A comparison of QAM and CAP equalizers for VDSL

The two single-carrier line codes which have recently been considered for VDSL standards are quadrature amplitude modulation (QAM) and carrierless amplitude/phase modulation (CAP). They have the same signal constellation and transmitted spectral shape. However, experiments have shown that the equalizer convergence rates for QAM and CAP can be different. These convergence rates can impact both the training sequence length requirements and the equalizer tracking ability for time-varying channels. We investigate a variety of equalizer architectures and algorithms and study the training sequence equalizer convergence behavior of QAM- and CAP-based VDSL systems by using both theoretical analysis and Monte Carlo computer simulations. We show that the particular equalizer structure and channel have important effects on the convergence behavior. Based on our simulations for various VDSL test loops, we recommend using the CAP matched filtered phase-splitting equalizer due to its good convergence speed and relatively low complexity