Uplink Multicarrier Code-division Multiple-access Systems Research Articles

Time complexity analysis of GA-based variants uplink MC-CDMA system

Complexity plays a very significant role in real-time problems. A genetic algorithm (GA)-based multiple input multiple output for an uplink multi-carrier code-division multiple-access (MC-CDMA) receiver is being considered as an important pillar in real-time wireless communication problems. Bit error rate (BER) and minimum mean square error (MMSE) are well-known system performance evolution parameters to estimate the real-time system standards. Sometimes, the multiple solutions give the same BER and MMSE, and then, the complexity is one of the core aspects that will outline the suitable candidate solution. In this article, the proposed drift analysis is being used for the complexity analysis among these GA-based variations. It is observed that the time complexity of the proposed GA-based MC-CDMA with weight relation is less as compared to without weight relation, due to empowering of an exceptional relationship to find out the required solution.

Approximate expression of bit error rate in uplink MC-CDMA systems with equal gain combining

Uplink multicarrier code-division multiple-access (MC-CDMA) with equal gain combining (EGC) over Nakagami fading channels is considered. An improved expression which is a feasible alternative for the bit error rate (BER) performance evaluation of MC-CDMA signals is proposed. Simulated annealing algorithm is employed to obtain the optimum value of the coefficients belonging to the proposed expression. Numerical examples show that the performance curves computed by the improved expression are in good agreement with the results obtained by the exact expression. Thus, the proposed expression can improve the accuracy of BER performance evaluation that has been realized by the approximate expression.

A novel CFO estimator with joint bisection-searching and complexity-reduction technique for uplink MC-CDMA systems

This paper presents a simple scheme, which speeds the estimation of carrier frequency offset (CFO) as well as reducing the estimation complexity for multicarrier code-division multiple-access (MC-CDMA) uplink. Based on the concept of bisection method, the CFO is efficiently estimated by way of locating the peak value in the output power spectrum of the minimum output energy (MOE) detector. Complexity reduction results from the proposed data vector partition associated with an innovative data processing technique for the MOE detector. Simulation results show that the expected advantage of the proposed scheme can be achieved at the expense of a reasonable performance loss even in the presence of a large CFO.

Error Rate Analysis of Uplink MC-CDMA Systems under Imperfect Channel Estimation

Theoretical error rate performance of wireless communication systems are usually determined assuming that the perfect channel state information (CSI) is available at the receiver. However, in actual practice, the channel gains at the receiver are obtained via using some channel estimation (CE) techniques. Due to inherent presence of noise, the CE is not perfect resulting in the performance degradation. In this paper, we evaluate the error rate performance of an uplink multicarrier code-division multiple-access (MC-CDMA) system, considering different modulation techniques, where CE is performed using pilot symbol assisted (PSA) minimum mean-square error (MMSE) CE technique. The symbol error rate (SER) analysis of an uplink MC-CDMA system using multiuser detection techniques, such as MMSE and zero forcing (ZF), is presented under imperfect CE. Simulated results for SER are also shown to confirm the accuracy of the analytically derived results.

Iterative joint data detection and channel estimation for uplink MC-CDMA systems in the presence of frequency selective channels

This paper is concerned with joint multiuser detection and multichannel estimation (JDE) for uplink multicarrier code-division multiple-access (MC-CDMA) systems in the presence of frequency selective channels. The detection and estimation, implemented at the receiver, are based on a version of the expectation maximization (EM) algorithm and the space-alternating generalized expectation-maximization (SAGE) which are very suitable for multicarrier signal formats. The EM-JDE receiver updates the data bit sequences in parallel, while the SAGE-JDE receiver reestimates them successively. The channel parameters are updated in parallel in both schemes. Application of the EM-based algorithm to the problem of iterative data detection and channel estimation leads to a receiver structure that also incorporates a partial interference cancelation. Computer simulations show that the proposed algorithms have excellent BER end estimation performance.

