Research On Fast And Slow Link Adaptive Transmission In Mobile Communication Systems

Link adaptive technology is a very suitable technology to improve the performance of mobile wireless communication system in fading channel, and it is an effective method to improve the system’s spectrum efficiency and transmission reliability. The state of the channel is the main basis of how to adapt the link, for example, using different series of quadrature amplitude modulation techniques in different signal to noise ratio range. But there are two problems:(1) Corresponding to the same target bit error rate or outage probability, the signal to noise ratio range is different in different fading channel types; (2) The sending end basically need the channel feedback from the receiving end. This process will be introduce the time delay, and the feedback of channel state information become outdated information, according to the outdated channel state information to determine the transmission strategy and that may degrade the performance of mobile communication system.In view of the above, this paper has carried out research on the link adaptive transmission technology, and has made some progress, which is embodied in the following aspects:(1)Currently, in mobile communication system mainly use fast link adaptive transmission. The results show that the performance of slow link adaptive transmission is better than fast link adaptive transmission and feedback frequency is lower in the fast-moving environment or the channel estimation error is larger. Therefore this paper carried out the slow link adaptive transmission study. By analyzing the physical connection between the slow link adaptation and the channel type, a slow link adaptation method based on channel type judgment is proposed. This method judges the channel type by probability density series expansion approximation distribution method. On this basis, then the adaptive thresholds of slow link adaption are determined by assuming that the average bit error probability (BEP) is equal to the BEP target. So, the adaptive QAM modulation series is easy to determine. Simulation results shows that the proposed method has significant advantages over the method in the literature and the achieved spectrum efficiency and outage probability of our proposed method is about 10%to 20%that of the method in the literature.(2) In order to deal with the influence of the outdated channel state information on the performance of the link adaptation, a fast link adaptive method based on channel prediction is proposed. The proposed method firstly uses the specific conditional probability density function to predict the signal to noise ratio of the wireless channel, that is using the outdated wireless channel signal to noise ratio to predict the signal to noise ratio when transmit. On this basis, then the adaptive thresholds of fast link adaption are determined by assuming that the instant bit error probability (BEP) is equal to the BEP target. So, the adaptive QAM modulation series is easy to determine. The simulation results show that the proposed method can effectively deal with the impact of outdated channel state information on the transmission performance of the system, and improve the performance of the system.