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This paper deals with the comparison of two multi carrier modulation techniques over existing copper twisted pairs: the Discrete Multi Tone (DMT) modulation and the Multi Carrier Code Division Multiple Access (MC-CDMA) modulation. The first form has been proposed for Asymmetrical Digital Subscriber Line (ADSL) [3]. However in ADSL, the wire pairs suffer from disturbances such as transients, radio-frequency noise. Among them, impulse noise is considered to be one of the most damaging impairments. Then, our proposal intends to limit the effects of such a noise thanks to the use of the latter form of modulation which combines multi carrier technique with spread spectrum. In order to improve the performances of the system proposed still further, various detection techniques are studied. In this context, we show that the MC-CDMA technique presents better performances than the normalised ADSL technique under identical simulation parameters (data rate, bandwith, transmitted power, coding, interleaver).

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.

Abstract-The data that is transmitted in DSL systems 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 and 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 bursts. 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.