Fine timing recovery for QAM modem receiver

A QAM modem receiver performs fine-timing on a baseband signal sampled at two complex points per symbol. Based on this information, the algebraic signs of the second derivative of the baseband signal are estimated. These signs, computed separately for the real and imaginary axes, are used to a make local decision regarding convexity/concavity of the baseband signal. By accumulating these local decisions over an appropriate time interval, a global decision is reached regarding the sampling point position relative to the baseband signal maxima/minima. Based on this global decision, a correction command is issued to the modem's analog front end to either advance or delay the timing.