A Milestone in Wi-Fi / LTE-U Coexistence
Today is an important milestone for unlicensed spectrum coexistence - the Wi-Fi Alliance (WFA) has released its plan for testing how well LTE-Unlicensed coexists with Wi-Fi.
This culminates many months of work by many expert engineers within the WFA and its membership, including CableLabs staff. The outcome is that we now have a definitive set of tests, based on real-world consumer data, against which to judge LTE-U – and we can move past the competing technical studies that were the hallmark of 2015.
The WFA and its staff are to be commended for bringing all sides to the table on this issue of such importance for broadband consumers everywhere. The test plan, developed in record-time, is a product of compromise by all sides, and LTE-U proponents participated robustly in the process. There are a number of tests that CableLabs supported as important that ultimately were not adopted. But the final product is nevertheless essential – both in validating coexistence performance of any LTE-U device proposed for deployment, and as a sign that diverse industry interests can work toward solutions as wireless access becomes ever more important for consumers.
CableLabs will continue to be engaged as the WFA moves to implement this plan with authorized test labs. We look forward to a transparent process with results reported publicly by the WFA. As we move to this implementation phase, it is worth describing what the test plan does, in order to understand why it is so important.
At a high level, the test plan does the following:
- Checks that LTE-U devices select the most lightly used channel, as LTE-U proponents say they will do;
- Ensures that new Wi-Fi networks can access the channel when LTE-U is active;
- Measures the impact to Wi-Fi throughput and latency from LTE-U; and,
- Ensures that LTE-U adapts its use of the spectrum in response to variation in consumer use of Wi-Fi, as occurs in the real world, in real time.
And it does all of this at signal levels that have been shown with real-world data to be reflective of consumer use of Wi-Fi hotspots. These tests are necessary due to the well-documented shortcomings in the LTE-U Forum coexistence specification, and the lack of standardized test procedures to date, which has yielded vastly different coexistence conclusions. For more information on our views of the test procedures, see Jennifer Andreoli-Fang’s contribution to the August workshop of the WFA, which is available here.
Reasonable compromises have been made by all sides in developing this test plan. It is time to move forward using the outcome of this process, in full, as the sole source of reliable determinations of LTE-U coexistence.
Coming Up for Air on LTE-U Coexistence: An Update
If you have been tracking CableLabs’ work to ensure that the introduction of LTE into unlicensed spectrum does not do disproportionate harm to Wi-Fi consumers, you may have noticed that it has been a while since we wrote in this blog on the topic. That’s not because we’ve moved on, however. To the contrary, we’ve been hard at work with industry stakeholders.
News of the Week: The Wi-Fi Alliance Workshop
This week, CableLabs contributed to the second Wi-Fi Alliance (WFA) LTE-U Coexistence Test Workshop, just as we did at the first WFA event. The proceedings served as an update on work within the WFA – of which CableLabs is a member, along with 600 or so other companies, including the proponents of LTE-U – to develop consensus-based technical procedures to validate the coexistence performance of LTE-U devices. This is occurring primarily through the development of a test plan, of which a draft was released this week for informational purposes.
As Edgar Figueroa, WFA CEO noted at the event, the coexistence test plan is not complete. Since the test plan is to be used in its entirety as a holistic suite of tests that LTE-U devices must pass, we must reserve final judgment until it is complete. But it looks promising at this stage.
Principles for LTE-U Coexistence Testing
As we outlined at the WFA workshop alongside Google, Broadcom, and Comcast, any satisfactory test plan must:
- Specify a rigorous and realistic set of test cases that validates LTE-U ability to coexist fairly with Wi-Fi;
- Not simply be a demonstration of equipment’s ability to satisfy the LTE-U Forum Coexistence and CSAT specifications, or vendor solutions and claims, which are insufficient to prove coexistence;
- Include clear, quantified pass/fail metrics to ensure that LTE-U does not disproportionately degrade Wi-Fi performance; and,
- Be conducted in an open manner, with results available to the public.
