For the past several decades radio system and circuit designers have struggled with the challenges associated with developing cost effective solutions for very high crest factor digital data wireless transmission systems. On the one hand, spectrum is the scarce resource requiring optimization through the use of linear power amplifiers (PAs), yet on the other hand the high costs associated with linear PAs are burdening the build out of wireless networks and system operators are demanding lower cost solutions. Better in this context means systems that cost less to build out and operate with substantially increased power added efficiency in order to contain operating costs given the ever increasing cost of supplying power to the networks.
Modern portable wireless terminals are marvels of semiconductor, material science, communications theory, and packaging technologies. Consumers take for granted that voice and relatively high data rate links reliably support their calls as they navigate our motorways making one handoff after another seamlessly through the cellular maze. This paper makes no attempt to examine the signal processing and network management associated with cellular telephony. It does, however, attempt to summarize the techniques historically applied to address the downlink side burdens on linearity.
The focus quickly reduces to the linear power amplifier as it applies to the transmission of high data rate message traffic. Markets drive the pace and direction of technology and in our era the resource in greatest scarcity is the wireless spectrum required to support growing traffic demand. From Shannon’s law one can readily deduce that increasing bandwidth and/or signal to noise ratio are requisite tasks for increasing capacity. It is possible to significantly enhance the bandwidth efficiency of a system with additional encoding of the message during modulation to increase the number of bits per symbol. Any such gains in bandwidth utilization generally accrue at the expense of increased system complexity and associated cost. The standard metric for bandwidth efficiency is the number of bits per second transmitted per Hertz of consumed bandwidth. This number exceeds unity for many modern air interface standards.