In wireless systems, power amplifier (PA) linearity and efficiency are often two parameters that must be weighed. Engineers are looking for an efficient and flexible Volterra-based adaptive predistortion technology that can be used to achieve high linearity in wideband RF power amplifiers. This article will provide an overview of the different digital predistortion techniques and introduce an adaptive algorithm unique to an innovative DPD linearization circuit.
In wireless systems, power amplifier (PA) linearity and efficiency are often two parameters that must be weighed. Fortunately, Volterra-based adaptive digital predistortion (DPD) linearization circuits enable high linearity and high efficiency for RF PAs in wireless systems. This adaptive digital pre-distortion scheme extends the linear range of the power amplifier, while reducing the crest factor, which can drive the RF PA more powerfully, and is more efficient, while meeting the transmission spectrum efficiency requirements and modulation accuracy requirements.
This new digital predistorter has been integrated into Texas Instruments' GC5322 integrated transmitter solution. Millions of gate-specific signal processors (ASSPs) are fabricated in a 0.13-micron CMOS process and include digital up-conversion, crest factor reduction, and digital pre-distortion. This "modulation-agnostic" processor supports a 30 MHz signal bandwidth. For third-generation (3G) cell phone signals, the ratio of peak power to average power (PAR) can be reduced by 6dB. For Orthogonal Frequency Division Multiplexing (OFDM), 4 dB can be improved while meeting adjacent channel power ratio (ACPR) and error vector magnitude characteristics. It is possible to correct nonlinearities up to order 11 and achieve PA storage effects of 200 ns. For a variety of RF PA topologies, ACPR is generally improved by more than 20 dB, and power efficiency is increased by more than 4 times. For general base stations, static power loss can be reduced by as much as 60%. This flexible Volterra-based preprocessor is optimized for a variety of RF architectures, modulation standards, and signal bandwidth.
Non-constant envelope modulation schemes used in 3G and other emerging air interface standards are more spectrally efficient, but the peak-to-average signal ratio is higher and the PA's fallback is necessarily higher. This reduces PA efficiency and increases the cooling and operating costs of the base station. The RF PA with lower efficiency generally accounts for 30% of the total base station system cost, and the environmental impact is quite significant. With the continuous development of “greenâ€, energy-efficient technologies and increasing energy costs, as well as the ever-increasing spectral efficiency and signal bandwidth requirements, and the evolving standards, the power amplifier linearity becomes the next Key design issues for a generation of base stations. Over the years, a large number of power amplifier linearization techniques have been proposed and implemented, such as RF feedforward, RF feedforward, and RF/IF predistortion and post-distortion. Among them, the adaptive DPD scheme has proven to be the most efficient and cost-effective compared to traditional analog/RF linearization techniques. The ever-increasing computing power of DSP/ASSP makes digital pre-distortion an increasingly attractive option.
The GC5322 transmit solution combines digital up-conversion (DUC), crest factor reduction (CFR), and DPD in a highly integrated ASSP with real-time adaptive control from Texas Instruments' C67x DSP built-in software. The transmitter can be optimized for a variety of RF architectures and supports a variety of air interface standards including CDMA2000, WCDMA, TD-SCDMA, MC-GSM, WiMAX, and Long Term Evolution (LTE) handset standards. This flexible pre-compensator can be used effectively with a variety of power topologies, such as Class A/B or Doherty amplifiers, designed to support communication systems with signal bandwidths up to 30 MHz. This article is divided into two articles that focus on the hardware implementation of the DPD solution.
