| The increasing demand for high quality services of voice, data, image, and video transmission calls for high data rate of wireless transmission. OFDM technique can provide high data rate transmission over wireless channel, and is a promising technique for future wideband communication. However, there is a serious interference between neighbor cells when OFDM is used in wideband cellular communication system. At the edge of a cell, the downlink interference received has the same power (the overlap of two cells) and double power (the overlap of the three cells) as the signal power received, therefore the SINR received by a mobile station in this situation is lower than 0dB. For future cellular communication system based on OFDM, it is difficult for synchronization, channel estimation and demodulation at such a low SINR. For solving this problem, this thesis presented the following 4 algorithms. 1) The adaptive subcarrier cluster modulation and spectrum spread algorithm. This algorithm automatically changes the order of QAM modulation and spread factor to obtain the best transmission data rate and required BER performance according to channel condition. With the given channel bandwidth, when the channel is bad, and the BER at the receiver is too high, the receiver will request the transmitter to reduce the order of the QAM modulation, sacrifice the system source for its transmitting speed; when the order of modulation is reduced to its minimum, and the BER is still high, the system will increase the spectrum spread factor, decrease the transmitting speed to satisfy the BER requirement. When the channel is good, the system will decrease the spectrum spread factor, and increase the QAM modulation order to increase the transmitting date rate. The simulation results show: for AGWN channel, with this algorithm, BER can be kept lower than 10-3 when SNR changes from 0dB to 30dB. The data rate increases with the increase of SNR. When the SNR is higher than 20dB, the data rate increases fast. 2) The adaptive subcarrier cluster allocation algorithm. This algorithm divides subcarrieres in groups (clusters), and sort the sucarrier groups according to their transmission performance. The best subcarrier groups are used for transmission. In this way, the subcarrier utilization efficiency is increased. The simulation results show: The algorithm can improve BER. This improvement is remarkable when the channel is good. 3) The adaptive subcarrier cluster beamforming algorithm. This algorithm adjust the maximum and zero direction of antenna array by adjusting the weights to suppress interference from neighbor cells and improve the SINR at the receiving point. The simulation results of 8 antennas'array show: The SINR before QAM demodulation is much higher than the space SINR at receiving point, and doesn't change with the variation of the space SINR at the receiving point. 4) An integrated algorithm which combines the previous three algorithms: At the base station, the adaptive subcarrier cluster modulation and spectrum spread module adaptively adjusts the modulation order and the spectrum spread factor for each subcarrier cluster, and the subcarrier cluster allocation module adaptively choose the best subcarrier clusters for a user to maximize the spectrum efficiency and data rate; at the mobile station, the adaptive subcarrier cluster beamforming module adaptively adjusts the maximal and the minimal directions of antenna array. The simulation results show: by using this algorithm, the transmission performance is improved greatly. |
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