If the signals are fixed, as you suggest, there will be interference until you are fairly close unless you have something like a tube around the sensor to limit where it gets input from. +/- 40 degrees is quite a range for a signal to be received from, especially with the ability of materials to bounce a signal back into the field. The big problem with IR as a whole is anyone in the stands with a 38khz remote can mess with any robots that are facing toward the stands. I do agree that if emitters are on the field they will be fixed, but that limits it to probably 4 sensors unless there are walls on the field to stop stray IR.