Critical Communication 10.12.3 Interference Cancelation Mitigating the interference with either network-based or terminal-based techniques has been identified as a promising complementary solution to improve the SINR. The received interference from neighboring base stations or terminals can be reduced in order to improve the SINR. 10.12.4 HARQ (Hybrid Automatic Repeat Request) for High Reliability The use of retransmissions is a way to increase reliability at the expense of latency. HARQ is an error correction mechanism based on retransmissions. The receiver pro- duces either an ACK for the case of error-free transmission, or a NACK if errors are detected. Upon reception of a NACK message, the desired packet will be sent again. The contributors to the HARQ RTT (Hybrid Automatic Repeat Request Round Trip delay Time) are: BTS and UE processing time, the propagation time, the backhaul delay, and the TTI size. In the context of a flexible design, the duration of the ACK/NACK is also flexible and configurable per UE. Such a scheme enables the HARQ RTT to be set not according to the "worst condition UE" in the cell, but optimized per link. The latency can be also reduced by exploiting early feedback based on a prediction of the detection success. The practical system-level reliability is studied in system- and link-level simulations [13, 14]. First, system-level simulations are run to find out the SINR distribution. Sec- ond, the link-level performance as a function of the SINR is shown for the control and data channels. System-level radio reliability can be evaluated by combining these two simulations. The evaluation approach is shown in Figure 10.68. The system simulations follow the urban macrocell environment. The studies show that the 5th percentile cell edge SINR values at 4 GHz are 14.5 dB in the downlink and 3.2 dB in the uplink. The corresponding values at 700 MHz are 9.4 and 8.0 dB. We note that the higher band GHz) has a higher downlink SINR due to a more limited inter- ference propagation, while the lower b