GPP RAN WG1 is considering the potential benefits of extending MIMO support from eight antenna ports to 16, 32 or 64 antenna ports in Rel-13. This includes investigation of enabling Rel-13 mobile devices with the ability to measure and feed back CSI corresponding to 16, 32 or 64 transmit antenna ports. The potential benefits of going from 8 to 64 antenna ports come from the increased beamforming gains and multiplexing gains. The beamforming gains are due to the increased focusing of the energy (in three dimensions) with a larger number of antenna ports towards a single user. The multiplexing gains come from the ability (of a larger number of antenna ports) to form non-interfering beams to different users in the 3D-spatial domain that allows for efficient multi-user transmission (MU-MIMO). The polarization dimension LTE Small Cell Optimization 16 TXRU 32 TXRU 64 TXRU Mean user throughput Cell edge user throughput Figure 11.14 System performance comparison of 16 TXRU, 32 TXRU and 64 TXRU AAS base- stations with 8 TXRU conventional base-station. In all cases, a 64-element cross-polarized antenna array arranged in 8 rows and 4 columns is used. The results assume reciprocity-based operation in TDD particularly helps in multiplexing multiple streams to a single user (SU-MIMO). A critical factor in achieving these gains is the accuracy of the feedback CSI. This effect is shown in Figure 11.15. The system performance attained with Rel-12 mobile devices that are capable of 8-port CSI feedback is compared with the performance attained with hypothetical Rel-13 mobile devices capable of 16-port CSI feedback. In both cases, a Massive-MIMO base-station with 16 TXRUs driving an array of 64 cross-polarized antenna elements (arranged in eight rows and four columns) is considered. The achievable performance is a strong function of the fidelity of CSI feedback and if CSI feedback is restrictive the performance improvement may not be significant. The SU-MIMO transmissions are, for the most practical purposes, limited to two streams 