You avoid interference in the same way you avoid it for radio wave communication: multiplexing. It could be time-based (time slots for each sender), frequency-b

You avoid interference in the same way you avoid it for radio wave communication: multiplexing. It could be time-based (time slots for each sender), frequency-based (different colors) or code-based (look up CDMA). Plus, you already have 'spatial multiplexing' because light doesn't pass through walls as opposed to RF.