Trellis coded modulation employing lower dimensionality convolutional encoder

DETAILED DESCRIPTION Coding gain is improved in accordance with the principles of the present invention by employing an N-dimensional trellis coded modulator incorporating an N'-dimensional convolutional encoder where N' is less than N.
The additional coding gain is realized as a shaping gain.
This unique modulator encodes a digital signal by generating a sequence of N-dimensional symbols as a function of the digital signal using an N'-dimensional convolutional encoder within the trellis coded modulator more than once (integer N/N' times) during each N-dimensional symbol interval.
This allows the trellis coded modulator to produce an output of higher dimensionality than would normally be expected from the lower dimensionality convolutional encoder.
In a specific embodiment of the invention, a two-dimensional (2D) convolutional code is used in a four-dimensional (4D) trellis coded modulation scheme.
The trellis coded modulator uses four-dimensional constellation mapping.
In another embodiment, the N-dimensional trellis encoder is used as the inner coder of a concatenated code while an N-dimensional constellation mapper converts the coder output into a sequence of N-dimensional symbols or J P-dimensional signal points where J and P are integers whose product is N.
Each P-dimensional signal point is transmitted in a so-called "signalling interval.
" J P-dimensional signal points are then transmitted in an N-dimensional symbol interval.