Channel Spacing
Channel spacing is the difference between the centre frequencies of channels laid side by side within a band; in sub-GHz short range devices it is typically chosen between 25 kHz and a few hundred kHz. If the spacing is not sufficiently larger than the signal bandwidth, a transmission on the neighbouring channel leaks into your own receiver as noise.
A channel plan balances two things at once: how many independent channels fit into the band, and how far adjacent channel interference is suppressed. As the spacing narrows the channel count rises, but so does the chance of two neighbouring transmissions disturbing each other.
- Adjacent channel selectivity describes how far the receiver can suppress a strong transmission one channel away, and it is expressed in dB.
- The modulation choice has a direct effect: filtered schemes such as GFSK suppress the sidebands and therefore give cleaner neighbourhood at the same spacing.
- Frequency drift eats into the spacing: a crystal drifting with temperature can carry the signal on top of the neighbouring channel.
In the field this looks like the following: two sets operating in the same band should in theory not affect each other, but if one transmits very close to the other and very strongly, adjacent channel interference appears. The solution is not to change the channel plan but to separate the transmitters physically and to place the receiver antennas at separate positions.
Context where this term is used: Wireless remote control installation: pairing, relay, safety