| Wide-band wireless communication is a roll booster for developing information society. It is making people change living mode, e.g. , People will be able to enjoy the new vogue of digital surf , video on command and video conference anywhere and anytime.MIMO systems enable the data throughput increasing linearly with the minimum numbers of antennas at transmitter and receiver, so that it is one of the most attractive techniques in LTE,B3G and future 4G systems. OFDM is also a key technique for wide-band wireless communications because of its high spectral efficiency and capability to combat multipath fading.This dissertation researches on key techniques of physical layers based on LTE MIMO-OFDM systems,including space diversity, power allocation, beamforming for multiuser MIMO systems and so on. Major contributions of the dissertation are as follows:1. The dissertation proposes a new diversity method -SDA(small space array antenna diversity algorithm), which can obtain more diversity gain without any channel state information (CSI) at the transmitter. For the small-space array antenna, the spatial channel appears to be high correlation. Under this scenario, space- frequency block codes (SFBC) scheme can not obtain significant diversity gain. Scheduling the symbols in frequency domain, we improved its bit-error performance of small-space array antenna MIMO systems. The results of LTE downlink simulation shows that its bit-error performance can be significantly improved.2. In order to improve the bit error performance of a wideband mobile communication system to increase its system throughput, we propose a power allocation algorithm, i.e. pilot assistant power allocation (PA2) algorithm. Based on user's feedback information, the optimal power allocation between pilots and data symbols is obtained for transmission at the transmitter, so that the system can acquire much higher data throughput, where the optimal value for power allocation is derived from theoretical analysis. Taking the system complexity into account, we quantify the optimal power values with a group of fixed values for implementation. Simulation results indicate that the system performance is remarkably improved by the proposed PA2 algorithm compared with the conventional ones. 3. A vector quantization (VQ) codebook based beamforming algorithm (CBF) for time division duplex (TDD) system is proposed. The optimal beamforming vector for a user can be obtained at the receiver of base station and their index in VQ codebook is sent to the user for its receiving beamforming by restoring the vector with the codebook,which can avoid complex calculation for the beamforming vector. Through theoretic analysis, the low bound of quantization error of CBF algorithm is deduced. The algorithm is very suitable for beamforming at user receiver, which can significantly improve the system performance with very low cost of feedback information.4. To reduce the interference between the downlink signals so as to improve the throughput of multiuser MIMO system, a multiuser beamforming algorithm is proposed, which based on the minimal signal leakage(MSL) criterion,named as codebook based maximizing signal to leakage and noise ratio (CB-SLNR). The base station finds out all the optimal beamforming vectors for all the user from the codebook by means of MSL criterion, and use them to transmit downlink signals; at the same time finds their indexes in the codebook to send all the users via control channels; every user receive the downlink signal with the beamforming vectors restored from the codebook with the indexes. Since every user can restore the beamforming vectors of downlink signals, it can realize an optimal receiving based on the MMSE detection algorithm, to remove the interference from other downlink signals. Simulation shows the CB-SLNR algorithm can significantly increase data throughput. Moreover, it can reduce the complexity of beamforming vector computation at base station and users.
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