Space Division Multiple Access Research Articles

Space-Division Multiple Access Based on Time-Modulated Array

A novel space-division multiple-access (SDMA) method is proposed by the multibeam characteristic of the time-modulated array (TMA). The TMA generates the fundamental and harmonic components pointing to different directions. For the uplink, the users in different directions transmit the signals with the same carrier frequency to the base station with the TMA. Then, the base station distinguishes the users by the generated harmonic components. For the downlink, signals of different users are modulated to different carrier frequencies beforehand. Because of the modulation of the TMA, the downlink signals are transmitted to different directions with the same carrier frequency. Compared to the traditional SDMA systems implemented by the Butler matrix beamforming or the digital beamforming, the proposed method implements the SDMA with a single RF channel, which has advantage in the hardware complexity and cost. The uplink and downlink schemes are analyzed, and their drawbacks are discussed. A simple TMA is constructed to verify its effectiveness.

Distributed spatial reuse distance control for basic safety messages in SDMA-based VANETs

The periodic generation and transmission of basic safety messages (BSM) place a heavy burden on the load of a vehicular ad hoc network (VANET), where there also remain multiple packet collisions undetected due to the hidden terminals. In this paper, we propose a distributed scheme that controls the spatial reuse distance to improve the efficiency of BSM transmissions in space-division multiple access (SDMA) based VANETs. With the proposed SDMA structure, only vehicles located sufficiently far apart are allowed to reuse the same time slot to send their BSMs. The signal interference is thus reduced while the hidden-node problem is essentially alleviated. Multiple transmissions per SDMA road segment or ‘cell’ are also enabled using the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), giving rise to a much better use of both space and bandwidth. To guarantee that the actual number of BSM transmissions does not exceed the maximum allowable in each SDMA cell, we further devise an adaptive scheme that adjusts the spatial reuse distance in accordance with the vehicle density. Because the global information of vehicle density is not available at individual vehicles, we propose a distributed algorithm that estimates the vehicle density and makes consensus to enhance the accuracy of spatial reuse distance estimation. As the transmission range is controlled accordingly, the mutual interference among the SDMA cells is further reduced. Importantly, the developed control algorithm can be distributively implemented by each vehicle with limited information exchange. To optimize the performance of the proposed solution, we also determine the optimal bandwidth utilization that maximizes the newly-defined criterion termed as ‘safe range,’ an important figure-of-merit in vehicular safety applications. Simulation results confirm the clear advantages of our proposal over the available approaches in terms of safety range, packet reception rate, end-to-end delay and BSM inter-arrival time in realistic network scenarios.

Resource allocation for multiple input multiple output‐orthogonal frequency division multiplexing ‐based space division multiple access systems

The problem of the system throughput maximisation and the fairness issue among users is considered for the downlink multiuser multiple input multiple output-orthogonal frequency division multiplexing system with MMSE precoding. Since finding the optimal solution by using the exhaustive search has very high computational burden, the authors propose a low-complexity user selection strategy plus the optimal power allocation in this study. With resorting to the mathematical simplification process, the proposed user selection scheme is designed to compare the related value of SINR. The power allocation is performed after the user selection. However, the power allocation is not in interference-free environment but in the presence of crosstalk, which is a nonlinear non-convex optimisation problem. In this study, the authors use an optimisation technique to obtain the optimal solution of the constrained non-convex nonlinear problem by replacing the objective function with a difference of two convex functions. A bisection-based strategy to solve this problem with interference-awareness to find local optimal solutions is presented in this study. Simulation results show that the performance of the proposed algorithm is close to that of the optimal solution. The trade-off between the system throughput and the fairness among users is also considered.

An SDMA-based MAC protocol for wireless ad hoc networks with smart antennas

A wireless ad hoc network consists of a set of wireless devices. The wireless devices are capable of communicating with each other without the assistance of base stations. Space Division Multiple Access (SDMA) is a new technology designed to optimize the performance of current and future mobile communication systems. In this paper, an SDMA-based MAC protocol (S-MAC) for wireless ad hoc networks with smart antennas is proposed. The proposed protocol exploits the SDMA system to allow reception of more than one packet from spatially separated transmitters. Using SDMA technology provides collision-free access to the communication medium based on the location of a node. The proposed protocol solves the hidden terminal problem, the exposed terminal problem, and the deafness problem. Simulation results demonstrate the effectiveness of the proposed S-MAC in improving throughput and increasing spatial channel reuse.

