Applying list output Viterbi algorithms to a GSM-based mobile cellular radio system

Digital mobile radio systems based on the pan-European standards GSM and DCS 1800 use concatenated coding schemes consisting of an inner error correcting convolutional code and an outer error detecting block code to ensure an appropriate performance. An improvement of this performance was achieved by applying list generating algorithms supplying a list of possible convolutional decoded sequences to the block decoder which checks them for correctness. Two list generating algorithms were investigated, namely the serial list output Viterbi algorithm and the soft output Viterbi algorithm with a novel list generating unit. Applying these algorithms yielded a gain of 3 dB without any bandwidth expansion nor additional delay. The only supplementary cost was a slightly increased computational effort. Therefore, list output algorithms seem to be a well-suited mean for improving the performance of GSM-based mobile radio systems. Concerning speech transmission, this improvement was achieved at the expense of a larger residual frame error rate because of the poor error-detecting capabilities of the used block code.