Power Amplifier Nonlinear Distortion Research Articles

FPGA Implementation of a Real-Time NARMA-Based Digital Adaptive Predistorter

This paper presents the design of a nonlinear autoregressive moving average (NARMA) digital adaptive predistorter (DAPD) for power amplifier (PA) linearization consisting of a low-complex closed-loop architecture. Both the predistortion function and the adaptation algorithm are fully implemented in a field-programmable gate array (FPGA) device, without the need for using any additional coprocessor. The proposed predistortion architecture is capable to compensate for both the PA nonlinear distortion and memory effects. Moreover, this DAPD allows almost real-time adaptation without interrupting the normal transmission. The computational complexity introduced by this DAPD is studied in this paper. The proposed theoretical design is implemented in an FPGA, whereas the linearization performance of the DAPD is validated through simulated and experimental results.

Linear Precoding of OFDMA Signals to Minimize Their Instantaneous Power Variance

The large instantaneous power fluctuations of orthogonal frequency division multiple access (OFDMA) signals necessitate significant power backoff to minimize the effects of nonlinear power amplifier distortion. Is there an optimum linear block precoding that significantly reduces the instantaneous power variance of OFDMA signals? For contiguous and block-interleaved non-contiguous frequency mappings we find that there is a power variance-minimizing linear block precoding which requires a slightly lower power backoff than that for DFT precoding (which in the form of SC-FDMA, is used for the uplink signaling in the 3GPP-LTE standard). The optimum precoding is not unique, and in its simplest form it imposes only mild linear filtering and a moderate noise enhancement penalty if the receiver uses linear equalization.

Comparative analysis of influence of non-linear distortion in power amplifier in wireless communications systems with single and multiple orthogonal carrier frequencies

The paper considers the impact of power amplifier’s non-linearity on wireless communication systems with a single carrier (SC) and multiple orthogonal subcarriers (OFDM). Detailed analysis is provided for the impact of non-linearity on generation accuracy of the transmitted signal and the shape of the output spectrum. It is demonstrated that in order to meet standard requirements to the signal generation accuracy and the spectrum mask compliance, a SC system typically has a moderate gain of 1–1.5 dB in comparison with an OFDM system. It is concluded that a decision on the modulation type selection for a wireless communication system should include additional factors beyond the power amplifier non-linearity impact analysis.

EVM simulation and analysis in digital transmitter

The error vector magnitude (EVM) is extensively applied as a metric for digital transmitter signal quality compliance in modern communication systems. This article is focused on the effects of local oscillator (LO) phase noise and nonlinear distortion of power amplifier on EVM. This article contributes to below aspects. First, the relationships between EVM and two effects, LO phase noise and nonlinear distortion of power amplifier, are derived and expressed. Second, to simplify the expression, the third-order intermodulation distortion (IMD3) is used to calculate the EVM. Then, an expression for the EVM is derived based on the digital transmitter model that considers local oscillator phase noise and nonlinear distortion of power amplifier. Finally, the math formula of bit error rate (BER) versus EVM is given which can be easier and more useful to predict BER, according to analysis of the relationship between EVM and signal to noise rate (SNR), inspired by the works of Rishad, Md. Shahriar and AHM, 2006. Simulations are carried out to display the performance of EVM based on these relationships.

Receiver Cancellation Technique for Nonlinear Power Amplifier Distortion in SDMA–OFDM Systems

Space-division multiple access (SDMA) and orthogonal frequency-division multiplexing (OFDM) can be combined to design a robust communications system with increased spectral efficiency and system capacity. This combination is one of the most promising candidates for future wireless local area network implementations. However, one drawback of OFDM systems is the high peak-to-average power ratio, which imposes strong requirements on the linearity of power amplifiers (PAs). Such linearity requirements translate into high back-off that results in low power efficiency. In order to improve power efficiency, a PA nonlinearity cancellation (PANC) technique is introduced in this paper. This technique reduces the nonlinear distortion effects on the received signal. The performance of the new technique is evaluated with simulations, which show significant power efficiency improvements. To obtain meaningful results for comparison purposes, we derive a theoretical upper bound on the bit error rate performance of an SDMA-OFDM system subject to PA nonlinearities. In addition, a novel channel estimation technique that combines frequency- and time-domain channel estimation with PANC is also presented. Simulation results show the robustness of the cancellation method also when channel estimation is included.