Therefore, when a given symbol of interest is considered, {HEAD ~ the other symbols} of interest may be considered as {TAIL ~ interfering symbols} 
considered as
Therefore , when a given symbol of interest is considered , { HEAD ~ the other symbols } of interest may be considered as { TAIL ~ interfering symbols }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL