Multiple Access and the Cellular Principle

This chapter discusses the simultaneous communication of multiple users. We start out by discussing multiple access between a single base station (access point) and multiple mobile stations (handsets, users). In particular, Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) allow a simple assignment of the resources to different users; Code Division Multiple Access (CDMA) is relegated to Chapter 18. Basic aspects of queuing theory, including the Erlang B and Erlang C model, are then introduced, and we show that due to the trunking gain, the admissible ratio of offered traffic to number of available channels increases (for a given outage probability) when the number of available channels increases. We then discuss various schemes for multiple access for packet radio, including Aloha, slotted Aloha, Carrier Sense Multiple Access (CSMA) and packet reservation multiple access. A brief discussion of routing in packet radio is also given, though a more extensive discussion is relegated to Chapter 22. Duplexing, in particular Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD), allows the separation of transmitted and received data. We then describe the cellular principle. In order to increase the number of simultaneous links on which communication can take place, an area is divided into multiple cells, each of which has one base station. In order to limit interference, the same time-frequency resource can only be reused in cells separated by a certain minimum reuse distance; the number of cells that are all using disjoint time-frequency resources is called the cluster size. For the simple case of hexagonal cells, explicit equations for cluster size are given. Various methods for increasing capacity, including overlay structures, fractional loading, and partial frequency reuse, are also discussed. The information theory of broadcast channels (downlink) and multiple-access channels (uplink) is also briefly described.