PREDICTIVE JITA

Publishing Venue

Motorola

Related People

Abstract

JITA (Just In Time Audio) is a process within the iDEN system that allows synchronization of voice packet transmissions between the XCDR and the EBTS in respect to RF time slots (TDMA RF interface from EBTS to SU). The goal of JITA is to reduce the amount of time the packet will wait at the EBTS before it is transmitted over its assigned RF slot. Normally JlTA is performed at the begin- ning of a call, after a cell handover and due to changes in Tl propagation delay. JITA is performed in several steps, depending on the initial misalign- ment of voice packet arrival time to the BR and the boundary of the available RF slot. Each JITA step causes a discontinuity in the inbound voice stream and could cause audio distortion ("thunks"). As a result, minimizing JITA requests will improve audio quality.

m M-LA Technical Developments

PREDICTIVE JITA

by Michael Bychowsky, Michael Kloos and Yakov Simkin

BACKGROUND

  JITA (Just In Time Audio) is a process within the iDEN system that allows synchronization of voice packet transmissions between the XCDR and the EBTS in respect to RF time slots (TDMA RF interface from EBTS to SU). The goal of JITA is to reduce the amount of time the packet will wait at the EBTS before it is transmitted over its assigned RF slot. Normally JlTA is performed at the begin- ning of a call, after a cell handover and due to changes in Tl propagation delay. JITA is performed in several steps, depending on the initial misalign- ment of voice packet arrival time to the BR and the boundary of the available RF slot. Each JITA step causes a discontinuity in the inbound voice stream and could cause audio distortion ("thunks"). As a result, minimizing JITA requests will improve audio quality.

THE PROBLEM

  Since the XCDR needs to corrupt the input PCM stream in order to perform the delay adjust- ment, audible discontinuities can be detected by the subscriber user. This is especially noticeable shortly after a handover since the call is in process and could potentially be transmitting speech. Since the XCDR has no knowledge of the packet's desti- nation after it is sent, it cannot use any previous information to predict the delay. Instead, each call is originated from the XCDR asynchronously to its assigned RF time slot.

THE SOLUTION

  By determining from call history the timing delay between an audio channel and the RF time slot, initial packets can be sent out to match this delay. This will help in reducing the dependency on JlTA adjustments to correct to the proper delay. Furthermore, if the initial guess of the packet delay was incorrect, JITA adjustments will be made to correct the timing. This reduction in required JI'IA adjustments will help improve audio quality by not introducing any discontinuities in the incoming PCM stream. Figure 1 (Predictive JITA Messaging Protocol) shows the proposed protocol that will allow the XCDR to determine which BRfSlot com- bination it is communicating with. Converged delay values will be stored in the XC...