e- station is employing 8 transceivers in azimuth only. It shows that within a similar form factor doubling the number of transceivers can provide a significant performance improvement in certain scenarios. This performance is achievable with the existing LTE-A specifications. As shown in Figure 11.13 the AAS architecture has some impact on system performance but this alone should not be construed as a well-rounded evaluation of architecture options. 11.5.2 Reciprocity-Based Approaches The reciprocity-based approaches do not rely on CSI feedback for controlling the amplitude and phases at the transceivers driving the transmit antenna array. Instead they depend on the LTE Macro Cell Evolution No VS, 8 TXRU VS sub-array, 16 TXRU VS full-connection, 16 TXRU Cell avg (full buffer) Cell edge (full buffer) Figure 11.13 Vertical sectorization system performance gains (16 TXRU AAS) comparing to 8 TXRU passive antenna array for sub-array and full-connection architectures uplink transmissions from the mobile devices to determine the amplitude and phases required at the transceivers for directing the downlink data transmission to the mobile device. There- fore, an arbitrary number of transmit antenna ports can be supported using this approach. An instantaneous channel between a base-station antenna array and a mobile device is always reciprocal but several practical challenges remain - for example, a mobile device may have a single transmit antenna but multiple receive antennas, the transmit and the receive RF chain responses may not be equalized perfectly at the base-station and there will be difficulty in predicting other transmission parameters (rank, MCS). Nevertheless, reciprocity remains a practical and feasible deployment option for especially TDD. Figure 11.14 shows the improve- ment of data channel performance obtained by increasing the number of transceivers from to 64. In all cases, a 2D rectangular cross-polarized array with eight rows and four columns is assumed. 11.6 Further LTE Multi-antenna Standardization 3