Microwave Engineering Europe - November 2008 - (Page 24) 24 MMICs — RF FRONT END The ballooning RF front end in multi-standard handsets Efficient, wide-band power amplifiers support high-performance, cost-effective transmission circuits in mobile handsets, enabling them to work with multiple standards, across all frequency bands in use around the world. By Martin Wilson, Chief Scientist and Founder, Nujira Ltd 3 G and 4G standards use complex modulation schemes that increase data throughput in the operators’ expensive spectrum but have a dramatic impact on the power consumption of RF transmitters and hence handset battery life. Power requirements are aggravated by always-on usage modes adopted by mobile users with the encouragement of networks. This is leading analysts like ABI Research to predict that the adoption of technologies like WiMAX could reduce battery life by up to two-thirds. Since these new standards are multiband, handsets also use more and more power amplifiers (PAs) in an effort to maintain efficiency, increasing design complexity exponentially, and leading to runaway bill-of-materials costs. There is a real risk that the air interface in a 4G handset will turn into a bulky, power hungry monster. Figure 1: Main curve shows drain efficiency versus power output during a W-CDMA transmission for a typical PA. Dashed curve shows the probability distribution of the instantaneous output power value. Conventional RF amplifiers are can consume as much as half the power in a high speed modem. The hurdle here is the lack of efficient, wide-band power amplifiers that meet the performance demands of the transmission side within the system cost constraints. High Accuracy Tracking (HAT™) power modulators offer handset designers the opportunity to use wideband PAs without compromising the efficiency of the overall RF front end. RF power amplifiers Modern complex modulation schemes have the disadvantage of requiring linear power amplification, which compromises overall system efficiency because PAs are much less efficient when backed off from maximum power. GSM signals operate at constant amplitude, so that the power amplifier can always be in saturated mode. Newer standards like UMTS, WiMAX and further evolved standards such as Long Term Evolution (LTE), are based on a variety of different channel coding and modulation techniques (CDMA, OFDM), but they all require faithful reproduction of the amplitude of the transmitted RF signal. In general the higher the data rates, the higher the peak to average power ratio (PAPR) and the more difficult the amplification process becomes. This non constant amplitude modulation means that the amplifier cannot be run up to its saturated output power capability as in GSM, resulting in lower efficiency. RF PAs are classic AB class amplifiers, which offer most efficient operation when the RF envelope waveform is closest to peak power. Efficiency is a function of the RF signal’s crest factor, more specifically by the peak-to-average power ration (PAPR). This in turn is determined by the type of modulation and coding scheme. In a W-CDMA mobile, the PA usually operates far below the peak power, with a PAPR Figure 2: Conventional multimode 3G RF sub system using single mode PAs. Microwave Engineering Europe ● November 2008 ● www.mwee.com http://www.mwee.com
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