Downlink Of Multiuser Systems Research Articles

Hybrid Beamforming for Reconfigurable Intelligent Surface based Multi-User Communications: Achievable Rates With Limited Discrete Phase Shifts

Reconfigurable intelligent surfaces (RISs) have drawn considerable attention from the research community recently. RISs create favorable propagation conditions by controlling the phase shifts of reflected waves at the surface, thereby enhancing wireless transmissions. In this paper, we study a downlink multi-user system where the transmission from a multi-antenna base station (BS) to various users is achieved by an RIS reflecting the incident signals of the BS towards the users. Unlike most existing works, we consider the practical case where only a limited number of discrete phase shifts can be realized by a finite-sized RIS. A hybrid beamforming scheme is proposed and the sum-rate maximization problem is formulated. Specifically, continuous digital beamforming and discrete RIS-based analog beamforming are performed at the BS and the RIS, respectively, and an iterative algorithm is designed to solve this problem. Both theoretical analysis and numerical validations show that the RIS-based system can achieve good sum-rate performance by setting a reasonable size of the RIS and a small number of discrete phase shifts.

Energy-Efficient Power Allocation with Hybrid Beamforming for Millimetre-Wave 5G Massive MIMO System

Massive multiple-input and multiple-output systems combined with hybrid beamforming technique is a key approach to achieve high data rate and extended cell coverage in the fifth generation (5G) cellular networks . Specifically, the precoding for analog/digital hybrid beamforming technique is absolutely essential for millimeter-wave (mm-wave) frequency based transmission for 5G, simply because the technique reduces hardware complexity and energy consumption for high-frequency design. In order to prove the aforementioned advantages, the present paper will investigate the energy efficiency (EE) and optimization of low complexity hybrid precoding (EELCHP) algorithm to enable the reduction of radio frequency (RF) chains within the base station (BS). The proposed solution will also incorporate high-resolution phase shifters (HRPS) specifically for downlink multi-user mm-wave system. The intelligence within the EELCHP algorithm for hybrid beamforming is based on the fully complex zero-forcing (ZF) precoding mechanism that allowed the maximization of energy efficiency by reducing the RF chains, without affecting the potentially increasing number of antennas. In addition, maximum EE is possible by eliminating the nonlinear concave fractional issue based on the Lagrange duality method. During the simulation process of the proposed method, we further realized that hardware complexity and power consumption can be reduced by allowing the RF chains to select only one beam per switch within the beamforming solution. Furthermore, with the implementation of EELCHP algorithm, more energy is saved due to low complexity system by reducing the required RF chain.

Practical Hybrid Beamforming With Finite-Resolution Phase Shifters for Reconfigurable Intelligent Surface Based Multi-User Communications

In this paper, we study the reconfigurable intelligent surface (RIS) based downlink multi-user system where a multi-antenna base station (BS) sends signals to various users assisted by the RIS reflecting the incident signals of the BS towards the users. Unlike most existing works, we consider the practical case where only the large-scale fading gain is required at the BS and a limited number of phase shifts can be realized by the finite-sized RIS. To maximize the sum rate, we propose a hybrid beamforming scheme where the continuous digital beamforming and discrete RIS-based analog beamforming are performed at the BS and the RIS, respectively. An iterative algorithm is designed for beamforming and theoretical analysis is provided to evaluate how the size of RIS influences the achievable rate. Simulation results show that the RIS-based system can achieve a good sum-rate performance by setting a reasonable size of RIS and a small number of discrete phase shifts.

Broadcast Coding and Successive Refinement for Layered UE Cooperation in Multi-User Downlink

A multi-user downlink system is considered, in which a base station (BS) serves multiple user equipments (UEs). It is assumed that, to improve the downlink performance, the UEs can exchange compressed versions of the downlink received signals on out-of-band device-to-device (D2D) communication links. Since the D2D links from a UE to the other UEs may have different channel qualities across the receiving UEs, this letter proposes a layered cooperation strategy which combines broadcast coding and successive refinement quantization techniques. With this approach, a UE can have a better reconstruction of the downlink signal of the transmitting UE than another UE with worse D2D channel quality. The problem of jointly optimizing the downlink precoding and layered UE cooperation strategies is tackled, and numerical results show the performance gains of the proposed layered scheme as compared to baseline schemes with no or single-layer cooperation.

