1 CMPT 371 Data Communications and Networking Spread Spectrum

1

CMPT 371Data Communications and

Networking

Spread Spectrum

Janice Regan © 20052

Spread SpectrumAnalog signal based on analog or digital dataBegin with data encoded in a narrow band

signal. Spread data over wider bandwidth

Can move from one narrow band to anotherCan share multiple narrow bands with other

signals


Spread Spectrum ConceptInput fed into channel encoder

Narrow bandwidth analog signal around central frequency

Signal modulated using a spreading sequence/codeSequence often generated by pseudorandom number

generator

Increases bandwidth significantlySpreads each signal throughout the spectrum

Receiver uses same sequence/code to demodulate signal

Demodulated signal fed into channel decoder


Model Spread Spectrum System

Stallings 2003: Figure 9.1


GainsMany users can share same higher bandwidth

with little interferenceCan hide/encrypt signalsCan reduce each signal’s susceptibility to

jamming, noise and interferenceOnly receiver who knows spreading code can

retrieve signalNeed to jam a wide bandwidth to guarantee that a

particular narrow band signal is jammedNoise or interference at a particular frequency does

less damage to the signal


Spread SpectrumFrequency hopping

Broadcast in a narrow frequency band whose central frequency moves from one narrow frequency band to another on a regular basis

Order of switching between bands is based on a pseudorandom sequence

Direct SequenceSignal is multiplied by a chipping code (multiple chips per

input signal bit)

Code Division Multiple Access (CDMA)Signal is spread over a wide band shared with many other

similar signals


Frequency Hopping Spread Spectrum (FHSS)

Signal broadcast over pseudorandom series of frequencies

Receiver uses same pseudorandom series to know which frequencies to listen for

Receivers often use autocorrelation to synchronize pseudorandom sequence with transmitter.

Noise or jamming on one frequency affects only a few bits being transmitted at that frequency (usually these bits can be recovered using error correction)


Frequency Hopping



Pseudorandom Sequences

Generated by algorithm which repeatedly produces a particular known series of numbers of length n that appear to have random properties

An initial seed is used to choose where in the known sequence the particular random sequence will begin.

Algorithm is deterministic but resulting pseudorandom sequence will pass reasonable tests of randomness

Need to know algorithm and seed to predict sequenceAlgorithm will be part of protocol definitionSeed will be determined based on assigned channel,

serial number or other basis


FHSS OperationTypically 2k carrier frequencies/channelsChannel spacing corresponds with bandwidth of input

The channel to use in a particular time interval, Tc, is determined by the next k digits in the pseudorandom spreading sequence

Each channel used for fixed time interval (300 ms in IEEE 802.11)

During each time interval L (L may be fractional) bits are transmitted using some M=2L level encoding scheme on one of the 2k carrier frequencies


FHSS System (Transmitter)



FHSS using BFSK

Filtering for a single sideband)}]1[5.0{2cos(5.0)( 0 tffbfAts iidt

sequencespreadingtf

bitiththeofvalueb

hopithduringfrequencyf

signaltheofamplitudeAwhere

tftfbfAts

i

i

i

iid

)2cos(

)1(

)2cos()}]1[5.0{2cos()( 0

tffbfA

tffbfAts

ii

iid

}]1[5.0{2cos(5.0

)}]1[5.0{2cos(5.0)(

0

0


FHSS System (Receiver)



FHSS using BFSK

Filtering for a single sideband

)}]1[5.0{2cos(25.0)( 0 tfbfAts i

sequencespreadingtf

signalreceiveds

signaldtransmitteswhere

tftffbfA

tftstftsts

i

r

dt

iii

iridtc

)2cos(

)2cos()}]1[5.0{2cos(

)2cos()()2cos()()(

0

tfffbfA

tfffbfAts

iii

iiid

}]1[5.0{2cos(25.0

)}]1[5.0{2cos(25.0)(

0

0


Slow and Fast FHSSTc is the chipping duration or the length of

time each channel is used for a single series of k bits in the spreading or chipping sequence. Frequency is shifted every Tc seconds

