Multicarrier Direct-sequence Code-division Research Articles

Redundant Residue Number System Assisted Multicarrier Direct-Sequence Code-Division Dynamic Multiple Access for Cognitive Radios

A redundant residue number system (RRNS)-assisted multicarrier direct-sequence code-division dynamic multiple-access (MC/DS-CDDMA) scheme is proposed for application in cognitive radios (CRs). Taking the advantages of both multicarrier direct-sequence code-division multiple access and RRNS, the RRNS MC/DS-CDDMA has a low complexity for implementation and a high flexibility for reconfiguration and is robust to dynamic spectrums. Associated with the RRNS MC/DS-CDDMA, in this paper, the so-called receiver multiuser diversity aided multistage minimum mean-square error multiuser detector (RMD/MS-MMSE MUD) is considered for signal detection. Specifically, three types of RMD/MS-MMSE MUDs are proposed: Type-I.1, Type-II.1, and Type-I.2 RMD/MS-MMSE MUDs. In these MUD schemes, the Type-I.1 MUD carries out the joint detection of all the subcarrier signals using both the observations and the channel state information (CSI) of all the subcarriers. In the Type-I.2 and Type-II.1 RMD/MS-MMSE MUDs, the embedded MMSE-MUDs are implemented subcarrier by subcarrier independently. Furthermore, in the Type-I.2 RMD/MS-MMSE MUD, the autocorrelation matrices used by the MMSE-MUDs are free from CSI. Explicitly, both Type-II.1 and Type-I.2 RMD/MS-MMSE MUDs are suitable for operation in dynamic spectrum environments. Furthermore, our simulation results show that the preceding three types of RMD/MS-MMSE MUDs are capable of making the RRNS MC/DS-CDDMA systems achieve similar error and throughput performance. Owing to the foregoing merits, the RRNS MC/DS-CDDMA employing either Type-I.2 or Type-II.1 RMD/MS-MMSE MUD may be considered one of the highly promising dynamic multiple-access schemes for application in CR systems.

Multi-User Resource Allocation for a Distributed Multi-Carrier DS-CDMA Network

This paper addresses an adaptive multi-user resource allocation for a distributed multi-carrier direct sequence-code division multiple access (MC DS-CDMA) network. The packet throughput, which turns out to be nonconvex and nondifferentiable, is considered to measure the system performance. A sub-optimal non-cooperative power control game is proposed to adaptively allocate the transmit power, available subchannels and alphabet size by minimizing the transmit power with a transmit power constraint and a packet throughput requirement. Also, the paper investigates the effect of channel estimation error on the adaptive resource allocation in a distributed MC DS-CDMA network, and shows that using the Gaussian approximation for the signal-dependent noise can lead to a nontrivial discrepancy in system performance.

A family of minimum mean square error multiuser detectors for three-domain spread multi-carrier direct sequence-code division multiple access

This study presents a family of multiuser detectors (MUDs) for the uplink of spatial-frequency-time-domain (SFT-domain) spread, multi-carrier, direct sequence code division multiple access systems. More specifically, the family is comprised of a joint SFT MUD and six separate MUDs. The proposed joint SFT MUD achieves a significant performance gain because it is based on minimum mean square error (MMSE) decision criteria. For practical realisation, the main contribution of the study is that the authors further reduce the computational complexity by decomposing the complete received signal into several low-dimension signal components and implementing the corresponding separate MMSE MUDs to facilitate symbol detection. The authors also analyse the computational complexity and mean square error bound of the proposed MUDs. The results of simulations demonstrate that the joint SFT MMSE MUD outperforms the other proposed schemes; however, it requires the largest amount of computation. Meanwhile, the performance of the separate MMSE MUDs is effective and close to that of the joint SFT MMSE MUD, but the computational overhead is modest.

