Digital Communications with Emphasis on Data Modems

3DIGITAL COMMUNICATIONS

3.1 INTRODUCTION

A functional description of a communication system is shown in Figure 3.1. Generally the information source data is represented by discrete‐time and discrete‐amplitude digital data; however, if the information source is represented by continuous‐time and continuous‐amplitude analog data, then the source encoder includes time‐domain sampling and amplitude quantization. The resulting data is referred to as the source or user bit of duration T_b. Depending upon the nature of the data and the system performance requirements, the resulting digital information sequence may then undergo data compression. For example, if the information source is a facsimile system, then data compression is applied to eliminate redundant or unnecessary data or, in the case of voice data, the sampled voice signal is typically processed using a voice‐coding algorithm to minimize the required amount of data to be transmitted. The resulting source coded data sequence undergoes channel coding to mitigate impact of distortion on the received signal. Examples of channel coding are forward error correction (FEC) coding, data interleaving, and pseudo‐noise (PN) spread‐spectrum encoding. The FEC‐coded output consists of one digit per T_cb seconds where T_cb is the code‐bit duration. When direct‐sequence spread‐spectrum (DSSS) is applied each interval of duration T_c seconds ...