed nodes. This
    output: all trigger nodes within these victim nodes
    Estimate d as mentioned;                                                           method is dedicated for proactive jamming attacks, which
    Set γ = (10τ − 8τ 2 − τ −d − 1)/2 ; // upper bound of error                        sacrifices significant packet delivery ratio due to the unnec-
    probability for each test
              τ ln n(d+1)2
    Set t = (τ −γ(d+1))2 ;                               // number of rows
                                                                                       essarily long routes selected, though the effects of jamming
    Construct a (d, z)-disjunct matrix using ETG algorithm with t rows, and            signals are avoided. We compare the end-to-end delay be-
    divide all the n victim nodes into t groups accordingly {g1 , g2 , · · · , gt };   tween each sensor node and the base station, of the selected
    // For each round, conduct group testing on m groups                               routes by evading the jammed areas detected by JAM, with
        using m different channels (radios). The testing
        is asynchronous in that, the m groups tested in                                that of the ones evading only trigger nodes. Although there
        parallel do not wait for each other to finish the                              are many existing routing protocols for unreliable network
        testing, instead, any finished test j will trigger
        the test j + m, i.e., the tests are conducted in m                             environments, the aim of this experiment is to show the
        pipelines.                                                                     potential of this service to various applications, instead of
    for i = 1 to ⌈t/m⌉ do
         Conduct group testing in groups gim+1 , gim+2 , gim+m in parallel;            being a dedicated routing protocol.
          If any nodes in group gj with j ∈ [im + 1, im + m] detects jamming              Three key parameters for routing could be the number
          noises, the testing in this group finishes and start testing on gj+m ;
                                                                                       of Jammers J, jamming range R, jamming threshold θ.
          If no nodes in group gj detect jamming noises, while at least one other
          test in parallel detects jamming noises, let all the nodes in group gj       As mentioned earlier, θ indicates the aggressiveness of the
                                                                                                                                    θ 1
          resend 3 more messages to activate possible hidden jammers.                  attacker and the triggering range r ≈ rs ( θ′ ) ξ . Therefore,
          If no jamming signals are detected till the end of the predefined round
          length (L), return a negative outcome for this group and start testing on
                                                                                       with rs , θ′ and ξ as fixed network inputs, the effect of θ
          gj+m ;                                                                       can be exactly indicated by studying the effect of r instead.
    end
                                                                                          The whole network has n = 1500 nodes and sensor
              Algorithm 2: Asynchronous Testing                                        transmission range rs = 50. The results with respect to the
                                                                                       three parameters J ∈ [1, 20], R ∈ [100, 200], r ∈ [50, 150]
                                                                                       are included in Fig.9(a), 9(b) and 9(c) respectively. Notice
6       E XPERIMENTAL E VALUATION                                                      that for each experiments, the other two parameters are
                                                                                       set as the median value of their corresponding intervals.
6.1       Overview                                                                     Therefore, R = 150 for Fig.9(c), which matches the
As a lightweight distribute trigger-identification service, our                         extreme case R = r. Furthermore, for the nodes that are in
solution will be experimentally evaluated from four facets:                            jammed areas for JAM and that are triggers for our method,
    •   in order to show the benefit of this service, we compare                        in another word, unable to deliver packets to or from the
        it with JAM [11] in terms of the end-to-end delay and                          base station, we count the delay as n + 1, which is an
        delivery ratio of the detour routes from the base station                      upperbound of the route length.
        to all the sensor nodes, as the number of sensors n,                              As shown in Fig. 9(a) and 9(b), when j and R increases,
        sensor range rs , and number of jammers J vary within                          the routing delay goes up, which is quite reasonable since
        practical intervals.                                                           the jamming areas get larger and more detours have to be
    •   in order to show the acceleration effect of the clique-                        taken. The length of routes based on JAM quickly climbs
        independent set in this solution, we compare the                               up to the upperbound, while that of our trigger method
        complexity of this solution to our previous centralized                        is much lower and more stable, specifically keeps less
        one [7], with varying the above four parameters,                               than 900 seconds. When triggering range r is small, as
        where both jamming and triggering range R and r                                in Fig.9(c), the end-to-end delay of Trigger-based routing
        are assumed to be known beforehand.                                            is much smaller than the other, while as r increases the two
    •   in order to show the accuracy of estimating the jam-                           approaches each other, since more victim nodes are triggers
        ming range by using the polygon disk cover algorithm,                          now.
        we provide the estimated jamming ranges as well as
        the error rate to the actual values.                                           6.3 Improvements on Time Complexity
    •   in order to show its performance and robustness                                In our previous work [7], we proposed a preliminary idea of
        towards tricky attackers, we provide its false posi-                           this trigger detection, and provided a disk-based solution.
        tive/negative rate, when taking into account those two                         However, its high time complexity limits its usage in real-
        advanced jammer models, as well as the estimation of                           time networks. As mentioned above, the time complex-
        R.                                                                             ity of our new clique-based detection is proved to be
The simulation is developed using C++ on a Linux Work-                                 asymptotically lower than the previous, while the message
station with 8GB RAM. A 1000 × 1000 square sensor field                                 complexities are approaching each other.
is created with uniformly distributed n sensor nodes, one                                 Although the computational overhead for estimating R
base station and J randomly distributed jammer nodes. All                              is asymptotically huge, the phase is not the key part of our
the simulation results are derived by averaging 20 random                              scheme, and can be easily improved by machine learning
instances.                                                                             techniques. Therefore, in this section, we assume that both
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                              1400          JAM                                                                            JAM                                                                       1400        JAM
   average end-to-end delay




