Performance evaluation of the future narrowband digital terminal and MELP over concatenated Internet and CDMA cellular data networks

Scope of study. This study describes the performance evaluation of the Future NarrowBand Digital Terminal (FNBDT), a new system being developed by the government supporting secure multimedia communications. FNBDT provides a network independent architecture for communications over concatenations of wired and wireless networks. FNBDT uses the new MIL-STD-3005 MELP speech coder for secure voice communication. This research investigates issues related to transmission of real-time FNBDT formatted voice data across mixed Internet and CDMA cellular data networks with the goal of providing recommendations for a robust high quality FNBDT system. Empirical studies and probabilistic models are presented to simulate and evaluate FNBDT system performance in these mixed IP packet network environments. As packets traverse the Internet and CDMA wireless links they experience congestion on Internet routers, deep fades as a result of multi-path effects, and Doppler frequency shift effects related to mobile unit speeds on the wireless link. These network abnormalities results in packet loss, burst loss, excessive jitter delays, and out-of-order packet delivery.; Findings and conclusions. Results show that the FNBDT signaling plan and MELP are particularly susceptible to these types of packet errors which seriously degrade voice quality. However, quality improvements are presented that improve FNBDT quality to an acceptable level including frame replacement strategies for packet loss, forward error correction (FEC) for recovery of FNBDT synchronization loss, and an adaptive jitter buffer scheme to accommodate network delay jitter. Quality measures including the frequency weighted distortion measure specifically designed for MELP, informal listening tests, the perceptual evaluation of speech quality (PESQ) adapted by the ITU-T, and a self-organizing map (SOM) neural network are presented for the evaluation of overall FNBDT system quality.