Minimum Mean Square Error Combining Research Articles

Performance of multicarrier complementary-coded CDMA under frequency-selective Nakagami-m fading channels

Multicarrier complementary-coded code division Multiple Access (MC CC-CDMA) is becoming an attractive multiple access technique for high data rate transmission in future wireless communication systems. MC CC-CDMA systems transmitting over frequency-selective channels suffer from multiple access interference (MAI) owing to non-ideal correlation properties of complementary codes (CC). MC CC-CDMA with equalization has recently gained much attention for its ability to offer an excellent performance than traditional systems in frequency-selective fading channels. In this paper, the authors present an analytical study and investigation of MC CC-CDMA downlink system using different combining schemes. The use of parallel interference cancellation (PIC) under frequency-selective Nakagami-m fading channels is also analyzed. A comparison among different combining schemes is provided to show the impact of PIC with minimum mean square error combining (MMSEC) and maximal ratio combining (MRC) on the performance of MC CC-CDMA system. The analytical and simulation results show that the combination of general combining schemes with PIC provides an efficient solution to suppress MAI in downlink MC CC-CDMA system than conventional MC-CDMA systems using Walsh codes under frequency-selective channels.

Comparative Study of Open-Loop Transmit Diversity Schemes with Four Antennas in DFT-Precoded OFDMA Using Turbo FDE and Iterative Channel Estimation

This paper compares the block error rate (BLER) performance of open-loop (OL) transmit diversity schemes with four antennas in the uplink for discrete Fourier transform (DFT)-precoded orthogonal frequency division multiple access (OFDMA). One candidate is a quasi-orthogonal (QO) - space-time block code (STBC) in which four-branch minimum mean-square error combining (MMSEC) is achieved at the cost of residual inter-code interference (ICI). Another candidate is a combination of STBC and selection diversity called time switched transmit diversity (TSTD) or frequency switched transmit diversity (FSTD). We assume a turbo frequency domain equalizer (FDE) associated with iterative decision-feedback channel estimation (DFCE) at a receiver to reduce the influences of the residual ICI and channel estimation (CE) error. From the extensive link-level simulation results, we show that a combination of STBC and TSTD (FSTD) is suitable as four-antenna OL transmit diversity scheme for DFT-precoded OFDMA using turbo FDE and iterative DFCE.

Analysis on the Performance of Doppler Diversity Receiver with Different Combining Methods

The Doppler frequency shift can do great harm to the performance of OFDM systems, so how to design the OFDM receiver under mobile environment is an important topic. An efiective Doppler diversity receiving technology for OFDM was proposed by Kim B C, which can achieve much better Bit Error Rate (BER) performance than the conventional OFDM receiver. In this article, we make research on the performance of the Doppler diversity receiving technology when using difierent combining methods, including the Orthogonality Restoring Combining (ORC) method, the Equal Gain Combining (EGC) method, the Maximal Ratio Combining (MRC) method and the Minimum Mean Square Error Combining (MMSEC) method. The research and simulation results show that in the Doppler diversity receiver, the BER performance of the ORC method is worse than that of the method without diversity, while the BER performance of the EGC method, the MMSEC method and the MRC method are better than that of the method without diversity. Moreover, the MRC method can achieve the best BER performance among the four combining methods mentioned above.

Perceptually adaptive MC-SS image watermarking using GA-NN hybridization in fading gain

This paper proposes an optimized multicarrier (MC) spread spectrum (SS) image watermarking scheme using hybridization of genetic algorithms (GA) and neural networks (NN). Data embedding is done in the mutually independent host components using the distinct code patterns that are assigned to the different watermark bits. GA determines the gradient thresholds for the pixel intensities to partition the host image into the edge, the smooth, and the texture regions as well as determines the watermark embedding strengths. The goal is to optimize the imperceptibility and the data hiding capacity. A minimum mean square error combining (MMSEC) decoder is used and the weight factors are calculated using NN through training/learning. Stable decision variables thus obtained for the watermark bit detection are partitioned into the multiple groups to improve decoder performance by canceling out the multiple bit interfering effect. Simulation results show the relative performance gain achieved in this method compared to the existing other works including the biologically inspired approaches.

Multi-User Interference Cancellation in Complementary Coded CDMA with Diversity Gain

This letter analyzes the performance of complementary coded code division multiple access (CC-CDMA) systems in frequency selective fading channels. Based on the evaluation of various combining algorithms for the correlation functions of M element sequences of a CC, a per-carrier parallel interference cancellation scheme based on minimum mean square error combining (MMSEC) is proposed for CC-CDMA to achieve a frequency diversity gain and multi-user interference resistance. Both analytical and simulation results verify the superiority of the proposed scheme. It is shown that combining algorithms and correlation properties of element sequences have a significant impact on the performance of CC-CDMA systems in frequency selective fading channels.

Performance of MC-CDM Systems With MMSEC Over Rayleigh Fading Channels

In this paper, we investigate the performance of multicarrier code-division multiplexing (MC-CDM) systems with minimum mean square error combining (MMSEC) over a Rayleigh fading channel. An approximated expression on the symbol error rate (SER) for MMSEC is derived. The approximated expression provides an accurate approximation of the true SER, particularly for systems with higher modulation and a small spreading factor. Through the derived error performances, we also calculate the achievable diversity order. MC-CDM systems with MMSEC achieve the same asymptotical diversity order of 1, regardless of the spreading factor. Within practical signal-to-noise ratio (SNR) ranges, the effective diversity order is further increased as a larger spreading factor is used. The derived analytical expressions are verified by numerical and simulation results.

