TRANSLATION TABLE AND CONSTELLATION DESIGN FOR A PCM MODEM SUBJECTED TO ALTERNATING ROBBED BIT SIGNALING

摘要

The receiver of a PCM modem utilizes the V.90 TRN1d trainingsignal for detecting whether any of the six slots of the receivedsignal may be subject to alternating robbed bit signaling (ARBS),and what the alternating robbed bit signaling pattern may be.This is accomplished by accumulating the received level over aplurality of frames for each slot and by comparing the receivedlevels for each slot to an average level for that slot. If thedifference between the received levels and the average levelsexceeds a threshold, the slot is determined to be subject to ARBS.Where a slot is subject to ARBS, DIL sequence signals for framesof the alternating robbed bit signaling slot having LSB=0 andLSB=1 are accumulated separately (if available) in order togenerate two translation tables (TRT0, TRT1) for that slot. Wherethe DIL sequence is found only in the frames having thealternating robbed bit signaling slot having LSB=0 or havingLSB=1, only one translation table can be generated. Differentalgorithms are provided for designing a constellation for thealternating robbed bit signaling slot depending on (a) whether twotranslation tables were generated for the slot, (b) whether theframe-to-TRT correspondence is known, and (c) where only onetranslation table is generated, whether the translation tablegenerated was TRT0 or TRT1.