The current draft of the WFA test plan appears to be on a path to meet these criteria. Some important test cases have yet to be written – for example, verifying that Wi-Fi consumers will still be able to choose their preferred network even in the presence of LTE-U – and we are working with the WFA to see that this and other unfinished pieces proceed expeditiously to completion.
Once the test plan is complete and verified to be effective, LTE-U devices will start proceeding through the tests. If a device passes the full suite of tests, stakeholders should (ideally) be assured that Wi-Fi consumers will not be harmed by its deployment. If a device fails a part of the WFA tests, then it will be clear that improvement in its coexistence technology is needed before it is introduced to the marketplace.
An Evolution in Tone
The cautious optimism of the day is in contrast to the tone of the LTE-U debate in 2015. Then, the discourse was marked by competing technical studies and at-odds assertions. Now, a common, industry-led technical engagement may produce definitive results. A couple of takeaways come to mind:
First, given the substantial effort of the unlicensed community to develop test procedures within the WFA, it should be clear to all that the bare-bones test cases in the LTE-U Forum Coexistence Specification are vastly inadequate – as we have been saying for some time.
And second, given the progress to date, industry collaboration must be given time to play out. Though it is proceeding expeditiously, work within the WFA is not done, and can only serve to validate LTE-U coexistence as a whole work product.
That is an important point that was missed in press coverage of the FCC’s recent grant of an experimental license to Qualcomm. In a blog post, Julius Knapp of the FCC was careful to distinguish between those independent experiments and the collaborative coexistence process of the WFA. Any testing that is based on a draft, incomplete set of technical procedures cannot, by definition, yield meaningful information on LTE-U coexistence. The independent experiments that will be conducted by Qualcomm under this grant of temporary authority are much different than the collaborative industry process that remains underway at the WFA.
We certainly hope that the work in the WFA will lead to meaningful assurance that Wi-Fi consumers will not be harmed. For that to occur, the WFA test plan is likely to only be a start, even if it is proven to be rigorous in its final form. It is possible that evolution in LTE-U technology will be needed as well. That is how the development of License Assisted Access LTE (LAA-LTE) has occurred in 3GPP, the mobile standards body.
In 3GPP, a robust technical debate has led to improvements in coexistence features, with listen-before-talk procedures that should make the technology more friendly to its neighbors in shared, unlicensed spectrum. Work is just now beginning to simulate the efficacy of LAA coexistence properties. If the WFA coexistence testing model proves to be effective in protecting wireless consumers, it may also be adapted to LAA.
LAA is scheduled to be part of 3GPP’s Release 13 in March, and work within the WFA to develop the LTE-U coexistence test plan should be nearing completion around that time as well. Stay tuned… it looks like 2016 will see some major developments for wireless consumers.
Jennifer Andreoli-Fang and Bernie McKibben also contributed to this article.
Rob Alderfer is Vice President of Technology Policy at CableLabs.
Fair LTE-U Coexistence Far From Proven In CableLabs / Qualcomm Testing
CableLabs has been working hard to ensure that the introduction of LTE into unlicensed spectrum is a win for wireless broadband, and that it does not disrupt the Wi-Fi services that consumers have come to rely on. We have covered in prior posts why that is a challenge, since LTE-U can take advantage of Wi-Fi’s inherent politeness. Here we will review recent technical work we did with a major proponent of LTE-U, Qualcomm Technologies, and explain why that effort only reinforces our concerns.
In brief, we observed that current LTE-U prototype equipment is quite primitive — it is really just in mock-up state at this point — and incapable of demonstrating important coexistence features. The vendor-promised features, most of which are not required or even identified by the LTE-U Forum in its latest specifications, are not yet working to enable fair and reliable coexistence and confidence in testing. In addition, we found that claims of its ability to share fairly rest on a seemingly faulty understanding of how Wi-Fi shares spectrum. For CableLabs, this reinforces the need for a collaborative and open standards development process.
