Multicarrier Code-division Multiple-access Signals Research Articles

Multiple-Access Interference and Multipath Influence Mitigation for Multicarrier Code-Division Multiple-Access Signals

Sidelobe raising, caused by multipath and multiple-access interference (MAI), hinders the signal detection and time-delay estimation of multicarrier code-division multiple-access (MC-CDMA) signals, which are crucial for localization and tracking applications as well as the sampling-instant configuration of demodulators. Therefore, we propose a method based on reiterative minimum mean-square error (RMMSE) filters to mitigate the MAI and multipath influence. The RMMSE filter is modified to adapt to the multicarrier modulation and symbol involvement characteristics of MC-CDMA signals. The superiority of the proposed method in terms of sidelobes suppression, compared with existing matched filter, least square estimator, least mean square, and single-carrier RMMSE methods, is verified through simulations in the presence of MAIs and short-time-delay multipath. The results show the advantages of the proposed method in terms of high temporal resolution and robustness to MAI influence. This study could contribute toward multipath detection and aid multipath mitigation techniques in strong MAI scenarios.

Iterative Invertible Clipping Method for PAPR Reduction in Wireless MC-CDMA Communication Systems

Background & Objective: One of the disadvantages of Multi-carrier Code Division Multiple Access (MC-CDMA) system is the high value of Peak to Average Power Ratio (PAPR). This paper proposes an approach called Iterative Invertible Clipping Method (IICM) to address the PAPR for MC-CDMA signal based on wireless communication system characteristics. This is mainly concentrated in clipping function making use of a polynomial of degree three at the transmitter, and its inverse at the receiver, to recover the original signal. Method: A Fast Fourier Transform (FFT)-based frequency domain filter is employed to reduce the overall peak regrowth which is supported by measuring the Adjacent Channel Power Ratio (ACPR) after soft clipping and filtering. This technique is compared with Ragusa method using a polynomial of degree 5. Result: The simulated results of the proposed method showed better performance in terms of PAPR reduction, Bit Error Rate (BER), and computational complexity requiring two iterations only. The Complementary Cumulative Complementary Density Function (CCDF) is served to measure and improve PAPR performance of the system. Conclusion: The channel coding process is applied to strengthen the obtained results in the Additive White Gaussian Noise (AWGN) channel and fading channel (Rayleigh). The High-Power Amplifier (HPA) is used for validation purposes.

A Novel Detected Peak Samples Combining Technique for PAPR Minimization of MC-CDMA Signal

A new signal scrambling technique is proposed to minimize the peak to average power ratio (PAPR) of multi carrier code division multiple access (MC-CDMA) signal using novel detected peak samples combining technique. This new approach minimizes PAPR by combing the MC-CDMA symbol with a number of copies of the detected peak sample which belong to the same MC-CDMA symbol. The analytical and simulation results show that by the right selection of the number of peak samples; the PAPR for 64 subcarrier system reduces by 4.9 dB at the complementary cumulative distribution function = 10−3. The PAPR can be further reduced by increasing the number of peak samples but it could compromise the data speed. The implementation of this technique is very simple when compared to the existing techniques like selective mapping, partial transmit sequence and tone reservation as it requires lesser number of Inverse Fast Fourier Transform/Fast Fourier Transform blocks, no side information and low processing delay.

Evolution of Digital Iterative Receiver for Multicarrier Code-Division Multiple-Access Signals with Low Peak-to-Mean Envelope Power Ratio Value

Evolution of Digital Iterative Receiver for Multicarrier Code-Division Multiple-Access Signals with Low Peak-to-Mean Envelope Power Ratio Value

Dynamic User Grouping-Based NOMA Over Rayleigh Fading Channels

With the development of wireless communications and internet of things (IoT), non-orthogonal multiple access (NOMA) has been considered as one of effective schemes to meet the rapidly growing user access requirements. Two types of NOMA, i.e., NOMA-2000 and power-domain NOMA (PD-NOMA), have been proposed in recent years. The first one is a superposition of a set of orthogonal frequency division multiple access (OFDMA) signals and a set of spread-spectrum multicarrier-code division multiple access (MC-CDMA) signals while the second one is a direct superposition of a MC-CDMA signal and an OFDMA signal. This paper proposes a dynamic user grouping method for classifying users and then compares the performance between NOMA-2000 and PD-NOMA over Rayleigh fading channels. Simulation results show that the PD-NOMA can always exhibit lower bit error rate (BER) than the NOMA-2000 under different signal-to-noise ratios. Under the Rayleigh fading environment, the performance of the PD-NOMA after user grouping is better than that of NOMA-2000.

