Coded modulation techniques
BCM is a technique for generating multi-dimensional signal constellations which have both large distances (i.e. good error performance) and also regular structures allowing an efficient parallel demodulation architecture called staged decoding. It obtains a sub-set of the Cartesian product of a number of elementary (i.e. low dimensional) signal sets by itself. The staged construction allows demodulation algorithms based on projection of the signal set into lower-dimensional, lower-size constellations. These algorithms lend themselves quite naturally to a pipelined architecture.
Multi‑dimensional signals with large distances can be generated by combining algebraic codes of increasing Hamming distance with nested signal constellations of decreasing Euclidian distance.
Compared to TCM (see § 2), BCM schemes give smaller coding gains. However BCM often requires a lower demodulator complexity than TCM with the same performance and it lends itself to a parallel demodulator architecture, which might prove to be a bonus if high processing speeds are necessary.
The construction of practical BCM schemes can be based on the “step partitioning” of the signal constellation. Table 2 gives the coding gains over the uncoded reference system corresponding to the same number of net information bits per transmitted symbol, in the case of BCM families based on one-step partitioning (or “B-partition”) and on two‑step partitioning (or “C‑partition”). The results are relevant to additive white Gaussian noise (AWGN) channels.
TABLE 2
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