A Transcoding-Free Multiple Description Coder for Voice over Mobile Ad-Hoc Networks

Abstract

Abstract-We propose a new multiple description (MD) coder design based on the Adaptive Multi-Rate Wideband (AMR-WB) coder that can support transcoding-free communication between an ad-hoc network and another network that supports the AMR-WB codec. The encoder of the MD coder consists of the standard AMR-WB coder and a bit-stream splitting block that splits the AMR-WB bit-stream into two balanced descriptions. The decoder consists of a bit-stream substitution block that substitutes the missing bits, when only one description is received, to construct a valid AMR-WB frame that can be decoded using the standard AMR-WB decoder. We show that the performance of the new MD coder is better than a previous non-transcodingfree MD coder based on AMR-WB when transcoding is required and it is significantly better than using a single description of AMR-WB over the ad-hoc network supporting transcoding-free communication. I. VOICE OVER AD-HOC NETWORKSMobile Ad-hoc Networks (MANETs) are formed by mobile wireless hosts without the need of an existing infrastructure, unlike wireless cellular systems which require a centralized control and support system at the base station. Most of the wireless systems deployed today are centralized systems, wherein the nodes connected to the network communicate through an access point or a base station. MANETs do not need such a centralized support and can be deployed any place where nodes want to talk to each other or someone else through their neighbors. Since the nodes now depend on each other for communication, mobility of nodes implies that the routes for data transfer between two nodes are not fixed.MANETs are seen as future networks for Personal Area Networks (PAN), military environments, emergency operations and in an office or conference environments. Voice communication is essential in all of these scenarios. Real time communication over a MANET is a challenging problem because of the many transient characteristics of the network that arise due to the flexibilities that a MANET offers. The constantly changing routes between the sender and receiver nodes, lack of time synchronization between nodes in the network, broken links and delays involved in establishing Path diversity can be used to improve end-to-end connectivity between the nodes in a MANET. Using a path diversity scheme not only improves fault tolerance but also reduces overall packet losses and end-to-end delays. However, sending multiple copies of the same packet is inefficient usage of bandwidth. To improve the efficiency of using multiple paths, a source coding diversity method like Multiple Description (MD) coding can be used. In MD coding, multiple descriptions/bit-streams of the source are created in such a way that each description can be used to reconstruct the source with acceptable quality and two or more descriptions can be combined to give a better quality reconstruction.With different communication networks using different speech codecs, merging of the networks requires cross tandeming / transcoding of speech, w...

Multiple-Description Speech Coding Using Speech-Polarity Decomposition

“…Such extensions have been proposed for FS-1016, MELP, G.723.1 [5], G.711 [6], G.729 [8], and AMR-WB [8], [7], [9]. The common thread throughout this work is the insightful division of the standardized (or slightly modified) encoder output into m descriptions so that when taken together, those descriptions produce speech with a quality near that of the standardized coder, and when fewer than m descriptions are available, they can still be decoded to produce somewhat lower-quality speech.…”

“…The work in [5] has no rate penalty, but achieves this by doubling speech coding frame sizes thus incurring quality penalties. The results in [8], [7], [9] have no quality penalties, but the transmission of redundant information leads to rate penalties ranging from 9% to 20%. Reference [6] offers a family of designs that include one with a quality penalty and no rate penalty, one with a rate penalty and no quality penalty, and several intermediate cases that allow one to trade-off the two penalties.…”