How to make the error of the Gardner timing error detector not depend on signal amplitude?
The maximum absolute error term is roughly 2 times the input power for the Gardner TED. Having a fixed (or predictable) input power, through the use of an AGC, means you can scale the error term to adjust the speed at which corrections will be performed.
With M-QAM modulations (for M &gt; 4), the input power will vary depending on the symbol, and thus the maximum error term will also vary. This means that symbols with less energy will move the timing corrections more slowly than symbols with more energy. Given a proper initial synchronization sequence, this should not be an issue when maintaining the lock. Proper care should be taken to make sure the lowest energy symbol can still make the timing offset move if necessary.