Division Multiple Access Research Articles

Effect of antenna array on the uplink orthogonal multicarrier DS‐CDMA system with imperfect power control

AbstractIn this paper we analyze the effects of antenna array (AA) and imperfect power control on the performance of the uplink synchronous and/or asynchronous orthogonal multicarrier (MC) direct sequence‐code division multiple access (DS‐CDMA) system in multipath Rayleigh fading channels. Bit error rate (BER) performance is evaluated in terms of the number of antennas, the number of total subcarriers, power control error (PCE), and the number of users. Our numerical results show that the available user capacity of synchronous uplink is more than 1.5 times higher than that of asynchronous uplink, even though PCE increases. Copyright © 2009 John Wiley & Sons, Ltd.

Linear-Dispersion Division Multiple-Access for MIMO systems

Comprehensive study on novel Linear-Dispersion Division Multiple-Access (LDDMA) for multi-user uplink Multiple-Input Multiple-Output (MIMO) systems is proposed. In the new multiplexing scheme, each user’s information symbol is dispersed by a User-Specific Matrix (USM) both in space and time domain and linearly combined at base-station side. And a simple random search algorithm, based on capacity maximization criteria, is developed to generate a bank of USMs. Simulation results are presented to demonstrate the advantages of LDDMA. When the Bit Error Rate (BER) reaches 10−3, the performance gains are 3dB and 5dB, compared with Time-Division Linear Dispersion Codes (TD-LDC) and BLAST, respectively.

Local search multiuser detection

In this work, a wide analysis of local search multiuser detection (LS-M UD) for direct sequence/code division multiple access (DS/CDMA) systems under multipath channels is carried out considering the performance-complexity trade-off. It is verified the robustness of the LS-M UD to variations in loading, E b / N 0 , near–far effect, number of fingers of the Rake receiver and errors in the channel coefficients estimates. A compared analysis of the bit error rate (BER) and complexity trade-off is accomplished among LS, genetic algorithm (GA) and particle swarm optimization (PSO). Based on the deterministic behavior of the LS algorithm, it is also proposed simplifications over the cost function calculation, obtaining more efficient algorithms (simplified and combined LS-M UD versions) and creating new perspectives for the M UD implementation. The computational complexity is expressed in terms of the number of operations in order to converge. Our conclusion pointed out that the simplified LS (s-LS) method is always more efficient, independent of the system conditions, achieving a better performance with a lower complexity than the others heuristics detectors. Associated to this, the deterministic strategy and absence of input parameters made the s-LS algorithm the most appropriate for the M UD problem.

A Capacity Analysis Method for Uplinks in DS/CDMA Cellular Systems with Imperfect SIR-Based Power Control and Multipath Fading

The letter proposes a method to analyze the system capacity of uplinks with imperfect SIR-based power control in direct sequence/code division multiple access (DS/CDMA) cellular systems with the multipath fading. It considers the received signal-to-interference ratio (SIR) as a random variable due to imperfect SIR-based power control. Based on assumptions regarding the average received SIR, a set of linear equations for the average received power on each uplink is derived. An outage-based system capacity can be then obtained according to the feasibility of the average received power vector and the average bit error rate. Finally, by applying central limit theorem, a closed-form solution for system capacity is approximately derived. Results show that numerical results of the closed-form solution have a good match with simulation results, which supports the validity of the proposed capacity analysis method.

Asynchronous cooperative transmission mechanism using distributed Orthogonal Frequency Division Multiplexing for mobile ad hoc networks

In this article, we propose an asynchronous cooperative transmission mechanism for Mobile ad hoc Networks (MANETs). The proposed cooperative transmission mechanism employs distributed Unitary Space-Frequency Coded-Orthogonal Frequency Division Multiplexing (USFC-OFDM), which does not require any channel estimation and has a lower decoding complexity, and therefore can provide better reliability and robustness when mobile nodes move fast. Moreover, it can achieve a larger diversity gain and multiplexing gain, and can thus combat fast time-varying and frequency-selective channel fading, and tolerate imperfect synchronisation among mobile nodes. To support USFC-OFDM, a Time Division Multiple Access-based Cooperative Medium Access Control protocol and an on-demand Cooperative Dynamic Source Routing protocol are also proposed. Through simulation results, we show that the proposed USFC-OFDM-based cooperative transmission mechanism can significantly improve the network performance in terms of packet delivery ratio and bit error rate in highly dynamic mobile environments.

On transmission techniques for multi‐antenna W‐CDMA systems

AbstractIn this paper we present and evaluate a new pre‐processing scheme for Multiple Input Multiple Output (MIMO) channels. Its performance is compared with other MIMO schemes proposed for the downlink of Wideband‐Code Division Multiple Access (W‐CDMA) systems, namely a variation of the Beam Forming (BF), entitled Beam‐Selective Transmit Diversity (BSTD) or post‐processing schemes, such as the Alamouti‐like MIMO or the Vertical Bell Laboratories Layered Space‐Time (V‐BLAST) MIMO scheme. It is assumed that the Base Station (BS) has M ⩾ 2 transmit antennas and the Mobile Station (MS) receiver has space enough to accommodate N = 2 uncorrelated receive antennas.It is shown that the proposed pre‐processing scheme allows receivers with very low complexity, contrarily to the case where a post‐processing approach is followed, simplifying the MS receiver. Therefore, the proposed M × N MIMO pre‐processing scheme can be seen as an alternative to post‐processing schemes. It is also shown that the pre‐processing scheme for MIMO channels tends to achieve the best performance in most scenarios, being followed by the BSTD, which allows interference cancellation and provides diversity. Copyright © 2007 John Wiley & Sons, Ltd.

Computer simulation of Rayleigh fading channels in MC-CDMA systems

For Multi-Carrier-Code Division Multiple Access (MC-CDMA) systems, it is usually assumed that the fading of the subcarriers is frequency non-selective and independent of each other. This paper shows that the two assumptions are incompatible. In fact, the MC-CDMA signals at each subcarrier undergo fading that are highly correlated. Based on this observation, this paper develops a simulation algorithm for Rayleigh fading channels via frequency-domain correlation function, which incorporates the Doppler effect simultaneously. Numerical examples are presented to demonstrate the effectiveness of the new algorithm, with the conclusion that the independence assumption of subcarrier fading overrates the system performance.

Performance Analysis of Multiuser Detectors for Uplink Quasi-synchronous MC-CDMA Systems

This paper analyses and compares the average bit error rate (BER) of different multiuser detectors (MUD) in the uplink of a multicarrier code- division multiple access (MC-CDMA) system. In particular, maximum likelihood, zero-forcing, minimum mean-square error and interference cancellation-based multiuser detectors have been analysed for the special case of uncorrelated subcarriers. The derived BER expressions are based upon previous results on diversity combining and also on recent findings on multiple input multiple output (MIMO) architectures. The subcarrier correlation is considered in the context of physical parameters currently under discussion for future wireless systems to give an indication up to what extent the assumption of uncorrelated subcarrier fading is plausible.

Performance of wavelet based Multi‐Chip Rate DS‐CDMA signals over multi‐path Nakagami‐m fading channels

AbstractMulti‐Chip Rate/Direct Sequence‐Code Division Multiple Access (MCR/DS‐CDMA) technique using scaled chip waveforms has been designed as an alternative to multi‐data rate DS‐CDMA techniques having constant chip rates. In this work, the probability of error (Pe) expression for MCR/DS‐CDMA signals is derived over multi‐path Nakagami‐m fading channels to investigate the effects of chip waveforms on it. This paper also proposes the use of orthogonal wavelets as chip waveforms of MCR/DS‐CDMA signals over the considered channel. For numerical calculations, Daubechies‐22 (D22) wavelet is used because its side lobes are 40 dB below its main lobe in frequency domain. D22 is compared with a Square Root Raised Cosine (SRRC) chip waveform. In the numerical calculations, only first four scales of the chip waveforms relating to four different chip/data rates are considered. The results for the Pe performance and the capacity (the number of user per Hertz for a same Pe level) show that D22 significantly outperforms the SRRC chip waveform at all data rates, due to low cross correlations among different scales of orthogonal wavelets. Besides, by increasing the number of scales, the advantage of the use of orthogonal wavelets will increase furthermore. Copyright © 2007 John Wiley & Sons, Ltd.

Performance of optimum wavelet for DS‐CDMA chip waveform over multi‐path Rayleigh fading channels

AbstractIn this paper, the probability of error (Pe) expression of asynchronous direct sequence‐code division multiple access (DS‐CDMA) signals using band‐limited chip waveforms is derived over multi‐path Rayleigh fading channels. In receiver, a matched filter‐based rake receiver in conjunction with maximum ratio combiner (MRC) is considered. Numerical values for the Pe are calculated for various chip waveforms including an optimum wavelet waveform. Analytical results are verified by conducting simulations. Results show that the optimum wavelet‐based scheme outperforms time‐limited raised cosine, half sine, rectangular and band‐limited square‐root raised cosine chip waveform‐based schemes in terms of the Pe and the capacity defined as the number of users per Hertz for a same Pe level. Copyright © 2006 John Wiley & Sons, Ltd.