Autonomous algorithms for centralized and distributed interference coordination: a virtual layer-based approach

Interference mitigation techniques are essential for improving the performance of interference limited wireless networks. In this paper, we introduce novel interference mitigation schemes for wireless cellular networks with space division multiple access (SDMA). The schemes are based on a virtual layer that captures and simplifies the complicated interference situation in the network and that is used for power control. We show how optimization in this virtual layer generates gradually adapting power control settings that lead to autonomous interference minimization. Thereby, the granularity of control ranges from controlling frequency sub-band power via controlling the power on a per-beam basis, to a granularity of only enforcing average power constraints per beam. In conjunction with suitable short-term scheduling, our algorithms gradually steer the network towards a higher utility. We use extensive system-level simulations to compare three distributed algorithms and evaluate their applicability for different user mobility assumptions. In particular, it turns out that larger gains can be achieved by imposing average power constraints and allowing opportunistic scheduling instantaneously, rather than controlling the power in a strict way. Furthermore, we introduce a centralized algorithm, which directly solves the underlying optimization and shows fast convergence, as a performance benchmark for the distributed solutions. Moreover, we investigate the deviation from global optimality by comparing to a branch-and-bound-based solution.

Improved Spectral Efficiency With Acceptable Service Provision in Multiuser MIMO Scenarios

In this paper, we study the downlink resource assignment problem of maximizing the total data rate subject to minimum satisfaction guarantees in multiservice scenarios assuming the use of multiuser (MU) multiple-input-multiple-output (MIMO) techniques. With the use of MU MIMO techniques, the same frequency resource can be shared by different terminals at the same time by assigning different spatial channels to each terminal. Different from single-user (SU) access per frequency resource where the resource assignment problem consists of searching for the best association between frequency resources and terminals, with MU access per resource, we aim at finding which space-division multiple-access (SDMA) group should be associated with each frequency resource. The first contribution of this paper is the formal presentation of this generalized problem as an optimization problem. Then, after some mathematical manipulations, we managed to convert this problem from a nonlinear integer to an integer linear problem (ILP) that can be optimally solved by standard techniques instead of resorting to brute-force methods. Motivated by the high computational complexity of the optimal solution, we propose a low-complexity algorithm. By analyzing outage rate and total data rate performance metrics, we show that the proposed solution presents a good performance-complexity tradeoff compared with the method for obtaining the optimal solution.

Intrinsic interference mitigating coordinated beamforming for the FBMC/OQAM based downlink

In this work, we propose intrinsic interference mitigating coordinated beamforming (IIM-CBF)-based transmission strategies for the downlink of multi-user multiple-input-multiple-out (MIMO) systems and coordinated multi-point (CoMP) systems where filter bank based multi-carrier with offset quadrature amplitude modulation (FBMC/OQAM) is employed. Our goal is to alleviate the dimensionality constraint imposed on the state-of-the-art solutions for FBMC/OQAM-based space division multiple access that the total number of receive antennas of the users must not exceed the number of transmit antennas at the base station. First, two IIM-CBF algorithms are developed for a single-cell multi-user MIMO downlink system. The central idea is to jointly and iteratively calculate the precoding matrix and decoding matrix for each subcarrier to mitigate the multi-user interference as well as the intrinsic interference inherent in FBMC/OQAM-based systems. Second, for a CoMP downlink scenario where partial coordination among the base stations is considered, the application of coordinated beamforming-based transmission schemes is further investigated. An appropriate IIM-CBF technique is proposed. Simulation results show that when the number of transmit antennas at the base station is equal to the total number of receive antennas of the users, the proposed IIM-CBF algorithm outperforms the existing transmission strategies for FBMC/OQAM-based multi-user MIMO downlink systems. Moreover, we evaluate the performances of the IIM-CBF schemes in the downlink of multi-user MIMO systems and CoMP systems where the total number of receive antennas of users exceeds the number of transmit antennas at the base station. It is observed that by employing the IIM-CBF techniques, FBMC/OQAM systems achieve a similar bit error rate (BER) performance as its orthogonal frequency division multiplexing with the cyclic prefix insertion (CP-OFDM)-based counterpart while exhibiting superiority in terms of a higher spectral efficiency, a greater robustness against synchronization errors, and a lower out-of-band radiation. In the presence of residual carrier frequency offsets, FBMC/OQAM systems provide a much better performance compared to the CP-OFDM-based system, which corroborates the theoretical analysis that FBMC/OQAM systems are more immune to the lack of perfect synchronization. In addition, numerical results with respect to the convergence behavior of the IIM-CBF techniques are presented, and the computational complexity issue is also addressed.

