Space-time Block Coded Orthogonal Frequency Division Multiplexing System Research Articles

A Novel LMMSE-EM Channel Estimator for High Mobility STBC-OFDM System

In time-selective fading channel, the Alamouti orthogonality principle is lost due to the variation of channel from symbol-to-symbol in space–time block-coded orthogonal frequency division multiplexing (STBC-OFDM) system and causes co-channel interference (CCI) effects. To combat the CCI effects, various signal detection schemes have been proposed earlier by assuming that a priori channel state information (CSI) is known to the receiver. However, in practice, the CSI is unknown and therefore accurate estimation of channel is required for efficient signal detection. In this paper, by exploiting circulant properties of the channel frequency response (CFR) autocorrelation matrix [Formula: see text], we propose an efficient low complexity linear-minimum-mean-square-error (LMMSE) estimator. This estimator applies an expectation–maximization (EM) iterative process to reduce the computational complexity significantly. Finally, we compare the proposed LMMSE-EM estimator with conventional least square (LS) and LMMSE estimator in terms of performance and computational complexity. The simulation results show that the proposed LMMSE-EM estimator achieves exactly the same performance as the optimal LMMSE estimator with much lower computational complexity.

Iterative Channel Estimation for Higher Order Modulated STBC-OFDM Systems with Reduced Complexity

In this paper, a frequency domain Expectation-Maximization (EM)-based channel estimation algorithm for Space Time Block Coded-Orthogonal Frequency Division Multiplexing (STBC-OFDM) systems is investigated to support higher data rate applications in wireless communications. The computational complexity of the frequency domain EM-based channel estimation is increased when higher order constellations are used because of the ascending size of the search set space. Thus, a search set reduction algorithm is proposed to decrease the complexity without sacrificing the system performance. The performance results of the proposed algorithm is obtained in terms of Bit Error Rate (BER) and Mean Square Error (MSE) for 16QAM and 64QAM modulation schemes.

Generalised closed-form symbol error rate analysis for Orthogonal Space-Time Block Coded OFDM system

In this paper, the Symbol Error Rate (SER) performance for Orthogonal Space- Time Block Coded (OSTBC) Orthogonal Frequency Division Multiplexing (OFDM) systems over Nakagami-m fading channels is analysed. A novel closed-form SER expression is proposed, which incorporates the Gauss hypergeometric function and Appell hypergeometric function into the conventional Probability Density Function (PDF) approach. The proposed exact closed-form SER expression is a generalised solution since it perfectly captures OSTBCOFDM systems' performances when having different antenna configurations that employ various modulation schemes and which experience various fading conditions. Finally, Monte Carlo simulation results are provided to demonstrate the exact match between the simulation results and the proposed analytical expressions.

Characterisation of impulse noise effects on space-time block-coded orthogonal frequency division multiplexing (OFDM) signal reception

This correspondence presents the characterisation of impulse noise effects on the space-time block-coded orthogonal-frequency-division-multiplexing (STBC-OFDM) symbol reception, in which the conventional OFDM system is combined with the antenna-diversity/spatial-diversity in order to achieve the high data transmission rates. The weak impulse noise may be combated by the longer OFDM symbol duration, in which the total energy of noise spreads among the simultaneously transmitted OFDM sub-carriers irrespective of the impulse noise energy distribution. Specifically, the focus of the presented work is on “noise bucket” effect, in which the STBC-OFDM system is considered to be working in the impulsive environment. It is demonstrated by simulation results that for the occurrence of a significant number of impulses per symbol duration, the noise distribution at the input of the receiver’s final decision device is approximately Gaussian. Therefore for simulation purpose, the impulsive environment may be generated by the mere incorporation of equivalent Gaussian noise. Besides, some simulation results are presented using 16-QAM / STBC-OFDM system to give some insight into the symbol error rate variations with respect to the different number of sub-carriers and characteristics of impulsive environment. Key words: Impulse noise, space-time block-coded (STBC), orthogonal-frequency-division-multiplexing, Gaussian distribution, i nverse discrete Fourier transform (IDFT), M-ary QAM.

Interference mitigation for asynchronous cooperative STBC-OFDM systems

This paper addresses the problem of interference mitigation in cooperative Space Time Block Coded Orthogonal Frequency Division Multiplexing (STBC-OFDM) systems in the presence of asynchronism. This scheme first preprocesses the received ST codewords to convert the equivalent fading matrix into a suboptimal ordering upper triangular form based on low complexity permutation QR decomposition, and then suppresses the InterCarrier Interference (ICI) and InterSymbol Interference (ISI) by exploiting Successive Interference Cancellation (SIC) technique. Simulation results show that the performance of the proposed algorithm slightly outmatches or asymptotically approaches to that of the existing Minimum Mean Square Error (MMSE) detector depending on the magnitude of the Carrier Frequency Offsets (CFOs) but with less complexity.

Analytical Performance Evaluation of Space Time Coded MIMO OFDM Systems Impaired by Fading and Timing Jitter

An analytical approach is presented to determine the impact of time selective fading, timing jitter and additive white Gaussian noise (AWGN) on the bit error rate (BER) performance of an orthogonal frequency- division multiplexing (OFDM) system with differential quaternary phase-shift keying (DQPSK), differential phase- shift keying (DPSK) and quaternary phase-shift keying (QPSK) modulation. The expression for the conditional BER conditioned on a given timing error and fading, is derived and the average BER is evaluated. The BER performance results are evaluated for different values of fading and jitter. The performance of the OFDM system in Rayleigh and Rician fading channels is also compared. Analytical approach is also developed to evaluate the BER performance of a quasi-orthogonal- space-time block coded OFDM system having multiple transmitting and single receiving antennas. The analysis is extended for a multiple- input multiple-output (MIMO)-OFDM system using the selection method for combining multiple receiving antennas, which offers significant improvement in the system performance. The effects of increase in number of OFDM subcarriers and increase in Doppler frequency are also investigated.

