ennas are observed to be capacity optimal. This is motivated by the fact that low SNR is a power-lim- ited region. It is therefore optimal to allocate the sum transmit power to the best eigenmode using single-stream transmission. In this context, we observe that the maximum eigenmode for copolarized antennas is greater than for cross-polarized antennas. This structure arises from dual properties: trace constraint on the chan- nel power matrix HH and higher condition number (as shown in Figure 11.27) of copolarized antennas. In Figure 11.29, we compare the performance of the SVD-water-filling strat- egy to that of MMSE equal power allocation. XPD antennas are a better choice for SNRs greater than 6 dB. To isolate the gains of water-filling power allocation from SVD precoding, throughput CDF of SVD with water filling is compared to that without water filling in Figure 11.30. It is seen that benefits of water filling are primarily at low SNRs. At high SNRs, the water-filling strategy converges to equal power allocation. 11.4.3.5 Precoding with Unitary Codebooks and Bit Loading Next, we present the results with codebook-based precoding. This is done along with dual-stream or single-stream transmit mode selection. Mode selection can be viewed as a water-filling strategy with just three quantized power levels of 0, 0.5, and 1. In the codebook-based strategy, the terminal reports only the codebook index selected from the three codebooks, specified in IEEE 3GPP TS 36.211 V8.0.0 [8]. The codebook is selected by the terminal such that the achievable rate is maxi- mized. To reduce feedback, the terminal reports only on the bit loading per stream. In computation, the terminal also takes into account single-stream or dual-stream mode selection. A natural question to ask is whether the performance loss is significant with codebook-based precoding as compared to optimal precoding. We observe that it is not significant (Figure 11.31). This result is intuitively explained on the follow- ing basis. Channel inversion at the receive