In order to obtain high transmission rates DSL systems have to cope with stationary and non-stationary disturbances, as well as the non-ideal characteristic of the twisted pair channel. To mitigate non-stationary interference, a forward error correction (FEC) scheme is employed that consists of a Reed-Solomon outer code and interleaving. By augmenting the interleaving delay, protection against increasingly long noise bursts can he achieved. however, when radio frequency ingress (RFI) also affects the subscriber loops, the required interleaving delay may potentially increase to undesired levels. The additional delay that is required for combined impulse noise and RFI protection can be lowered by using more redundant RS codes, or by employing RFI cancellation techniques. In the latter case, additional digital cancellation is also necessary in order to mitigate the effect of RFI on the transmitted data. This paper proposes a digital RFI cancellation scheme for DMT-DSL systems that is based on information exchange between the DSL demodulator and the RS decoder at the receiver. It is shown that the technique can be seamlessly incorporated in current DMT-DSL systems, since no changes are required at the transmitter. The method can partly cancel the effect of RFI, and can help bridge the gap between the required interleaving delay when only impulse noise is present and the increased delay when RFI also affects transmission.

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.