Efficient Neural Network DPD architecture for Hybrid Beamforming mMIMO

This paper presents several different Neural Network based DPD architectures for hybrid beamforming (HBF) mMIMO applications. They are formulated, tested and compared based on their ability to compensate nonlinear distortion of power amplifiers in a single user (SU) and multiuser (MU) Fully‐Connected (FC) HBF mMIMO transmitters. The proof‐of‐concept is provided with a 64 × 64 FC HBF mMIMO system, with 2 RF chains. The complexity of DPD solution is reduced by using a single Real‐Valued Time‐Delay Neural Network with two hidden layers (RVTDNN2L) instead of using as many different DPD blocks as there are RF chains in the HBF mMIMO transmitter and it is shown that the proposed architecture better compensates nonlinear distortion compared to the traditional memory polynomial DPD. Two RVTDNN2L DPD architectures are developed and tested for linearization of MU FC HBF mMIMO systems, and it is also shown that the proposed RVTDNN2L DPD architecture efficiently linearizes MU FC HBF mMIMO transmitters in terms of Normalized Mean‐Squared Error (NMSE) and Error Vector Magnitude (EVM)

Digital Predistortion in Large-Array Digital Beamforming Transmitters

In this article, we propose a novel digital predistortion (DPD) solution that allows to considerably reduce the complexity resulting from linearizing a set of power amplifiers (PAs) in single-user large-scale digital beamforming transmitters. In contrast to current state-of-the art solutions that assume a dedicated DPD per power amplifier, which is unfeasible in the context of large antenna arrays, the proposed solution only requires a single DPD in order to linearize an arbitrary number of power amplifiers. To this end, the proposed DPD predistorts the signal at the input of the digital precoder based on minimizing the nonlinear distortion of the combined signal at the intended receiver direction. This is a desirable feature, since the resulting emissions in other directions get partially diluted due to less coherent superposition. With this approach, only a single DPD is required, yielding great complexity and energy savings.Comment: 8 pages, Accepted for publication in Asilomar Conference on Signals, Systems, and Computer

Técnicas de equalização para MIMO massivo com amplificação não linear

The dawn of the new generation of mobile communications and the trafic explosion that derives from its implementation pose great challenge. The milimeter wave band and the use of massive number of antennas are technologies which, when combined, allow the transmission of high data rate, functioning in zones of the electromagnetic spectrum that are less explored and with capability of allocation of dozens of GHz of bandwidth. In this dissertation we consider a massive MIMO millimeter wave system employing a hybrid architecture, i.e., the number of transmit and receive antennas are lower than the number of radio frequency chains. As consequence, the precoder and equalizers should be designed in both digital and analog domains. In the literature, most of the proposed hybrid beamforming schemes were evaluated without considering the effects of nonlinear amplifications. However, these systems face non-avoidable nonlinear effects due to power amplifiers functioning in nonlinear regions. The strong nonlinear effects throughout the transmission chain will have a negative impact on the overall system performance and thus its study and the design of equalizers that take into account these effects are of paramount importance. This dissertation proposes a hybrid iterative equalizer for massive MIMO millimeter wave SC-FDMA systems. The user terminals have low complexity, just equipped with analog precoders based on average angle of departure, each with a single radio frequency chain. At the base station it is designed an hybrid analog-digital iterative equalizer with fully connected architecture in order to eliminate both the multi-user interference and the nonlinear distortion caused by signal amplification during the transmission. The equalizer is optimized by minimizing the bit error rate, which is equivalent to minimize the mean square error rate. The impact of the saturation threshold of the amplifiers in the system performance is analysed, and it is demonstrated that the iterative process can efficiently remove the multi-user interference and the distortion, improving the overall system performance.O surgimento de uma nova geração de comunicações móveis e a explosão de tráfego que advém da sua implementação apresenta grandes desafios. A banda de ondas milimétricas e o uso massivo de antenas são tecnologias que, combinadas, permitem atingir elevadas taxas de transmissão, funcionando em zonas do espectro electromagnético menos exploradas e com capacidade de alocação de dezenas de GHz para largura de banda. Nesta dissertação foi considerado um sistema de MIMO massivo de ondas milimétricas usando uma arquitectura híbrida, i.e., o número de antenas para transmissão e recepção é menor que o número de cadeias de radiofrequência. Consequentemente, o pré-codificador e equalizadores devem ser projectados nos domínios digital e analógico. Na literatura, a maioria dos esquemas híbridos de beamforming são avaliados sem ter em conta os efeitos de não linearidade da amplificação do sinal. No entanto, estes sistemas sofrem inevitavelmente de efeitos não lineares devido aos amplificadores de potência operarem em regiões não lineares. Os fortes efeitos das não-linearidades ao longo da cadeia de transmissão têm um efeito nefasto no desempenho do sistema e portanto o seu estudo e projecto de equalizadores que tenham em conta estes efeitos são de extrema importância. Esta dissertação propõe um equalizador híbrido para sistemas baseados em ondas milimétricas para MIMO massivo com modulação SC-FDMA. Os terminais de utilizador possuem baixa complexidade, equipados apenas com pré-codificadores analógicos baseados no ângulo médio de partida, cada um com uma única cadeia de radiofrequência. Na estação base é projectado um equalizador iterativo híbrido analógico-digital com arquitectura completamente conectada de modo a eliminar a interferencia multi-utilizador e a distorção causada pela amplificação do sinal aquando da transmissão. O equalizador é optimizado minimizando a taxa de erro de bit, o que é equivalente a minimizar a taxa de erro quadrático médio. O impacto do limiar de saturação dos amplificadores no desempenho do sistema é analisado, e é demonstrado que o processo iterativo consegue eliminar de modo eficiente a interferência multi-utilizador e a distorção, melhorando o desempenho do sistema.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

