Transmission Technique And Adaptive Modulation For Massive MIMO Systems

As one of the key technologies in 5G,massive multiple-input multiple-output(MIMO)has attracted a lot of attention.By deploying a large number of antenna arrays at the base station(BS),massive MIMO can effectively improve the system's energy and spectral efficiency.As the number of BS increases,the effect of small-scale fading can be averaged out,the channels between the different users and the BS become asymptotically orthogonal,and simple linear processing schemes at the BS can achieve the close performance of dirty paper coding scheme.Although there are many potential advantages,massive MIMO is still facing many challenges,one of which is the hardware complexity and cost.How to utilize the advantages of massive MIMO,to simplify the transmission technology and reduce the complexity of hardware,is the focus of our study.First,we consider an uplink massive MIMO system with dual-antennas users.And we in-vestigate the performance of several popular diversity transmission schemes,such as orthogonal space-time block coding(OSTBC),maximum eigenvalue-transmit beamforming(TB)and trans-mit antenna selection(TAS).In order to obtain the closed-form mathematical expression,we derive the lower bound of achievable sun rate and then give asymptotic power scaling criterion when the number of base stations antenna M tends to infinity.The analytical expressions show that the in-vestigated diversity schemes can achieve the same power scaling criterion as the system with only single-antenna users,indicating a rather pessimistic conclusion that using multiple antennas of the users does not bring much performance gain.Therefore,advanced transmission schemes should be designed to take full advantage of the additional spatial degrees offered by multi-antenna users.Then,considering the hardware cost of large-scale MIMO systems,enabled by the cost-effective variable phase shifts,a limited RF chains connected with a large number of base-station antennas,that is,hybrid precoding scheme is proposed.In order to overcome the limitation of the current research on hybrid precoding in multicasting systems,we propose a hybrid precod-ing scheme suitable for any channel model.Firstly,the traditional semi-definite relaxation(SDR)method is used to obtain the optimal full digital precoding.Then,based on the alternation optimiza-tion technique,the digital precoding and the analog precoding are jointly designed by minimizing the Euclidean distance between them and full digital precoding.The numerical results show that the performance of the proposed hybrid precoding scheme is very close to that of the full digital precoding under the condition of the limited radio frequency links.Finally,exploiting the wise feature of channel hardening(i.e.,the effect of small-scale fading is averaged out)in massive MIMO systems,we propose a low-complexity multi-user adaptive modulation scheme for uplink,which only needs large scale shadow fading.Also,we derive the closed-form expressions of average spectral efficiency(ASE)and bit error probability(BEO).Compared with the traditional fast adaptive modulation(FAM)which requires instaneous CSI,our proposed scheme greatly reduces the feedback rate and overhead with slight decrease of the performance.Subsequently,in order to further enhance the performance of the adaptive modulation system,a joint multi-user adaptive modulation and power control scheme is proposed.