cture. The use of HARQ adds increased robustness to the system and a spectral efficiency gain. Two re- transmission strategies are supported: incremental Redundancy (IR) and chase combining. The basic idea of the chase combining scheme is to transmit an identical version of an erroneously detected data HSDPA Table B.2 Layer 1 transmission-retransmission link budget (3GPP) Delay event Delay HSDSCH transmission 2.0 ms (3 TS) Before the UE sends ACK/NACK on the uplink HSDPCCH 5.0 ms (7.5 TS) For ACK/NACK transmission 0.67 ms (1 TS) From reception of ACK/NACK until HSSCCH transmission 3.0 ms (4.5 TS) For HSSCCH transmission before starting the HSDSCH transmission 1.33 ms (2 TS) Total 12 ms packet before the decoder combined the received copies weighted by the SNR prior to decoding. With the IR scheme, additional redundant information is incrementally transmitted if the decoding fails on the first attempt, by means of different puncturing schemes used in the coding of the retransmitted data packets. In the case of HSDPA, IR with a one-third punctured turbo code would typically be used for the retransmissions, although it has the drawback of requiring higher memory buffers in the UE than chase combining. During the scheduling phase, the MAC-hs layer will give priority to retransmissions over new RLC packets, which will be transmitted with the same code as the original transmission. HARQ can be used in the stop-and-wait mode or in the selective repeat mode. Stop-and-wait is simpler, but waiting for the receiver's acknowledgement reduces efficiency; thus multiple stop-and-wait HARQ processes are often done in parallel in practice. When one HARQ process is waiting for an acknowledgement, another process can use the channel to send more data. There are a few aspects to consider for the HARQ mechanism: If Node B receives an ACK from a UE, everything is fine. If Node B receives a NACK from a UE, it means that the packet was received, but could not be detected properly. In this situation, Node B should retransmit using increm