Evaluation of receiver performance over VDSL noisy links

• Main Author: Cherif, Mohamed Khaled
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/10369

VDSL environment is hostile because of the severe propagation loss due to the difficulty to propagate a high frequency signal over unshielded twisted pairs. Important noise effects ranging from crosstalk to other environment-related additive noises such as Radio Frequency Interference and impulse noise can also be a serious harm. Several algorithms have been developed to mitigate the joint effect of channel distortion resulting in intersymbol interference, and additive noise. These algorithms try to recover the transmitted data sequence with a small number of errors suitable for data transmission. Converging these algorithms for VDSL channels is not feasible in many cases, especially when the start-up process has to be done blindly, without the help of a training sequence. Generally, in blind mode the equalizers need a special setup in order to (adaptively) compensate for the channel amplitude and phase distortions. As a consequence of the nature of the VDSL environment, the LMS algorithm was shown not to converge and the number of errors at the slicer output is very high after switching to the Decision- Directed (DD) mode. Among the attractive algorithms for VDSL applications there are the Reduced Constellation Algorithm (RCA) and the Constant Modulus Algorithm (CMA). Both have been tested for bandwidth-efficient CAP-modulated VDSL links in this thesis. A similar and improved algorithm is the Multi-Modulus Algorithm (MMA) [1], which is found to be more suitable for severe channels. A dual-algorithm receiver has been tested afterwards, it combines the MMA algorithm to initially converge the receiver and then it switches the LMS in the Decision Directed mode, to get a fine convergence. The maximum achievable VDSL data rates were reported for this receiver for VDSL ANSI standard test loops.