Space Division Multiple Access Systems Research Articles

Grouping methods for orthogonal frequency- and space-division multiple access in a radio-on-fiber radio-space transmission system

This paper proposes a radio-on-fiber (RoF) radio-space transmission system. We consider the orthogonal frequency- and space-division multiple access system, in which the orthogonal frequency-division multiple access (OFDMA) and space-division multiple access (SDMA) are combined. In this system, two frequency-channel scheduling schemes are also proposed for OFDMA, and two grouping methods are proposed for SDMA. The performance of the proposed systems is analyzed by computational simulations. It is shown that the random grouping method does not degrade the performance compared with a more complicated grouping method. In the proposed frequency-channel scheduling schemes, the control station estimates all users' signal-to-noise ratios (SNRs), and these schemes allow the system to assign subcarriers to different users according to their channel conditions so that each user can transmit on subcarriers where it has high SNR. Simulation results show that users can avoid frequency-selective fading and achieve higher bit-error-rate performance.

MBER Space-Time Decision Feedback Equalization Assisted Multiuser Detection for Multiple Antenna Aided SDMA Systems

This paper proposes a space-time decision feedback equalization (ST-DFE) assisted multiuser detection (MUD) scheme for multiple receiver antenna aided space division multiple access systems. A minimum bit error rate (MBER) design is invoked for the MUD, which is shown to be capable of improving the achievable bit error rate performance and enhancing the attainable system capacity over that of the standard minimum mean square error (MMSE) design. An adaptive implementation of the MBER ST-DFE assisted MUD is proposed using a stochastic gradient-based least bit error rate algorithm, which is demonstrated to consistently outperform the classical least mean square (LMS) algorithm, while achieving a lower computational complexity than the LMS algorithm for the binary signalling scheme. Our simulation results demonstrate that the MBER ST-DFE assisted MUD is more robust to channel estimation errors as well as to potential error propagation imposed by decision feedback errors, compared to the MMSE ST-DFE assisted MUD

Minimum bit-error rate design for space-time equalization-based multiuser detection

A novel minimum bit-error rate (MBER) space-time-equalization (STE)-based multiuser detector (MUD) is proposed for multiple-receive-antenna-assisted space-division multiple-access systems. It is shown that the MBER-STE-aided MUD significantly outperforms the standard minimum mean-square error design in terms of the achievable bit-error rate (BER). Adaptive implementations of the MBER STE are considered, and both the block-data-based and sample-by-sample adaptive MBER algorithms are proposed. The latter, referred to as the least BER (LBER) algorithm, is compared with the most popular adaptive algorithm,known as the least mean square (LMS) algorithm. It is shown that in case of binary phase-shift keying, the computational complexity of the LBER-STE is about half of that required by the classic LMS-STE. Simulation results demonstrate that the LBER algorithm performs consistently better than the classic LM Salgorithm, both in terms of its convergence speed and steady-state BER performance.

An SDMA Approach with Preamble Subcarrier Assignment for IEEE802.11a-Based OFDM Signals

In this letter, we present a space division multiple access (SDMA) approach for IEEE802.1la-based system employing pre-fast Fourier transform (FFT) adaptive array antenna (AAA) at base station (BS). As the core idea, we propose a preamble subcarrier assignment method to generate different preambles for different users using the same signal burst structure defined by IEEE802.11a, by which BS can effectively distinguish each user from other users and accurately estimate the channel impulse response (CIR) for each user. In this way, SDMA can be easily realized with no significant change in IEEE802.1la-based system. The performance of the proposed SDMA system is evaluated by computer simulation using a realistic spatio-temporal indoor wireless channel model.

Traffic Models and Blocking Probabilities for Two-Fold and Three-Fold SDMA Communication Systems

Spectrum efficiency is a constant challenge in the design of wireless networks. Space-division-multiple-access (SDMA) is a promising approach to achieve higher spectral efficiency which reuses bandwidth via multiplexing signals based on their spatial signature. Several different studies have shown that SDMA can effectively improve system capacity in a mobile environment. In this paper, we present a new Markov chain traffic model for a duplicate-at-last (DL) approach [IEE Proceedings on Communication 146 (1999) 303] in two-fold and three-fold SDMA systems. Simplified blocking probability formulations for two-fold and three-fold SDMA are also derived. Simulations based on a common method of spatial separation check for channel allocation in SDMA are presented to evaluate the probability of successfully creating two-fold and three-fold SDMA channels. The simulation, as well as analytical, results indicate that the SDMA system can reduce the blocking probability of the calls and result in more traffic loading than a traditional cellular system. The results also show that our simplified approaches not only can reduce the computational complexity, but can also accurate approximate two-fold and three-fold SDMA performance.