The Wi-Fi community has long sought the same collaborative standards development process for LTE-U that LAA has enjoyed (License Assisted Access LTE is the flavor of unlicensed LTE being developed in the mobile standards body, 3GPP). But in the absence of LTE-U standards development, CableLabs has engaged directly with the promoters of LTE-U in an attempt to do the fundamental research required to find coexistence solutions. Far from converging on solutions, however, our work to date on LTE-U has raised more questions than answers.
Lessons Learned with Qualcomm’s LTE-U
CableLabs recently concluded a brief technical engagement at Qualcomm’s campus in San Diego, which was preceded by a lengthy negotiation of what we would be allowed to test on site. Ultimately, the scope of the plan was much narrower than our guidance and focused on a limited set of basic coexistence tests. It certainly was not the fulsome research that we recommended and is required to address the concerns of Wi-Fi technologists, which we summarized in a presentation to the Wi-Fi Alliance last week.
We began this limited test hoping that we would be able to independently validate the definitive statements of LTE-U proponents that it has been ‘proven to coexist’, and is ‘more friendly to Wi-Fi than Wi-Fi is to itself’.
Unfortunately, our main conclusion from the three weeks we spent on site at Qualcomm is that there is no basis for definitive technical statements about LTE-U coexistence. The reason for this is surprisingly simple: LTE-U is in a prototype phase of development, and does not possess the features that its proponents have noted are important to coexistence.
For instance, Qualcomm has noted that their LTE-U solution will sense the spectrum for Wi-Fi activity and adjust its duty cycle ‘on’ time for rapidly changing congestion conditions. But that is not what we were shown. What we saw was an LTE-U prototype that must have its duty cycle manually programmed; it has no adaptation capabilities at all. There were other issues as well: For example, the equipment did not natively use the 5 GHz band that is targeted for LTE-U, and it only supported a single user device. We will refrain from going on at length here, but as is apparent in the photo below, LTE-U requires substantial further development. In short, we were surprised to see that the state of the art plainly won’t work in the real world, despite assurances to the contrary and claims of comprehensive testing.
Importance of a Common Research Framework
Since LTE-U equipment is not mature, it should come as no surprise that coexistence research leaves much to be desired as well. Statements that LTE-U is ‘more friendly to Wi-Fi than Wi-Fi is to itself’ necessarily rely on a baseline understanding of how Wi-Fi shares the spectrum with other Wi-Fi networks. But, in our three weeks at Qualcomm, engineers spent the majority of the time grappling with that crucial baseline information. The test setup at Qualcomm was uncontrolled and provided strangely imbalanced measurements. Afterwards, a CableLabs engineer replicated the setup at our Colorado facilities with the same make of Wi-Fi equipment, and within a half hour obtained balanced results, suggesting that problematic baseline measurements were somehow endemic to Qualcomm’s research environment. CableLabs and its members regularly test, configure, and operate Wi-Fi networks, and the behavior we observed on-site in San Diego seems quite out of the ordinary. Selected baseline measurements are shown in Figure 2 below, including the expected balanced baseline we observed in our Colorado lab.
This highlights the fundamental problem with the LTE-U coexistence research done to date: There is no common technical framework in which stakeholders are working, which makes it very difficult, if not impossible, to interpret research results across studies. This is apparent in our limited work with Qualcomm, and in Qualcomm’s prior studies, which also reflect baseline imbalances and call into question the research conclusions of LTE-U proponents.
Since we spent most of our time at Qualcomm working to diagnose apparent problems with the research environment, we did not come close to executing against the already modest test plan developed at the outset. We did however take some limited measurements of Wi-Fi behavior in the presence of LTE-U (which was tuned to 50% duty cycle ‘on’ time, so is only a narrow representation of possible real-world LTE-U configurations).
