l Pulse amplitude RF Signal Propagation and Reception The first part of this chapter described the various techniques that are used to encode and modulate an input data stream onto a radio frequency carrier. The following four sections discuss the various factors that impact on the transmission, propagation and reception of radio waves, which will enable an estimate to be made of the power requirements for a given wireless networking application. The key factors are transmitter power, antenna gain at the transmitter, propagation or link losses, antenna gain once again at the receiving station, and finally receiver sensitivity. Taken together these factors make up the link budget - the balance of power plus gain required to compensate for losses in the link SO that sufficient signal strength is available at the receiver to allow data decoding at an acceptable error rate. Transmitter Power Every RF transmitter generates a certain amount of power (PTX), which is the first major factor in determining the range of a radiated signal. Transmitter power is measured in one of two ways, either in the familiar unit of Watts (or milliwatts) or alternatively using a relative unit called "dBm". The power in dBm is calculated as dBm = 10 X log 10 (Power in milliwatts), SO a transmitter of 100 mW (0.1 Watts) is equivalent to 20.0 dBm (Table 4-16). The dB (or dBm) unit is useful for two reasons. First, when considering the various factors affecting signal strength, these effects can be easily combined when using dB units by simply adding the relevant dB numbers together. Second, it is easy to translate dB into relative power by remembering that +3 dB represents a doubling of power and -3 dB similarly a halving of power. The additive rule applies here too, SO -6 dB is 1/4 the power, -9 dB is 1/8 and SO on. Radio Communication Basics Table 4-16: Power in mW and dBm Power (m W) Power (dBm) Transmitter power levels for typical wireless networking products are in the region of 100 milliwatts to 1 Watt (20 to 30 dBm). For example, in t