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LTE-U in 5Ghz: An Introduction

Ericsson RBS 6402 Indoor Picocell

Ericsson RBS 6402 Indoor Picocell

Estimated reading time: 3 minutes, 46 seconds. Contains 756 words

I’m trying to wrap my brain around LAA-LTE. I understand what it’s trying to do, but not why. I mean that makes sense to me. Users are already choosing Wi-Fi over LTE when it's available. LAA will be used for downlink data only and most study's show that Wi-Fi use is significantly greater than mobile data usage anyways. Also, the carriers OWN their own spectrum, and unlike 5GHz, they control it, and have the ability to manage it. 

So, why encroach on unlicensed spectrum? They'll have to abide by the power constraints in 5GHz, so in-building, small-cell deployments seem to be the way to go. Does this mean lots of indoor carrier pico-cells right next to existing APs? 

I wanted to learn a bit more about LAA and this is what I found so far...

What is LAA? It stands for “Licensed Assisted Access”. Basically, the carriers want to use 5GHz to supplement their “primary” LTE cells with a secondary cell in unlicensed 5GHz. This cell (in the initial stages of LAA-LTE at least) will be for downlink data transmission only. The “License Assisted” part means that the 5GHz cell is linked to the “Licensed” cell via “Carrier Aggregation” (think channel-bonding.. sort of). But, all control functions reside in the licensed spectrum cell and can even disable the 5GHz secondary channel if necessary. At issue here is not the use of 5GHz, which is unlicensed and so by definition anyone can use, but HOW it will be used.

In countries other than United States, China and South Korea, there is a regulatory requirement to “Listen-Before-talk”, or LBT. Since we have no requirement for that in the U.S. carrier manufactures are looking for other ways to place nice with Wi-Fi. The “how” is still up for debate. But, one thing seems clear, they won’t be using CSMA/CA. Qualcomm has a white-paper about this specific issue. Nokia has one to, but you have to buy it. No thanks, I'll just stick with Qualcomm's. Some of their proposed “Coexistence Mechanisms” are:

Channel Selection - attempts to pick the cleanest channel based on Wi-Fi and LTE measurements. Basically, it uses energy detection to see if how used the channel is.

Pretty straight forward, it scans the medium to find the channel that has the least usage. So, in small, to moderately-sized WLAN it should’t be a problem. Especially with DFS enabled.

Carrier-Sensing Adaptive Transmission (CSAT) - "...the cell senses the medium for a duration (around 10s of msec to 200msec) and according to the observed medium activities, the algorithm gates off LTE transmission proportionally."

This one is interesting. If the medium is found to be pretty saturated, and a “clean” channel is not available the cell will fall back to CSAT. LAA-LTE uses an “on-off” duty-cycle pattern. When the medium is heavily used, CSAT will change the duty-cycle timing it will use. It will still use the 5GHz spectrum, but settle on the "least harmful" time-cycle. It also uses only primary channels to mitigate inference with QoS traffic. However, this duty-cycle is directly proportional to the throughput of wi-fi client. In other words, throughout is reduced.

CableLabs.com has a nice piece on this and actually shows the correlation of the LAA-LTE duty-cycle  on wireless clients. 

Opportunistic SDL - “Since the anchor carrier in license band is always available, the SDL carrier in unlicensed band can be used on an opportunistic base.”

The Supplemental Downlink (SDL) can be enabled, or disabled, on-demand as necessary. In theory, if the primary LTE cell in the licensed spectrum can handle the traffic, the 5GHz secondary cell is turned off. Then if unlicensed offload is required the secondary cell can be turned on. So, LAA-LTE may not always be present. If the carrier's primary channel can handle the load 5GHz won't be used. Question is, will the carriers actually do that?

Qualcomm's white paper claims "due to the coexistence safeguards. In fact, the Wi-Fi performance improves by about 10%, since the neighboring LTE-U Picos can finish transmission faster and incur less interference instead". I don't know about that, and 10% over what?

I'm skeptical about the affect LAA will have on WLANs, but I'm willing to learn and be proven wrong. Also, this is just one more thing we as WLAN professionals have to account for in our wireless designs. 802.11 is already a pretty inefficient protocol and adding more overhead to 5GHz doesn't seem like it will improve anything except the carriers sense of well-being.

Qualcomm White paper:
LTE in Unlicensed Spectrum: Harmonious Coexistence with Wi-Fi

CableLab.com article: 
Wi-Fi vs. Duty Cycled LTE: A Balancing Act

Cellular Exploitation on a Global Scale

Wow, this is probably one of the worst security exploits in a series of recent massive security exploits. Matt Solnik at Accuvant Labs broke the news on this OTA exploit. This was a few months ago, but I’ve only really recently heard of it.

