recovered from the other tone, the wide separation assuring that the probability of both tones being affected is very small. Pulse Modulation Methods Several wireless network standards specify the use of pulsed rather than continuous transmission of a carrier wave, and a number of specific modulation techniques are used in these systems. Pulse Position Modulation In pulse position modulation (PPM), each pulse is transmitted within a reference time frame, and the information carried by the pulse is determined by the specific transmission time of the pulse within its frame. For example, a 4-PPM system will define four possible positions for a pulse within the reference frame, with each possible position coding one of four input data symbols (Table 4-14). More generally, an m-PPM system will have m possible pulse transmission slots within a frame. An 8-PPM modulation system is shown in Figure 4-27. PPM is specified in the IrDA standard at a 4 Mbps data rate, and is also used in Impulse Radio (IR as opposed to Ir!). Radio Communication Basics Table 4-14: Data Symbols for 4-PPM Modulation Input data symbol 4-PPM data symbol Pulse position Code Word Pulse position Code Word (000) (100) (001) (101) (010) (110) (011) (111) Pulse sequence Code sequence (000) (011) (101) (100) Figure 4-27: 8-PPM Modulation Pulse Shape Modulation Pulse shape modulation (PSM) encodes the input data stream in the shape of the transmitted pulse. The simplest form of PSM is pulse amplitude modulation (PAM) in which, typically, two or four distinct pulse amplitudes are used to encode data symbols, as shown in Table 4-15. Similarly, pulse width modulation (PWM) uses the width of transmitted pulses and more generally, PSM may use some other pulse shape characteristics - such as the derivative of the pulse waveform - to encode data onto the pulse train. Pulse amplitude and pulse shape modulation are candidates for use in the ultra wideband radio physical layer of the ZigBee specification. Chapter Four Table 4-15: PAM Encoding Table Input data symbo