Multicarrier Direct-sequence Code-division Multiple-access System Research Articles

Performance of a novel evolutionary genetic-based multi-user detector for multi-carrier CDMA communication systems

In this paper, we investigate the performance of a modified evolutionary multi-user detector (MUD) in multi-carrier direct-sequence code-division multiple-access (MC-CDMA) communication systems over frequency-selective fading channels. The genetic algorithm (GA), which tries to reduce the complexity in optimal detection due to greedy exhausted searching, performs using the adequate sub-optimal multi-user detector for MC-CDMA systems. In this paper, we propose a forcing mutation scheme for the genetic algorithm (GA) to perform a forcing mutation evolutionary multi-user detector (FME-MUD) and to largely reduce computational complexity. Simulation results show that with six generations, the proposed FME-MUD scheme outperforms the conventional GA-MUD, especially at heavy system loads.

Soft Decision Error Assisted Layered Multiuser Detectors for MIMO 2D Spread MC DS-CDMAs

In this paper, we present two layered multiuser detectors (MUDs) for MIMO frequency-time-domain (FT-domain) multi-carrier (MC) direct sequence code division multiple access (DS-CDMA) systems with an antenna array at the base station. We assume that multiple users are active and individually utilize multiple transmit antennas in the MC DS-CDMA system. The users are organized into groups, and each user is assigned a unique Time-domain (T-domain) signature code. Moreover, users in the same group share a unique F-domain signature code. As a result, they can exploit the T-domain and F-domain signature codes to spread their multiple symbols in parallel, and then transmit the FT-domain spread signals from the corresponding multiple antennas over the fading channels to the base station. However, because of the non-ideal channel effect and/or the use of non-orthogonal signature codes, the FT-domain spread MC DS-CDMA system is affected by multiple access interference (MAI) in the same way as CDMA-like systems. To mitigate the effects of MAI and improve the system's performance, we propose two layered MUDs that exploit the layered soft decision errors. Specifically, in a trade-off between the performance and the computational complexity, only the soft decision errors of one user/one group are used in the proposed layered MUDs. The results of simulations and a complexity analysis demonstrate that the proposed layered MUDs outperform existing approaches and the computational complexity is modest.

One-User/One-Group Soft-Decision Aided Multiuser Detection for 2D Spread MC DS-CDMAs

We consider the uplink of a multiuser, multiple-input multiple-output (MIMO), frequency-time-domain spread, multi- carrier (MC), direct sequence code division multiple access (DS-CDMA) system. As other CDMA-like systems, the multiple access interference (MAI) effect still exists in such an MC DS-CDMA system. To mitigate the MAI effect, we propose user-based and group-based layered detection schemes. Specifically, to enable a trade-off between the performance and the computational complexity, the schemes only use one user's/group's soft decisions for user-based/ group-based layered detection. The results of simulations demonstrate that the proposed schemes outperform existing approaches, and their computational complexity is modest.

One-user/one-group soft-decision aided layered detection for multiuser MIMO 2D spread multi-carrier DS-CDMA systems

We consider the uplink of a multiuser, multiple input multiple output (MIMO), frequency-time-domain spread, multi-carrier (MC), direct sequence code division multiple access (DS-CDMA) system. In this two-domain (2D) spread system, users are arranged in groups. Each user is assigned a unique time-domain (T-domain) signature code and also shares a frequency-domain (F-domain) signature code with users in the same group. Moreover, each user employs his/her individual T-domain signature code and the shared F-domain signature code to spread multiple symbols, and then sends the spreading signals in parallel from multiple transmit antennas to the antenna array at the base station. However, the multiple access interference (MAI) effect, which results from the non-orthogonality of signature codes and is the main performance limitation, still exists in such an MC DS-CDMA system. To mitigate the MAI effect and improve the performance, we propose user-based and group-based layered detection schemes. Specifically, to enable a trade-off between the performance and the computational complexity, the schemes only use one user's/group's soft decisions for user-based/group-based layered detection. The results of simulations demonstrate that the proposed schemes outperform existing approaches, and their computational complexity is modest.

