chapter 7, CDMA in Cellular Telephony

Excerpt

7.1 Introduction

Direct-sequence spreading was initially considered for defense applications such as jamming-resistant communication and low probability of intercept communication. The realization that it could also be a multiple-access technique led to its exploration and eventual adaptation as a mode for achieving mobile wireless communication or cellular telephony. This chapter provides an overview of basic issues in cellular telephony and a description of how CDMA is typically administered.

7.2 Cellular Telephony Basics

Today, widespread use of wireless communication has been made possible by the adoption of a cellular structure. As in conventional landline telephony, the connection between two users for a voice conversation requires a communication path or channel between them. For example, if a link is to be established between two telephones on a small campus using conventional telephony, it is done through a private exchange or telephone switch. If the telephones are widely separated, by a few hundred kilometers for example, coordination among several switches is required to establish the connection. When a call is initiated, the calling instrument first contacts its switch through a wired connection.

In the world of wireless telephones, a similar need exists for switches to link the calling and the called parties. The calling phone uses a wireless link to communicate to a wireless receiving station (called the base station) within its reach. The base station is connected to a switching center, which is essentially a telephone exchange. The initial information from the calling telephone indicates to the switching station who it intends to call. The switch then contacts the destination telephone to issue a ring signal. If it so happens that the called telephone is also a mobile unit in the vicinity of the base station the calling phone is in contact with, then the same base station will be used to establish a wireless link with the called phone. When the destination telephone responds, the call is established. A key thing to note is that the power of each mobile telephone is limited, which also limits its reach.