Multicarrier CDMA Systems Research Articles

A Simple Code Allocation Algorithm Based on Asymptotic Analysis of MIMO MC-CDMA Systems in a Multi-Cell Environment

Owing to frequency diversity gain and simplicity, a chip interleaved multi-carrier code division multiple access (MC-CDMA) system has been considered in a multi-cell environment, and combining it with multiple input multiple output (MIMO) schemes offers high spectral efficiency. In spite of the advantages, the system is highly affected by inter code interferences. To control them, this letter analyzes the asymptotic performance of a MIMO MC-CDMA system with a symbol level MMSE receiver in a multi-cell environment and presents an appropriate multiplexed code ratio satisfying a required error rate.

Doppler Diversity in MC-CDMA Systems with T-ZCZ Sequences for Doppler Spread Cancelation

Doppler diversity has been proven effective to combat time variation caused by Doppler spread in single carrier systems. However, it is not efficient to directly apply Doppler diversity into Multi-Carrier Code Division Multiple Access (MC-CDMA) systems because Inter-Carrier-Interference (ICI) increases with the artificial frequency shifts in diversity branches. In this paper, a novel Doppler diversity scheme in MC-CDMA with Three Zero Correlation Zones (T-ZCZ) sequences is proposed to further improve the performance of Doppler diversity. Particularly, zero correlation zones are employed in frequency domain for ICI cancelation caused by Doppler spread, which confirms the validity of the contribution to the wideband wireless communications in high speed mobile environment.

Channel Tracking and Signal Detection for MC-CDMA in a Time-Varying Environment

In this paper, a joint channel estimation and multiuser detection method is presented for uplink multicarrier code-division multiple access systems in time-varying environments. Unlike the conventional channel-tracking schemes, which generally need to insert pilot signals in every orthogonal frequency-division multiplexing symbol, the proposed approach requires only one pilot symbol at the beginning of each data frame. Based on the fact that the channel coefficients of two successive symbols are highly correlated, subsequent channel tracking and signal detection are recursively carried out. A major feature of the proposed approach is that it enables robust error propagation control and is capable of effectively detecting and tracking abrupt channel changes. Simulation examples demonstrate the robustness of the proposed algorithms.

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.

Reverse Link Outage Probabilities of Multicarrier CDMA Systems with Beamforming in the Presence of Carrier Frequency Offset

The outage probability of reverse link multicarrier (MC) code-division multiple access (CDMA) systems with beamforming in the presence of carrier frequency offset (CFO) is studied. A conventional uniform linear array (ULA) beamformer is utilized. An independent Nakagami fading channel is assumed for each subcarrier of all users. The outage probability is first investigated under a scenario where perfect beamforming is assumed. A closed form expression of the outage probability is derived. The impact of different types of beamforming impairments on the outage probability is then evaluated, including direction-of-arrival (DOA) estimation errors, angle spreads, and mutual couplings. Numerical results show that the outage probability improves significantly as the number of antenna elements increases. The effect of CFO on the outage probability is reduced significantly when the beam-forming technique is employed. Also, it is seen that small beamforming impairments (DOA estimation errors and angle spreads) only affect the outage probability very slightly, and the mutual coupling between adjacent antenna elements does not affect the outage probability noticeably.

A Low-Power Multicarrier-CDMA Downlink Baseband Receiver for Future Cellular Communication Systems

In this paper, design and implementation of a baseband receiver integrated circuit (IC) for a downlink multi-carrier code-division multiple access (MC-CDMA) system are presented. This MC-CDMA system aims to provide higher data transmission capacity than the current wide-band CDMA systems in mobile cellular communication environments. The proposed chip provides a robust tracking mechanism for synchronization errors and an accurate channel estimation strategy to overcome the challenge of outdoor fast-fading channels. Besides, low-power and low-complexity architecture design techniques are adopted to satisfy mobile receiver needs. Experimental results of the designed baseband receiver integrated circuit demonstrate its superior system performance and great reduction in power consumption. The chip was fabricated in a 0.18-mum CMOS technology with a core area of 2.6 mm times 2.6 mm. It can support up to 21.7-Mbps uncoded data rate in a 5-MHz bandwidth. When running at 5.76 MHz, its power consumption is as low as 9.9 mW from a supply voltage of 1.1 V.

