Mahmoud Abdelaziz

REDUCED-COMPLEXITY POWER AMPLIFIER LINEARIZATION FOR CARRIER AGGREGATION MOBILE TRANSCEIVERS

Spurious intermodulation components have recently been identified as a major problem in carrier aggregation mobile transmitters with multi-band power amplifiers (PAs). This article presents novel adaptive digital predistortion (DPD) solutions with reduced complexity in both the predistortion processing and the feedback paths, to tackle this problem. Compared with conventional DPDs which aim to linearize the whole transmit bandwidth, the proposed technique aims at mitigating only those intermodulation components which are most problematic from the spurious emission limit perspective. The proposed technique is verified with extensive simulations in various 3GPP LTE-A carrier aggregation scenarios, showing that the intermodulation spurs can be efficiently mitigated below the spurious emission limit with relatively small back-offs.

Mobile transmitter digital predistortion: Feasibility analysis, algorithms and design exploration

This article addresses intermodulation challenges in carrier aggregation (CA) and multicluster type transmission scenarios in mobile transmitters. In such transmission schemes, emerging in 3GPP LTE-Advanced mobile cellular radio evolution, the spectrum of the signal entering the transmit power amplifier (PA) is of non-contiguous nature and thus severe intermodulation is created which may violate the spurious emission mask. To satisfy the stringent emission requirements and limits, devices may need to considerably back off their transmit power, compared to nominal value of ;23dBm, but this will reduce the uplink coverage. As an alternative, feasibility of digital predistortion (DPD) is explored in this article. A DPD solution is developed to control the most critical intermodulation components from terminal emission mask perspective with reduced complexity compared to conventional DPD solutions. The DPD is based on the Augmented Parallel Hammerstein (APH) architecture which can handle IQ imbalance and local oscillator leakage in addition to the PA nonlinearity while using simple digital linear estimation techniques. Furthermore, digital design exploration is carried out for the predistortion algorithm, implying that the needed computational resources are close to what is already available in most advanced mobile platforms and chipsets in the market.

Sub-band digital predistortion for noncontiguous transmissions: Algorithm development and real-time prototype implementation

This article proposes a novel, reduced complexity, block-adaptive digital predistortion (DPD) technique for mitigating the spurious emissions that occur when amplifying spectrally noncontiguous signals with a nonlinear power amplifier (PA). The introduced DPD solution is designed for real-time scenarios where a loop delay exists in the DPD system. By a proper choice of the DPD parameters, the technique is shown to be robust against arbitrarily long loop delays while not sacrificing DPD linearization performance and convergence speed. Moreover, the proposed DPD solution has lower complexity compared to previously proposed solutions in the literature while giving excellent linearization performance in terms of mitigating the spurious emissions. Real-time implementations of the algorithm on the WARP platform are developed, including considerations for several key trade-offs in the hardware design to balance the robustness, performance and complexity. The simulations and real-time FPGA experiments evidence excellent and robust performance in real-life situations with highly nonlinear PAs and arbitrary loop delays.

Low-Complexity Subband Digital Predistortion for Spurious Emission Suppression in Noncontiguous Spectrum Access

Noncontiguous transmission schemes combined with high power-efficiency requirements pose big challenges for radio transmitter and power amplifier (PA) design and implementation. Due to the nonlinear nature of the PA, severe unwanted emissions can occur, which can potentially interfere with neighboring channel signals or even desensitize the own receiver in frequency division duplexing transceivers. In this paper, to suppress such unwanted emissions, a low-complexity subband digital predistortion solution, specifically tailored for spectrally noncontiguous transmission schemes in low-cost devices, is proposed. The proposed technique aims at mitigating only the selected spurious intermodulation distortion components at the PA output, hence allowing for substantially reduced processing complexity compared with classical linearization solutions. Furthermore, novel decorrelation-based parameter learning solutions are also proposed and formulated, which offer reduced computing complexity in parameter estimation as well as the ability to track time-varying features adaptively. Comprehensive simulation and RF measurement results are provided, using a commercial LTE-Advanced mobile PA, to evaluate and validate the effectiveness of the proposed solution in real-world scenarios. The obtained results demonstrate that highly efficient spurious component suppression can be obtained using the proposed solutions.

Digital Mitigation of Transmitter-Induced Receiver Desensitization in Carrier Aggregation FDD Transceivers

Carrier aggregation transmissions in frequency division duplexing devices reduce the duplexing distance between the transmitter (TX) and receiver (RX) bands. As a consequence, the spurious intermodulation distortion products created by the nonlinear RF front-end of the TX may easily extend over to the RX band, potentially causing own RX desensitization. In this paper, we propose an efficient and computationally feasible adaptive digital identification and cancellation technique to mitigate the RX desensitization. We first show that the spurious leakage signal at own RX band depends on an equivalent leakage channel that models the overall signal leakage path including the TX nonlinearities, duplexer filter responses, and RX path. The parameters of the equivalent leakage channel can be efficiently estimated with the least squares or the recursive least squares algorithm, using the actual digital transmit data as a reference, and then used to regenerate and cancel the leakage interference from the received signal. The performance of the proposed technique is evaluated with extensive computer simulations, as well as with practical real-world RF measurements, demonstrating excellent calibration properties with up to 19-dB improvement in singal-to-interference plus noise ratio of the desired received signal.

