2.2 Multiple Access Wireless Communications

In cellular and PCS wireless communications systems a multitude of users access and share network resources (frequency bandwidth) to obtain different types of services, including voice, messaging, and data. The goals of these multiple access communications systems are to provide communications services in a near-universal geographical coverage while minimizing both subscriber stations and network equipment, deployment, and operational costs. Because regulatory agencies have allocated limited bandwidth to these services, a crucial goal of these solutions is to achieve high spectral efficiency, traditionally measured in Erlangs/megahertz/unit service area for voice applications and in bits/second/megahertz/unit service area for data applications. The cellular concept pioneered by Bell Labs in the 1970s makes use of multiple fixed stations, or cells that each serve a number of mobile subscribers within a limited geographical area. When a subscriber moves between cells, over-the-air messaging is used to handoff the call between cells, ensuring its continuity. The first such system in North America was called AMPS. Similar analog systems were also deployed in different parts of the world, including the Nordic Mobile Telephone (NMT) in Scandinavia, and the Total Access Communications System (TACS) used in the United Kingdom, China, and other countries. The spectrum chosen for these systems was in the 800 900-MHz band. The frequency band allotted to each system was then divided according to a scheme called FDMA.