A FREQUENCY HOPPING SPREAD SPECTRUM TRANSMISSION SCHEME FOR by liuhongmei

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									A FREQUENCY HOPPING SPREAD
SPECTRUM TRANSMISSION SCHEME
FOR UNCOORDINATED COGNITIVE
RADIOS


     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
   radios
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
             receiver
                                                           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
                                                        Receive                           information
                                                                                           collection
                                                                      Tx, Rx’s
                                                                      antenna
    3. New FHSS transmission
   Spreading
       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
        receivers
   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
        transmit or receive
       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



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


                                                                     available             j1≠j2
                                                                      channel

                                                                            Transmitter

     Match

                                                                                Receiver
             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
              spreading
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
              spreading
                            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
    uncoordinated cognitive radios
       Tolerate spectrum sensing errors
       No need of coordination assumptions
       Use FHSS spreading gain to combat spectrum sensing errors
        and to avoid interfering primary users
       Resolve the “chicken-and-egg” problem: provide a way for
        CRs to initiate communications in uncertain spectrum
       Simulations demonstrate reliable performance even in large
        spectrum sensing errors

								
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