MC-CDMA Signals Research Articles

Parameter estimation of MC-CDMA signals based on modified cyclic autocorrelation

An algorithm, based on frequency domain accumulation for the cyclic autocorrelation of MC-CDMA signals, is proposed to estimate the signal parameters: useful data duration, symbol duration, and chip duration in multipath fading channel. Firstly the cycle autocorrelation of received MC-CDMA signals is computed, then frequency domain accumulation is used to reduce the influence of noise. At last, through interval measurement between the peak pulses in different slices, parameters mentioned above can be estimated. Meanwhile, for the estimation of symbol duration, a novel accumulative average method is developed: by averaging amplitudes of the spectral lines in each column parallel to the time-delay axis in the three-dimensional diagram, the symbol duration can be obtained. The cyclic autocorrelation expression of MC-CDMA signal has been derived, and simulation results show that the proposed algorithm is effective and works well under the condition of low signal-to-noise ratio (SNR).

English

The most favorable technique for high speed data transmission is Multicarrier Code Division Multiple Access. The MC-CDMA signals, which are characterized through PAPR, which can reduce the efficiency of the system. In this paper, we have to reduce the PAPR of an MC-CDMA system using PTS (Partial Transmit Sequence) through suboptimal combination algorithms (BPSO&BFO), which uses binary phase factors, for different number of subcarriers and for different length of sub blocks of PTS. I. I n tro d uct io n Mainly, the transmission of multi-carrier is to divide the entire bandwidth into smaller bandwidths. Each bandwidth with a different sub-carrier frequency, such that each of these narrow- band signals is protected from frequency selective fading and the data-rates are improved as compared to single-carrier system as we say that the total bandwidth can be increased significantly.MC-CDMA is a combination of both OFDM and CDMA that can provide protection from frequency selective fading and time dispersion. The CDMA part that provides both multiple access ability as well as spread each user signal over the frequency domain provide protection cover from the impact of frequency selective fading. The OFDM provides spreading across time domain of each spreading code's chip which decreases the effect of inter-symbol interference. So that we are able to get high data rate for transmission purpose. As we see, MC-CDMA is a powerful technique which has multiple accesses, but not free from problem. MC-CDMA signal has large peak to average ratio (PAPR) which largely limits its applications. High PAPR values causes a critical problem to the power amplifier (PA) used at transmitter. The efficiency performance of power amplifier decreases as PAPR increases. At transmitter side, the signal suffers from non-linear distortion. At receiver side, it degrades Bit Error Rate performance. This implements the use of power amplifier with large linear range so that the cost increases. Therefore it is necessary to reduce the impact of PAPR by means of PAPR reduction schemes. PTS is one of them most suitable probabilistic distortion less technique due to its less complexity.

Peak to Average Power Ratio Reduction in Mc Cdma System by Using Pulse Shaping Technique

One of the drawback of multicarrier code division multiple access (MC CDMA) is the high peak to average power ratio (PAPR). In this paper A novel scheme based on pulse shaping described for PAPR reduction. Here three different pulse shaping filters (Raised cosine, Sinc power pulse and Nyqist pulse) are described for PAPR reduction in MC CDMA system. The method is based on the proper selection of the pulses for shaping the different sub carriers of the MC CDMA modulation scheme. It is shown by computer simulation that the scheme achieves the significant improvement in PAPR reduction in MC CDMA system. Its implementation complexity is much low in comparison to the previous published methods. Thus pulse shaping technique can be used not only to shape the spectrum of the transmitted signal but also to reduce its PAPR. In computer simulation It is observed that by using nyquist pulse near about 6.8 db PAPR is obtained. Which is considerable PAPR reduction than a PAPR value of Original MC CDMA signal

MC CDMA Papr Reduction Techniques Using Discrete Transforms And Companding

High Peak to Average Power Ratio (PAPR) of the transmitted signal is a serious problem in multicarrier modulation systems. In this paper a new technique for reduction in PAPR of the Multicarrier Code Division Multiple Access (MC CDMA) signals based on combining the Discrete Transform either Discrete Cosine Transform (DCT) or multi-resolution Discrete Wavelet Transform (DWT) with companding is proposed. It is analyzed and implemented using MATLAB. Simulation results of reduction in PAPR and power Spectral Density (PSD) of the MC CDMA with companding and without companding are compared with the MC CDMA with DCT and companding, DWT and companding systems. The new technique proposed is to make use of multi-resolution DWT in combination with companding in order to achieve a very substantial reduction in PAPR of the MC CDMA signal.

