Comparison of M2-QAM and M-PAM VSB single-carrier systems for digital TV transmission via CATV

The transmission of digital TV pictures has become important topic. For terrestrial broadcasting, a multi-carrier system consisting of a large number of orthogonal carriers (orthogonal frequency division multiplexing (OFDM)) has been proposed. In geographical areas where the cable TV (CATV) distribution system is already widely spread, the CATV distribution network is the main candidate for broadcasting digital TV signals. As the impulse response duration of the CATV transmission channel is much shorter than the delay spread of the terrestrial broadcast channel and the linear distortion is much smaller, it is not evident that OFDM is still the preferred modulation for a CATV environment. The authors have previously pointed out that a single-carrier (SC) QAM system with ideal decision-feedback equalization (DFE) is about 2 dB better than OFDM, when transmitting digital TV signals over the CATV network. The present paper investigates the performance of M²-QAM and M-PAM VSB single-carrier (SC) systems. The authors compute the degradation in SNR (as compared to a non-distorting channel) when using either ideal decision feedback equalization (with adaptive filtering) or reduced-complexity feedback equalization (with fixed filtering). They show that the performance of the reduced-complexity equalizer is sensitive to a timing offset (both QAM and VSB) and a carrier offset (VSB only). This indicates that the reduced-complexity equalizer should be used in combination with a synchronizer that carefully controls the phase of the carrier generator (VSB) and sampling clock (VSB and QAM)