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.

As the signal is spread over a large frequency band, the Power Spectral Density becomes very small. Other communications systems may not suffer from this kind of communications.
CDMA can provide for Multiple Access or Random Access. A large number of codes can be generated, so a large number of users can be permitted to transmit. This kind of multiple access can operate without centralized control.

Figure: CDMA: three users share the same radio channel, but their signals can be separated because each user uses a different code (cf: FDMA and TDMA)
(See also: Throughput of DS-CDMA combined with random access)
Confidentiality: without knowing the spreading code, it is difficult to recover the transmitted data. Moreover, as the spectral density is small, the signal may remain undetected.
Spreading and despreading makes the signal robust against interference. This also holds for multipath self interference.
Fading rejection: as the bandwidth can be made much larger than the coherence bandwidth of the channel, the system is less susceptible to deep fades at particular frequencies. (see also previous point)

CDMA Schemes

Various spread-spectrum techniques have been proposed:

Direct-Sequence
Frequency-Hopping
Multi-Carrier
Ultra Wide Band (UWB) techniques using very steep pulses at well defined instants. This is sometimes called "Time Hopping"

Applications of spread spectrum are in

Military Systems. This is the oldest known application. It is popular for security reasons.
Positioning Systems. High bandwidth signals allow accurate measurements of propagation delays. This is used to estimate the the distance to a transmitter. (Note: the location of a strong spread spectrum can be estimated much more accurately that a that of a narrowband radio emitter. This can be a disadvantage in military applications)
Cellular radio. It is mainly used to combat dispersion and to provide multiple access.
Wireless LANs. If conventional modulation would be used frequency management for many coexisting links can be very difficult. Moreover, narrowband transmission could be impaired by deep local fades.

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.

JPL's Wireless Communication Reference Website

Chapter: Analog and digital transmission

Spread Spectrum Transmission

CDMA Schemes

Further reading

Implementation Example for Broadband CDMA

JPL's Wireless Communication Reference Website © 1993, 1995.