Reference signal design for demodulation of higher-order MU-MIMO in 3D-MIMO systems

Large number of antennas in transmitter and receiver side of 3D-MIMO can provide finer spatial granularity of beams and larger beamforming gain, and the number of simultaneously served UEs may also be increased compared with conventional LTE system. To support increased multi-user capacity in 3D-MIMO, DMRS needs to be further enhanced because DMRS only support up to 4 layers multi-user MIMO (MU-MIMO) transmission in current LTE specification, each layer associated with one DMRS (demodulation reference signal) port, and among the four MU-MIMO layers, two layers are orthogonal, and others are quasi-orthogonal. In this paper, link level evaluation of several enhance alternatives are analysed, considering mobility, beamforming algorithm, and orthogonal/quasi-orthogonal design. The results show that orthogonal DMRS transmission provides significant channel estimation gain than quasi-orthogonal. Therefore, in order to guarantee performance of large multiplex number of UEs, increased number of orthogonal DMRS ports is preferred. Larger orthogonal cover code (OCC) length provides good performance at lower mobility, at the same time maintaining less resource overhead, therefore slightly preferred for 3D-MIMO deployment.