Crosstalk (or inter-channel interference) is a major source of channel impairment for Multiple Input Multiple Output (MIMO) communication systems, such as Digital Subscriber Line (DSL) communication systems.
As the demand for higher data rates increases, DSL systems are evolving toward higher frequency bands, wherein crosstalk between neighboring transmission lines (that is to say, transmission lines that are in close vicinity such as twisted copper pairs in a cable binder) is more pronounced (the higher frequency, the more coupling).
A MIMO system can be described by the following linear model:Y(f)=H(f)×(f)+z(f)  (1),wherein the N-component complex vector X, respectively Y, denotes a discrete frequency representation of the symbols transmitted over, respectively received from, the N channels, wherein the N×N complex matrix H is referred to as the channel matrix: the (i,j)-th component of the channel matrix H describes how the communication system produces a signal on the i-th channel output in response to a signal being transmitted to the j-th channel input; the diagonal elements of the channel matrix describe direct channel coupling, and the off-diagonal elements of the channel matrix describe inter-channel coupling (also referred to as crosstalk coefficients),and wherein the N-component complex vector Z denotes additional noise present over the N channels, such as Radio Frequency Interference (RFI), thermal noise and alien interference.
Different strategies have been developed to mitigate crosstalk and to maximize effective throughput, reach and line stability. These techniques are gradually evolving from static or dynamic spectral management techniques to multi-user signal coordination (or vectoring).
One technique for reducing inter-channel interference is joint signal precoding: the transmit data symbols are jointly passed through a precoding matrix before being transmitted over the respective communication channels. The precoding matrix is such that the concatenation of the precoder and the communication channel results in little or no interference at the receiver. This is achieved by adding to the original signal an anti-phase signal that is the inverse of an estimate of the aggregate crosstalk signal.
A further technique for reducing inter-channel interference is joint signal post-processing: the received data symbols are jointly passed through a crosstalk cancellation matrix before being detected. The crosstalk cancellation matrix is such that the concatenation of the communication channel and the post-coder results in little or no interference at the receiver. This is achieved by subtracting from the received signal an estimate of the aggregate crosstalk signal.
Signal precoding is particularly suited for downstream communication (towards customer premises), while signal post-processing is particularly suited for upstream communication (from customer premises). Either technique is often referred to as signal vectoring.
Signal vectoring is typically performed at a traffic aggregation point, whereat all the data symbols that are to be concurrently transmitted and/or received are available. For instance, signal vectoring is advantageously performed within a Digital Subscriber Line Access multiplexer (DSLAM).
The choice of the vectoring group, that is to say the set of communication lines, the signals of which are jointly processed, is rather critical for achieving good crosstalk cancellation performances. Within that group, each communication line is considered as a disturbing line inducing crosstalk into the other communication lines of the group, and the same communication line is considered as a victim line incurring crosstalk from the other communication lines of the group. Crosstalk from lines that do not belong to the vectoring group is treated as alien noise and is not canceled.
Ideally, the vectoring group should match the whole set of communication lines that physically and noticeably interact with each other. Yet, limited vectoring capabilities and/or specific network topologies may prevent such an exhaustive approach, in which case the vectoring group would include a subset only of all the interacting lines, thereby yielding limited crosstalk cancellation performances.
The performance of signal vectoring depends critically on the component values of the precoding or cancellation matrix, which component values are to be computed and updated according to the actual and varying crosstalk channels.
A method for estimating the crosstalk coefficients comprises the steps of:                simultaneously transmitting a plurality of mutually orthogonal crosstalk pilot sequences over respective ones of a plurality of disturber channels,        measuring errors induced over a victim channel while the pilot sequences are being transmitted,        correlating the error measurements with respective ones of the plurality of crosstalk pilot sequences, thereby yielding a plurality of correlated error measurements,        estimating the crosstalk coefficients from the plurality of disturber channels into the victim channel based on respective ones of the plurality of correlated error measurements.        
That is, transceiver units send downstream or upstream pilot sequences. Error samples, measuring both the amplitude and phase of interference and noise over the victim channel, are fed back to a vectoring controller. The error samples are correlated with a given pilot sequence in order to obtain the crosstalk contribution from a specific disturber line. To reject the crosstalk contribution from the other disturber lines, the pilot sequences are made orthogonal, for instance by using Walsh-Hadamard sequences comprising ‘+1’ and ‘−1’ digits. The crosstalk estimates are used for updating the precoding or cancellation matrix. The process can be repeated as needed to obtain more and more accurate estimates.
This prior art method has been adopted by the International Telecommunication Union (ITU) for use with VDSL2 transceivers, and is described in the recommendation entitled “Self-FEXT Cancellation (vectoring) For Use with VDSL2 Transceivers”, ref. G.993.5 (April 2010).
In this recommendation, the pilot signals are sent on the so-called SYNC symbols, which occur periodically after every 256 DATA symbols.
On a given disturber line, a representative subset of the active carriers (or tones) of the SYNC symbol, which are 4-QAM modulated, are rotated by 0 or 180 degrees according to the respective digit ‘+1’ or ‘−1’ of the pilot sequence. The remaining carriers of the SYNC symbol keeps on carrying the SYNC-FLAG for On-Line Reconfiguration (OLR) message acknowledgment.
On a given victim line, error samples, which comprise both the real and imaginary part of the frequency error on a per tone or group-of-tones basis, quantized with a certain number of bits (typically 16), are measured and reported for a specific SYNC symbol to the vectoring control unit for further crosstalk estimation.
Thus, the DSLAM shall transmit and receive the SYNC symbols over the respective vectored lines synchronously (super frame alignment) so as pilot signal transmission and interference measurements are carried out simultaneously over the respective transmission lines. Also, super frame alignment keeps regular DATA symbols from being impaired by pilot signal transmission over a new joining line, the crosstalk of which is not yet canceled.
The orthogonality requirement implies that the length of the pilot sequences is lower-bounded by the maximum number of subscriber lines to be jointly processed: the more lines, the more lengthy the pilot sequences, the longer the crosstalk measurements.
This crosstalk estimation method can be severely biased by power control commands, whereby the transmit power of one or more carriers is dynamically adjusted on account of e.g. new noise conditions. Indeed, §10.5.2 of 6.993.2 ITU recommendation (VDSL2, February 2006) states that “the χ(bi), gi and tssi values shall be applied to the SYNC symbol in the same way as they are applied to DATA symbols in showtime”.
However, most crosstalk estimation algorithms that correlate the pilot sequences with the reported error samples assume that the transmit power level do not vary over time as it would affect the correlation. Considering the typical size of a vectoring group, which may comprises hundreds of lines, and the corresponding pilot sequence length, this assumption may not hold true in the field as the probability that power adaptation is carried out over one given line during the span of a pilot sequence exponentially increases with the size of the vectoring group.
A solution would be to fully disable on-line power adaptation both at the DSLAM and the CPE. This means that the line is unable to adapt itself to new noise conditions: if noise increases (e.g., RFI, residual crosstalk), there is a risk for errors, meaning a risk for line retrain.
Another solution would be to keep track of each and every carrier gain change, and to account for them in the correlation process. Clearly, this may be too costly for a large vectoring system.
Still another solution would be to just accept the errors induced by the carrier gain changes, relying on another crosstalk measurement round to get better crosstalk estimates.