In principle, each broadcaster has a different radio frequency (planned by the public authority) in a common reception area to avoid interference from each other.
However still there are two problems: spurious radiation of adjacent channels and fringe reception.
The exceptionally long-range reception means that the receiver may be tuned to more than one transmitter (transmitting at same frequency) at the same time.
In analogue transmission, even the transmitters transmitting the very same program interfere each other because of phase differences of the incoming signal, but in digital transmission the transmitters transmitting the same program in the same channel may reinforce each other.
In order to reduce the interference from the fringe area transmitters transmitting in the same channels, a method named frequency offset is often used.