Symmetry-Embedded Spreading Sequences for Multicarrier CDMA

It was shown by Slimane that the bit error rate (BER) performance of the downlink of multicarrier code-division multiple-access (MC-CDMA) systems can be improved significantly by employing quadrature spreading sequences instead of binary Walsh-Hadamard sequences. Inspired by Slimane's work, we propose a class of complex-valued spreading sequences, which are mathematically derivable from the Kronecker product of some basic matrices. The good algebraic structure in the proposed spreading sequences, when applied in a BPSK based MC-CDMA system, can be exploited for multiple access interference (MAI) suppression, for order-2 diversity combining, and for the reduction of peak-to-average power ratio. The new spreading sequences outperform the Slimane's quadrature sequences, as evidenced by various numerical results.

FIR Zero-Forcing Multiuser Detection and Code Designs for Downlink MC-CDMA

This paper focuses on multiuser detection for multicarrier code-division multiple-access (MC-CDMA) systems, employing cyclic-prefixed (CP) or zero-padded (ZP) transmission techniques. For both systems, we consider either the classical linear finite-impulse response (L-FIR) receiving structures or, when the transmitted symbols are improper, the proposed widely linear FIR (WL-FIR) ones, which process both the received signal and its complex-conjugate version. With regard to both CP- and ZP-based downlink configurations, it is shown that, if the number of users does not exceed a given threshold and their codes are appropriately designed, L-FIR and WL-FIR universal zero-forcing (ZF) multiuser detectors can be synthesized, which, in the absence of noise, guarantee perfect symbol recovery for each user, regardless of the underlying frequency-selective channel. In particular, some spreading code examples are provided, which satisfy the design rules. Finally, numerical simulations are carried out to show that the theoretical considerations developed herein provide useful guidelines for practical MC-CDMA system designs.

On Intercell Interference and Its Cancellation in Cellular Multicarrier CDMA Systems

The handling of intercell interference at the cell border area is a strong demand in future communication systems to guarantee efficient use of the available bandwidth. Therefore, this paper focuses on the application of iterative intercell interference cancellation schemes in cellular multicarrier code division multiple access (MC-CDMA) systems at the receiver side for the downlink. First, the influence of the interfering base stations to the total intercell interference is investigated. Then, different concepts for intercell interference cancellation are described and investigated for scenarios with several interfering cells. The first approach is based on the use of the hard decision of the demodulator to reconstruct the received signals. This does not require the higher amount of complexity compared to the second approach which is based on the use of the more reliable soft values from the decoding process. Furthermore, the extrinsic information as a reliability measure of this soft iterative cancellation process is investigated in more detail based on the geographical position of the mobile terminal. Both approaches show significant performance gains in the severe cell border area. With the soft intercell interference cancellation scheme, it is possible to reach the single-user bound. Therefore, the intercell interference can be almost eliminated.

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 for an MC-CDMA System over Single- and Multiple-Cell Environments in Correlated-Nakagami-m Fading

This paper investigates system performance for an MC-CDMA (multi-carrier coded-division multiple-access) system, in which an operating environment with both single-cell and multiple-cell configurations and correlated-Nakagami-m statistics for fading channels are adopted. It is worthwhile noting that applying the joint characteristic function to determine the jpdf (joint probability density function) with a generalized Laguerre polynomial yields a simpler method. The traditional difficult methods for explicitly obtaining the jpdf are avoided for the sake of simplification. Some new closed-form formulas for average BER (bit-error rate) with statistical calculation of MAI (multiple-access interference) for MC-CDMA system operation in multi-cell environments were obtained in this study. For achieving accuracy in the derived formulas, an example of an MC-CDMA system with a dual-receiver branch operating in a multiple-cell structure with 12 surrounding cells is presented.

