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