A system and method for shifting the relative phases of a PCM data frame and a network RBS frame by one or more symbols is disclosed. The method includes determining whether or not RBS is present. If RBS is present, the slots affected by RBS are identified. Next it is determined what data slots, if any, are affected by encoding. The PCM data frame is shifted so the RBS constraint and the encoding constraint do not coincide at any particular data slot. For one particular PCM upstream modulation scheme, the digital modem adjusts the relative phases of the data-mode frame and network RBS frame such that a minimum number of trellis-modified symbols fall on network RBS affected slots. This information is transmitted to an analog modem. The amount of shift can be conveyed to an analog modem in the same data sequence used to send upstream constellation sets and mapping parameters during an initial training sequence. The present invention improves the performance of any PCM modem that employs a data frame structure in which not all symbols are subject to the same constraints, for example, due to trellis coding. |