| 摘要 |
Signal points from a PCM-derived constellation are selected for transmission via a modulation technique which employs different levels of redundancy coding-including the possibility of no redundancy coding-for respective different sub-constellations of the overall PCM-derived constellation. The coding that is employed for at least one of the sub-constellations is carried out independently from any coding that is employed for any of the other sub-constellations. In preferred embodiments, the sub-constellations are non-overlapping portions of the overall PCM-derived constellation, the redundancy codes are trellis codes, and the trellis codes employed in conjunction with sub-constellations having increasingly smaller minimum distance between signal points provide respectively increasing amounts of decibel gain in that minimum distance in order to compensate for that increasingly smaller minimum distance. The boundary between a pair of sub-constellations is at an amplitude position which is as high as possible consistent with whatever constraints and requirements exist, this having been found to provide a marked improvement in error rate performance. In various embodiments, the PCM-derived constellation and the overall modulation technique are designed so as to accommodate the possibility that the telecommunications system in which the modulation technique is implemented may include so-called bit robbing and so-called digital attenuation.
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