Multicarrier CDMA Systems Research Articles

Bit Error Rate Analysis for MC-CDMA Systems in Nakagami-"Equation missing" Fading Channels

Multicarrier code division multiple access (MC-CDMA) is a promising technique that combines orthogonal frequency division multiplexing (OFDM) with CDMA. In this paper, based on an alternative expression for the Q-function, characteristic function and Gaussian approximation, we present a new practical technique for determining the bit error rate (BER) of multiuser MC-CDMA systems in frequency-selective Nakagami-m fading channels. The results are applicable to systems employing coherent demodulation with maximal ratio combining (MRC) or equal gain combining (EGC). The analysis assumes that different subcarriers experience independent fading channels, which are not necessarily identically distributed. The final average BER is expressed in the form of a single finite range integral and an integrand composed of tabulated functions which can be easily computed numerically. The accuracy of the proposed approach is demonstrated with computer simulations.

Performance of Asynchronous MC-CDMA Systems with Maximal Ratio Combining in Frequency-Selective Fading Channels

The bit error rate (BER) performance of the asynchronous uplink channel of multicarrier code division multiple access (MC-CDMA) systems with maximal ratio combining (MRC) is analyzed. The study takes into account the effects of channel path correlations in generalized frequency-selective fading channels. Closed-form BER expressions are developed for correlated Nakagami fading channels with arbitrary fading parameters. For channels with correlated Rician fading paths, the BER formula developed is in one-dimensional integration form with finite integration limits, which is also easy to evaluate. The accuracy of the derived BER formulas are verified by computer simulations. The derived BER formulas are also useful in terms of computing other system performance measures such as error floor and user capacity.

On the Performance and Capacity of an Asynchronous Space–Time Block-Coded MC-CDMA System in the Presence of Carrier Frequency Offset

In this paper, the bit-error rate (BER) performance and capacity of asynchronous space-time block-coded (STBC) multicarrier code-division multiple-access (MC-CDMA) systems in the presence of carrier frequency offset (CFO) between the transmitter and receiver oscillators are analyzed. The exact BER expression when using equal gain combining (EGC) and the approximate BER expression when using maximum ratio combining (MRC) are derived. These BER expressions are verified through simulations. Using these derived expressions, the achievable system capacity satisfying a minimum BER requirement can be studied for the two cases when EGC and MRC are used and, hence, it is possible to compare the achievable capacity of STBC MC-CDMA systems with that of MC-CDMA systems. It is concluded that small CFO has an insignificant effect on the BER and capacity of STBC MC-CDMA systems and that this range of CFO is important in transceiver design. Besides, STBC MC-CDMA systems with multiple receive antennas can achieve higher capacity than that of the MC-CDMA systems; this amount can be obtained analytically using the theoretical BER expressions derived.

Single Carrier Cyclic Prefix-Assisted CDMA System with Frequency Domain Equalization for High Data Rate Transmission

Multiple-access interference and interfinger interference limit the capacity of conventional single-carrier DS-CDMA systems. Even though multicarrier CDMA posses the advantages of conventional CDMA and OFDM, it suffers from two major implementation difficulties such as peak-to-average power ratio and high sensitivity to frequency offset and RF phase noise. A novel approach based on single-carrier cyclic prefix-assisted CDMA has been proposed to overcome the disadvantages of single-carrier CDMA and multicarrier modulation. The usefulness of the proposed approach for high-speed packet access with simplified channel estimation procedures are investigated in this paper. The paper also proposes a data-dependent pilot structure for the downlink transmission of the proposed system for enhancing pilot-assisted channel estimation in frequency domain. The performance of the proposed pilot structure is compared against the data-independent common pilot structure. The proposed system is extensively simulated for different channel parameters with different channel estimation and equalization methods and the results are compared against conventional multicarrier CDMA systems with identical system specifications.

Multiuser Detection Techniques Using Maximum Likelihood Sphere Decoding in Multicarrier CDMA Systems

When performed using an exhaustive search, the maximum likelihood (ML) joint detection of all users in a multicarrier code-division multiple-access (MC-CDMA) system has a prohibitive complexity, growing exponentially with the number of users and the number of bits in each modulation symbol. In this paper, a novel ML multiuser detection algorithm is proposed, the complexity of which is a polynomial function of the number of users and is independent of the modulation size. The MC-CDMA system is modeled as a sphere packing lattice and a low-complexity optimum lattice decoder, the sphere decoder, is applied to jointly detect all users. Suboptimum simplifications, based on the orthogonal projection of the received signal on a facet of the lattice constellation, are also proposed to further decrease the complexity. Simulation results are shown with up to 64 users transmitting 16-quadrature-amplitude modulation symbols.

