Research On The Channel Estimations Of Power Line Communication System Based On OFDM

LPLC technology as a kind of high-speed broadband communication technology, cantake advantage of the distribution of a wide range of existing low-voltage power lines as atransmission medium, which has many advantages such as low cost, wide range, high speed,convenient connection and so on, becoming a hotspot in research of communication today.But because the power line is not designed as a dedicated communications medium, thepower line channel environment is very terrible，with serious noise disturbance, difficult tomatch the impedance and attenuation changing a lot. OFDM modulation technology has manyadvantages such as strong anti-interference ability, high spectrum efficiency and theadvantages of high speed transmission, so that it can be effective against the bad channelenvironment of low-voltage power line, as well as meet the needs of high-speedcommunication under the condition of limited bandwidth in low voltage power line, thusbecoming a popular choice for the LPLC technology modulation and demodulation. Usingcoherent OFDM system researchers must know the channel state information whendemodulating, and the channel state information is obtained by channel estimation. As thelow-voltage power line’s channel transmission characteristics change over time, so real timeto estimate the channel accurately will directly affect the entire communication systemperformance.The attenuation, impedance and the noise in Low-voltage power line channel has thecharacteristics of the complex and changeable. In order to effectively track the channelchange, the pilot frequency based channel estimation algorithm requires send pilot frequentlywhich cause taking up a lot of spectrum resources, and the decision feedback algorithm hashigh spectrum efficiency but can appearing error diffusion. Then the paper introduces an improved algorithm which based on the mismatch sequence of decision feedback, get thechannel initial estimate by mismatch sequences at the same time also according to the need inthe process of decision feedback inserting mismatch sequences. Using the goodautocorrelation properties of mismatch sequences to strengthen the anti-noise performanceand the estimation precision of estimation algorithm, also using the decision feedbackprinciple in the estimation process at the same time, not only improve the spectrum efficiencyof the system and restrain of the error diffusion problems in decision feedback algorithm, butalso can effectively monitor the changes of power line channel. Simulation results show thatthe improved algorithm has good estimation performance, in bit error rate of10-2orders ofmagnitude, the improved algorithm is better than decision feedback algorithm about8db.In order to solve the estimated delay problems because of high complexity of thesubspace channel estimation algorithm, based on subspace of decision feedback algorithm ispresented. The algorithm uses decision feedback algorithm at the beginning of the channelestimation, at the same time running the subspace decomposition algorithm. After a numberof channel state information is obtained by decision feedback estimated, the channel stateinformation obtained by subspace decomposition can be used. Not only avoiding the errordiffusion problems when using decision feedback algorithm, but also solving the time delayproblem due to the subspace decomposition algorithm is too complex. Simulation resultsshow that the improved algorithm has good estimation performance, in bit error rate of10-2orders of magnitude, the improved algorithm is better than decision feedback algorithm about11db.