Improvement of VoIP packet loss concealment for CELP-based coder

Abstract

Recently, as the packet-switched network is growing and bandwidth of the network is broadening, transmission of real-time data such as voice becomes important issue. However, because of "best-effort" characteristic, internet cannot guarantee complete transmission of voice. Of some factors affecting the QoS of VoIP, packet loss is a major source of degradation of voice quality. Packet loss is defined as the discarded or dropped packet due to various network environments. If decoder of VoIP detects the loss of packet, decoder generates new signal to conceal the packet loss. Various codecs such as G.711, G.729, G.723.1, AMR-NB, and AMR-WB, are used in VoIP and each codec has a unique packet loss concealment algorithm. Since the packet loss concealment algorithms in the codecs are different one another, the performance of PLC algorithms embedded in the codecs is different. This thesis discusses the performance of PLC algorithms of above codecs and compared the performance of the PLC algorithms. We analyzed which codec shows the best performance at burst packet loss and how many frames can be coupled with one packet for each codec. G.729 coder provide good performance at random packet loss. On the other hand, for burst packet loss, PLC algorithm of G.729 is very poor. To let G.729 PLC algorithm be robust to burst packet loss, this thesis provide three modules to improve G.729 PLC algorithm. The three modules are adaptive gain controller, adaptive memory update, and adaptive bandwidth expansion. Adaptive gain controller minimizes the error propagation phenomenon. Adaptive memory update and adaptive bandwidth expansion improve speech quality by providing slowly-changing speech characteristics for lost frames.