Iterative Semiblind Multiuser Receivers for a Space–Time Block-Coded MC-CDMA Uplink System

Iterative multiuser detection and space-time coding are two promising techniques to improve the capacity and performance of coded multiuser systems in wireless channels. In this paper, we present iterative multiuser detection schemes for a space-time block-coded multicarrier code-division multiple-access system with multiple transmit and receive antennas. We consider a more general case of an uplink system in the presence of both intra- and intercell interferences. We propose two types of iterative semiblind space-time receivers for such an uplink environment. The first is based on the minimum mean-square error criterion and the second is a hybrid scheme based on a combination of parallel interference cancellation and linear multiuser detection. These iterative receivers are derived, using a subspace approach, which utilizes known users' information for the computation of log-likelihood ratios (LLRs) while blindly suppressing the unknown interference. The LLRs are refined successively during the iterative process by using the extrinsic information available through decoding of all known users. A turbo code is used for channel coding. Simulation results in a frequency-selective Rayleigh-fading environment are presented to verify the performance of the proposed schemes.

Constellation Rearrangement: Enhancement for Multilevel Modulation Formats and Transmit Diversity

Radio link adaptivity will be a key feature of the air interfaces of future mobile communication systems. This adaptivity includes techniques such as fast scheduling, hybrid ARQ, transmit diversity and adaptive modulation and coding. The performance of adaptive modulation generally suffers from the power inefficiencies of multilevel modulation formats. This is due to the variations in bit reliabilities caused by the bit-mapping onto the signal constellation. This article presents a concept, called constellation rearrangement (CoRe), which improves the multilevel modulation power efficiency by the joint application with transmit diversity schemes. It is shown by a theoretical analysis for 16-QAM that the presented concept can equalize variations in bit reliabilities by employing different mapping rules for the transmission over the diversity branches. This significantly improves the receiver block error rate performance and, hence, the performance of the adaptive modulation and coding. This is proven by simulations at link-level in a multicarrier CDMA system for AWGN and fading channels employing turbo coded transmission.

Interleaving scheme for multicarrier CDMA system

In MultiCarrier Code-Division Multiple-Access (MC-CDMA) system, the received signals scattered in the frequency domain are combined to get frequency diversity gain. However, the frequency diversity gain is limited because of correlation between subcarriers. A novel interleaving scheme for MC-CDMA system is proposed in this paper. A circular shifting register is introduced into each subcarrier branch to decrease the correlation between subcarriers. By using interleaving, frequency diversity gain of system is increased. System structure and model with interleaver are discussed. In the case of multiple users, Parallel Interference Cancellation (PIC) technique is also introduced. Computer simulations demonstrate the performance of proposed scheme, and the performance comparison of MC-CDMA with interleaver and conventional MC-CDMA system is shown as well.

Merging Multicarrier CDMA and Oscillating-Beam Smart Antenna Arrays: Exploiting Directionality, Transmit Diversity, and Frequency Diversity

In this paper, a novel merger of multicarrier code-division multiple access (MC-CDMA) and smart antenna arrays is introduced. Here, a group of Q carriers in the MC-CDMA system is applied to its own M-element smart antenna array at the base station (BS). The smart antennas are located in close proximity to one another. We generate a transmit diversity gain at the receiver by carefully moving (oscillating) the antenna array's pattern. The pattern oscillation is achieved by applying appropriate time-varying phases to array elements of each smart antenna. The beam pattern oscillation ensures a mainlobe at the position of the intended user and small oscillations in the beam pattern. This beam pattern oscillation leads to a time-varying channel with a controllable coherence time; hence, a transmit diversity benefit, in the form of a time diversity benefit, is available at the receiver. Employing MC-CDMA with the proposed smart antenna at the BS, we achieve: 1) directionality which creates high network capacity via space-division multiple access; 2) a transmit diversity gain which supports high performance at the receiver in the mobile unit; and 3) increased capacity and performance via MC-CDMA's ability to support both CDMA and frequency diversity benefits, respectively.

Performance of an MC-CDMA system based on the complex wavelet packet and pre-equalisation technique

A novel complex wavelet packet-based MC-CDMA (CWP-MC-CDMA) system is proposed, based on the analysis of the principle of multi-carrier code division multiple access (MC-CDMA) and the use of the optimised complex wavelet packet. By making use of the reciprocity between uplink and downlink channels under the time-division duplex (TDD) schemes, the authors adopt the pre-equalisation technique to simplify the receiver architecture and suppress the multi-access interference (MAI) effectively. The system performance is investigated in multipath Rayleigh fading channels. The CWP-MC-CDMA scheme with the energy-controlled pre-equalisation technique is also developed due to the greater energy consumption in pre-compensating for the weak sub-carriers. The theoretical analysis and simulation results show that the proposed CWP-MC-CDMA system has a superior bit-error rate (BER) performance to that of the conventional MC-CDMA system based on DFT (DFT-MC-CDMA), and the system with pre-equalisation also outperforms other detection-combining techniques, such as equal gain combining (EGC), maximum ratio combining (MRC), orthogonality restoring combining (ORC) and minimum mean square error combining (MMSEC).

