Can a Wi-Fi radio detect Duty Cycled LTE?

Joey Padden
Distinguished Technologist, Wireless Technologies

Jun 24, 2015

For my third blog I thought I’d give you preview of a side project I’ve been working on. The original question was pretty simple: Can I use a Wi-Fi radio to identify the presence of LTE?

Before we go into what I’m finding, let’s recap: We know LTE is coming into the unlicensed spectrum in one flavor or another. We know it is (at least initially) going to be a tool that only mobile network operators with licensed spectrum can use, as both LAA and LTE-U will be “license assisted” – locked to licensed spectrum. We know there are various views about how well it will coexist with Wi-Fi networks. In my last two blog posts (found here and here) I shared my views on the topic, while some quick Googling will find you a different view supported by Qualcomm, Ericsson, and even the 3GPP RAN Chairman. Recently the coexistence controversy even piqued the interest of the FCC who opened a Public Notice on the topic that spawned a plethora of good bedtime reading.

One surefire way to settle the debate is to measure the effect of LTE on Wi-Fi, in real deployments, once they occur. However to do that, you must have a tool that can measure the impact on Wi-Fi. You’d also need a baseline, a first measurement of Wi-Fi only performance in the wild to use as a reference.

So let’s begin by considering this basic question: using only a Wi-Fi radio, what would you measure when looking for LTE? What Key Performance Indicators (KPIs) would you expect to show you that LTE was having an impact? After all, to a Wi-Fi radio LTE just looks like loud informationless noise, so you can’t just ask the radio “Do you see LTE?” and expect an answer. (Though that would be super handy.)

To answer these questions, I teamed up with the Wi-Fi performance and KPI experts at 7Signal to see if we could use their Eye product to detect, and better yet, quantify the impact of a co-channel LTE signal on Wi-Fi performance.

Our first tests were in the CableLabs anechoic chamber. This chamber is a quiet RF environment used for very controlled and precise wireless testing. The chamber afforded us a controlled environment to make sure we’d be able to see any “signal” or difference in the KPIs produced by the 7Signal system with and without LTE. After we confirmed that we could see the impact of LTE on a number of KPIs, we moved to a less controlled, but more real world environment.

Over the past week I’ve unleashed duty cycled LTE at 5GHz (a la LTE-U) on the CableLabs office Wi-Fi network. To ensure the user/traffic load and KPI sample noise was as real world as possible… I didn’t warn anybody. (Sorry guys! That slow/weird Wi-Fi this week was my fault!)

In the area tested, our office has about 20 cubes and a break area with the majority of users sharing the nearest AP. On average throughout the testing we saw ~25 clients associated to the AP.

We placed the LTE signal source ~3m from the AP. We chose two duty cycles, 50% of 80ms and 50% of 200ms, and always shared a channel with a single Wi-Fi access point within energy detection range. We also tested two power levels, -40 dBm and -65dBm at the AP so we could test with LTE power above and just below the Wi-Fi LBT energy detection threshold of -62dBm.

We will have more analysis and results later, but I just couldn’t help but share some preliminary findings. The impact to many KPIs is obvious and 7Signal does a great job of clearly displaying the data. Below are a couple of screen grabs from our 7Signal GUI.

The first two plots show the tried and true throughput and latency. These are the most obvious and likely KPIs to show the impact and sure enough the impact is clear.

Figure 1 - Wi-Fi Throughput Impact from Duty Cycle LTE

Figure 2 - Wi-Fi Latency Impact from Duty Cycle LTE

We were able to discern a clear LTE “signal” from many other KPIs. Some notable examples were channel noise floor and the rate of client retransmissions. Channel noise floor is pretty self-explanatory. Retransmissions occur when either the receiver was unable to successfully receive a frame or was blocked from sending the ACK frame back to the transmitter. The ACK frame, or acknowledgement frame, is used to tell the sender the receiver got the frame successfully. The retransmission plot shows the ratio of retransmitted frames over totaled captured frames in a 10 second sample.

As a side note: Our findings point to a real problem when the LTE power level at an AP is just below the Wi-Fi energy detection threshold. These findings are similar to those found in the recent Google authored white paper attached as Appendix A to their FCC filing.

Figure 3 - Channel Noise Floor Impact from Duty Cycle LTE

Figure 4 - Wi-Fi Client Retransmission Rate Impact of Duty Cycle LTE

Numerous other KPIs showed the impact but require more post processing of the data and/or explanation so I’ll save that for later.

