Profile detection in multiuser digital subscriber line systems

Abstract

Multiuser transmission methods for digital subscriber line (DSL) systems have become of interest with the potential for increased data rate and loop reach. These methods often assume that the set of crosstalk interferers, called the crosstalk profile, and their associated channel responses are known. For DSL systems, the interferers are often uncoordinated, so that in a dynamic environment where DSL transmitters can energize and deenergize, the crosstalk profile cannot be transmitted to the user of interest. While the crosstalk channel estimation problem in a dynamic environment can be intractable for general transmission systems, channel and crosstalk analysis can make use of the specific DSL environment. Namely, the physical channels in a DSL system do not change rapidly, and hence estimates of the crosstalk channel can be saved for future reference. For this reason, we introduce the concept of a channel profile. We develop several algorithms to detect the crosstalk profile and investigate the asymptotic behavior of the new algorithms. Simulations show that for typical crosstalk interference scenarios, the observation time to determine the correct crosstalk profile at probability of error less than 10 3 can be less than 2 ms.

Computationally Efficient Partial Crosstalk Cancellation in Fast Time-Varying DSL Crosstalk Environments

“…Firstly, as p increases a bigger fraction of the error is due to the residual crosstalk of the dominant crosstalkers for the AI scheme (compare (A.5) and (A.6)). Secondly, the condition number of H 0 k increases as predicted by (7), and therefore, the error is bigger for the first-order terms of the power-series expansion of H 0 k . Thirdly, as p increases, the number of elements that should be eliminated from H −1 k decreases in the RI method, and therefore, the resultant matrix is a better approximation for the perfect crosstalk canceller.…”

“…The crosstalk couplings between pairs are considered to be inversely proportional to the square of the distance between the centers of the corresponding circles in Figure 2. 7 As a worst-case scenario, we select a tightly packed subset of pairs at the center of the binder. The crosstalk couplings are normalized so that they are equal to the one percent worstcase model for tangent circles (e.g., pairs 1 and 2, 1 and 3, 2 and 7, etc.).…”

“…Note that although the twisted-pair channel does not change quickly [7], the crosstalk profile can change very rapidly in DSL systems. These profile time variations can be due to a variety of causes.…”

“…Most obviously, they can be 2 EURASIP Journal on Advances in Signal Processing caused by quiescent modes in DSL transmitters. For example, a protocol, which reduces the transmitted power or switches the modem to an idle state when there is no information to be sent, would not only save money for the transmitter, but would also reduce the crosstalk power in the loop plant and allow other rate-adaptive modems to increase their rates [7]. Such a power-reduction scheme would have dramatic effect on the crosstalk profile as a significant share of traffic over DSL lines is due to Internet web browsing including variable-rate multimedia traffic.…”