Pilot-Based Channel Estimation for OFDM Systems by Tracking the Delay-Subspace

In orthogonal frequency division multiplexing (OFDM) systems over fast-varying fading channels, channel estimation and tracking is generally carried out by transmitting known pilot symbols in given positions of the frequency-time grid. The traditional approach consists of two steps. First, the least-squares (LS) estimate is obtained over the pilot subcarriers. Then, this preliminary estimate is interpolated/smoothed over the entire frequency-time grid. In this paper, we propose to add an intermediate step, whose purpose is to increase the accuracy of the estimate over the pilot subcarriers. The presented techniques are based on the observation that the wireless radio channel can be parametrized as a combination of paths, each characterized by a delay and a complex amplitude. The amplitudes show fast temporal variations due to the mobility of terminals while the delays (and their associated delay-subspace) are almost constant over a large number of OFDM symbols. We propose to track the delay-subspace by a subspace tracking algorithm and the amplitudes by the least mean square algorithm (or modifications of the latter). The approach can be extended to multiple input multiple output OFDM or multicarrier code-division multiple-access systems. Analytical results and simulations prove the relevant benefits of the novel structure.

Performance analysis of multicarrier CDMA systems with parallel and serial concatenated coding in fading channels

The authors present a bit error rate performance analysis of multicarrier code division multiple access (MC-CDMA) systems with turbo and serial concatenated convolutional coding (SCCC) in multipath fading channels. The performance analysis is done for maximal ratio combining and minimum mean square error combining detection in the downlink system. Upper bounds to the average bit error probability are presented for a punctured turbo code and for an SCCC code of similar decoding complexity. These analytical bounds are derived for fully interleaved Rayleigh fading channels. The bit error rate performance is also verified by simulations in the regions of low signal-to-noise ratios. The analytical and simulation results illustrate the relative merits of the turbo and SCCC codes for MC-CDMA systems and their suitability to achieve very low error rates in wireless data applications.

On frequency domain equalization for MC-CDMA in Nakagami fading channels

In this letter, we evaluate the performance of multicarrier code division multiple access (MC-CDMA) in terms of average bit error probability (BEP) in Nakagami fading channels. The results are applicable to MC-CDMA systems employing coherent demodulation with maximal-ratio combining (MRC) or equal gain combining (EGC) reception. The effects of fading parameters and number of users are presented. The accuracy of the proposed analysis is demonstrated by computer simulations. The BEP performance of the EGC receiver in the uplink is highly influenced by the fading parameter compared with that of the MRC receiver. The EGC receiver outperforms the MRC receiver in the downlink, but the MRC receiver gives almost the same performance as the EGC in the uplink. Copyright © 2004 AEI

Multi-Rate Access Schemes for MC-CDMA

In order to accommodate different types of traffic in future wireless communications, it is necessary to consider a system, which can operate satisfactorily at multiple transmission rates. Due to its capability to cope with the hostile frequency selective fading, that limits transmission rate, and its suitability to handle multi-rate data, multi-carrier CDMA (MC-CDMA) has recently drawn considerable attention as a suitable candidate for supporting multimedia services in wireless communications. Multi-rate access schemes where users are able to transmit at different data rates on MC-CDMA systems are presented in this paper. Four multi-rate access schemes: uncoded fixed spreading length (UFSL), coded fixed spreading length (CFSL), multi-code fixed spreading length (MFSL) and variable spreading length (VSL) schemes are proposed. With these schemes, different traffic such as voice, video and high rate data can be transmitted seamlessly through one MC-CDMA infrastructure. A chip-level minimum mean square error combining (MMSEC) technique is employed for joint energy combining and interference cancellation purpose. The performance of these schemes with MMSEC is compared by both theoretical derivations and simulation results under frequency selective Rayleigh fading channels.

Interference cancellation for mobile dispersive channels

A robust interference canceller for Multi-Carrier Code Division Multiple Access (MC-CDMA) using Orthogonal Frequency Division Multiplexing (OFDM) in Rayleigh fading is proposed. This interference canceller is robust in the sense that it cancels Inter-Carriers Interference (ICI) and is suitable for use in dispersive channels. To come up the effects of the signal dispersion, Doppler shifts and delay spreads on the performance of MC-CDMA systems over mobile fading channels, this interference canceller exploits the merit of the orthogonal signaling and pilot signals to evaluate the channel parameters. This interface canceller is well suited to work in iterative turbo interference cancellation.

