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