Homeplug uses the same OFDM modulation as 802.11 wireless. The Intellon chips contain dedicated hardware for doing real-time FFTs of the powerline signal. The actually circuitry is not that complicated. Here's a picture of an older 14Mb Homeplug device. The circuitry is very similar for 85Mb. The long back thing at the bottom of the photo has been replaced with an analog chip from Intellon.