Bit error rate performance of Haar wavelet based scale‐code division multiple access (HW/S‐CDMA) over the asynchronous AWGN channel

AbstractIn this paper, we study a recently proposed multirate system, called wavelet based scale‐code division multiple access (W/S‐CDMA). W/S‐CDMA depends on the code, time and scale orthogonality introduced by pseudo‐noise (PN) sequences, and wavelets. In this system, the channel is partitioned into different scales, and each scale into time slots. In addition, the PN sequences are used in each scale to identify multiple users. In W/S‐CDMA, each user is assigned a specific scale and PN sequence, and transmits its successive information symbols with its PN sequence and the wavelets in that scale. More symbols are transmitted in finer scales. We analyse the bit error rate performance of Haar wavelet based S‐CDMA (HW/S‐CDMA) over an asynchronous additive white Gaussian noise (AWGN) channel by using a conventional detector for deterministic PN sequences. The performance of the system is compared to that of an equivalent multirate CDMA (MR‐CDMA) system for Gold and Kasami PN sequences. Results show that HW/S‐CDMA outperforms MR‐CDMA. In addition, because of its suitable format HW/S‐CDMA is also capable of employing the optimal PN sequence families with limited number of sequences such as Kasami, Bent, etc. repeatedly in different scales. Copyright © 2006 John Wiley & Sons, Ltd.

High-performance RF signals transmission in SCM/WDMA radio-on-fiber bus link using optical FM method in presence of optical beat interference

In the near future, various wireless services such as a wireless local area network and an intelligent home appliance will be widely used at home or office. To connect among many radio base stations such as a home or office network, a subcarrier multiplexing/wavelength division multiple access (SCM/WDMA) radio-on-fiber (RoF) bus link is excellent in simplicity and flexibility. However, a degradation of RF transmission performance due to optical beat interference (OBI) is a severe problem. This paper proposes the reduction method of OBI using an optical frequency modulation (OFM) method in the SCM/WDMA RoF bus and theoretically analyzes the availability of this proposed method. Using the OFM method, the power spectral density of the OBI can be reduced and the improvement of carrier power to noise power ratio before FM demodulation using the proposed method is obtained at approximately 4 and 9 dB compared to the intensity modulation/direct detection method when FM index /spl beta//sub FM/ is 1 and 10, respectively.

Algoritimos Heurísticos Aplicados à detecção Multiusuário DS/CDMA

The characteristics of nine heuristic algorithms based on genetic evolution theory and local search, applied to a DS/CDMA (Direct Sequence/Code Division Multiple Access) multi-user detection problem in Flat Rayleigh fading channel are introduced. The algorithms comparisons through the performance versus computational complexity tradeoff are carried out. The computational complexity is determined in terms of the number of operations to reach the Maximum Likelihood (ML) performance. The effects on the performance of the errors in the estimates of the channel coefficients are also considered, as well as the performance degradation due to overload. There are three contributions in this work: a) a compared analysis of the evolutionary, evolutionary with cloning and local search algorithms applied to MuD detector in Flat Rayleigh channels; b) a systematic comparison of these algorithms in terms of the same simulation scenario and performance analysis base; c) a more fair and precise complexity analysis for the MuD detector in terms of the number of the computational operations.

MMSE multiuser detection for array multicarrier DS-CDMA in fading channels

Reception of asynchronous, multicarrier direct-sequence-code division multiple access (DS-CDMA) in time-varying, multipath radio channels with use of a receiving antenna array is investigated. Interference reducing minimum mean squared error (MMSE) receivers are discussed, and by considering the time-variation of the channel, a modified structure is derived which is efficient for channels experiencing small-scale fading. A blind implementation of this receiver is then proposed. Subspace concepts are applied to formulate a tracking, composite channel vector estimator which operates effectively in fading situations, even when high levels of interference are present. Both the modified MMSE weight matrix and diversity combining weights are generated from these channel estimates. Simulations of the proposed receiver show it to have superior performance over a standard MMSE receiver which is periodically re-evaluated to permit it to follow the channel variations due to small-scale fading. Furthermore, a hybrid MMSE receiver is proposed which applies different processing methods depending on each transmitters mobility, resulting in improved performance.