Performance analysis of multicell multiuser MIMO MC DS/CDMA system with MMSE user-ordered SIC technique

In this paper, we study the performance of co-channel interference (CCI) cancellation technique based on minimum mean square error (MMSE) user-ordered successive interference cancellation (UOSIC) for multi input multi output (MIMO) multicarrier (MC) direct sequence code division multiple access (DS/CDMA) system for multi cell setting. Though MC DS/CDMA possesses several advantages, interference limits the capacity of the system. Mitigating the interference can directly swell the system capacity. In this contribution, we consider K co-channel users arbitrarily distributed in an L cell uplink and downlink communication system with both the base stations (BSs) and the mobile stations (MSs) equipped with two antennas each. Also, we assume that, both the BSs and MSs employ space time block code (STBC) based on the transmission matrix (G2). Our simulation study shows that, MMSE UOSIC provides better achievable bit-error rate (BER) than MMSE maximum likelihood (ML) and MMSE successive interference cancellation (SIC) technique due to optimal ordering among users and iterative interference cancellation (IC).

Throughput improvement of STS-based MC DS-CDMA system with adaptive modulation

The throughput enhancement of space-time spreading (STS)-based multicarrier direct sequence code division multiple access (MC DS-CDMA) system is investigated in this paper. Adaptive modulation is performed to improve the data throughput of the system in Rayleigh fading channel. In this contribution, an analytical approach is proposed to compute a new expression for the bit error rate (BER) performance of the STS-based MC DS-CDMA system for QAM data. The other contribution of the paper is deriving a new analytical expression for the throughput enhancement and the BER performance of the adaptive modulated STS-based MC DS-CDMA system over Rayleigh fading channel. The novel expressions presented in this paper bring out the facility of evaluation of the proposed system with no need for more calculations and simulations.

Multifunctional MIMO systems: A combined diversity and multiplexing design perspective

In this treatise we investigate the design alternatives of different multiple-input multiple-output schemes while considering the attainable diversity gains, multiplexing gains, and beamforming gains. Following a brief classification of different MIMO schemes, where the different MIMO schemes are categorized as diversity techniques, multiplexing schemes, multiple access arrangements, and beamforming techniques, we introduce the family of multifunctional MIMOs. These multifunctional MIMOs are capable of combining the benefits of several MIMO schemes and hence attaining improved performance in terms of both their bit error rate as well as throughput. The family of multifunctional MIMOs combines the benefits of both space-time coding and the Bell Labs layered space-time scheme as well as those of beamforming. We also introduce the idea of layered steered space-time spreading, which combines the benefits of space-time spreading, V-BLAST, and beamforming with those of the generalized multicarrier direct sequence code-division multiple access concept. Additionally, we compare the attainable diversity, multiplexing, and beamforming gains of the different MIMO schemes in order to document the advantages of multifunctional MIMOs over conventional MIMO schemes.

Layered Steered Space–Time-Spreading-Aided Generalized MC DS-CDMA

We present a novel trifunctional multiple-input-multiple-output (MIMO) scheme that intrinsically amalgamates space-time spreading (STS) to achieve a diversity gain and a Vertical Bell Labs layered space-time (V-BLAST) scheme to attain a multiplexing gain in the context of generalized multicarrier direct-sequence code-division multiple access (MC DS-CDMA), as well as beamforming. Furthermore, the proposed system employs both time- and frequency-domain spreading to increase the number of users, which is also combined with a user-grouping technique to reduce the effects of multiuser interference. Further system performance improvements can be attained by serially concatenating our proposed scheme with an outer code that is amalgamated with a unity-rate code for the sake of improving the achievable decoding convergence behavior of the proposed system, which is evaluated with the aid of extrinsic information transfer charts. We also propose a novel logarithmic likelihood ratio (LLR) postprocessing technique to improve the iteratively detected system's performance. Explicitly, the proposed system can attain a second-order spatial diversity gain and a frequency diversity gain of order V, where V is the number of subcarriers. Additionally, the proposed system attains a beamforming gain and a multiplexing gain that is twice that of a single-input-single-output system. Furthermore, after I = 10 decoding iterations and employing an interleaver depth of Dint = 160 000 bits, a time-domain spreading factor of Ne = 4, and V = 4 subcarriers, the overloaded system supporting K = 8 users requires an Eb/N0 that is only about 0.45 dB higher than the single-user system.

