Power amplifier (PA) linearity and efficiency are often two parameters that must be traded off in a wireless system. Fortunately, a Volterra-based adaptive digital-predistortion (DPD) linearizer circuit can enable RF PAs in wireless systems to achieve high efficiency with good linearity. This adaptive digital predistortion solution extends the linear range of power amplifiers and, in combination with crest factor reduction, enables RF PAs to be driven harder and more efficiently while meeting transmit spectral efficiency and modulation accuracy requirements. Part 1 of this two-part series will provide an overview of different digital predistortion techniques and introduce the unique adaptation algorithm that is at the heart of this innovative DPD linearizer.

The new digital predistorter has been incorporated in the model GC5322 integrated transmit solution from Texas Instruments (www.ti.com). The multi-million-gate application- specific signal processor (ASSP) is fabricated with 0.13-micron CMOS technology and incorporates digital upconversion, crest factor reduction, and digital predistortion. The “modulation-agnostic” processor supports signal bandwidths to 30 MHz. It can reduce peak-to-average power ratios (PARs) for thirdgeneration (3G) cellular signals by as much as 6 dB. It provides a 4-dB improvement for orthogonal-frequencydivision- multiplex (OFDM) signals while meeting adjacent-channel-power- ratio (ACPR) and error-vectormagnitude (EVM) specifications. It can correct for up to 11th-order nonlinearities and PA memory effects to 200 ns. It typically provides greater than 20-dB ACPR improvement and a more than four-time increase in power efficiency for a variety of RF PA topologies, resulting in as much as 60-percent reduction in the static power consumption for typical basestations. This flexible Volterra-based predistorter can be optimized for a variety of RF architectures, modulation standards, and signal bandwidths.

Nonconstant-envelope-modulation schemes like those used in 3G and other emerging air interface standards are spectrally more efficient, but have high peak to average signal ratios, necessitating a higher level of PA backoff. This decreases the PA efficiency and increases the cooling and operational costs of the base-stations. Lower efficiency RF PAs typically account for as much as 30 percent of the overall base station system cost and have a considerable environmental footprint. Increasing push toward “green,” energy-efficient technologies combined with rising energy costs and increasing spectral efficiency and signal bandwidth requirements of current and evolving wireless standards make power amplifier linearity a crucial design issue in next-generation base stations. A variety of power amplifier linearization techniques such as RF feedforward, RF feedback, and RF/IF predistortion and post-distortion have been proposed and implemented over the years. Of these, adaptive DPD schemes have proven to be the most efficient and cost effective compared to traditional analog/RF linearization techniques. Increasing DSP/ASSP computational capacities make digital pre-distortion an ever more attractive option.

The GC5322 transmit solution combines digital upconversion (DUC), crest factor reduction (CFR), and DPD in a highly integrated ASSP, with real-time adaptation control provided by software residing in a model C67x DSP from Texas Instruments. The transmit device can be optimized for a variety of RF architectures and supports multiple air interface standards including CDMA2000, WCDMA, TD-SCDMA, MC-GSM, WiMAX, and the Long Term Evolution (LTE) cellular standard. The flexible pre-distorter can be used efficiently with a variety of power amplifier topologies such as Class A/B or Doherty amplifiers, and is designed to support communication systems with signal bandwidths as wide as 30 MHz. This two-part article focuses on the hardware implementation of the DPD solution.

Wireless communications systems based on 3G CDMA as well as multicarrier systems using such methods as OFDM often handle signals with high PARs or crest factors. The nonconstant- envelope-modulation techniques such as quadrature amplitude modulation (QAM) employed in such systems have stringent error-vectormagnitude (EVM) requirements. Such requirements call for a PA with highly linear amplitude and phase response. PAs typically have a limited linear range of operation. PA nonlinearities cause intermodulation distortion in the transmitted signal, leading to spectral splatter and reduction in adjacent-channel power ratio (ACPR). A simple solution to this problem is to back off the level of the input signal to the PA so that the resulting output signal lies completely within the amplifier’s linear operating region. Unfortunately, PA power efficiency decreases considerably at lower input power levels, making this a less-thanoptimal solution. Moreover, even advanced, efficient amplifier topologies such as Doherty PAs suffer considerable nonlinearities even at backed-off power levels, resulting in poor EVM and ACPR performance.

