Research On Space-time Transmission Schemes For Multiple Antenna Multicasting

The increasing demand of wireless multimedia, especially mobile operators'and users'intense need for Mobile TV promote the development of multimedia broadcast/multicast service. In another aspect, the frequency spectrum efficiency and transmission reliability of wireless communication systems can be improved by multiple antenna technology without extra frequency bandwidth and no additional power consumption. When multiple antenna technology is applied into multicasting, it is called multiple antenna multicasting. Through transmit beamforming technology, the transmission rate improves as the transmitting antenna increases, and approaches to the capacity of multicasting system. However, it needs ideal channel state information at the transmitter. It is unrealistic. Since that, we pay attention on the adaptive transmitting schemes and performance evaluation under the nonideal channel state information. If any user in the multicast group could not receive the data correctly, the base station needs to retransmit the message to all the users, which leads to a waste of wireless resource. To decrease the probability of retransmission, it is valuable to study the outage performance of the multicast system.First, this paper studies the adaptive transmission schemes when the base station can only obtain the delayed channel state information for the mobility of the users. Because of the mismatch of the channel state information, the traditional max-min beamforming (MMBF) method will result into high outage probability. To solve this problem, the capacity of multicast system is estimated by using delayed channel state information and Doppler frequency shift. Through adopting novel MMBF method which combined with capacity estimation, the system performance approaches that under the ideal channel state information when the users'speed is low. However, as the users'speed is high, the performance of the proposed method decreases rapidly. What's more, the mode switch between the novel MMBF method and OSTBC method is studied in this paper, and the user-group-separation transmission method is proposed for the scene that the users in the same multicast group have quite different speeds. Simulation results show that when the users'speed is high, the OSTBC method is likely to be chosen. The user-group-separation transmission method outperforms OSTBC method at small number of transmitting antennas and low transmitting power. The outage performance of multiple antenna multicasting system is also studied in this paper. We assume that the users are uniformly distributed in the cell and the open-loop OSTBC method is adopted at the base station. The outage probability formulation of the multiple antenna multicasting system is derived first. Through numerical simulation, the correctness of the derivation is verified. Under some requirement of the outage probability, the available transmission rate increases as the number of the transmitting antennas increases, and it decreases as the number of the users or the cell radius increases. When the number of the transmitting antennas is limited, the multiple-slot transmission scheme is used to improve the diversity gain. Simulation results show that with a large cell radius, a large number of users and a small number of transmitting antennas, the optimum slot-number is larger than one and the multiple-slot multicast transmission scheme outperforms the conventional single-slot multicast transmission scheme obviously in terms of transmission rate.