A Unified Analysis of Diversity Exploitation in Multicarrier CDMA

Since the introduction of multicarrier code-division multiple access (MC-CDMA), several publications have presented various detection concepts for efficient and robust multiuser communications in the up- and downlink. System concepts, which either exploit channel knowledge at the transmitter or at the receiver, have been investigated. The approach, which exploits channel knowledge at the transmitter and at the receiver simultaneously, referred to as combined-equalization, is a new approach that targets a common optimization of data transmission and reception. In this paper, we introduce a unified analysis for optimally exploiting diversity. We apply this analysis exemplarily to MC-CDMA systems exploiting any of the aforementioned concepts. It should be noted that this analysis could be applied to numerous other systems that exploit diversity. The presented approach analytically derives the theoretical lower performance bounds for pre-, post-, and combined-equalization achievable with MC-CDMA in up- and downlink. This fundamental investigation of lower bounds will serve as a benchmark for future optimization of MC-CDMA in multiuser scenarios. Moreover, we exemplarily present multiuser performance results with combined-equalization for the uplink which approach the lower bound derived in this paper.

Adaptive MC-CDMA Multiple Channel Estimation and Tracking Over Time-Varying Multipath Fading Channels

Multicarrier code-division multiple access (MC-CDMA) combines multicarrier transmission with direct-sequence (DS) spread spectrum techniques. In this study, several possible channel models of MC-CDMA systems are considered as a multi-state switching Markov process to match the time-varying multipath fading channel. A novel multiple channel estimator, which consists of several possible channel models and can be self-adjusted to fit different channel uncertain conditions, is proposed for MC-CDMA systems to effectively estimate channel coefficients in the frequency domain over time-varying multipath fading channels. Based on the proposed channel estimation, an enhanced minimum mean-square error (MMSE) equalizer is employed for symbol detection of MC-CDMA systems. Simulation results demonstrate that the proposed multiple channel estimator can track the channel more accurately than the conventional channel estimators. With the consideration of channel estimation error as a channel noise to improve symbol detection, the enhanced equalizer based on the proposed multiple channel estimator performs much better than those based on the conventional channel estimators in bit error rate (BER).

Minimum Variance Channel Estimation in MC-CDMA Systems: Bias Analysis and Cramér–Rao Bound

We present a comprehensive performance analysis of the minimum variance channel estimator for multicarrier code-division multiple access systems. We provide novel highly accurate closed form expressions for the bias due to the additive noise as well as the finite data record mean-square error of the channel estimates. In addition, we derive the corresponding Crameacuter-Rao bound that assumes the knowledge of only the spreading code of the desired user

A New Channel Estimation Using Data-Dependent Pilot Symbols for MC-CDMA Systems With Chip Interleaving

We propose a new channel-estimation method in a multicarrier-code division multiple access system with chip interleaving. This new method works with code-multiplexed pilot channels and data-dependent pilot symbols, where the reliability of the estimated channel gain is increased by the noise averaging effect. Simulation results show that the performance of the proposed method is comparable to the minimum mean-square error estimation method with less computation complexity

Estimation of Carrier Frequency Offset for Multicarrier CDMA Uplink

This paper presents a training-based estimation method of carrier frequency offset (CFO) for multicarrier code-division multiple-access (MC-CDMA) uplink. The basic assumption is the same as in Morellis' work (OFDMA), i.e., active users in the network are classified into two categories: reference (synchronized) users (RUs) and new (asynchronized) users (NUs). But, we consider the system that RUs use CDMA to share the wireless medium. NUs employ time-division multiple-access (TDMA) for the training purpose. Our major idea is to perform the CFO estimation in the desired signal subspace that is orthogonal to the RU subspace. The proposed approach is investigated both in the subband and interleaved MC-CDMA systems. It is shown that implementation of the CFO estimation together with the training-pattern design could be relatively simple when users do not experience the considerable frequency-selective fade. In the highly frequency-selective fading scenario, an adaptive training-scheme is proposed to make the CFO estimator robust. We also consider NUs operating in the frequency-division multiple-access (FDMA) training mode. Due to the presence of inter-NU-interference, the estimation performance may be not as good as in the TDMA training mode. The performance degradation may be severe when RUs operate in the interleaved MC-CDMA. In this case, we propose an inter-NU-interference self-canceling training-pattern and use simulations to demonstrate its impact on the estimation performance

