| Multiple-input multiple-output (MIMO) antenna system has gained considerable research interest for future broadband wireless communication services due to its transmission rate and high reliability. At the same time, orthogonal frequency division multiplexing (OFDM), known as one of the most promising multicarrier modulation schemes, has also received extraordinary research emphasis from all over the world. By transforming a wideband multi-path channel into multiple parallel narrowband flat fading channels, OFDM mitigates frequency selectivity of wireless environment, benefits from combating against channel fading and multi-path interference. The powerful attractive combination of MIMO and OFDM techniques, or MIMO-OFDM, will impact the evolution of wireless communication systems such as WLAN, UWB, WiMAX, and is a leading candidate for future fourth generation (4G) wireless communication systems, and has been adopted in IEEE Standard for Local and metropolitan area networks.In this document, a MIMO-OFDM testbed of WiMAX system is firstly described, which is used to study practical issues in both RF and baseband models. This testbed is software reconfigurable and flexible to alter the system parameters such as carrier frequency, band-width, frame format, and transmission/ reception antennas structures. Our research scopes include baseband signal processing algorithm development, hardware design, measurements and field trials for rapid prototyping.By comparing the measured data with the simulation results in different channels, including AWGN (Added White Gaussian Noise) channel and multipath channel, the results show that the performance of this WiMAX testbed system is consistent with the theoretical performance. Thus, it can serve as reliable platform for many baseband algorithms development, such as synchronization, channel estimation and space time coding. It can also be used to evaluate the WiMAX system performance in several kinds of practical circumstances.The above sections are all about open-loop communication system and its implementation on WiMAX Testbed, which is a realization of open-loop communication. It is known that STBC is not spectral efficient since the maximum rate is 1 for a 2by2 MIMO system, which transmits two symbols over two time slots. Therefore, in adaptive systems to achieve the maximal spectral efficiency, we need to know the channel state information (CSI) at the transmitter to adjust the transmitter accordingly, i.e. the modulation scheme, the power and even the coding rate.When feedback is added to the system, the transmitter may be able to have knowledge of the channel between itself and the receiver. Because the channel changes quickly in a highly mobile scenario, closed-loop transmission schemes tend to be feasible primarily in fixed or low-mobility environment. In practical systems, perfect CSI (Channel State Information) is definitely not attainable due to reasons such as imperfect channel estimation, quantization of feedback information and the time-varying nature of the wireless channel causing the CSI to be outdated. Thus the transmitter only has access to partial CSI.This next few parts of the document focus on the close-loop communication scheme, namely the adaptive MIMO-OFDM Eigenmode Transmission System (EMTS) and its implementation on WiMAX Testbed, including the following subtopics: the EMTS system structure, the main algorithms, implementation issues and the field measurement results.In the last chapter of thesis, we summarized the whole thesis and pointed out further research directions for WiMAX systems. |
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