Spread Spectrum

The term spread spectrum describes a modulation technique which makes the sacrifice of bandwidth in order to gain signal-to-noise performance. Basically, the SS system is a system in which the transmitted signal is spread over a frequency much wider than the minimum bandwidth required to send the signal. The fundamental premise is that, in channels with narrowband noise, increasing the transmitted signal bandwidth results in an increased probability that the received information will be correct. If total signal power is interpreted as the area under the spectral density curve then signals with equivalent total power may have either a large signal power concentrated in a small bandwidth or a small signal power spread over a large bandwidth.

From a system viewpoint, the performance increase for very wideband systems is referred to as "process gain". This term is used to describe the received signal fidelity gained at the cost of bandwidth. The numerical advantage is obtained from Claude Shannon's equation describing channel capacity:

C=W log2 (1+ S/N)

where, C = Channel capacity in bits, W = Bandwidth in Hertz, S = Signal Power, and N = Noise Power

From this equation the result of increasing the bandwidth becomes apparent. By increasing W in the equation, the S/N may be decreased without decreased BER performance. The process gain (GP) is what actually provides increased system performance without requiring a high S/N. This is described mathematically as:

GP = BWRF/RINFO

where, BWRF = RF Bandwidth in Herz and RINFO = Information rate in bits/second.

The baseband signal is spread out to BWRF over the channel (see Fig. 1). Then at the receiving end, the signal is de-spread by the same amount by a correlation with a desired signal generated by the spreading technique (more on the different spreading techniques later). When the received signal is matched to the desired signal the baseband/information signal is retrieved.

Fig. 1 Bandwidth Spreading

Signal Spreading works quite well in situations with strong narrowband interference signals since the SS signal has a unique form of frequency diversity. The actual signal spreading may be achieved with one of three basic techniques. These include: direct sequence, frequency hopped and pulsed FM or hybrid forms.

More Information on Spread Spectrum

• A more detailed look at Spread Spectrum
• Spread Spectrum Scene