Power amplifiers for wireless applications are expected to provide exceptional linearity and efficiency in order to handle the complex waveforms used in modern communications systems. Rather than building RF power amplifiers with more pure performance, which adds cost, lowers efficiency and creates reliability issues, designers today can also opt for adding digital processing power with the use of digital-predistortion (DPD) techniques that can help maximize power-amplifier (PA) efficiency, increase reliability, and reduce operating costs.

Digital techniques offer many advantages in cost, power consumption and reliability when compared to analog methods. Because of these advantages, older narrow band, single-carrier, triple conversion systems are being replaced with wide band, multi-carrier transmitters enabled by digital signal processing (DSP) and DACs that produce direct IF, or even direct RF outputs to the RF amplifier.

Wireless systems are providing users with a host of services and benefits. Unfortunately, the benefits of advanced wireless technologies often come at the expense of increased power consumption and cost of operations. Modern cellular and wireless technologies—especially digital RF communications networks—transmit and receive more data, more video, and more voice than ever before. New standards such as HSDPA, HSUPA, 1xEVDO, WiMAX, and Long-Term Evolution (LTE) necessitate greater power usage, create more and larger RF waveform peaks, and allow for larger data bursts. Consequently, modern wireless devices are producing RF signals with unprecedented peak-to-average ratios (PARs) and the potential for distortion within an already crowded RF spectrum.

With power consumption and modern PARs at all-time highs, power amplifiers are being strained like never before—and causing transients and cost inefficiencies as a result. Bigger amplifiers that can accommodate more power inflate capital expenses in the short-term and operating expenses in the long-term. Larger, more costly batteries are required for the same amount of backup capability. And greater power consumption and production intensifies thermal and electrical conditions, which can create reliability problems.

When working with power amplifiers that support advanced wireless technologies, designers and network operators can choose one of two paths: They can add more brawn, or they can add more brains. Whereas the former effectively adds to the aforementioned cost and reliability concerns, the latter is prompting new strategies for digitally predistorting waveforms prior to power amplification for maximum efficiency and tight spectrum control. With the right test instruments, digital-predistortion (DPD) techniques can be honed to allow for smaller, more efficient power amplifiers—reducing development and operating costs while improving network and device reliability.

Whether it is a high-power satellite ground station, a multi-carrier cellular base station or even a low-power mobile system, modern transmitters employ a variety of predistortion techniques to reduce out-of-channel interference and optimize operating efficiency. One of the most popular and effective distortion reduction methods is Adaptive DPD.

This approach uses a sample of the transmitter's output to calculate error vectors and generate correction coefficients, which are then used to predistort the incoming signal. To reduce analog circuitry distortions, the signal in the chain is kept in digital format for as long as possible.

Figure 1 shows how a portion of an amplifier's output signal is tapped, then down-converted and digitized. The digitized signal is used to feed the DSP circuitry, which performs analysis of the non-linearities present in the signal and creates non-linear correction coefficients. These non-linear coefficients are used to alter the incoming in-phase (I) and quadrature (Q) signals in the transmit chain. The signal, now predistorted and with PAR reduction applied, is fed to the amplifier after being converted back to the analog realm by the DAC, as seen in the transmit chain. The resulting output signal exhibits reduced spectral distortion and improved adjacent-channel-leakage-ratio (ACLR) performance than the signal without predistortion techniques.

Digitally predistorted amplifiers offer improved spectral efficiency with much higher power-added efficiency (PAE) than previous feed-forward architectures, dramatically reducing heat concerns, improving reliability, and lowering operating costs. This approach has branched beyond cellular base stations, and we are now seeing feedback linearization for cellular handsets, satellites and even adaptive phased-array radars.

This scenario creates a wide variety of troubleshooting challenges not seen in traditional analog systems, however. Digital artifacts may be introduced into the transmit chain by the ADC and DAC, or by any DSP performed on the signal prior to analog conversion in the transmit path. These artifacts are frequently transient in nature and are difficult or impossible to capture using conventional spectrum analyzers. They may only occur rarely and can cause frequency-domain effects in the adjacent and alternate channels. Effective troubleshooting of transient frequency-domain signals requires not only the detection of the problem, but also the ability to trigger on it and capture a record for analysis.

Characterizing these systems presents new challenges as well. In the development stage, a variety of predistortion and PAR reduction methods may be tested and optimized prior to the availability of the entire transmit chain. The signals in the feedback path must often be captured using test equipment, and calculation of the new non-linear distortion coefficients is performed in offline software prior to the availability of completed hardware (ASICs or FPGAs). Correction algorithms using these coefficients are then applied to the initial I and Q signals and the result is loaded into arbitrary waveform generators (AWGs) to test their performance.

The rate of signal and power changes is also problematic. Since many wireless signals employ a burst format (such as 1xEV, HSxPA, and WiMAX signals), use pulsed waveforms (such as radar, RFID/NFC, and Zigbee signals), or rely on adaptive techniques (with changes in coding or modulation rate), the RF power level changes quickly. Often, these changes occur faster than the feedback loop can respond. Unlike previous linearization architectures, such as feed-forward amplifiers, the amplifier is blind to fast temporal changes while the feedback loop is sensing and adapting to these changes. This can lead to unintended signal performance that can be damaging to network reliability and operation.