As seen in Figure 3 below, the impact of LTE-U depends on what your comparison point is: To conclude that LTE-U coexists better than Wi-Fi, one would need to lower the bar as much as possible — using an imbalanced baseline and referencing only the lower end in the analysis. This would clearly be a skewed approach, and even when doing so, it still doesn’t tell a conclusive story — reference the first case in the figure below, where the presence of LTE-U degrades Wi-Fi more than either baseline case. We would submit that the better approach is to diagnose the problems with the baseline measurements, rather than using unexplained results to justify definitive conclusions.
Furthermore, the in-home research detailed in a recent blog post by our principal architect, Jennifer Andreoli-Fang, made it clear that LTE-U is likely to have a disproportionately negative impact to Wi-Fi when the baseline is properly calibrated. An open standards process with common research methods is clearly needed to drive greater consistency and confidence in results.
We certainly hope that Qualcomm’s LTE-U solution will move from prototype to product in the near future, so that the Wi-Fi community can attempt to validate its coexistence efficacy in an open and comprehensive fashion. But that would only be one necessary step on the path toward equitable spectrum sharing. As we have detailed before, the LTE-U Forum coexistence specification leaves substantial room for different vendor and carrier approaches, which are likely to do disproportionate harm to Wi-Fi.
While this quite limited testing at Qualcomm’s facilities raised more questions than answers for us, CableLabs remains fully committed to rectifying the shortcomings in unlicensed LTE coexistence. Indeed, we have seen more hopeful progress in the development of the global standard form of the technology, LAA-LTE, which we are cautiously optimistic is on a path to coexist well. The more aggressive approach taken by LTE-U clearly poses significant challenges, but we see promise in the open standards process and the particular technical choices of LAA as the basis for a more effective and comprehensive solution.
Reliable coexistence in unlicensed spectrum requires a broadly supported agreement on specific solutions. That is why a collaborative, open standards development process is so important — that is how Wi-Fi is developed. Its success is self-evident in the marketplace.
Rob Alderfer is Vice President of Technology Policy at CableLabs.
Wi-Fi vs. Duty Cycled LTE-U: In-Home Testing Reveals Coexistence Challenges
Rob Alderfer, VP Technology Policy, CableLabs and Nadia Yoza-Mitsuishi, Wireless Architect Intern, CableLabs also contributed to this article.
In our last blog on Wi-Fi / LTE coexistence, we laid out the dangers attending the apparent decision of a few large carriers to go forward with the carrier scale deployment of a non-standard form of unlicensed LTE in shared spectrum. This time, we will review some of the testing conducted by CableLabs recently to explain why we are worried. We covered this material at a recent Wi-Fi Alliance workshop on LTE-U coexistence, along with the broader roadmap of research that we see as needed to get to solutions that are broadly supported.
Before we begin, let’s recap: there are different approaches to enabling LTE in unlicensed spectrum: LAA-LTE, and LTE-U. LAA is the version that the cellular industry standards body (3GPP) has been working on for the past year, and its coexistence measures appear to be on a path similar to what is used in Wi-Fi: “listen before talk,” or LBT. In contrast, LTE-U, the technology being developed for the US market, is taking a wholly different approach. LTE-U has not been submitted for consideration to a collaborative standards-setting body like 3GPP, instead it is being developed by a small group of companies through a closed process. LTE-U uses a carrier-controlled on/off switch that “duty cycles” the LTE signal. It turns on to transmit for some time determined by the wireless carrier, then it switches off (again, at the discretion of the carrier) to allow other technologies such as Wi-Fi the opportunity to access the channel.
CableLabs has tested duty-cycled LTE-U in our lab, our office environment, and most recently in a residential environment (test house) to research how technologies are experienced by consumers. Our research to date has raised significant concerns about the impact LTE-U will have on Wi-Fi services. Today we will review our most recent research, which was conducted in our test house.
