In this dissertation, various aspects of statistical voice/data multiplexing have been studied, and new results have been obtained in three areas that attract recent research interests.
First, the on-off speech statistics of conversational speech have been measured using an accurate speech detector and a large data base. Based on the measurement results, probability density functions of silence and talkspurt durations have been modeled approximately by two weighted geometric probability density functions. Then, for any value of hangover or fill-in, important speech parameters such as speech activity, and average silence and talkspurt durations have been calculated based on the model of probability density function of silence durations, and compared with those measured. Directly measured values of speech parameters and those calculated have been shown to be in good agreement for a practical range of hangover and fill-in time. For a large hangover time greater than 200 ms, silences and talkspurts have been fit by an exponential distrubution and a constant-plus-exponential distribution in the continuous time domain, respectively. On the other hand, for a large fill-in time greater than 200 ms, silences and talkspurts have been modeled by a constant-plus-exponential distribution and an exponential distribution, respectively. With both large hangover and fill-in values, the talkspurt model agrees closely with the measured data, but the silence model does not agree as closely as the talkspurt model.
Second, the performances of embedded adaptive differential pulse code modulation (ADPCM), embedded adaptive delta modulation (ADM), and the same systems with a delayed-decision scheme have been studied with real speech over a wide dynamic range. The embedded ADPCM and ADM coders have been obtained by modifying the conventional ADPCM and ADM coders. The basic scheme of the embedded ADPCM coder is based on the ADPCM originally proposed by Cummiskey et al. For embedded ADM systems, three representative ADM systems have been chosen. They are continuously variable slope DM (CVSD), constant factor DM (CFDM) and hybrid companding DM (HCDM). Among these embedded coders, the performance of the embedded HCDM is superior to the other coders over a wide range of transmission rate from 16 kbits/s to 64 kbits/s. When the delayed-decision scheme is applied to the embedded ADPCM, the performance is improved significantly at all transmission rates. But, in the embedded ADM systems with 16 KHz sampling rate, the performance improvement resulting from delayed decision is not so much drastic as is in the embedded ADPCM with the same number of delayed samples.
Finally, new statistical voice/data multiplexers have been proposed. The structures of these multiplexers are based on a new queueing method, a frame management scheme and an integrated flow control algorithm. For the queueing purpose, two separate voice and data buffers are used to queue voice and data packets independently. Each queue discipline is first-in first-out (FIFO). Four versions of master frame management have been considered. They are fixed boundary (FB), movable boundary for data traffic (MBD), movable boundary for voice traffic (MBV), and movable boundary for both voice and data traffics (MBB). For the integrated flow control algorithm, speech coding techniques such as variable rate coding and embedded coding have been considered. To evaluate the performances of the proposed multiplexers, new analytical methods have been proposed. For the performance analysis, a two-state Markov model has been chosen to reflect the frame-by-frame correlation of talkspurt activity for the same talker. For a data traffic model, three representitive models have been considered. They are Poisson, batch Poisson, and mixed Poisson arrival processes. By using these input traffic models, analytical formulations for the performance analysis have been made from the equivalent queueing models of the proposed multiplexing systems. Then, numerical results on the performance of the multiplexers have been obtained by using the Gauss-Seidel iteration method. These results show that there exist tradeoffs between data and voice traffic performances for a given input traffic load and the output channel rate. To study various aspects of the proposed voice/data multiplexing systems, computer simulation has been done with real speech. Two types of the statistical voice/data multiplexers have been studied; one employs a variable rate ADPCM coder and the other an embedded ADPCM coder with a delayed-decision scheme. The performance results of these multiplexers have been obtained with various system parameter values. Both simulation and analysis results show that the quality of received speech is largely dependent on the average input rate of data traffic in statistical voice/data multiplexing with MBB frame management.
