RFI Cancellation in VDSL

Very High-speed Digital Subscriber Line (VDSL) is a modern way of using the telephone network for high-speed digital communication. One of the problems with this technique is that the frequency range used for VDSL signalling is shared with radio amateurs and broadcast AM radio. A transmitter in the proximity of a wire will induce an electric signal in the wire this is called Radio Frequency Interference (RFI). This thesis describes how the RFI enters the VDSL system and investigates how the VDSL system is affected by it and how the effects can be reduced. In VDSL moderate levels of RFI can be handled in the digital domain by vari-ous cancellation techniques. However, if the RFI level is higher than what the VDSL receiver was designed for clipping can occur. It is components such as amplifiers and Analog to Digital Converters (ADC) in the VDSL receiver that causes the clipping. This thesis assumes that the ADC is the limiting component. There are two types of solutions to the RFI problem. The first is to remove the RFI before is comes to the ADC in the receiver. The second is to increase the dynamic range of the ADC, so that clipping is avoided. The following methods are investigated: an adaptive quadrature RFI canceller to remove the RFI before the ADC and two different dual range ADC methods to increase the dynamics of the ADC, simulations are used to compare the methods. The investigation shows that it is practically impossible to completely remove the RFI from the system before it enters the digital domain. This means that there is still a need for the RFI cancellation techniques in the digital domain. The best solution to the RFI problem is to use the adaptive quadrature RFI canceller together with a digital domain RFI canceller. The problem with this solution is that the implementation can introduce noise if it is not designed in a careful way. A more space efficient method that is almost as good is to use a dual range ADC in combination with the digital domain RFI canceller.Validerat; 20101217 (root