April 12, 2016
To even a casual observer, it’s obvious that the world’s appetite for wireless data is voracious and seemingly never satisfied. Globally we consumed 8 Exabytes per month in 2014. By 2020 the monthly average will spike to 71 Exabytes (figure 1), and there simply isn’t enough licensed spectrum to support this amount of growth.
Licensed spectrum is also very expensive – commercial wireless carriers in the U.S. spent a record-setting $49 billion on spectrum in the AWS-3 auctions, and it’s expected that competition (and pricing) for the upcoming 600 MHz auction will easily break that mark. Given these economics, it should come as no surprise that commercial carriers have been aggressively moving forward with plans to deploy a modified form of LTE into the unlicensed bands.
A Potential Solution: LTE-Unlicensed
Known generally as LTE-Unlicensed, the technology is an adaptation of standard LTE that’s been modified to help it co-exist with the standards we normally associate with unlicensed spectrum – notably Wi-Fi™. LTE-Unlicensed uses techniques such as transmitter duty-cycling (ceasing transmission for a period of time) and listen-before-talk (avoiding transmission when other RF signals are detected in the channel) to co-exist with IEEE 802.11ac systems and other incumbent technologies.
LTE-Unlicensed will be part of the upcoming 3GPP Release 13 standard, and will be deployed in the U-NII (Unlicensed National Information Infrastructure) bands (figure 2) that span most of the 5 GHz spectrum. Trials are already underway in the U.S., and once the standard is released wide deployment is expected over the next two years – primarily in dense urban centers and public venues, such as shopping centers and entertainment arenas and stadiums. For IT managers tasked with tracking down interference to existing wireless networks, identifying LTE-Unlicensed signals will require some creative testing strategies.
Spectrum Analysis Techniques
Because of its transmitter duty-cycling and listen-before-talk behavior, LTE-Unlicensed cannot be analyzed by existing LTE analyzers. Eventually, solutions for direct testing of LTE-Unlicensed will be available, but for now testing will need to be done using general purpose test equipment such as spectrum analyzers like the one shown in figure 3. Unfortunately, LTE-Unlicensed and 802.11ac Wi-Fi are both OFDM (Orthogonal Frequency Division Multiplexing) signals, which support high-order modulation such as QAM. Channel bandwidths for both LTE-Unlicensed and 802.11ac can vary from 20 MHz to 80 MHz.
When viewed on a spectrum analyzer, OFDM signals will produce a characteristic “Bart’s Head” trace, but without demodulation it’s impossible to determine if an observed OFDM signal is LTE-Unlicensed or 802.11ac. Combining a spectrum analyzer with a smartphone or tablet Wi-Fi sniffer app (figure 4) is a workaround for this – if you see a Bart’s Head on the spectrum analyzer and your Wi-Fi sniffer doesn’t show a Wi-Fi SSID for that channel, it’s probably an LTE-Unlicensed transmitter. Be aware that not all Wi-Fi sniffer apps will identify SSIDs set to non-broadcast, so be sure to get an app that supports non-broadcast detection.
Also, since both LTE-Unlicensed and 802.11ac transmissions are not 100% duty-cycle, you should choose a high-quality spectrum analyzer capable of tracking transient signals, such as the Spectrum Master™ MS2720T.
Anritsu offers a number of solutions and educational tools to help field engineers and technicians hunt and locate interference sources. To learn more, visit a specialized interference hunting web page.