11 Oct 2014
Software defined power amplifiers using envelope tracking
Jeremy Hendy VP Marketing Nujira describes how envelope tracking technology can be used to create RF power amplifiers where the characteristics are defined by software
Envelope Tracking, ET, is transforming the RF front end in mobile handsets. The energy efficiency benefit of operating an RF power amplifier, PA, in ET mode is widely known.
However, envelope tracking is also delivering a range of other useful system benefits. One such benefit is increasing the control that system designers have over the behaviour and performance of the power amplifier.
Power amplifier performance has traditionally depended on the inherent small-signal RF characteristics of the PA, and there has been little that could be done in terms of optimisation outside of the RF domain. This ‘uncontrolled’ analogue nature of PA devices also causes other problems for system designers. power amplifier behaviour is not stable - small signal RF response varies particularly in response to temperature, process, supply voltage, and as a function of the age of the device.
This variability means that power amplifiers have to have significant analogue margin designed-in, which inevitably compromises performance. These compromises ultimately impact on the user experience of mobile handsets - degrading battery life, call quality, coverage and data rates.
Envelope tracking opportunities
In contrast, the performance of an envelope tracking power amplifier is not as ‘self-contained’ as it is for traditional fixed supply PAs. In fact envelope tracking transforms the power amplifier from an uncontrolled ‘pure’ analogue component into a software-defined device, where its behaviour is defined in the digital domain, and can even be changed dynamically based on operating conditions.
The key to optimising the performance of the power amplifier dynamically under software control is the envelope tracking shaping table – a look-up table in the baseband, which maps the RF signal amplitude to the instantaneous PA supply voltage. This article will describe the function and performance of the envelope tracking shaping table and explain how it can be utilised by system designers to unlock the full performance of the power amplifier.
What is ET and where does the shaping table fit in?
The objective of Envelope Tracking is to improve the efficiency of power amplifiers carrying high peak to average power, PAPR, signals. The drive to achieve higher data throughput within limited spectrum resources requires the use of complex modulation schemes, such as OFDM, with high PAPR – often up to 10 dB. To successfully transmit these signals requires a highly linear amplifier, with a high peak power to accommodate the wide dynamic range of the signal. Consequently, at the average power level, the PA is significantly “backed off”. Unfortunately, the efficiency of traditional fixed supply PAs when operated under these conditions is very poor.
In an envelope tracking system, the supply voltage is dynamically adjusted to track the RF envelope at high instantaneous power. Here, the power amplifier operates with high efficiency in compression. Since the PA is in compression, the output signal amplitude is determined almost entirely by the supply voltage, rather than the input signal level or the small-signal gain of the PA.
Conversely, when the instantaneous RF power is low in the “troughs” of the waveform, the supply voltage is held substantially constant and the power amplifier output characteristics are primarily determined by the instantaneous input power (linear region) and the small-signal performance of the PA. A transition region in which both supply voltage and input power influence the output characteristics exists between these two extremes (see Figure 1).
Figure 1 - Instantaneous efficiency vs supply voltage - IsoGain Shaping
With a fixed supply power amplifier there are two primary performance metrics, efficiency and linearity, both of which depend on the PA design (e.g. supply voltage, bias and matching) and the instantaneous signal level at the input. Linearity can in turn be broken down into gain (AM:AM) and phase (AM:PM) distortion components. Since the power amplifier design and the supply voltage are fixed at design time, the only “run time variable” is the instantaneous signal amplitude at the PA input, making the PA effectively a 2-port device (input and output).
In envelope tracking mode however, the PA’s fundamental output characteristics, including power, efficiency, gain, phase, now depend on two “run time” inputs - instantaneous RF input power and instantaneous supply voltage – turning the PA into a 3-port device (input, supply and output).
In an ET system the mapping between instantaneous RF envelope and applied supply voltage is defined by the contents of a non-linear envelope ‘shaping table’ in the digital signal processing path which generates the envelope reference signal (see Figure 2).
Figure 2 - Envelope Tracking power amplifier system
In a typical 4G application, the shaping table would be implemented as a lookup table with 64-128 entries, interpolated to 14 bit precision, allowing the supply voltage to be accurately defined for every possible I/Q sample amplitude. The shaping table is applied to the signal on a sample-by-sample basis, typically at hundreds of megasamples per second (6x the RF channel bandwidth is a good rule-of-thumb).
The shaping table, which maps the instantaneous RF output power to supply voltage, has to be created at design time based on the type of power amplifier being used and the system performance requirements. The shaping table does not need to be created for each individual part on the production line. Instead it is possible to fix the shaping table at design-time, and then just calibrate out analogue gains and offsets in the envelope and RF paths on the production line.
Benefits of the shaping table
In most applications, the PA has to simultaneously meet several key metrics - efficiency, output power, gain, in-band linearity (EVM) and out-of-band linearity (ACLR and RX band noise), potentially over a wide power control range. These metrics are all interlinked and ultimately improving the performance in one area means trading off performance in another. For example, PA efficiency and gain vary with supply voltage - a higher supply voltage gives you more gain through the PA, but lower efficiency. At lower voltages the reverse is true.
‘Optimising’ a PA design therefore, comes down to finding the right balance between these metrics to give you the best overall system performance.
With fixed supply power amplifiers, designers have had very limited control over these metrics and PAs have had to be ‘over-designed’ in terms of inherent linearity at the expense of efficiency. As envelope tracking turns the PA from a two-port into a three-port device, with the supply pin acting as an additional high bandwidth control input, it gives envelope tracking designers more freedom to define the trade-offs required to optimise PA performance.
In fact the benefit of making a power amplifier a three-port device goes further. By modulating the instantaneous supply voltage at high speed, and synchronously with the instantaneous signal amplitude, designers can actually control linearity using the shaping table – after all, amplifier linearity can be thought of as the variation of gain as a function of the input signal amplitude.
At each instantaneous power level, a higher supply voltage will give more gain (at the expense of efficiency), and a lower supply voltage will give more efficiency (at the expense of gain). This technique can be used in the shaping table to control the desired AM:AM characteristic of the PA.
In contrast to a linear power amplifier, when the envelope tracking PA is operating in compression/saturation, increasing the input amplitude does not give you any more output power – that’s the definition of saturation. The only way to get more output power is to increase the supply voltage. In other words, the output amplitude is directly controlled by the supply voltage.
The profile of the shaping table controls how the supply voltage varies with power amplifier input signal amplitude. So the AM:AM distortion of the PA (i.e.. how its output signal amplitude changes as a function of the input signal amplitude) is therefore directly controlled by the shaping table. A “kink” in the shaping table will give a corresponding “kink” in the AM:AM response and degrade linearity.
Phase is not directly controllable by the supply voltage in the same way as the gain of the PA, but the phase response (AM:PM distortion) of the PA in envelope tracking mode can certainly be characterised, and to some extent controlled, via the shaping table – but this does also depend on the design of the PA bias and matching networks.
The ability to directly control and trade-off gain and efficiency, and indirectly control phase, allows the shaping table to control a wide range of PA performance characteristics, as illustrated in Figure 3. The three “primary trade-offs” (efficiency, average gain, and linearity) are represented by the three points of the triangle, with secondary trade-offs in the central portion of the pyramid.
Figure 3 - System characteristics influenced by the shaping table
Characterising the envelope tracking PA
Before defining the shaping table, we need to measure the PA’s fundamental characteristics (output power, efficiency, gain, phase) over the full range of instantaneous supply voltages and RF input powers. Figure 4 compares potential measurement methods.
|Figure 4 Comparison of PA characterisation methods|
|Test methodology requirements||PA current measurement||Supply impedance||Supply bandwidth requirements||Correlation with ET operation||Parameters measured|
|Swept CW testing||Bench PSU||Low (decoupling Capacitor)||Low (Bench PSU)||Poor, due to PA die heating||Gain (AM:AM), Efficiency|
|Pulsed CW RF /DC testing||Instrumentation grade current probe, ~5 µs resolution||Low (decoupling Capacitor)||Low (Bench PSU)||Good, if short pulses (~10 µs, 10% duty cycle).||Gain (AM:AM), Efficiency|
|Dynamic supply modulation||Challenging – high BW with high common mode voltage current sense||Requires low impedance dynamic supply (no decoupling)||High (~60 MHz BW)||Excellent||Gain (AM:AM), Phase (AM:PM), Efficiency, with fixed ET shaping table|
|PA surface capture||Challenging – high BW with high common mode voltage current sense||Requires low impedance dynamic supply (no decoupling)||High (~60 MHz BW)||High||Gain (AM:AM), Phase (AM:PM), Efficiency, for any ET shaping table|
In principle this characterisation could be carried out using a CW network analyser and a variable DC supply, but results are typically poor due to thermal effects, ranging errors and drift in phase measurements. It is also far too slow to allow load pull techniques to be used.
An alternative approach is to use a pulse characterisation methodology, using ATE controlled standard test equipment. This avoids the need for a high bandwidth low impedance supply and is sufficiently fast for load pull to be viable, but has the drawback that it is difficult to make accurate phase measurements.
The last approach is to use real waveforms and to vary the shaping table to allow all combinations of input power and supply voltage to be measured. This requires an envelope tracking supply modulator, but is very fast, allows accurate phase information to be gathered and can also be used to characterise memory effects.
By post-processing multiple data points captured over a range of supply voltages and RF input powers, it is then possible to analyse the efficiency, gain and phase data as a set of 3D surfaces, enabling the PA designer to visualise the amplifier characteristics, and make suitable design trade-offs during performance optimisation.
It is even possible to collect the entire surface of the PA with a single measurement, rapidly switching between multiple envelope tracking shaping tables to capture the behaviour of the PA over a wide range of supply voltages and input powers. Nujira’s ET Surface Explorer toolchain is defined specifically for this task, and can capture the entire surface of the PA in a few seconds.
Once the power amplifier surfaces of gain, phase and efficiency have been captured (see Figure 5), they can be used to establish parameters around a static (i.e. memory-less) simulation model of the PA, having output power, phase and efficiency as outputs, and input power and supply voltage as inputs. Once the shaping table is defined, this model can then be used to predict PA system performance parameters such as ACPR, EVM and efficiency for a wide variety of test waveforms and power levels, enabling many of the shaping table trade-offs to be evaluated in software, rather than on the RF bench (see Figure 6).
Figure 5 – Envelope tracking power amplifier gain, phase and efficiency surfaces, captured using Nujira ET Surface Explorer
Figure 6 – Envelope tracking power amplifier shaping table design and simulation, using Nujira ET Surface Analyzer
Designing the shaping table
One “obvious” envelope tracking shaping table is the maximum efficiency shaping, which attempts to minimise the current consumption of the power amplifier by selecting a supply voltage which maximises the instantaneous efficiency of the PA at all signal levels, as shown in Figure 8.
However, the downside of this approach is that the power amplifier gain (which is strongly influenced by the instantaneous supply voltage) is almost always non-linear, resulting in significant AM:AM distortion, as shown in Figure 10.
While this distortion can readily be corrected with adaptive Digital Pre-Distortion, as found in typical wireless infrastructure applications, the size and power consumption penalties of DPD make it unattractive for lower power applications.
An alternative mapping of particular interest is ‘Iso-Gain’ shaping, in which the instantaneous supply voltage is chosen to achieve a particular constant PA gain (see Figure 9).
With this mapping, the envelope tracking power amplifier system achieves low AM:AM distortion despite operating in compression over much of the envelope cycle as shown in Figure 7. The equivalent trajectory for fixed supply operation is also shown in Figure 9 – from this it is apparent that envelope tracking can actually be used to linearise a PA, reducing ACPR and EVM.
Figure 7 - Instantaneous efficiency vs supply voltage – Iso-Gain shaping
Figure 8 - Instantaneous efficiency vs supply voltage – Optimum Efficiency shaping
Figure 9 - Envelope tracking power amplifier gain characteristics - Iso-Gain shaping
Figure 10 - Envelope tracking power amplifier gain characteristics - Optimum Efficiency shaping
Figure 11 - RF O/P power - Supply voltage mapping - Iso-Gain shaping table
Figure 12 - RF O/P power - Supply voltage mapping – Optimum Efficiency shaping table
The system trade-off associated with using the shaping table to linearise the power amplifier is a small loss of efficiency (compare Figure 7 and Figure 8) for a substantial improvement in linearity (compare Figure 9 and Figure 10). With LTE PAs, the efficiency penalty for operating in Iso-Gain is typically only 1-2%.
The designer can create an IsoGain table with (in principle) any desired PA gain, although with most power amplifiers there is a usable operating range of 4-5 dB. The choice of Iso-Gain will in the first instance depend on the maximum required output power, and the maximum available input power. However, depending on the specific PA characteristics other considerations may also apply, such as the minimum PA supply voltage, and any limitations on the envelope tracking swing range.
To maintain the same Iso-Gain over a range of power control levels will require an Iso-Gain value which maximises the flat (controlled) area of the gain characteristic. For example, in Figure 9, it can be seen that with the highlighted 26 dB Iso-Gain trajectory, a slight “kick-up” in the gain occurs at low instantaneous power levels as the power amplifier drops out of compression. This will introduce some AM:AM distortion in the troughs of the signal, which may not be significant at high average power levels, but may degrade ACLR as the average power level is reduced. By contrast, choosing a slightly higher IsoGain, eg 26.2 dB, would allow the IsoGain also to be maintained at lower output powers, at the cost of a slight decrease in efficiency.
A third option for mapping a shaping table is known as Constant Compression. This involves designing a shaping table which always runs the PA at a particular operating point, e.g. 3 dB of compression. For each supply voltage, this can be calculated from the gain surface/curves. Although similar to “max efficiency” mapping, it is computed from the gain surface, not the efficiency surface. It will normally introduce distortion, but isn’t necessarily the highest efficiency.
