U.S. Patent No. 6,952,408, directed to frequency hopping with time division multiplexing.
The claim recites “changing” between RF channels, but the prior art indicated frequency hopping as optional through a reference to the GSM standard (page 15). Any differences are in unclaimed implementation (pages 15-17).
Hindsight: a creative argument based on claiming an event where the prior art shows that event as optional. Claim 1 appears to be overly broad, which may have allowed the lawsuit but hurt in the validity analysis.