The teachings herein disclose methods and apparatus that simplify impairment correlation estimation for received signal processing, based on determining, for any given processing interval, which impairment contributors should be considered in the estimation of overall received signal impairment correlations. Each data structure indicates for each of multiple contexts, occurrences of symbols that have the same context and are in the corresponding one of the frames. The respective coil drive signals have the same shape, but may have a different amplitude and phase, controlled by a controller on the basis of characteristic information in a memory as well as user input information.