Details
Simulcast solves a specific problem: covering a large area on one channel, so that users never have to think about which site they are near.
The problem
A municipality with three repeaters on three hills has three coverage areas. Without simulcast each uses a different frequency, so a driver crossing between them must change channel — and will, at some point, forget. A call goes unanswered because the receiving radio is listening to a site the caller is no longer using.
Simulcast removes the channel change. Every site transmits the same frequency at the same moment. To the radio, the entire region is one cell.
What makes it hard
Two transmitters radiating the same signal from different places is normally a recipe for interference. It works only when three conditions are met simultaneously:
1. Frequency synchronisation — the carriers must be within a few hertz of each other, which means a common high-stability reference, in practice GPS-disciplined. 2. Timing synchronisation — the modulation must leave every site within microseconds of the same instant, which requires equalising the delay through each backhaul path. 3. Deliberate overlap design — the region where two signals arrive at comparable strength has to be placed where it does least harm, and kept narrow.
Get all three right and the user experiences one seamless coverage area. Get the third wrong and there is a band of degraded reception exactly between two sites, which is worse than having no second site at all.
What it needs
- GPS-disciplined repeaters at every site
- Backhaul with controlled, measurable and stable latency — IP, fibre or microwave
- Detailed RF planning with terrain data, before any equipment is ordered
- Field measurement across the whole coverage area, particularly in overlap regions
- Periodic re-measurement, because buildings and vegetation change the picture
The field measurement is not optional. Simulcast is the one architecture where the design cannot be fully validated on paper, and where a plausible-looking plan can produce a genuinely unusable zone.
Where it earns its cost
Municipal field operations across a district, highway and motorway operations along a corridor, energy distribution networks, large industrial complexes with separated plants, and public-safety organisations that cannot rely on users switching channels correctly under stress.
Where the area is smaller or the user population tolerant of channel changes, a conventional multi-site or wide-area design costs less and does the job.
How TechnoRF approaches it
Coverage objective first, then terrain-based prediction, then site selection, then a synchronisation and backhaul design, then licensing, then installation and — always — a measured verification across the coverage area including every overlap zone. The report showing measured signal levels against the plan is part of the handover. Get in touch.
Frequently Asked Questions
What is the difference between simulcast and multi-site?
Where the switching happens. In a multi-site system each site uses its own frequency and the radio changes channel as the user moves, which is simpler but produces a moment of transition and requires roaming to be configured. In simulcast every site transmits the same frequency at the same instant, so the radio never switches — the coverage area behaves as one very large cell.
What happens where two sites overlap?
That overlap region is the entire engineering problem. Two identical signals arriving with a small time difference interfere, and where the difference approaches half a bit period the receiver cannot decode either. The fix is frequency and timing synchronisation to within microseconds, usually disciplined by GPS, plus deliberate design of where the overlap zones fall.
Is simulcast more expensive than multi-site?
In infrastructure, yes: synchronisation equipment, precise timing distribution, and considerably more RF design and field measurement. In operation it is often cheaper, because it uses fewer frequencies and removes an entire category of user error. If your users forget to change channel at the district boundary, simulcast removes that failure permanently.
Does simulcast work with DMR?
Yes. DMR simulcast is well established and widely deployed, and the timing requirement is met with GPS-disciplined repeaters and a synchronised backhaul. TETRA simulcast is likewise standard practice in large public-safety networks. The technology is not the constraint; the RF planning is.