LTE networks and signals present a range of problems for the RF front end in handsets as the signal characteristics differ greatly from previous 2G and 3G standards. Key metrics such as battery life, antenna performance, network coverage and thermal management are negatively affected by issues in the RF front end. Central to these challenges is the RF power amplifier (PA), its performance and power consumption.
Envelope Tracking (ET) is a new power modulation technique being used to optimise the performance and efficiency of the PA to help overcome the technical challenges presented by complex LTE signals. Indeed ET has largely been accepted by mobile handset manufacturers as the de facto standard RF PA power supply modulation architecture for LTE phones due to enter the market in 2014.
Alongside ET, some chipset companies are looking at a signal processing technique, Digital PreDistortion (DPD), as a way to optimise PA performance. Although many assume the two techniques perform similar tasks, it is important to clarify that DPD offers no efficiency benefits. DPD is a linearisation technique, and so ET and DPD are performing different functions in RF front end design. It is possible to use both techniques in isolation or together.
However, DPD is not an easy technology to develop. For designers there are important questions to resolve, such as: how do ET and DPD interact? How do I implement them in isolation? How do I use them together? If I have ET, do I need DPD, and vice versa?
As a result we have seen several different approaches from LTE chipset vendors. Some are not implementing DPD at all, some have implemented DPD but are not yet using it, others are using ET on its own and some are using DPD in conjunction with ET. Why is this? What might this split in approaches mean for the industry? This article looks at the technical advantages and disadvantages of DPD in the RF front end of handsets, the wider implications of vendors choosing the DPD route or not, and how DPD and ET can be used together most effectively.
Efficiency vs linearity
ET is a very fast power supply modulation technique that improves the energy efficiency of RF power amplifiers (PAs). It replaces the traditional fixed DC supply voltage to the RF PA with a dynamic supply voltage, which closely tracks the instantaneous amplitude, or "envelope" of the transmitted RF signal (See Figure 1).
RF PAs in handsets are typically operated in a classic Class AB configuration, and are only at their most efficient when the RF envelope waveform is close to peak power. This is not a problem with such traditional signals as 2G GSM, where information is encoded only in the phase of the signal - the amplitude is constant, and the PA can operate in this high efficiency mode all the time. GSM PAs consequently have typical efficiencies of 50-55%. However, as data rates increase from 2G to 3G and 4G, the increased spectral efficiency forces information to be encoded in the amplitude, as well as the phase, of the signal. When amplifying RF signals with high crest factors such as 4G LTE waveforms, the average efficiency of the PA drops significantly, with figures of 20-25% being common.
Modulating the supply voltage dynamically, in synchronisation with the envelope of the transmitted RF signal, ensures that the output device stays in saturation – its most efficient operating region - for a large portion of time, by providing just the minimum instantaneous supply voltage to the PA on a sample-by-sample basis. This can restore the PA efficiency to 50-55%, even for high crest factor 4G signals, offering the promise of 4G performance with 2G battery life.
This efficiency gain is a major benefit for product designers. However, if an ET PA is operated in maximum efficiency mode, then it will introduce distortion that compromises the linearity of the PA. So although you may be achieving maximum PA efficiency, some form of linearisation will be needed to correct this distortion (See Figure 2).
Why do we care about distortion?
In an RF Power Amplifier, there are several types of distortion which need to be considered and controlled. Amplifier distortion products falling within the bandwidth of the signal being transmitted will degrade the Error Vector Magnitude (EVM) of the signal at the receiver, reducing coverage and data rate. Higher frequency distortion products outside the transmit channel may cause interference to other users in neighbouring channels, and are usually constrained by Adjacent Channel Leakage Ratio (ACLR) regulatory specifications. For Frequency Division Duplex (FDD) systems, such as FD-LTE, distortion from the PA which spreads from the transmit band into the receive band can also degrade the sensitivity of the handset’s receiver, despite attenuation from the duplex filter – with more than 40 bands in use for LTE, this requires analysis of many different frequency offsets. Additional co-existence requirements, such as WiFi and GPS receivers in the same handset, place yet more constraints on the amplifier performance.
Managing PA distortion, without unduly sacrificing power consumption, is therefore a major consideration for designers of chipsets, PAs, and end-products alike.