A Spectrum-Efficient Multicarrier CDMA Array-Receiver with Diversity-Based Enhanced Time and Frequency Synchronization

This paper proposes a spectrum-efficient spatio-temporal array-receiver for multi-carrier CDMA systems named MC-STAR. First, we derive a new post-correlation model for MC-CDMA that characterizes the structure of the channel in space, time and frequency. Based on this model, we introduce a new multi-carrier array-receiver with rapid and accurate joint synchronization in time and frequency. There, we exploit jointly the spatial, temporal and frequency diversities as well as the intrinsic inter-carrier correlation (termed hereafter frequency gain) to improve the channel identification and the synchronization operations. In addition, based on a new link/system-level performance analysis, with a band-limited chip waveform assumption, we provide a comparative performance study of MC-STAR over two multi-carrier CDMA air-interface configurations, namely MT-CDMA and MC-DS-CDMA, in the most realistic operating conditions. Link/system-level results confirm the advantages of MT-CDMA in increasing throughput and bandwidth efficiency. The current trend is to design radio air-interfaces with flat fading subcarriers. In contrast, with MC-STAR we show that the positive effects of multipath diversity and frequency gain over large strongly-overlapping subcarriers is more significant than the negative effects of multipath and multi-carrier interference.

A Novel Joint Detection of Frequency Offset and MAI for MC-CDMA Systems

While multicarrier CDMA techniques are used to reduce interference and improve the performance of the system in fading channel, carrier frequency offset and multiple access interference remain major obstacles for a multicarrier CDMA system's performance. In this article we propose a novel joint detection method called modified maximum likelihood multistage parallel interference canceller (M-ML-MPIC) to eliminate the frequency offset and MAI simultaneously. The main idea of our approach is to combine PIC multiuser detection and frequency offset estimation based on a maximum likelihood function with guard interval. We discuss our scheme and report on its performance using a set of simulation experiments.

Performance of space-frequency combining at the antenna array of a MC-CDMA system

In this paper, a multicarrier code-division multiple-access (MC-CDMA) system employing antenna array at base station with quaternary phase shift keying (QPSK) modulation is proposed. This receiver adopts a space-frequency two-dimensional (2-D) Rake receiver structure. Based on the detailed analysis of the interference characteristics of the proposed system, the bit error rate (BER) performance of the system is provided. With regard to spatial domain combining, the optimum and suboptimum combining weight is derived, while the suboptimum set of weights is simplified on sense of that only the knowledge of array vector of desired user alone is sufficient for the combining. Simulation results verify the analysis and show that better performance is achieved for the proposed antenna array MC-CDMA system than the corresponding single antenna approach.

Design of spreading codes with high diversity gain for multicarrier CDMA systems

AbstractThe paper proposes a designing guideline for the spreading codes of the multicarrier code division multiple access (MC‐CDMA) systems based on the evaluation of the performance of the maximum‐likelihood multi‐user detection (ML‐MUD) in the frequency selective fading channels. Two parameters of the spreading codes, the minimum Hamming distance and the minimum product distance, are found to bound the performance of the system. According to this, a new kind of spreading codes, which are constructed by offsetting the phase of conventional orthogonal codes, is proposed to maximize the diversity gain. Simulation results show that full diversity order can be obtained even for full loaded systems. The proposed spreading codes can also improve the performance of single‐user detectors. Copyright © 2007 John Wiley & Sons, Ltd.