Downlink Multi-carrier Code Division Multiple Access Research Articles

Novel Metric-Based PAPR Reduction Schemes for MC-CDMA Systems

This paper proposes the use of a metric-based peak-to-average power ratio (PAPR) reduction method and the joint spreading code allocation and subcarrier scrambling (JSCASS) scheme in downlink multicarrier code-division multiple-access (MC-CDMA) systems. Subcarrier scrambling is employed in MC-CDMA systems to introduce additional disturbances to the correlated patterns among subcarriers. In particular, the variance of the time-domain signal power is derived from an approximate closed-form expression. Given that PAPR values are monotonically related to the proposed metric values, the PAPR of a downlink MC-CDMA signal can be efficiently reduced by assigning the lowest metric value to the spreading code allocation and subcarrier scrambling pattern. The proposed JSCASS scheme adopts the cross entropy (CE) method to jointly optimize the spreading code allocation and scrambling pattern. To reduce implementation cost, a sequential spreading code allocation and subcarrier scrambling (SSCASS) method is proposed by optimizing the spreading code allocation with the tree-search method (TSM) and by determining the scrambling pattern with the CE method. Simulation results show that the performance gap between the proposed JSCASS and SSCASS schemes is within 0.2 dB. Moreover, both schemes outperform the conventional method, which simply allocates spreading codes by 6 dB when the number of subcarriers is 512.

Multi-user Performance of Relay-Aided MC-CDMA Downlink System Employing Transmitter Preprocessing Based on Feedback of Vector Quantized CSI

The achievable bit-error-rate (BER) performance of a relay-aided downlink (DL) multi-carrier code division multiple access (MC-CDMA) cellular system, where multi-user interference (MUI) and inter-relay interference (IRI) are the dominant channel impairments is investigated in this paper, principally from a transmitter signal processing perspective. Specifically, in this paper, we investigate the considered system by employing three cooperation strategies. The MUI and IRI are mitigated with the aid of the transmitter preprocessing (TP) operated at the base station (BS) and relays, respectively. The TP operated at the BS that suppresses the MUI at the relays is based on the maximum-signal-to-interference-plus-noise ratio approach, while that operated at the relays that mitigates the IRI relies on the classical transmit minimum-mean-square-error principle. Further, the TP in our work is constructed with the aid of vector-quantized channel state information (VQ-CSI) acquired via feedback channels. Our simulation study shows that relay-aided cooperative DL MC-CDMA system employing TP, results in better achievable BER, as the TP helps in achieving the relay-diversity by mitigating the DL-MUI and IRI at the relays and DMSs, respectively. Furthermore, our study demonstrates that the resultant BER performance of the VQ-CSI based TP remains close to that achieved with the perfect CSI based TP. Hence, we advocate that the feedback of the VQ-CSI to the transmitter for conceiving the preprocessing can be considered as an efficient approach, particularly in a frequency division duplex type wireless system.

Cooperative communication‐aided multi‐carrier code division multiple access downlink transmission with transmitter preprocessing: performance results

In this correspondence, the authors study the performance of multi-user transmitter preprocessing (MUTP)-assisted cooperative downlink (DL) transmission for multi-carrier code division multiple access (MC-CDMA) system, where multi-user interference (MUI) and inter-relay interference (IRI) are the principal channel impairments. Relaying is facilitated with the aid of fixed infrastructure-based relays that perform additional signal processing like formulating the preprocessing matrix for mitigating MUI in addition to forwarding the information they receive from the base station (BS). Specifically, in this study, the authors analyse the performance of cooperative communication-aided DL MC-CDMA system by employing three cooperation strategies: (i) all users supported by a single relay with dominant MUI at the relay and mobile stations (MSs); (ii) each user supported by a single relay where, MUI and IRI are dominant at the relays and MSs, respectively, as well as weak IRI at the MSs; and (iii) all users supported by L relays where MUI is dominant at the relays, and MUI and IRI are dominant at the MSs. Our simulation study shows that MUTP-aided cooperative DL transmission results in better achievable bit error rate than the multi-user detection-aided system as the MUI at the relays and MUI and IRI at the MSs are perfectly eliminated.

