\brief This function does Alamouti combining on RX and prepares LLR inputs by skipping pilots, PBCH and primary/secondary synchronization signals.
@param frame_parms Frame descriptor structure
@param rxdataF_comp Compensated channel output
@param dl_ch_mag First squared-magnitude of channel (16QAM and 64QAM) for LLR computation. Alamouti combining should be performed on this as well. Result is stored in first antenna position
@param dl_ch_magb Second squared-magnitude of channel (64QAM only) for LLR computation. Alamouti combining should be performed on this as well. Result is stored in first antenna position