                                                                                    average end-to-end delay




                                                                                                                                                                         average end-to-end delay
                                         Trigger                                                               1400     Trigger                                                                               Trigger
                              1200                                                                                                                                                                   1200
                                                                                                               1200
                              1000
                                                                                                                                                                                                     1000
                              800                                                                              1000
                                                                                                                                                                                                      800
                              600
                                                                                                               800
                              400                                                                                                                                                                     600

                              200                                                                              600                                                                                    400

                                     2      4      6   8   10   12   14   16   18                                 100      120      140     160     180      200                                              60        80      100       120    140
                                                   number of jammers                                                              jamming range R                                                                        triggering range r


                                (a) Average end-to-end delay by J                                                (b) Average end-to-end delay by R                                                      (c) Average end-to-end delay by θ

Fig. 9. Benefits for routing


R and r are known beforehand, and validate the theoretical                                                                                                                                                    J=5
                                                                                                                                               Actual R                 50                               60         70            80        90       100
results through simulations on network instances with var-                                                                                     Estimated R         51.9542                           61.378    72.5228       80.7886   92.9285   104.826
ious settings. Specifically, the network size n ranging from                                                                                     R                    3.91%                            2.29%      3.60%         0.99%     3.25%     6.21%
450 to 550 with step 2, transmission rs from 50 to 60 with                                                                                                                                                    J=10
                                                                                                                                               Actual R                 50                               60         70            80        90       100
step 0.2 and number of jammers J from 3 to 10 with step                                                                                        Estimated R         52.9438                           63.496    73.4763       82.4191   93.9339   104.202
1. Parameter values lower than these intervals would make                                                                                       R                    5.88%                            5.83%      4.96%         3.02%     4.37%     4.21%
                                                                                                                                                                                                              J=15
the sensor network less connected and jamming attack less                                                                                      Actual R                 50                               60         70            80        90       100
severe, while higher values would lead to impractical dense                                                                                    Estimated R         51.6574                          65.5034    73.5997       83.4615   96.6998    107.21
                                                                                                                                                R                    3.31%                            9.17%      5.14%         4.33%     7.44%     7.21%
scenarios and unnecessary energy waste.
   Since the length of each reactive attack is equal to the
transmission delay of the object sensor signal, note that in                                                                              Fig. 11.    Estimation error of R

