Multiple Access for Broadband Wireless Networks

Several emerging communication technologies hold the key to approaching the information theoretic limits of wireless multiple access channels. This article offers a brief review of those technologies and their promise to meet future demand for wireless data rate. One of the defining features of the information technology industry is the unrelenting exponential growth of the " bandwidth, " or data rate capacity, sustained by data communication networks and, in particular, the Internet. Likewise, the global demand for wireless " bandwidth " exhibits, now and in the foreseeable future, strong exponential growth. New wired network " bandwidth " is created when new physical resources (cable, fiber, routers, etc.) are added to the network. In sharp contrast, wireless communication requires sharing a finite natural resource: the radio frequency spectrum. The radio frequency (RF) bandwidth allocated by regulatory agencies to cellular wireless services has steadily grown in the last two decades. For example, in the United States, the 40 MHz allocated to first-generation cellular tele-phony in 1983 grew to 50 MHz with the advent of the second generation at the end of the 1980s, and to 170 MHz in 1995. While pressure will continue on regulatory agencies to release more and more bandwidth for wireless access, competing interests and fundamental physical laws dictate that the RF bandwidth allocated to mobile wireless services will cease to grow significantly in the not-too-distant future. The exponential demand for wireless " band-width " coupled with the very limited supply of RF bandwidth direct a potent spotlight on physical-layer communications engineering. Although bandwidth is not equal to " band-width, " they are intimately related through the laws of information theory. As shown by Claude Shannon in 1948, the maximum spectral efficiency (bits per second per Hertz) a channel can sustain depends on its statistical behavior and various design parameters such as transmitted power. The fundamental limitations on reliable data transmission posed by Shannon's capacity laws keep the realm of wireless communication beyond the reach of Moore's law on the exponential growth of computing capabilities. Fortunately, technological advances in integration enable designs whose data rates are closer and closer to the information theoretic limits. Those limits are much harder to analyze for wireless channels than for wired channels because not only are the channel models more complex but, as we will see later, the distributed nature of radio waves offers new challenges and opportunities. The viewpoint of this article …