No abstract

In current DSL systems, crosstalk is a major problem. In the case of in-domain crosstalk and AWGN, it has been shown that the crosstalk can be effectively mitigated using a linear zero-forcing canceler. Furthermore, the complexity can be reduced by only canceling the crosstalk from major crosstalkers on each tone, which is reffered to as partial cancellation. However, such approach does not work e.g. in the case of alien crosstalk. As alien crosstalk is spatially correlated, the zero-forcing canceler performs very poorly, hence an alternative linear canceler has to be examined. In this paper, we demonstrate that an MMSE crosstalk canceler provides much improved performance than the zero-forcing canceler in this scenario and additionally, we also present an efficient algorithm to perform partial MMSE partial cancellation.

a b s t r a c tPer-tone equalization has been proposed as an alternative to time domain equalization for DMT receivers in DSL modems. It optimizes the bit rate performance of the receiver as each tone can be equalized independently. It has also been shown that using variable length equalizers can significantly reduce the total number of equalizer taps and hence the run-time complexity, without compromising performance. For a given transmit power loading, it has been shown that the equalizer taps can be allocated optimally using a dual decomposition based approach with per-tone exhaustive searches over all possible equalizer lengths. However, a more general approach is needed when optimal transmit power allocation is also considered to maximize the overall bit rate, where in addition the per-tone exhaustive searches are replaced by a more efficient procedure. In this paper, a sparse approximation based resource allocation algorithm is presented to allocate equalizer taps and transmit power over tones and maximize the overall bit rate. This algorithm is shown to provide efficient allocations at a relatively low computational cost.

Per-tone pulse shaping has been proposed as an alternative to time domain spectral shaping for DMT transmitters, e.g. VDSL modems. It shapes the spectrum of individual tones such that the stop band energy of each tone can be minimized. This in particular enables transmitter to use more tones without violating the PSD mask constraint for data transmission. In pertone pulse shaping based DMT transmitters a fixed length pulse shaping filter is typically used for every tone. The tones in the middle of the pass band however, contribute less to the overall stop band energy, so that using a high order pulse shaping filter for these tones does not result in a significant reduction of the stop band energy. As a result a significant number of pulse shaping filter taps are wasted on the tones in the middle of the pass band and do not bring any performance gain. Using a variable length pulse shaping filter which is designed such that the PSD mask constraint is not violated can then significantly reduce the total number of pulse shaping filter taps without compromising performance. In this paper, a resource allocation technique is presented for variable length pulse shaping filter design using a dual problem formulation. This optimally solves the problem of pulse shaping filter tap distribution over tones for given PSD mask constraints, with a relatively low complexity. In per-tone pulse shaping based DMT transmitters a fixed length pulse shaping filter is typically used for every tone. RESOURCE ALLOCATION IN DMT TRANSMITTERS WITH PER-TONE PULSE SHAPINGThe tones in the middle of the pass band however, contribute less to the overall stop band energy, so that using a high order pulse shaping filter for these tones does not result in a significant reduction of the stop band energy. As a result a significant number of pulse shaping filter taps are wasted on the tones in the middle of the pass band and do not bring any performance gain. Using a variable length pulse shaping filter which is designed such that the PSD mask constraint is not violated can then significantly reduce the total number of pulse shaping filter taps without compromising performance. In this paper, a resource allocation technique is presented for variable length pulse shaping filter design using a dual problem formulation. This optimally solves the problem of pulse shaping filter tap distribution over tones for given PSD mask constraints, with a relatively low complexity.

Per-tone pulse shaping has been proposed as an alternative to time domain spectral shaping for Discrete MultiTone (DMT) transmitters, e.g. VDSL modems. This enables the transmitter to use more tones without violating the Power Spectral Density (PSD) mask constraint for data transmission. The computational complexity of the per-tone pulse shaping and transmit power is evenly distributed over tones, however, resources (computational complexity and power) can be better exploited by using different filter lengths for different tones and by resorting to power loading. For a fixed pulse shaping filter length, the contribution of a particular tone to the stop band energy depends on the power allocated to the tone and on the distance of the tone from the band edges. The use of high order pulse shaping filters for the tones at the band edges (as well as the use of lower power) will reduce their contribution to the out of band PSD, whereas for the tones at the middle of the band these factors will have less effect on the out of band PSD. Therefore, the combination of both power loading and a variable length pulse shaping filter can be used to achieve a high data rate under resource and PSD constraints. In this paper we present an algorithm to optimally allocate the resources i.e. power and filter taps, using a dual problem formulation. This solves the problem of optimally distributing power and filter taps over tones for a given PSD mask constraint, with a relatively low complexity.