our trigger detection, only one message is broadcast by each
sensor in the testing groups. Therefore, it is reasonable to
                                                                                                                                          than the other. It has slightly more communication over-
predefine the length of each testing round as a constant.
                                                                                                                                          heads (10 messages per victim nodes) but is still affordable
We set this as 1 second, which is far more enough for
                                                                                                                                          to power-limited sensor nodes.
any single packet to be transmitted from one node to its
neighboring nodes. Henceforth, the time cost shown in
Fig. 6.3 only indicates the number of necessary rounds to                                                                                 6.4 Accuracy in Estimating Jammer Properties
find out all the triggers, and can be further reduced. The                                                                                 Though the estimate of jamming range R is only to provide
message complexity is measured via the average message                                                                                    an upperbound for R, such that the testing teams obtained
cost on each sensor node.                                                                                                                 accordingly are interference-free, we are also interested in
   As shown in Fig. 10(a) and 10(b), this clique-based                                                                                    the accuracy of this estimation. As shown in Fig. 11, we
scheme completes the identification with steadily less than                                                                                investigate the error rate ∆R for R = [50, 100] when there
10 seconds, compared to the increasing time overhead with                                                                                 are respectively J = 5, 10, 15 jammers.
more than 15 seconds of the disk-based solution, as the                                                                                      Two observations are straightforward from these results:
network grows denser with more sensor nodes. Meanwhile,                                                                                   (1) all the estimated values are above the actual ones,
its amortized communication overheads are only slightly                                                                                   however, less than 10% difference. This meets our require-
higher than that of the other solution, whereas both are                                                                                  ment for a tight upperbound of R. (2) the error rates in
below 10 messages per victim node. Therefore, the new                                                                                     case of fewer jammers are relatively lower than those with
scheme is even more efficient and robust to large-scale                                                                                    more jammers. This is because jammers could have large
network scenarios.                                                                                                                        overlaps in their jamming areas, which introduces estimate
   With the sensor transmission radius growing up, the time                                                                               inaccuracies. Thanks to the accurate estimation of R, the
complexity of the disk-based solution gradually ascends                                                                                   overall false positive/negative rate is quite small, as to be
(Fig. 10(d) and 10(c)) due to the increased maximum degree                                                                                shown next.
∆(H) mentioned in the above analysis. Comparatively,
the time cost of clique-based solution remains below 10
seconds, while the message complexity still approximates                                                                                  6.5 Robustness to Various Jammer Models
the other one.                                                                                                                            In order to show the precision of our proposed solution
   Since sensor nodes are uniformly distributed, the more                                                                                 under different jamming environments, we vary the two
jammer nodes placed in the networks, the more victim                                                                                      parameters of the jammer behaviors above: Jammer Re-
nodes are expected to be tested, the identification complex-                                                                               sponse Probability α and Testing Round Length/Maximum
ity will therewith raises, as the performance of disk-based                                                                               Jamming Delay L/X and illustrate the resulted false rates
scheme shows in Fig. 10(f) and 10(e). Encouragingly, the                                                                                  in Fig. 12(a) and 12(b). To simulate the most dangerous
proposed scheme can still finish the identification promptly                                                                                case, we assume a hybrid behavior for all the jammers,
with less than 10 seconds, which grows up much slower                                                                                     for example, the jammers in the simulation of Fig. 12(a)
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                                  30                                                                                            20                                                                                               50




                                                                                                   number of message per node
                                              Disk-based                                                                                   Disk-based                                                                                        Disk-based
                                            Clique-based                                                                        18       Clique-based                                                                            45        Clique-based
                                  25
    time complexity (sec)




                                                                                                                                                                                                        time complexity (sec)
                                                                                                                                16                                                                                               40
                                                                                                                                14                                                                                               35
                                  20
                                                                                                                                12                                                                                               30
                                  15                                                                                            10                                                                                               25
                                                                                                                                 8                                                                                               20
                                  10                                                                                                                                                                                             15
                                                                                                                                 6
                                                                                                                                 4                                                                                               10
                                  5
                                                                                                                                 2                                                                                                5

                                            460        480        500        520        540                                              460      480        500       520   540                                                      50         52         54         56          58       60
                                                    number of sensor nodes n                                                                     number of sensor nodes n                                                                                 triggering range r


                                                  (a) # Rounds by n                                                                            (b) # Messages by n                                                                               (c) # Rounds by r

                                  20                                                                                            40
    number of messages per node




                                              Disk-based                                                                                   Disk-based                                                                            14          Disk-based
                                  18        Clique-based                                                                        35       Clique-based                                                                                      Clique-based


                                                                                                   time complexity (sec)




                                                                                                                                                                                                        time complexity (sec)
                                  16                                                                                                                                                                                             12
                                                                                                                                30
                                  14
                                                                                                                                25                                                                                               10
                                  12
                                  10                                                                                            20                                                                                                8
                                   8                                                                                            15                                                                                                6
                                   6                                                                                            10                                                                                                4
                                   4
                                                                                                                                5                                                                                                 2
                                   2
                                       50         52         54         56         58         60                                     3     4       5     6         7    8    9        10                                              3      4        5        6      7        8        9   10
                                                         triggering range r                                                                        number of jammers J                                                                                number of jammers J


                                                  (d) # Messages by r                                                                          (e) # Rounds by J                                                                                 (f) # Messages by J

Fig. 10. Time and Message complexity


                                                                                                                                                                                                 0.3
not only launch the jamming signals probabilistically, but                                                                                                                                                 fp
                                                                                                                                                                                                0.25       fn
also delay the jamming messages with a random period
                                                                                                                                                                                                 0.2
                                                                                                                                                                                   False Rate



of time up to 2L. On the other hand, the jammers in the
                                                                                                                                                                                                0.15
simulation of Fig. 12(b) respond each sensed transmission
                                                                                                                                                                                                 0.1
with probability 0.5 as well. All the simulation results are
                                                                                                                                                                                                0.05
derived by averaging 10 instances for each parameter team.                                                                                                                                        0
   As shown in both figures, we consider the extreme                                                                                                                                                    0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
                                                                                                                                                                                                                                 Jammer Response Probability p
cases where jammers respond transmission signals with a
probability as small as 0.1, or delay the signals to up to                                                                                                                                       (a) Probabilistic Jammer Response
10 testing rounds later. This actually contradicts with the
                                                                                                                                                                                                 0.3
nature of reactive jamming attacks, which aim at disrupting                                                                                                                                                fp
                                                                                                                                                                                                           fn
                                                                                                                                                                                                0.25
the network communication as soon as any legitimate trans-
                                                                                                                                                                                                 0.2
                                                                                                                                                                                   False Rate




mission starts. The motivation of such parameter setting is
                                                                                                                                                                                                0.15
to show the robustness of this scheme even if the attackers
                                                                                                                                                                                                 0.1
sense the detection and intentionally slow down the attacks.                                                                                                                                    0.05
The overall false rates are below 20% for any parameter                                                                                                                                           0
values.                                                                                                                                                                                                0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
                                                                                                                                                                                                                                Round Length / Max Jamming Delay
   In Fig. 12(a), when α > 1/2 which corresponds to prac-
tical cases, we find that the false negative rates generally                                                                                                                                            (b) Random Jamming Delay
decrease from 10% to 5% as α increases. Meanwhile the
false positive rate grows gently, but is still below 14%, this                                                                                                Fig. 12. Solution Robustness
is because as more and more jamming signals are sent, due
to their randomized time delays, more and more following
tests will be influenced and become false positive. In Fig.
12(b), considering the practical cases where L/X > 1/2,                                                                                                       mitigation, both of which have been well studied and
both rates are going down from around 10% to 1%, since                                                                                                        developed with various defense schemes. On the one hand,
the maximum jamming delay becomes shorter and shorter                                                                                                         a majority of detection methods focus on analyzing specific
compared to the testing round length L, in which case,                                                                                                        object values to discover abnormal events, e.g., Xu et.
the number of interferences between consecutive tests is                                                                                                      al [16] studied a multi-model (PDR, RSS) to consistently
decreasing.                                                                                                                                                   monitor jamming signals. Work based on similar ideas
                                                                                                                                                              [17][15][14] improved the detection accuracy by investigat-
                                                                                                                                                              ing sophisticated decision criteria and thresholds. However,
7                            R ELATED W ORKS                                                                                                                  reactive jamming attacks, where the jammer node are not
Existing countermeasures against jamming attacks in WSN                                                                                                       continuously active and thus unnecessary to cause huge de-
can be categorized into two facets: signal detection and                                                                                                      viations of these variables from normal legitimate profiles,
IEEE TRANSACTION ON MOBILE COMPUTING                                                                                             14



