Performance analysis of MC-CDMA system in multipath fading channel

This paper proposes a chaos-based direct-sequence spread-spectrum (CDSSS) communication system using M-ary PSK modulation in combination with MIMO-OFDM technique. This combination aims at improving the performance as well as capacity of the conventional CDSSS systems over multipath fading channels. In the transmitter, the data is modulated by an M-PSK modulator whose output is fed to a MIMO encoder. Two resulting signals are spread by directly multiplying with chaos-NRZ sequences and then put into the corresponding OFDM modulators. The output signals are transmitted through a multipath fading channel using multiple transmitting and receiving antennas. In the receiver, beside the use of OFDM demodulation, chaotic despread-spectrum, and M-PSK demodulation, a V-BLAST algorithm with Minimum mean squared error (MMSE) equalization is applied to decode and recover the data. Schemes for the transmitter and receiver are described in detail and their BER performance over the multipath Rayleigh fading channel is evaluated by means of numerical simulations. The obtained results prove that the performance and capacity of the proposed system are significantly enhanced and can be adjusted by changing the values of M-ary level, spreading factor or the number of used antennas.

In this paper, we propose an adaptive multistage parallel interference cancellation (PIC) structure for multipath fading channels. Similarly to the previous partial interference cancellation (IC) approach suggested in Divsalar et al. (1998), only part of the multiaccess interference (MAI) estimate is canceled at each stage. However, the weights employed in this proposed scheme are derived from an LMS algorithm which tries to minimize the mean-square error between the actual signal received and its replica. The complexity of the proposed adaptive multistage PIC scheme is much lower than that of linear multiuser detectors. Performance of the proposed structure in single-path Rician and multipath Rayleigh fading channels is evaluated by simulations. Our results show that the proposed technique outperforms the existing interference cancellation methods in multipath fading channels.

Adaptive multiuser DFE with Kalman channel estimation for DS-CDMA systems in multipath fading channels

As an effective technique for combating multipath fading and for high-bit-rate transmission over wireless channels, orthogonal frequency division multiplexing (OFDM) is extensively used in modern digital television terrestrial broadcasting (DTTB) systems, such as the OFDM based Digital Video Broadcasting for Handheld (DVB-H) system, to support high performance bandwidth-efficient multimedia services. OFDM based DVB-H system is very sensitive to the nonlinear distortion at the transmitter and the carrier frequency offset at the receiver. The objective of this paper is to research and analyze the performance of the OFDM based DVB-H system in the presence of carrier frequency offset and nonlinear distortion under multipath fading channel. A closed form of symbol error probability is derived for the OFDM based DVB-H system in the presence of carrier frequency offset and nonlinear distortion under multipath fading channel. Theoretical results show closely matching with those obtained by simulations for the OFDM based DVB-H system in the presence of carrier frequency offset and nonlinear distortion under multipath fading channel.

Modulation classification, an intermediate step between signal detection and demodulation, is commonly deployed in many modern wireless communication systems. Although many approaches have been introduced in the last decades for identifying the modulation format of the incoming signal, they have the obstacle of mining radio characteristics for most traditional machine learning algorithms. To effectively handle this limitation, we propose an accurate modulation classification method by exploiting deep learning for being compatible with constellation diagram. A convolutional neural network (CNN), namely CRNet, is developed to proficiently learn the most relevant radio characteristics from transformed gray-scale constellation image by cross-residual connection, a novel structure for associating the intrinsic information between two processing flows specified by regular and grouped convolutional layers. Based on the experimental evaluation, CRNet achieves the classification rate of approximately 90% at +10 dB signal-to-noise ratio (SNR) under a multipath Rayleigh fading channel and further performs more accurately than some existing deep models for constellation-based modulation classification.

