How Spread Spectrum Works

The term Spread Spectrum describes a modulation technique that makes the sacrifice of bandwidth in order to gain signal-to-noise performance. The fundamental premise is that, in channels with narrow-band 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 wide-band systems is referred to as "process gain". This term is used to describe the received signal fidelity gained at the cost of bandwidth.

Nyquist sampling theorem expresses relationship between bandwidth and maximum data transmission speed. It provides the theoretical maximum bound on the maximum speed at which data can be sent. In general, for system with a bandwidth of B hertz and using K different states, the maximum data rate D = 2Blog2K

In practice, noise limits maximum data transmission rate to less than maximum allowed by Nyquist sampling theorem. Claude Shannon extended Nyquist's theorem to specify the maximum data rate that could be achieved over a transmission system that introduces noise. Claude Shannon's equation describing in a system with bandwidth B in hertz, signal power S and noise power N, the channel capacity C =B log2 (1+ S/N).

By increasing B in the equation, the signal-to-noise ratio (S/N) may be decreased without decreased channel capacity. The process gain is what actually provides increased system performance without requiring a high signal-to-noise ratio. The process gain is described mathematically as:

process gain = Radio Frequency Bandwidth in Hertz / Information rate in bits per second

The base-band signal is spread out to RF Bandwidth 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 base-band/information signal is retrieved.

The most common signal spreading techniques are direct sequence, frequency hopped or a hybrid of the two forms. Two other techniques, time hop and chirp, exist as well but they are rarely used in commercial networks.