EMA: Energy-Efficiency-Aware Multiple Access

To improve the energy efficiency (EE) of an orthogonal frequency-division multiple access system with multiple transmit antennas supporting multiple single-receive-antenna users, we propose an EE-aware multiple access (EMA) scheme. The EMA selects an energy-efficient channel access method that is either time-division multiple access (TDMA) or space-division multiple access (SDMA) for each subband. For the SDMA, the near optimal number of SDMA time slots is derived. Numerical results verify that the EE of pure TDMA and SDMA can be significantly improved through the proposed EMA scheme.

Space-Polarization Division Multiple Access System with Limited Feedback

This paper proposes a space-polarization division multiple access (SPDMA) system that has limited feedback channels. The system simultaneously serves data streams to multiple mobile users through dual-polarized antenna arrays, by using pre-determined sets of precoding vectors that are orthogonal in both space and polarization domains. To this end, a codebook whose elements are sets of the precoding vectors is systematically designed based on the discrete Fourier transform (DFT) matrix and considering the power imbalance of polarized channels. Throughput of the SPDMA system is evaluated and compared to that of space division multiple access (SDMA) system, according to the various parameters including cross polarization discrimination (XPD). The results show that the throughput of SPDMA system outperforms that of SDMA in the environments of high XPD with many mobile users.

상향링크 공간 분할 다중 접속 시스템에서 사용자 스케쥴링, 송신 전력 제어, 수신 빔포밍에 관하여

ABSTRACT In this paper, we investigate the user scheduling, transmit beamforming, and receive beamforming of uplink space division multiple access (SDMA) systems where multiple users are allowed to transmit their signal to a base station (BS) using the same frequency band simultaneously. The BS performs a receive beamforming using the predetermined pseudo-random pattern and select users with a specific criterion. Especially, in this paper, we propose the threshold-based transmit power control, in which a user decrease its transmit power according if its generating interference to other users’s signal is larger than a predetermined threshold. Assuming that the TDD system is used, the channel state information (CSI) can be obtained at each user from pilot signals from the BS. Simulation results show that the proposed technique significantly outperforms the existing user scheduling algorithms. 키워드 : 다중안테나, 셀룰라 네트워크, 빔포밍, 전력 제어, 사용자 스케쥴링 Key word : MIMO, cellular networks, beamforming, power control, user schedulingJournal of the Korea Institute of Information andCommunication Engineering

Visible light communication based on space-division multiple access optical beamforming

We propose and demonstrate a visible light communication (VLC) scheme based on space-division multiple access (SDMA) optical beamforming to accommodate multiple user devices in the VLC based on optical beamforming. SDMA optical beamforming is a technique which separates light-emitting diode light spa­tially and focuses each part on different target devices simultaneously. We show the experimental results of the VLC signal amplitudes, the optical power densities, and the bit-error rate performance as a function of transmission distance before and after the SDMA optical beamforming. The results show that the VLC signal amplitudes and optical power densities are improved by 8-2 and 3.8-5 dB, respectively, with the help of SDMA optical beamforming.

SDMA-Based Aeronautical Machine-to-Machine Communications under SINR Constraints

In order to achieve an efficient usage on a limited spectrum for aeronautical machine-to-machine (M2M) communications, in this paper, a space division multiple access- (SDMA-) based architecture is proposed, where the random access of short messages noisy (SMN) M2M communications is considered between autopilot systems onboard and the ground control center for the air traffic precise control, that is, full 4-dimensional trajectory management. It shows that using a zero-forcing beamforming combined with a low complexity aircraft grouping strategy, the SDMA-based mechanism is able to improve the spectrum efficiency under signal to interference plus noise ratio (SINR) constraints.

