Towards voice over ad hoc networks: An adaptive scheme for packet voice communications over wireless links

In a no-guarantees network such as an ad hoc network, nodes are expected to adapt to changes in network and channel conditions. As a step toward supporting voice communications in such environments, we propose an adaptation scheme that takes both voice coding (i.e., compression) and modulation as parameters. In such severe environments, the main goal is to maintain a call as opposed to providing excellent quality. To this end, our adaptation scheme demands the least cost in terms of network bandwidth when network conditions are good. As network conditions worsen, more bandwidth is needed so that an acceptable quality can be maintained. This approach is a balance between providing reasonable quality and less resource consumption (with its implication on decreasing network load thus not affecting other connections). Even though our ultimate goal is supporting voice over ad hoc networks, in this paper we present extensive simulation results over a single hop scenario. Unlike other adaptation schemes involving source coding and modulation schemes in the literature, our scheme is specific to the nature of voice, thus allowing for better perceived quality without imposing high costs. Our adaptation scheme is investigated under a Rayleigh fading channel modeled as a finite state Markov channel (FSMC). The scheme is generic enough to work for any packet voice system including VoIP over wireless. Results show that the scheme allows for supporting many more calls when compared to a non-adaptive scheme.