# A FREQUENCY HOPPING SPREAD SPECTRUM TRANSMISSION SCHEME FOR by liuhongmei

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```									A FREQUENCY HOPPING SPREAD
SPECTRUM TRANSMISSION SCHEME
FOR UNCOORDINATED COGNITIVE

Xiaohua (Edward) Li and Juite Hwu
Department of Electrical and Computer Engineering
State University of New York at Binghamton
{xli, jhuw1}@binghamton.edu
http://ucesp.ws.binghamton.edu/~xli
Contents

1. Introduction
2. System model
3. New FHSS transmission for cognitive
4. Demodulation and Performance analysis
5. Simulations
6. Conclusions
1. Introduction
   Cognitive radios (CRs)
   Detect and utilize spectrum white spaces
   Should avoid interfering primary users
   A major issue: “Chicken-and-Egg Problem”
   CRs are initially not synchronized (e.g., in picking spectrum)
for transmission
   Transmission is required to negotiate such synchronization
   Our goal
   Develop a transmission scheme for uncoordinated CRs,
tolerable to spectrum/channel uncertainty and spectrum
sensing errors
Introduction (cont’)

   Basic idea: Frequency-hopping over uncertain
spectrum slots
   CR transmitters and receivers hop over available
spectrum slots
   Hopping pattern determined by:
   Spreading codes (shared)
   Spectrum detection results (independent)
   Channel selection rules (shared)
Introduction (cont’)
   Assumptions
   CR transmitters and receivers do have
   Some common spreading codes
   A common channel selection rule
   Common procedure of adapting transmission
parameters, such as symbol rate, modulations, etc
   CR transmitters and receivers do not
   have common spectrum white space information
2. System model
FI 1             Frequency
F0                  F1
segment
f 0,0 f 0,1       f 0, J 1        f1, J 1                 f I 1, J 1 Frequency band

   Spectrum slots for frequency hopping
    Divide the spectrum into I segments
    Divide each segment into J frequency bands
    Each band is a basic slot for frequency hopping,
which we call “channel”
   CR transmitters and receivers know slot structure,
but do not know which slot is available in each time
2. System Model
   Major problem                                                 Far away
Tx                               Rx
     A channel may be available to a                                   Noise
transmitter but unavailable to a                                  source
1, if channel fi , j available
   Define parameters:                            ai , j  
0, else
1, if f i , j detected available          1, if f i , j detected available
                                          
ti , j                     to transmitter ri , j                        to receiver
0, else                                   0, else
                                          

P[tij  rij ]  Pd  0
2. System Model
F0                        F1                                 FI 1

Segmentation-based               f 0,0 f 0,1       f 0, J 1               f1, J 1                             f I 1, J 1

{
{
spectrum detection:
Transmit                               Channel
When the CR transmits in      Receive                              information
collection
a channel, it also collects
Tx, Rx’s
information about the                     antenna
channels of next segment.                       F0                        F1                               FI 1

f 0,0 f 0,1       f 0, J 1               f1, J 1                             f I 1, J 1

{
{
Transmit                            Channel
collection
Tx, Rx’s
antenna
3. New FHSS transmission
   To transmit a sequence s k , k  0,           , K 1
   Each symbol spreaded into M chips
s k  s k ,m
   This procedure is identical to CR transmitters and
   Spectrum slot selection
   Each chip is to be transmitted via a channel of ith segment
Fi
i   kM  m I , m  0,              , M 1

   Transmitters and receivers use a common binary sequence
cn to determine channel selectability in this segment
Tx: ui, j  ti , j c kM m J  j ,            1, if fi , j is selectable
ui , j  
0, else

Rx: wi, j  ri , j c kM m J  j ,            1, if fi , j is selectable
wi , j  
0, else
   Channel selection rule
   There may be many channels selectable in each segment
   Each CR Tx or Rx needs to select one channel to
   Distributed channel selection means Tx and Rx may
choose different channels  synchronization problem
   Smart channel selection rule can alleviate this problem
   A simple rule: choose the first available channel of this segment
   Secondary transmitter use fi,j1 if ui,j1=1
   Secondary receiver use fi,j2 if wi,j2=1
   Successful transmission→ Tx and Rx selected the same
channel, i.e., j1=j2

Emit message
Transmitter

noise
f 0,0 f 0,1                                                             f 0, J 1

available             j1≠j2
channel

Transmitter

Match

f 0,0 f 0,1                                            f 0, J 1

available   j1=j2
channel
Illustration of multiple CR transmissions using our scheme
Signal
y
y
y       y
power   y                   y       y
y       y       x
User A’s Symbol
y       y                          decoding
y
y
y       x
x
Signal          Noise          Detection for user A
x
x               Signal collision
x
power   x
x
User B’s Symbol
x
4. FHSS demodulation and
performance analysis
   FHSS/MFSK demodulation
   Vector symbol model for FHSS/MFSK signals
T
s k ,m   sk ,m,0 , sk ,m,1 ,
                          , sk ,m, L 1  ,


 s0,0,0       s0,1,0      s0, M 1,0      s1,0,0          sK 1, M 1,0 
 s            s0,1,0      s0, M 1,1                                      
 0,0,1                                                                    
                                                                          
                                                                          
 s0,0, L1   s0,1, L1   s0, M 1, L1   s1,0, L1       sK 1, M 1, L1 
   FHSS/MFSK received signal model
x k ,m  I j1  j2 G i , j 2s k ,m  v i , j 2 ,
Baseband channel matrix
 xk ,m,0                     gi , j2 ,0                    sk ,m,0   vk ,m,0 
                                                                                    
               I j1  j2                                                         
 xk ,m, L 1                              gi , j2 , L 1   sk ,m, L 1  vk ,m, L 1 
                                                                                    
1, if   j1  j2
I j1  j2                             Frequency slot synchronization
0, if   j1  j2
indicator function
   Demodulations: coherent demodulation
M 1                    M 1                                     M 1
yk   G   H
i , j2   x k ,m   G    H
i , j2   G i , j2 I j1  j2 s k ,m   G iHj2 vi , j2
,
m0                     m0                                       m0

M 1                                     M 1
yk ,l   gi , j2 ,l I j1  j2 s k ,m   gi*, j2 ,l vi , j2 .
2
Element-wise
description
m 0                                     m 0

2
Coherent: Maximum                           arg max yk ,l
Likelihood detection                                t  0, , L 1

   Demodulations: non-coherent demodulation
M 1
yk ,l   | xk , m,l |2
m0
4. FHSS demodulation and
performance analysis
   Performance analysis
   Major issue: Tx and Rx may use difference frequency slots
 channel mismatch
   SNR for coherent demodulation
ˆ
M 2 s2
 coherent   
M  v2
M 1
where M   I j1  j2 is the number of matched
ˆ
m0

frequency slot selections among M selections.
   Performance is limited by the correctness of
frequency-selection
   Assume mismatch probability pd be the probability that
there is mismatch in the first j channels
   With our simple channel selection rule

Pj  1  1  pd 
j
   Average channel mismatch probability
1 J 1 
PJ   1  1  pd   .
j

J j 0           
   For every M transmissions, number of correct matches

 1 J 1
ˆ  M 1  P   M 1   1  1  p  j  

M           J                       d
 J j 0                 
5. Simulations
BER as functions of SNR under various mismatch probability
0
Spreading gain M=40                     10

Symbol amount
K=100
Segments I=20                            -1
10
Bits error rate

J=100
channels/segment
-2
10         pd=0
pd=0.02
pd=0.05
pd=0.1
pd=0.3
-3
10
0    2         4       6         8         10     12       14    16
Signal to noise ratio (dB)
5. Simulation
verious spreading gain when pd=0.1
0
10
Mismatch pd≒0.1
Symbol amount K=100
Segments I=20
J=100                                    -1
10
channels/segment
Bits error rate

M=40
M=30
-2
10            M=20

-3
10
0   2     4          6         8         10     12   14   16
Signal to noise ratio (dB)
6. Conclusions
   Developed an FHSS-FSK transmission scheme for