Error probability analysis of multicarrier direct sequence code division multiple access system under imperfect channel estimation and jamming in a Rayleigh fading channel

The performance of a multicarrier direct sequence code division multiple access (MC-DS-CDMA) system in fading environment has been extensively analysed in the literatures. The analysis that is available in the literatures did not take into account the impact of jamming and channel estimation errors on the performance of the system. In this study, closed-form expressions of the average bit error probability (BEP) of the MC-DS-CDMA system with channel estimation errors in the presence of partial band, broadband and multitone jamming are derived in a Rayleigh fading channel. The analysis shows that the magnitude of the normalised correlation between the actual and the estimated complex fading channel gain, A, greatly affects the performance of the MC-DS-CDMA system. For large signal-to-noise ratio, the average BEP with A=1 and without multiple access interference (MAI) tends to zero. When A≠1, the BEP of the system exhibits an error floor even without MAI. This error floor increases as the number of users increases. It is also shown that increasing the number of the system subcarriers enhances the system performance against jamming.

A novel Radon-based multi-carrier direct sequence code division multiple access transceiver design and simulation

Multi-carrier direct sequence code division multiple access (MC-DS-CDMA) has emerged recently as a promising candidate for the next generation broadband mobile networks. Multipath fading channels have a severe effect on the performance of wireless communication systems even those systems that exhibit efficient bandwidth, like orthogonal frequency division multiplexing (OFDM) and MC-DS-CDMA; there is always a need for developments in the realisation of these systems as well as efficient channel estimation and equalisation methods to enable these systems to reach their maximum performance. A novel MC-DS-CDMA transceiver based on the Radon-based OFDM, which was recently proposed as a new technique in the realisation of OFDM systems, will be used here as a basic building block in the design of MC-DS-CDMA transceiver to increase the orthogonality against the multipath frequency selective fading channels. Simulation results are provided to demonstrate the significant gains in performance and simplicity due to the proposed techniques.

Performance Analysis of MIMO MC-DS/CDMA System Using Chaotic Spreading Sequence

1 Abstract— This paper presents a novel chaotic spreading sequence for multiple input multiple output multi-carrier direct sequence code division multiple access (MIMO MCDS/CDMA) systems. The effect of multiple access interference can be mitigated by choosing the spreading sequences with appropriate cross-correlation properties. The performance of the system is analysed in multiuser scenario with the aid of simulation. The simulation results show that the proposed chaotic code spreading approach achieves a significant improvement in system utility and combats multiple access interference (MAI). The analysis reveals that the proposed system achieves significant performance improvement compared to Walsh Hadamard spreading code in MIMO MCDS/CDMA systems. Index Terms—MC-DS/CDMA, Chaotic code, MIMO, Spreading sequence, MAI

A joint power control and interference avoidance code assignment strategy for downlink MC-DS-CDMA with 2D spreading

Maintaining the quality-of-service (QoS) requirement by transmission power adjustment while eliminating interference is an important but difficult task in the downlink multicarrier direct-sequence code division multiple access (MC-DSCDMA) systems with two-dimensional (2D) spreading. To achieve this goal, we integrate the power control (PC) mechanism and the interference avoidance code assignment strategy. This joint policy is to firstly eliminate interference via an interference avoidance (IA) code assignment strategy and then maintain the QoS via a power control mechanism. To facilitate the code assignment strategy, a new PC-based multiple access interference (MAI) coefficient is defined through performance analysis. Thanks to the help of the PC-based MAI coefficient, we can evaluate each candidate code by quantitatively predicting the amount of interference produced by a possible code assignment event. With this interference pre-evaluation, the PC mechanism can not only maintain the QoS with less transmission power but also eliminate extra MAI in the system. The simulation results show that the proposed joint scheme can maintain a predefined signal quality and significantly improve the call admission rate while largely reducing the power consumption. The contribution of this paper fills up the missing link between the power control and code assignment in the 2D-spread MC-DS-CDMA systems.

The effects of spatial diversity and imperfect channel estimation on wideband MC-DS-CDMA and MC-CDMA

In our previous work, we compared the theoretical bit error rates of multi-carrier direct sequence code division multiple access (MC-DS-CDMA) and multi-carrier code division multiple access (MC-CDMA). To ensure a fair comparison, we constrained both schemes to the same bandwidth, information rate, and energy-per-bit, and these constraints resulted in a possible trade-off between diversity gain and channel estimation errors between the two schemes. While only a single-input single-output (SISO) system was considered in our previous work, in this paper, we extend the comparison to a multiple-input multiple-output (MIMO) system which employs Alamouti space-time block coding at each sub-carrier frequency to achieve spatial diversity. We consider only those cases where MC-CDMA has higher frequency diversity than MC-DS-CDMA. Since increases in diversity yield diminishing gains, we conclude that the addition of spatial diversity to this multi-carrier comparison benefits MC-DS-CDMA more than MC-CDMA. To determine whether these gains for MC-DS-CDMA are enough to offset the difference in frequency diversity between the two schemes, we derive closed-form expressions for the bit error probabilities of both schemes, and we compare the MIMO results against those of the SISO system for different information rates, number of users, and number of pilot symbols per channel estimate.

Blind Multiuser Detection in Frequency Domain for MC-CDMA Systems Using Particle Swarm Optimization

In digital communication systems, the constant modulus algorithm (CMA) for blind multiuser detector has some disadvantage such as slow convergence speed and weight phase rotation. In this paper, we put forward a novel receiver, which combines CMA blind adaptive multiuser detection and particle swarm optimization (PSO) technique in frequency domain for multi-carrier direct sequence code division multiple access (MC DS-CDMA) systems. By this approach we present an adaptive multistage structure based on PSO technique. In this structure, the weights are obtained by PSO technique between the received signal and its estimate through a CMA. The resulting weights contain reliability information for the hard decisions made in the previous stage. The simulation results show that BER performance of the PSO-based CMA blind multiuser detector is superior to that of steepest decreasing constant modulus algorithm (SDCMA), linearly constrained constant modulus algorithm (LCCMA), CMA based multiuser detection and is worthy due to its significant detection performance to achieve desirable improvement in convergence speed and tractability.

An Adaptive Multistage Multiuser Detector for MC-CDMA Communication Systems Using Evolutionary Computation Technique

The main targets of multi-carrier direct sequence code division multiple access (MC-DS-CDMA) mobile communication systems are to overcome the multi-path fading influences as well as the near-far effect and to increase its capacity. Many optimal and suboptimal multi-user detection approaches have recently been proposed and analyzed in literature. Unfortunately, most of them share the drawback of requiring a practical solution. Therefore, we have presented an adaptive multistage interference cancellation structure based on the particle swarm optimization (PSO) approach in this paper, and have effectively eliminated the multi-access interference (MAI) and near-far effect, and quickly converges to global optimal solution. Simulation results show that the proposed scheme can outperform some of the existed interference cancellation methods in both the additive white Gaussian noise and the multi-path fading channels.

Performance analysis of multicarrier code select CDMA system for PAPR reduction in multipath channels

Multicarrier direct sequence code division multiple access (MC DS-CDMA) is an attractive technique for achieving high data rate transmission. This is valid regardless of whether or not the potentially large peak-to-average power ratio (PAPR) is an important factor for its application. On the other hand, code select CDMA (CS-CDMA) is an attractive technique with constant amplitude transmission of multicode signal regardless of subchannels. This is achieved by introducing a code select method. In this paper, we propose a new multiple access scheme based on the combination of MC DS-CDMA and CS-CDMA. The proposed scheme, which we call MC CS-CDMA, includes as special cases the subclasses of MC DS-CDMA and CS-CDMA. This paper investigates the performance of these systems over a multipath frequency selective fading channel using a RAKE receiver with maximal ratio combiner. In addition, the PAPR of the proposed system is compared with that of both MC DS-CDMA and CS-CDMA. Simulation results demonstrate that the proposed system provides better PAPR reduction than MC DS-CDMA, at the expense of the complexity of the receiver and the number of available users. The numerical result demonstrates that the proposed system has better performance than MC DS-CDMA due to the increased processing gain and time diversity gain.

