Algogrithms for link adaptation in GPRS

In the General Packet Radio Service (GPRS) development of GSM, four error correcting coding schemes have been defined to allow for variations in radio channel conditions. In this paper, two algorithms for link adaptation (i.e. choosing the most suitable coding scheme for transmission) are discussed. The first one is based on the estimated C/I (ECI) of the transmission and the second one on block error rate (BLER). The results of the study indicate that algorithms based on estimated C/I are superior to those based on block error rate. The improvement in terms of throughput per user is in the order of 10%. Both algorithms are superior to any fixed coding scheme. One problem that is apparent when using link adaptation is the degree of fairness in the system. Unfortunately, both the proposed algorithms show disturbing variations in throughput between different users. This is true also for the ideal case, where throughput is maximized by basing the choice of coding scheme on actual C/I.