Sharing the airwaves

Stage 2 of 7 · The signal3 of 6 in this stage5 min read

Spectrum is finite and everyone wants some. Every wireless system on a show is an answer to one question — how do several signals occupy the same band without destroying each other — and there are only a few answers. The band rules themselves are on the frequency map.

First

Simplex, half duplex, full duplex

Three words for who can talk, and when. Simplex is one direction only: a radio mic transmitter sends, the receiver listens, and there is no path back. Half duplex is both directions but one at a time, which is what a walkie-talkie is doing when you press the button and why you say "over". Full duplex is both directions simultaneously, like a phone call — or a DECT intercom beltpack where you can hear the stage manager while you are still talking.

This is not academic on a show. A simplex radio mic cannot be told anything by its receiver, which is exactly why older systems make you walk to the performer to change a frequency. A link with a return path can push a frequency change, read battery state, and mute the pack from front of house.

rule

If a system can tell a beltpack anything — battery, frequency, mute — it is not simplex. That return path is the feature you are paying for.

Then

FDMA, TDMA, CDMA, OFDMA

All four divide the same resource: a block of spectrum, over a period of time. Draw that as a grid — frequency up the side, time along the bottom — and the difference between them is just which way they slice it.

Change the method below and watch the four users move.

Access method

FDMA is what a conventional radio mic rig is. Every channel owns a frequency for the whole show, which is why coordination is a frequency-planning exercise and why intermodulation matters — the products land in the gaps between the carriers.
TDMA needs everyone to agree on time. Slots only work if every device shares a clock, which is why time-division systems are strict about synchronisation.
CDMA does not sound like anything to a receiver without the code. Several users transmit over the whole band at once and are separated mathematically rather than by tuning.
OFDMA is what modern wireless mostly is. Wi-Fi 6 and 5G both allocate blocks of time and frequency per user, which is why they degrade so much more gracefully in a crowded room than older systems.

Which brings us to

WMAS

Wireless Multichannel Audio Systems is the current change to how radio mics use spectrum. A conventional system gives every channel its own narrow carrier, with guard space either side that carries nothing — spectrum spent on separation rather than on audio.

WMAS instead puts up one wide carrier — up to 20 MHz under the ETSI standard — and shares it between many audio channels, using the time and frequency division ideas above rather than one-carrier-per-mic. ETSI expresses the capability as three or more audio channels per MHz.

It is standardised in ETSI EN 300 422-1, which defines the WMAS transmit mask and caps the bandwidth. The FCC aligned US rules with it in February 2024, following a rulemaking petition Sennheiser filed in 2018.

The practical consequences are bigger than the channel count. A wide shared carrier is naturally bidirectional, so the link can carry telemetry and control back to the pack; and because channels are allocated inside one block rather than hunted between broadcasters, coordination becomes an allocation problem rather than a search.

rule

WMAS trades many narrow carriers for one wide one, and gets back the spectrum that used to be spent on the gaps — plus a return path to the transmitter.

It is not a firmware update. WMAS is a different air interface; it needs hardware built for it at both ends.
The regulator still decides. Legal bandwidth and power are national. Check the frequency map for the country before assuming a wide carrier is permitted there.
Wide does not mean immune. A 20 MHz carrier sitting on top of a broadcaster is still sitting on top of a broadcaster.

Then plan it properly.

Frequency legality by country is on the RF map, and the third-order intermod checker shows which products from your own channel list land back on channels you are using.

Check yourself

1 question on the mechanism rather than the trivia. Nothing is scored and you can answer again — the explanation under a wrong answer is the point of it.

1Why can two radio mics on legal, licence-exempt frequencies still interfere with each other?

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You can now answer

Simplex or duplex?
FDMA vs TDMA vs OFDMA?
What is WMAS?

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Where to go next

These pages are a chain rather than a library — the next one along usually answers the question this one raised.

the practice Experience architecture What all of this is actually for. Wireless Which radio, and why LoRa or Wi-Fi or BLE? Video Why the picture is black Why is the screen black? Talkback Getting the crew to hear each other Partyline or matrix? Control DMX on the wire Is one fixture one unit load?

Everything on this page, in the index

Each of these has its own entry with sources, ports, gotchas and an open gap where a field is missing.

protocol DECT The radio behind most modern wireless intercom beltpacks. It sits in a band reserved for it in m… standard ETSI EN 300 422-1 Wireless microphones; Audio PMSE up to 3 GHz; Part 1: Class A Receivers