Power and Beam Optimization for Uplink Millimeter-Wave Hotspot Communication Systems

We propose an effective interference management and beamforming mechanism for uplink communication systems that yields fair allocation of rates. In particular, we consider a hotspot area of a millimeter-wave (mmWave) access network consisting of multiple user equipment (UE) in the uplink and multiple access points (APs) with directional antennas and adjustable beam widths and directions (beam configurations). This network suffers tremendously from multi-beam multi-user interference, and, to improve the uplink transmission performance, we propose a centralized scheme that optimizes the power, the beam width, the beam direction of the APs, and the UE - AP assignments. This problem involves both continuous and discrete variables, and it has the following structure. If we fix all discrete variables, except for those related to the UE-AP assignment, the resulting optimization problem can be solved optimally. This property enables us to propose a heuristic based on simulated annealing (SA) to address the intractable joint optimization problem with all discrete variables. In more detail, for a fixed configuration of beams, we formulate a weighted rate allocation problem where each user gets the same portion of its maximum achievable rate that it would have under non-interfered conditions. We solve this problem with an iterative fixed point algorithm that optimizes the power of UEs and the UE - AP assignment in the uplink. This fixed point algorithm is combined with SA to improve the beam configurations. Theoretical and numerical results show that the proposed method improves both the UE rates in the lower percentiles and the overall fairness in the network

Low-Complexity Hybrid Beamforming for Massive MIMO Systems in Frequency-Selective Channels

Hybrid beamforming for frequency-selective channels is a challenging problem as the phase shifters provide the same phase shift to all of the subcarriers. The existing approaches solely rely on the channel's frequency response and the hybrid beamformers maximize the average spectral efficiency over the whole frequency band. Compared to state-of-the-art, we show that substantial sum-rate gains can be achieved, both for rich and sparse scattering channels, by jointly exploiting the frequency and time domain characteristics of the massive multiple-input multiple-output (MIMO) channels. In our proposed approach, the radio frequency (RF) beamformer coherently combines the received symbols in the time domain and, thus, it concentrates signal's power on a specific time sample. As a result, the RF beamformer flattens the frequency response of the "effective" transmission channel and reduces its root mean square delay spread. Then, a baseband combiner mitigates the residual interference in the frequency domain. We present the closed-form expressions of the proposed beamformer and its performance by leveraging the favorable propagation condition of massive MIMO channels and we prove that our proposed scheme can achieve the performance of fully-digital zero-forcing when number of employed phase shifter networks is twice the resolvable multipath components in the time domain.Comment: Accepted to IEEE Acces

An alternating direction algorithm for hybrid precoding and combining in millimeter wave MIMO systems

Millimeter-wave (mmWave) technology is one of the most promising candidates for future wireless communication systems as it can offer large underutilized bandwidths and eases the implementation of large antenna arrays which are required to help overcome the severe signal attenuation that occurs at these frequencies. To reduce the high cost and power consumption of a fully digital mmWave precoder and combiner, hybrid analog/digital designs based on analog phase shifters are often adopted. In this work we derive an iterative algorithm for the hybrid precoding and combining design for spatial multiplexing in mmWave massive multiple-input multiple-output (MIMO) systems. To cope with the difficulty of handling the hardware constraint imposed by the analog phase shifters we use the alternating direction method of the multipliers (ADMM) to split the hybrid design problem into a sequence of smaller subproblems. This results in an iterative algorithm where the design of the analog precoder/combiner consists of a closed form solution followed by a simple projection over the set of matrices with equal magnitude elements. It is initially developed for the fully-connected structure and then extended to the partially-connected architecture which allows simpler hardware implementation. Furthermore, to cope with the more likely wideband scenarios where the channel is frequency selective, we also extend the algorithm to an orthogonal frequency division multiplexing (OFDM) based mmWave system. Simulation results in different scenarios show that the proposed design algorithms are capable of achieving performances close to the optimal fully digital solution and can work with a broad range of configuration of antennas, RF chains and data streams.info:eu-repo/semantics/acceptedVersio