Performance analysis of downlink NOMA over η–µ and κ–µ fading channels

Non-orthogonal multiple access (NOMA) is recognised as an improved multiple access technique as compared with OMA technique to fulfil the requirements of fifth-generation wireless communication systems. In this study, the outage probability and sum-rate analysis of downlink multi-user NOMA system over the generalised fading channels, i.e. η–µ and κ–µ, are presented. Specifically, the outage probability of NOMA is analysed with fixed target rate and OMA rate. The mathematical expressions for the outage probability and sum rate with a fixed set of power allocation coefficients are derived and verified through the simulation results. The impact of system parameters such as number of users, power allocation coefficients, fading parameters etc. on the performance of NOMA system is investigated. NOMA with given fixed target rate offers better outage performance as compared with OMA. However, NOMA with OMA rate offers better outage performance for the near user only. The sum rate and near user rate of NOMA improves with signal-to-noise ratio; however, the far user rate does not follow the same trend.

Energy-Efficient Base Station Association and Beamforming for Multi-Cell Multiuser Systems

This paper investigates the joint base station (BS) association and beamforming for energy efficiency maximization in coordinated multi-cell multiuser downlink systems. In particular, we assume that only the channel distribution information is known to the BSs. The considered problem is difficult to be solved optimally due to the non-smooth and non-convex functions in the formulation. Therefore, we propose an iterative suboptimal algorithm to solve the problem efficiently based on the successive convex approximation (SCA). More specifically, the convex approximation of the original problem at each iteration can be solved efficiently by the second-order cone programming and the solution obtained by the proposed algorithm satisfies the generalized Karush-Kuhn-Tucker (KKT) conditions. To facilitate the implementation of decentralized beamforming, we transform the convex approximation problem at each iteration of the SCA into an equivalent form, which is amenable to applying the alternating direction method of multipliers (ADMM). By combining the SCA and the ADMM, a decentralized energy-efficient beamforming algorithm is proposed. Numerical results are presented to show the performance of the proposed algorithms.

Symbol-Level Hybrid Precoding in mmWave Multiuser MISO Systems

Recently, symbol-level precoding has emerged as a novel efficient solution in multi-user multi-antenna downlink systems, since it can exploit multi-user interference (MUI) and transform it into useful signal. In this correspondence, we attempt to apply symbol-level precoding into the well-known hybrid precoding architecture in multi-user multi-input single-output (MU-MISO) millimeter wave (mmWave) communication systems. Particularly, we aim to design the symbol-varying digital precoder and the analog precoder with low-resolution phase shifters (PSs) to optimize symbol detection performance under the transmit power constraint and the discrete phase constraint. To handle this difficult joint optimization problem, we propose to minimize the Euclidean distance between the optimal fully-digital precoder and the hybrid precoder. An efficient iterative algorithm is developed to solve this problem. Simulation results demonstrate the advantages of symbol-level precoding and validate the effectiveness of our proposed symbol-level hybrid precoding algorithm.

Construction of 1-Bit Transmit-Signal Vectors for Downlink MU-MISO Systems With PSK Signaling

We consider a downlink multi-user multiple-input single-output system where the base station has a large number of antennas with cost-effective 1-b digital-to-analog converters. For this system, we first identify that assigning the zero-power to some transmit signals (in short, zero-power allocation) can yield a non-trivial symbol-error-rate performance gain by alleviating an error-floor problem. Likewise the related works on 1-b precoding, finding an optimal transmit-signal vector (encompassing precoding and zero-power allocation) requires an exhaustive search due to its combinatorial nature. Motivated by this, we propose a low-complexity two-stage algorithm to construct such transmit-signal vector directly. In the first stage, we derive a feasible transmit-signal vector via iterative-soft-thresholding algorithm whose output ensures that each user's noiseless observation belong to a desired region (called decision region). In the second stage, a bit-flipping algorithm is performed to refine the feasible vector so that each user's received signal is more robust to additive Gaussian noises. Simulation results demonstrate that the proposed method can yield a more elegant performance-complexity tradeoff than the existing 1-b precoding methods.

A user pairing method to improve the channel capacity for multiuser MIMO channels in downlink mode based on NOMA

This paper investigates applications of multiple-input multiple-output (MIMO) in non-orthogonal multiple access (NOMA) for downlink multiuser systems, where the total number of transmitter antennas is equal to or less than receiver antennas in each user. We also consider a user pairing scheme involving 3 users per cluster with non-overlapping frequency band for users with similar channel gains. The integration of beamforming and user pairing along with successive interference cancellation (SIC) can reduce occurred interference and maximize the sum rate in MIMO-NOMA systems. The novelty of this work is to provide a solution for achieving better performance and massive connectivity in NOMA with 3 users in each cluster by increasing the number of transmitter and receiver antennas. The analytical expressions of the MIMO-NOMA transmission scheme are derived in the ergodic sum capacity term. Also, the impact of the number of users in a cluster on the ergodic capacity of the desired NOMA scheme is evaluated. Finally, computer simulation results are performed to show the superiority of MIMO-NOMA over MIMO-OMA and MIMO-(NOMA-OMA).

Secrecy Performance of Two-Level Superposition Transmission in Untrusted Multiuser Wireless Downlink Systems

In an untrusted multiuser downlink system (UTMS) where a desired user is opportunistically selected and all the other users are potential eavesdroppers (Eves), the system performance with security requirement will deteriorate as the number of users grows. In this letter, we have proposed two-level superposition transmission (SuT) in which secure signals are superimposed with ordinary signals that are not necessarily to be secure. The ordinary signals play as jamming signals but are made not decodable at Eves with appropriate power allocation at the sender. We have proved that SuT achieves the full diversity order in secure transmission in UTMS. We also have shown that average secrecy rate with SuT obeys growth rate loglog ${K}$ as the number of users ${K}$ grows in UTMS. Numerical investigation is used to verify the proof and derivations obtained in this letter.

Support Vector Machine-Based Transmit Antenna Allocation for Multiuser Communication Systems.

In this paper, a support vector machine (SVM) technique has been applied to an antenna allocation system with multiple antennas in multiuser downlink communications. Here, only the channel magnitude information is available at the transmitter. Thus, a subset of transmit antennas that can reduce multiuser interference is selected based on such partial channel state information to support multiple users. For training, we generate the feature vectors by fully utilizing the characteristics of the interference-limited setup in the multiuser downlink system and determine the corresponding class label by evaluating a key performance indicator, i.e., sum rate in multiuser communications. Using test channels, we evaluate the performance of our antenna allocation system invoking the SVM-based allocation and optimization-based allocation, in terms of sum-rate performance and computational complexity. Rigorous testing allowed for a comparison of a SVM algorithm design between one-vs-one (OVO) and one-vs-all (OVA) strategies and a kernel function: (i) OVA is preferable to OVO since OVA can achieve almost the same sum rate as OVO with significantly reduced computational complexity, (ii) a Gaussian function is a good choice as the kernel function for the SVM, and (iii) the variance (kernel scale) and penalty parameter (box constraint) of an SVM kernel function are determined by 21.56 and 7.67, respectively. Further simulation results revealed that the designed SVM-based approach can remarkably reduce the time complexity compared to a traditional optimization-based approach, at the cost of marginal sum rate degradation. Our proposed framework offers some important insights for intelligently combining machine learning techniques and multiuser wireless communications.

Multi-User Analog Beamforming in Millimeter Wave MIMO Systems Based on Path Angle Information

We aim to design an analog-only beamforming scheme for downlink multi-user mm-wave systems to optimize the beamforming gain and the inter-user interference at the same time. Traditional analog beamforming schemes, such as the beam selection method, use the array response vector corresponding to the strongest path of the channel to generate a beam pointing to the user. In multi-user systems, such schemes will lead to large inter-user interference, especially when the users are closely located. In this paper, we formulate a multi-objective problem to strike a balance between the beamforming gain and the inter-user interference. To solve the problem, we first use the weighted-sum method to transform the multi-objective problem into a single-objective problem. Then, we use the semi-definite programing technique to make the analog beamforming with constant-magnitude constraints tractable. Furthermore, to alleviate the effects of the channel estimation and feedback quantization errors, we design a robust beamforming scheme to provide robustness against imperfect channel information. We first develop a channel error model for the scattering clustered channel model, which can serve as a general channel error model for the mm-wave channels. Then, we formulate a multi-objective problem using the stochastic approach to suppress the interference and enhance the beamforming gain at the same time. The simulation results show that our proposed non-robust multi-user analog beamformer outperforms the traditional analog beamforming method when the SNR is high and our proposed robust beamformer can provide up to 109% improvement in the sum-rate compared with the beam selection method.

