ided by using Differential Phase Shift Keying (DPSK). This does not require the genera- tion of a local carrier at the demodulator. In DPSK, the information is conveyed by changes in phase between digits instead of by the phase deviation of each digit from a reference carrier. If a greater number of phase differences, say 4, 8, or 16, are used, then the corre- sponding systems are called DQPSK, 8-DPSK, and 16-DPSK, respectively. n/4-DQPSK, another modulation technique, is a variant of DQPSK. In n/4-DQPSK an additional phase shift of /4 radians is inserted in each symbol. Another hybrid modulation scheme is Quadrature Amplitude Modulation (QAM), where both amplitude and phase are varied to represent bits. The spectrum utilization is greatly improved in QAM. Several higher-level QAM schemes (16-QAM and 64-QAM) are more bandwidth efficient. Attenuation Attenuation is an inherent property of any channel that makes the signal power decrease with increasing distance. For a general transmission medium (guided or unguided), attenu- ation can be defined as (2.13) where A is the attenuation, PT is the transmitted power, and PR is the received power. Equa- tion 2.13 can be expressed in dB: (2.14) For transmission lines, coaxial cables, and waveguides, received power is generally represented by the following equation: PR=e-20dPT (2.15) where a is the attenuation coefficient (dependent on the medium) and d is the distance between the transmitter and receiver (channel length). It is important to mention that attenuation varies with frequency depending on channel characteristics. For example, if an optical fiber is used as the transmission medium, an optical signal frequency of 193 THz will be attenuated less than 1 at 225 THz. For wireless (free-space) transmission, attenuation is dependent on the nature of the terrain as well as on the radio frequency used and the distance between the transmitter and receiver. The received power based on a free-space propagation model can be given by the following equation: (2.16) Data Transm