Adaptive, Asynchronous Incremental Redundancy (A2IR) with Fixed Transmission Time Intervals (TTI) for HSDPA

A major effort is underway to support the evolution of UMTS to meet the rapidly developing needs associated with wireless Internet applications. The support of packet-switched high-speed data users is provided by means of a new shared channel called the High Speed - Downlink Shared Channel (HS-DSCH) that preferably serves one packet data user at a time in a time-multiplexed manner. Several performance-enhancing technologies are included in the High Speed Downlink Packet Access (HSDPA) proposal to ensure high peak and average packet data rates while supporting circuit switched voice and packet data on the same spectrum. These techniques include Adaptive modulation and coding (AMC), Hybrid ARQ (HARQ), Fat-pipe scheduling, Fast cell site selection (FCSS) and Multiple input multiple output (MIMO) antenna techniques. Hybrid ARQ techniques may be classified in terms of the combining method used (Chase or Incremental Redundancy), the protocol timing (synchronous or asynchronous) and adaptivity (if modulation/code-rate is constrained at time of retransmission or not). In previous work [5} and [6], it was demonstrated that adaptive, asynchronous, incremental redundancy (a2IR) provided substantial gains over any of the other HARQ options considered for HSDPA. In that work adaptive operation was enabled by potentially varying the time duration of transmission depending on the quality information being reported. In some instances it may be desirable to keep the transmission time interval (TTI) fixed but enable adaptive operation by varying the number of channelization codes assigned to the scheduled user(s). In this paper, we cover the design and performance aspects of A2IR in the code domain and contrast it against the performance of other non-adaptive hybrid ARQ schemes. We alsp analyze some of the tradeoffs that are unique to A2IR with a fixed TTI. A2IR with a fixed TTI is now part of the HSDPA standard for UNTS Release 5.