Power Allocation for Energy Efficiency Optimization in Multi-User mmWave-NOMA System With Hybrid Precoding

Recently, the non-orthogonal multiple access (NOMA) scheme has been applied in millimeter-wave (mmWave) communication system to support more users and further improve the performance. In this paper, an energy-efficient power allocation (PA) scheme is designed for a downlink multi-user mmWave-NOMA system with hybrid precoding (HP), where two typical HP architectures, namely the fully-connected HP architecture (FHPA) and the sub-connected HP architecture (SHPA), are both considered. Firstly, we pair users in term of their channel difference and correlation. Then, analog beamforming schemes are proposed for the system with fully-connected and sub-connected HP architectures, respectively. Based on this, a two-step HP design with the proposed analog beamforming and zero-forcing precoding is presented. With these results, the optimization problem is formulated to maximize the energy efficiency (EE) of the system. This is a non-convex optimization problem, and can be approximately decomposed into independent convex sub-problems by applying the fractional programming theory. Using the coordinate descent method, the closed-form solutions of each sub-problem are derived. On these basis, an effective iterative algorithm is proposed to obtain the suboptimal power allocation. Simulation results verify the effectiveness of the proposed PA scheme, it has the same EE performance as the existing scheme with relatively low complexity, and can obtain the EE close to the exhaustive search scheme as well as the chaotic accelerated particle swarm optimization scheme. Moreover, the proposed analog beamforming obviously outperforms the conventional finite resolution analog beamforming, and the system with SHPA has higher EE than that with FHPA.

Quantized Constant Envelope Precoding With PSK and QAM Signaling

Coarsely quantized massive multiple-input multiple-output (MIMO) systems are gaining more interest due to their power efficiency. We present a new precoding technique to mitigate the multi-user interference and the quantization distortions in a downlink multi-user MIMO system with coarsely quantized constant envelope (QCE) signals at the transmitter. The transmit signal vector is optimized for every desired received vector taking into account a relaxed version of the QCE constraint. The optimization is based on maximizing the safety margin to the decision thresholds of the receiver constellation modulation. Due to the linear property of the objective function and the constraints, the optimization problem is formulated as a linear programming problem. The simulation results show a significant gain in terms of the uncoded bit error rate compared to the existing precoding techniques.

Exploiting high rate differential algebraic space‐time block code in downlink multiuser MIMO systems

This paper considers a multiuser multiple-input multiple-output (MIMO) space-time block coded system that operates at a high data rate with full diversity. In particular, they propose to use a full rate downlink algebraic transmission scheme combined with a differential space-time scheme for multiuser MIMO systems. To achieve this, perfect algebraic space-time codes and Cayley differential transforms are employed. Since channel state information (CSI) is not needed at the differential receiver, differential schemes are ideal for multiuser systems to shift the complexity from the receivers to the transmitter, thus simplifying user equipment. Furthermore, orthogonal spreading matrices are employed at the transmitter to separate the data streams of different users and enable simple single user decoding. In the orthogonal spreading scheme, the transmitter does not require any knowledge of the CSI to separate the data streams of multiple users; this results in a system which does not need CSI at either end. With this system, to limit the number of possible codewords, a sphere decoder is used to decode the signals at the receiving end. The proposed scheme yields low complexity transceivers while providing full rate full diversity with good performance. Simulation results demonstrate the effectiveness of the proposed scheme.

MIMO Radar and Cellular Coexistence: A Power-Efficient Approach Enabled by Interference Exploitation

We propose a novel approach to enable the coexistence between Multi-Input-Multi-Output (MIMO) radar and downlink multiuser multi-input single-output communication system. By exploiting the constructive multiuser interference (MUI), the proposed approach tradeoff useful MUI power for reducing the transmit power, to obtain a power efficient transmission. This paper focuses on two optimization problems: a) Transmit power minimization at the base station (BS), while guaranteeing the receive signal-to-interference-plus-noise ratio (SINR) level of downlink users and the interference-to-noise ratio level to radar; b) Minimization of the interference from BS to radar for a given requirement of downlink SINR and transmit power budget. To reduce the computational overhead of the proposed scheme in practice, an algorithm based on gradient projection is designed to solve the power minimization problem. In addition, we investigate the tradeoff between the performance of radar and communication, and analytically derive the key metrics for MIMO radar in the presence of the interference from the BS. Finally, a robust power minimization problem is formulated to ensure the effectiveness of the proposed method in the case of imperfect channel state information. Numerical results show that the proposed method achieves a significant power saving compared to conventional approaches, while obtaining a favorable performance-complexity tradeoff.