A turbo SDMA receiver for strongly nonlinearly distorted MC-CDMA signals

This paper considers the use of space division multiple access (SDMA) techniques for the uplink of multi-carrier code division multiple access (MCCDMA) systems, where the transmitted signals face strong nonlinear distortion effects. The signal transmitted by each mobile terminal (MT) is submitted to a nonlinear operation consisting of a clipping device followed by a frequency-domain filtering operation, so as to reduce the envelope fluctuations and peak-to-mean envelope power ratio (PMEPR) while maintaining the spectral occupation of conventional MC-CDMA signals. At the base station, an iterative receiver employing multiple antennas jointly performs turbo multi-user detection and the estimation and cancellation of nonlinear distortion effects. Performance results show that the proposed receiver structures can achieve good performances that are very close to those obtained with linear transmitters, even for high system load and/or when a very low-PMEPR MC-CDMA transmission is intended for each MT.

On the Peak Factors of BPSK and QPSK Modulated MC-CDMA Signals Employing WH Sequences and WH-Based Complementary Sequences

In this paper, the inverse discrete Fourier transform (IDFT) results of oversampled Walsh Hadamard (WH) sequences and WH-based complementary (CP) sequences are derived, based on which, the peak factors (PF) of MC-CDMA signals are investigated for both the two sequence sets. The upper bounds of PF for both BPSK and QPSK modulation are presented in this paper and the PF bounds presented in B.J. Choi and L. Hanzo (2003) can be regarded as special cases of the bounds derived here. Compared with accurate computing which requires prohibitive computations, the upper bounds obtained provide a more effective way to estimate the PF of BPSK and QPSK modulated MC-CDMA signals employing WH or WH-based CP sequences. In addition, based on the IDFT properties of WH sequences, a simplified search for optimum WH subset in the context of BPSK modulation is derived.

Analytical evaluation of nonlinear effects in MC-CDMA signals

The MC-CDMA signals (multicarrier coded division multiple access) have strong envelope fluctuations which make them very prone to nonlinear effects. These effects can be both intentional (such as the ones inherent to a nonlinear signal processing for reducing the envelope fluctuations) or not (such as the ones inherent to a nonlinear power amplification). In this paper we present an analytical tool for the performance evaluation of nonlinear effects in MC-CDMA signals which takes advantage of the Gaussian-like behavior of MC-CDMA signals with a large number of subcarriers and employs results on memoryless nonlinear devices with Gaussian inputs so as to characterize statistically the signals at the output of the nonlinear device. This characterization is then used for an analytical computation of the SIR levels (signal-to-interference ratio) and the BER performance (bit error rate). A set of numerical results is presented and discussed, showing the accuracy of the analytical BER performance analysis in the presence of nonlinear effects

Analysis of ESPRIT-Based Algorithms for Blind Frequency Offset Estimation on MC-CDMA Downlink

In this letter, we investigate ESPRIT-based approaches for blind frequency offset estimation on MC-CDMA downlink. By analyzing the signal subspace structures of the MC-CDMA signals, the natures of the spreading codes and the number of users determine the feasibility of the ESPRIT-based approaches for the blind estimation task.