An enhancement to Iso-Gain shaping is “FlexiGain”™. This is a method of doing Crest Factor Reduction without any additional signal processing. It starts with an Iso-Gain table, but then deliberately introduces distortion at the peaks, in the form of a controlled “soft clipping”, which in some ways emulates what the PA does with a fixed supply voltage – the big difference is that this form of crest factor reduction is much more stable and predictable, since you are not relying on the small-signal RF characteristics of the PA to introduce soft clipping.
While FlexiGain gives you a slight improvement in PA efficiency, this tends to be marginal since you actually spend very little time at the peaks – the bigger benefit comes from reducing the envelope tracking supply swing range, which may significantly improve the efficiency of the ET power supply. The downside of FlexiGain is that you are pushing the PA harder into compression at the peaks, which may give you an unusual phase response.
The shaping table can also be used to significantly reduce bandwidth in the envelope path. Immediately after the magnitude calculation, the envelope tracking path signal has a very high bandwidth in the troughs of the modulated waveform, caused by the signal amplitude falling to zero. By applying a minimum voltage in the shaping table, the bandwidth requirements are significantly reduced (see Figure 13).
A smooth transition between the linear and compressed regions results in a lower bandwidth requirement for the envelope amplifier, whereas sharp transitions will increase the supply bandwidth – which is usually one of the constraints of an envelope tracking system. Shaping tables should always be monotonic – changes in direction of the voltage can cause significant ‘spikes’ in the bandwidth of the envelope path.
Figure 13 - The shaping table significantly reduces bandwidth in the envelope path
The shaping table can also be used to control noise transfer in the RF front end. For FD-LTE systems, noise in the receive band is a significant limiting factor for handset performance, particularly when the terminal is a long way from the base-station. Envelope tracking introduces an additional noise source – the ET path – which gets mixed with the RF waveform, and ultimately finds its way to the receiver via the duplex filters, where it acts as an additional noise source which de-sensitises the receiver.
However, the amount of TX noise transferred through the PA is a function of how much the PA is in compression - so maximum efficiency also means maximum noise transfer. Using the shaping table it is possible to trade off efficiency and noise dynamically in software, altering the trade off dependent on the signal strength received from the basestation. By backing off the shaping table slightly (e.g. to a higher IsoGain with less compression) the power supply rejection of the ET PA is improved, and less of the envelope tracking path noise is transferred to the RF path.
The use of multiple shaping tables can also be used to deliver further performance optimisation. Using multiple tables can enable dynamic control based on a variety of different operating conditions. Potential variables include battery level, RF frequency, temperature and whether the RF signal is under high VSWR (or load mismatch) conditions.
Using sensors on these variables can allow for dynamic optimisation of the shaping table based on operating conditions. By using multiple shaping tables in this way even further performance gains in terms of efficiency or output power can be eked out of the PA. For designers this also means not having to leave quite as much “analogue” margin in their design.
Power of software defined PAs
Compared to fixed supply power amplifiers, the process of designing envelope tracking-enabled systems does introduce additional complexity and requires the use of more sophisticated characterisation techniques. However, as this article has shown there are significant system benefits that can be gained from this additional complexity.
Envelope tracking opens up a series of performance trade-offs that designers can manipulate to dynamically deliver optimal RF performance in any given situation. Accurate PA characterization data can be used to produce multidimensional simulation models of the PA, enabling designers to have far more subtle control over performance optimisation. With this approach, designers can easily explore performance trade-offs and optimise the design from a laptop, not in the lab.
These capabilities have simply not been available to PA designers in the past. The concept of software defined radio has, until now, always stopped short of the PA. With the envelope tracking shaping table, the benefits and capabilities of software defined control are now finally coming to the PA. As the complexity of cellular networks continues to increase with LTE-Advanced and 5G on the horizon, greater software control over power amplifiers will be critical in ensuring that the RF front end can continue to deliver the optimum performance over a wide range of operating conditions.
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About the author
Jeremy Hendy is VP of Sales and Marketing at Nujira and he brings the company considerable experience of semiconductor sales and marketing across multiple technologies for wireless communication and digital video. Previous positions include Marketing Director of wireless USB start-up Artimi, VP Marketing for Aspex Semiconductor, and Strategic Technology Director of Cadence’s Wireless and Multimedia business unit. He started his career with Texas Instruments, and holds a first class honours degree in Electronic Engineering from the University of Liverpool.
Nujira Ltd is the world leader in Envelope Tracking technology and solutions for powering energy efficient 4G cellular terminals, base stations and digital broadcast transmitters. Since the Company’s formation in 2002, Nujira has developed the most extensive and complete patent portfolio around Envelope Tracking, with over 200 patents filed or granted worldwide. Nujira’s corporate headquarters is located in Cambridge, UK. The Company currently has design centres in Cambridge and Bath, England and in Edinburgh, Scotland, and sales locations in North America, Germany, Japan and China.
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