rom Figure 12.41 it can be seen that the condition number varies between 3 and 30 dB, depending on the instantaneous radio channel fading conditions. In case of more than two streams, some other figure of merit could be substituted, for example, the so-called ellipticity statistic which is the ratio of geometrical and arithmetical means of the stream signal powers [10]. To reach maximum spectral efficiency the ideal radio channel would have equal power for all streams, in other words S = S2 = ... = Sk. In this case, the information streams are completely orthogonal and do not interfere with each other. The reason why this is not possible in real-world conditions is mainly threefold: instantaneous channel fading, long-term antenna correlation and antenna power imbalance. All of these factors increase the condition number and thus increase the SNR difference between streams. Even when the entries of the channel matrix have no correlation or power imbalance on average, the random channel fading results with high probability in non-orthogonal information streams at any given time instant. Figure 12.42 shows a measurement example with a real base station, 20 MHz FD LTE and a real UE where the average SNR = 25 dB is constant and antenna correlation is artificially altered with a fading simulator. The terms low, medium and high correlation refer to the standardized 3GPP channel models in 3GPP TS 36.101. At the beginning of the measurement, antenna LTE Key Performance Indicator Optimization High correlation Medium correlation Low correlation Stream 1 DL throughput Stream 2 DL throughput Total dual-stream DL throughput Average condition number Time (seconds) Figure 12.42 The 2x2 spatial multiplexing throughput for different correlations, SNR is 25 dB through- out the measurement. Carrier frequency is 2.6 GHz and bandwidth is 20 MHz correlation is high at both receive and transmit ends of the 2x2 system, and correspondingly one stream carries almost all of the 40 Mbps throughput. At 60 seconds time the correlation is switc