VLSI Implementation of a Soft-Output Signal Detector for Multimode Adaptive Multiple-Input Multiple-Output Systems

This paper presents a multimode soft-output multiple-input multiple-output (MIMO) signal detector that is efficient in hardware cost and energy consumption. The detector is capable of dealing with spatial-multiplexing (SM), space-division-multiple-access (SDMA), and spatial-diversity (SD) signals of 4 × 4 antenna and 64-QAM modulation. Implementation-friendly algorithms, which reuse most of the mathematical operations in these three MIMO modes, are proposed to provide accurate soft detection information, i.e., log-likelihood ratio, with much reduced complexity. A unified reconfigurable VLSI architecture has been developed to eliminate the implementation of multiple detector modules. In addition, several block level technologies, such as parallel metric update and fast bit-flipping, are adopted to enable a more efficient design. To evaluate the proposed techniques, we implemented the triple-mode MIMO detector in a 65-nm CMOS technology. The core area is 0.25 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with 83.7 K gates. The maximum detecting throughput is 1 Gb/s at 167-MHz clock frequency and 1.2-V supply, which archives the data rate envisioned by the emerging long-term evolution advanced standard. Under frequency-selective channels, the detector consumes 59.3-, 10.5-, and 169.6-pJ energy per bit detection in SM, SD, and SDMA modes, respectively.

Throughput analysis of transmit-nulling SDMA with limited feedback

We recently proposed a precoder codebook for a transmit-nulling space-division multiple access (TN-SDMA) to share spectrum with existing wireless services. Since a portion of the spatial subspaces of a multiantenna broadcast channel is used to eliminate the interference to coexisting systems, TN-SDMA could benefit the efficiency of spectrum usage from the coexistence of different systems in the same band while it always yields lower throughput per unit bandwidth than the orthogonal SDMA (called per user unitary and rate control (PU2RC)) that utilizes all the spatial subspaces for data transmission. This study aims to theoretically quantify the throughput loss of TN-SDMA relative to PU2RC and to analyze the effect of the main system parameters (signal-to-noise ratio (SNR) and the numbers of transmit antennas, users, and feedback bits) on the throughput loss. We derive the theoretical upper bound of the throughput loss of TN-SDMA relative to PU2RC, with the same feedback bits (codebook size). The throughput loss is lower with more transmit antennas, fewer users, lower SNR, or fewer feedback bits. It is interesting to note that the throughput loss converges to an upper limit with an increase in the SNR, which indicates that the SNR has a comparatively minor effect on the throughput loss in the high-SNR region. We also derive the required additional number of feedback bits for TN-SDMA to achieve the throughput of PU2RC (i.e., zero throughput loss). We find that the throughput achieved is feasible at the cost of a practically small number of additional feedback bits.

Multiuser Detection in SDMA–OFDM Wireless Communication System Using Complex Multilayer Perceptron Neural Network

The space division multiple access–orthogonal frequency division multiplexing (SDMA–OFDM) wireless system has become very popular owing high spectral efficiency and high load capability. The optimal maximum likelihood multiuser detection (MUD) technique suffers from high computational complexity. On the other hand the linear minimum mean square error (MMSE) MUD techniques yields poor performance and also fails to detect users in overload scenario, where the number of users are more than that of number of receiving antennas. By contrast, the differential evolution algorithm (DEA) aided minimum symbol error rate (MSER) MUD can sustain in overload scenario as it can directly minimizes probability of error rather than mean square error. However, all these classical techniques are still complex as these do channel estimation and multiuser detection sequentially. In this paper, complex multi layer perceptron (CMLP) neural network model is suggested for MUD in SDMA–OFDM system as it do both channel approximation and MUD simultaneously. Simulation results prove that the CMLP aided MUD performs better than the MMSE and MSER techniques in terms of enhanced bit error rate performance with low computational complexity.

Neural network-based multiuser detection for SDMA–OFDM system over IEEE 802.11n indoor wireless local area network channel models

Space division multiple access – orthogonal frequency division multiplexing-based wireless communication has the potential to offer high-spectral efficiency, system performance and capacity. This article proposes an efficient blind multiuser detection (MUD) scheme using artificial neural network models such as the radial basis function. The proposed MUD technique is consistently outperforming the existing minimum mean square error and minimum bit error rate (MBER) MUDs with the performance close to the optimal maximum likelihood (ML) detector. Besides that, the computational complexity of the proposed one is comparatively lower than both the MBER and ML detectors. Further, it can also outperform MBER MUD in the overload scenario, where the number of users is more than that of the number of receiving antennas simulation-based study showing BER performance and complexity are carried out to prove the efficiency of the proposed techniques. This analysis is carried through the IEEE 802.11n standard channel models, which are designed for indoor wireless local area network applications of bandwidth up to 100 MHz at frequencies 2 and 5 GHz.

