Digital Communications with Emphasis on Data Modems

14MODEM TESTING, MODELING, AND SIMULATION

14.1 INTRODUCTION

Perhaps the most useful and certainly the most ubiquitous tool for evaluating any communication system is a good uniform number generator; a close runner‐up is a good Gaussian number generator. Once this tool is available, it is used in virtually every simulation program to  perform a variety of tasks. These tasks include: random source‐data generation, pseudo‐noise (PN) sequence generation, generation of various types of fading channels, and additive white Gaussian noise (AWGN) generation. Various methods of generating uniformly distributed random numbers are discussed in the following paragraph. With this background, the generation of other forms of random variables is examined and subsequent examples of system simulations focus on their applications in accurately characterizing system performance.

In terms of network terminology, the communication link is used here to include all of the functions of the physical layer between a data source and a data sink as depicted in Figure 14.1. In this context the channel may include, for example, a satellite repeater or bent‐pipe that does not process the received uplink signal for information content. Also, whenever there is a linear relationship between the transmitter and channel filtering, it is often convenient to treat the cascaded response as one filter. When this is done, however, care must be taken in the manner that various noise sources are added throughout the ...