Nonlinear Multiantenna Detection Methods

A nonlinear detection technique designed for multiple-antenna assisted receivers employed in space-division multiple-access systems is investigated. We derive the optimal solution of the nonlinear spatial-processing assisted receiver for binary phase shift keying signalling, which we refer to as the Bayesian detector. It is shown that this optimal Bayesian receiver significantly out-performs the standard linear beamforming assisted receiver in terms of a reduced bit error rate, at the expense of an increased complexity, while the achievable system capacity is substantially enhanced with the advent of employing nonlinear detection. Specifically, when the spatial separation expressed in terms of the angle of arrival between the desired and interfering signals is below a certain threshold, a linear beamformer would fail to separate them, while a nonlinear detection assisted receiver is still capable of performing adequately. The adaptive implementation of the optimal Bayesian detector can be realized using a radial basis function network. Two techniques are presented for constructing block-data-based adaptive nonlinear multiple-antenna assisted receivers. One of them is based on the relevance vector machine invoked for classification, while the other on the orthogonal forward selection procedure combined with the Fisher ratio class-separability measure. A recursive sample-by-sample adaptation procedure is also proposed for training nonlinear detectors based on an amalgam of enhanced κ-means clustering techniques and the recursive least squares algorithm.

Bounds on the Outage-Constrained Capacity Region of Space-Division Multiple-Access Radio Systems

Space-division multiple-access (SDMA) systems that employ multiple antenna elements at the base station can provide much higher capacity than single-antenna-element systems. A fundamental question to be addressed concerns the ultimate capacity region of an SDMA system where in a number of mobile users, each constrained in power, try to communicate with the base station in a multipath fading environment. In this paper, we express the capacity limit as an outage region over the space of transmission rates R1, R2, ..., Rn from the n mobile users. Any particular set of rates contained within this region can be transmitted with an outage probability smaller than some specified value. We find outer and inner bounds on the outage capacity region for the two-user case and extend these to multiple-user cases when possible. These bounds provide yardsticks against which the performance of any system can be compared.

Impact of successive interference canceler on the performance of ubiquitous antenna‐based SDMA system

AbstractIn this paper, in order to improve the packet error rate of a ubiquitous antenna‐based SDMA (Space Division Multiple Access) system, the application of a successive interference canceler (SIC) is proposed. In a ubiquitous antenna‐based SDMA system, radio base stations (RBS) distributed throughout the service area are connected by radio‐on‐fiber links to the central control station (CCS). By performing all signal processing functions such as wireless modulation and demodulation at the CCS, simultaneous reception of several signals transmitted in the same frequency range is made possible and hence the frequency utilization efficiency can be enhanced. When an SIC is applied to the ubiquitous antenna system, the magnitude relationship of the power received by the user is different for each RBS as the RBSs are distributed at several locations. Hence, strength comparison of the received power is not easy. In the SIC proposed in this paper, the SINR (Signal to Interference plus Noise Ratio) after MMSE is estimated, and then the interference is eliminated according to size. A computer simulation is performed assuming a QPSK‐OFDM (Quadrature Phase Shift Keyed–Orthogonal Frequency Division Multiplex) signal with a coding rate of 1/2 in accordance with the IEEE 802.11a specification. It is made clear that by the use of the proposed scheme the required transmitted power satisfying the requirement of an average BER of 10−4 can be reduced by about 7.2 dB, and that with a packet transmission success rate of 0.9 it can be reduced by 8 dB. © 2004 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 87(12): 10–25, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20076

A study on channel efficiency for SDMA with an adaptive array

AbstractRecently, in order to improve channel use efficiency, the SDMA (Space Division Multiple Access) system has been proposed. In this paper, an adaptive array is applied to a cellular system and the SDMA system with MMSE‐based weight control performed for fixed channel assignment and dynamic channel assignment is discussed. In addition to the conventional blocking rate and the forced call termination rate, the ISR (Incomplete Service Rate) is defined as a new QoS (Quality of Service) and its characteristics are evaluated. Further, the occupation rate within the home cell by the SDMA system is studied with random assignment. It is found that the capacity increases by a factor of 2.08 with a blocking rate of 3% when the SDMA system is combined with a fixed channel assignment of nine cell repetition. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(11): 75–83, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10069

Performance enhancement and compensation of carrier frequency offset using CRA‐AA for uplink in SDMA system

AbstractRecently, the Space Division Multiple Access (SDMA) scheme has been proposed to improve channel utilization efficiency. SDMA requires separation and detection schemes in the base station because serious interference develops among all of the users accommodated by the same channel. In this paper, we propose the Channel Response Assisted Adaptive Array (CRA‐AA) that estimates the channel response in the uplink and uses this result to indirectly control the directivity, and compare its performance to those of MMSE adaptive array and joint detection. We also study a scheme to compensate for the carrier frequency offsets generated at each user. We found that CRA‐AA had superior performance in the presence of both fast fading and carrier frequency offsets. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(5): 81–91, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.1164

More realistic analysis for blocking probability in SDMA systems

The proliferating demands for mobile services propel the development of increasingly sophisticated techniques to overcome wireless radio resource limitations. With the introduction of cell splitting and sectorisation in cellular communications systems, system capacity can be significantly increased. Space division multiple access (SDMA) technology along these lines has therefore gained a great deal of attention recently; intensive research on resource allocation for SDMA systems has been undertaken in the last few years. In the paper, the author carries out a realistic performance analysis for some resource allocation schemes in SDMA systems and obtains more accurate analytical results for blocking probability.