It’s possible to exploit bad carrier management client software and remotely compromise most smartphones on the planet. Seriously, scary stuff.

Here is the pdf of his slides

Listen to the Risky Business podcast where he talks about the exploit
(The interview starts at 29:15)

Concerns on LTE-U and Wi-Fi Co-Existance

Estimated reading time: 4 minutes, 22 seconds. Contains 874 words


A few days ago Andrew Von Nagy tweeted an article by Patrick Moorhead on Forbes.com, "How LTE-U In Unlicensed Spectrum Helps Carriers Make Money". He was not positive on the article:

After I read the article I replied on Twitter about the apparently one-sided nature of the article and went on to make insinuations of being in the industry's pocket and such. Mr. Moorhead was not amused and replied with his assertion that we were just trolls.

Well, such is the life on the Interwebs. I felt bad and replied assuring him the intent wasn't to troll. We were just venting our frustration that there was no real mention of the concerns regarding LAA-LTE co-existing with Wi-Fi. And that it seems that all we see in the media about LAA-LTE is how it's going to solve all the carriers problems and will even improve everyone's wi-fi. So, after some back and forth - he DM'd me his e-mail and asked me send him my concerns and I took him up on it. I'm thankful the rather than dismissing me he was genuinely interested in my opinion.

So, here is the gist what I put together in the e-mail explaining why as a wireless guy I have serious concerns about the this:

Wi-Fi/802.11 is a "polite", but inefficient protocol. It's half-duplex, only one client can transmit at a time, has inherent overhead that will only be exacerbated by non-wifi interference, etc. Sometimes I'm amazed it works at all! 

Here are the main concerns I have with the introduction of LAA-LTE:

  • Does not play by 802.11 rules.
  • LBT (Listen Before Talk/Transmit) is not a given in the US.
  • Wi-Fi clients deal with interframe spaces, physical carrier sense (sensing for energy on the medium), Virtual Carrier Sense (NAV timer, based on the duration field in MAC header), and finally let's add a random back off timer to. Unlike 802.11 clients LAA-LTE radios don't know about the timers, random, or otherwise. This can lead to LAA causing delay and decrease in throughput. Carrier sense is not enough.
  • In wi-fi, voice and other latency sensitive frames are given weighted opportunity to access the medium. We already have to deal with contention from devices that follow the protocol. Now we add LAA-LTE and we have to deal with devices that don't, and will be deployed on a large scale.
  • LTE duty-cycles are an issue as well. CableLabs has done research showing the delay and reduction of throughput cause by varying duty cycles. When duty cycles are small wi-fi clients don't have enough time to contend for the medium. When the duty-cycle increases latency for wi-fi client follows.
  • We are already starting to see usage increase in 5GHz and it is difficult enough when 802.11 is contending with itself. Adding another technology that does not follow the rules will only make it more difficult for WLANs to operate effectively. 
  • Another issue I wonder about is how will devices choose between LAA-LTE and Wi-Fi? Right now most devices choose wi-fi over cellular for data traffic when available. When LAA and Wi-Fi are co-located how is the decision made?
  • Also, why now? More than half of all mobile data is over wi-fi. This will only increase. Where is the benefit for the carriers/users? Because of the power restrictions in 5GHz LAA-LTE will most likely only be deployed using small cells - just like Wi-Fi. So, the most likely places that LAA will be deployed will be areas already served (and probably over-served) by wi-fi. 
  • The control plane for LAA-LTE lies in the carriers licensed spectrum where they have full control of the medium. They will now have easy access to unlicensed spectrum for free and also their own licensed spectrum to fall back on. Wi-Fi users don't have that advantage and have nowhere to go. So, LAA starts off with an inherent advantage over wi-fi.
  • Healthcare already has a difficult time with wi-fi. Most healthcare WLAN designs do not support DFS channels (UNII-2, UNII-2e) due to the scanning and channel change requirements if RADAR detected. Because of this I'm guessing LAA will not be deployed in those channels. LAA could become even more disruptive in those environments due to the limited number of channels. This, and other dense enterprise deployments may be at risk.

The mobile carriers are looking to grab all the spectrum they can possibly use. There is nothing that will keep them from running over any other users. In the end it's all about making money for the carriers. The same ones that spend billions on licensed spectrum that is theirs alone to use as they please. Nothing wrong with making money, but not at the expense of a useful and expanding technology that is now - even in the nascent stages of 802.11ac - growing rapidly.

04-06-2015 UPDATE:

Tell me again how we can trust the carriers to make LAA-LTE work as a "good neighbor" with Wi-Fi?