cannot be efficiently tackled by these methods. In addi-         embedded in the plane using O(|V |2 ) area units such that
tion, some recent works proposed methods for detecting          its vertices are at integer coordinates and its edges consist
jammed areas [11] and directing normal communications           of line segments of the form x = i or y = j, for any
bypass possible jammed area using wormhole [18]. These          integers i and j.
solutions can effectively mitigate jamming attacks, but their      Theorem A.1: Clique-Independent Set problem is NP-
performances rely on the accuracy of detection on jammed        hard on Unit Disk Graph.
areas, i.e. the transmission overhead would be unnecessarily         Proof: Given an instance G′ = (V ′ , E ′ ) of such a MIS
brought up if the jammed area is much larger than its           problem, whose optimal value is denoted as M IS(G′ ), we
actual size. On the other hand, mitigation schemes which        construct an instance G = (V, E) of the CIS problem as
benefit from channel surfing [13], frequency hopping and          follows:
spatial retreats[12], reactively help legitimate nodes escape                     ′
                                                                   • Embed G in the plane in the way mentioned above
from the jammed area or frequency. Unfortunately, being               [22].
                                                                                                        ′
lack of pre-knowledge over possible positions of hidden            • For each node vi ∈ V , attach two new nodes vi1
reactive jammer nodes, legitimate nodes cannot efficiently             and vi2 to it and form a triangle Ni = {vi1 , vi2 , vi3 },
evade jamming signals, especially in dense sensor network             where each edge of this triangle Ni is of a unit length
                                                                            √
when multiple mobile nodes can easily activate reactive               r = 33 .
jammer nodes and cause the interference. For the sake of           • Since each nodes vi is incident to at most three
overcoming these limitations above, in [7] we studied on              edges, for all edges (vi , u), · · · , (vi , v), move their
the problem of identification trigger nodes with a short               endpoint from vi to different vij s, e.g., (v1 , u) changes
period of time, whose results can be employed by jamming-             to (v11 , u) and (v1 , v) to (v12 , v). Afterwards, for
resistent routing schemes, to avoid the transmissions of              each of such edges e = (u, v), assume that it is
these trigger nodes and deactivate the reactive jammer                of length t, we divide it into t pieces and replace
nodes. In this paper, we complete this trigger identification          each piece with a concatenation of 2 triangles (not
procedure as a lightweight service, which is prompt and               necessarily equilateral), as shown in Fig. 13(b). There-
reliable to various network scenarios.                                fore, any edge eij = (vi , vj ) ∈ E ′ of length |eij |
                                                                      becomes a concatenation of 2|eij | 3-cliques, denoted
8   D ISCUSSION              C ONCLUSIONS                                                             |e |,1 |e |,2
                      AND                                             as {c1,1 , c1,2 , c2,1 , · · · cijij , cijij }. Because of the
                                                                            ij    ij     ij
One leftover problem to this service framework is the                 triangles Ni s, the two triangles at each corner of Fig.
jammer mobility. Although the identification latency has               13(b) may need slight stenches, which can be done in
been shown small, it would not be efficient toward jammers             polynomial time.
that are moving at a high speed. This would become an              • The resulting graph G is then a unit disk graph with
                                                                                    √
interesting direction of this research.                               radius r = 33 .
   Another leftover problem is the application of this ser-
vice. Jamming-resistent routing and jammer localizations                                            V1
are both quite promising, yet the service overhead has to
be further reduced to for real-time requirements.
   As a summary, in order to provide an efficient trigger-
                                                                                      V2                           V4
identification service framework, we leverage several op-
timization problem models and provide corresponding al-
                                                                                                    V3
gorithms to them, which includes the clique-independent
                                                                                           (a) G′ = (V ′ , E ′ )
problem, randomized error-tolerant group testing, and min-
                                                                                                    N1
imum disk cover for simple polygon. The efficiency of this
framework is proved through both theoretically analysis
toward various sophisticated attack models and simulations
under different network settings. With abundant possible                    N2                                          N4
applications, this framework exhibits huge potentials and
deserves further studies.

