A Combined Coding and Modulation Approach for Communication over Dispersive Channels

The concept of combining the design of the error-correcting-coding approach and the modulation format is treated in general terms and its effectiveness is demonstrated by a specific implementation referred to as Codem I. A new decoding algorithm is presented, which has the interesting property that the only channel measurement information utilized is the relative reliability of each received digit. The effectiveness of this decoding technique is demonstrated by computer simulation over three different channel models for dispersive channels. Comparisons of the performance of Codem I with the more conventional 16-tone (four-phase DPSK) HF modem are obtained by actual field results as well as by computer simulations. Improvement in error probability in the region of two orders of magnitude is demonstrated when both systems are operating under similar channel conditions and at equal data rates. A further improvement is demonstrated when channel measurement information is used to reject a small percentage (typically less than 3 percent) of codewords that are considered unreliable.