Transmitter and Receiver Techniques（发射机和接收机技术）.pdf

Chapter 6 Transmitter and Receiver Techniques 6.1 Introduction Electrical communication transmitter and receiver techniques strive toward obtain- ing reliable communication at a low cost, with maximum utilization of the channel resources. The information transmitted by the source is received by the destina- tion via a physical medium called a channel. This physical medium, which may be wired or wireless, introduces distortion, noise and interference in the transmitted information bearing signal. To counteract these effects is one of the requirements while designing a transmitter and receiver end technique. The other requirements are power and bandwidth efficiency at a low implementation complexity. 6.2 Modulation Modulation is a process of encoding information from a message source in a man- ner suitable for transmission. It involves translating a baseband message signal to a passband signal. The baseband signal is called the modulating signal and the passband signal is called the modulated signal. Modulation can be done by varying certain characteristics of carrier waves according to the message signal. Demodu- lation is the reciprocal process of modulation which involves extraction of original baseband signal from the modulated passband signal. 101 6.2.1 Choice of Modulation Scheme Several factors influence the choice of a digital modulation scheme. A desirable modulation scheme provides low bit error rates at low received signal to noise ratios, performs well in multipath and fading conditions, occupies a minimum of bandwidth, and is easy and cost-effective to implement. The performance of a modulation scheme is often measured in terms of its power efficiency and bandwidth efficiency. Power efficiency describes the ability of a modulation technique to preserv