How We Did Our In-Home Tests
Proponents of LTE-U claim that the technology will be ‘more friendly to Wi-Fi than Wi-Fi is to itself.’ So we decided to test that statement, using our test house to approximate a real-world environment, with technology that would comply with the LTE-U Forum Coexistence Specification v1.2.
(Note that just this week, the LTE-U Forum released a new version of their Coexistence Specification. We’re still looking at it along with the rest of the Wi-Fi community, since it is again a product of their closed process, but we don’t think it changes much for our purposes here. And judging by the discussion at this week’s Wi-Fi Alliance workshop, much work remains to get to reliable coexistence.)
To do our tests, we went to Best Buy and bought two identical off-the-shelf Wi-Fi APs, and we have a LTE signal generator that we can program to cycle the signal on and off – “duty cycling,” in the parlance of the LTE-U Forum.
We first established a baseline of how fair Wi-Fi is to itself. We had our off-the-shelf APs send full buffer downlink traffic (i.e., an AP to a Wi-Fi device), and took throughput, latency and packet loss measurements. We found that the two APs shared the spectrum about equally – throughput was roughly 50:50, for instance.
Next, we replaced one of the APs with LTE-U and repeated our tests, using various duty cycle configurations (LTE ‘on’ times). To make things simpler, we made sure that all of our measurements were done at the relatively strong signal level (-62 dBm) that is used by the LTE-U Forum as the level for detecting Wi-Fi.
What Our Research Found
If our Wi-Fi AP performed better in the presence of LTE-U, that would validate the claims made about it being a better neighbor.
But, we unfortunately did not find that. Quite the opposite, in fact. Our results showed that Wi-Fi performance suffered disproportionately in the presence of LTE-U. Wi-Fi throughput (“speed”) is degraded by 70% when LTE-U is on only 50% of the time, for instance. And more aggressive LTE-U configurations (longer ‘on’ times) would do even more damage, as seen in Figure 1.
Why does LTE-U do disproportionate damage to Wi-Fi? The primary reason is that it interrupts Wi-Fi mid-stream, instead of waiting its turn. This causes errors in Wi-Fi transmissions, ratcheting down its performance. We ran tests across a range of duty cycles to explore this effect. In our test case of two networks sharing with each other, to maintain Wi-Fi at 50% throughput, LTE-U could be on for no more than 35% of the time, as seen in Figure 2.
Does this mean that if LTE-U were to limit its duty cycle to 35% that Wi-Fi would perform acceptably? Unfortunately, it is not that simple. Our test case is admittedly limited: We are showing only two networks sharing the spectrum here, but in reality there can sometimes be hundreds of Wi-Fi APs within range of each other. If LTE-U took 35% of the airtime when sharing with 5 Wi-Fi networks, or 50 for that matter, the Wi-Fi experience would surely suffer significantly. And the problem gets worse if there are multiple LTE-U networks as well, which seems likely.
The effect that LTE-U would have across a range of real-world circumstances is, frankly, unknown. It has not been researched – by anyone. What we do know, based on our work here, is that the 50% LTE-U ‘on’ time considered by the LTE-U Forum when two networks are sharing (see Section 6.2.1 of their coexistence spec) does not yield proportionate throughput results.
That’s the effect of LTE-U on throughput, which is important. But equally important is latency, or the amount of delay on the network, which can have a dramatic impact on real-time applications like voice and video conferencing. We tested Wi-Fi latency across different duty cycles, and the results are seen in Figure 3 below. What’s important to note is that while the two Wi-Fi APs we tested can co-exist while providing smooth operation of real-time communications, that doesn’t appear to be the case if LTE-U is present. Even if LTE-U is only on 50% of the time, it degrades real-time Wi-Fi communications in a way that would likely be irritating to Wi-Fi users. And if LTE-U is on 70% of the time, then latency reaches levels where even latency-tolerant applications, like web page loading, are likely to become irritating.
