Multi-user Multiple-input Multiple-output Wireless Research Articles

Diversity Order and Measure of MIMO Antennas in Single-User, Multiuser, and Massive MIMO Wireless Communications

In this letter, the diversity order (DO) and the diversity measure (DM) of multiple-input multiple-output (MIMO) antennas in generic single-user, multiuser, and massive MIMO wireless communications are calculated for several practical case studies. These performance metrics are calculated from the antenna correlation coefficient (ACC) matrix. The DO is calculated from the rank of the ACC matrix, and the DM is obtained from the trace and Frobenius norm of the ACC matrix. Numerical results are provided. In particular, it is shown that though the DM is always less than the DO, it incorporates the effect of antenna correlation on the diversity gain in a better way.

Asynchronous Designs for Multiuser MIMO Wireless Powered Communication Networks

This paper considers multiuser multiple-input multiple-output (MIMO) wireless powered communication networks (WPCNs). In this system, a multiantenna power beacon (PB) transfers wireless energy to several multiantenna users, and then the users transmit data to a multiantenna base station. Time durations for the energy harvesting (EH) operations at each user can be differently set based on the users’ channel conditions, which results in an asynchronous protocol on the EH and the information transmission. We maximize the sum rate performance of the asynchronous WPCN by jointly optimizing the energy precoding matrices at the PB, the information precoding matrices, and the EH time durations at the users. By using convex optimization techniques, the optimal algorithm for the sum rate maximization problem is provided with analytical expressions of the optimal precoding matrices. Simulation results verify that the proposed optimal algorithm outperforms conventional schemes.

Precoding Design and Power Allocation in Two-User MU-MIMO Wireless Ad Hoc Networks

In this paper, we consider the precoding design and power allocation problem for multi-user multiple-input multiple-output (MU-MIMO) wireless ad hoc networks. In the first timeslot, the source node (SN) transmits energy and information to a relay node (RN) simultaneously within the simultaneous wireless information and power transfer (SWIPT) framework. Then, in the second timeslot, based on the decoder and the forwarding (DF) protocol, after reassembling the received signal and its own signal, the RN forwards the information to the main user (U1) and simultaneously sends its own information to the secondary user (U2). In this paper, when the transmission rate of the U1 is restricted, the precoding, beamforming, and power splitting (PS) transmission ratio are jointly considered to maximize the transmission rate of U2. To maximize the system rate, we design an optimal beamforming matrix and solve the optimization problem by semi-definite relaxation (SDR), considering the high complexity of implementing the optimal solution. Two sub-optimal precoding programs are also discussed: singular value decomposition and block diagonalization. Finally, the performance of the optimization and sub-optimization schemes are compared using a simulation.

Sum-Rate Maximization for Multiuser MIMO Wireless Powered Communication Networks

This paper investigates multiuser multiple-input–multiple-output (MIMO) wireless powered communication networks where a multiantenna hybrid access point (H-AP) transfers wireless energy to multiantenna users in a downlink phase, and the users utilize the harvested energy for their information transmission to the H-AP in an uplink phase. By employing space-division multiple-access techniques, we propose an optimal algorithm that jointly computes the downlink energy precoding matrices, the uplink information precoding matrices, and time allocation between the downlink and the uplink phases for maximizing the uplink sum-rate performance. To this end, we first obtain the optimal energy and information transmit covariance matrices with given time allocation. Then, the optimal time allocation can be efficiently identified by a simple line search method. Simulation results verify that the proposed joint optimal algorithm significantly improves the average sum-rate performance, compared with a conventional scheme that determines time allocation and precoding matrices separately.

Multiuser MIMO Wireless Energy Transfer With Coexisting Opportunistic Communication

This letter considers spectrum sharing between a multiuser multiple-input multiple-output (MIMO) wireless energy transfer (WET) system and a coexisting point-to-point MIMO wireless information transmission (WIT) system, where WET generates interference to WIT and degrades its throughput performance. We show that due to the interference, the WIT system suffers from a loss of the degrees of freedom (DoF) proportional to the number of energy beams sent by the energy transmitter (ET), which, in general, needs to be larger than one to optimize the multiuser WET with user fairness consideration. To minimize the DoF loss in WIT, we further propose a new single-beam energy transmission scheme based on the principle of time sharing, where the ET transmits one of the optimal energy beams at each time. This new scheme achieves the same optimal performance for the WET system, and minimizes the impact of its interference to the WIT system.

