A square distance-based byte-erasure method for reduced-delay protection of DSL systems from non-stationary interference

Abstract

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 that cope with both stationary and non-stationary disturbers. In current DSL systems, protection against nonstationary interference, comprising of Impulse Noise and Radio Frequency Ingress (RFI), is achieved using a Forward Error Correction (FEC) scheme consisting of a Reed-Solomon 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 the Reed-Solomon codes. This paper presents a byte-erasure technique that allows erasure decoding to be employed. The method locates the possibly erroneous bytes in each Reed-Solomon 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. This way, 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 Radio Frequency Ingress, 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.

Digital interference cancellation for DSL using information from the RS decoder

“…As can be seen, the flow of information is bidirectional. The DSL system provides the reliability metrics that are used by the byte-erasure algorithm to erase bytes before RS decoding is performed [3]. When RFI is also present, the RS decoder needs to convey the values of the corrected bytes to the DSL system as well.…”

“…In this paper it is assumed that the receiver has the ability of erasure decoding in order to reduce the end-to-end delay. The erasure method that is employed in this paper is the squaredistance method of References [3,7], that processes each tone in order to provide erasure information to the decoder. By examining each symbol in each tone it is possible to distinguish between tones (and, consequently, bytes) corrupted by RFI and impulse noise, as described in more detail in Reference [8].…”

“…The interleaving depth is determined by the length of the noise impulses and the characteristics of RFI. Although use of erasures [1][2][3] can reduce significantly the end-to-end delay, there is still a low bound caused by the length of impulse noise. If the end-toend delay is above the acceptable values, it can be lowered by increasing the parity of the Reed-Solomon (RS) code, at the expense of the useful rate that is delivered to the user.…”

Impulse noise detection techniques for retransmission to reduce delay in DSL systems

“…Thus, the sample should be erased, but it is not. This event is called missdetection [11]. The miss-detection probability is denoted by .…”

“…If is too low, the system can be too sensitive and generate more unnecessary retransmissions. It is known that with the erasure marking procedure, RS code can provide an error correction capability of ( ) [11]. In this case, a proper guess is to let .…”