So, we have seen in our research that LTE-U causes disproportionate harm to Wi-Fi, even when it is configured to share roughly 50:50 in time – which is not a given, as we have noted before. That’s because LTE-U interrupts Wi-Fi signals instead of waiting its turn through the listen-before-talk approach that Wi-Fi uses. We discussed the importance of using listen-before-talk in our last blog, and now you can see why we think it is important for LTE-U.
This research shows that LTE-U is not, in fact, a better neighbor to Wi-Fi than Wi-Fi is to itself, as its proponents claim. What should we make of these competing results? Is one claim right, and the other wrong? It’s not really that simple – the answer depends on how LTE-U is configured and deployed, and what coexistence features it actually adopts.
This highlights the need for the open and collaborative R&D that we have long been urging, so that we can find solutions that actually work for everyone. That has been happening with LAA, the 3GPP standard form of unlicensed LTE, where there is room for cautious optimism on its ability to coexist well with Wi-Fi. Hopefully LTE-U proponents will move toward actual collaboration.
Jennifer Andreoli-Fang is a Principal Architect in the Network Technologies group at CableLabs.
Rob Alderfer is VP Technology Policy and Nadia Yoza-Mitsuishi is Wireless Architect Intern, both at CableLabs, also contributed to this article.
Can LTE-U Forum Specification Provide Fair Coexistence?
At CableLabs, we have been deeply engaged in ensuring that Wi-Fi and other technologies continue to thrive as LTE mobile technology is newly brought in to unlicensed spectrum. We have participated in the international standards process at 3GPP, and we are glad to say that there has been some progress by that body to lay the groundwork for reliable coexistence with Wi-Fi.
But unfortunately, when it comes to development of LTE-U – a different form of unlicensed LTE being developed outside of international standards for the US market – we haven’t been in the “club” -- and a small club it is. Back in 2014, Verizon created the LTE-U Forum along with Alcatel-Lucent, Ericsson, LG Electronics, Qualcomm, and Samsung. The goal of this ad-hoc group is to develop a form of unlicensed LTE, using different, and lesser, coexistence features than are required to protect European and Japanese broadband consumers.
Let’s recap: We know that there are different approaches to enabling LTE in the unlicensed band. Licensed-Assisted Access (LAA) is the version that the cellular industry standards body (3GPP) has been working on for the past year, and is nearing completion. Its coexistence measures start with the most fundamental coexistence procedure, “listen before talk”, or LBT. LBT refers to the 20+ pages of very specific algorithmic channel access contained in the nearly 3,000 page IEEE standard for Wi-Fi, which ensures that diverse users of shared spectrum do not talk over one another, and that all have equal and fair chance of accessing the spectrum. Many companies with a stake in the ground are participating; work is currently going on at the 3GPP to attempt to achieve a consensus-driven, fair coexistence protocol. Though work remains in the final stretch to realize win-win outcomes, it appears that 3GPP is headed toward adopting Wi-Fi-like coexistence features, as the IEEE urged at their recent presentation to the group in Beijing. If realized, that would ensure compatibility between LAA and Wi-Fi – at least in Europe and Japan. We welcome the collaborative process within 3GPP, and would encourage active collaboration with outside stakeholders as well, since this is shared unlicensed spectrum, after all. In particular, 3GPP should work more closely with the IEEE in order to ensure the 3GPP LAA specifications incorporate fair coexistence when completed.
In contrast, LTE-U, the non-standard US version, is taking a wholly different approach – both in process, and in substance. LTE-U is being developed outside of the 3GPP standards process, even though the same companies in the LTE-U Forum are also active in 3GPP. And it is taking an entirely different approach to coexistence, using a carrier-controlled on/off switch known as “duty cycling” instead of reliably fair listen-before-talk. Here, LTE turns on to transmit for some time determined by the wireless carrier, then switches off for some period of time, again determined by the carrier. It is during this “off” period that other users such as Wi-Fi can have the chance to access the channel, illustrated in Figure 1.
