MULTIPLE-ACCESS IN WIRELESS DIGITAL NETWORKS

General principles for the design of a multiple-access system for large numbers of terminals transmitting to a single hub station are discussed. The importance of understanding the nature of the traffic to be carried by the network is emphasized. After some discussion of multiple access options for steady traffic and for slowly varying traffic, the use of random-access protocols for rapidly varying traffic is explained. Two general random-access protocols have been used in a variety of data networks, ALOHA and CDMA. Although these two techniques have different origins and are generally thought of as separate, they are in fact but different ways of looking at the same basic signals. We show that the use of multiple spreading codes in a CDMA network is not necessary in order to achieve multiple access capability. A single code can greatly reduce the complexity of a CDMA system. We introduce a spread-spectrum version of an ALOHA channel (Spread ALOHA) which is equivalent to a CDMA channel with a common spreading code for all users. The equivalence we demonstrate opens the door to a variety of techniques commonly used in ALOHA channels which can significantly increase both the throughput and the efficiency of the spread-spectrum channel.