An adaptive radio link protocol with enhanced data rates for GSM evolution

In this article we address the problem of link adaptation in a wireless data system. Link adaptation is necessary in order to match the data rate to time-varying channel and interference conditions. We present a robust radio link protocol (RLP) based on the concept of incremental redundancy (IR). Here, redundant data, for the purpose of error correction, is transmitted only when previously transmitted packets of information are received and acknowledged to be in error. The redundant packet is combined with the previously received (errored) information packets in order to facilitate error correction decoding. If there is a decoding failure, more redundancy is transmitted. It is shown here that an RLP built using the IR concept is more robust and has better throughput than link adaptation schemes using explicit channel measurements such as instantaneous or average signal-to-noise or signal-to-interference ratio. We study the performance of an implementation of a IR-based RLP for EDGE (enhanced data services for GSM evolution) data and demonstrate its superior throughput and robustness properties. The penalty paid for increased robustness and higher throughput is additional receiver memory and higher delay. IR based RLP has already been standardized for IS-136+ packet data and is being actively considered for EDGE standardization.