Multicarrier spread spectrum watermarking for secure error concealment in fading channel

This paper proposes a novel multicarrier spread spectrum (SS) watermarking scheme for the application of image error concealment using multicarrier-code division multiple access (MC-CDMA) with binary phase shift keying (BPSK) transmission in Rayleigh fading channel. The goal is achieved by embedding important information (image digest) which is extracted from the original image itself, and is used to introduce sufficient redundancy in the transmitted image. Half-toning technique is applied to obtain image digest from its low-resolution version. At the decoder side, data demodulation as well as watermark decoding are done using minimum mean square error combining (MMSEC) strategy. The extracted image digest is used to correct the damaged regions. The integration of SS watermarking with the existing SS modulation not only simplifies the design but also offers significant performance improvement for error concealment in fading channel. Authorized users having the knowledge of code patterns for SS watermarking can only perform the error concealment operation and the method is secured. Experimental results duly support the effectiveness of the proposed scheme.

Variable power and rate allocation using simple CQI for multiuser OFDMA-CDM systems

This paper presents variable power and rate allocation (PRA) for a multiuser orthogonal frequency division multiple access code division multiplexing (OFDMA-CDM) system which exploits both orthogonality among users and symbol-level diversity. To predict the link performance, we analyze the postprocessing SNR as the transmit power (TP) varies when the minimum mean square error combiner (MMSEC) is employed. It is shown from analysis that the MMSEC SNR is nonlinear in TP, but is bounded by the harmonic and the arithmetic means of the subcarrier SNRs. From the observation, we propose two variable PRA methods using a simple channel quality indication and a required TP estimation method. Simulation results show that the proposed methods not only guarantee the target performance but also increase the throughput at the same or slightly increased feedback overhead compared with the fixed TP allocation case.

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.

Combining Coded Signals with Arbitrary Modulations in Orthogonal Relay Channels

We consider a relay channel for which the following assumptions are made. (1) The source-destination and relay-destination channels are orthogonal (frequency division relay channel). (2) The relay implements the decode-and-forward protocol. (3) The source and relay implement the same channel encoder, namely, a convolutional encoder. (4) They can use arbitrary and possibly different modulations. In this framework, we derive the best combiner in the sense of the maximum likelihood (ML) at the destination and the branch metrics of the trellis associated with its channel decoder for the ML combiner and also for the maximum ratio combiner (MRC), cooperative-MRC (C-MRC), and the minimum mean-square error (MMSE) combiner.

A Study on Forward Link Capacity in MC-CDMA Cellular System with MMSEC Receiver

This paper focuses on the MC-CDMA (Multi Carrier-Code Division Multiple Access) with the MMSEC (Minimum Mean Square Error Combining) receiver, which is a good candidate of a transmission scheme for beyond 3G systems. This paper evaluates the forward link capacity using the MMSEC receiver in the MC-CDMA cellular system, which employs TDM (Time Division Multiplex) transmission for multiple users. In this paper, the PDF (Probability Distribution Function) of the SINR (Symbol to Interference plus Noise energy Ratio) after MMSEC under multi-cell environment are calculated with the various number of the code division multiplexes. Based on the PDF, the numerical relation can be derived between the peak rate of the adaptive transmission and the average transmission rate per sector.

Evaluation of MC-CDMA with Frequency Interleaving Technique in Frequency Selective Fading Channel

In the Multi Carrier (MC)-CDMA system, the frequency diversity gain is obtained by its being spread in the frequency domain. The frequency interleaving technique can improve the frequency diversity gain. In this paper, the bit error rate (BER) performance in the MC-CDMA system which adopts the frequency interleaving scheme in the frequency selective fading channel is evaluated by computer simulation. In this simulation, orthogonal restoration combining (ORC) and minimum mean square error combining (MMSEC) are considered as frequency equalization combining techniques. This paper shows that BER performance with the frequency interleaver is better than without it in various environments.

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).

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.

Performance of multicode DS-CDMA using frequency domain equalisation in frequency selective fading channel

For the reception of multicode direct sequence (DS)-CDMA signals, the MMSE frequency domain equalisation is applied instead of RAKE combining. The achievable BER performance in a frequency selective Rayleigh fading channel is evaluated by computer simulation. It is shown by computer simulation that the DS-CDMA with MMSE frequency domain equalisation outperforms the DS-CDMA with RAKE combining and shows only slight performance degradation compared to the MC-CDMA with minimum mean square error combining (MMSEC).

Performance Improvement by Microdiversity for Wideband Signals in Frequency Selective Fading Indoor Channels with Co-Channel Interference

A comprehensive analytical bit-error-rate (BER) model is presented to analyse the performance of antenna-microdiversity for wideband BPSK modulated signals in the frequency selective fading multipath channel, specified by its complex impulse response. The model includes the disturbance by intersymbol interference (ISI) and co-channel interference (CCI), as well as the channels‘ impact on the carrier phase- and clock recovery in the receiver. The channel impulse responses at the antenna elements are determined by taking into account the direction of arrival of the individual paths. Computational BER- and SNIR-gain results (SNIR = signal-to-noise+interference-ratio) show that a substantial performance improvement is achieved with antenna combining for wideband signals which suffer ISI and/or CCI. For the indoor multipath channel with exponentially decaying power delay profile, the performance enhancement is compared for several antenna combining schemes. Quasi-coherent equal gain combining (QCEGC) is proposed as an novel EGC scheme based on a less accurate phase estimation technique. For wideband signals, QCEGC shows a slight performance degradation when compared to maximal ratio combining or minimum mean square error combining (MMSEC), but has a much lower implementation complexity. In the channel with CCI, where the best performance is achieved with MMSEC, QCEGC performs very poor.