Lecture 8: Spread Spectrum

• Principle of spread spectrum• Frequency hopping spread spectrum• Direct sequence spread spectrum• Direct sequence CDMA systems

Ben Slimaneslimane@kth.se

What is Spread Spectrum?

• Spread spectrum techniques are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth

• Spread spectrum methods:– Frequency hopping spread spectrum– Direct sequence spread spectrum– Time hopping spread spectrum

Spread Spectrum

• At the transmitter side:– Input is fed into a channel encoder

• Produces analog signal with narrow bandwidth

– Signal is further modulated using sequence of digits

• Spreading code or spreading sequence • Generated by pseudonoise, or pseudo-random number

generator

– Effect of modulation is to increase bandwidth of signal to be transmitted

Spread Spectrum

• At the receiving end:– digit sequence is used to demodulate the spread

spectrum signal– Signal is fed into a channel decoder to recover data

Spread Spectrum

Spread Spectrum

• What can be gained from apparent waste of spectrum?– Immunity from various kinds of noise and

multipath distortion– Can be used for hiding and encrypting signals– Multiple access capability

• Several users can independently use the same wider bandwidth with very little interference

Frequency Hopping Spread Spectrum (FHSS)

• Signal is broadcast over seemingly random series of radio frequencies– A number of channels allocated for the FH signal– Width of each channel corresponds to bandwidth of input

signal

• Signal hops from frequency to frequency at fixed intervals– Transmitter operates in one channel at a time– Bits are transmitted using some encoding scheme– At each successive interval, a new carrier frequency is

selected

Frequency Hoping Spread Spectrum

• The frequency sequence is dictated by the spreading code

• Receiver should hop synchronously with the transmitter to be able to recover the message

• Advantages– Eavesdroppers hear only unintelligible blips

– Attempts to jam signal on one frequency succeed only at knocking out a few bits

Frequency Hoping Spread Spectrum

Frequency Hopping Spread Spectrum

Frequency Hopping Spread Spectrum

• Slow-frequency-hop spread spectrum– The hopping duration is larger or equal to the

symbol duration of the modulated signal

Tc >= Ts

• Fast-frequency-hop spread spectrum– The hopping duration is smaller than the

symbol duration of the modulated signal

Tc < Ts

Slow Frequency-Hop SS

Fast Frequency-Hop SS

FHSS Performance Considerations

• Large number of frequencies used

• Results in a system that is quite resistant to jamming– Jammer must jam all frequencies– With fixed power, this reduces the jamming power

in any one frequency band

Direct Sequence Spread Spectrum (DSSS)

• The modulated signal is spread by a spreading waveform (spreading code)

• The spreading code spreads the signal over a wider frequency band – Spread is in direct proportion to number of bits per symbol

used

• The spreading code is usually periodic with a period larger or equal to the symbol duration of the modulated signal

DSSS Using BPSK

Direct Sequence Spread Spectrum (DSSS)

Spectrum of DS Spread Spectrum Signal

Code-Division Multiple Access (CDMA)

• CDMA is multiple access scheme that allows many users to share the same bandwidth– 3G (WCDMA), IS-95

• Basic Principles of CDMA– Each user is assigned a unique spreading code– The processing gain protects the useful signal and

reduces interference between the different users

PG = (Bandwidth after spreading)/(Bandwidth before spreading)

CDMA for Direct Sequence Spread Spectrum

CDMA Example

Spreading Sequences

• Spreading sequences are very important in the design of spread spectrum communication

• Two categories of Spreading Sequences – PN sequences– Orthogonal codes

• FHSS systems– PN sequences most common

• DSSS CDMA systems– PN sequences– Orthogonal codes

PN Sequences

• PN sequences are periodic but appear random within one period

• PN sequences are very easy to generate– Generated using LFSR

• PN sequences are easy to re-generate and synchronize at the receiver

• PN sequences have good random properties• PN sequences converge to a Gaussian process when

the period tends to infinity

Spreading in Cellular CDMA Systems

• Cellular CDMA systems use two layers of spreading

• Channelization codes (orthogonal codes)– Provides orthogonality among users within the

same cell

• Long PN sequences (scrambling code)– Provides good randomness properties (low cross

correlation) – Reduces interference from other cells