Successive beamforming, multiuser detection and diversity reception for multicarrier DS-CDMA system with antenna array

Code-Division Multiple-Access (CDMA) systems are interference limited, and therefore efficient interference management is necessary to enhance the performance of a CDMA system. In this paper, a successive beamforming (spatial filtering), linear decorrelating MultiUser Detection (MUD, temporal filtering) and diversity reception structure for uplink multicarrier Direct Sequence CDMA (DS-CDMA) system with antenna array are proposed. By beamforming, the antenna array suppresses interference according to the distinct array signature. Subsequently, linear decorrelating MUD is applied to separate the signals of different users and eliminate Multiple Access Interference (MAI). Finally, the decorrelated signals at different subcarriers that belong to the same user are combined to achieve frequency diversity. Simulation results show that the proposed structure offers significant Bit Error Rate (BER) performance improvement by successively exploiting the space-time-frequency processing.

BER-Optimal Linear Parallel Interference Cancellation for Multicarrier DS-CDMA in Rayleigh Fading

In this paper, we consider the design and bit-error performance analysis of linear parallel interference cancellers (LPIC) for multicarrier (MC) direct-sequence code division multiple access (DS-CDMA) systems. We propose an LPIC scheme where we estimate and cancel the multiple access interference (MAI) based on the soft decision outputs on individual subcarriers, and the interference cancelled outputs on different subcarriers are combined to form the final decision statistic. We scale the MAI estimate on individual subcarriers by a weight before cancellation. In order to choose these weights optimally, we derive exact closed-form expressions for the bit-error rate (BER) at the output of different stages of the LPIC, which we minimize to obtain the optimum weights for the different stages. In addition, using an alternate approach involving the characteristic function of the decision variable, we derive BER expressions for the weighted LPIC scheme, matched filter (MF) detector, decorrelating detector, and minimum mean square error (MMSE) detector for the considered multicarrier DS-CDMA system. We show that the proposed BER-optimized weighted LPIC scheme performs better than the MF detector and the conventional LPIC scheme (where the weights are taken to be unity), and close to the decorrelating and MMSE detectors.

Capacity improvement of multicarrier DS-CDMA system with antenna array

An antenna array based base station receiver for multicarrier Direct Sequence Code Division Multiple Access (DS-CDMA) system is proposed. The main advantage of the receiver is that the spatial diversity is achieved by combining signals of array elements. Based on the detailed analysis of multiuser interference and noise characteristics, the performance of the proposed receiver is analyzed. Theoretical analysis shows significant performance improvement in terms of system capacity due to the use of antenna arrays compared with the conventional single antenna multicarrier DS-CDMA approach. Simulation results confirm the theoretical analysis.

Performance of Adaptive Multistage Fuzzy-Based Partial Parallel Interference Canceller for Multi-Carrier CDMA Systems

In this paper, we propose an adaptive multistage fuzzy-based partial parallel interference cancellation (FB-PPIC) multiuser detector for multi-carrier direct-sequence code-division multiple-access (MC-CDMA) communication systems over frequency selective fading channels. The partial cancellation tries to reduce the cancellation error in parallel interference cancellation (PIC) schemes due to the wrong interference estimations in the early stages and thus outperforms the conventional PIC (CPIC) under the heavy load for MC-CDMA systems. Therefore, in this paper, the adaptive cancellation weights are inferred from a proposed multistage fuzzy inference system (FIS) to perform effective PPIC multiuser detection under time-varying frequency selective fading channels in MC-CDMA systems. Simulation results show that the proposed adaptive fourstage FB-PPIC scheme outperforms both CPIC and constant weight PPIC (CW-PPIC) schemes, especially in near-far environments.

Performance Analysis of Parallel Interference Cancellation in Multicarrier DS-CDMA Systems

In this paper, we present and analyze the performance of a parallel interference cancellation (PIC) scheme for multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) systems. At each cancellation stage in the proposed PIC scheme, on each subcarrier, a weighted sum of the soft outputs of the other users in the current stage is cancelled from the soft output of the desired user (rather than making hard bit decisions of the other users and regenerating and canceling the interfering signals) to form the input to the next stage. At the last stage, the interference cancelled outputs from all the subcarriers are maximal ratio combined (MRC) to form the decision statistic. The scheme has the advantage of not requiring the amplitude estimates of the other users. We derive analytical expressions for the bit error rate (BER) at different stages in the proposed PIC scheme on Rayleigh fading channels. Analytical results are found to agree well with the simulation results. The results show that the proposed PIC scheme offers better interference suppression capability than the conventional matched filter (MF) receiver. We also obtain bounds on the coded BER of the proposed PIC scheme for a convolutionally coded multicarrier DS-CDMA system where the PIC-MRC output feeds a soft decision Viterbi decoder.

