Tuesday, February 24, 2015

LAA-LTE and the Threat to Wi-Fi

There is a big raging debate in the Wi-Fi industry right now about LAA-LTE.   This is a proposal by cellular carriers to use the 5 GHz unlicensed band to perform data offload, but using a LTE protocol and not Wi-Fi.   Of course the mobile carriers are going to proclaim how wonderful LAA-LTE is for their network operations and sugar-coat the whole Wi-Fi co-existence problem.

LTE, as a protocol, is designed to work on a licensed spectrum where you do not have to compete with, or even share, the resources with another network.  The "LAA" here stands for "licensed assisted access", which is a wonderful Orwellian term that sounds really good but actually means the complete opposite of what it sounds like.   In this context, "licensed assisted access" means that your traffic is traveling over unlicensed spectrum.

Whether licensed or not, wireless radio technology, being half duplex, is all about collision avoidance.  LTE uses time division multiplexing, generically known as TDMA.  TDMA, and all of its variations, are designed to work under the central premise that the network coordinates the usage of the spectrum, in order to prevent collisions from occurring between two or more radio transmissions on the same frequency.  Wi-Fi, by contrast, works by clients "contending" for the space and the right to transmit.  Most people don't understand that when you have a network with an AP and 20 client devices, the AP has to compete for the next transmission slot along with all of the clients, and while QoS modifies the contention rules, it is only a statistical advantage, not a guarantee. 

Wi-Fi and LTE are fundamentally conflicting and mutually exclusive architectures. Even if LAA-LTE allocated a large portion of the spectrum time to "external Wi-Fi", early studies (see http://www.cablelabs.com/wi-fi-vs-duty-cycled-lte/) have already shown that latency in Wi-Fi increases astronomically when an LAA-LTE network is present, even under light duty cycles. There are also no guarantees or requirements that LAA-LTE has to be configured to "play nicely" in the 5 GHz sandbox.

It is ironic that 2.4 GHz and 5 GHz used to be considered "garbage bands" that nobody in the cellular space wanted to get close to, until other companies figured out how to use the bands effectively (and make money from it). Now, these same carriers want to save money on licensed bands by seizing control over "free" unlicensed bands with LAA-LTE.

At the same time, the way the FCC rules are currently structured, the bands are unlicensed and thus there really isn't anything that the industry can do to stop it. If the 800 lb gorillas in the room want to bully their way into taking over the spectrum, they have the ability to make a real go of it.

One idea I've had on this subject is to take the proposed 5.9 GHz band that Congress wants to open up and dedicate those channels to applications like LAA-LTE, to keep it unlicensed yet, by mutual agreement, separated from Wi-Fi. Such an approach is unfortunately way too logical and reasonable to likely gain any traction.

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