Say I'm considering a long maturity fixed rate swap, for instance 20 years paid semi annually. Now I want to find the fixed rate for this hypothetical swap. I understand that this fixed rate is going to be predicated on discount rates based on the current term structure of the LIBOR yield curve (or whatever reference rate).
Because of the very long maturity of the swap, however, I'll need to have LIBOR discount rates for payments that will be made very far in the future. Now it may be the case that LIBOR rates are only quoted up to 1 year.
So I need the discount rate for the final payment at the end of the swap, in the distant future, to compute the swap fixed rate. However I don't have a LIBOR discount rate for that payment because it isn't quoted up to that maturity.
To compute the swap fixed rate, how do I find the appropriate discount factors beyond what's quoted in the LIBOR curve?