Preface
Wireless communications is growing at a phenomenal rate. From 1991 to 1999, the number of subscribers increased from about 25 million to over 250 million. Incredibly, over the next seven years, the number. of subscribers is expected to quadruple, to over 1 billion [ 1]. That growth rate is faster than that of any other consumer electronics product and is similar to that of the Internet.

Originally, wireless communications were motivated by and intended for mobile voice services. Later on, the first analog systems were improved with digital techniques, providing increased robustness and subscriber capacity. In the near future, digital systems will be augmented to try to meet users’ insatiable need for even greater capacity and high-speed mobile data services.

Wireless communications rely on multiple-access techniques to share limited radio spectrum resources. These techniques, which use frequency, time, and power to divide the precious radio spectrum, are described in standards and are highly regulated. As such, infrastructure and subscriber manufacturers can be different and interchangeable.