Research On Characteristics And Wavelet Multi-Carrier Modulation In Low Voltage Power Line Channels

Power line communication(PLC)is referred as the communication via indoor low voltage power line grids.It is a new broadband communication technology which consists of terminal,coupler,repeater,and power line network.PLC has merits of convenient construction,low investment,wide range,and high data rate.It can not only provide access network and internet network for the areas with no optical fiber and xDSL networks,but also construct local area network for family and/or a building.It can even apply to special organization to form a private communication network.Therefore,PLC recently becomes a hot research topic for its potential applications.The transmission characteristics of low voltage power line grids are different from other wired communication networks.The power line grid is originally designed for power transmission,rather than for the purpose of communication;hence it is necessary to investigate the communication characteristics of a power line channel.In this dissertation,the characteristics of power line channels are studied and the transmission characteristics of indoor low voltage power line grids are analyzed.The noise properties of several household appliances and indoor low voltage power line grids are simulated and measured.A wavelet multicarrier PLC system is developed for its reduced noise method by wavelet shrinkage technique which makes use of wavelet transform and takes advantage of the localization both in time and frequency.The main innovations and contributions of this dissertation are listed below.1.The transmission characteristics of low voltage power line channels are investigated using the bottom to top modeling method through the transmission line principle and chain scattering matrix theory.The transmission function is deduced and the transmission characteristic is analyzed for indoor low voltage power line grids.The transmission characteristics of bus topology,tree topology,star topology and loop topology are simulated and measured.The effects of branch structures,lengths,and specifications on transmission characteristics are studied.The influences of different topologies,branch structures and load on transmission characteristics are also investigated for low voltage power line grids.2.The noises produced in several household appliances and existed in power line grids can be categorized as the background noise and the impulse noise.The background noise and impulse noise are modeled using auto regressive model and improved Markov chain respectively.For the background noise,the relative errors between model simulation and realmeasurement are 2.68% and 2.25% by auto regressive model through Hamming window and Blackman window cutoff respectively.For the impulse noise,the relative errors of modeling evaluation and real measurement are 2.12% and 2.09% by improved Markov chain through Hamming window and Blackman window cutoff respectively.The noise values in two modeling methods are in accordance very well with noise measurement.3.A time-frequency filter designing method is put forward for background and impulse noise mitigation by a reliable noise detector in the wavelet domain with local variance analysis.The noise in the power line channels can be reduced by wavelet transform.The filtering is applied only to the detected noisy samples and it can reduce the noise level by proper operation in accordance with variance characteristics.4.A low complexity and high flexibility multi-carrier spread spectrum(MC-SS)system is proposed.The performances of the MC-SS system with carrier frequency errors over multipath frequency selective fading channel with time varying behavior and various sources of noise are investigated.The system with variable channel spacing and coherent detection at the receiver can effectively combat the multipath fading.It is demonstrated in experimental results that this method not only removes background and impulse noises,but also reduces bit error rate(BER)for better service quality in PLC.5.A multi-carrier spread spectrum system based on orthogonal wavelet aimed at characteristics of indoor low voltage power line channel is provided.The multi-carrier modulation is fulfilled by the way that different orthogonal wavelet basis are used to modulate signal in different sub-bands in power line channels.The provided modulation method can not only overcome the shortcomings of sensitivity for frequency shift and high peak to average power ratio(PAPR)in OFDM based on Fourier transformation,but also remove the noise in received signals and realize the multi-carrier,multi-user and multi-class communication.