Techniques of downstream rate balancing for vectored digital subscriber line systems: case study of the ITU G.9700 standard

Abstract

As the DSL (Digital Subscriber Line) technology reaches its physical bandwidth limit and the Fiber-to-the-Home (FTTH) model becomes a potential substitute, new techniques and standards are developed to cope with the requirements from the Internet access industry. In this context, the newly agreed G.9700 (G.fast) standard, from the International Telecommunication Union (ITU), brings to the copper plant a renewal of its lifespan with the use of techniques such as Vectoring. Although Vectoring is a powerful tool for DSL to become a competitive technology for optical fiber, it must deal with the problem that subscribers still may get differing rates at their CPEs (Customer Premises Equipment), caused mainly by physical aspects of the channel. With that in mind, this work proposes methods of balancing the achievable rates delivered to user lines at Downstream. This is done by using different column sortings of the channel matrix, in scenarios that utilize the Tomlinson-Harashima Precoder (THP) up to 200 MHz. Simulations using each rate balancing method on six measured cables are made in order to show each method’s progress. These simulations are extended to the Near-Far Scenario, which tries to resemble the situations where some lines have greater length than others, that is, differing distances between the CPE and the DSLAM (Digital Subscriber Line Access Multiplexer). The proposed methods yielded promising balancing result, with reductions of up to 95.79% of the standard deviation for the Swisscom I51 cable, when comparing results of a genetic algorithm method with those of the THP.