Performance evaluation of some basic ARQ schemes in the presence of multiple user interference

This paper investigates the throughput efficiencies of some well known ARQ schemes when used for error control on channels where multiple user interference (MUI) plays a very significant role, such as code division multiple access or CDMA channels. The paper also investigates the reliability of some Hamming codes used as error detecting codes in ARQ schemes on CDMA channels. All of the results are presented as functions of the number of users present in the CDMA systems considered. Fritchman (1967) channel models of the MUI present in a CDMA environment are used to calculate parameters such as the undetected error probability of the Hamming codes used for error detection in the ARQ schemes. The throughput efficiencies of the various ARQ schemes are also calculated for random error channels such as the binary symmetric channel (BSC) and compared to that for the channels with MUI. The results show that the ARQ schemes considered all have higher throughput efficiencies on the channels with MUI when compared to channels with random errors at an average bit error rate comparable to that of the channels with MUI. In order to achieve reliability in ARQ schemes used on channels with MUI, codes with shorter block lengths seem to improve the reliability substantially when compared to longer block lengths. We compare block lengths of 255 and 1023 bits. The results indicate that the ARQ schemes are more reliable on channels with MUI when compared to channels with completely random errors