Distributed & Centralized Power Control Algorithms for Very High-Speed Digital Subscriber Lines (VDSL) Upstream Transmission Adnan Rashdi†, Noor M. Sheikh† and A.U.H. Sheikh †Department of Electrical Engineering, University of Engineering & Technology, Lahore, Pakistan Department of Electrical Engineering, King Fahad University of Petroleum & Minerals, Dhahran, Saudi Arabia Phone: +92-42-6829300, e-mail: adnanrashdi@ieee.org, deanee@uet.edu.pk, asrarhaq@kfupm.edu.sa. Abstract i — Very High-Speed Digital Subscriber Lines upstream transmission suffers from strong far end crosstalk (FEXT) by the shortest lines in a multiuser communication environment. This problem called “Near-Far Effect” drastically reduces the upstream capacity on the longer lines. Power control algorithms effectively handle this problem and maximize the achievable data rate region, given an average power constraint for each user. This paper examines distributed and centralized power control algorithms and compares their performance in a frequency selective multiuser interference channel considering VDSL upstream transmission environment. I. INTRODUCTION Digital Subscriber Lines (DSL) is a local access technology that brings high speed data connection to home via ordinary telephone twisted pairs. VDSL is the latest and the most advanced member in the family of DSL. The DSL transmission environment is traditionally thought of as a single user environment because each user is connected to the central office via a pair of dedicated wires (see Fig.1). However, a central office typically serves hundreds of thousands of homes, and twisted pairs from different homes are bundled together on the way to central office. In the bundled environment, because of the physical proximity, the twisted pairs emit electromagnetic interference into each other. Such interference is called crosstalk and is a major issue in DSL systems. DSL experience two types of crosstalk: near end crosstalk (NEXT) generated by transmitter on the same side of the receiver and far end crosstalk (FEXT) generated by transmitter on the opposite side of the receiver. For this reason, the DSL environment is more accurately modeled as a multiuser environment. The effect of crosstalk on the performance of DSL systems is more severe in a distributed topology where loops carrying the DSL signals significantly vary in length and thus give rise to “Near-Far Effect” (see Fig.2) as in case of code division multiple access (CDMA) wireless systems. In CDMA near far problem arises when the user close to the base station limits the performance of the user far from the base station without power control. In DSL if the transmit power spectral densities (PSDs) of all the users are the same in the upstream direction, the far end crosstalk (FEXT) from a short loop is larger than that from a long loop. The excessive crosstalk from the short loop thus reduces the data rate of the long loop, which is already smaller than the data rate of the short loop even in the absence of crosstalk because the loop attenuation increases as the loop length increases. Power control of DSL systems like in CDMA can be quite useful in mitigating crosstalk resulting from near far effect. But the power control in DSL differs in two important aspects from that in wireless systems. Fig.1. DSL Crosstalk Environment Fig.2. VDSL Upstream Transmission Environment First, although DSL transmission environment varies from line to line, it does not vary over time. Fading and mobility are not the issues. Consequently, the assumption of perfect channel knowledge is realistic and is made here. On the other hand, unlike the usual flat-fading assumption in wireless, the DSL lines are severely frequency selective. Thus the optimal power scheme needs to consider not only the total amount of power allocated to each user, but also the allocation of power over frequencies. Nevertheless, power control schemes designed for wireless systems [2] [3] can still provide considerable insight. This paper investigates the performance of two power control algorithms for VDSL upstream transmission. A distributed power control (DPC) algorithm iterative waterfilling (IWF) [6][7] and a centralized power control (CPC) algorithm based on multiuser discrete bit loading [8][9] have been considered. The organization of this paper is as follows. Section II introduces the system model and formulates the problem mathematically. Section III reviews DPC algorithm IWF. Section IV presents CPC algorithm and Section V compares the performance of both the algorithms. Finally, concluding remarks are given in Section VI. Central Office Long Line Short Line User 1 Users 2 . . . Central Office Upstream Downstream . . . . Binder . . FEXT NEXT Twisted Pair User 1 User 2