width. As discussed in the following sections, four major techniques are used to allocate capacity: Space Division Multiplexing, Fre- quency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and spread spectrum (implemented as Code Division Multiple Access [CDMA] in 2G networks and as either W-CDMA or OFDM in 3G cellular networks). Space Division Multiplexing The cellular concept involves subdividing coverage areas. Mobile telephony is not a new invention. It has been around since the early 1950s, but at that time, two things limited its availability to the mass market. First, we were using very high- powered antennas. So, when we relayed a signal, it would have strength over a COV- erage area of perhaps 100 miles (161 km) in diameter. Second, at that time, spectrum management agencies issued very few frequencies (perhaps one or two dozen) that could be used for purposes of mobile communications. In the rela- tively large coverage area of 100 miles (161 km) or so, only 12 to 24 channels were available. The majority of these few channels were reserved for critical services, Chapter 13 Wireless Communications Basics such as police and other emergency services, as well as for those who could afford a body builder to carry those big phones that operated at low frequencies over long stretches. In the 1970s, two things changed. The first was the advent of the low-power transmitter receiver tower, which reduced the coverage area to a cell size that was only about 8 miles (13 km) in diameter. Second, the regulatory agencies made available large amounts of spectrum for use in support of mobile communications, and depending on the location, anywhere from 600 to 1,000 channels were made available to service providers. The cellular architecture depends on cells organized into a reuse pattern. In the traditional analog cellular network, the cellular reuse pattern is n = 7 and is often depicted as a honeycomb configuration of what are called seven cells (see Figure 13.3). The idea of spectrum reuse is that