Research On Crosstalk Cancellaion Technology In VDSL System

With the rapid development of Internet, Internet business is becoming important access business for many users. As a result, bandwidth demand is several times increasingly. The BN (Backbone Network) and CPE (Customer Premises Equipment) develop quickly, but the AN (access network) is stagnating and becomes the bottleneck of entire network. At present, the domestic broadband access is xDSL (Digital Subscriber Line System). VDSL (Very-high-bit-rate Digital Subscriber Line) access technology is the most pertinent among all DSL technologies for its high transmission rate.Crosstalk is the important factor to affect DSL system performance. In VDSL system, crosstalk is usually more than 10 to 20 dB to noise. There are two kinds of crosstalk, one is near end crosstalk (NEXT), the other is far-end crosstalk (FEXT). Because VDSL system takes use of FDM, so NEXT can be cancelled by filter. For FEXT can’t be divided with received signal by the same way, FEXT has been a important constrained factor and lead to worse transmitted performance. For this reason, measures must be taken to cancel the FEXT. In this paper, only FEXT is considerate.This paper is aimed to do research on the FEXT cancellation in downstream VDSL system. First, the VDSL system is reviewed in the second chapter, including the system model, twisted pair transmission model, different kinds of interference in VDSL system, as well as the conventional methods to reduce negative influence caused by crosstalk. And then, method based on precoding for crosstalk elimination is introduced in the third chapter. This method takes use of precoding technique in MIMO (Multiple-input Multiple-output) system to eliminate crosstalk. At last, Chapter 4 is about limited feedback techniques for crosstalk cancellation. In an actual system, CO (Central Office) can’t get the perfect channel state information to process transmitted signal. Users must feedback information to help the CO obtain precoding matrix. In the limited feedback system, how to reduce system overhead without performance decrease is the key point. The author has proposed a new method of crosstalk cancelation combining the codebook and channel quantization. The proposed method has nearly the same performance as the scheme which requires perfect channel state information (PCSI), but the amount of feedback decreases greatly. In the last part of this paper, conclusion has been made for this paper, as well as next step of the project.