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