Frequency-Domain Pre-Equalization for MC-CDMA/TDD Uplink and Its Bit Error Rate Analysis

SUMMARY In multi-carrier code division multiple access (MCCDMA) uplink (mobile-to-base station), since different users’ signals go through different frequency-selective fading channels, large multi-access interference (MAI) is produced. The use of frequency-domain equalization reception can only partially restore the orthogonality among different users’ signals, resulting in a severe degradation in the bit error rate (BER) performance. Hence, frequency-domain pre-equalization transmission, which equalizes the MC-CDMA signal before transmission, is recently attracting attention. In this paper, we present a generalized minimum mean square error (GMMSE) frequency-domain pre-equalization transmission suitable for MC-CDMA/TDD uplink. The pre-equalization weight is derived based on the method of Lagrange multipliers. The theoretical analysis of BER performance using the GMMSE frequency-domain pre-equalization transmission in a frequency-selective Rayleigh fading channel is presented and the result is confirmed by computer simulation.

Neural network-based techniques for efficient detection of variable-bit-rate signals in MC-CDMA systems working over LEO satellite networks

For a few years, multicarrier modulations have been proposed as valuable alternatives with respect to state-of-the-art single carrier ones due to their resilience against interference and channel distortion effects. In particular, many studies are arising about multicarrier-CDMA (MC-CDMA) techniques that provide improved robustness and flexibility with respect to single-carrier spread-spectrum techniques. Satellite communications considered the employment of MC-CDMA for multimedia transmission very recently, especially for what concerns the delivery of variable-bit-rate services over low-earth-orbit (LEO) satellite networks. To this aim, the problems to be faced are mainly related to the development of efficient methodologies for satellite channel estimation, equalization and multi-user detection, in order to exploit in an optimal way the natural diversity inherent to MC-CDMA. In this paper we compare two different neural-network-based approaches for efficient reception of MC-CDMA signals in the case of asynchronous, multi-user, and variable-bit-rate transmission over LEO satellite channels. The first approach introduces neural networks for supporting receiver decision. The second more sophisticated approach exploits neural networks for joint channel estimation and symbol detection. Simulation results will be presented that demonstrate the improved effectiveness of the proposed methodologies, with respect to state-of-the-art MC-CDMA detection techniques, both in terms of reduced BER and in terms of low computational complexity.

Peak power reduction scheme based on subcarrier scrambling for MC-CDMA systems

A new PAPR (peak-to-average power ratio) reduction scheme is proposed for MC-CDMA (multicarrier-CDMA) using subcarrier scrambling. Based on the fact that the PAPR of the MC-CDMA signal is highly dependent on the scrambling pattern of the subcarriers irrespective of users' data symbols, the optimum fixed scrambling patterns for each combination of user spreading codes are searched to achieve the minimum PAPR. This enables the system to dynamically allocate the optimum scrambling pattern during system operation. The proposed scheme achieves a significant reduction in PAPR compared to random scrambling, especially for a small number of users. A simple suboptimum scrambling scheme is also proposed where a common scrambling pattern is used irrespective of the code combinations.

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.

Derivation of New Expressions of Bit Error Rate for MC-CDMA System in Nakagami Fading Channel

Abstract In this paper, new expressions for the bit error rate (BER) of MC-CDMA signals operating in Nakagami fading channel are derived. The proposed formulas can be easily computed numerically and hence, can be usefully exploited in the performance evaluation of digital mobile radio systems. Also, we show that MC-CDMA suffers from the near far problem. The effects of the near-far problem on the performance of MC-CDMA system is evaluated for different cases of the communication channel. Simulation results show that the error probability deteriorates considerably as the number of users in the system increase. It is also noticed that the performance deteriorates substantially in the Nakagami fading channel for little values of fading parameter as compared to the non-fading case due to the near-far problem, which results in a severe reduction of the multiple access capability of the MC-CDMA system. This problem has already been noticed in DS-CDMA.

Bandwidth efficiency of MC-CDMA signals

The bandwidth efficiency of MC-CDMA signals is evaluated and compared to that of DS-CDMA signals. For the same available bandwidth, the achievable processing gain of the two systems with and without filtering is evaluated and then compared.