The Elements of Digital Communication System

This chapter briefly covers some general topics on digital signal transmission and detection theory. It discusses the line coding, which includes encoding and pulse shaping. The chapter describes some fundamental techniques in digital signal detection. Receive symbol timing and frame timing must be synchronized with the transmitted signal. The chapter describes how to establish this synchronization. In contrast to the analog signal scrambling, the scrambling of digital signals is easily carried out by using logic circuits or a processor. There are several different ways of scrambling: addition of pseudonoise (PN), permutation, and transformation. The public key cryptosystem uses a secret key only for decryption. In a mobile radio communication system, multiplexed transmission is done in the downlink. In recent years, some methods such as the turbo codes and low density parity check (LDPC) systems are realized to attain the channel capacity under a practically low error rates.