An Improved FD-DFE Structure for Downlink VLC Systems Based on SC-FDMA

Visible light communication (VLC) technology is considered to be an attractive solution for future wireless indoor multimedia communication. To reduce the peak-to-average power ratio, single-carrier frequency division multiple access (SC-FDMA) with time domain asymmetrical clipping is applied for the downlink multi-user VLC system. An improved frequency-domain decision feedback equalizer (FD-DFE) structure is proposed for the SC-FDMA -based VLC system in this letter. Different kinds of subcarrier distributions are considered in the analyses and simulations. Thanks to the clipping noise information on the even subcarriers, which can reduce the decision error, the proposed algorithm provides a lower bit error rate and better transmission performance compared with traditional schemes.

QoS-Aware Utility-Based Resource Allocation in Mixed-Traffic Multi-User OFDM Systems

This paper deals with the joint subcarrier and power allocation problem in a downlink multi-user orthogonal frequency division multiplexing system subject to user delay and minimum rate quality-of-service (QoS) requirements over a frequency-selective multi-carrier fading channel. We aim to maximize the utility-pricing function, formulated as the difference between the achieved spectral efficiency and the associated linear cost function of transmit power scaled by a system-dependent parameter. For a homogeneous system, we show that the joint resource allocation can be broken down into sequential problems while retaining the optimality. Specifically, the optimal solution is obtained by first assigning each subcarrier to the user with the best channel gain. Subsequently, the transmit power for each subcarrier is adapted according to water-filling policy if the global optimum is feasible, else it is given by a non-water-filling power adaptation. For a heterogeneous system, an optimal solution needs exhaustive search and hence, we resort to two reduced-complexity sub-optimal algorithms. Algorithm-I is a simple extension of the aforementioned optimal algorithm developed for a homogeneous system, while Algorithm-II further takes into consideration the heterogeneity in user QoS requirements for performance enhancement. Simulation results reveal the impacts of user QoS requirements, number of subcarriers and number of users on the system transmit power.

Mitigation of self‐interference and multi‐user interference in downlink multi‐user MIMO system

Here, the authors develop a method of simultaneous optimisation of precoder and equaliser for the multi-user multiple-input multiple-output (MIMO) system by minimising outage probability. In order to do that, the authors first characterise the signal-to-interference-plus-noise ratio of downlink multi-user MIMO system in the presence of both multi-user interference and self-interference. The closed-form expression for the outage probability is derived in terms of the eigenvalues of an indefinite weight matrix of a quadratic norm. Next, the authors develop an active-set algorithm-based precoder and equaliser by minimising the derived outage probability expression. This work investigates the effect of various system parameters such as the number of users, the number of multi-path channels, the number of transmitting and receiving antenna elements, and the additive noise variance on the outage probability and bit error rate. Simulation results validate the theoretical analysis for multiple system configurations. Lastly, the authors show that the designed precoder and equaliser improves the overall system performance.

Resource Allocation in Non-Orthogonal and Hybrid Multiple Access System With Proportional Rate Constraint

Non-orthogonal multiple access (NOMA), which has attracted a lot of attention recently due to its superior spectral efficiency, could play a vital role in improving the capacity of future networks. In this paper, a resource allocation scheme is developed for a downlink multi-user NOMA system. An optimization problem is formulated to maximize the sum rate under the total power and proportional rate constraints. Due to the complexity of computing the optimal solution, we develop a low complexity sub-optimal solution for a two-user scenario and then extend it to the multi-user case by proposing a user-pairing approach as well as a number of power allocation techniques that facilitate dealing with a large number of users in NOMA system. Simulation results support the effectiveness of the proposed approaches and show the close performance to the optimal one. In addition, we propose a new hybrid multiple access technique that combines the properties of NOMA and the orthogonal frequency division multiple access. Simulation results show that the proposed hybrid method provides better performance than NOMA in terms of the overall achievable sum rate and the coverage probability.