Hybrid Transceiver Optimization for Multi-Hop Communications

Multi-hop communication with the aid of large-scale antenna arrays will play a vital role in future emergence communication systems. In this paper, we investigate amplify-and-forward based and multiple-input multiple-output assisted multi-hop communication, in which all nodes employ hybrid transceivers. Moreover, channel errors are taken into account in our hybrid transceiver design. Based on the matrix-monotonic optimization framework, the optimal structures of the robust hybrid transceivers are derived. By utilizing these optimal structures, the optimizations of analog transceivers and digital transceivers can be separated without loss of optimality. This fact greatly simplifies the joint optimization of analog and digital transceivers. Since the optimization of analog transceivers under unit-modulus constraints is non-convex, a projection type algorithm is proposed for analog transceiver optimization to overcome this difficulty. Based on the derived analog transceivers, the optimal digital transceivers can then be derived using matrix-monotonic optimization. Numeral results obtained demonstrate the performance advantages of the proposed hybrid transceiver designs over other existing solutions.Comment: 32 pages, 6 figures. This manuscript has been submitted to IEEE Journal on Selected Areas in Communications (special issue on Multiple Antenna Technologies for Beyond 5G

Hybrid Beamforming via the Kronecker Decomposition for the Millimeter-Wave Massive MIMO Systems

Despite its promising performance gain, the realization of mmWave massive MIMO still faces several practical challenges. In particular, implementing massive MIMO in the digital domain requires hundreds of RF chains matching the number of antennas. Furthermore, designing these components to operate at the mmWave frequencies is challenging and costly. These motivated the recent development of hybrid-beamforming where MIMO processing is divided for separate implementation in the analog and digital domains, called the analog and digital beamforming, respectively. Analog beamforming using a phase array introduces uni-modulus constraints on the beamforming coefficients, rendering the conventional MIMO techniques unsuitable and call for new designs. In this paper, we present a systematic design framework for hybrid beamforming for multi-cell multiuser massive MIMO systems over mmWave channels characterized by sparse propagation paths. The framework relies on the decomposition of analog beamforming vectors and path observation vectors into Kronecker products of factors being uni-modulus vectors. Exploiting properties of Kronecker mixed products, different factors of the analog beamformer are designed for either nulling interference paths or coherently combining data paths. Furthermore, a channel estimation scheme is designed for enabling the proposed hybrid beamforming. The scheme estimates the AoA of data and interference paths by analog beam scanning and data-path gains by analog beam steering. The performance of the channel estimation scheme is analyzed. In particular, the AoA spectrum resulting from beam scanning, which displays the magnitude distribution of paths over the AoA range, is derived in closed-form. It is shown that the inter-cell interference level diminishes inversely with the array size, the square root of pilot sequence length and the spatial separation between paths.Comment: Submitted to IEEE JSAC Special Issue on Millimeter Wave Communications for Future Mobile Networks, minor revisio

State-of-the-art assessment of 5G mmWave communications

Deliverable D2.1 del proyecto 5GWirelessMain objective of the European 5Gwireless project, which is part of the H2020 Marie Slodowska- Curie ITN (Innovative Training Networks) program resides in the training and involvement of young researchers in the elaboration of future mobile communication networks, focusing on innovative wireless technologies, heterogeneous network architectures, new topologies (including ultra-dense deployments), and appropriate tools. The present Document D2.1 is the first deliverable of Work- Package 2 (WP2) that is specifically devoted to the modeling of the millimeter-wave (mmWave) propagation channels, and development of appropriate mmWave beamforming and signal processing techniques. Deliver D2.1 gives a state-of-the-art on the mmWave channel measurement, characterization and modeling; existing antenna array technologies, channel estimation and precoding algorithms; proposed deployment and networking techniques; some performance studies; as well as a review on the evaluation and analysis toolsPostprint (published version