Duration of signal element is Ts secondsIf Tc Ts transmission by slow FHSSIf Tc < Ts transmission by fast FHSSGenerally fast FHSS gives improved

performance in noise (or jamming)


Slow FHSS: MFSK (M=4, k=2)



Fast FHSS MFSK (M=4, k=2)



FHSS Performance ConsiderationsTypically large number of frequencies used so there

are many more channels than levelsProvides excellent protection against jamming, noise

and interferenceTo jam a single channel need to broadcast a signal

with the bandwidth of the channel and some power level P

To jam FHSS must jam all channels simultaneously, broadcasting a power equal to the number of channels * P (expensive and difficult)


Direct Sequence Spread Spectrum (DSSS)Each bit multiplied by multiple bits of a spreading

sequenceSignal is spread across a frequency band wider than

that of the original signal. If each data bit is multiplied by n bits of the spreading sequence the bandwidth of the spread signal is n times the bandwidth of the original signal

One method:Combine input with spreading code using XORData rate equal to data rate of the original spreading code

Performance similar to FHSS


DSSS Transmitter



Direct Sequence Spread Spectrum



DSSS Receiver


Direct Sequence Spread Spectrum



DSSS using BPSK

Multiply data by chipping sequence to get transmitted data

Multiply received data by chipping sequence to recover initial data

sequencespreadingtc

streamdatatdwhere

tftAdts cd

)(

)(

)2cos()()(

)2cos()()()( tftctAdts ct

)()2cos()()()()()( tstftctctAdtcts dct


DSSS Using BPSK



Approximate Spectrum: DSSS Signal



CDMA Code Division Multiple Access Start with data signal rate D (Called bit data rate) Break each bit into k chips by multiplying by a k bit

user code (known as a Walsh code) Channel has chip data rate kD chips per second User code (Walsh code) is orthogonal to all other

possible user codes User code 1 * User code 2 = 0 User code 1 * User code 1 = signal for user 1

Signals for several users can be added and sent as a single signal within the same band (multiplexed)


CDMA user code and data



CDMA ExplanationConsider a user communicating with a base stationBase station knows user A’s codeAssume communication already synchronized Base station receives a message from A and wants to

decode it. To extract the signal from A the basestation multiplies the signal by A’s code

Decoder ignores other sources by using A’s code to decodeFor all other stations code station I * code station A = 0 so

only the signal for station A remains


CDMA for DSSSWhen the basestation sends messages to n

users each message multiplied by a different orthogonal Walsh code sequence, those signals are added before transmission.

At each receiving station, the signal for that station is extracted by multiplying by that stations Walsh code.


CDMA: two-senders, eight bit Walsh codes

Walsh Code 2

Walsh Code 1

DataStation 1

DataStation 2

Data multiplied by Walsh Code

Data multiplied by Walsh Code(Sum of all stations)

Transmitted data


CDMA: eight bit Walsh codes

Walsh Code 1

Received Data multiplied by Walsh Code

Decoded Received DataStation 1

-2 -2 -2

2

-2

2 2 2

Receive Data (Sum)


Summary of Channel PartitioningCDMA (Code Division Multiple Access) Used mostly in wireless broadcast channels such as cellular

phones All users share same frequency band. Information from each

user is spread throughout that frequency band Each user has their own orthogonal Walsh code ‘chipping’

sequence to encode data. encoded signal = (original data) X (Walsh code) Encoded signals from each channel are added, the summed

signal is transmitted The orthogonal property of Walsh codes guarantees that

(ignoring transmission errors) multiplying the received signal by a Walsh code will extract the data for the channel encoded using that Walsh code from the received (summed) signal.

Decoded signal = (received summed signal X Walsh code)


CDMA in a DSSS Environment


Documents

1 CMPT 371 Data Communications and Networking Spread Spectrum