Digital predistortion for mitigating spurious emissions in spectrally agile radios

Spectrally non-contiguous transmissions pose serious transceiver design challenges due to the nonlinear PA. When two or more non-contiguous carriers with close proximity are amplified by the same PA, spurious emissions inside or in the vicinity of the transmitter RF band are created. These spurious emissions may violate emission limits or otherwise compromise network coverage and reliability. Lowering the transmit power is a straightforward remedy, but it will reduce throughput, coverage, and power efficiency of the device. To improve linearity without sacrificing performance, several DPD techniques have recently been proposed to target the spurious emissions explicitly. These techniques are designed to minimize the computational and hardware complexity of DPD, thus making them better suited for mobile terminals and other lowcost devices. In this article, these recent advances in DPD for non-contiguous transmission scenarios are discussed, with a focus on mitigating the spurious emissions in the concrete example case of non-contiguous dual-carrier transmission. The techniques are compared to more traditional DPD approaches in terms of their computational and hardware complexities, as well as linearization performance.

Low power implementation of digital predistortion filter on a heterogeneous application specific multiprocessor

Power-constrained mobile radio communication transmitters drive their transmit power amplifiers close to their saturation regions, which results in nonlinear intermodulation distortion that is especially harmful in multi-cluster and carrier aggregation transmission scenarios. Digital predistortion is a method for linearizing the transmitter and suppressing the most harmful spurious emissions at the transmitter power amplifier output. This paper describes a programmable implementation of a digital predistortion filter on a heterogeneous Transport Trigger Architecture (TTA) multiprocessor. The predistortion algorithm is based on a parallel Hammerstein polynomial model and the experimental results show that the proposed programmable architecture is capable of linearizing a 20 MHz LTE carrier in realtime with a power consumption that is suitable for mobile devices.

Mobile GPU accelerated digital predistortion on a software-defined mobile transmitter

We present the design exploration and the performance evaluation of a mobile transmitter digital predistortion (DPD) module on a mobile GPU. Digital predistortion is a widely used technique for suppressing the spurious spectrum emission caused by the imperfection of power amplifier and radio frequency (RF) circuits in a real wireless transmitter. Considering the parallel architecture, numerous computing cores and programmability of GPU, in this work, a DPD design based on augmented parallel Hammerstein structure is implemented on a mobile GPU integrated in an Nvidia Jetson TK1 mobile development board, targeting at a mobile transmitter. The algorithm level and data level parallelism are carefully explored for efficient mapping of the DPD algorithm and full utilization of the mobile GPU resources. We analyze the throughput and timing performance of our implementation and verify the functionality of DPD experimentally on a novel software-defined mobile terminal. The results show that our proposed mobile GPU driven digital predistortion design not only achieves real-time high performance, but also offers programmability and reconfigurability for design upgrading and extension.

Parallel Digital Predistortion Design on Mobile GPU and Embedded Multicore CPU for Mobile Transmitters

Digital predistortion (DPD) is a widely adopted baseband processing technique in current radio transmitters. While DPD can effectively suppress unwanted spurious spectrum emissions stemming from imperfections of analog RF and baseband electronics, it also introduces extra processing complexity and poses challenges on efficient and flexible implementations, especially for mobile cellular transmitters, considering their limited computing power compared to basestations. In this paper, we present high data rate implementations of broadband DPD on modern embedded processors, such as mobile GPU and multicore CPU, by taking advantage of emerging parallel computing techniques for exploiting their computing resources. We further verify the suppression effect of DPD experimentally on real radio hardware platforms. Performance evaluation results of our DPD design demonstrate the high efficacy of modern general purpose mobile processors on accelerating DPD processing for a mobile transmitter.

Low-Complexity, Sub-Band DPD with Sequential Learning: Novel Algorithms and WARPLab Implementation

Digital predistortion (DPD) is an effective way of mitigating spurious emission violations without the need of a significant backoff in the transmitter, thus providing better power efficiency and network coverage. In this paper an iterative version of the IM3 sub-band DPD, proposed earlier by the authors, is presented. The DPD learning is iterated between the higher and lower IM3 sub-bands until a satisfactory performance is achieved for both of them. A sequential DPD learning procedure is also presented in order to reduce the hardware complexity when higher order nonlinearities are incorporated in the DPD learning. Improvements on the convergence speed of the adaptive DPD learning are also achieved via incorporating a variable learning rate and training from previous values. A WARPLab implementation of the proposed DPD is also shown with excellent suppression of the targeted spurious emissions.