Lecture 8: Spread Spectrum Principle of spread spectrum Frequency hopping spread spectrum Direct sequence spread spectrum Direct sequence CDMA systems Ben Slimane slimane@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 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