Paulo Alexandre Crisóstomo Lopes,



The power line does not present constant characteristics, because they depend on the equipment type and number that is connected. Noise, line impedance and attenuation vary in time, having a relationship with the hour of the day. The noise spectrum is relevant up to 145 kHz, where it shows the higher observed values. Also the power line channel is characterized by deep spectral nulls in the channel transfer function (a very important problem in
PLC) and by time-dispersive fading. In order to overcome these difficulties adaptive OFDM modulation is being developed for PLC systems. Several systems like ADSL, WLAN and DVB already use OFDM modulation. OFDM stands for Orthogonal Frequency Division Multiplexing. It implements a multicarrier modulation technique were the bits to be transmitted are multiplexed throw several subbands or carriers. This allows that each carrier be treated separately in order to compensate the effects of the channel. It also allows the use off adaptive loading techniques that distribute the signal power and the number of bits in an uneven way throw the subcarriers according to the characteristics of the channel and in order to optimize the resulting bit rate and error probability. The multiple carriers are in fact implemented throw efficient IFFT and FFT operations. This requires synchronization at the receiver, but the adding of a circular prefix reduces the accuracy required, and the removal of inter symbolic interference.


Date: 2006-Feb-17     Time: 14:00:00     Room: 336

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