Why LTE-U Is The Next Great Interference Threat To Wi-Fi
Radio-frequency interference is the carbon monoxide of the wireless world – invisible, tasteless, odorless, arriving without warning, and with the ability to really ruin your day – unless you know the symptoms and have the right tools at your disposal. And that’s just what we have for you in this new series of articles from noted wireless expert Craig Mathias of Farpoint Group.
As wireless LANs have matured over the past fifteen years, we’ve gone from speed being the number one concern for IT administrators to approaching wired level speed and stability. In this series, we are looking at the next great challenge for the wireless LAN industry: interference. Before continuning with this post, be sure to catch up with what interference is and how you can detect it.
It’s important to understand up front that wireless LANs based on Wi-Fi have a built-in mechanism that’s designed to deal in part with the challenges of interference. This is known as spread-spectrum radio, and is specified in FCC and other international rules for using the unlicensed bands.
Basically, spread-spectrum, as the name implies, spreads a given transmission across a broad range of frequencies, creating a robust and somewhat redundant signal that, to an improved degree, resists interference. Spread-spectrum is a wideband (analogous to broadband) technique.
Potential narrowband interferers might damage a part of this wideband transmission, but signal-processing techniques often enable any damage to be corrected without requiring a retransmission. And since transmit power is also spread as well, spread-spectrum signals are “good citizens” with respect to the use of radio waves. Two wideband signals can usually even overlap in time and space without interfering with once another.
Wi-Fi also includes coexistence mechanisms that make it an even better citizen. The most important of these is called clear channel assessment (CCA), or “listen before talk”, which involves listening for traffic before a transmission is attempted. Wi-Fi systems wait until the channel is clear before jumping in – but, as we’ll see, other systems aren’t so kind.
Still, there’s no magic here – lots of independent Wi-Fi systems all competing for time on the same set of channels (think: channels 1, 6, and 11 in the 2.4 GHz. band) can really slow one another down, as can rogue APs and even high-powered Wi-Fi systems that, while still operating within the rules, can easily overwhelm many Wi-Fi networks. Incompatible (non-Wi-Fi) products like cordless phones, wireless cameras, IoT devices, and even some lighting systems and many more completely-legal devices can have a similar effect.
Bluetooth is a common potential source of interference, given its popularity in wireless keyboards, mice, and headsets. Bluetooth also uses spread-spectrum radio, but of a different form – something called frequency-hopping. In this case a narrowband signal literally hops from one channel to another, avoiding interference in many cases. And, since it’s narrowband and usually involves short, low-power, and infrequent transmissions, Bluetooth usually isn’t a big problem for Wi-Fi.
Perhaps the biggest threat now on the horizon for Wi-Fi is a form of the cellular technology LTE (often called “4G”) that actually operates on the unlicensed bands. It’s called, for that reason, LTE-U, but is more formally known as LAA-LTE, and is designed to be deployed indoors just like Wi-Fi. But LTE-U isn’t Wi-Fi; it’s in fact incompatible to the point the LTE-U can essentially lock out Wi-Fi entirely. It does this by using a shorter listen-before-talk period than Wi-Fi specifies, so LTE-U will always grab free airwaves ahead of Wi-Fi. This isn’t good.
Fortunately, the LTE and Wi-Fi communities are talking about the problem, and LTE-U isn’t widely deployed – at least not yet.
Once again the issue here is really the free-for-all nature of the unlicensed bands. So it’s fair to ask: Given the increasing possibility (likelihood?) of RF interference, can we really do anything beyond the detection of potentially interfering signals to maximize the effectiveness of Wi-Fi today – and, more importantly, going forward? Why, yes we can. More on that next week.
All Posts In This Series: