WMAS Technology Aims To Fit More Into Less Spectrum

Post-FCC auction search for more-efficient use of remaining spectrum enters another phase

If you were to look for a definition of WMAS online, you’d encounter links to home pages for Tennessee Wildlife Areas, the Watershed Management Authorities in Iowa, or an Adult Contemporary FM station in Springfield, MA, before you found a reference to Wireless Multichannel Audio Systems. Even so, according to stakeholders in the RF universe, the technology could be a game-changer for managing wireless signals in a narrower, denser spectral environment.

WMAS technology, already an ETSI standard in Europe, allows dense multiplexing of audio channels onto a single wideband RF carrier, offers an easier and more flexible wireless workflow, and provides greater protection to other services sharing the same frequency band.

Specifically, WMAS would let wireless users operate in a wider bandwidth than Federal Communications Commission rules currently allow, enabling more wireless microphones to operate in that spectrum.

WMAS has been on the FCC’s radar for more than four years, with wireless-systems manufacturers recommending various ways to approach its regulation in the U.S. In April 2021, the agency issued a Notice of Proposed Rule Making (NPRM) asking for public comment.

Efficient Use of Shared Spectrum

The goal of the technology is to more effectively and efficiently utilize existing available RF spectrum without impacting the spectrum rights of other authorized users, such as broadcast licensees, white-space–device users, or current wireless-microphone users that share the frequency bands with wireless-microphone operations. Among other things, what WMAS proponents seek is authorization to use 6-MHz channels when available, such as a locally vacant UHF TV channel or in the 657-663–MHz segment of the 600-MHz duplex gap, with at least three audio channels per megahertz of spectrum to ensure that the spectrum is used efficiently. To accomplish this, the FCC would have to agree to revise its existing Part 74 and Part 15 wireless-microphone technical rules to reference relevant portions of the 2017 ETSI wireless-microphone standard. Hence, the NPRM.

The NPRM came about as a result of a petition for rulemaking filed in August 2018 by Sennheiser, requesting that the FCC “define a new class of wireless microphone called a Wireless Multichannel Audio System (WMAS) that digitally combines the signals of multiple low-power auxiliary station devices into one radio-frequency channel. Sennheiser requests that such systems be permitted to operate with a maximum channel bandwidth of 6 MHz, rather than 200 kHz as the rules currently allow, and that they be permitted to operate in the TV bands, the 600-MHz duplex gap, and in the 941.5-944–MHz, 944-952–MHz, and 1435-1525–MHz bands.”

If signals from multiple devices are digitally combined into a 6-MHz RF channel, according to Sennheiser’s proposal, intermodulation issues would be eliminated, denser use of the spectrum would be permitted, and power density across the channel would be reduced.

Sennheiser’s Joe Ciaudelli: “[WMAS] is only part of the equation in addressing the needs of content-creation professionals. [As with] any wireless device, access to suitable spectrum is required.”

It’s a big ask, though not a novel one: in Europe, WMAS is already permitted under applicable European Telecommunications Standards Institute (ETSI) standards.

“We applaud that the commission seems to be entertaining our Petition for Rulemaking to authorize WMAS,” says Joe Ciaudelli, director, spectrum and innovation, Sennheiser, “but it’s only part of the equation in addressing the needs of content-creation professionals. [As with] any wireless device, access to suitable spectrum is required. Sennheiser will continue to work with the commission on developing long-term solutions.”

The company is joined in endorsing WMAS by other wireless-systems manufacturers, notably Shure, which with processing developer Waves, jointly submitted an ex parte filing to the FCC that reaffirmed their backing of WMAS but also noted differences in their applications from those of Sennheiser.

“We worked with incumbents in the TV-UHF band, the primary spectrum band for wireless mics, to alleviate any concerns they may have, especially with the unlicensed use of WMAS,” explains Prakash Moorut, global head of spectrum and regulatory affairs, Shure. “The majority of our users operate wireless mics without a license, and we view FCC allowing unlicensed WMAS a ‘must,’ even if it means putting some additional constraints on unlicensed WMAS.”

Shure’s Prakash Moorut: “We view FCC allowing unlicensed WMAS as a ‘must.’”

To illustrate its points, in June 2022, Shure, with the support of NAB and Paramount Global, conducted a demonstration at New York’s Ed Sullivan Theater (which is owned by CBS, part of the Paramount brand) to illustrate the co-channel and adjacent-channel impact of WMAS systems on conventional analog and digital narrowband wireless-microphone systems. The outcome, according to Shure, demonstrated that operating a legacy narrowband wireless microphone in the adjacent channel of a WMAS signal was successful in a real-world scenario. However, “Shure also demonstrated by using a 2-MHz and 6-MHz WMAS signal that co-channel operation between WMAS and legacy narrowband wireless microphones was unsuccessful in the scenario tested.”

Sennheiser filed a response in which it described the NAB/Paramount concerns as “extremely unlikely, hypothetical scenarios”: “Sennheiser has demonstrated that WMAS actually decreases the required co-channel separation distance and likelihood of interference when compared to traditional narrowband microphones deployed in similar situations.”

Different, But Not So Much

The differences in approaches to WMAS implementation are significant but not necessarily mutually exclusive. For instance, according to Martin Brandenburg, project manager, WMAS, Sennheiser, the company’s system would operate with a total of 50 mW of transmit power per RF broadband channel — equivalent to the transmit power of a single conventional wireless microphone — regardless of the number of audio channels. Shure, on the other hand, has lobbied to have power scale higher with the wider bandwidth. For example, a conventional narrowband mic currently has a bandwidth of 200 kHz and is limited to 50 mW for unlicensed use; Shure has requested that a 2-MHz WMAS operate at 100 mW for unlicensed operators.

Sennheiser’s Martin Brandenburg: “Within the capacity limit of the system, there will no longer be any scaling of the total transmit power by the number of devices used.”

“Within the capacity limit of the system,” he explains, “there will no longer be any scaling of the total transmit power by the number of devices used. Furthermore, this transmission power will be distributed over the entire RF broadband channel, so the spectral power density is lower by a factor of 30 (6 MHz) or 40 (8 MHz), compared to a single wireless microphone with a bandwidth of 200 kHz. This low spectral-power density means that an RF channel can be reused more easily. This would, for example, be an asset in multi-studio broadcast-production facilities.”

In contrast, Shure contends that its scalable approach allows introduction of WMAS technologies for both licensed and unlicensed use, with lower maximum bandwidth and power for the latter, and ensures spectral efficiency by imposing an efficiency requirement of four audio channels per MHz.

“We see potential for WMAS solutions to be scalable in order to fit a variety of real-world use scenarios and allow compatibility with legacy systems,” says Nick Wood, senior director, wireless products, Shure.

Shure’s Nick Wood: “We see potential for WMAS solutions to be scalable to fit a variety of real-world use scenarios.”

“Shure and NAB/Paramount believe that this approach fairly balances the desire of wireless-microphone users to employ WMAS and broadcasters’ need for assurance that they will be able to continue operating narrowband systems,” states the company’s ex parte filing to the FCC. “Using this approach, Shure and NAB/Paramount submit that all wireless-microphone and UHF-spectrum stakeholders would be able to realize significant public-interest benefits.”

In particular, adds Wood, “broadcast sports is a multi-user and multichannel application for wireless and can benefit from the simplification of consolidating multiple wireless-audio links in a robust and spectrally efficient system.”

Despite the traction it has already achieved in pro-audio applications in Europe, WMAS has a hard row to hoe in the U.S., in terms of regulation and measurement. But what the technology offers to a drastically constrained RF landscape will likely make the effort worth it.