The receiver compares the difference between the phase sector of {HEAD ~ the present symbol} and the phase sector of {TAIL ~ a preceding symbol} 
The receiver compares the difference between the phase sector of { HEAD ~ the present symbol } and the phase sector of { TAIL ~ a preceding symbol }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL