CEN 342Introduction to Data Transmission

Chapter 9Spread Spectrum

Dr. Mostafa Hassan DahshanComputer Engineering DepartmentCollege of Computer and Information SciencesKing Saud University

mdahshan@ccis.ksu.edu.sa

Spread Spectrum

Important form of encoding for wireless communications

Analog or digital data analog signal

Initially designed for military

Jamming, interception more difficult

Concept of Spread Spectrum

Input fed to channel encoder

Produce analog signal, narrow bandwidth

Modulated using spreading sequence / code

Generated using pseudorandom number

Effectively increase bandwidth significantly

Spread spectrum of signal to be transmitted

Receiver demodulate with same sequence

Signal fed into channel decoder recover data

Concept of Spread Spectrum

Advantages

Signal gains immunity from•noise •multipath distortion •jamming

Security, hide and encrypt signalcan only be recovered knowing spreading code

Same higher bandwidth can be used by many users with little interference

e.g. CDM/CDMA in cellular telephony

Thus, spectrum not wasted

Pseudorandom Numbers (PN)

Generated by algorithm using initial seed

Deterministic, not actually random

Same seed produces same number

However, good algorithm pass many reasonable tests for randomness

Unless algorithm and seed are knownnumber (sequence) cannot be predicted

Only receiver can decode signal

Frequency Hopping Spread Spectrum (FHSS)

Signal broadcast over (seemingly) random series of radio frequencies

Hop from freq to another over fixed intervals

Receiver follow same freq series, intervals

Eavesdropper hear unintelligible blips

Jamming one freq only damage few bits

FHSS Basic Approach

Number of channels allocated for FH signal

2k carrier frequencies, one for each channel

Channel width related to input bandwidth

Frequencies sorted as permuted table

PN used to index frequencies table

Binary data modulated FSK or PSK

Result centered on some base frequency

FHSS Basic Approach

Each interval, k bits of PN select frequency

This freq is modulated with FSK/PSK signal

Produce signal centered on new carrier

FHSS Using MFSK

MFSK commonly used with FHSSFor one signal element MFSK

fi = fc + (2i − 1 − M) fdfc = carrier frequencyfd = difference frequency (between fc and fi)M = number of different signal elements = 2L

L = number of bits per signal element

( ) ( )cos 2 , 1is t A f t i Mπ= ≤ ≤

FHSS Using MFSK

MFSK signal modulated with FHSS carrier

Translated to new channel every Tc sec

For data rate Rbit duration T = 1/R sec

signal element duration Ts = LT

Slow FHSS Tc ≥ Ts

Fast FHSS Tc < Ts

Example

M = 4 frequencies encode 2 bits at a time

MFSK bandwidth Wd = 2M fdUsing FHSS with k = 2, 2k = 4 channels

Each channel with bandwidth Wd

Total bandwidth for FHSS: Ws = 2kWd

Slow FHSS: Tc = 2 Ts = 4 Tb

channel held for duration of two signal elements

Fast FHSS: Ts = 2 Tc = 2 Tb

signal element represented in two channels

Example – Slow FHSS

Example – Fast FHSS

FHSS Performance

For MFSKEb / Nj = (Eb Wd) / Sj

Wd = bandwidth of MFSK signal

Nj = jamming noise per hertz

Sj = jamming power (Nj = Sj / Wd in this case)

Eb = signal energy per bit

FHSS Performance

FHSS: jammer must jam all 2k frequencies

Jamming power reduced to Sj / 2k

Gain in S/N (processing gain)Gp = 2k = Ws / Wd

Ws = FHSS signal bandwidth

FHSS has strong resistance to jamming

Direct Sequence Spread Spectrum (DSSS)

Each input bit represented by multiple bits

Spreading code spreads signal wider band

Freq band proportional to number of bits10-bit spreading code 10 times > bandwidth

Input combined with spread code by XORinput 0: spreading code unchanged

input 1: spreading code inverted

DSSS – Example (4 bit code)

DSSS Using BPSKBPSK signal

To produce DSSS signalmultiply c(t) = PN sequence (0 = −1, 1 = 1)

receiver multiply again by c(t): (c(t) × c(t) = 1)

( ) ( ) ( )cos 2d cs t A d t f tπ= ( )1 binary 1

1 binary 0d t

⎧= ⎨−⎩

( ) ( ) ( ) ( )cos 2 cs t A d t c t f tπ=

( ) ( ) ( ) ( ) ( ) ( ) ( )cos 2 c ds t c t A d t c t c t f t s tπ= =

DSSS Performance

Gain in signal to noise ratioGp = Tb / Tc ≈ Ws / Wd

Ws = FHSS signal bandwidth

Tb = duration of 1 bit of input signal

Tc = duration of 1 bit of spreading code

Jamming resistance very close to FHSS

Code Division Multiple Access (CDMA)

Multiplexing technique with spread spectrum

Start with data signal with rate D

Break bit into k chips using fixed pattern

Pattern unique for each user (user code)

New channel rate = kD chips/s

CDMA – Example

User A code cA = <1, -1, -1, 1, -1, 1>

User B code cB = <1, 1, -1, -1, 1, 1>

User C code cC = <1, 1, -1, 1, 1, -1>

If A wants to send bit 1:transmit chip code <1, -1, -1, 1, -1, 1>

If A wants to send bit 0:transmit chip code <-1, 1, 1, -1, 1, -1>

i.e. 1’s complement (1, -1 inverted)

CDMA – Example

Decoding function for user u on receiver SSu(d) = d1×c1+d2×c2+d3×c3+d4×c4+d5×c5+d6×c6

If A sends 1d = <1, -1, -1, 1, -1, 1>

SA = 1×1+(-1×-1)+(-1×-1)+1×1+(-1×-1)+1×1= 6

If A sends 0d = <-1, 1, 1, -1, 1, -1>

SA = -1×1+1×-1+-1×1+1×-1+1×-1+-1×1= -6

CDMA – Example

If user B send 1, receiver using SA

d=<1, 1, -1, -1, 1, 1>

cA = <1, -1, -1, 1, -1, 1>

SA <1, 1, -1, -1, 1, 1> = 1×1+1×-1+-1×-1+-1×1+1×-1+1×1= 0

Same result if B sends 0

Orthogonal Codes

If A, B transmit same time, SA is usedonly A signal is received, B is ignored

If A, B transmit same time, SB is usedonly B signal is received, A is ignored

SA(cB) = SB(cA) = 0

Codes of A, B are called orthogonal

Orthogonal CodesOrthogonal codes are not always available

More commonly, SX(cY) is small if X ≠ Y

Thus, can distinguish when X = Y, X ≠ Y

In the previous exampleSA(cC) = SC(CA) = 0

SB(cC) = SC(cB) = 2

signal makes small contribution instead of 0

Receiver can identify signal of user even if other users transmitting at same time

CDMA Limitations

Receiver can filter unwanted userseither 0 or low-level noise

However, system will break down ifmany users compete for channel

signal power from some users is too high because some users are very near to receiver

CDMA for DSSS

CDMA for DSSS

n users, each using different PN sequence

For each user, data di(t) modulated BPSK

Produce signal with bandwidth Wd

Multiplied by spreading code ci(t)

CDMA for DSSS

All signals + noise received by receiver

Multiplied by spread code of user 1: c1(t)

BW of user 1 narrowed to original

BW of other users Ws + noise not narrowed

Unwanted signal energy remains spread

Wanted signal concentrated

Recovered by demodulator, band-pass filter