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24 September 2015

CableLabs pipes up again to defend WiFi from “unfair” LTE-U

A blog at CableLabs from its principal architect, Jennifer Andreoli-Fang, makes it abundantly clear that US MNOs are planning to hijack WiFi spectrum, and effectively act as a block on WiFi performance.

The main accusations are that LTE-U is being developed outside of international standards for the US market, and no WiFi entity is being invited to join the process. Instead it is a club run by Verizon, Alcatel-Lucent, Ericsson, LG Electronics, Qualcomm, and Samsung – all dependent upon reducing the influence of WiFi, and promoting LTE.

But she says that the alternative LTE-LAA could end up going down the right track, not only incorporating “listen before talk” (LBT) but likely to be convinced to do it in a manner consistent with fair coexistence as seen by WiFi players.

Her blog asks what the agenda is behind a two pronged, unaligned attack on what has so far been WiFi dominated spectrum.

LTE-LAA (Licensed-Assisted Access) is being put together by the cellular industry standards body (3GPP) and is nearing completion. Coexistence measures are already embedded in 20 plus pages of very specific algorithmic channel access, in almost 3,000 page of the IEEE standard for WiFi. But even if this reaches an agreed standard this year, it will be another 9 months before it is built into chips and 18 months before it can be put into networks, and then it will take 5 or 6 years until it becomes ubiquitous within those networks.

The two approaches are in danger of making WiFi incompatible with LTE in the US, but compatible in Europe and Japan. As Faultline has said all along, the argument for LTE-U is being made on emotional grounds, not technical, and this is being treated like a lobbying process, a piece of PR, rather than a technical discussion by the cellular equipment and chip majors.

LTE-U takes an entirely different approach to coexistence from LTE-LAA, using a carrier-controlled on/off switch known as “duty cycling” instead of reliable listen-before-talk. 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 WiFi can have the chance to access the channel.

Andreoli-Fang also pointed out that 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. She expected the coexistence specification would contain requirements which the LTE-U Forum has talked about, such as its July presentation to the IEEE, or its recent letter to the FCC. But unfortunately, it contains none of this detail. So the attempt is to hoodwink the FCC into prematurely approving it, before it knows what it is.

In other words the eventually standard would detail a simple ability to duty cycle when it senses other operators are present, but there is no requirement to share time fairly or avoid interrupting WiFi transmissions mid-stream. In its “off state” LTE-U can still send discovery signals which will also interfere with WiFi. Andreoli-Fang says the design point is to ensure that WiFi can hit at least 4 Mbps, not the gigabit speeds that it reaches today, and there is no sign of a Listen Before Talk process being added, instead it Listens and Talks Anyway, talking over WiFi. Hardly polite.