I'm trying to implement an OFDM system. I decided to use the lowest frequency carrier as a constant tone with a cyclic prefix so that I could lock the receiver to this by adjusting RX frequency to achieve zero real signal on this channel, IOW a single sine cycle with a cyclic prefix before it starts.
It locks and tests very well on the bench wired straight through, and when I connect up through a transmission line it locks well too. However, the cyclic prefix discontinuity has now moved along the carrier by about 90 degrees due to the frequency dependant phase shift of the transmission line.
Well, of course it would. Oops, should have thought of that.
But what to do? How can I lock so that the cyclic prefix discontinuity is always in the right place, ie just after the FFT sampling has ended?
I don't much care about the phases of the various carriers, as I can correct for that on TX or RX as needed, but the cyclic prefix discontinuity needs to be in the right place.
Any ideas? Preferably practical ones, I'm a bit resource limited. Is there some way of locking onto this discontinuity? It disappears into the mush as far as I can tell on the 'scope when I start using the data carriers.