Baseband trellis codes with a spectral null at zero

A method is described for modifying classical N-dimensional trellis codes to provide baseband codes that combine a spectral null at DC with significant coding gain. The information rate of the classical code is decreased by one bit, and this extra redundancy is used to keep the running digital sum bounded. Equivalently, if the rate is held constant, then twice as many signal points are needed, causing a power penalty of 6/N dB. Baseband trellis codes are presented for several information rates together with complete spectral plots and performance comparisons. A method of constructing baseband codes with multiple spectral nulls is also described. >