JPL's Wireless Communication Reference Website

Chapter: Analog and digital transmission

Spread Spectrum Transmission

In Spread Spectrum communication, the bandwidth occupancy of a single transmitted signal is much higher than in systems using conventional modulation methods. This band-spreading is achieved by selecting appropriate transmission waveforms with a wide bandwidth. A very popular method is to multiply the user data signal with a fast code sequence, which mostly is independent of the transmitted data message. In the case that multiple users share the same portion of the radio spectrum but use different codes to distinguish their transmissions, we speak of Code Division Multiple Access (CDMA)

The ratio between the bandwidth of the user signal and the transmit bandwidth is called the "spreading factor".

Spreading does not affect the performance in 'ideal' channels without interference and without dispersion or fading: As we know the the theory of the matched filter for antipodal signals, the BER in an AWGN channel depends on the signal-to-noise ratio E_b/N_0 but is independent of the exact waveform chosen for transmission.

However, under less ideal circumstances spread spectrum can have advantages.

CDMA Schemes

Various spread-spectrum techniques have been proposed: Applications of spread spectrum are in

Further reading

Typical implementation problems are

Implementation Example for Broadband CDMA

The infopad research project implemented a broadband CDMA system for downlink transmission, with a monolithic radio receiver. The rake filtering is done at base band.

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