Adaptive, asynchronous incremental redundancy (A2IR) with fixed transmission time intervals (TTI) for HSDPA

In anticipation of the high demand for wireless data services, several wireless standards are evolving to support packed data more efficiently. In UMTS, a new, shared channel called the high speed downlink shared channel (HS-DSCH) that multiplexes packet data users has been defined. Several performance-enhancing technologies are included in high speed downlink packet access (HSDPA): adaptive modulation and coding (AMC), hybrid ARQ (HARQ), scheduling etc. HARQ algorithm plays a critical role in HSDPA performance. HARQ techniques are classified in terms of the combining method (chase or incremental redundancy, IR), protocol timing (synchronous or asynchronous) and adaptivity (if modulation/code-rate can be changed for retransmission or not). In previous work (Das et al. 2001), an adaptive, asynchronous, incremental redundancy (A²IR) scheme was proposed and shown to provide substantial gains over other HARQ options. In this paper, the design and performance aspects of A²IR in the code domain (as against the time domain in Das et al.) are considered. Due to the higher bandwidth of the UMTS channels compared to cdma2000 or HDR, it is expected that code-division multiplexing (CDM) of users will be needed to "fill the data pipe" instead of pure time-division multiplexing. The paper also analyzes tradeoffs that are unique to implementing A²IR with CDM. A²IR with a fixed TTI is now part of UMTS Release 5.