Optical fiber have serious disadvantages, but it gives reliable and very high quality digital link, length depend on how much fiber drone could have onboard. -

Optical fiber have serious disadvantages, but it gives reliable and very high quality digital link, length depend on how much fiber drone could have onboard. - For quadrocopters practical up to 30 km; for terrain drones (wheel or caterpillar), could be 60 km or even 90 km.