<P> The increased capacity in a cellular network, compared with a network with a single transmitter, comes from the mobile communication switching system developed by Amos Joel of Bell Labs that permitted multiple callers in the same area to use the same frequency by switching calls made using the same frequency to the nearest available cellular tower having that frequency available and from the fact that the same radio frequency can be reused in a different area for a completely different transmission . If there is a single plain transmitter, only one transmission can be used on any given frequency . Unfortunately, there is inevitably some level of interference from the signal from the other cells which use the same frequency . This means that, in a standard FDMA system, there must be at least a one cell gap between cells which reuse the same frequency . </P> <P> In the simple case of the taxi company, each radio had a manually operated channel selector knob to tune to different frequencies . As the drivers moved around, they would change from channel to channel . The drivers knew which frequency covered approximately what area . When they did not receive a signal from the transmitter, they would try other channels until they found one that worked . The taxi drivers would only speak one at a time, when invited by the base station operator . This is, in a sense, time - division multiple access (TDMA). </P> <P> The first commercially cellular network, the 1G generation, was launched in Japan by Nippon Telegraph and Telephone (NTT) in 1979, initially in the metropolitan area of Tokyo . Within five years, the NTT network had been expanded to cover the whole population of Japan and became the first nationwide 1G network . </P> <P> To distinguish signals from several different transmitters, time - division multiple access (TDMA), frequency - division multiple access (FDMA), code - division multiple access (CDMA), and orthogonal frequency - division multiple access (OFDMA) were developed . </P>

What is the principal role of a tower within a cellular system