WiFi repeater deployment for improved communication in confined

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					       WiFi repeater deployment for improved
      communication in confined-space urban
                  disaster search
Alexander Ferworn1, Nhan Tran1,2, James Tran1, Gerry Zarnett1, Farrokh Sharifi2
                 1
                  Network-Centric Applied Research Team
                      Department of Computer Science
                   2
                    Department of Mechanical Engineering
                            Ryerson University
                           Toronto, ON, Canada

                                                      monitored at the perimeter of the space being
Abstract                                              entered.
         We examine the use of WiFi repeaters
to extend the range of wireless devices employed               If a robot or similar device is to be used,
in confined space search in an Urban Search           communication and control of the devices are
                                                      always a challenge. Typically, robots are
and Rescue (USAR) environment. Confined
                                                      equipped      with    a tether       that    allows
spaces are often encountered by emergency
                                                      communication between a responder and the
responders at disaster sites. They typically are
too small or hazardous to be searched by              device. Another option that avoids tether
humans but may be accessible by rescue robots         management issues is the use of wireless control.
controlled and monitored from outside the space.      In this study we examine the use of a WiFi
                                                      network used for control and monitoring
         We employed a purpose-built confined         employing relaying units called “pucks” to
space training area constructed of steel              extend the communication range of interaction
reinforced sewer tubing to determine strategies       possible. We employ a purpose-built confined
for maintaining a usable signal for devices           space training area constructed of steel
deployed     within     spaces     with similar       reinforced sewer tubing to simulate the effects of
                                                      rebar and other debris often encountered in
characteristics at disaster sites.
                                                      USAR environments.
Keyword: USAR, WiFi, repeater, rescue
robots, confined space                                2        Tethered Teleoperation
1        Introduction                                          Most Urban Search and Rescue (USAR)
         Emergency responders are often faced         robots currently in operation fall into two
with confined spaces. Soon after the initial entry    categories, tethered or analog radio frequency
of the search site searchers find themselves          communication [1, 2]. Tethered robots benefit
blocked due to debris or other physical barriers      from having the possibility of a continuous
present that are due to the events of the disaster.   power feed, dependable communications, [3]
While there may be much debris, there are             and relatively easy retrieval from the search
usually many voids or other confined spaces that      space. The problems with tethered robots are
may contain casualties or lead to larger spaces       typically associated with tether management.
and access paths. Confined spaces are small and       Hard cable tethers require large cumbersome
usually hazardous spaces typically inaccessible       spools, and the tether itself is present as a load
by humans but may be accessible by means such         on the robot Lighter tethers may be broken,
as rescue robots. These robots are often quite        cause snagging or become tangled in the robot
small and can be remotely controlled and              itself [4].     All these factors are further
exacerbated with multiple robots in one confined              The use of analog and digital wireless
search space.                                        communication for USAR robots has been
                                                     examined by many investigators in the literature
2.1      Analog Radio Control                        [1,4-6, 9] Tunnel rat[9] was deployed for sewer
                                                     reconnaissance with the use of analog RF. It
                                                     suffered from significant breakup and signal
         Robots communicating via wireless           degradation. Similar implementations in
means are used when possible because they are        confined space proved that it was nearly
often much easier to deploy. Most of these           impossible to maintain communications beyond
robots use analog radio communication[5] This        two meters into a search tunnel. There have
form of teleoperation eliminates many problems       been example of using robots deployed as relays
encountered by tethered robots, but wireless         to maintain steady communication when entering
robots encounter a completely different set of       buildings and other relatively open and
problems. Analog Radio Frequency (RF) signals        unobstructed hazardous environments [1,4,5] but
suffer    from    interference,     scatter,   and   this technique may not be applicable in more
attenuation.[6] Security is also a major problem.    demanding settings.
Any other device capable of reproducing the
same RF signal can take control of the system or              Player[6],    primarily    a   protocol
inadvertently “jam” the signal so that control and   implemented as a TCP socket server that allows
monitoring are lost.                                 devices on the network to access other sensors
                                                     on the network. Player does not have any direct
2.2      WiFi Selection                              application to confined space search. However, it
                                                     may be adapted to provide a middle layer to the
       An    alternative to analog       RF          ad hoc network to distributed control for robots
communication is the use of digital wireless         and sensory feedback.
communication. Perhaps most popular among
the many alternatives is the use of WiFi             Confined space search imposes different
communications.                                      challenges than other environments. Spaces are
                                                     always unstructured, usually very small and
         WiFi (IEEE 802.11) systems provide a        debris filled which causes havoc to RF signals
very attractive framework for use within an          both analog and digital.
USAR environment because of their simple
architecture, robustness, expandability and their    Low-cost and effective repeater “pucks” can be
cost effectiveness. By using WiFi we inherit a       easily be fabricated from WiFi components and
reliable network architecture, and the ability to    conveniently powered by batteries.
set up an ad hoc network using inexpensive off-
the-shelf equipment.                                 The use of relaying pucks to extend signal range
                                                     may be ideal for confined space search due to
         The use of WiFi’s Digital-spread-           their low maintenance and small size. We are
spectrum      (DSS)      mitigates  interference,    aware there are many robots systems that have
intentional or unintentional jamming,[4] and may     been developed with WiFi communication and
automatically resolve many problems that plague      some even use relay units to extend their range,
analog RF communication.                             but we are not aware of one that is design to
                                                     address issues      encountered     in   USAR
          An additional advantage that we explore    environments in confined space search.
in this paper is that the range of a WiFi networks
may be easily extended through the use of
repeating transmitters that share all the            4       Confined Space Testing
characteristics of the other WiFi components of
the network.
                                                              With the collaboration of the Ontario
                                                     Provincial Emergency Response Team (PERT)
                                                     of the Ontario Provincial Police (OPP), we
3        Related Work                                employed a purpose-built confined space training
                                                     and testing facility as a simulated disaster site
exhibiting the radio characteristics           often   meters directly in front of the access point
associated with confined space.                        indicated (figure 1). This acted as a base where
                                                       all communication would be received.
         The facility is constructed of steel
reinforced sewer tubing meant to simulate                        The measurement of the signal strength
structures and materials often found in urban          and quality was done starting at the access point
areas and typically at disaster sites. The facility    to the facility. The signal quality was recorded at
is composed of a series of interconnecting pipes       50 cm increments. The analog RF signal testing
forming a rectangle shape 13.45 meters by 11.4         was accomplished using 2.4Ghz analog
meters, depicted by figures 1 and 2. We have           transmitter/receiver pair transmitted the signal
repurposed the facility to provide a suitable          from an infrared camera. For the WiFi signal
location for analog and digital RF testing.            tests (with repeater) we employed commercial
                                                       routers and repeaters. These were D-Link DI-624
                                                       and D-Link DWL-2100AP respectively. Two
                                                       laptops with IEEE802.11b capability were used
                                                       to transmit video.