Interference Cancellation with Space Diversity for Downlink MC-CDMA Systems

Modern wireless communications require an efficient spectrum usage and high channel capacity and throughput. Multiple-input and multiple-output (MIMO), Linear equalizers, multi-user detection and multicarrier code-division multiple access (MC-CDMA) are possible solutions to achieve spectral efficiency, high channel capacity, eliminate multiple access interference (MAI), eliminate Inter symbol interference (ISI) and robustness against frequency selective fading. In this paper, we combine all these techniques and investigate BER performance. We propose a low complexity receiver structure for Single-input Multiple-output (SIMO) downlink MC-CDMA systems. It employs an interference cancellation scheme to suppress the interference caused by the multipath fading channel. Also, the proposed scheme is developed for MIMO MC-CDMA system. The performance analysis of Downlink MIMO MC-CDMA systems with V-BLAST over frequency selective fading channel is investigated under various number of transmit and receive antennas. The simulation results show proposed SIMO equalization with parallel interference cancellation scheme is effective in reducing the ISI and the MAI. It improves the performance significantly and the simulation results show that MIMO MC-CDMA with V-BLAST multi-user detection provides high data rate and the BER significant improvement.

Parallel interference cancellation with different linear equalisation and Rake receiver for the downlink MC-CDMA systems

The multi-carrier code division multiple access (MC-CDMA) is becoming a very attractive multiple access technique for high-rate data transmission in the future wireless communication systems. MC-CDMA systems transmitting over multipath channels suffer from inter-symbol interference (ISI) and multiple access interference (MAI). Recently, MC-CDMA with equalisation has attracted much attention for its ability to obtain an excellent performance even in strong frequency-selective fading channels. In this study, the proposed architecture uses the minimum mean-square error (MMSE) with parallel interference cancellation (PIC) for downlink MC-CDMA system and compared with another system based on the rake receiver with PIC for downlink MC-CDMA system. A comparison between such architectures is presented. The effect of the tentative decision functions and the effect of the loading users on the performance of the two proposed receivers are discussed and presented in this study. Simulation results show that the combination of tentative decisions with PIC and equalisation provides an efficient solution to suppress the MAI and ISI in downlink MC-CDMA systems over frequency-selective fading channels.

Joint detection, channel estimation, and interference cancellation in downlink MC-CDMA communication systems using complex-valued multilayer neural networks

Data detection in the presence of interference is one of the main challenges in multicarrier code division multiple access (MC-CDMA) communication systems. In this paper, a new detection technique for downlink MC-CDMA systems is proposed. This technique uses complex-valued multilayer neural networks at the receiver side. With the new definition for desired responses (±(1+J) instead of ±1, where $$ J = \sqrt {{ - 1}} $$ ), the convergence rate is increased (in the training process) regarding to saturation of imaginary output and the performance is increased because of increasing Euclidean distance of output neuron inputs in two states of desired outputs (with factor of $$ \sqrt {2} $$ ). The performance of the proposed method is improved further by considering two various saturation coefficients (in the activation function of output layer) in the training and test processes. Since the last performance improving lead to low convergence rate, this effect is compensated by correcting the coefficient of training rate in the output layer. Simulation results confirm the high convergence rate, low computational complexity, and also good performance of the proposed method in wide range of SNRs.

Adaptive BER-constraint-based power allocation for downlink MC-CDMA systems with linear MMSE receiver

To facilitate the efficient allocation of limited transmit power, a bit error rate (BER) constraint based power allocation scheme is proposed in this paper for the downlink of multi-carrier code division multiple access (MC-CDMA) systems with linear minimum mean square error (MMSE) receiver. According to the instantaneous channel conditions, the proposed scheme can adaptively allocate the required minimum transmit power on each channel to meet the BER constraint. The simulation results show that the proposed scheme is efficient in reducing the transmit power while meeting BER constraint, and thereby improve the system performance.

SAGE Based Suboptimal Receiver for Downlink MC-CDMA Systems

An iterative joint data detection and channel estimation algorithm for downlink multi-carrier code division multiple access (MC-CDMA) systems is proposed. The resulting algorithm is a space alternating generalized expectation-maximization (SAGE) algorithm which updates the data sequences serially and the channel parameters in parallel, leading to a receiver structure that also incorparates interchannel interference cancellation. Its performance is compared with various Minimum Mean Square Error (MMSE) estimators for a multipath frequency selective fading channel using simulations. It is illustrated that the proposed SAGE algorithm performs better than the MMSE estimators considered in this paper.

PAPR reduction in SFBC MIMO MC-CDMA systems via user reservation

The combination of multicarrier code-division multiple access (MC-CDMA) with multiple-input multiple-output technology is attractive for broadband wireless communications. However, the large values of the peak-to-average power ratio (PAPR) of the signals transmitted on different antennas can lead to nonlinear distortion and a subsequent degradation of the system performance. In this article, we propose a PAPR reduction scheme for space-frequency block coding MC-CDMA downlink transmissions that does not require any processing at the receiver side because it is based on the addition of signals employing the spreading codes of inactive users. As the minimization of the PAPR leads to a second-order cone programming problem that can be too cumbersome for a practical implementation, some strategies to mitigate the complexity of the proposed method are also explored.

Interference Cancellation with RZF Equalization for the Downlink MC-CDMA Systems

The Multi-Carrier Code Division Multiple Access (MC-CDMA) is becoming a very attractive multiple access technique for high-rate data transmission in the future wireless communication systems. The advantage of MC-CDMA systems that is work in frequency domain. Recently, MC-CDMA with any equalization scheme has attracted much attention for its ability to obtain an excellent performance even in strong frequency selective fading channels. In this paper, the two proposed architecture of the downlink MC-CDMA is based on the regularized zero forcing equalizer (RZF) with parallel interference cancellation (PIC) RZF-PIC) and the rake receiver with parallel interference cancellation (PIC) (RAKE-PIC) to suffer from the intersymbol interference (ISI) and multiple access interference (MAI). Two schemes are implemented in frequency domain. The effects of the tentative decision functions, the user loading on the performance of the proposed receivers architecture, and the number of cancelled users are discussed in this paper. Simulation results show the advantageof the RZF equalizer result over rake receiver and show that the combination of PIC and equalization provides an efficient solution to suppress the MAI and ISI in downlink MCCDMA systems over frequency selective fading channels.

Parallel Interference Cancellation with MMSE Equalization for the Downlink MC-CDMA Systems

The Multi-Carrier Code Division Multiple Access (MC-CDMA) is becoming a very attractive multiple access technique for high-rate data transmission in the future wireless communication systems. MC-CDMA systems transmitting over multipath channels suffer from intersymbol interference (ISI) and multiple access interference (MAI). Recently, MCCDMA with equalization has attracted much attention for its ability to obtain an excellent performance even in strong frequency selective fading channels. In this paper, the proposed architecture uses the minimum mean square error (MMSE) with parallel interference cancellation (PIC) for downlink MC-CDMA system and compared with another system based on the rake receiver with PIC for downlink MC-CDMA system. A comparison between such architectures is presented. The effect of the tentative decision functions, the number of cancelled users and the effect of the loading users on the performance of the two proposed receivers are discussed and presented in this paper. Simulation results show that the combination of tentative decisions with PIC and equalization provides an efficient solution to suppress the MAI and ISI in downlink MC-CDMA systems over frequency selective fading channels.

Partial Equalization for MC–CDMA Systems in Non-Ideally Estimated Correlated Fading

Multicarrier code-division multiple access (MC-CDMA) can support high data rates in next-generation multiuser wireless communication systems. Partial equalization (PE) is a low-complexity technique for combining the signals of subcarriers to improve the achievable performance of MC-CDMA systems in terms of their bit error probability (BEP) and bit error outage (BEO) in comparison with maximal ratio combining, orthogonality restoring combining, and equal-gain combining techniques. We analyze the performance of the multiuser MC-CDMA downlink and derive the optimal PE parameter expression, which minimizes the BEP. Realistic imperfect channel estimation and frequency-domain (FD) block-fading channels are considered. More explicitly, the analytical expression of the optimum PE parameter is derived as a function of the number of subcarriers, number of active users (i.e., the system load), mean signal-to-noise ratio (SNR), and variance of the channel-estimation errors for the aforementioned FD block-fading channel. We show that the choice of the optimal PE technique significantly increases the achievable system load for the given target BEP and BEO.

