[{MS:"MS1",fileName:"file1",RSCP:75,EcNo:10},{MS:"MS2",fileName:"file1",RSCP:76,EcNo:11},{MS:"MS3",fileName:"file1",RSCP:77,EcNo:12,},{MS:"MS4",fileName:"file1",RSCP:78,EcNo:13,},{MS:"MS1",fileName:"file2",RSCP:782,EcNo:132,}]

db.collection.aggregate([{$match:{fileName:"file1"}},{$addFields:{EcNo:{$cond:[{$eq:["$MS","MS1"]},"$$REMOVE","$EcNo"]},RSCP:{$cond:[{$eq:["$MS","MS2"]},"$$REMOVE","$RSCP"]}}}])