Realization Of A Low-Voltage Powerline OFDM Communication System And The Study Of Its Equalization And Synchronization

Recently, Powerline Communication (PLC) as one of the lowest cost manner for signal transmission, attracts increasing attention, and is developing with high speed. The wide coverage and the convenient access of powerline network grant itself a unique advantage as access network. But the noises, the attenuation and the multipath propagation inhering in the powerline make it very difficult to realize such kind of communication system. Considering the fact that the Orthogonal Frequency Division Multiplexing (OFDM) technique is an effectiveness modulation technology to overcome the difficulty caused by the powerline channel influences, it was introduced into the PLC research. The main objectives of this thesis are to develop this communication system and to carry out the relevant studies on the developed system. It contains the following aspects: to build a powerline OFDM communication system model, to develop an effective equalization method for signal transmission and to investigate OFDM symbol timing in the experimental environment. Both the research and the experiment results of this thesis show that to communicate with 220V powerline network is definitely feasible. The thesis first introduces the basic theory and the realization aspects of OFDM, and then demonstrates the feasibility and advantage of using OFDM in PLC. The thesis realized a prototype of a powerline OFDM communication system which can be practically used in a laboratory environment. The structure of the system and the functions of each part of the developed system are all given in detail. The whole structure of the system software and the specification for some key points are explained as well. With the developed system as a research platform, channel estimation and OFDM symbol timing are carried out. An adaptive equalization algorithm in the frequency domain for PLC OFDM system based on the LMS algorithm is presented. Its effectiveness over the fixed tap equalization method is verified by the comparison of the experiment results. As to the convergence property of the presented algorithm, a proper range of the step size in the algorithm is found by the experiments to compromise between the convergence speed and the stability. Experiments in synchronization indicate that noises existed in the powerline channel, especially the impulse noises, are the main obstacles to the OFDM symbol timing. Taking a real OFDM signal as an example, the performances of three kinds of timing methods --- double sliding window method, delay and correlation algorithm, and modified delay and correlation algorithm are compared. The proper threshold values for each method are found. It has been found that, of the three methods, the double sliding window method takes the least computation with the least stability in the PLC environment. The other two methods are far more stable. The modified delay and correlation method gives the best result in stability, but needs the greatest computation, which means that a higher computing speed for the receiver is required.