3.3.2 Coded modulation
This method is a technique that combines coding and modulation which would have been done independently in the conventional method. Redundant bits are inserted in multi-state numbers of transmitted signal constellations (see Appendix 1). This is known as coded modulation.
Representative examples of coded modulation are block coded modulation (BCM), trellis coded modulation (TCM) and multi-level coded modulation (MLC or MLCM). In BCM, levels are coded by block codes whereas TCM uses only convolutional codes. On the other hand, different codes can be used for each coded level in MLCM, so MLCM can be seen as a general concept that includes BCM and to some extent TCM. These schemes require added receiver complexity in the form of a maximum likelihood decoder with soft decision. Tables 1a and 1b provide indications of expected performances.
A technique similar to TCM is the partial response, sometimes called a duo-binary or correlative signalling system. A controlled amount of intersymbol interference, or redundancy is introduced into the channel. Hence, the signal constellation is expanded without increasing the transmitted data bandwidth. There are various methods utilizing this redundancy to detect and then correct errors to improve performance. This process is called “ambiguity zone detection” or AZD.
Additional information on BCM, TCM, MLCM and partial response with AZD is given in Appendix 2.
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