A Recursive ICI Cancellation Technique for Cooperative STBC-OFDM Systems with Carrier Frequency Offsets

In this letter, a recursive ICI cancellation technique for cooperative space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) systems is proposed to mitigate the intercarrier interference (ICI) caused by multiple carrier frequency offsets (CFOs) in cooperative transmission. The proposed technique is shown to mitigate the noise enhancement effect on the STBC-OFDM signals caused by multiple CFOs, and to be effective in reducing ICI especially when the system has a large FFT size and multiple CFOs.

Bandwidth-efficient OFDM transmission with iterative cyclic prefix reconstruction

For orthogonal frequency division multiplexing (OFDM), cyclic prefix (CP) should be longer than the length of channel impulse response, resulting in a loss of bandwidth efficiency. In this paper, we describe a new technique to restore the cyclicity of the received signal when the CP is not sufficient for OFDM systems. The proposed technique efficiently restores the cyclicity of the current received symbol by adding the weighted next received symbol to the current received symbol. Iterative CP reconstruction (CPR) procedure, based on the residual intersymbol interference cancellation (RISIC) algorithm, is analyzed and compared to the RISIC. In addition, we apply the CPR method to Alamouti space-time block coded (STBC) OFDM system. It is shown that in the STBC OFDM, tail cancellation as well as cyclic reconstruction of the CPR procedure should be repeated. The computational complexities of the RISIC, the proposed CPR, the RISIC with STBC, and the proposed CPR with STBC are analyzed and their performances are evaluated in multipath fading environments. We also propose an iterative channel estimation (CE) method for OFDM with insufficient CP. Further, we discuss the CE method for the STBC OFDM system with the CPR. It is shown that the CPR technique with the proposed CE method minimizes the loss of bandwidth efficiency due to the use of CP, without sacrificing the diversity gain of the STBC OFDM system.

Efficient ISI cancellation for STBC OFDM systems using successive interference cancellation

In this letter, we propose an efficient detection scheme for space-time block coded (STBC) orthogonal frequency-division multiplexing (OFDM) with insufficient cyclic prefix (CP). The proposed scheme employs successive interference cancellation (SIC) and cyclic prefix reconstruction (CPR) concepts. Simulation results present that the proposed scheme outperforms the conventional scheme for STBC OFDM systems.

Space-Time Channel Estimation and Performance Analysis for Wireless MIMO-OFDM Systems With Spatial Correlation

This paper treats channel estimation in multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with correlation at the receive antenna array. A two-step channel estimation algorithm is proposed. Firstly, the iterative quadrature maximum likelihood based time delay and spatial signature estimation is presented by utilizing special training signals with a cyclic structure. The receive spatial correlation matrix of the vector valued channel impulse response is formulated as a function of the spatial signature, the time delay, and the pulse shaping filter. The joint spatio-temporal (JST) filtering based minimum mean squared error channel estimator is derived by virtue of the spatial correlation. In addition, the effect of channel estimation errors on the bit error probability performance of the space-time block coded OFDM system over correlated MIMO channels is derived. The Cramer-Rao lower bound on the time delay estimate is provided for a benchmark of the performance comparison. The performance of proposed algorithms is illustrated based on analysis and computer simulations. The JST channel estimator achieves significant gains in the mean squared error compared to the temporal filtering. It also enables remarkable savings in the pilot symbol power level.

Iterative cyclic prefix reconstruction and channel estimation for a STBC OFDM system

In order to prevent a loss of spectral efficiency due to the use of a cyclic prefix (CP), a CP reconstruction (CPR) method, such as residual intersymbol interference cancellation (RISIC), has recently been developed. In this letter, we apply the RISIC method to an Alamouti space-time block coded (STBC) orthogonal frequency division multiplexing (OFDM) system with insufficient CP. It is shown that in the STBC OFDM, tail cancellation as well as cyclic restoration of the RISIC should be repeated. An iterative channel estimation method for the STBC OFDM system with CPR is proposed. The performance of the proposed method is evaluated by computer simulation in a multipath fading environment.

Iterative receivers for space-time block-coded OFDM systems in dispersive fading channels

We consider the design of iterative receivers for space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) systems in unknown wireless dispersive fading channels, with or without outer channel coding. First, we propose a maximum-likelihood (ML) receiver for STBC-OFDM systems based on the expectation-maximization (EM) algorithm. By assuming that the fading processes remain constant over the duration of one STBC code word and by exploiting the orthogonality property of the STBC as well as the OFDM modulation, we show that the EM-based receiver has a very low computational complexity and that the initialization of the EM receiver is based on the linear minimum mean square error (MMSE) channel estimate for both the pilot and the data transmission. Since the actual fading processes may vary within one STBC code word, we also analyze the effect of a modeling mismatch on the receiver performance and show both analytically and through simulations that the performance degradation due to such a mismatch is negligible for practical Doppler frequencies. We further propose a turbo receiver based on the maximum a posteriori-EM algorithm for STBC-OFDM systems with outer channel coding. Compared with the previous noniterative receiver employing a decision-directed linear channel estimator, the iterative receivers proposed here significantly improve the receiver performance and can approach the ML performance in typical wireless channels with very fast fading, at a reasonable computational complexity well suited for real-time implementations.