Research On Multiuser Diversity And Precoding In Wireless Communications Systems

With the rapid development of cellular wireless communications, Internet and multimedia servieces, the requirement of the system capacity of the wireless communications increases speedily. The space-time processing techniques, represented by MEMO, are considered as one of core techniques for 4G/B3G systems. MIMO systems with space-time coding could not just be fit for the Internet and multimedia services, but increase the coverage area and improve the system reliability. At present, users expect higher system capacity and better quality of services, but the available wireless Spectral resource is limited. On the other hand, the multi-path and time-varing characteristics of the wireless channel cause significant damage to transmit signals. MIMO techniques have attracted more and more attentions because it can tremendously increase the spectrum efficiency and improve the transmission reliability of wireless communication systems. Based on the platform of multiuser MIMO downlink system and distributd MIMO system, our research mainly focuses on the system model and transmission strategy.For the resource allocation and scheduling of downlink multi-user multiple input multiple output (MU-MIMO) system, a multi-user proportional fair scheduling scheme based on genetic algorithms (GA) is proposed. By adding some good-gene individuals to the initial population and keeping its gene stable, the convergence of GA is accelerated. Specially, the base station exploits Block Diagonalization (BD) precoding technique to eliminate the inter-user interference. To guarantee the fairness of the user transmission speeed while maximizing the throughput performance, a subset of users is selected to serve at one time slot. Moreover, the impact of feedback error on the channel state information is analyzed. Simulation results show that both schemes can achieve a good tradeoff between fairness and throughput with low computational complexity compared to other scheduling schemes.The combined Block Digonalization precoding and Geometric Mean Decomposition (BD-GMD) scheme is developed to eliminate the multi-user interference and improve the system performance of bit error rate (BER) in a multiuser multiple input multiple output (MU-MIMO). For the resource allocation and scheduling of BD-GMD system, the impacts of the system from the equal power allocation and water-filling algorithm are compared. According to the subspace characteristics, a multi-user scheduling scheme based on maximizing the system throughput with low computational complexity is proposed. At the meantime, the brute-force search algorithm and conventional greedy algorithm are analyzed and compared with the proposed algorithm. Simulation results show that the proposed algorithm based on the orthogonal projection can reduce the computational complexity while maintaining the system throughput. For the fairness among internal users, a novel proportional fair scheduling algorithm is proposed in this paper. This algorithm, inserting a new parameter to denote users' past channel status information, could schedule multi-user better and guarantee fairness among internal users by considering both current and past channel status information.In order to mitigate co-channel interference and maximize the sum capacity in a multiuser MIMO downlink system, a novel system model based on oblique projection is presented and the expression of the system capacity with water-filling power algorithm is deduced. The simulation results have proved that, at the high signal-to-noise ratio (SNRs), the capacity performance of the system with oblique projection is better than that of BD-GMD system, but at low SNRs, the situation is opposite. The result obtained by system simulation means the system with oblique projection precoding could provide better capacity performance with a worse system channel.In recent years, distributed antenna system has obtained more and more attention due to its promising improvement in coverage, power efficiency and channel capacity. And the distributed MIMO system, which combines MIMO technique and the distributed antenna system, becomes to be a hot research area. However, most of those papers are only concerned those aspects which are mentioned above, but no considering multiuser diversity. This paper, applying the BD pre-coding to the distributed MIMO system, proposes a novel multi-user scheduling algorithm, which researches the capacity performance of downlink distributed MIMO systems from the aspect of multi-user diversity.