Improving packet loss probability in the UMTS high-speed downlink

According to the current design of the hybrid automatic repeat request (HARQ) protocol which is used in UMTS release 5 for the high speed downlink shared channel (HS-DSCH), a mobile terminal which fails to decode a packet will transmit a physical-layer negative acknowledgement (NACK). If the base station misinterprets this NACK as a positive acknowledgement (ACK), there is no further opportunity for a physical-layer retransmission of the packet. An error recovery scheme is described here to handle this case by means of a further uplink signal called Revert (REV) in addition to ACK and NACK. The mobile terminal transmits REV if it detects that it has received a new transmission instead of the expected retransmission of an earlier negatively-acknowledged packet. Simulations show that this error recovery scheme allows the transmission power of NACKs to be significantly reduced for a given packet loss probability, or the packet loss probability to be significantly reduced for a given NACK transmission power. The physical-layer implementation and protocol operation of the error recovery scheme are examined in detail, and it is shown that all potential error cases are able to be handled satisfactorily.