In addition to the above plots, I have a couple of anecdotal results to share. First our Wi-Fi controller was continuously changing the channel on us making testing a bit tricky. I guess it didn’t like the LTE much. Also, we had a handful CableLabs employees figure out what was happening and say “Oh! That’s why my Wi-Fi has been acting up!” followed by various defamatory comments about me and my methods.

Hopefully all LTE flavors coming to the unlicensed bands will get to a point where coexistence is assured and such measures won’t be necessary. If not — and we don’t appear to be there yet — it is looking pretty clear that we can detect and measure the impact of LTE on Wi-Fi in the wild if need be. But again, with continued efforts by the Wi-Fi community to help develop fair sharing technologies for LTE to use, it won’t come to that.


Approaches to Increasing Wireless Spectrum

Rob Alderfer
VP, Technology Policy

May 13, 2014

At the 2014 Cable Show, one of the hottest topics was Wi-Fi. It doesn’t take long to realize how important access to this suddenly precious resource really is: Studies indicate that the number of wirelessly connected devices will triple by 2020, to a mind-blowing 30 billion devices.

[Related: Wireless Spectrum Infographic]

And with each new device that connects, less space is available in the wireless spectrum. The FCC recently voted to increase usable wireless spectrum in the 5GHz band, but this is a small fraction of what will be needed to support the growing wireless culture.

Spectrum 1

How do we get to a point where there is plenty of spectrum for everyone?  There are multiple approaches.

Reallocating Spectrum

Historically, the government has sought to meet new wireless needs by reallocating spectrum from one service to another. The idea has generally been that each service needs its own exclusive spectrum in order to avoid interference, and as technology changes the FCC needs to shuffle the spectrum portfolio.

For those that can remember all the way back to 2009, the digital television transition was an example of this – the FCC recaptured some spectrum used for TV broadcasts and repurposed it for wireless broadband.  Believe it or not, this process started about 15 years earlier – so reallocation takes time. Not exactly ideal in a world where technology seems to evolve daily.

The FCC already sees new opportunities for further reallocation of TV spectrum to broadband – but hopes to speed it up this time by creating a new two-sided market for spectrum. In essence, they will buy spectrum back from broadcasters in order to put it in the hands of our smartphones and tablets.

The FCC hopes a one-time-only auction will incentivize broadcasters to relinquish licensed spectrum in exchange for presumably very large sums. They get cash in their pocket for giving up space that was under-used, more wireless spectrum is available, and everybody’s happy – right?

Possibly. There are a lot of unknowns at this point. Some questions were answered at the May 15 meeting of the FCC, but not everyone can be happy all of the time. Hopefully things will move faster than the last 15-year process, even if keeping up with wireless growth is a near-impossible task.

Sharing Spectrum

Another approach to meeting wireless demand is to open up new spectrum by sharing it among many services, rather than shifting spectrum from one service to another. This is possible because wireless technology has come a long way. Wi-Fi is a great example – it is a fundamentally “polite” technology, since it listens to the airwaves before it transmits. It was built to share.

Without the need to shift services from one band of spectrum to another, spectrum sharing lends a greater ability to keep up with wireless growth. And since a lot of that growth is occurring over Wi-Fi, that’s a real opportunity. The FCC’s action in March to expand Wi-Fi access to part of the 5 gigahertz (GHz) band is a case in point. And there’s more that can be done in 5 GHz, and in other bands like 3.5 GHz.

But the value of spectrum sharing depends on the specifics. We still need to protect against interference, but we also need to be sure that we’re not overly protective and discourage productive use of the spectrum. We don’t want Wi-Fi jamming military naval radars, for example, but neither do we want Wi-Fi to be confined to Kansas. (No offense to Kansas.)

It’s a balance. And CableLabs is doing the R&D to ensure that the right balance is struck.

Increasing Wi-Fi Spectrum: A Roadmap

Ultimately, it will take a combination of approaches to achieve the national goal of wireless spectrum growth.

slides for James2

When CableLabs helped the FCC with research to increase the 5GHz band, there was a concerted effort around interference issues. This collaborative research will serve as the technology roadmap for interference issues surrounding both reallocation and sharing of Wi-Fi spectrum. It’s critical that spectrum policy moves forward quickly and effectively to benefit everyone who uses it – and that’s all of us.

Rob Alderfer is a Principal Strategic Analyst for CableLabs, where he guides technology policy and strategy across the industry.