Application of a Recurrent Neural Network to Space Deversity in SDMA and CDMA Mobile Communication Systems

Linear and non-linear adaptive algorithms are investigated for Space Division Multiple Access (SDMA). SDMA is one of the emerging techniques for multiple access of users in mobile radio, which uses spatial distribution of users for their differentiation. The performance of the linear Square Root Kalman (SRK) algorithm for SDMA is compared to that of the non-linear Recurrent Neural Network (RNN) technique. The proposed SDMA-RNN technique is evaluated over Rician fading channels, and it shows improved Bit Error Rate (BER) performance in comparison with the linear SRK-based technique. The performance of SDMA-RNN is also compared with that of Code Division Multiple Access (CDMA) systems, showing that it could be used as a viable alternative scheme for multiple access of users. Finally, a Hybrid CDMA-SDMA system is proposed combining CDMA and SDMA-RNN systems. Hybrid CDMA-SDMA exhibits a very good potential for increase in the capacity and the performance of mobile communications systems.

Theoretical model to determine the blocking probability for SDMA systems

Antenna array technology has attracted the attention of the research community as a means to increase system capacity and improve the signal reception. Space division multiple access (SDMA) is a multi-access scheme based on the use of antenna arrays to separate users by exploiting their positions in space. Several works have been carried out to examine the improvement in the system capacity provided by SDMA. A theoretical model to determine the blocking probability for SDMA is derived. A closed-form linear system of equations is obtained whose numerical solution gives the blocking probability. The formulation is employed to assess the capacity gain improvement of a single-cell system under specific conditions. It is found from the results that SDMA is not efficient for low traffic loads, whereas it is so for high traffic.

Blind multiuser detection in space division multiple access systems

Cet article presente et propose des techniques de detection aveugle multiutilisateur pour les systemes d’acces multiple par repartition dans l’espace. Dans ce cas les utilisateurs peuvent etre separes a partir de leur position dans l’espace, a l’aide d’une antenne en reseau adaptative. L’algorithme du module constant (CMA), employe dans l’adaptation des coefficients de ponderation, est combine avec un concept de decorrelation explicite, dans le but de recuperer tous les utilisateurs. Pour ameliorer la vitesse de convergence et l’erreur residuelle en regime permanent, deux nouvelles approches sont introduites. Une premiere contribution propose une version en moindres carres de la technique CMA pour la detection multiutilisateur. Une methode adaptative est ensuite proposee pour ameliorer la decorrelation explicite. Des resultats de simulation illustrent les gains de performances.

Information Capacity of the Space Division Multiple Access Mobile Communication System

An expression for the Shannon capacity lower bound of the space division multiple access (SDMA) mobile communication channel with interference is shown. The bound is tightly approaching the Shannon capacity of the orthogonal SDMA system over Gaussian channel with no interference. The information capacity of the SDMA system with adaptive MMSE receivers is presented as well. The SDMA system with MMSE receivers achieves at least 50% of the orthogonal SDMA system capacity for the input signal to noise ratio of practical interest in cellular mobile communications (10–30dB).

Experimental and theoretical capacity analysis of space-division multiple access (SDMA) with adaptive antennas

The paper focuses on TDMA air interface techniques combined with spatial multiplexing through the space division multiple access (SDMA) method. First, results from field trials performed for a variety of operational scenarios provide experimental proof of the SDMA concept for two users. Then, the capacity of the space and time division multiple access scheme is studied with the help of simulations, but from the more general and practical point of view of the produced/achieved radiation-pattern characteristics. Up to now, the analyses for the capacity of SDMA systems have ignored the critical effects that parameters, such as the scattering of the environment, the mutual coupling among the array elements or signal mismatches, have on the produced radiation pattern. The straightforward method employed in the analysis presented here takes into account these effects and hence produces a more realistic capacity assessment. The presented results for different radiation pattern beamwidths and average sidelobe levels, different frequency reuse distances and additional interference cancellation techniques highlight the trade-offs between different performance criteria and the implications to the overall system design.

Smart antennas in wireless systems: uplink multiuser blind channel and sequence detection

Space-division multiple-access (SDMA) schemes have been proposed to increase the capacity of wireless communication systems by simultaneously transmitting and receiving multiple co-channel signals through different spatial channels. We address the uplink (remote users to the base station antenna array) blind channel and sequence identification problem for an SDMA system. We show that multiuser blind identification can be accomplished by exploiting the spatial and temporal diversities of an antenna array system. In particular, a recursive estimation algorithm is developed to recover multiple signals from intersymbol interference (ISI) and co-channel interference (CCI) by taking advantage of a special structure of the array output and the finite-alphabet property of digital communication signals. The implementation of the proposed approach in practical applications is discussed, and field experiments have been conducted to demonstrate the effectiveness of the proposed algorithm.