Joint multi-user interference and clipping noise cancellation in uplink MC-CDMA system

In this paper, an iterative method is proposed to jointly cancel multi-user interference and clipping noise in uplink Multi-Carrier Code Division Multiple Access (MC-CDMA) systems. Clipping is the simplest method to overcome high peak-to-average power ratio of multi-carrier signals but it makes the signals distorted. Reconstruction methods use non-distorted samples to reconstruct distorted samples in the receiver but multi-user interference causes the methods do not work properly because all the received samples are distorted due to clipping and interference and so there is no undistorted samples to be used in recovering clipped samples. On the other hand, multi-user interference cancellation methods do not work properly because clipping in the transmitter is a non-linear process and therefore this process increases the amount of multiple access interference (MAI). In this paper, we propose a joint MAI and clipping noise cancellation method. In the proposed iterative method in each iteration a MC-CDMA detection method accompany with a clipping and filtering process and then by using the received signal as reference signal, an approximation of error due to clipping and residual interference is computed. Simulation results show that the proposed method improves the performance of clipped MC-CDMA systems in uplink for synchronous/asynchronous links.

Improved ML Channel Estimation for Uplink MC-CDMA Systems in Closely Spaced Multipath Channels

In order to attain near-single user performance in uplink multicarrier code- division multiple-access (MC-CDMA) systems, multiuser detection (MUD) methods may be employed which rely on simultaneous estimation of the channel frequency responses of multiple users. Pilot symbol assisted (PSA) channel estimation is needed in fast fading channels and it can be performed either by applying maximum likelihood (ML) criterion or minimum mean-squared error (MMSE) criterion. The performance of ML estimation technique degrades significantly in the case of fractionally spaced (FS) multipath channels where dominant paths are closely spaced with respect to the time resolution of the system. In such situation, the number of effective paths (which contribute more towards signal power) becomes considerably less than the actual number of multipaths at low and moderate SNR values. We propose an improved ML estimation method which considers only effective paths during the estimation process. The proposed method performs nearly identical to the MMSE estimation method and it can also provide significant reduction in the computational complexity when a large number of users are accommodated in the system.

Blind adaptive constrained moe receiver for uplink MC-CDMA systems with real signaling in multi-cell environments

A blind adaptive constrained minimum output energy (MOE) receiver has been proposed for the uplink multicarrier code-division multiple access (MC-CDMA) system in a multicell environment when some users, including the desired user, employ real-valued modulation such as binary phase-shift keying (BPSK) or amplitude-shift keying (ASK). Since the imaginary part of residual interference at the filter output does not affect the decision of the desired user's data in this case, the proposed receiver has been designed to perfectly eliminate the real part of residual in-cell interference using the information of the spreading sequence and to reduce the real part of residual intercell interference by minimizing the constrained output energy. Results of analysis and simulation show that the proposed blind adaptive algorithm converges faster and has a higher steadystate signal-to-interference and noise ratio (SINR) than those of conventional schemes.

Iterative and Diversity Techniques for Uplink MC-CDMA Mobile Systems With Full Load

Iterative and diversity techniques are two of the most effective techniques for uplink multicarrier code-division multiple-access (MC-CDMA) systems. However, there is still no knowledge on how to efficiently combine these techniques to design high-performance uplink receivers when there is a complexity constraint. In this paper, we compare the performance and the complexity of MC-CDMA systems with and without iterative detectors and multiple-receive-antenna arrays. Through extensive simulation, we demonstrate that the following four combinations are good tradeoffs between complexity and performance: a single-antenna receiver with an iterative parallel-interference-cancellation (PIC) detector initialized by a matched filter (MF) with three iterations, a two-antenna receiver with a minimum-mean-square-error multiuser detector or an iterative PIC detector initialized by an MF with two iterations, and a four-antenna receiver with a simple MF detector. Therefore, noniterative detectors with multiple-receive-antenna arrays can be used to replace iterative detectors and reduce complexity, which is a promising solution for fourth-generation (4G) uplink MC-CDMA systems, where multiple-receive antennas are available at the base stations. In this paper, we also briefly discuss pilot-aided channel estimation using the weighted-delay-profile technique and investigate the impact of channel-estimation error in different environments.

Multistage RLS Subcarrier-Combining Method for Uplink Quasi-Synchronous MC-CDMA

In order to improve the performance of multicarrier code division multiple access (MC-CDMA) in the uplink channel, quasi-synchronous MC-CDMA with minimum-mean-square-error (MMSE)-based subcarrier-combining method is effective. However, conventional MMSE methods require a lot of pilot symbols for derivation of optimal weights. In this paper, in order to reduce the number of pilot symbols for uplink MC-CDMA systems, we propose a multistage recursive-least-square (MS-RLS) method for the derivation of the weights for the MMSE subcarrier combining. In the MS-RLS, the received symbols are stored in the buffer memory, and the RLS algorithm is repeatedly performed with a multistage structure by using the same pilot symbols. In this method, the weights and the inverse matrices of correlation matrices of the adjacent subcarriers are utilized for the initial values of the next-stage weight updating. Through computer simulation, we confirm that the performance of MC-CDMA in the uplink channel is improved by using the proposed MS-RLS method, even with short pilot symbols.