BER Degradation of MC-CDMA at High SNR with MMSE Equalization and Residual Frequency Offset

Multicarrier Code Division Multiple Access (MC-CDMA) is an attractive technique for high speed wireless data transmission in view of its advantages over orthogonal frequency division multiplexing. In this paper, we analyze the performance of fully loaded downlink MC-CDMA systems with minimum mean square error (MMSE) equalizer in the presence of residual frequency offset (RFO) in multipath Rayleigh fading channels. We first show that as the SNR is increased beyond a value, referred as threshold SNR, the performance degrades. We then analyze the cause for this behavior and propose a remedy to prevent the degradation by regularizing the coefficient(s) of the equalizer, and use the regularized equalizer for SNRs beyond the threshold value. We suggest two methods for estimating this SNR, one gives close to the true value but requires the knowledge of RFO and the channel state information (CSI), while the other gives an approximate value but requires only CSI. We show that the regularization based on the approximate value also prevents the degradation, but the performance at higher SNRs is slightly poorer compared to that with the better estimate. Numerical and simulation results are provided to support the analysis.

Transmission Strategies for MISO Downlink MC-CDMA Mobile Systems

This paper proposes two space-frequency schemes with a multi-user pre-filtering technique for downlink (DL) multicarrier code division multiple access (MC-CDMA) systems. We consider the use of antenna arrays at the base station (BS) and a single antenna at the mobile terminal (MT) and derive the proposed multi-user pre-filtering technique that modulates the transmitted signal to eliminate the effects of multiple access interference (MAI) and channel distortions at the mobile terminals, while maintaining low MT complexity. Two types of detectors are considered at the MT: simple despreading and single user equalizers. The performances of the proposed schemes are compared to those of other transmit signal design approaches that have been recently proposed for DL MC-CDMA, considering both typical indoor and pedestrian scenarios, and channel coding based on UMTS specifications.

Blind and semi-blind equalization of downlink MC-CDMA system exploiting guard interval redundancy and excess codes

This paper introduces new blind time domain equalization (TEQ) techniques for downlink multicarrier code division multiple access (MC-CDMA) systems. This equalization exploits the guard interval (GI) redundancy together with the excess codes (EC) to restore the structural properties of MC-CDMA signal destroyed by multipath fading channels. Indeed, since in CDMA based wireless systems, the number of users per cell is significantly less than the spreading factor, a base station can set apart a subset of the codes, the excess codes that will not be used for spreading. It is shown that the joint use of the EC and GI redundancy improves significantly the equalization performance compared to the GI only or EC only cases. A second contribution consists in exploiting the previous information together with the one given by a pilot sequence in a semi blind scheme to further improve the performance. Finally, we propose a new adaptive algorithm for the estimation and tracking of the equalizer that has a faster convergence rate compared to existing algorithms of similar complexity order. Simulation based performance evaluations are given at the end of this paper to assess the effectiveness of the proposed methods in MC-CDMA.

BER Performance of Downlink MC-CDMA with ORC in Nakagami-m Fading Channel

In this letter, we derive an exact bit error rate (BER) expression for downlink multi-carrier code division multiple access (MC-CDMA) systems with orthogonal restoring combining (ORC) in Nakagami-m fading channel. A simple approximated expression is also provided. For uncoded and coded MC-CDMA systems, the BER expressions are calculated based on the moment generating function (MGF) of the combined fading random variable. The derived analytic expressions are verified by simulation results.

Robust MOE detector for MC-CDMA downlink systems under carrier frequency offset

In this paper, a performance analysis of carrier frequency offset (CFO) on the minimum output energy (MOE)-based detectors in multi-carrier code-division multiple-access (MC-CDMA) downlink systems is first presented. Furthermore, a blind decision-directed (DD) MOE detector for enhancing robustness against CFO and the finite sample effect is proposed. Numerical study confirms the effectiveness of the proposed method and the performance improvement over conventional detectors.

Code Selection Method for Downlink MC-CDMA Over Frequency-Selective Fading Channel

A spreading code-allocation procedure using simple least squares (LS) channel estimation is presented to reduce multiple-access interference (MAI) in downlink multicarrier code-division multiple-access (MC-CDMA) systems. The spreading codes chosen by the proposed method are in a consistent sequence.

Complexity Reduction for MC-CDMA With MMSEC

The minimum mean square error combining (MMSEC) scheme performs best among various equalization and combining schemes for downlink multicarrier code-division multiple access (MC-CDMA). However, it is also the most computationally complex to realize as it involves the complex matrix inversion operation of a large matrix. In this paper, a new method is proposed to calculate the equalizer coefficient for MMSEC based on reduced-size matrices. It is shown that as long as the number of users is even, the matrix inversion size is reduced to at least , where is the number of subcarriers.

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.