A symbol detector converts {HEAD ~ initial symbol estimates} of received symbols to {TAIL ~ soft estimates} for decoding 
A symbol detector converts { HEAD ~ initial symbol estimates } of received symbols to { TAIL ~ soft estimates } for decoding
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH