Multicarrier Direct-sequence Code-division Multiple Access Research Articles

Joint estimation of correlated multi-carrier DS-CDMA mobile fading channels based on optimal filters

This paper deals with the joint estimation of rapidly time-varying and correlated Rayleigh fading channels in synchronous multi-carrier direct-sequence code-division-multiple-access (MC-DS-CDMA) systems. Usually, when the multiple carrier fading channels are modelled by autoregressive (AR) processes, they can be estimated separately by means of an optimal Kalman filter. However, a loss in performance can be expected when the channels are correlated. To take into account these correlations, the multiple carrier fading processes are stored in a vector, modelled as vector AR process, and estimated jointly by means of an optimal Kalman filter. Nevertheless, this requires the simultaneous estimation of the AR parameter matrices in the vector AR process. To avoid a non-linear approach such as the extended Kalman filter (EKF), this estimation issue can be solved by using dual Kalman filters. A comparative study on channel estimation is carried out between the proposed joint estimation scheme, the separate estimation counterpart and the standard least mean square (LMS) based estimator.

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.

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.

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 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.

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 PSO‐based parallel interference canceller for MC‐CDMA communication systems

AbstractThis work analyses the performance of particle swarm optimisation (PSO) algorithm‐based parallel interference cancellation (PIC) multiuser detectors exploited in multicarrier direct‐sequence code‐division multiple‐access (MC‐DS‐CDMA) communication systems over frequency‐selective fading channels. The PSO, which attempts to reduce the complexity in optimal detection caused by the greedy exhausted searching, achieves adequate sub‐optimal multiuser detection in CDMA systems. Simulation results reveal that with a lower generation of Gn = 5, 10, the constant weight PIC (CWPIC) scheme outperform the conventional PIC (CPIC) particularly at heavy load. However, with higher generation Gn = 20 in PSO, the CPIC schemes contrarily outperform the CWPIC for all system loads. Copyright © 2008 John Wiley & Sons, Ltd.

Trade-Off Between Diversity and Channel Estimation Errors in Asynchronous MC-DS-CDMA and MC-CDMA

This paper investigates the trade-off between diversity and channel estimation errors for asynchronous multi-carrier CDMA (MC-CDMA) and multicarrier direct-sequence CDMA (MC-DS-CDMA). We construct a side-by-side comparison between the two systems by assuming equal operating and channel conditions. Both systems perform pilot-symbol- aided channel estimation and maximal ratio combining at the receiver, where the weighting coefficients are determined by the channel estimates at each sub-carrier. Closed-form expressions for the probability of error of both systems are derived under two separate channel scenarios. These two scenarios assume a frequency-selective Rayleigh fading channel, where the coherence bandwidth is equal to either the bandwidth of a MC-DS-CDMA sub-band or the bandwidth of a MC-CDMA sub-band. Finally, we compare the bit error rates of the two systems for different information rates, number of users, and number of pilot symbols used in each channel estimate.

Performance of a Complete Complementary Code-Based Spread-Time CDMA System in a Fading Channel

In this paper, we study the performance of complete complementary code-based spread-time code-division multiple access (CC-ST-CDMA) in frequency-nonselective and frequency-selective fading channels. The theoretical bit-error-rate (BER) expressions conditioned on the number of users and the sequence length (and the number of resolvable paths for multipath frequency-selective fading) are obtained. The theoretical approximation of the BER for the system using long-sequence-length complete complementary codes is investigated as well. The simulations are conducted over flat-fading and two-path frequency-selective fading channels and have been used to verify the theoretical analysis. The results show that the proposed CC-ST-CDMA system has a comparable BER performance with complete complementary code-based multicarrier direct-sequence code-division multiple access (CC-MC-DSCDMA) for a flat-fading channel. However, for a frequency-selective fading channel, the CC-ST-CDMA is found to be more resistant, resulting in about 2-3 dB better performance than the CC-MC-DSCDMA.

A Multicarrier-CDMA Space–Time Receiver With Full-Interference-Suppression Capabilities

This paper proposes a low-complexity multicarrier-code-division-multiple-access (MC-CDMA) space-time receiver with full-interference-suppression capabilities. First, we derive a complete model of the interference which takes into account multiple-access, intersymbol, and intercarrier interferences. Based on this model, we introduce a new multicarrier-interference-subspace-rejection (MC-ISR) receiver and analyze its performance in an unknown time-varying Rayleigh channel with multipath, carrier offset, and cross correlation between subcarrier channels. We also propose a realistic implementation of this receiver, which includes an efficient strategy for carrier-offset recovery in a multicarrier- and multiuser-detection scheme. In addition, based on the Gaussian assumption, we derive a link-/system-level performance analysis of the MC-ISR over the two MC-CDMA air-interface configurations, which are the multitone CDMA and the multicarrier direct-sequence CDMA, and validate it by simulations. The gains in the throughput, which are attainable by the MC-ISR, are significant and evaluated in this paper as functions of the air-interface configuration, the number of subcarriers, and the modulation order.

Zero-Forcing and Minimum Mean-Square Error Multiuser Detection in Generalized Multicarrier DS-CDMA Systems for Cognitive Radio

In wireless communications, multicarrier direct-sequence code-division multiple access (MC DS-CDMA) constitutes one of the highly flexible multiple access schemes. MC DS-CDMA employs a high number of degrees-of-freedom, which are beneficial to design and reconfiguration for communications in dynamic communications environments, such as in the cognitive radios. In this contribution, we consider the multiuser detection (MUD) in MC DS-CDMA, which motivates lowcomplexity, high flexibility, and robustness so that the MUD schemes are suitable for deployment in dynamic communications environments. Specifically, a range of low-complexity MUDs are derived based on the zero-forcing (ZF), minimum mean-square error (MMSE), and interference cancellation (IC) principles. The bit-error rate (BER) performance of the MC DS-CDMA aided by the proposed MUDs is investigated by simulation approaches. Our study shows that, in addition to the advantages provided by a general ZF, MMSE, or IC-assisted MUD, the proposed MUD schemes can be implemented using modular structures, where most modules are independent of each other. Due to the independent modular structure, in the proposed MUDs one module may be reconfigured without yielding impact on the others. Therefore, the MC DS-CDMA, in conjunction with the proposed MUDs, constitutes one of the promising multiple access schemes for communications in the dynamic communications environments such as in the cognitive radios.

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.

An Interference Avoidance Code Assignment Strategy for Downlink Multi-Rate MC-DS-CDMA with TF-Domain Spreading

Frequency selective fading may affect the orthogonality of the spreading codes in the multi-carrier direct sequence code division multiple access (MC-DS-CDMA) systems. In this paper, we define a new performance metric called the multiple access interference (MAI) coefficient for the MC-DS-CDMA system to quantitatively predict the impact of inter-code interference with the time- and frequency-domain spreading in a frequency selective fading channel. With the help of MAI coefficient, a novel interference avoidance code assignment strategy is proposed. By jointly considering the incurred MAI effect and the blocking probability in the code tree structure, the proposed interference avoidance code assignment method can effectively reduce the MAI for the multi-rate MC-DS-CDMA system, while maintaining very good call blocking rate performance.

Design and Performance Analysis of an Adaptive Receiver for Multicarrier DS-CDMA

An adaptive parallel interference cancelation (APIC) scheme is proposed for the multicarrier direct sequence code division multiple access (MC-DS-CDMA) system. Frequency diversity inherent in the MC system is exploited through maximal ratio combining, and an adaptive least mean square algorithm is used to estimate the multiple access interference. Theoretical analysis on the bit-error rate (BER) of the APIC receiver is presented. Under a unified signal model, the conventional PIC (CPIC) is shown to be a special case of the APIC. Hence the BER derivation for the APIC is also applicable to the CPIC. The performance and the accuracy of the theoretical results are examined via simulations under dierent design parameters, which show that the APIC outperforms the CPIC receiver provided that the adaptive parameters are properly selected.

Space‐time diversity‐enhanced QoS provisioning for real‐time service over MC‐DS‐CDMA based wireless networks

AbstractIn order to support the quality‐of‐service (QoS) requirements for real‐time traffic over broadband wireless networks, advanced techniques such as space‐time diversity (STD) and multicarrier direct‐sequence code division multiple access (MC‐DS‐CDMA) are implemented at the physical layer. However, the employment of such techniques evidently affects the QoS provisioning algorithms at the medium access control (MAC) layer. In this paper, we propose a space‐time infrastructure and develop a set of cross‐layer real‐time QoS‐provisioning algorithms for admission control, scheduling, and subchannel‐allocations. We analytically map the parameters characterizing the STD onto the admission‐control region guaranteeing the real‐time QoS. Our analytical analyses show that the proposed algorithms can effectively support real‐time QoS provisioning. Also presented are numerical solutions and simulation results showing that the STD can significantly improve the QoS provisioning for real‐time services over wireless networks. Copyright © 2007 John Wiley & Sons, Ltd.

A Multiple-Rate, Multicarrier Direct-Sequence CDMA System with 2-D OVSF Codes in Fading Channels

In this paper, a multiple-rate, multicarrier direct- sequence code-division multiple-access (MC/DS-CDMA) system with the use of two-dimensional orthogonal variable spreading factor (2-D OVSF) codes is studied. They are the first 2-D codes that can be generated by the tree structure commonly used by one-dimensional OVSF codes in wideband-CDMA. The 2-D OVSF codes also preserve orthogonality among code matrices with different spreading factors in the code tree. The performances of the proposed system with RAKE receivers employing equal-gain and maximal-ratio combining methods are analyzed and compared. Our results show that our multiple-rate MC/DS-CDMA system with the 2-D OVSF codes is more suitable for a non-fading additive white Gaussian noise channel or a Rician weak-fading channel. However, in a Rayleigh fading channel, the system performance gets worse because the orthogonality of the 2-D OVSF codes is destroyed by the strong fading effect.

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.

Vector Channel Estimation and Multiuser Detection for Multicarrier DS CDMA in Time and Frequency Selective Fading Channels

In a multicarrier direct-sequence code-division multiple access (MC DS CDMA) system, different fading channels for different users and/or different carriers are correlated in general; thus a vector channel model is more appropriate than disjoint scalar channel models. For multiuser MC DS CDMA systems, we propose (1) a generalized vector autoregressive model which accounts for correlation between different user/carrier fading channels, (2) the use of a two-phased algorithm to obtain the proposed model's parameters, and (3) a receiver structure that consists of a generalized decorrelator followed by maximal-ratio combining (MRC) of uncorrelated carrier channel outputs of each user. The estimated fading coefficients provide the necessary quantities to MRC. The computer simulation results show that the proposed scheme has performance close to the case in which the channel is perfectly known, and outperforms separate scalar channel estimation case.

On the performance of multicarrier DS-CDMA with imperfect power control and variable spreading factors

Multicarrier direct-sequence code-division multiple access (MC-DS-CDMA) becomes an attractive technique for the future fourth-generation (4G) wireless system because it can flexibly adapt transmission rates by changing both time and frequency spreading factors and possesses many physical-layer advantages in dispersive fading channels. However, power control errors (PCE) and the complete multiple access interference (MAI) from all the intersubcarriers may significantly degrade the performance of the MC-DS-CDMA system. In this paper, we propose an analytical method to evaluate the joint effects of the PCE and the complete MAI on the multirate MC-DS-CDMA system. From analysis and simulation, we obtain some important insights into the performance issues of the MC-DS-CDMA system. First, the effect of PCE can exacerbate the impact of the complete MAI on the MC-DS-CDMA system, or vice versa . For BER=10-3 in a considered case, the joint effect of the complete MAI and PCE further degrades the performance by 2.1 dB compared with the sum of the degradation from the complete MAI and the PCE individually. Second, increasing frequency or time-domain spreading gain can improve the performance of the MC-DS-CDMA system, but the system also becomes more sensitive to power control errors. Third, a larger PCE can possibly make the frequency-domain diversity diminish faster than the gain obtained from the time-domain spreading although an MC-DS-CDMA system with a larger frequency-domain spreading gain (M) is usually better than that with a larger time-domain spreading gain (Go). In our example, for the standard deviation of PCE (sigmae) equal to 0 dB, the BERs with (M,Go)= (4, 16) and (16, 4) are 9.3times10-4 and 3.7times10-5, respectively, while for sigmae=4 dB, the BER performances of the two cases are all in the order of 10-3