Modification for the Generalized Quasi–Orthogonal Space Time Block Codes

Abstract

In the recent years, demand for mobile communication systems with high data rates has increased significantly. To satisfy this huge communications demand search for new methods has been going that exploiting the limited resources such as bandwidth and power as efficient as possible. MIMO (Multiple input and Multiple Output) systems is one of the techniques with multiple antenna elements at both link ends that is an efficient solution for future wireless communications systems as they provide high data rates by exploiting the spatial domain under the constraints of limited bandwidth and transmit power. Space-Time Block Coding (STBC) is a MIMO transmit strategy which exploits transmit diversity. STBCs can be divided into two main classes, namely, Orthogonal Space-Time Block Codes (OSTBCs) and Non-Orthogonal Space-Time Block Codes (NO-STBCs). The Quasi-Orthogonal Space-Time Block Codes (QO-STBCs) belong to class of NO-STBCs and have been an intensive area of research. The O-STBCs achieve full diversity with low decoding complexity, but at the cost of some loss in data rate. Full data rate is achievable in connection with full diversity only in the case of two transmit antennas. For more than two transmit antennas full data rate can be achieved with QSTBCs with a small loss of the diversity gain.The main aim of this work is to provide a unified theory of QSTBCs for four transmit antennas and one receive antennas. The dissertation consists of two main parts: In the first part detailed analyses of the QSTBCs transmission without any channel knowledge at the transmitter is given and in the second part transmission with QSTBCs assuming partial channel state (CSI) information at the transmitter is illustrated.In the first part of this dissertation a definition of QSTBC for four transmit antennas is given. It is shown that different QSTBCs are obtained by linear transformations and that already known codes can be transformed into each other. The (4 × 1) MIMO channel in the case of applying quasi-orthogonal codes can be transformed into an equivalent highly structured virtual (4 × 4) MIMO channel matrix. The structure of the equivalent channel is of key importance for the performance of the QSTBCs. The off-diagonal elements of the virtual channel matrix are responsible for some signal self-interference at the receiver. The closer these off-diagonal elements of the virtual channel matrix are to zero, the closer is the code to an orthogonal code. Based on the six different self-interference parameter, performance of corresponding six QOSTBC types has been analysed. After that, effect of decoding method on the performance of the code is illustrated. Finally, new QOSTBC is proposed whose decoding complexity is very less compared to conventional QOSTBC without losing any performance. In the second part of the dissertation, transmit antenna shuffling (TAS) method is demonstrated to improve the QSTBC transmission when partial CSI is available at the transmitter. It is shown that QSTBCs can achieve full diversity and nearly exact orthogonality with a small amount of feedback bits returned from the receiver back to the transmitter. Later on, TAS method is demonstrated on the new QOSTBC code to improve its transmission strategy when some feedback is available at the transmitter. Lastly, performance of both conventional and proposed QOSTBC is compared.