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Cutting 4G Handset RF Front Ends Down to Size

Fri, 02/13/2009 - 5:22am
High Accuracy Tracking (HAT) technology could provide a practical and easy to implement solution for increased efficiency in 4G handset design.

By Julian Hildersley, Nujira

According to ABI Research the battery life for WiMAX and other 4G enabled devices could end up at only a third of that of 3G. The major villain here has been identified as the RF power amplifier. Creating an efficient broadband cellular network air interface for LTE (Long Term Evolution) wireless technology is hard, especially if it is to work globally. LTE standards are not only inherently less power efficient from a RF transmission design point of view, but different frequency bands have been licensed LTE around the world, all of which need to be supported if a handset is to achieve global coverage. There is a real risk that the air interface in a 4G handset will turn into a bulky, power hungry monster. The battery life issue is further complicated by the fact that newer services will often be based on faster uplink speeds, and are sometimes 'always on'.


click to enlarge

Figure 1. Multiple band PA solution
So far, ten different FDD frequency bands and four different TDD frequency bands have been defined in 3GPP that can be used for LTE, and it is likely that more bands will be added to this list such as 700 MHz in the US. A 4G handset will be required to transmit on the appropriate band anywhere in the world. This implies five or more conventional power amplifiers (PAs) for a broadband cellular RF interface seeking to cover all ten LTE bands — adding several dollars to the bill of materials (see Figure 1.)

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 DPD
Conventional 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 Tracking
A 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.

click to enlarge

Figure 2. PA solution utilising HAT technology
Envelope tracking as a technique for improving power efficiency of amplifiers was first described by Bell Labs in 1937, and has been featured in RF design textbooks ever since. What has been lacking is an effective commercial implementation, because of the difficulty of making a power supply modulator capable of achieving the accuracy, bandwidth and noise specifications necessary at a level of conversion efficiency that delivers a significant energy saving for the system as a whole.

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.
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