Frequency-domain digital predistortion for Massive MU-MIMO-OFDM Downlink

Digital predistortion (DPD) is a method commonly used to compensate for the nonlinear effects of power amplifiers (PAs). However, the computational complexity of most DPD algorithms becomes an issue in the downlink of massive multi-user (MU) multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM), where potentially up to several hundreds of PAs in the base station (BS) require linearization. In this paper, we propose a convolutional neural network (CNN)-based DPD in the frequency domain, taking place before the precoding, where the dimensionality of the signal space depends on the number of users, instead of the number of BS antennas. Simulation results on generalized memory polynomial (GMP)-based PAs show that the proposed CNN-based DPD can lead to very large complexity savings as the number of BS antenna increases at the expense of a small increase in power to achieve the same symbol error rate (SER)

Theoretical analysis of nonlinear amplification effects in massive MIMO systems

To fulfill 5th Generation (5G) communication capacity demands, the use of a large number of antennas has been widely investigated, and the array gain and spatial multiplexing that are offered by massive multiple input multiple output (mMIMO) have been used to improve the capacity. Fully digital architectures are not feasible for a large number of antennas, and hybrid analog/digital systems have emerged as options to retain a high number of antennas without as many radio frequency (RF) chains. However, these systems have, as consequences, non-avoidable nonlinear effects due to power amplifiers functioning in nonlinear regions. The strong nonlinear effects throughout the transmission chain will have a negative impact on the overall system’s performance. Being able to access this impact is very important. For this purpose, we propose analytical and semi-analytical tools that allow for the evaluation of the nonlinear effects of a hybrid analog/digital orthogonal frequency-division multiplexing (OFDM) system. The proposed analysis starts with the characterization of the power amplifier’s (PA) nonlinear response. This response is then used to derive a semi-analytic bit error rate expression. The theoretical tools are validated by using numerical results from two different cases: in the first one, the nonlinear PA response is assumed to follow an analytical model found in the literature and, in the second, the used nonlinear polynomial model mimics the response of a real amplifier. Using these two scenarios, the proposed tools are shown to be accurate making it possible to predict the nonlinearities’ penalties in hybrid analog/digital OFDM systems and/or to assess the optimal operation point for a specific nonlinear amplifier.publishe