                                                       4.2      Analog Video Test

                                                                When a robot is sent into an
                                                       environment instead of a person, often, the video
                                                       signal is the only means of monitoring the
                                                       robot’s situation. Therefore it is critical that the
                                                       video feedback is of a high quality. For our RF
                                                       signal tests, we used a small analog RF wireless
                                                       camera system. The wireless camera transmitted
                                                       video signals via RF to a receiver that output the
      Figure 1. Top view of confined space
                                                       video to a monitor. The video receiver and
                                                       monitor were located at the base station. The
                                                       wireless camera was moved through Path 1 until
                                                       the video signal was completely lost. Then the
                                                       position where the signal loss occurred was
                                                       recorded.

                                                       4.3      WiFi Signal Strength Test

                                                                The router was setup at the base station.
                                                       A laptop equipped with signal strength
                                                       measuring software was connected to the
                                                       router’s wireless network. The laptop was moved
                                                       through Path 1 at 0.5m increments and readings
                                                       of the signal strength were recorded until the
                                                       signal was lost. The position of the signal loss
      Figure 2. Actual Facility shown during
                                                       was recorded.
                   construction

                                                       4.4      WiFi Video Test
4.1      Test Setup
                                                                This test examined video transmission
         After consultation with PERT members          through WiFi transmission. For this test, the
we elected to conduct testing in the same manner       router and two laptops were used. Laptop 1 and
as a deployment would dictate in operation. The        the router were at the base station. Laptop 2,
confined space was completely sealed with 1.5          equipped with a webcam was moving through
cm thick steel doors, except the access point. A       the structure along Path 1. Both laptops were
master communication point was set up 15               connected on the same network through the
router. Laptop 2 was broadcasting the video feed     the WiFi signal range can be extended to cover a
to Laptop 1. When Laptop 1 could not receive         search space through the use of a repeater puck.
the video signal from Laptop 2 indicating the        It is clear that analog RF is insufficient in this
network connection between Laptop 1 and              type of hostile communication environment and
Laptop 2 was lost, the position of Laptop 2 was      only provided up to 1.50 m coverage into the
recorded.                                            search space. Digital RF was considerably better
                                                     with a consistent range of 12.0 m inside the
5        Trials Results                              tunnels.


          Tests reviewed how disruptive
reinforced concrete can be to analog RF
communication.        Consistent results showed
reception for a clear video signal at 1.0 meter
inside the structure and a very weak signal at 1.5
m. The signal was completely lost at 2.0 m.




                                                                      Figure 4. Puck

                                                             With the use of one puck constructed
                                                     from readily available materials (Figure 4) we
                                                     were able to increase the operational range to
                                                     cover approximately 80 % of the search space.

          Figure 3. Signal Penetration               We see that the miniaturization of repeater pucks
                                                     and improved power efficiency will be important
          While the WiFi network also                factors for future work. We plan to test this
communicated at 2.4ghz, it did considerably          system in many different confined spaces to see
better than the analog system. We were able to       how signal range is affected by different
broadcast from inside the structure along Path 1     structures.      Of particular interest is the
(figure 3) up to the 12.0 m point. At 12.5 meters,   development of appropriate algorithms for the
the connection to network was dropped and the        effective deployment of pucks to maximize the
video feed stopped.                                  effective search distance.
          The thirds set of tests with the puck
repeater demonstrated that while the signal was
still lost at 12.0 m, communication could be
                                                     References
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