Performance comparison of MRC and EGC on a MC-CDMA system with synchronization errors over fading channels

This work derives the average bit error rate (BER) of the uplink and downlink multicarrier code division multiple access (MC-CDMA) systems using maximum ratio combining (MRC) and equal gain combining (EGC) with synchronization errors over fading channels. The derived equation can simultaneously incorporate the parameters of the fading channel and all of the synchronization errors, including frequency offset, carrier phase jitter, and timing jitter. Numerical results indicate that those two combining schemes on the uplink and downlink MC-CDMA systems are degraded by all of the normalized synchronization errors over 10?2. The comparison outcomes between MRC and EGC reveal that the MRC generally outperforms EGC in the uplink MC-CDMA system. However, EGC achieves better performance when the number of users is small, the normalized synchronization errors are low and the signal to noise ratio (SNR) is high. In the downlink system, EGC mainly outperforms MRC when the SNR and the number of users are gradually increased and the normalized synchronization errors are low. Therefore, the selection of MRC or EGC depends on the SNR, the synchronization errors and the number of users in uplink and downlink MC-CDMA systems.

Nonblind and Semiblind Space-Frequency Multiuser Detection for Multirate MC-CDMA Systems

Multicarrier code-division multiple access (MC-CDMA) is a promising candidate for the air interface of future wireless communication systems. After the development of a unified signal model that applies to the different multirate MC-CDMA schemes studied in this paper, a novel space-frequency multiuser detection scheme for multirate uplink MC-CDMA systems with multiple receive antennas is proposed. In particular, a space-frequency minimum mean-squared error (MMSE)-based hybrid serial/parallel interference cancellation receiver (SF-MMSE/HIC) is developed which produces the SF-MMSE combiner as a byproduct. The signal processing of this new detector is jointly carried out in space and frequency domains which leads to the advantage of combating the interference resulting from different sources simultaneously. Based on subspace estimation, a semiblind implementation of the receiver is introduced for slow fading channels. Furthermore, the impact of subcarrier interleaving is investigated for variable spreading factor (VSF) MC-CDMA systems. Based on the characteristic function (CF) of a complex Gaussian random vector (CGRV), a method for analyzing the bit error probability (BEP) performance is presented. Several representative simulation examples of the detector are provided to demonstrate the performance of the proposed receiver. In a fully loaded system, the proposed receiver can offer performance close to the single-user bound. Moreover, the results indicate that multicode and VSF MC-CDMA schemes give very similar performance

Performance Evaluation of Uplink MC-CDMA Systems with Residual Frequency Offset

This letter evaluates the performance of an uplink multicarrier-code division multiple access (MC-CDMA) system when the frequency offsets of all users are random variables and the frequency offset for the desired user is compensated. The analysis confirms that performance degradation due to frequency offset is negligible if the estimation error of normalized frequency offset for the desired user is less than 10 -1 .

Performance of Up-link MC-CDMA System with Frequency Offset and Imperfect Channel Estimation Simultaneously over Nakagami Fading Channels

In this paper, effects of carrier frequency offset on performance of uplink MC-CDMA (Multi-Carrier Code Division Multiple Access) system in Nakagami fading channel are investigated through the theoretical analysis and Monte Carlo computer simulations. Both perfect maximal-ratio combining (MRC) and equal gain combining (EGC) receivers are analyzed; the impact of imperfect channel fading estimation on the performance of MRC is also explored. The performance of MC-CDMA system is also compared with that of the conventional single-carrier DS-CDMA system. Our results indicate that the performance of MC-CDMA system is sensitive to even small values of carrier frequency offset and that the performance of MC-CDMA system improves as number of subcarriers increases. In perfect channel fading estimation, the overall performance of MRC is superior to EGC. However, when imperfect or inaccurate channel fading estimation exists, which leads to serious performance degradation, EGC becomes superior to MRC.