By Julian Hildersley, Nujira
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Figure 1. Multiple band PA solution
A further issue is that 3G/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. These schemes have the disadvantage of requiring linear power amplification, which compromises overall system efficiency because RF PAs are much less efficient when backed off from saturated maximum power. GSM signals operate at constant amplitude, so that the power amplifier can be in saturated mode. Newer UMTS, WiMAX and further evolved standards such as LTE are based on a variety of different channel coding and modulation techniques (CDMA, OFDM, etc), with increasing channel bandwidths, and high peak-to-average power ratios (PAPR). Essentially the newer the standard, the higher the data through put, the higher the PAPR and the lower the RF amplifier efficiency.
Doherty and DPDConventional techniques for improving PA efficiency are inherently narrowband, and are unable to span more than a single band. Building Doherty solutions, which are now common in base stations, would require variants to cover each different frequency, and PAPR value, as these designs have inherently narrow bandwidth. Doherties also typically need Digital Pre Distortion (DPD) to achieve the linearity specifications. The DSP processing required by DPD would mop up much, if not all, the power saved in the relatively low power handset transmission circuits. In an industry where cost is king these techniques don't hit the spot.
High Accuracy TrackingA technique known as High Accuracy Tracking (HAT) has already become popular in the network, and shows great potential in the handset too. Instead of optimising a final RF stage power transistor supplied by constant voltage, the supply is changed dynamically, modulated in synchronisation with the envelope of the incoming RF signal. Doing this ensures that the output device stays in saturation its most efficient operating region.
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Figure 2. PA solution utilising HAT technology
HAT envelope tracking can make a significant contribution to the power efficiency of the PA, improving this from the 15% of traditional amplifiers to as much as 45%. The argument for envelope tracking in the handset is about reducing the BOM cost as well as about extending battery life. Replacing the DC/DC converter with a HAT solution can greatly simplify the RF front end of a handset by enabling multi mode, multi band PAs to be used. The HAT based RF front end design in Figure 2 uses just two PAs but covers the same frequency spectrum as the seven PA design shown in Figure 1, matching or exceeding its performance and efficiency, and saving an estimated 30% the BOM cost of multi-banding the RF circuit.
RF subsystem design for 4G handsets is a challenge because new standards are inherently less efficient and the need to support a very wide range of frequency bands. HAT technology could provide a practical, and easy to implement solution to enhance the efficiency of the subsystem, more than compensating for the efficiency lost through compromises like the use of wide-band PA devices. It may be destined to be one of the key enabling technologies for 4G mobile. .
Julian Hildersley is vice president for handset development at Nujira, www.nujira.com.