Here we propose that each UE (UE1 and UE2) feeds back quantized ampli- tude and phase values of its estimated radio channels, h, h12/h21, and h22. For this purpose, one might use the UL control-shared channel, which allows each UE to send in UL (in addition to PMIs) up to 200-b control information per subframe to the eNB. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Codebook Number of Layers U Index 1/11/19 12/11/19 Figure 7.21 Codebook for transmission on antenna ports {0,1}. In the case of LTE release 8, there are some good reasons for the limited CB size: Feedback overhead from UEs to eNBs in the UL is expensive in terms of resources and power consumption and therefore it should be minimized. For LTE baseline single-user (1 X 2, 1 X 2) or multiuser (1 X 2, 2 X 2) sys- items and the assumption of 2/2 spaced AEs (with a as the RF-wavelength), beam patterns will be quite broad. At the same time, in cellular networks we can expect a high number of users per cell, SO we should exploit mul- tiuser diversity by applying user grouping. In combination with the broad beam pattern, there is a good chance to find UEs feeding back mutual orthogonal PMIs SO that user grouping will be simple and effective. For DL MU-MIMO, we have a X 4 system, SO we can apply receive diversity or interference rejection combining at the UE side, which allows suppressing residual intracell interference caused by improper precoding. At the same time, there are a number of good reasons for more advanced pre- coding schemes for future LTE enhancements: The goal of active IF management, based on interference-free CAs, is to increase spectral DL efficiencies by several hundred percent. Therefore, the additional (hopefully moderate) UL feedback overhead can be justified by large performance gains. © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes Even in the case of 2/2 spacing, user grouping will grow increasingly difficult with an increasing number of AEs because resulting b