Also known as: SFN, Single-frequency network
A single-frequency network (SFN) has multiple transmitters broadcast the identical signal on the same frequency, tightly synchronised in time so their overlapping coverage reinforces rather than clashes.1 It is made possible by OFDM: the modulation’s guard interval absorbs the delayed copies arriving from neighbouring transmitters as constructive multipath instead of interference.
How it works
In an ordinary network, two transmitters on the same channel would jam each other wherever their coverage met. OFDM changes the arithmetic. It divides the signal across many narrow subcarriers and prepends each symbol with a guard interval — a short copy of the symbol’s tail. Any echo that arrives within that guard window, whether it is a reflection off terrain or a copy from a second transmitter, lands inside the same symbol period and adds coherently at the receiver. Provided every transmitter radiates bit-for-bit the same waveform at the same instant, a distant transmitter is indistinguishable from a natural echo, and the receiver treats the sum as constructive multipath.
Two conditions make this work. The transmitters must be frequency- and time-locked, which in practice means GPS-disciplined references so their clocks and symbol timing agree to a fraction of the guard interval. And the network’s inter-transmitter spacing must keep differential delays within that guard window — a longer guard interval permits wider spacing at the cost of some throughput. Get either wrong and the delayed copies fall outside the guard window and become inter-symbol interference instead of reinforcement.
In practice
SFNs save spectrum. A multi-frequency network (MFN) gives each transmitter its own channel to avoid mutual interference, consuming several channels to cover a region; an SFN covers the same region on a single channel, freeing the rest for other services. Digital broadcasting adopted the technique widely: DAB digital radio, DVB-T and DVB-T2 television, Japan’s ISDB-T, and ATSC 3.0 all run SFNs, and HD Radio uses on-channel boosters on the same principle. The SFN is the digital-broadcast cousin of analog land-mobile simulcast, which likewise keys several transmitters together on one frequency — but where analog simulcast must carefully manage overlap distortion, OFDM’s guard interval handles it by design.
Relevance to SDR
An SDR receiving a digital-broadcast multiplex in an SFN region is, without any special handling, demodulating the coherent sum of several transmitters — one reason OFDM broadcasts stay robust indoors and in cluttered terrain. Understanding SFNs explains why a strong digital signal can originate from more than one tower at once, and why signal-quality metrics behave differently from a single-transmitter link. SFN broadcasting sits outside GopherTrunk’s land-mobile trunking scope, but it is documented here as the OFDM network architecture behind the digital-radio and television signals an SDR user encounters alongside trunked voice.
Sources
-
Single-frequency network — Wikipedia, for the identical-signal same-frequency design, GPS synchronisation, and the SFN-versus-MFN spectrum trade. ↩