MOSx and Voice Outage Rate in Wireless Communications

Average packet loss rate (PLR) or average mean opinion score (MOS) are often used performance indicators for voice communications. However, even for a fixed average PLR, the delivered voice quality depends on the location of the packet losses as well as the distribution of packet losses due to the differences in voice codecs, their packet loss concealment schemes, the difficulty of concealing packet losses in unvoiced- to-voiced and voiced-to-unvoiced transition regions, and the difficulty of concealing successive packet losses. The result is that there is a distribution of achieved MOS values for a fixed average PLR and that the average MOS value may not capture the voice communications performance. Using the PESQ-MOS, we study the distribution of MOS values for wireless voice communications over additive white Gaussian noise (AWGN) and multipath fading channels using G.711 and G.729 voice codecs and their packet loss concealment schemes. Based on the distribution of PESQ-MOS, we define a quality indicator referred to as the MOS_x, which is the MOS value that a user can expect to achieve or exceed x% of the time, where MOS and x are chosen to correspond to an acceptable voice outage rate. The MOS_x is then used to evaluate the performance of G.711 and G.729 codecs over frequency selective fading and AWGN channels for voice over IEEE 802.11 a wireless LANs.