Performance analysis of an improved multi‐carrier CDMA system under frequency‐selective Rayleigh fading channels

AbstractIn this article, an analytical method is proposed to study an improved orthogonal multi‐carrier DS/CDMA system, which uses QPSK for both spreading and carrier modulations without redundant sub‐carriers and time interleavers in each sub‐channel. The article concerns in particular such a system and its performance under frequency‐selective Rayleigh fading channels with various delay‐power profiles. The bit error probabilities under varying multipath‐diversity orders (using maximal ratio combing RAKE reception) and different numbers of resolvable multipaths are derived. It has been shown that the system, despite of its structural simplicity, performs well in mitigating frequency‐selective fading. The performance is also evaluated taking into account various other parameter sets, such as numbers of users and sub‐carriers, lengths of PN codes and the number of fingers in an RAKE receiver, etc. Copyright © 2003 John Wiley & Sons, Ltd.

Performance analysis of concatenated space-time coding with two transmit antennas

In this paper, we show how Alamouti's simple but useful transmit diversity scheme for two antennas can be combined with a standard outer error-correcting code to achieve a stronger concatenated space-time coding scheme. By introducing a matrix formalism that allows us to interpret the transmission channel as a rotation in an Euclidean space, it can be easily shown that this scheme with two transmit (TX) and L/sub r/ receive (RX) antennas is equivalent to a simple RX antenna setup with 2L/sub r/ RX antennas. Analytical formulas for pair error probabilities will be derived for the time and/or frequency flat fading and for the ideally interleaved Rayleigh fading channel as well as for the correlated fading channel. As a practical example, we study how the performance of a Walsh-Hadamard coded multicarrier code-division multiple-access system depends on the correlation bandwidth of the channel and the number of RX and TX antennas.

Achieving high-capacity wireless by merging multicarrier CDMA systems and oscillating-beam smart antenna arrays

We establish the network capacity (measured in terms of number of users) of a wireless system merging multicarrier code-division multiple-access (MC-CDMA) and smart antennas with oscillating-beam patterns. The MC-CDMA component supports high performance (in a probability-of-error sense) via frequency diversity and high network capacity via code division. The smart antenna with oscillating-beam pattern further enhances performance via transmit diversity (in the form of an induced time diversity) and further enhances network capacity via spatial division. The proposed merger has been shown to achieve a very high performance by exploiting a two-dimensional time-frequency diversity. We demonstrate the impressive network capacity gains achieved by this merger.

Hierarchical wideband multicarrier CDMA systems: Reverse link analysis

The reverse link performance of hierarchical wideband multicarrier code-division multiple-access (MC-CDMA) systems is studied. An MC-CDMA system divides the system bandwidth into several equal narrow subbands that are used to transmit multiple signal waveforms in parallel. MC-CDMA systems are known to be robust to multipath fading and narrowband interference. We propose a hierarchical MC-CDMA system where the microcell(s) occupies a fraction of the available subbands. It is shown that such an architecture is a viable way of supporting microcell traffic while protecting the existing macrocell traffic. The effects of microcell Nakagami fading and power control error are also studied.

An iterative maximum SINR receiver for multicarrier CDMA systems over a multipath fading channel with frequency offset

A robust iterative multicarrier code-division multiple-access (MC-CDMA) receiver with adaptive multiple-access interference (MAI) suppression is proposed for a pilot symbols assisted system over a multipath fading channel with frequency offset. The design of the receiver involves a two-stage procedure. First, an adaptive filter based on the generalized sidelobe canceller (GSC) technique is constructed at each finger to perform despreading and suppression of MAI. Second, pilot symbols assisted frequency offset estimation, channel estimation and a RAKE combining give the estimate of signal symbols. In order to enhance the convergence behavior of the GSC adaptive filters, a decisions-aided scheme is proposed, in which the signal waveform is first reconstructed and then subtracted from the input data of the adaptive filters. With signal subtraction, the proposed MC-CDMA receiver can achieve nearly the performance of the ideal maximum signal-to-interference-plus noise ratio receiver assuming perfect channel and frequency offset information. Finally, a low-complexity partially adaptive (PA) realization of the GSC adaptive filters is presented as an alternative to the conventional multiuser detectors. The new PA receiver is shown to be robust to multiuser channel estimation errors and offer nearly the same performance of the fully adaptive receiver.