 transmitting station C, prevents a nearby station B from transmitting although B's intended receiving station A is out of range of station C's transmission. The later sections of this chapter look at digital modulation techniques and the factors affecting RF propagation and reception, as well as the practical implications of these factors in actual wireless network installations. Spread Spectrum Transmission Spread spectrum is a radio frequency transmission technique initially proposed for military applications in World War II with the intention of making wireless transmissions safe from interception and jamming. These techniques started to move into the commercial arena in the early Radio Communication Basics Baseband signal Spread Spreading function signal Frequency Frequency De-spread Narrowband Baseband interference signal Spread signal De-spreading function Interference Frequency Frequency Figure 4-3: A Simple Explanation of Spread Spectrum 1980s. Compared to the more familiar amplitude or frequency modulated radio transmissions, spread spectrum has the major advantage of reducing or eliminating interference with narrowband transmissions in the same frequency band, thereby significantly improving the reliability of RF data links. Unlike simple amplitude or frequency modulated radio, a spread spectrum signal is transmitted using a much greater bandwidth than the simple bandwidth of the information being transmitted. Narrow band interference (the signal I in Figure 4-3) is rejected when the received signal is "de-spread". The transmitted signal also has noise-like properties and this characteristic makes the signal harder to leavesdrop on. Types of Spread Spectrum Transmission The key to spread spectrum techniques is some function, independent of the data being transmitted, that is used to spread the information signal over a wide transmitted bandwidth. This process results in a transmitted signal bandwidth which is typically 20 to several 100 times the information bandwidth in commercial applications, or 100