Técnicas de equalização iterativa para arquiteturas híbridas sub-conectadas na banda de ondas milimétricas

Mestrado em Engenharia Eletrónica e TelecomunicaçõesThe millimeter wave communications and the use of a massive number of antennas are two promising technologies that being combined allow to achieve the multi Gb/s required by future 5G wireless systems. As this type of systems has a high number of antennas it is impossible to use a fully digital architecture, due to hardware limitations. Therefore, the design of signal processing techniques for hybrid analog-digital architectures is a requirement. Depending on the structure of the analog part the hybrid analog-digital architectures may be fully connected or sub-connected. Although the fully connected hybrid architectures allow to connect all RF chains to any antenna element, they involve a high cost due to its structural and computational complexity. As such, the sub-connected hybrid architectures become more attractive, since either at the hardware level or from the computational point of view they are less demanding. In this dissertation, we propose a hybrid iterative block multiuser equalizer for sub-connected millimeter wave massive MIMO systems. The user terminal transceiver has low-complexity and as such employ a pure analog random precoder, with a single RF chain. For the base station, a sub-connected hybrid analog-digital equalizer is designed to remove the multiuser interference. The hybrid equalizer is optimized using the average bit-error-rate as a metric. Due to the coupling between the RF chains in the optimization problem the computation of the optimal solution is way too complex. To address this problem, we compute the analog part of the equalizer sequentially over the RF chains using a dictionary built from the array response vectors. The proposed sub-connected hybrid iterative multiuser equalizer is compared with a recently proposed fully connected hybrid analog-digital approach and with the fully digital architecture. The results show that the performance of the proposed scheme is close to the fully connected hybrid approach after just a few iterations.As comunicações na banda das ondas milimétricas e o uso massivo de antenas são duas tecnologias promissoras que, sendo combinadas permitem alcançar elevadas taxas de transmissão, na ordem dos multi Gb/s, exigidas pelos futuros sistemas sem fios da 5G. Como estes sistemas possuem um número elevado de antenas, torna-se impossível o uso de uma arquitetura totalmente digital devido às limitações de hardware. Desta forma, é necessário projetar técnicas de processamento de sinal para arquiteturas híbridas analógico-digitais. Dentro das arquiteturas híbridas, foram propostas duas formas de lidar com a parte analógica, que são, a forma totalmente conectada e a forma sub-conectada. Embora as arquiteturas híbridas totalmente conectadas permitam interligar todas as cadeias RF a qualquer elemento de antena, estas envolvem um elevado custo devido à sua complexidade estrutural e computacional. Assim sendo, as arquiteturas híbridas sub-conectadas tornam-se mais atraentes pois são menos exigentes do ponto de vista computacional, bem como ao nível do hardware. Nesta dissertação, é proposto um equalizador iterativo para um sistema com uma arquitetura hibrida sub-conectada, com múltiplos utilizadores e um número massivo de antenas a operar na banda das ondas milimétricas. Os terminais dos utilizadores têm baixa complexidade e utilizam pré-codificadores aleatórios analógicos puros, cada um com uma única cadeia RF. Para a estação base, projetou-se um equalizador híbrido analógico-digital de arquitectura sub-conectada, para remover a interferência multiutilizador. O equalizador híbrido é otimizado usando a taxa média de erro de bit como métrica. Devido ao acoplamento entre as cadeias de RF no problema de otimização, o cálculo das soluções ótimas possui elevada complexidade. Para ultrapassar este problema, calculou-se a parte analógica de cada cadeia de RF do equalizador de forma sequencial, usando um dicionário construído a partir da resposta do agregado de antenas. Compara-se o equalizador iterativo híbrido para sistemas multiutilizador de arquitectura sub-conectada proposto com uma abordagem híbrida analógica/digital totalmente conectada, recentemente proposta na literatura e com uma arquitetura totalmente digital. Os resultados mostram que o desempenho do esquema proposto aproximasse da abordagem híbrida totalmente conectada após apenas algumas iterações