ratio combining. In the uplink, power control has been disabled and transmission bandwidth is a constant 90 PRBs. One PDCCH symbol is used in the downlink. It can be seen that at low SNR the performance curves are within 3 dB of each other, especially if the slightly larger L1/L2 overhead of the uplink is compensated. Downlink spectral efficiency (throughput per PRB) of the 2x2 system is slightly better due to four- path diversity. At high SNR the downlink is clearly better than uplink to dual-stream MIMO transmission with 64QAM. 9 A common counter-argument among practitioners in the field is that TCP is the protocol used by end users. While this is surely correct, this section is about radio interface performance, not end-to-end performance. TCP throughput testing is suitable for end-to-end performance testing, but this is a separate topic beyond the scope here. 10 In downlink, the SINR value measured internally by most commercial UEs (and reported to drive test tools) can be very unreliable, unless suitable calibration measures are used. LTE Small Cell Optimization Average SNR versus uplink/downlink throughput, 20 MHz system Downlink 2 X 2 dynamic OL-MIMO Uplink 1 X 2 MRC Average SNR (dB) Figure 15.7 Example throughput measurement of a commercial Cat3 modem and base station at 20 MHz For example, for the measured UE model, SNR > -2 dB is sufficient to reach 10 Mbps downlink throughput. For uplink, the evaluation of SNR target should take into account the number of PRBs allocated at a given SNR which in turn depends on the link adaptation implementation of the eNodeB. Assuming 90 PRBs, SNR = -5 dB achieves 4 Mbps uplink throughput. On the other hand, assuming that uplink link adaptation trades off the number of PRBs to higher power per PRB, say, by allocating 9 PRBs, this would provide 10 dB SNR gain thus shifting the SNR operating point to -5 + 10 = +5 dB. Since the uplink throughput at SNR = 5 dB is about 16 Mbps for 90 PRBs, then for 9 PRBs the uplink throughput would be around one-tenth of that, or 1.6 Mbps.