Uncoded bit error rate of eCall modem through AMR codec and AWGN channel

Data communications through the voice channel of the cellular network are important for telematics applications such as automatic vehicle crash notification, an implementation of which is the pan-European eCall system scheduled for deployment in 2015. The cellular voice channel includes the vocoder which is designed for compressing speech waveforms and is a hindrance for data communications. No mathematical model exists for the vocoder as a data communications channel. Thus, in this paper, the uncoded bit error rate (BER) is found by simulation using the bit-exact C-code implementation of the vocoder. The adaptive multi-rate (AMR) vocoder is used in GSM, the network on which eCall runs, and is the one used in these simulations. The BER is first presented for the vocoder-only channel at different AMR compression rates. Then it is presented for the vocoder and additive white Gaussian noise (AWGN) channel. Finally, an attempt is made to find the optimum receiver filter for the vocoder channel.