802.16e (WiMAX)Speech activity detection (SAD)VoIPPacket dropping probability
This paper studies the performance of voice streams over 802.16e networks. We assume the voice source employs a speech activity detection (SAD) mechanism so no packets are generated when the source stays in silence state and packets are periodically generated when it stays in talkspurt state. The SAD mechanism can be described by a two-dimensional Markov chain. Without violating what have been standardized, we suggest to use extention subheaders in uplink generic MAC header of each MAC protocol data unit (MPDU). The Type field bit 0 in the header is used as an indicator of the presence of grant management subheader which is used for the subscriber station (SS) to ask for uplink bandwidth. In addition, the Type field bit 1 is an indicator of the presence of packing subheader which is for multiplexing the shorter packets from individual voices into a longer packet. By this way, the MAC overhead can be significantly reduced. On the other hand, as the TDD frame structure allows for dynamical boundaries between downlink and uplink subframes, a feature extremely suitable for voice streams with SAD, we investigate the performance merit of using dynamical boundary for voice streams with SAD. For given system parameters and the constraint of packet dropping probability no more than 1%, our results show that the movable boundary scheme can achieve 2 more voice coonnections in comparison with the fixed boundary scheme.