Abstract
Predistortion (PD) lineariser for microwave power amplifiers (PAs) is an important topic of research. With larger and larger bandwidth as it appears today in modern WiMax standards as well as in multichannel base stations for 3GPP standards, the relatively simple nonlinear effect of a PA becomes a complex memory-including function, severely distorting the output signal. In this contribution, two digital PD algorithms are investigated for the linearisation of microwave PAs in mobile communications. The first one is an efficient and low-complexity algorithm based on a memoryless model, called the simplicial canonical piecewise linear (SCPWL) function that describes the static nonlinear characteristic of the PA. The second algorithm is more general, approximating the pre-inverse filter of a nonlinear PA iteratively using a Volterra model. The first simpler algorithm is suitable for compensation of amplitude compression and amplitude-to-phase conversion, for example, in mobile units with relatively small bandwidths. The second algorithm can be used to linearise PAs operating with larger bandwidths, thus exhibiting memory effects, for example, in multichannel base stations. A measurement testbed which includes a transmitter-receiver chain with a microwave PA is built for testing and prototyping of the proposed PD algorithms. In the testing phase, the PD algorithms are implemented using MATLAB (floating-point representation) and tested in record-and-playback mode. The iterative PD algorithm is then implemented on a Field Programmable Gate Array (FPGA) using fixed-point representation. The FPGA implementation allows the pre-inverse filter to be tested in a real-time mode. Measurement results show excellent linearisation capabilities of both the proposed algorithms in terms of adjacent channel power suppression. It is also shown that the fixed-point FPGA implementation of the iterative algorithm performs as well as the floating-point implementation.
Highlights
Future mobile communication systems are intended to provide multimedia communications which require high-speed broadband transmissions
The simplicial canonical piecewise linear function (SCPWL) which is suitable for modeling static nonlinearities is presented
A non-iterative method known as the image coordinate mapping (ICM) method [28] is employed for identifying the SCPWL PD
Summary
Future mobile communication systems are intended to provide multimedia communications which require high-speed broadband transmissions. Orthogonal frequency division multiplexing (OFDM) modulation is extensively employed and proposed for many broadband systems (e.g., WLAN, WiMax [1, 2], LTE of 3GPP [3]) due to its spectral efficiency and robustness in multipath environments The drawback of such schemes is their high peakto-average power ratio (PAPR), which requires the transmitter system to be highly linear, especially the power amplifiers (PAs), in order to avoid nonlinear distortion. [13,14,15,16] reported implementation of LUT-based digital PDs on DSP/FPGA hardware and validated on real PAs in measurement testbeds Another example of a partial hardware implementation of a parametric model PD is reported in [17], where the training algorithm of a memory polynomial PD is implemented on a Texas Instruments’ floating-point digital signal processor (TMS320C67xx).
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