Abstract—Crosstalk is the major limiting issue in very high bit- rate digital subscriber line (VDSL) systems in terms of bit-rate or service coverage. At the central office side, joint signal processing accompanied by appropriate power allocation enables complex multiuser processors to provide near capacity rates. Unfortunately complexity grows with the square of the number of lines within a binder, so by taking into account that there are only a few dominant crosstalkers who contribute to main part of crosstalk power, the canceller structure can be simplified which resulted in a much lower run-time complexity. In this paper, a multiuser power control scheme, namely iterative waterfilling, is combined with previously proposed partial crosstalk cancellation approaches to demonstrate the best ever achieved performance which is verified by simulation results. Keywords—iterative waterfilling, partial crosstalk cancellation, run-time complexity, VDSL. I. INTRODUCTION ERY high bit-rate digital subscriber line (VDSL) offers multi-ten-Mbps services by using up to 20 MHz region in the ordinary telephone copper twisted pairs. High frequency application imposes some distortions such as attenuation, crosstalk, impulsive and radio noises which limit capacity of the access channel. In theses systems, crosstalk due to electromagnetic induction of neighborhood lines is the major concern. It arises in near-end (NEXT) and far-end crosstalk (FEXT) types, wherein NEXT refers to the crosstalk created at the same side of the cable while FEXT is generated at the other side. NEXT is usually much stronger than FEXT since FEXT is attenuated as it travels along the loop length. Using time or frequency division duplexing (TDD and FDD as here assumed) circumvents generation of self NEXT (other VDSL users' NEXT). Other systems' NEXT usually don't impinge on VDSL systems substantially, as they occupy much narrower bandwidth, consequently self FEXT cancellation becomes the target of VDSL crosstalk cancellation schemes, especially as a result of short length VDSL lines. FEXT can be very Manuscript received November 20, 2004. This work was supported by the Iran Telecommunication Research Center (ITRC), Tehran, Iran. H. Bagheri is with the Electrical Engineering Department, Sharif University of Technology, Tehran, Iran, 11365 (phone: 021-616-5984; fax: 021-805-1956;e-mail: hbagheri@mehr.sharif.edu). H. Emami is with the Electrical Engineering Department, Sharif University of Technology, Tehran, Iran, 11365 (e-mail: h_emami@mehr.sharif.edu ). M. R. Pakravan is with the Electrical Engineering Department, Sharif University of Technology, Tehran, Iran, 11365 (e-mail: pakravan@sharif.edu). destructive in near-far scenarios in which strong signal of near-end transmitter destroys attenuated signal of far-end transmitter in the way to their target points. This might occur only in upstream transmission to central office (CO) for VDSL systems; however Asymmetric DSL (ADSL) systems may suffer from it in the downstream direction [1]. Impairments due to high frequency usage make telephone channels severely frequency selective, as a result, intersymbol interference (ISI) occurs which leads to imperfect detection. ISI effect can be mitigated by dividing the spectrum into N approximately flat subchannels, which is called discrete multitone (DMT) modulation. By applying well known water- filling (WF) algorithm, the transmitter can distribute its power among subchannels according to their signal to noise ratios (SNRs) in order to maximize its bit-rate. Multiuser power control and multiuser detection (MUD) based schemes are the two general approaches to reduce crosstalk destructive effects. While the former tries to vary power spectral density (PSD) of the users in order to lessen generation of crosstalk, the latter attempts to cancel the existing one. Although MUD based approaches outperform the avoidance ones, they suffer from their run-time complexity (~ multi billions multiplications/second) which is not currently realizable and may remain infeasible economically for several years [2]. The main part of crosstalk power seen by each user comes from a few numbers of its neighboring lines in the binder. In addition, crosstalk cancellation may lead to large performance gains only in a small sub-set of tones. Taking these points into account, large run-time complexity reduction can be achieved via partial crosstalk cancellation (canceling only dominant crosstalkers in space and frequency domains). In this paper, a multiuser power control procedure, known as iterative waterfilling (IWF), is joined with existing partial crosstalk cancellation (PCC) algorithms to achieve near capacity rates, which is not considered before. It also, leads to larger run-time complexity reduction at the expense of slightly higher initialization complexity in comparison to previous algorithms wherein power control schemes have not been applied. Various simulation results show that the iterative algorithm almost always converges after 1~2 iterations, so the initialization complexity increases marginally. It must be noted that the initialization complexity is not taken into account, in crosstalk cancellation complexity calculations due to almost static nature of DSL channels. The rest of the paper is organized as follows. Section II Iterative Joint Power Control and Partial Crosstalk Cancellation in Upstream VDSL H. Bagheri, H. Emami, M. R. Pakravan V World Academy of Science, Engineering and Technology 2 2005 21