The minimum mean-square error (MMSE) criterion is often used for channel estimation in wireless OFDM systems. The MMSE channel estimators perform better than the least-square (LS) criterion-based techniques but their computational complexity is one of the main drawbacks for practical implementation, especially when the number of subcarriers is large. To address this issue while maintaining the performance advantage, we present a new simplified MMSE channel estimator for OFDM systems in multipath fading channels. The proposed method is primarily based on the rank-reduction of the channel correlation matrix in the frequency domain. Simulation results show that the proposed method achieves almost the same performance as the full-rank MMSE method, while significantly reducing the computational complexity.

We evaluate the performance of the internally coded partial-band DS-CDMA scheme first introduced in [1], in a multipath Rayleigh fading channel. The scheme of [1] is based on frequency-hopping (FH) and direct-sequence (DS) spread spectrum techniques. The total bandwidth is divided into N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> subbands. The output of a super-orthogonal encoder and a pseudorandom (PN) sequence selects one subband, in which the user’s data is sent in the form of DS-CDMA signal using the second PN sequence. We consider soft detection at the receiver front end. We obtain the upper and lower bounds on the bit error rate. For comparison, we also consider the conventional DS-CDMA system. It is shown that for the same spectral efficiency the proposed multiple access method outperforms significantly the conventional DS-CDMA system over the fading channel as well without requiring any extra bandwidth.

This paper presents the performance analysis of compound chaotic sequence (CCS)-based noise reduction differential chaos shift keying (NR-DCSK) system under multipath Rayleigh fading channel conditions. The special characteristics of chaotic sequences are their deterministic randomness behaviour that adds security and multipath immunity to the data when used as a carrier in communication systems. In this paper, the chaotic sequences are generated by combining the outputs of chaotic maps, such as logistic map, Chebyshev map, Bernoulli shift map, tent map, etc., leading to new complex sequences known as CCSs. This sequence possesses more randomness, overcomes severe interference levels encountered during transmission and provides higher multipath immunity compared with those of pseudo-noise (PN) codes. Since NR-DCSK is a spread spectrum technique, its performance in wireless multipath fading channels has important considerations. The CCS is used as a carrier in NR-DCSK systems, which leads to improved bit error rate (BER) performance. Comparisons of simulation results to theoretical BER expressions of additive white Gaussian noise (AWGN) and Rayleigh fading channels have been carried out to test the efficiency of the proposed CCS-based NR-DCSK system.

In this paper, we propose a multi-carrier initial-condition-index-aided differential chaos shift keying (MC-ICI-DCSK) scheme tailored for multipath fading channels. In the proposed scheme, the ICI bits are carried by the index of the initial condition of the chaotic sequences while the modulated bits are carried by positive/negative state of the chaotic signal. To adapt the ICI method for multipath fading channels, we design a cross-correlation-value-based detection scheme for ICI detection. In this detection scheme, multiple carriers can be combined for ICI detection. Meanwhile, ICI and modulated bit detections remain independent of each other. The theoretical bit-error-rate (BER) expression of MC-ICI-DCSK scheme is derived over multipath Rayleigh fading channels. Furthermore, the data rate, spectral efficiency, energy efficiency, and complexity of the proposed scheme are analyzed. Simulation results not only verify the accuracy of the theoretical analysis, but also demonstrate the superiority of the proposed MC-ICI-DCSK scheme. Consequently, the proposed MC-ICI-DCSK can be deemed to be a promising scheme for low-power and low-cost short-range wireless communications.

This paper proposes a frequency-domain iterative soft decision feedback equalization (SDFE) scheme for multiple-input---multiple-output orthogonal frequency division multiplexing communication systems in multi-path fading channels. Error propagation can seriously affect the performance of the adaptive decision feedback equalization. We propose an iterative SDFE to not only combat error propagation but also be suitable for multilevel modulation systems. Under a Gaussian approximation of the a priori information and the SDFE output, the output of an equivalent AWGN channel and expressions for the a posteriori log likelihood ratio (LLR) and the extrinsic LLR are derived for multilevel modulation. In addition, we also investigate the iterative receivers by employing irregular low-density parity check codes (LDPC) and turbo codes. The performance of the LDPC-coded iterative receiver is better than that of the turbo-coded iterative receiver. Some simulation examples are given to show the effectiveness and comparisons of the proposed receiver.

A spectrally efficient OFDM transmission technique that does not require a cyclic prefix for combating intersymbol interference (ISI) is introduced. For achieving this, partial response coding (PRC) is combined with OFDM transmission scheme. PRC is regarded as one of the best known methods for combating ISI and for achieving the Nyquist signaling rate. In this paper the performance of OFDM system using Class IV PRC is analyzed over fading channels. Simulations show that OFDM system using the proposed PRC scheme perform much better than standard systems in multipath fading channels and such a simple coding technique improves the spectral efficiency also

This study attempts to develop a time-scale deconvolution filter for optimal signal reconstruction of nonstationary processes with a stationary increment transmitted through a multipath fading and colored noisy channel with stochastic tap coefficients. A deconvolution filter based on wavelet analysis/synthesis filter bank is proposed to solve this problem via a three-stage filter bank. A fractal signal transmitted through a multipath fading and noisy channel is provided to demonstrate the design procedure's effectiveness and to exhibit the signal reconstruction performance of the proposed optimal time-scale deconvolution filter.

In this paper, we analyze the performance of orthogonal frequency division multiplexing (OFDM) and carrier interferometry OFDM (CI/OFDM) for high-speed data rate wireless communication systems. For CI/OFDM realization, digital implementation of CI/OFDM structure is used in which CI code spreading and carrier allocation are separately processed by simple inverse fast Fourier transform. Based on previous work, a novel design of these systems is studied for multipath Rayleigh fading channel by using minimum mean square error equalization. The performance of OFDM and CI/OFDM wireless communication systems is compared based on peak to average power ratio, operating distance and bit error rate characteristics over independently additive white Gaussian noise and multipath Rayleigh fading channels. Simulation results show that CI/OFDM system is very useful for high-speed data rate wireless communication system and also provides wider operating range even in the presence of nonlinearity of the high power amplifier.

In multi-user wireless communication system, there is huge demand for higher data rates. One promising solution to achieve it is through the amalgamation of Code Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) technologies which results in Multi Carrier-CDMA (MC-CDMA) system. In CDMA system, the multiple access interference (MAI) can be reduced through proper selection of spreading codes. Bit error rate (BER) is an effective measure to evaluate the interference performance of a multi-user communication system. In this paper the BER and Peak to Average Power Ratio (PAPR) of MC-CDMA system is analyzed for different orthogonal spreading codes such as Walsh codes, complementary Golay codes and Orthogonal Gold codes in Rayleigh multipath fading channel employing Low Density Parity Check (LDPC) channel coding technique.

This paper proposes likelihood function generation of complexity-reduced maximum likelihood detection with QR decomposition and M-algorithm (QRM-MLD) suitable for soft-decision turbo decoding and investigates the throughput performance using QRM-MLD with the proposed likelihood function in multipath Rayleigh fading channels for orthogonal frequency and code division multiplexing (OFCDM) with multiple-input multiple-output (MIMO) multiplexing. Simulation results show that by using the proposed likelihood function generation scheme for soft-decision turbo decoding following QRM-MLD in 4-by-4 MIMO multiplexing, the required average received signal energy per bit-to-noise power spectrum density ratio (E/sub b//N/sub o/) at the average block error rate (BLER) of 10/sup -2/ at a 1-Gbps data rate is significantly reduced compared to that using hard-decision decoding in OFCDM access with 16 QAM modulation, the coding rate of 8/9, and 8-code multiplexing with a spreading factor of 8 assuming a 100-MHz bandwidth. Furthermore, we show that by employing QRM-MLD associated with soft-decision turbo decoding for 4-by-4 MIMO multiplexing, the throughput values of 500 Mbps and 1 Gbps are achieved at the average received E/sub b//N/sub o/ of approximately 4.5 and 9.3 dB by QPSK with the coding rate of R = 8/9 and 16 QAM with R = 8/9, respectively, for OFCDM access assuming a 100-MHz bandwidth in a twelve-path Rayleigh fading channel.