Year

2006

Degree Name

Doctor of Philosophy (PhD)

Department

School of Electrical, Computer and Telecommunications Engineering - Faculty of Informatics

Abstract

Multiple-Input Multiple-Output (MIMO) transmission has recently emerged as one of the most significant technical breakthroughs in modern communication with a chance to resolve the bottleneck of traffic capacity in the future wireless networks. Communication theories show that MIMO systems can provide potentially a very high capacity that, in many cases, grows approximately linearly with the number of antennas. Space-time processing is the main feature of MIMO systems. Space-Time Codes (STCs) are the codes designed for the use in MIMO systems. Among a variety of STCs, orthogonal Space-Time Block Codes (STBCs) possess a much simpler decoding method over other STCs. Because of that, this thesis examines orthogonal STBCs in MIMO systems. Furthermore, Complex Orthogonal STBCs (CO STBCs) are mainly considered in this thesis since they can be used for PSK/QAM modulation schemes, and therefore, are more practical than real STBCs. The thesis starts with the backgrounds on MIMO systems and their capacity, on STBCs, and on some conventional transmission diversity techniques. These backgrounds are essential for the readers to overview the up-to-date scenario of the issues related to this thesis. After reviewing the state of the art of the issues related to this thesis and indicating the gaps in the literature, the thesis proposes three new maximum rate, order-8 CO STBCs. These new CO STBCs are amenable to practical implementations because they allow for a more uniform spread of power among the transmitter antennas, while providing better performance than other conventional codes of order 8 for the same peak power per transmitter antenna. Based on the new proposed CO STBCs, multi-modulation schemes (MMSs) are proposed to increase the information transmission rate of those new codes of order 8. Simulation results show that, for the same MMSs and the same peak power per transmitter antenna, the three new codes provide better error performance than the conventional CO STBCs of the same order 8. In addition, the method to evaluate the optimal inter-symbol power allocation in the proposed codes in single modulation as well as in different MMSs for both Additive White Gaussian Noise (AWGN) and flat Rayleigh fading channels is proposed. It turns out that, for some modulation schemes, equal power transmission per symbol time slot is not only optimal from the technical point of view, but also optimal in terms of achieving the best symbol error probability. The MMSs, which increase the information transmission rate of CO STBCs, and the method to examine the optimal power allocation for multi-modulated CO STBCs mentioned here can be easily generalized for CO STBCs of other orders. Constructions of square, maximum rate CO STBCs are well known. However, codes constructed via the known methods include numerous zeros, which impede their practical implementation, especially in high data rate systems. This disadvantage is partially overcome by the three new CO STBCs of order 8 mentioned above. Nevertheless, these codes still contain zeros which are undesirable or the design method is neither general nor easy yet. By modifying the Williamson and the Wallis-Whiteman arrays to apply to complex matrices, we discover two construction methods of square, order-4n CO STBCs from square, order-n codes. Applying the proposed methods, we construct square, maximum rate, order-8 CO STBCs with no zeros, such that the transmitted symbols equally disperse through transmitter antennas. These codes have the advantages that the power is equally transmitted via each transmitter antenna during every symbol time slot and that a lower peak power per transmitter antenna is required to achieve the same bit error rates as in the conventional CO STBCs with zeros. The combination of CO STBCs and a closed loop transmission diversity technique using a feedback loop has received a considerable attention in the literature since it allows us to improve performance of wireless communication channels with coherent detection. The thesis proposes an improved diversity Antenna Selection Technique (AST), referred to as the (N + 1;N;K) AST/STBC scheme, to improve further the performance of such channels. Calculations and simulations show that our technique performs well, especially, when it is combined with the Alamouti code [7]. While the combination between STBCs and a closed loop transmission diversity technique in the case of coherent detection has been intensively considered in the literature, it seems to be missing for the case of differential detection. The thesis thus proposes two ASTs for wireless channels utilizing Differential Space-Time Block Codes (DSTBCs), referred to as the AST/DSTBC schemes. These techniques improve significantly the performance of wireless channels using DSTBCs (with differential detection). The proposed AST/DSTBC schemes work very well in independent, flat Rayleigh fading channels as well as in the case of perfect carrier recovery. Does this conclusion still hold in the case of correlated, flat Rayleigh fading channels or in the case of imperfect carrier recovery? To answer this question, first, we propose here a very general, straightforward algorithm for generation of an arbitrary number of Rayleigh envelopes with either equal or unequal power, in wireless channels either with or without Doppler frequency shift effects. The proposed algorithm can be applied to the case of spatial correlation, such as with antenna arrays in Multiple Input Multiple Output (MIMO) systems, or spectral correlation between the random processes like in Orthogonal Frequency Division Multiplexing (OFDM) systems. It can also be used for generating correlated Rayleigh fading envelopes in either discrete-time instants or a real-time scenario. The proposed algorithm is not only more generalized and more precise, but also overcome all shortcomings of the conventional methods. Based on the proposed algorithm, the performance of our AST/DSTBC techniques proposed for systems utilizing DSTBCs in spatially correlated, flat Rayleigh fading channels is analyzed. Finally, the thesis examines the effect of imperfect carrier phase/frequency recovery at the receiver on the bit error performance of our AST/DSTBC schemes. The tolerance of differential detection associated with the proposed ASTs to phase/frequency errors is then analyzed. These analyses show that our ASTs not only work well in independent, flat Rayleigh fading channels as well as in the case of perfect carrier recovery, but also are very robust in correlated, flat Rayleigh fading channels as well as in the case of imperfect carrier recovery. The thesis is concluded with useful recommendations on the issues examined here and with a number of future research directions.

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