A FREQUENCY HOPPING SPREAD SPECTRUM TRANSMISSION SCHEME FOR UNCOORDINATED COGNITIVE RADIOS Xiaohua...
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Transcript of A FREQUENCY HOPPING SPREAD SPECTRUM TRANSMISSION SCHEME FOR UNCOORDINATED COGNITIVE RADIOS Xiaohua...
A FREQUENCY HOPPING SPREAD SPECTRUM TRANSMISSION SCHEME FOR UNCOORDINATED COGNITIVE RADIOS
Xiaohua (Edward) Li and Juite HwuDepartment of Electrical and Computer Engineering
State University of New York at Binghamton{xli, jhuw1}@binghamton.edu
http://ucesp.ws.binghamton.edu/~xli
Contents
1. Introduction2. System model3. New FHSS transmission for cognitive
radios4. Demodulation and Performance analysis5. Simulations6. 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, toler
able 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
0F 1F 1IF
0,0f 0,1f 0, 1Jf 1, 1Jf 1, 1I Jf
Frequency segment
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 A channel may be available to a
transmitter but unavailable to a receiver
Define parameters:
Tx Rx
Noise source
Far away
,
,
1, if detected available
to transmitter
0, else
i j
i j
f
t
,
,
1, if detected available
to receiver
0, else
i j
i j
f
r
,
,
1, if channel available
0, else
i j
i j
fa
[ ] 0ij ij dP t r P
2. System Model0F 1F 1IF
0,0f 0,1f 0, 1Jf 1, 1Jf 1, 1I Jf {
Tx, Rx’s antenna
TransmitReceive
{
Channel information collection
0F 1F 1IF
0,0f 0,1f 0, 1Jf 1, 1Jf 1, 1I Jf {
Tx, Rx’s antenna
TransmitReceive
{Channel
information collection
Segmentation-based spectrum detection:
When the CR transmits in a channel, it also collects information about the channels of next segment.
3. New FHSS transmission
Spreading To transmit a sequence Each symbol spreaded into M chips
This procedure is identical to CR transmitters and receivers
, 0, , 1k k K s
,s sk k m
Spectrum slot selection Each chip is to be transmitted via a channel of ith segment F
i
Transmitters and receivers use a common binary sequence cn to determine channel selectability in this segment
, 0, , 1i kM m I m M
, ,Tx: ,i j i j kM m J ju t c ,
,
1, if is selectable
0, else
i j
i j
fu
, ,Rx: ,i j i j kM m J jw r c ,
,
1, if is selectable
0, else
i j
i j
fw
Channel selection rule There may be many channels selectable in each segment Each CR Tx or Rx needs to select one channel to transm
it or receive Distributed channel selection means Tx and Rx may cho
ose 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
0,0f 0,1f0, 1Jf
Transmitter
Receiver
available channel
j1≠j2
Em
it message
noise
0,0f 0,1f0, 1Jf
Transmitter
Receiver
available channel
j1=j2
Match
y
y
y
y
y
y
y
y
spreading
decoding
power
Noise
User A’s Symbol
Signal
x
x
x
x
x
x
x
spreadingpower
User B’s Symbol
y
y
y
y
y
y
Signal
x
x
Signal collision
Detection for user A
Illustration of multiple CR transmissions using our scheme
4. FHSS demodulation and performance analysis
, , ,0 , ,1 , , 1, , , ,T
k m k m k m k m Ls s s s
0,0,0 0,1,0 0, 1,0 1,0,0 1, 1,0
0,0,1 0,1,0 0, 1,1
0,0, 1 0,1, 1 0, 1, 1 1,0, 1 1, 1, 1
M K M
M
L L M L L K M L
s s s s s
s s s
s s s s s
FHSS/MFSK demodulation Vector symbol model for FHSS/MFSK signals
2
1 2
2
, ,0, ,0 , ,0 , ,0
, , 1 , , 1 , , 1 , , 1
i jk m k m k m
j j
k m L i j L k m L k m L
gx s v
I
x g s v
1 2
1 2
1 2
1, if
0, ifj j
j jI
j j
1 2, , 2 , , 2 ,k m j j i j k m i jI x G s v
Baseband channel matrix
FHSS/MFSK received signal model
Frequency slot synchronization indicator function
Demodulations: coherent demodulation
2 2 2 1 2 2 2
1 1 1
, , , , , , ,0 0 0
M M MH H H
k i j k m i j i j j j k m i j i jm m m
I
y G x G G s G v
2 1 2 2 2
1 12 *, , , , , , ,
0 0
.M M
k l i j l j j k m i j l i jm m
y g I g v
s
2
,0, , 1
arg max k lt L
y
Element-wisedescription
Coherent: MaximumLikelihood detection
Demodulations: non-coherent demodulation1
2, , ,
0
| |M
k l k m lm
y x
4. FHSS demodulation and performance analysis
Performance analysis Major issue: Tx and Rx may use difference frequency slots
channel mismatch SNR for coherent demodulation
2 2
coherent 2
ˆs
v
M
M
1 2
1
0
ˆwhere is the number of matched
frequency slot selections among selections.
M
j jm
M I
M
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
1 1j
j dP p
For every M transmissions, number of correct matches
1
0
11 1 .
Jj
J dj
P pJ
1
0
1ˆ 1 1 1 1J
j
J dj
M M P M pJ
Average channel mismatch probability
5. SimulationsSpreading gain M=40
Symbol amount K=100
Segments I=20
J=100 channels/segment
0 2 4 6 8 10 12 14 1610
-3
10-2
10-1
100
Signal to noise ratio (dB)
Bits
err
or r
ate
BER as functions of SNR under various mismatch probability
pd=0
pd=0.02
pd=0.05
pd=0.1
pd=0.3
5. Simulation
Mismatch pd≒0.1Symbol amount K=100Segments I=20J=100
channels/segment
0 2 4 6 8 10 12 14 1610
-3
10-2
10-1
100
Signal to noise ratio (dB)
Bits
err
or r
ate
verious spreading gain when pd=0.1
M=40
M=30M=20
6. Conclusions Developed an FHSS-FSK transmission scheme for unc
oordinated 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 s
pectrum sensing errors