Combined Power Control and Error-Control Coding in Multicarrier DS-CDMA Systems

We propose truncating the transmission power (allocating no power) for symbols with low channel gain, and tagging erasures on the corresponding symbols at the receiver. The motivation is that symbols with low channel gain are highly likely to be in error and yet, if transmitted, consume the energy resource and generate interference to other users. Truncating the power for those symbols has the effect of reducing the interference to other users and allocating more power on symbols with high channel gain (thereby reducing the error probability). Since block codes can correct twice as many erasures as errors, the coded performance can be improved by properly combining the power control with the error-control coding. In this letter, we analyze the performance of the Reed-Solomon-coded multicarrier direct-sequence code-division multiple-access systems with two power-control schemes. We show that the probability of incorrect decoding can be significantly improved by properly combining the power control with the error control coding.

A fast maximum likelihood sequence decoding method for multicarrier DS-CDMA using frequency spread coding

This paper focuses on a multicarrier direct-sequence code-division multiple-access system using frequency spread coding, which was proposed by Matsutani and Nakagawa in 1999. Although this system is attractive due to its inherent frequency diversity gain without time or frequency redundancy, it is disadvantaged by its exponential complexity in the maximum likelihood sequence decoding (MLSD). To cope with such a bottleneck, this paper proposes a fast MLSD method which consists of a searching process along the eigenvectors of a symmetric matrix and a refining process. The fast MLSD method is compared with the normal MLSD method via theoretical analysis and simulation results.

The Effect of Timing Jitter on MC-DS-CDMA

In this paper, we consider the effect of timing jitter on the performance of a multicarrier direct-sequence code-division multiple-access system for both uplink and downlink transmission, assuming orthogonal spreading sequences. Theoretical expressions are derived for the performance degradation caused by the timing jitter, in the presence of a multipath channel. Assuming an additive white Gaussian noise channel, perfect power control, and full load, it is shown that the performance degradation for the downlink transmission is independent of the number of subcarriers, of the spreading factor, and of the spectral contents of the jitter at the receiving mobile station, but only depends on the jitter variance. Under the same assumptions, we point out that, if the jitter spectra of all transmitting mobile stations are the same, the degradation on the uplink is the same as the degradation on the downlink.

Serial acquisition performance of single-carrier and multicarrier DS-CDMA over Nakagami-m fading channels

We investigate the issue of pseudo noise (PN) code acquisition in single-carrier and multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) systems, when the channel is modeled by frequency-selective Nakagami-m (1960) fading. The PN code acquisition performance of single-carrier and MC DS-CDMA systems is analyzed and compared when communicating over Nakagami-m fading channels under the hypothesis of multiple synchronous states (H/sub 1/ cells) in the uncertainty region of the PN code. In the context of MC DS-CDMA, the code acquisition performance is evaluated, when the correlator outputs of the subcarriers associated with the same phase of the local PN code replica are noncoherently combined by using equal gain combining (EGC) or selection combining (SC) schemes. The performance comparison of the above mentioned schemes shows that the code acquisition performance of the MC DS-CDMA scheme, especially when using the EGC scheme, is more robust, than that of single-carrier DS-CDMA schemes communicating over the multipath Nakagami-m fading channels encountered. However, our code acquisition performance comparison also shows that if the detection threshold was set inappropriately, the performance might be degraded, even if the channel fading becomes less severe.

MC DS/SFH-CDMA systems for overlay systems

We introduce the slow frequency hopping (SFH) technique to the multicarrier (MC) code-division multiple-access (CDMA) systems for overlay situations. Using lower chip rate, which results in a narrower spectrum for each carrier and hopping the signal in frequency, the MC direct sequence (DS)/SFH system achieves better performance than the MC DS CDMA system in most cases, especially when the bandwidth of the narrowband interference (NBI) is narrower than one subchannel. It also exhibits a stable performance against the variations of the NBI location and bandwidth. When there is no NBI, the two systems perform approximately the same. The evaluation is performed over a frequency selective Rayleigh fading channel, with both Gaussian approximation and Monte Carlo simulation.

Blind adaptive receiver for coded multicarrier DS-CDMA systems in multipath fading channels

The authors present a multicarrier asynchronous direct sequence code division multiple access (DS-CDMA) system which uses an adaptive blind algorithm for the optimum combining of the desired signal contributions from different carriers. The output of the transmitter convolutional encoder modulates multiple band-limited DS-CDMA waveforms which are parallel transmitted with different carrier frequencies. The performance of this system is compared to that of a conventional single-carrier DS-CDMA system with a RAKE receiver and a RAKE receiver with a sliding window decorrelator filter. In this latter case, the RAKE receiver uses pilot symbols to track the channel impulse response time variations. Results demonstrate a performance similar to that of a decorrelating detector in the context of a severe near–far situation while exhibiting lower complexity.

Performance analysis of single carrier and multicarrier DS-CDMA systems over generalized fading channels

Using a recently developed moment generating function-based approach for the performance evaluation of digital communications over fading channels, we present a unified approach for the exact performance analysis of binary direct-sequence code division multiple access (DS-CDMA) systems operating over generalized frequency-selective fading channels. The results are applicable to single carrier systems employing RAKE reception as well as to multicarrier DS-CDMA systems with frequency diversity. Aside from simplifying previous results both analytically and computationally, the proposed approach also gives a solution for many situations which heretofore defied a simple form. Copyright © 2001 John Wiley & Sons, Ltd.

Analysis of a multicarrier DS-CDMA code-acquisition system

We present two code-acquisition schemes for a multicarrier direct-sequence code-division multiple-access system, one that uses equal gain combining and the other that uses selection combining. The code-acquisition performance of the two multicarrier systems, as well as that of a single-carrier system, are analyzed in both nonfading and Rayleigh fading channels under the assumption that the receiver is chip-synchronized; the effect of partial-band interference (PBI) on the performance is also included. It is demonstrated that in an additive white Gaussian noise channel, the single-carrier system has a better code-acquisition performance than both multicarrier systems. However, in a Rayleigh fading channel, the code-acquisition performance of a multicarrier system with equal gain combining is better than that of the single-carrier system, while a multicarrier system with selection combining has the same performance as the single-carrier system. Further, the presence of PBI more severely affects the code-acquisition performance of the single-carrier system than those of both multicarrier systems. Finally, the code-acquisition performance of a multicarrier system with equal gain combining is always better than that of the selection combining system.

Convolutionally coded multicarrier DS-CDMA systems in a multipath fading channel. I. Performance analysis

This paper presents a multicarrier asynchronous direct-sequence code-division multiple-access (DS-CDMA) system wherein the output of a convolutional encoder modulates multiple band-limited DS-CDMA waveforms, which are transmitted in parallel at different carrier frequencies. The receiver detects and combines signals for the desired user and feeds a soft-decision Viterbi decoder. The performance of this system is compared to that of a conventional single-carrier DS-CDMA system with a RAKE receiver, assuming a slowly varying frequency-selective Rayleigh fading channel and assuming the presence of additive white Gaussian noise and multiple-access interference. Results will demonstrate similar performance at roughly equal receiver complexity.

Performance of a Modified Multicarrier Direct Sequence CDMA System

In this paper, we present an improved multicarrier direct sequence (DS) code division multiple access (CDMA) scheme by modifying the system originally proposed by Kondo and Milstein [13]. In this modified system, different spreading sequences multiplied by a data sequence modulate different carriers. This is to prevent the multiple access capability from reducing when the fading characteristics of different carrier frequencies are highly correlated. We have derived a formula which determines the mean values of the relative received signal strength in a single carrier DS CDMA rake system and in a multicarrier DS-CDMA system. We present results on the comparison of the bit error rate (BER) performance of the two systems including the effect of correlation between fading characteristics of different frequencies under various multipath fading conditions. The results indicate that with 50 users the modified multicarrier DS CDMA system can achieve an uncoded irreducible BER of 1.7 × 10−−3 with an average received signal-to-noise ratio per bit of 10 dB, which is better than 3.0 × 10−−3 achieved by the single carrier DS CDMA rake system, and also show that if multicarrier CDMA system is used with respect to single carrier CDMA system, the SNR gain is up to 4.5 dB for the uncode BER of 10−−3 being achieved.