PAR Reduction in SU/MU-MIMO OFDM Systems Using OPF Precoding Over the Nonlinearity of SSPA

Orthogonal frequency division multiplexing (OFDM) system suffers from a high peak-to-average (power) ratio (PAR) which leads to power inefficiency in radio frequency section and large performance degradation of system due to the nonlinearity of high power amplifiers (HPA). In this paper, a new downlink transmission scheme is designed based on orthogonal periodic function (OPF) precoding to achieve low PAR and investigated for single user and multi-user multiple-input multiple-output wireless systems. It is shown (through mathematical analysis and computer simulations) that the proposed transmission scheme can provide a significant PAR reduction using decorrelation, phase-rotation and orthogonality properties of OPF precoding, while it does not change power spectral levels or error performances due to the unitary property when compared with the normal OFDM systems with linear HPA. The OPF precoding outperforms existing precoding schemes when nonlinear solid state power amplifier (SSPA) is considered with several practical values of input back-off (IBO). Computer simulations show that the OPF precoding can achieve 3.1 and 4 dB PAR gain, while 2.2 and 1 dB SNR gain for IEEE 802.11a and DVB-T systems respectively. Moreover, OPF precoding leads to a total degradation improvement from 0.13 to 1.53 dB for 16 QAM, considering an SSPA with IBO = 8---4 dB respectively. OPF precoding does not increase computational complexity when compared with the existing precoding methods. It is shown that the performance of the proposed precoding is independent of modulation schemes and can work for any number of subcarriers.

Asynchronous Medium Access Protocol for Multi-User MIMO Based Uplink WLANs

Multi-user multiple-input multiple-output (MIMO) technology makes it possible for wireless nodes to successfully receive multiple packets from simultaneous transmitters in wireless networks. As it can provide more transmission opportunities without causing collisions, the network throughput performance can be dramatically improved. In this paper, we propose an asynchronous medium access control (MAC) protocol, which enables senders to independently start their transmissions if the access point (AP) can receive more simultaneous packets up to its multi-packet reception capability. This asynchronous protocol makes the multi-user MIMO channel more efficiently used, especially in wireless networks where transmission durations are dynamically varying due to different packet sizes and transmission rates. Through our performance analysis and extensive simulations, we show that the proposed asynchronous MAC protocol achieves significantly higher uplink throughput performance in multi-user MIMO wireless networks.

Performance analysis of distributed cluster-based MAC protocol for multiuser MIMO wireless networks

It is known that multiuser multiple-input multiple-output (MIMO) communication can enhance the performance of wireless networks. It can substantially increase the spectral efficiency of wireless networks by utilising multiuser interference rather than avoiding it. This paradigm shift has most impact on the medium access control (MAC) protocol because most existing MAC protocols are designed to reduce the interference. In this article, we propose a novel cluster-based carrier sense multiple access with collision avoidance (CB-CSMA/CA) scheme. The proposed scheme enables multiuser MIMO transmissions in wireless local area networks (WLANs) by utilising the multiuser interference cancellation capability of the physical layer. In this article we focus on the performance analysis of CB-CSMA/CA. We investigate saturation throughput applying optimum backoff parameters and in the presence of synchronisation errors. Furthermore, we study the impact of different clustering methods on non-saturation throughput. We show that CB-CSMA/CA improves throughput significantly compared to the CSMA/CA scheme used in the IEEE 802.11 system. It is a promising approach for a variety of network configurations including typical infrastructure WLANs as well as many other wireless cooperative networks.

Adaptive downlink multi-user MIMO wireless systems for correlated channels with imperfect CSI

Multiple-input multiple-output (MIMO) wireless antenna systems provide increases in capacity without the need for additional spectrum or power. However the capacity increase is limited when the number of antennas at the receiver is fixed or restricted (due to mobile size constraints for example). To overcome this limitation multi-user MIMO can be used, which allows several users to be served simultaneously in frequency and time. A disadvantage of these multi-user MIMO systems, when used in the downlink however, is that they need accurate channel state information (CSI) at the transmitter and also uncorrelated channels among users. In this paper we investigate methods to address the problems of multi-user MIMO systems in spatially correlated channels. We adopt the concept of angle between subspaces to characterize the inter-user spatial correlation and adapt the algorithm to those conditions. We also investigate the impact of the accuracy of CSI at the transmitter (CSIT) and whether more limited CSI such as channel correlation information alone can be used to provide good multi-user MIMO performance. Results are presented as various simulated capacity measures and we use them to make comparisons between the various multi-user MIMO configurations

Minimum Variance Linear Receivers for Multiaccess MIMO Wireless Systems with Space-Time Block Coding

We consider the problem of joint space-time decoding and multiaccess interference (MAI) rejection in multiuser multiple-input multiple-output (MIMO) wireless communication systems. We address the case when both the receiver and multiple transmitters are equipped with multiple antennas and when space-time block codes (STBCs) are used to send the data simultaneously from each transmitter to the receiver. A new linear receiver structure is developed to decode the data sent from the transmitter-of-interest while rejecting MAI, self-interference, and noise. The proposed receivers are designed by minimizing the output power subject to constraints that zero-force self-interference and/or preserve a unity gain for all symbols of the transmitter-of-interest. Simulation results show that in multiaccess scenarios, the proposed techniques have substantially lower symbol error rates as compared with the matched filter (MF) receiver, which is equivalent to the maximum likelihood (ML) space-time decoder in the point-to-point MIMO communication case.