Pilot-Assisted Channel Estimation for Orthogonal Multi-Carrier DS-CDMA with Frequency-Domain Equalization

Orthogonal multi-carrier direct sequence code division multiple access (orthogonal MC DS-CDMA) is a combination of time-domain spreading and orthogonal frequency division multiplexing (OFDM). In orthogonal MC DS-CDMA, the frequency diversity gain can be obtained by applying frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion to a block of OFDM symbols and can improve the bit error rate (BER) performance in a severe frequency-selective fading channel. FDE requires an accurate estimate of the channel gain. The channel gain can be estimated by removing the pilot modulation in the frequency domain. In this paper, we propose a pilot-assisted channel estimation suitable for orthogonal MC DS-CDMA with FDE and evaluate, by computer simulation, the BER performance in a frequency-selective Rayleigh fading channel.

Time- and Frequency-Domain-Spread Generalized Multicarrier DS-CDMA Using Subspace-Based Blind and Group-Blind Space–Time Multiuser Detection

Subspace-based blind and group-blind space-time (ST) multiuser detection (MUD) is invoked for a smart-antenna-aided generalized multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) system communicating over a dispersive ST Rayleigh fading channel, where the channel estimates are attained by using subspace-based blind techniques. Smart antennas are employed to provide increased degrees of freedom and, hence, to improve both the attainable single-user performance and the multiuser capacity. Furthermore, two adaptive subspace-tracking algorithms, i.e., the projection approximation subspace-tracking deflation (PASTd) algorithm and the noise-averaged Hermitian-Jacobi fast subspace tracking (NAHJ-FST) algorithm, are employed for the sake of reducing the computational complexity imposed by eigenvalue decomposition. When employing the NAHJ-FST algorithm, the group-blind MUD employing an antenna array having M = 2 elements exposed to uncorrelated fading achieved a 4.2-dB signal-to-noise-ratio (SNR) gain at a bit error rate (BER) of 10-4 compared with that employing a single antenna, which is an explicit benefit of its increased computational complexity.

Further performance analysis of the generalized MC DS-CDMA system in Nakagami- m fading channels

The generalized multicarrier direct sequence code-division multiple-access (MC DS-CDMA) is a well known model that can be extended to different MC DS-CDMA schemes by simply varying a single parameter called as normalized subcarrier spacing. In this paper, the effect of the rate of average power decay on the bit error rate (BER) performance of the generalized MC DS-CDMA system is presented. Two specific schemes known as multitone DS-CDMA and orthogonal MC DS-CDMA are considered for the BER performance analysis. Simulation results show that the rate of average power decay has an important effect on the BER performance comparisons between the multitone DS-CDMA system and the orthogonal MC DS-CDMA system.

Effects of Subcarrier Power Allocation on an Interference Avoidance Code Assignment Strategy for Multirate MC-DS-CDMA Systems

In this paper, we propose a joint subcarrier power allocation (SPA) and code assignment scheme for the synchronous multirate multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) system with time- and frequency-domain spreadings. Based on the newly defined metric multiple-access interference (MAI) coefficient, the proposed code assignment strategy can quantitatively predict the incurred MAI before assigning a spreading code. The SPA mechanism aims to maximize the received signal power. In addition to lowering the MAI, the proposed code assignment strategy further considers the compactness of the assigned codes in the entire 2-D tree structure. The simulation results show that the proposed joint SPA and code assignment strategy not only can reach a better received signal quality but can also achieve a high call admission rate.

Orthogonal Multi-Carrier DS-CDMA with Frequency-Domain Equalization

Orthogonal multi-carrier direct sequence code division multiple access (orthogonal MC DS-CDMA) is a combination of orthogonal frequency division multiplexing (OFDM) and time-domain spreading, while multi-carrier code division multiple access (MC-CDMA) is a combination of OFDM and frequency-domain spreading. In MC-CDMA, a good bit error rate (BER) performance can be achieved by using frequency-domain equalization (FDE), since the frequency diversity gain is obtained. On the other hand, the conventional orthogonal MC DS-CDMA fails to achieve any frequency diversity gain. In this paper, we propose a new orthogonal MC DS-CDMA that can obtain the frequency diversity gain by applying FDE. The conditional BER analysis is presented. The theoretical average BER performance in a frequency-selective Rayleigh fading channel is evaluated by the Monte-Carlo numerical computation method using the derived conditional BER and is confirmed by computer simulation of the orthogonal MC DS-CDMA signal transmission.