The efficiencies of traditional Class AB power amplifiers in use today range from about 5 to 10 percent when operated under back-off conditions. But with crest-factor reduction and adaptive DPD techniques, the efficiency can be improved by a factor of 3 to 5. Newer PA topologies, such as Doherty amplifiers, or even Class AB amplifiers with dynamic envelop tracking in combination with DPD, and newer device technologies, such as gallium nitride (GaN) or gallium arsenide (GaAs) power transistors, can be used to achieve efficiencies approaching 50 percent.

Current DPD implementations mostly use memory-less linearization techniques in which an instantaneous nonlinearity (the predistortion) is used to compensate for the instantaneous nonlinear behavior of the PA. Memory-less power amplifiers can be characterized by their amplitude and phase transfer characteristics, commonly referred to as AM-to-AM (or gain compression) and AM-to-PM characteristics. A generalized lookup table (LUT) can be used for the predistorter gain/phase correction for such a memory-less power amplifier. Figure 1 shows the gain compression and AM-PM characteristics for a typical Doherty PA. Because the gain and phase characteristics of a PA change with temperature, voltage, and component aging, adaptive control of the lookup tables is required for truly efficient and effective linearization.

For communication systems where the PA must support higher RF modulation bandwidths, the memory-less model proves to be inadequate since it is only amplitude dependent, not frequency dependent. PAs that must support large signal bandwidths exhibit significant memory effects due to the long time constants of components in the DC biasing networks and rapid thermal effects of the active devices. This causes the PA’s characteristics to vary as a function of earlier input levels, and necessitates the use of a predistortion architecture that can alleviate these memory effects.

Any efficient linearization scheme requires a highly accurate model for the predistorter, as well as for the PA if it uses a direct learning adaptation architecture. A variety of techniques have been proposed in the literature for modeling nonlinear systems with memory, with none providing a universal solution. As a result, model selection is challenging and dependent on the application. An efficient PA model must represent different types of non-linearities and memory effects in PAs with reasonable accuracy.

One of the more general models for time-invariant nonlinear systems with memory is the Volterra series. It consists of a sum of multidimensional convolutions, which in discrete time causal form can be written in the form of Eq. 1 with the conditions detailed in Eq. A, where the multidimensional matrices h1, h2, … hn are the nth-order Volterra coefficients that model the nonlinearities, and Mn is the finite-length memory of the nonlinearity. With the long memory depths (to 1 microsecond) and nonlinearity orders (to 11th order) to be considered for RF PAs, the above model becomes computationally intractable. Simplification schemes must be employed to yield a practical predistorter product. These simplifications can be placed into two basic approaches: algorithmic approaches and model-reduction approaches. For the first set of approaches, the generic Volterra model in Eq. 1 has a number of attractive arithmetic properties that can be exploited to come up with efficient implementations. For the modelreduction approaches, although a totally generic Volterra (or some other generic model) is desired, it is known that RF power amplifier models typically have a lot of Volterra terms that are insignificant for practical implementation. The terms may be dropped without measurable degradation of linearization performance.

A variety of different simplified pre-distortion systems, all using variations of the generalized model in Eq. 1, have been proposed in the current literature. A few of these systems are listed here:

1. Truncated Volterra^3,5 Direct-form, parallel-cascade, Vvector algebra based and a few other realizations of truncated Volterra systems have been proposed in the literature. These algorithmic reduction approaches are efficient at linearization, but are computationally complex and often intractable due to the large number of parameters to be estimated, making them unattractive for real-time implementations.

2. Wiener systems^6,7 A significant simplification of the Volterra model, the Wiener model consists of a linear filter followed by a memory-less nonlinearity. An LUT can be used to model the nonlinearity, and an FIR filter to model the linear filter. The effectiveness of Werner systems in modeling most RF power amplifiers is limited. The estimation of the model parameters is reasonably complicated, making it unattractive for real-time adaptation.

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