Research Of Opportunistic Beamforming For Multiuser MIMO-OFDMA Downlink System

Next-generation wireless communication systems require higher data rates and higher spectral efficiency. Orthogonal Frequency Division Multiplexing (OFDM) technology can effectively against inter-symbol interference caused by multi-path propagation, and has become one of the core technologies in the future high-speed wireless communications. Multiple-Input Multiple-Output (MIMO) technology can effectively increase the spectrum efficiency and improve the system data rate and channel capacity with no need for extra system bandwidth or change of system power, which has caused a great attention.In multi-user communications, OFDMA system can exploit multiuser diversity to enable a flexible radio resources allocation and simplify the receiver, which has been adopted in several modern wireless systems; multi-user MIMO technology can fully exploit the spatial dimension of radio resources. As one of the linear transmission schemes, opportunistic beamforming can make use of the disturbance of fading channel to enhance system throughput through multi-user diversity scheduling. Combining multi-user MIMO with OFDMA, multi-user MIMO-OFDMA system is a promising key solution for B3G system due to its potentials to improve the effectiveness and reliability for wireless links. When opportunistic beamforming technology is applied to the downlink of multi-user MIMO-OFDMA system, the resulting system is of great significance due to its potentials to fully use spacial dimension of wireless resources, to increase data rates and to improve cell edge performance.This dissertation mainly investigates in detail the opportunistic beamforming schemes in the multi-user MIMO-OFDMA downlink system. In order to guarantee the scheduling fairness and reduce the feedback amount of the downlink system, scheduling and feedback algorithms are designed and simulated. The work includes the following aspects.A general scalable model for a single cellular multi-user MIMO-OFDMA downlink opportunistic beamforming system is first developed. Some design considerations are further proposed including how to fully exploit the degrees of freedom and how to adapt to the situation where there are fewer users, as well as the scheduling and feedback design criterion. This part of the work lays the foundation for subsequent algorithm design and expansion research. In the proposed multi-user MIMO-OFDMA downlink framework, opportunistic beamforming is applied to clustered OFDMA. A fair scheduling scheme called Weighted Round-Robin (WRR) is proposed. Compared to the classical three scheduling algorithms as RR, MCI and PFS, simulation results show that using WRR scheme, the system can implement different scheduling strategies by changing the factor weights and WRR can satisfy users'QoS while guaranteeing system throughput and fairness. The proposed scheme is suitable for future wireless communication networks where multiple-class services access is provided.In the proposed multi-user MIMO-OFDMA downlink framework, multi-beam opportunistic beamforming is applied. Six kinds of feedback algorithms are proposed for clustered subcarriers method. Since the downlink system has a drawback that the total feedback amount increase linearly with the number of users, a threshold-based feedback (TBF) algorithm is further proposed. This algorithm constrains the average total feedback amount by a relatively small and fixed upper bound, which is of practical value.