Issues On MIMO Wireless Communication Systems: Channel Estimation, Cooperative Diversity And Limited Feedback

With the wide applications of wireless communication technology in recent years, the demand for wireless communication technology becomes greater, for example, larger system capacity, higher spectral efficiency and better quality of service. These urgent demands make the research about wireless communication technology very hot and many new technologies are being addressed, formed and developed. As one of these new technologies, Multiple-input multiple-output (MIMO) system can afford substantially higher link capacity than that of single-input single-output (SISO) system in the Shannon sense. However, there are many challenges to be coped with in practice. To address these challenges, this thesis investigates several key theories and technologies for MIMO wireless communication systems including MIMO channel estimation, cooperative diversity and MIMO downlink beamforming with limited feedback. The main content is listed as follows:MIMO systems can be combined with orthogonal frequency division multiplexing (OFDM) systems to improve the capacity and quality of wireless communications. We first study a channel estimation technique suitable to fast-varying channel by using a set of pilot tones in each OFDM block. The number of pilot tones in each OFDM block and their arrangement are first discussed, and the orthogonal structure of pilot tones is proposed under the condition of multiple transmitter antennas and multiple receiver antennas. The limitation for this channel estimation method based on space time processing is that it requires the channel to keep unchanged for duration of at least two OFDM symbols. To cope with this challenge, we propose a channel estimation method employing pilot structure in both space and frequency domain. By this method, different transmitting antennas can send pilot simultaneously and can perform estimation within one OFDM symbol. Optimality of this method in terms of minimum square error (MSE) is verified. The proposed method is more effective in fast fading environments compared to the space time channel estimation method. Next, we propose an adaptive channel estimation technique which adaptively estimate the channel frequency response directly in frequency domain based on the recursive least square (RLS) algorithm.Besides the channel estimation of MIMO OFDM systems, we also propose a method for blind tracking of narrowband MIMO channel by using sequential Monte Carlo method. The proposed method can track MIMO channel blindly by updating the importance weight at each symbol duration.Distributed MIMO technology proposed recently make wireless terminals which are limited by size, power, and hardware complexity share their antennas and form a virtual antennas array to achieve transmit diversity. In many practical scenarios not all nodes between the source and the destination which can decode information from the source take part in relay , here we propose a cooperative diversity method which randomly select a node from a set of available relays to forward information towards the destination and then analyze the outage probability of this method . We demonstrate that the upper bound of this outage probability almost does not vary to the number of potential relay nodes, but the bottom bound of this outage probability becomes much smaller as the number of potential relay nodes increases. So the system performance will benefit from the increasing number of potential relay nodes.We then study issues as how to cooperatively communicate between users in broadband wireless networks and propose a cooperative diversity method in which the destination note utilizes both the signal relayed by partner nodes and the signal directly transmitted from the source node to decode. The performance of the bit error rate (BER) for the method proposed with direct transmit from the source node to the destination node is analyzed and compared to the method without direct transmit. Results show the higher diversity gain of the proposed method than the cooperative method without the direct transmit.We further investigate the performance of interference-limited wireless relay networks. The wireless relay networks consist of several cooperation units (CUs, defined as one source and destination node pair with a relay node associated to it). All source nodes send their signals simultaneously and all relay nodes then forward their signals, causing co-channel interference both at the relay node and at the destination node in CUs. The outage probability of the link between the source node and the destination node in a CU over Rayleigh slow-fading channels with interference is analyzed for the decode-and-forward (DF) relaying strategy. The end-to-end outage probability in a CU and the signal-to-interference-plus-noise ratio (SINR) distribution expressions are derived in closed form. Simulation results indicate the effect of interference.Finally, we propose a feedback strategy based on SINR threshold for MIMO broadcast channels and analyze the sum feedback rate of the proposed strategy. By using the proposed strategy the base station receives feedback only from those users whose SINRs are above a threshold and the threshold is elaborately chosen and broadcast to all users in the cell by the base station. So not all users need send their channel direction information (CDI) and channel quality information (CQI) to the base station. It is proved that the downlink wireless systems using the proposed strategy can achieve the multiplexing diversity and multiuser diversity gain with greatly reduced the sum feedback rate.