Hi Kylie,
This has also been proposed by Minn et. al in their paper "On timing
offset estimation for OFDM systems". They showed that this method also
improves the variance of the estimator.
The modification is quite simple:
R(d) = 0.5 * sum over full window |r(d+m)|^2.
I think, normally the correlation power should always be less or equal
to the signal power. But if you have very small powers near zero that
can't be precisely described with your current operands' bitwidth, the
calculated power value may become smaller which yields a ratio greater
than 1.0.
However, we have never experienced such problems with our OFDM system
except for offline debugging scenarios. Over the air, the presence of
noise will probably avoid this problem.
Dominik