Signal Detection and Channel Estimation for Space-Time Block Coded Mobile Communication Systems

Abstract

The space time block coding (STBC) has been recognized as an efficient technique to exploit spatial diversity both in point-to-point and cooperative relaying systems due to its good performance with low complexity of decoding. However, most of the existing STBC based wireless communication systems rely on the assumptions that the channels remain quasi-static during the length of the code-word symbol periods and thus these systems are applicable only for time invariant flat fading channel. However, in practice the channel is both time as well as frequency selective i.e. doubly selective, making them not to be readily applicable to the real world scenario. In this thesis, we focus on low complexity channel estimation and signal detection techniques for both point-to-point and cooperative systems to improve the reliability with low cost as follows:

STBC/QO-STBC signal detection in time selective flat fading channel: The performance of STBC system suffers from inter-transmit- antenna-interferences (ITAI) due to the mobility of the cellular mobile handset. To improve the system performance, we have analyzed various conventional signal detection methods which include Alamouti, quasi- maximum likelihood detection (QMLD), maximum likelihood (ML) and QR decomposition (QRD) based ML (QRD-ML) method. We have proposed ZF and MMSE based order QRD (OQRD)and OQRD-parallel interference cancellation (OQRD-PIC) signal detection methods.

Similarly, the QOSTBC system suffers not only from ITAI but also from inter-symbol-interference (ISI) effects in time selective channel. To combat these interferences, we proposed two signal detection methods namely zero forcing-iterative interference cancellation-zero forcing-decision feedback equalization (ZF-IC-ZF-DFE) and
minimum mean square error-IIC-ZF-DFE (MMSE-IIC-ZF-DFE) and compared them with several conventional signal detection methods.

STBC-OFDM signal detection over doubly selective channel: The STBC system is sensitive to delay spread and thus it is limited to flat fading channel. Since, OFDM transforms the frequency selective channel into parallel flat fading channel,the STBC scheme can be combined with the OFDM technique to achieve optimal performance in frequency selective channel popularly known as (STBC-OFDM). However,in time selective channel, the STBC-OFDM system undergoes severe performance loss due to both co-channel interferences (CCI) and inter-carrier interferences (ICI). We reviewed various CCI cancellation methods e.g. Alamouti, successive interference cancellation (SIC), quasi-ML detection (QMLD), diagonalized zero forcing detection(DZFD), decision feedback detection (DFD), List-SIC and optimal ML methods and proposed a suboptimal order iterative decision feed detection (OIDF) method which provide comparable performance to the ML with a very low computational complexity. Furthermore, we proposed two efficient signal detection methods namely order block decision feedback equalization (OBDFE) and OBDFE-multistage parallel detection (OBDFE-MPD) to improve the STBC-OFDM system performance by cancelling both the CCI and ICI effects in highly mobile environment and compared it with several conventional signal detection methods.

STBC-OFDM channel estimation over doubly selective channel: Accurate channel estimation is required for efficient signal recovery at the receiver side. In this thesis, at first, we propose a low complexity but optimal linear minimum mean square error (LMMSE) mchannel estimation by exploiting the circulant properties of CFR autocorrelation matrix. Then, this LMMSE method applies the expectation maximization (EM) iterative process to estimate the channel in STBC-OFDM system. Finally, we propose a joint iterative channel estimation technique data detection to improve the overall system performance.

Signal detection of STBC based Cooperative AF relaying system under high mobility relay node : Finally, we have analyzed the impact of relay’s node mobilty on the performance of an amplify-and-forward (AF) relay assisted system employing Alamouti type STBC matrix transmission at the source. Due to the mobility of the relay nodes, the Alamouti’s orthogonality principle is lost and causes severe error floor. To combat this error floor, we have analyzed several conventional signal detection method which include Alamouti, ZFD,QMLD methods and derived their closed-form signal to noise ratio (SNR) expression. Based on this performance analysis, we have proposed a decision feedback detection (DFD) method which provides superior performance than the conventional methods under high mobility relay node.