Fractional-bit transmission with single-symbol TCM

Abstract

Summary form only given, as follows. Exploiting the bandwidth of a transmission channel to the largest possible extent is often only possible if a noninteger number of bits per symbol is sent. The authors consider the transmission of TCM-coded signals with m + p/q b/symbol, where q = 2k. They employ single-symbol TCM codes, together with simple block-coding schemes and signal constellations whose size is not a power of 2. This combination allows the symbol rate to be matched to the available channel bandwidth while retaining the advantages of single-symbol TCM, e.g., a small decoding delay and the availability of zero-delay tentative decisions for decision-directed receiver adaptation. In addition, the block code has the property that symbols with large amplitudes are sent less frequently, improving performance in the presence of nonlinear distortion. The block-coding operations become particularly simple for p = 1. As an example, the transmission of 5.25 b/symbol, which offers an alternative to the CCITT V.33 scheme for 14.4-kb/s modems, is discussed. Finally, the concept of reduced-state combined equalization and trellis decoding, which mitigates using a larger bandwidth, is generalized to the case of fractional-bit transmission.