A Blind Adaptive Technique for Cancelling ICI in OFDM-WPAN Systems

This paper proposes a hybrid OFDM/TDMA (Orthogonal Frequency Division Multiplexing/Time Division Multiple Access) scheduling method to obtain fine bit-rate granularity in wireless personal area networks (WPANs). A new blind adaptive receiver structure that combats the effects of inter-carrier interference (ICI) caused by users’ frequency offsets is also proposed to improve the system throughput. The proposed blind adaptive receiver is based on constant modulus (CM) criterion and requires no more knowledge of the frequency offsets of interferers’ signals than the conventional receiver. The advantage of our countermeasure is that frequency-guard or redundant data is not needed, thus the throughput performance will not degrade, and estimation of the frequency offset is also no longer necessary. In order to make the adaptive receiver realizable, we develop a soft-constraint-satisfaction (SCS) based algorithm to achieve faster convergence. Simulation results show that our proposal is capable of significantly reducing the influence of ICI.

Code-aided blind detection of the transmission rate for multirate direct-sequence CDMA systems

The problem of rate detection for a multirate direct sequence/code division multiple access (DS/CDMA) systems is considered in this paper. It is assumed that, in each data-frame, each user may transmit its data at one out of a set of available data-rates, depending, e.g., on the kind of data to be transmitted, on the desired quality-of-service, and on the channel state. Relying on the knowledge of the spreading codes of the user whose rate is to be detected, rate detection algorithms based on the application of the generalized likelihood ratio test are here proposed. Both the cases where either the rates of the interfering users have been already detected and are known to the receiver or these rates are not known to the receiver are separately considered. With regard to the performance assessment, the effectiveness of the proposed rate detection strategies is validated, for a single-path fading channel, through both computer simulations and, with regard to the case that the interfering users rates have already been acquired, through a Chernoff-bounding technique.

Schemes for capacity increase in multiple‐rate dual‐class DS/CDMA systems

AbstractThis paper presents and compares the capacities of multi‐cell and power constrained direct sequence‐code division multiple access (DS/CDMA) systems with different transmission or detection schemes. In the single‐class case (voice), we considered conventional and successive interference cancellation (SIC) schemes, determining the maximum number of users of each mode. Next, modeling and results proposed in the literature are extended for multi‐cell dual‐class systems (voice and data) and we compare the maximum data throughputs obtained with conventional, SIC and scheduling (transmission) schemes, for some user configurations. Copyright © 2005 John Wiley & Sons, Ltd.

An improved formula obtained by simulated annealing for the computation of maximum SINR for DS‐CDMA receiver with exponentially weighted despreading function

AbstractIt is well known that direct sequence‐code division multiple access (DS‐CDMA) receiver using exponentially weighted despreading function outperforms significantly receiver using rectangular despreading function. The maximum value of the signal‐to‐interference‐plus‐noise ratio (SINR) for this type of receiver is an important indicator to evaluate the bit error rate (BER) performance of the system. In this paper, we propose an improved formula obtained by using simulated annealing (SA) algorithm to calculate the maximum value of the SINR. It is shown that the results computed by the proposed formula are more accurate than the results obtained by the simplified formula available in the literature. Copyright © 2005 AEIT.

Iterative PIC detection and channel estimation for DS‐CDMA communications in AWGN and satellite/HAPS channels

AbstractThis paper presents a turbo multiuser detector for turbo‐coded DS‐code division multiple access (CDMA) systems, based on the utilisation of a parallel interference cancellator (PIC) and a bank of turbo decoders: in the proposed receiver the interference cancellation is performed after each constituent decoder of the turbo decoding scheme. Moreover, the soft outputs of the turbo decoders are used iteratively to improve the channel parameter estimation during the updating step, which is formally equivalent to one step of the expectation‐maximisation algorithm. By means of computer simulations, we will show that the proposed receiver achieves performance comparable with the one obtained by systems, which suppose perfect knowledge of the channel parameters for medium to high system loads, in AWGN channel. Finally, the proposed receiver is also applied in HAPS (high altitude platform station) and satellite channel, for different values of the elevation angle, affording remarkable performance. Copyright © 2005 AEIT.

Dual detection for optical code division multiple access communication

A dual optical detection scheme is proposed for optical code division multiple-access systems utilizing on-off-keying modulation technique. The proposed detector is developed by combining a correlation detector and a chip-level detector with the logical OR operation. In this way, the merits of two detectors can be fully exploited. Thus the dual detector can achieve the enhanced bit error rate performance in the presence of both multi-user interference and noise.