ience fading and hence the instantaneous link capacity is also time varying. Some form of link and rank adaptation has to be implemented in practical implementations. The adaptation algorithm is vendor dependent, which makes it difficult to give any guideline for parameter optimization. In multi-stream communication the signal power is shared between information streams, the sharing ratio is defined by the squared singular values of the n X m channel matrix. As mentioned, the signal powers of the individual streams fade as the UE moves, and thus it is not feasible to transmit multiple streams at every time instant. An example is shown in Figure 12.41 for a 2x2 narrow-band Rayleigh fading antenna system. The uppermost curve is the SNR of a 2x2 diversity system. The two other curves show the SNR of the first and second information stream. It can be seen that the SNR of the second stream is always less than the first one. Another point worth noting is that the amount of fading of the second stream is much higher than the first stream, which results in the second stream SNR occasionally dipping below 0 dB. At such points where the channel rank effectively drops to one, it is not possible for the receiver to successfully decode both information streams and, instead of splitting transmit power to two streams, it is more spectrally efficient to switch to single-stream diversity transmission with higher modulation order. In the downlink of the LTE system each subcarrier is subjected to fading as shown in Figure 12.41. To measure the suitability of the radio channel for MIMO transmission, one possible figure of merit is the condition number CN which in the present context can be defined as the ratio of stream signal powers, or for a dual-stream system CN 51/20 LTE Small Cell Optimization Received SNR of MIMO substreams and diversity signal (2 X 2 system) MIMO substream no. 1 MIMO substream no. 2 Single-stream with diversity Time (seconds) Figure 12.41 Signal power for 2x2 diversity and substreams of spatial multiplexing F