Research On Beamforming Algorithm For Group Scenario In Broadband Trunking System Downlink

As the specialized mobile communication system, the development of theory research and technology application in trunking communication system has been obviously far behind. The present narrow band digital trunking communication system is not able to fulfill the demand of broadband multi media business by every department. So, combine the Long Term Evolution in public network with the trunking communication system has become a route that must take by developing trunking communication system.In the public network Long Term Evolution system downlink, several users share the same time and frequency resource to transmit different information, which must cause the co-channel interference among several users. So, the main purpose of downlink beamforming is to eliminate the co-channel interference among several users. In the broadband trunking system group scenario, several users in the same group share the same frequency band to transmit the same signal, there is no co-channel interference. The present broadband trunking system transmits signals isotropically without using every user’s location information under the group scenario. This paper studies the application of beamforming technology in the broadband trunking system, in order to improve the energy efficiency of the system, the fair problem of the group users and QoS assurance based on priority.The paper mainly studies the design approach based on providing Quality of Service assurance to each of the receivers. Since the received SNR determines the maximum achievable data rate and the probability of error, it is an effective measure of the QoS. This paper firstly studies two optimization design problems: minimizing the total transmission power subject to the prescribed minimum received signal to noise ratios at each of the intended receivers and max-min signal to noise ratio fair problem subject to a transmitted power constraint. The two optimization problems are both NP-hard. The paper adopts semidefinite relaxation, dropping some constraints in the original optimization problem, transform the original optimization problem into semidefinite programming problem to achieve the approximation solution. When there is fewer users in the group, the approximation solution is very close to the optimal solution. Because of taking fairness into consideration, the max-min-fair multicast beamforming can achieve better system performance than the minimizing the total transmission power multicast beamforming.The paper proposes the multicast beamforming algorithms based on the priority on the foundation of the two algorithms above, which can fit in the scenario that demanding different QoS. Even though some users have bad channel state condition, after giving high priority, they can get much better performance than the ones with low priority. At last, on the assumption of the base station can achieve perfect channel state information, the paper studies the imperfect channel state information`s influence on the system performance. The simulation results show that, as the channel estimation error increases, in order to fulfill the constraints in the optimization problems, the base station must increase the transmitting power.