A 3G CDMA based wireless communication system and a multi-carrier system employing an OFDM method can often handle high PAR or crest factor signals. Non-constant envelope modulation techniques, such as quadrature amplitude modulation used in these systems, have strict error vector magnitude (EVM) requirements. Because of these requirements, PA is required for high linear amplitude and phase response. The linear working range of the PA is generally limited. PA nonlinearity can cause intermodulation distortion of the transmitted signal, resulting in a decrease in spectral cracking and adjacent channel power ratio (ACPR). A simple solution to this problem is to roll back the input signal level to the PA so that the resulting signal is completely in the linear working area of ​​the amplifier. Unfortunately, PA power efficiency drops considerably at lower input powers, making this approach inferior to the best approach. In addition, more advanced and efficient amplifier topologies (such as Doherty PA) exhibit considerable nonlinearity even at the back-off power level, resulting in poor EVM and ACPR performance.
When working in the retracted state, the efficiency of the conventional class AB amplifier currently used is between 5% and 10%. However, after using the crest factor reduction and adaptive DPD technology, the efficiency can be increased by 3 to 5 times. Newer PA topologies, such as Doherty amplifiers, or even class AB amplifiers with dynamic envelope traces combined with DPD, and newer device technologies such as gallium nitride (GaN) or gallium arsenide (GaAs) power transistors Used to achieve nearly 50% efficiency.
The next section of this article will discuss the need for a linearization scheme with a highly accurate model for the predistorter.
In wireless systems, power amplifier (PA) linearity and efficiency are often two parameters that must be weighed. Engineers are looking for an efficient and flexible Volterra-based adaptive predistortion technology that can be used to achieve high linearity in wideband RF power amplifiers. This article will provide an overview of the different digital predistortion techniques and introduce an adaptive algorithm unique to an innovative DPD linearization circuit.
The first part of this paper mainly introduces the GC5322 integrated emission scheme of Texas Instruments. Below we will continue to discuss the need for a linearization scheme for a highly accurate model of the predistorter.
Most current DPD implementations use memoryless linearization techniques in which instantaneous nonlinearity (predistortion) is used to compensate for the instantaneous nonlinear behavior of the PA. The memoryless power amplifier is characterized by its amplitude and phase transmission characteristics, which generally refer to AM to AM (ie, gain compression) and AM to PM characteristics. For this kind of memoryless amplifier, a general look-up table (LUT) can be used for pre-compensator gain/phase correction. Figure 1 illustrates the gain compression and AM-PM characteristics of a typical Doherty PA. Because the gain and phase characteristics of the PA vary with temperature, voltage, and component aging, adaptive control look-up tables are required to achieve truly efficient and efficient linearization.
For communication systems where the PA must support a higher RF modulation bandwidth, the no-memory mode proves not enough because it relies on amplitude rather than frequency. PAs that must support large signal bandwidths exhibit significant memory effects due to the large time constant of the components in the DC bias network and the rapid thermal effects of active devices. This causes the PA characteristics to change with earlier input levels, so a predistortion structure that reduces the memory effect is required.
Any efficient linearization scheme requires a highly accurate model of the predistorter. If the PA uses a direct learning adaptive architecture, a highly accurate model is also required. A large number of memory-based nonlinear system modeling techniques have been proposed in the literature. No single method can be a universal solution. Therefore, model selection is difficult and depends on the application. An effective PA model must be able to represent different types of nonlinear and memory effects with reasonable accuracy.
Volterra series is a more general time-varying nonlinear system model with memory. Including the sum of multidimensional convolutions, the discrete time causal form can be written as Equation 1, and Equation A gives the conditions in detail, where the multidimensional matrices h1, h2, ... hn are modeled nonlinear nth-order Volterra coefficients, and Mn is nonlinear and limited. Memory length. Given that RF PA allows for long memory depths (up to 1 microsecond) and non-linear levels (up to level 11), the above models are mathematically unworkable. A simplified solution must be used to obtain the actual pre-compensator product. These simplifications can be divided into two basic methods: arithmetic and model simplification. For the first, the general Volterra model in Equation 1 has many attractive arithmetic features that can be used to achieve an efficient implementation. For the model simplification method, although a complete general Volterra (or some other general model) is required, as is known, the RF power amplifier model generally has a large number of Volterra terms, which are meaningless in implementation. These items can be discarded without causing measurable deterioration in linear performance.
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