ving a better performance than chase combining (Frenger, Parkvall, and Dahlman 2001, 1829). However, the former implies a greater complexity in its implementation than the latter and requires a larger receiver buffer size. In LTE, HARQ retransmission strategy is only supported for share channels in uplink (physical uplink shared channel-PUSCH) and downlink (PDSCH), and it is based on multiple parallel stop-and-wait processes. Transport blocks are decoded upon arrival. An acknowledge (ACK) command is reported to the transmitter when a transport block is successfully decoded. On the other hand, a negative acknowledge (NACK) is reported in cases of unsuccessful decoding, implying a retransmission request for the particular transport block. Once an ACK/NACK message is received at the transmitter, its timing is used to associate it with its cor- respondent HARQ process. Code word in RM buffer Decoding error Decoding error Decoding error Successful decoding Figure 3.16 Example of incremental redundancy HARQ at code word level. © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer LTE downlink and uplink HARQ operations are based on different protocols. On the one hand, downlink HARQ is based on an asynchronous protocol, in which retransmissions may occur at any time. Additionally, each process is identi- fied with an explicit HARQ process number. On the other hand, uplink HARQ is based on a synchronous protocol, in which the retransmission instant is fixed by the initial transmission time and therefore the process number can be implicitly derived from it. 3.2.6 Channel-Aware Scheduling A fundamental characteristic of wireless channels is the fading of the received signal due to constructive and destructive interference between multipaths. Consequently, the short-term capacity of a channel changes randomly over time. In an OFDM- based technology like LTE, the signal level associated with each OFDM subcarrier also varies along the frequency domain. Additionally, in a multiuser environment, channel quality 