Unlicensed Spectrum Explained

Guest author Lewis Brewster is Broadcom’s vice president and general manager, wireless connectivity.

Surrounded by the traffic-snarled streets of Manhattan, New York City’s Central Park remains a leafy refuge where people can stroll, picnic, bike, and otherwise enjoy themselves, as long as they play by the rules and do not interfere with the rights of others. Imagine what would happen, then, if the park’s paths and roadways were opened to car and truck traffic?

There would be concerns, obviously. And one might be forgiven for being cautious if taxi drivers and truckers insisted that they, not the local government, should be trusted with setting the rules for sharing the paths and roadways. 

See also: How Google Might Challenge Mobile Carriers—First, With Cheap Roaming 

This is very similar to the situation we now face in the wireless communications world, as mobile operators proceed with plans to deploy, as soon as next year, a new proprietary service known as LTE-Unlicensed (LTE-U). LTE-U is designed to relieve growing congestion on the mobile carriers’ private networks by re-routing data traffic into the spectrum equivalent of public parks—the free, unlicensed spectrum that was set aside by the government to encourage innovation.

The unanswered question is: Will the incursion of big mobile phone carriers into the public airwaves degrade other wireless services that already use the spectrum, services such as consumer Wi-Fi?

Running The Spectrum

Almost 2 billion people worldwide now carry smartphones. They use these wireless devices to connect to social media and news apps, to send text messages, to stream videos and games, and yes, even to talk.

To keep pace with consumers’ seemingly insatiable appetite for 24/7 connectivity, mobile operators are constantly looking to acquire more licensed frequencies as they become available, and to use the frequencies they already own more efficiently. They’re also rushing ahead with plans to use more of the unlicensed public spectrum, without waiting for the established regulatory organizations to reach consensus on how to share those airwaves fairly.

The public spectrum has been the catalyst for such technological innovations as consumer Wi-Fi, television remote controls, microwave ovens, baby monitors, and wearable technologies. Billions of low-power Wi-Fi and Bluetooth devices today rely on those limited swaths of unlicensed airwaves. To minimize interference with radar, satellites, and other primary users of the public spectrum, all of these Wi-Fi consumer devices have been designed with built-in “politeness” protocols.

One such protocol is known as “listen before talk.” Essentially, this means that a Wi-Fi device has to listen and wait for a given frequency band to be clear before it communicates. It’s much like approaching a group of people talking at a cocktail party—listen first, and then speak.

Wi-Fi was also designed to listen carefully for extremely low power transmissions, the equivalent of two people whispering. Even if Wi-Fi does not hear another device communicating, it still has a built-in pause, called an initial defer time, just in case the other speaker wasn’t finished talking and was just taking a long breath.

Another protocol, known as exponential back-off, determines how long to wait, to minimize the probability of interrupting. At a cocktail party this is done using subtle social queues. In signal transmission it is done through the creation of randomly sized gaps prior to the next transmission.

In addition to helping competing signals share the frequency more effectively, these politeness protocols are also openly published, so that developers of products that use the unlicensed spectrum can inspect the protocols for fairness.

LTE-U, on the other hand, wasn’t designed with politeness protocols.

Forgetting Politeness

LTE-U isn’t “rude”—just smart.

It looks for the cleanest or least-used band, selects that, and then begins transmitting data. Once the band is selected, LTE-U monitors Wi-Fi or other wireless traffic on that channel, and decides how to share the spectrum using proprietary “duty cycle” algorithms that can only be accessed by LTE-U operators. Also, LTE signals are typically transmitted for a longer period of time than the short, bursty traffic of Wi-Fi.

In a sense, Wi-Fi’s built-in politeness protocols put it at a disadvantage when its signals compete with LTE-U.

The FCC has opened a proceeding on this matter. Some cellular advocates believe that LTE-U will be friendlier to Wi-Fi than even Wi-Fi is to itself, while traditional unlicensed advocates believe that LTE-U could severely degrade Wi-Fi. As a technology driven company with business interests in both infrastructure/small cell deployments and Wi-Fi, Broadcom believes that standardized and verified politeness protocols are necessary for achieving coexistence.

Supporters of LTE-U say the duty cycle protocols built into LTE-U might actually lead to greater spectrum efficiency and better Wi-Fi performance. But if it doesn’t, and there’s too much interference on the public airwaves, the LTE-U operators can always fall back to their exclusive, licensed spectrum. That isn’t an option for consumer Wi-Fi users.

Still, the need for more capacity is undeniable. There is a limited amount of spectrum available for commercial communication, and data traffic is expected to grow a thousandfold by the end of this decade. The next major phase of the Internet, the Internet of Things (IoT), will connect tens of billions of devices and lead to exponential growth of data traffic.

We are at a critical juncture for unlicensed public spectrum, in which friendly co-existence between new technologies and existing Wi-Fi deployments is paramount. It is vital that all parties efficiently use this public resource, without impairing or inhibiting the rights and abilities of others. Without robust politeness protocols built into global standards, of the kind already used by Wi-Fi devices, our public airwaves will be no picnic. 

Lead photo by Ervins Strauhmanis 

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