  In this paper, theoretical limits on time delay estimation are studied for
ultra-wideband (UWB) cognitive radio systems. For a generic UWB spectrum with
dispersed bands, the Cramer-Rao lower bound (CRLB) is derived for unknown
channel coefficients and carrier-frequency offsets (CFOs). Then, the effects of
unknown channel coefficients and CFOs are investigated for linearly and
non-linearly modulated training signals by obtaining specific CRLB expressions.
It is shown that for linear modulations with a constant envelope, the effects
of the unknown parameters can be mitigated. Finally, numerical results, which
support the theoretical analysis, are presented.
