In addition to being designed to successfully cope with stationary noise, crosstalk, and intersymbol interference, digital subscriber loop (DSL) systems need to be shielded from nonstationary disturbances, such as impulse noise and RF interference. Currently, deployed DSL systems achieve protection against nonstationary interference using a combination of Reed-Solomon (RS) codes and interleaving. However, interleaving results in delay. Long delays are undesirable in high-rate systems that support interactive applications. In this study, it is shown that the interleaving delay of DSL systems can be significantly reduced by performing erasure decoding of the RS codewords at the receiver. Three different techniques for determining the erasures are proposed. Use of the techniques results in a reduction of the interleaving delay that is required to mitigate worst-case impulse noise by up to a factor of 2, which is verified by simulation. Moreover, the techniques do not require any changes at the transmitter and therefore guarantee compatibility with currently deployed systems.

DSL systems achieve high rate transmission through copper pairs traditionally used for voiceband communication. In order to attain high rates, sophisticated system designs are employed to cope with both stationary and non-stationary disturbers. In current DSL systems, protection against nonstationary interference, comprising impulse noise and radio frequency ingress (RFI), is achieved using a forward error correction (FEC) scheme consisting of a Reed-Solomon (RS) code and interleaving. However, interleaving results in increased end-to-end delay. The end-to-end delay can be reduced by taking advantage of the erasure decoding property of RS codes. The paper presents a byte-erasure technique that allows erasure decoding to be employed. The method locates the possibly erroneous bytes in each RS codeword by comparing the square distance between each received carrier symbol of the interleaved stream and the closest point of the constellation to which the symbol belongs. Thus, the interleaving delay can be reduced by a factor up to 2 when impulse noise impairs transmission. The method can also be applied to systems affected by both impulse noise and RFI, since it can distinguish between the two sources of non-stationary interference. Finally, no change is required at the transmitter, and hence compatibility with currently deployed systems is guaranteed. A DMT-VDSL system is used as a particular example of the achieved reduction of the interleaving delay.

The data that is transmitted in DSL system is subject to corruption by impulse noise, i.e., noise bursts of high energy that interfere with the transmitted symbols. As DSL data rates increase the crosstalk mitigation techniques become more sophisticated, impulse noise limits service in terms of rate or delay. Because of the highly non-stationary nature of impulse noise, a combination of interleaving and Reed-Solomon coding is currently used to shield systems from noise burst. This paper presents a modified impulse noise protection algorithm that takes advantage of the improved performance of Reed-Solomon codes when the location of the impaired bytes is known. Without changing the structure of the encoder or the interleaver, it is shown that the delay, or equivalently the overhead due to forward error correction coding, can be reduced without compromising the immunity of the system to impulses. A DMT-VDSL system is used as a particular example of the improvement achieved using byte-erasure.

A significant portion of the end-to-end delay in high-rate DSL systems is due to the impulse noise protection scheme employed in order to shield those systems against random, non-stationary noise bursts of high energy that appear on the copper lines. Systems are protected from impulse noise using a combination of interleaving and Reed-Solomon codes. In order to lower the end-to-end delay without reducing the data rate that is available to the user, one needs to decrease the interleaver depth. This paper presents a way to achieve this reduction without compromising neither the robustness to noise bursts nor the data rate of the system. The proposed algorithm relies on the inner code used by many DSL systems and uses the metric provided by the inner code decoder at the receiver. A DMT-VDSL system is used as a particular example of the achieved reduction of the end-to-end delay.

The paper presents a numerical study of the impact of impulse noise on asymmetric digital subscriber lines (ADSL). Methods for simulating the effect of impulse disturbances on a discrete multitone system are first presented, and actual measured noise bursts are then used for the simulations as if they were deterministic signals, in order to characterize their effects on ADSL systems. It is shown that, while a combination of coding, interleaving, and 6-dB margin is adequate in protecting ADSL systems from isolated impulses, an impulse train with long duration can cause a significant number of error bits in the system. In this case, a tradeoff among the number of error seconds, the maximum reach, and the coding delay must be made.