Modeling Approaches for Active Antenna Transmitters

The rapid growth of data traffic in mobile communications has attracted interest to Multiple-Input-Multiple-Output (MIMO) communication systems at millimeter-wave (mmWave) frequencies. MIMO systems exploit active antenna arrays transmitter configurations to obtain higher energy efficiency and beamforming flexibility. The analysis of transmitters in MIMO systems becomes complex due to the close integration of several antennas and power amplifiers (PAs) and the problems associated with heat dissipation. Therefore, the transmitter analysis requires efficient joint EM, circuit, and thermal simulations of its building blocks, i.e., the antenna array and PAs. Due to small physical spacing at mmWave, bulky isolators cannot be used to eliminate unwanted interactions between PA and antenna array. Therefore, the mismatch and mutual coupling in the antenna array directly affect PA output load and PA and transmitter performance. On the other hand, PAs are the primary source of nonlinearity, power consumption, and heat dissipation in transmitters. Therefore, it is crucial to include joint thermal and electrical behavior of PAs in analyzing active antenna transmitters. In this thesis, efficient techniques for modeling active antenna transmitters are presented. First, we propose a hardware-oriented transmitter model that considers PA load-dependent nonlinearity and the coupling, mismatch, and radiated field of the antenna array. The proposed model is equally accurate for any mismatch level that can happen at the PA output. This model can predict the transmitter radiation pattern and nonlinear signal distortions in the far-field. The model\u27s functionality is verified using a mmWave active subarray antenna module for a beam steering scenario and by performing the over-the-air measurements. The load-pull modeling idea was also applied to investigate the performance of a mmWave spatial power combiner module in the presence of critical coupling effects on combining performance. The second part of the thesis deals with thermal challenges in active antenna transmitters and PAs as the main source of heat dissipation. An efficient electrothermal modeling approach that considers the thermal behavior of PAs, including self-heating and thermal coupling between the IC hot spots, coupled with the electrical behavior of PA, is proposed. The thermal model has been employed to evaluate a PA DUT\u27s static and dynamic temperature-dependent performance in terms of linearity, gain, and efficiency. In summary, the proposed modeling approaches presented in this thesis provide efficient yet powerful tools for joint analysis of complex active antenna transmitters in MIMO systems, including sub-systems\u27 behavior and their interactions

Active Transmitter Antenna Array Modeling for MIMO Applications

The rapid growth of data traffic in mobile communications has attracted interests to the Multiple-Input-Multiple-Output (MIMO) communication systems at millimeter-wave (mmWave) frequencies.\ua0 MIMO systems exploit active transmitter antenna arrays for higher energy efficiency and providing beamforming flexibility. The close integration of multiple PAs and antennas increases the transmitter analysis complexity. Moreover, due to the small antenna element spacing at mm-wave frequencies, isolators are too bulky and cannot be used. Therefore, including the effects of interactions between the antenna array and PAs is a significant aspect in the analysis of MIMO transmitters. For large active arrays, applying joint circuit and EM simulation tools for the analysis is a complicated and time-consuming task. In these occasions, behavioral models are the key to the fast and accurate evaluation of active transmitter antenna arrays.In this thesis, a technique for modeling the active transmitter antenna array performance is presented. The proposed model considers the effect of PAs nonlinearity as well as the coupling and mismatch in the antenna array. With this model, a comprehensive prediction of radiation pattern and signal distortions in the far-field is feasible. The model is experimentally verified by a mmWave active subarray antenna for a beam steering scenario and by performing over-the-air measurements. The measurement results effectively validate the modeling technique for a wide range of steering angles.\ua0\ua0 Furthermore, a linearity analysis is provided to predict transmitter performance in conjunction with beam-dependent digital predistortion (DPD) linearization. The study reveals the model potential in evaluating different DPD approaches as well as predicting the performance of linearized transmitters. The demonstration shows that the variation of nonlinear distortion versus steering angle depends significantly on the array configuration and beam direction.In summary, the proposed model allows for the prediction of the active transmitter antenna array performance in the early design stages with low computational effort. It can provide design guides for developing large-scale active arrays and can be employed for evaluating the DPD and transmitter linearity performance