Techniques for MIMO Wireless Networks over Fading تقنيات للشبكات اللاسلكية متعددة الإدخال و الإخراج عَبر قنوات الخفوت

In modern mobile communications, the recent researches are investigating worldwide efficient techniques for data transmission, exploiting the diversity concept in multiple domains over frequency-selective fading channels. In this framework, both system robustness and spectral efficiency should be met. Therefore, the claimed objective is to enhance the system capacity without increasing either the transmitted power or the transmission bandwidth. The main innovation of the thesis is related to design and implement advanced systems that significantly increase the capacity and profitably combine such technical advantages of the modern wireless mobile technologies .

The first contribution in this thesis proposes a novel combination between both the Polarized Multiple-Input Multiple-Output (MIMO) and the Pre-RAKE technologies. This innovative combination is considered as a strong candidate for the downlink wireless mobile communications. This is due to its capability of capacity enhancement as well as reducing both the complexity and the power consumption of the mobile terminal in addition to decreasing the spatial dimension of the MIMO system. Two advanced closed loop polarized MIMO Pre-RAKE systems are designed, analyzed and simulated in Frequency Division Duplex (FDD) mode. The first system exploits the Combined Transmit Diversity (CTD) technique. Therefore, it is called Adaptive Polarized MIMO Pre-RAKE CTD system. However, the Selection Transmit Diversity (STD) technique is utilized in the second system. Hence, it is called Adaptive Polarized MIMO Pre-RAKE STD system. Both systems have the capability to exploit the cross-polarized channels as resolved paths to improve the performance since the cross-pol Pre-RAKEs are used in addition to the co-pol Pre-RAKEs in the base station, which is called Node B in UMTS systems. Therefore, the proposed systems can achieve a significant performance gain while employing a simple MIMO receiver structure.

The proposed systems are compared with the single-polarization MIMO Pre-RAKE system, either employing STD or CTD schemes, and the advanced Pre/Post RAKE systems, presented in the recent researches. The comparison is performed in three technical directions; the receiver complexity, the spatial dimension and the BER performance. In this comparison, the proposed systems utilize both nonoverlapped and orthogonal carriers to separate between both co-pol and cross-pol Pre-RAKEs in the Node B. Also, the Pre/Post RAKE systems exploit different advanced adaptive algorithms to determine the weighting factors of both Pre-RAKE and Post-RAKE filters. These algorithms include Maximal Ratio Combining (MRC) algorithm, Principal Ratio Combining (PRC) algorithm and Optimum Solution Aglorithm (OSA). As will be shown, the proposed systems outperform the single-polarization MIMO Pre-RAKE system and the Pre/Post RAKE systems while utilizing simpler receiver structure and lower MIMO spatial dimension. This is in addition to mitigating the challenges of the single-polarization MIMO systems, such as self-interference, polarization mismatch and resolving cross-polarization power.

The proposed systems are also investigated for various transmit diversity degrees. The numerical results illustrate an interesting behavior of the proposed CTD scheme since its performance continues to improve without saturation as the number of transmitting dual polarized antennas increases. This demonstrates the potential of the proposed CTD system with respect to the foremost diversity techniques such as spectral diversity, temporal diversity and traditional open loop transmit diversity techniques.

The effect of polarization diversity characteristics including Cross-polarization discrimination (XPD), envelope correlation (?env) and Co-pol Power Factor (CPF) on the BER performance will be discussed. The proposed CTD system provides promising results in dense urban and suburban environments, where XPD approaches to 0 dB. Therefore, it has the capability to reduce both the complexity and the required spatial dimension of the Node B, which are considered as important practical requirements for the Node B implementation in dense environments.

In this thesis, the performance of the proposed systems will be studied using both random and orthogonal spreading codes between users. Also, a performance comparison will be presented in case of utilizing both nonoverlapped and orthogonal carriers between the co-pol and the cross-pol Pre-RAKEs. For slowly varying channels, a performance assessment for both systems will be investigated under different Feedback Information (FBI) rates. Besides, the proposed systems will be examined in case of excluding the cross-pol Pre-RAKEs in the Node B. Moreover, a performance comparison for the vehicular and the pedestrian environments will be presented. In addition, the proposed CTD system will be examined under different numbers of both channel paths (L) and Pre-RAKE fingers (Lt). For both systems, the simulation results provide good agreement with the numerical results.

The results, yielded by MIMO and multicarrier techniques, suggested to researchers to mix together such two innovative technologies. The second contribution in the thesis develops a novel adaptive Polarized MIMO Multi-carrier Spread Spectrum Code Division Multiple Access “adaptive Polarized MIMO MC-SS-CDMA” system for downlink mobile communications. The proposed system is considered as an efficient evolution to the MIMO multicarrier recent work. This is due to merging the advantages of multicarrier CDMA, closed loop, polarization diversity and MIMO technologies. The key feature of proposed system lies in its capability of exploiting diversity jointly in frequency, space and polarization domains. This is in addition to employing a simple practically implementation and low complex receiver owing to the utilization of both Discrete Fourier Transform (DFT) and closed loop techniques. Also, the proposed system has the capability to mitigate ISI and ICI due to the narrow bandwidths of the subcarriers. Moreover, it can achieve a high spectral efficiency by appropriate choosing the spectral expansion factor (F). Besides, it can also mitigate the challenges of the single-polarization MIMO systems including self-interference, polarization mismatch and resolving cross-polarization power.

In this thesis, the proposed adaptive polarized MIMO MC-SS-CDMA system will be examined in FDD mode for both macro urban and suburban environments. For the same transmission bandwidth, a performance comparison between both nonoverlapped and orthogonal Frequency Division Multiplexing (FDM) schemes will be presented. Also, the proposed system will be compared with both the advanced synchronous Space Time Block Code (STBC)-MIMO MC-SS-CDMA system and the closed loop single-polarization MIMO MC-SS-CDMA system. In the comparison, the STBC-MIMO MC-SS-CDMA system will utilize different equalization methods such as Equal Gain Combining (EGC), Minimum Mean Square Error (MMSE) and Minimum-BER (M-BER) approaches. However, the single-polarization MIMO MC-SS-CDMA system will exploit both orthogonal and non-orthogonal options between the MC-SS-CDMA blocks, transmitting their signals via the same antenna in the Node B. The comparison will be performed in BER performance, complexity and spatial dimension points of view. As will be shown, the proposed system introduces a significant performance gain as well as reducing the spatial dimension of the MIMO multicarrier system and simplifying the receiver implementation. The effect of the polarization diversity characteristics on the BER performance will be discussed. Also, the impact of excluding the cross-polarization MC-SS-CDMA blocks in the Node B will be investigated. In addition, the system performance will be evaluated under different Feedback Information (FBI) rates for slowly-varying channels. Finally, a performance comparison for vehicular and pedestrian environments will be presented.

In all proposed adaptive polarized MIMO systems, either employing Pre-RAKE or MC-SS-CDMA techniques, the receiver simplicity is not affected by the increase in the number of transmitting dual polarized antennas of the Node B.

Key words: Multi-Input Multi Output (MIMO), Pre-RAKE Technique, Multi-carrier Spread Spectrum Code Division Multiple Access (MC-SS-CDMA), Polarization Diversity, Transmit Diversity, Closed Loop Technique.