Earlier this year, the LTE-U Forum published a set of technical reports and specifications, including a document known as the “coexistence specification” (SDL Coexistence Specification v1.2, henceforth referred to as the “coexistence spec”), which governs the behavior of LTE-U. One might expect that the coexistence spec would contain requirements consistent with the various features that the LTE-U Forum has talked about on numerous occasions, such as their July presentation to the IEEE, or their recent letter to the FCC. But unfortunately, it contains none of this detail.
This is confusing, since we hear so much from LTE-U Forum companies about specific features that they envision and tests that have been run against parameters that they desire. So, to make things clear, Figure 2 below was taken directly from the LTE-U Forum spec, and contains the entirety of the coexistence requirements for LTE-U.
LTE-U Forum coexistence requirements simply entail an ability to duty cycle when it senses other operators are present. There is no requirement to share fairly in time, to avoid interrupting Wi-Fi transmissions mid-stream, or to adapt to different levels of Wi-Fi usage and traffic. And note that, in the ‘off’ state, LTE-U may still send discovery signals that can also interfere with Wi-Fi.
The LTE-U Forum coexistence specification does contain several test cases that envision configurations that are more specific. They are limited, however, and fail to reflect the range of circumstances in which LTE-U will be deployed, such as crowded street corners. But more importantly, these test cases in no way dictate how LTE-U will actually be deployed in the market, since that is how loosely the coexistence requirements are written. To quote directly, “Network should manage the duty cycle of LTE-U activity to pass the coexistence tests…”. It should. But it apparently isn’t required.
To ensure fair coexistence, the test cases should verify the amount of Wi-Fi performance impact due to LTE-U is limited and fair, based on the traffic demands on the channel. A few test cases check for Wi-Fi performance to be ≥ 4Mbps. But let’s be realistic, the 4Mbps performance target rather lowballs what Wi-Fi can do today. The latest generation of Wi-Fi is designed for gigabit speeds.
There are no requirements for determining or limiting duty cycle parameter values, and no rationale or method indicated to select these duty cycles in the test cases. Instead, the specification leaves the configurability of the duty cycle parameters to the discretion of the network operator or vendor, and an LTE eNodeB (the base station used in LTE networks) can pass the coexistence evaluation if it can simply show that it can manage the duty cycle. Consequently, whoever gets to control the duty cycle effectively controls the use of the channel. Now how is that for fairness?
We Need “Listen-Before-Talk”
The premise of LTE-U à la LTE-U Forum is to duty cycle the LTE in unlicensed bands in order to coexist with Wi-Fi. The fundamental problem here is that LTE-U does not “listen before talk,” a most basic politeness protocol; instead, LTE-U “listens and talks anyway” regardless of whether somebody else is talking or not. Put another way, once the LTE-U node determines the duty cycle parameters, the coexistence spec allows the node to transmit uninterrupted for an undefined period of time regardless of other network traffic on the channel.
There seems to be consensus all over the world that LBT is a fundamental coexistence requirement. The cellular industry itself (3GPP) has explicitly rejected non-LBT based approaches to the LAA standard, and regulations in the EU and Japan require LBT.
CableLabs encourages the LTE-U Forum to follow the 3GPP’s lead. The open standards development process has been successful in vetting new innovations and producing broadly supported solutions. True collaboration with IEEE and others will ensure that the unlicensed bands can continue their successful framework of self-regulation. Shared spectrum requires shared responsibility. The Wi-Fi industry is working to ensure unlicensed LTE is developed in a manner that maximizes use of spectrum and ensures win-win outcomes for consumers. Our goal is to continue the progress of wireless broadband, in all its forms.
Bernie McKibben, Distinguished Technologist, and Rob Alderfer, VP Technology Policy also contributed to this article.
Jennifer Andreoli-Fang is a Principal Architect at CableLabs.
Can a Wi-Fi radio detect Duty Cycled LTE?
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.
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.
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.