A joint cross-layer transmission design with time–frequency coded cooperation HARQ for underground coal mine MC-CDMA WSNs

In this paper, multicarrier code division multiple access (MC-CDMA) modulation is adapted to constitute wireless sensors to improve the monitoring performance of wireless sensor networks (WSNs) for underground coal mine. A subcarrier phase compensation algorithm based on selective mapping (SLM) is proposed to reduce the relatively high PAPR of MC-CDMA signal. To further improve the monitoring performance of the underground MC-CDMA WSNs, a joint cross-layer transmission with time–frequency coded cooperation hybrid automatic repeat request (HARQ) is also proposed. The proposed cross-layer transmission combines time–frequency coded cooperation of physical layer with HARQ of media access control (MAC) layer. In the proposed transmission, the cooperative sensor utilizes time–frequency coded cooperation method to retransmit the monitoring information of source sensor at each retransmission time to obtain the coding gain and spatial diversity gain. Simulation results show that the proposed joint cross-layer transmission for underground coal mine MC-CDMA WSNs based on SLM phase compensation has significantly reduced the PAPR of MC-CDMA signal and improved the monitor performance of the coal mine MC-CDMA WSNs.

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.

Intercarrier interference reduction in MC-CDMA system through second order duobinary coded phase rotated conjugate cancellation scheme.

Multi-carrier code division multiple access (MC-CDMA) technique is one of the strong candidates for next generation wireless mobile communication systems. Multi-carrier systems are very much sensitive to carrier frequency offset (CFO) results in intercarrier interference (ICI). To mitigate ICI without any spectral loss, a second order duobinary coded phase rotated conjugate cancellation algorithm is proposed in this paper. In the conventional phase rotated conjugate cancellation (PRCC) technique, one path carries the MC-CDMA signal with a phase spin of ϕ and the other path carries the conjugate of the first path signal with -ϕ phase spin. This artificial phase rotation allows the transmitter to tune the transmitted signals so that the ICI effects could be mutually cancelled at the receiver. Although the PRCC technique reduces the spectral efficiency, the limitation can be overcome by the joint second order duobinary coding scheme with PRCC technique. In the proposed method, the correlative coding between the binary symbols modulated on adjacent subcarriers is used to reduce the ICI without any spectral loss. Simulation results show that the proposed PRCC method provides better carrier to interference ratio (CIR) and bit error rate (BER) performances compared to the conventional conjugate cancellation (CC) technique.

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.

Spectral Sidelobe Decaying Property of Walsh-Hadamard Code in MC-CDMA Systems

In this letter, a spectral sidelobe-decaying property of the Walsh-Hadamard code employed in multicarrier code-division multiple access (MC-CDMA) systems is exploited. It is shown that the sidelobe-decaying characteristic of the MC-CDMA signal can be properly described by the code index of the underlined Walsh-Hadamard codeword.

Partial transmit sequences based on artificial bee colony algorithm for peak-to-average power ratio reduction in multicarrier code division multiple access systems

Multicarrier code division multiple access (MC-CDMA) is a very promising system for wireless communication. However, MC-CDMA signals have a high peak-to-average power ratio (PAPR), which causes signal distortion because of the use of a high-power amplifier (HPA) in the transmitter. Partial transmit sequences (PTSs) represent one of the most attractive PAPR reduction methods, but its high computational complexity in finding the optimal phase vector impedes practical implementation. In this paper, we propose a PTS based on an artificial bee colony (ABC) algorithm scheme (ABC-PTS) to reduce the computational complexity of the PTS in the MC-CDMA systems. Simulation results prove that the proposed ABC-PTS scheme shows a significant improvement in PAPR reduction performance, with a low computational complexity. In addition, the bit-error-rate performance of the MC-CDMA with the ABC-PTS and the conventional PTS is compared when the HPA and the linear amplifier are used.

Joint Turbo Equalization and Multiuser Detection of MC-CDMA Signals With Low Envelope Fluctuations

In this paper, we consider the uplink transmission in multicarrier code-division multiple-access (MC-CDMA) systems. As other multicarrier signals, MC-CDMA signals have high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR), which leads to amplification difficulties. This is particularly important for the uplink transmission, since an efficient low-cost power amplification is desirable at the mobile terminals (MTs). Moreover, the transmission over time-dispersive channels destroys the orthogonality between spreading codes, which might lead to significant multiple-access interference levels. To reduce the envelope fluctuations of the transmitted signals, while maintaining the spectral efficiency, the MC-CDMA signal associated to each MT is submitted to a clipping device, followed by a frequency-domain filtering operation. However, the nonlinear distortion effects can be high when an MC-CDMA transmitter with reduced envelope fluctuations is intended (e.g., a small clipping level and/or when successive clipping and filtering operations are employed). In this paper, we define an iterative receiver that jointly performs a turbo multiuser detection and the estimation and cancellation of the nonlinear distortion effects. Our performance results show that the proposed receiver structure allows good performances, very close to the linear receiver ones, even for high system load and/or when a PMEPR as low as 1.7 dB is intended for each MT.

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.

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 of Fractionally Spread Multicarrier CDMA in AWGN as Well as Slow and Fast Nakagami-<tex>$m$</tex>Fading Channels

In multicarrier code-division multiple-access (MC-CDMA), the total system bandwidth is divided into a number of subbands, where each subband may use direct-sequence (DS) spreading and each subband signal is transmitted using a subcarrier frequency. In this paper, we divide the symbol duration into a number of fractional subsymbol durations also referred to here as fractions, in a manner analogous to subbands in MC-CDMA systems. In the proposed MC-CDMA scheme, the data streams are spread at both the symbol-fraction level and at the chip level by the transmitter, and hence the proposed scheme is referred to as the fractionally spread MC-CDMA arrangement, or FS MC-CDMA. Furthermore, the FS MC-CDMA signal is additionally spread in the frequency (F)-domain using a spreading code with the aid of a number of subcarriers. In comparison to conventional MC-CDMA schemes, which are suitable for communications over frequency-selective fading channels, our study demonstrates that the proposed FS MC-CDMA is capable of efficiently exploiting both the frequency-selective and the time-selective characteristics of wireless channels.

Multiple-Access Interference Suppression for MC-CDMA by Frequency-Domain Oversampling

A technique for interference suppression in multicarrier code-division multiple-access (MC-CDMA) systems is proposed which exploits the structural differences in signals that arrive at the receiver with Doppler shifts or carrier offsets. Oversampling the received signal in the frequency domain and properly combining the samples provides the interference suppression. Frequency-domain oversampling is accomplished by using a time extension of the conventional MC-CDMA signal or unconsumed portions of the cyclic prefix. Furthermore, a receiver structure is introduced that despreads and combines groups of samples so that a linear minimum mean-squared error solution for combining the groups is easily found. This combining scheme increases the signal-to-interference ratio experienced by the desired user in the MC-CDMA system. In addition, the receiver performs well in severe near-far scenarios when there is sufficient frequency separation between the signals of the desired user and an interferer. Numerical results show that the proposed receiver significantly outperforms the conventional MC-CDMA receiver in many channel realizations.

Crest factors of complementary-sequence-based multicode mc-cdma signals

The crest factor properties of binary phase-shift keying modulated two- and four-code assisted multicarrier code-division multiple-access (MC-CDMA) employing complementary-sequence-based spreading sequences are characterized. More specifically, a complementary-sequence pair, a complementary-sequence-based subcomplementary code pair, and a Sivaswamy complementary code set are studied. It was found that the corresponding crest factors are bounded by 3 dB, which corresponds to the crest factor of a single sine wave. This low crest factor resulted in a lower power loss and a lower out-of-band power spectrum due to clipping when the time-domain signal was subjected to a typical nonlinear power amplifier, in comparison to those of Walsh code and orthogonal Gold code-spread MC-CDMA signals.

Multiuser Detection of Synchronous MC-CDMA in Multipath Fading Channels Using Hopfield Neural Networks

Sub-optimum multiuser reception using Hopfield Neural Network for synchronous Multicarrier Code-Division Multiple Access signals in a multipath fading channel is studied with respect to near-far ratio. We have shown that by the appropriate choice of Hopfield Neural Network parameters from the channel state information, the Hopfield network can collectively resolve the multipath fading effects and the multiple-access interference in the system. Moreover, the Hopfield Neural Network demonstrates multiple-access interference resilient performance regardless of the number of paths resolved at the receiver. We have also investigated the bit-error rate performance of the system with respect to channel estimation errors. Results show that performance of the proposed detection scheme is influenced by the correctness of the estimated channel state information.

Orthogonal multicarrier CDMA and its detection on frequency‐selective channels

AbstractMulticarrier code‐division multiple access (MC‐CDMA) first appeared in the literature in 1993. It combines conventional code‐division multiple access (CDMA) and orthogonal frequency‐division multiplexing (OFDM), and has several variants that depend on how this combination is done. Although there have been a significant number of publications on MC‐CDMA since its introduction, the literature is still very fragmented, and the terminologies used by different authors are inconsistent. In addition, there are also a number of confusions concerning the detection of MC‐CDMA signals on frequency‐selective channels. As an attempt to achieve mis goal, this paper gives a comprehensive review of MC‐CDMA, describes its different variants, and reviews the receiver techniques for orthogonal MC‐CDMA. It also discusses the relative merits of MC‐CDMA and of multicarrier time‐division multiple access (MC‐TDMA) and orthogonal frequency‐division multiple access (OFDMA) which are the two omer multiple access techniques usable in conjunction with multicarrier transmission.

Hopfield multiuser detection of asynchronous MC-CDMA signals in multipath fading channels

Multiuser reception using Hopfield neural network (HNN) for asynchronous multicarrier code-division multiple access (MC-CDMA) in a multipath fading channel is investigated. We have shown that by the appropriate choice of HNN parameters from the channel state information, the HNN can collectively resolve the multipath fading effects and the multiple-access interference in the system.