A PPENDIX A
NP- HARDNESS         OF   CIS   ON   UDG                                                            N3
                                                                                            (b) G = (V, E)
We prove the NP-hardness of this problem on UDG via a
polynomial-time reduction from the Maximum Independent          Fig. 13. Polynomial Time Reduction
Set problem on planar graph with maximum node degree
3 to it.                                                          The reduction is as follows:
   From [21], the Maximum Independent Set problem is NP-        (⇒): if G′ has a maximum independent set M , for
hard on planar graph with maximum degree 3, and from            each ui ∈ M , we choose cliques of two kinds in the
[22], any planar graph G with maximum degree 4 can be           corresponding instance G: (1) the clique Ni at ui ; (2)
IEEE TRANSACTION ON MOBILE COMPUTING                                                                                                          15



for each incident edge eij = (ui , uj ), choose cliques
                               |e |,2
{c1,2 , c2,2 , c3,2 , · · · , cijij }. Since the clique Nj at uj
   ij    ij     ij
                                                                      (infeasible by assert 1)
                                |e |,2                                or
shares a vertex with cijij , it cannot be selected. For any
edge ejk = (uj , uk ) where uj ∈ M and uk ∈ M , choose
                                        /             /                                   z − 1 + ln s + (d + 1) ln n
                                 |e |,2                                  p(1 − p)d ≥
cliques {c1,2 , c2,2 , · · · cjkjk }. It is easy to verify that all
              jk     jk                                                                     √          t
the cliques selected are vertex-disjoint from each other.                                           ln2 (snd+1 ) + 2(z − 1) ln snd+1
Assume that after embedding G′ into the plane, each                                           +
                                                                                                           t
node vi ∈ V ′ has coordinate (xi , yi ), then edge length
|eij | =∥ vi , vj ∥1 = |xi −xj |+|yi −yj |. Therefore if we have      Therefore, we can derived the lower bound
                                                                                (            )
an independent set of size |M | = k for G′ , we then have
                                                  ∑                               (d + 1)d+1
a clique independent set of size k ′ = k + (i,j)∈E ′ |eij |.              t≥2                  (z − 1 + ln s + (d + 1) ln n)
                                                                                      dd
(⇐): if G has a clique independent set of size k ′ , since the
lengths of the embedded edges are constant, then G′ has
                                                    ∑                   Corollary B.1: Given that each test has an indepen-
exactly an independent set of size k = k ′ − (i,j)∈E ′ |eij |.
                                                                      dent error probability γ, M is (d, z)-disjunct matrix with
The proof is complete.                                                      τ ln n(d+1)2 −2τ (d+1) ln 1
                                                                      t =          (τ −γ(d+1))2
                                                                                                      s
                                                                                                        with probability (1 − 1 ) for
                                                                                                                              s
                                                                      arbitrarily large s.
A PPENDIX B                                                                Proof: Substituting z by γt in the proof above com-
C ONSTRUCTION OF R ANDOMIZED E RROR -                                 pletes this proof.
TOLERANT d- DISJUNCT M ATRIX
  (Theorem )   B.1: M is (d, z)-disjunct matrix with t =              R EFERENCES
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