Abubaker Abdelhafiz

Digital Predistortion of LTE-A Power Amplifiers Using Compressed-Sampling-Based Unstructured Pruning of Volterra Series

This paper presents a method for pruning power amplifier (PA) behavioral models and digital predistorters based on compressed sampling theory. Using this method, the number of coefficients required by behavioral models and predistorters can be significantly reduced while achieving comparable performance in terms of both modeling accuracy and suppressing distortions. Hardware measurements obtained for a Doherty PA driven by a five-carrier 100-MHz wide long-term evolution-advanced signal demonstrate the capability of the proposed method to linearize a highly nonlinear PA prototype using a minimal number of coefficients, revealing the attractive properties of the proposed method and its desirable performance. Using the proposed technique, predistorters achieving similar linearization performance while requiring significantly less coefficients than the traditional models were demonstrated.

Lattice-based memory polynomial predistorter for wideband radio frequency power amplifiers

This study addresses the ill-conditioning problem of the memory polynomial (MP) model with application to the predistortion of highly non-linear power amplifiers with memory effects. A resource-efficient lattice-based MP structure built using the cascade of a MP generator and a lattice predictor is proposed to overcome the ill-conditioning of the MPs data matrix. The proposed model performances are benchmarked against those of the MP model as well as the orthogonal MP model. The experimental results demonstrate the suitability of the proposed predistorter as it achieves similar performance in the time and the frequency domains compared to the MP counterpart while alleviating its ill-conditioning problem.

A multi-stage concurrent dual-band DPD architecture for closely spaced carriers using a low bandwidth feedback loop

In this paper, a novel multi-stage digital predistortion (DPD) with constrained feedback bandwidth is proposed. The proposed DPD effectively compensates for the nonlinear distortion in closely spaced multi-carrier power amplifiers (PAs). By extracting the PA static nonlinearity characteristics from the bandwidth-constrained signals and separately processing the multi-carriers input, the proposed DPD reduces the feedback bandwidth, which results in a reduction of the sampling rate. Moreover, while the DPD reduces the spectral regrowth in the adjacent channels, it effectively suppresses the spectral regrowth within the off-carrier regions along the transmission bandwidth. Experimental results have validated improved performance on a 60 MHz LTE-advanced signal, even when the analog to digital converter sampling rate is constrained to 61.44 Msps. Thus, the proposed technique can significantly decrease the difficulties in system design and reduce implementation cost.

A lattice-based memory polynomial model for nonlinear MIMO transmitter behavioral modeling using fixed point arithmetic

Recently, there has been an accelerating trend towards the deployment of multiple-input multiple-output (MIMO) architectures for communication systems. While these architectures greatly enhance the channel capacity and the data rates delivered to users, they suffer from issues such as the presence of crosstalk and leakage between the signal paths. The presence of such effects greatly increases the number of coefficients required by behavioral models describing MIMO transmitters, which in turn exacerbates the ill-conditioning problem encountered in the coefficient-extraction process. This places a demand for a higher number of bits to be used when implementing digital predistorters (DPD) based on these models. To address this problem, a low-complexity lattice filter is proposed in this work to reduce the conditioning of the behavioral models used. Through experimental validation using a 2×2 MIMO transmitter setup, it was found that the proposed method greatly reduces the condition number of the model used and enables satisfactory modeling using only 16 bits.

Augmented Dual-Band Digital Predistorter for Reducing Cross-Band Intermodulation Distortion Using Predictive Injection Technique

In this paper, an augmented dual-band digital predistortion (DPD) technique for reducing the cross-band intermodulation distortion (IMD) using predictive injection technique is proposed to address some of the shortcomings of dual-band DPDs. The technique alleviates the need to observe the cross-band third-order IMD (IMD3) terms in the feedback loop by predicting the distortion terms and generating synthetic signals, which are then injected at the transmitter side. To highlight the issue, the wideband and dual-band DPD architectures and their respective limitations are briefly outlined. The theory behind the proposed concept is developed, and practical measurements performed using a Class AB power amplifier driven by a long-term evolution signal are provided to support the theory. The results obtained show that the proposed method achieves promising performance in mitigating the cross-band IMD3 issue faced by dual-band DPDs. This technique can be extended to mitigate higher order cross-band distortion as well.

Behavioral modeling of envelope tracking power amplifier using Volterra series model and compressed sampling

Radio frequency power amplifiers operate most efficiently in the compression region and become less efficient as signal peak-to-average power ratio increases. This is due to the amplifier spending more time operating below peak power and, thus operating below its maximum efficiency. However, the use of the envelope tracking technique can dramatically increase the efficiency of RF transmitters. In this work, a Volterra series-based behavioral model with compressed sampling is developed for an efficiency optimized envelope tracking power amplifier. The proposed method reduces the total number of coefficients needed by the Volterra series model and enables accurate behavioral modeling of nonlinear envelope tracking power amplifiers. Experimental results carried on a GaN based power amplifier driven by a four-carrier 18 MHz WCDMA signal demonstrate the effectiveness of the proposed model and approach.

A comparative study of wideband and dual-band digital predistortion architectures

In this paper, the wideband and dual-band architectures for digital predistorters (DPD) are compared in terms of the DPD models used, hardware implementations, complexity and linearization performance. The advantages and disadvantages of each architecture are outlined and hardware measurements are performed using an amplifier prototype to illustrate the differences between the two approaches.

Sparse channel estimation using adaptive filtering and compressed sampling

This paper discusses the topic of estimating sparse communication channels (i.e. having mostly zero entries) using classical adaptive filtering techniques and the recently-developedmethod of compressed sampling. For this purpose, the Least-Mean Squares (LMS) a variant of it known as 10-LMS are compared with the compressed sampling technique when used to estimate a communication channels having different levels of sparsity.

A Dual-Input Two-Box Model for Digital Predistortion of Envelope Tracking Power Amplifiers

This work proposes a novel two-box dual-input single-output digital predistorter (labeled as the LUT MISO predistorter) suitable for linearizing dynamic nonlinear envelope tracking power amplifiers. The LUT MISO predistorter is built by cascading a single-input memoryless nonlinear function with a two-input polynomial model with memory. The predistorted signal is synthesized as a function of the baseband complex waveform as well as the corresponding envelope-shaped signal. The LUT MISO predistorter is experimentally benchmarked against its single-box dual-input counterpart. It is shown that for the same number of coefficients, reduced spectrum regrowth and better adjacent channel power ratio were obtained with the LUT MISO predistorter. Moreover, the proposed predistorter was found to pass the ACPR requirements of the LTE standard while its conventional counterpart fails.