Research On Precoding And Feedback Techniques For SU/MU-MIMO Systems

The implementations of SU/MU MIMO technique extensively utilize the gains of both space diversity and multiplexing, hence increase system capacity effectively. The use of multiple-user-multiple-antenna codebook not only reduces the inter-user interference, but also reflects channels' characteristics in both long-time and short-time aspects. Limited feedback in MIMO systems greatly reduces the feedback overhead by employing quantization schemes, and minimizes the loss of performances. Therefore, extensive research in both precoding and feedback techniques for SU/MU MIMO systems are of critical importance in the development of wireless communication, which are also the hot points in both academic and industry.This paper mainly focuses on the following three techniques, CQI selection in SU-MIMO systems, CQI feedback scheme in MU-MIMO systems, and joint design of user scheduling and precoding in MU-MIMO systems. Traditional CQI selection schemes for SU-MIMO systems are only suitable for fixed channel models, and they take SNR as the only reference. Hence, they are impractical. This paper designs a CQI selection scheme to cover all the possible channel conditions in real systems. This scheme utilizes multipath spread as a new reference to reflect the current channel quality more precisely, so as to increase the accuracy of CQI selection. Simulations show that this scheme can perform better than previous schemes.In MU-MIMO systems such as LTE/LTE-A system, users need to feedback both SU CQI and MU CQI to enable the switch between SU and MU mode in BS. Traditional schemes linearly quantize SNR and SINR for SU CQI and MU CQI, respectively. However, such schemes cost great amount of feedback overhead while providing few improvement to the system. In this paper, a CQI feedback scheme, which adaptively switches between a differential sub-scheme and a ratio sub-scheme, is proposed and extensively analyzed. Theoretical and simulation results demonstrate that this scheme can increase the feedback accuracy while effectively reducing the system overhead. Based on that, this paper further designs an adaptive CQI feedback procedure for LTE/LTE-A system.Finally, this paper studies the joint design of user selection and precoding in MU-MIMO systems. A DFT structure is used in the codebook design to extend the orthogonality between the codewords, which in turn enhances the flexibility of user selection. To better utilize the codebook, this paper design a corresponding precoding scheme based on an orthogonal user group algorithm. It can be showed in simulations that the joint scheme directly increases the number of scheduled users, and is capable of maintaining larger system throughput.Future works may include: Calculation and feedback scheme for CQI/PMI considering channel estimation errors, optimal reconstruction of feedback in base stations, precoding scheme considering the QoS of different wireless services, as well as the limited feedback scheme in multi-cell coordination systems.