MC-OFDMA FOR MULTI-USER DETECTION SCHEMES IN POWERLINE COMMUNICATION

Support of multiple users in wideband powerline communication (PLC) is essential to enhance the dependence on such emerging systems. This paper presents novel adaptations of the space division multiple access (SDMA) techniques to PLC. Multicarrier data transmission and orthogonal frequency division multiple access (OFDMA) techniques are invoked to mitigate the technical challenges of PLC channel impairments and enhance the spectral efficiency. Performance is enhanced using orthogonal Walsh–Hadamard codes. Linear and nonlinear multi-user detection (MUD) schemes are investigated, including the optimal maximum likelihood (ML) methods and the near-optimal sphere detectors(SDs). Presented results indicate the potential of the proposed scheme to enhance wideband PLC systems.

Efficient complex radial basis function model for multiuser detection in a space division multiple access/multiple‐input multiple‐output–orthogonal frequency division multiplexing system

An adaptive multiuser detection (MUD) technique using the complex radial basis function (CRBF) network is proposed for space division multiple access–orthogonal frequency division multiplexing (SDMA–OFDM) system. Among various MUDs, the linear minimum mean-square error (MMSE) MUD suffers from poor performance and the maximum likelihood (ML) detector is restricted by high computational complexity. Hence, the cost function minimisation-based detector like minimum symbol error rate (MSER) is preferred because of significant performance gain over MMSE MUD and complexity gain over ML detector. Moreover, the MSER detector also has a potential of surviving in overload scenario, where the number of users are more than that of the number of receiving antennas. However, in all these techniques, the requirement of channel estimation adds an extra complexity whereas, the proposed CRBF detector approximates the channel parameters in training phase and detects signals in testing phase. It also has low complexity, better performance compared with MSER MUD and also supports overload scenario. Each neuron in the proposed CRBF network is assembled with ‘sech’ activation function, as this function can do better complex non-linear mapping than Gaussian activation. The simulation study and performance evaluation of CRBF MUD is investigated, considering both data and image transmission.

Downlink MIMO HetNets: Modeling, Ordering Results and Performance Analysis

We develop a general downlink model for multi-antenna heterogeneous cellular networks (HetNets), where base stations (BSs) across tiers may differ in terms of transmit power, target signal-to-interference-ratio (SIR), deployment density, number of transmit antennas and the type of multi-antenna transmission. In particular, we consider and compare space division multiple access (SDMA), single user beamforming (SU-BF), and baseline single-input single-output (SISO) transmission. For this general model, the main contributions are: (i) ordering results for both coverage probability and per user rate in closed form for any BS distribution for the three considered techniques, using novel tools from stochastic orders, (ii) upper bounds on the coverage probability assuming a Poisson BS distribution, and (iii) a comparison of the area spectral efficiency (ASE). The analysis concretely demonstrates, for example, that for a given total number of transmit antennas in the network, it is preferable to spread them across many single-antenna BSs vs. fewer multi-antenna BSs. Another observation is that SU-BF provides higher coverage and per user data rate than SDMA, but SDMA is in some cases better in terms of ASE.

An efficient power management scheme with SDMA for IEEE 802.15.3 WPANs

SUMMARYIEEE 802.15.3 is devised for short‐distance wireless connectivity within a wireless personal area network (WPAN). In WPANs, a power management (PM) mechanism is used for power saving. The PM enables devices to sleep for one or more superframes in order to conserve power. Space division multiple access (SDMA) is a new technology to optimize performance of current and future mobile communication systems. In this paper, we propose an SDMA‐based PM scheme, called enhanced min‐degree searching (EMDS) for WPANs. EMDS orders the streams among multiple devices to minimize the total wake‐up times. In this scheme, every device that can transmit data at the same time is checked using the SDMA. The total wake‐up times will be reduced substantially if the majority of the devices can transmit data at the same time. Simulation results show that the performance of the proposed EMDS outperforms that of the existing approaches. Copyright © 2013 John Wiley &amp; Sons, Ltd.