Spread spectrum communications is a method of spreading information signals (typically digital signals) so the frequency bandwidth of the radio channel is much larger than the original information bandwidth. There are various forms of spread spectrum communications. The most popular forms of spread spectrum include frequency hopping multiple access (FHMA) and code division multiple access (CDMA).

FHMA is an access technology where mobile radios may share radio channels by transmitting for brief periods of time on a single radio channel and then hopping to other radio channels to continue transmission. Each mobile radio is assigned a particular hopping pattern and collisions that occur are random and only cause a loss of small amounts of data that may be fixed through error detection and correction methods.

CDMA is a relatively new commercialized (verses militarized) modulation technique that is used in cellular and satellite systems. CDMA systems mix a relatively long digital code with a small amount of communication data to produce a combined signal that is spread over a relatively wide frequency band. To receive the signal, the long code is used to extract the original signal. Because the energy is spread over a wide bandwidth, multiple CDMA channels with different codes can co-exist with minimal interference.

Figure 1.12 shows how a single direct sequence spread spectrum communication channel can have several channels. In this example, there are 3 different code patterns that are used for communication channels. When a receiver uses the reference code, a...