Space-Time Block Coding for Multiple Antenna Systems

The demand for mobile communication systems with high data rates has dramatically increased in recent years. New methods are necessary in order to satisfy this huge communications demand, exploiting the limited resources such as bandwidth and power as efficient as possible. MIMO systems with multiple an- tenna elements at both link ends are 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 and high reliability. STBCs can be divided into two main classes, namely, Orthogonal Space-Time Block Codes (OSTBCs) and Non-Orthogonal Space-Time Block Codes (NOSTBCs). The Quasi-Orthogonal Space-Time Block Codes (QSTBCs) belong to class of NOSTBCs and have been an intensive area of research. The OSTBCs achieve full diversity with low decoding complexity, but at the price of some loss in data rate. Full data rate is achievable in connection with full diversity only in the case of two transmit antennas in case of complex-valued symbol transmission. For more than two transmit antennas full data rate can be achieved with QSTBCs with a small loss of the diversity gain. However, it has been shown that QSTBCs perform even better than OSTBCs in the SNR range of prac- tical interest (up to 20 dB) that makes this class of STBCs an attractive area of research. The main goal of this work is to provide a unified theory of QSTBCs for four transmit antennas and one (or more) receive antennas. The thesis consists of two main parts: In the first part we analyze the QSTBCs transmission without any channel knowledge at the transmitter and in the second part we an- alyze transmission with QSTBCs assuming partial channel state (CSI) information at the transmitter. For both cases, the QSTBCs are studied on spatially correlated and uncorrelated frequency flat MIMO channels applying a Maximum Likelihood receivers as well as a low complexity linear Zero-Forcing receivers. The spatial correlation is modelled by the so-called Kronecker Model. Measured indoor chan- nels are also used in our simulations to show the performance of the QSTBCs in real-world environment. In the first part of this thesis we give a consistent definition of QSTBCs for four transmit antennas. We show that different QSTBCs are obtained by linear transformations and that already known codes can be transformed into each other. We show that 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 vital importance for the performance of the QSTBCs. We show that 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 ma- trix are to zero, the closer is the code to an orthogonal code. Based on this self-interference parameter it can be shown that only 12 QSTBC types with different performance exist. In the second part of the thesis we provide two simple methods to improve the QSTBC transmis- sion when partial CSI is available at the transmitter. We propose two novel closed-loop transmission schemes, namely channel adaptive code selection (CACS) and channel adaptive transmit antenna selec- tion (CAAS). By properly utilization of partial CSI at the transmitter, we show that QSTBCs can achieve full diversity and nearly strict orthogonality with a small amount of feedback bits returned from the re- ceiver back to the transmitter. CACS is very simple and requires only a small amount of the feedback bits. With CAAS full diversity of four and a small improvement of the outage capacity can be achieved. The CAAS increases the channel capacity substantially, but the required number of the feedback bits increases exponentially with the number of available transmit antennas.