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Unlicensed radio spectrum has proven to be a tremendous success leading to an explosion of products and servicesJeffrey Krauss, president of telecommunications and technology policy of Rockville, MD that bring public interest benefits. Wi-Fi technology has decreased in cost and increased in deployment, thanks in part to standards developed by the IEEE. Bluetooth devices are widely used, and the ZigBee standard may provide a foundation for the Internet of Things. internet service providers (ISPs) deploy unlicensed point-to-point radios to bring Internet access to rural areas.

Licensed radio spectrum has also proven to be a tremendous success. Cell phones have evolved from clunky handsets with big batteries (bag phones) to small intelligent devices that provide mobile Internet access and ability to watch TV and movies. Standards developed by the 3rd Generation Partnership Project (3GPP) have fostered increasing capabilities, more efficient spectrum usage, lower battery drain and other benefits. Skipping the alphabet soup of previous generations of cell phone standards, we now have Long Term Evolution (LTE) products (sometimes called 3G and 4G) and we may soon see LTE-Advanced deployments (sometimes called 5G).

Until the last two or three years, the IEEE developed its standards, the 3GPP developed its standards, and there wasn’t much interaction. Certain spectrum bands were allocated for unlicensed use, other bands were allocated for licensed use, and they were separate.

Based on work underway at the 3GPP, that clean division is about to change.

LTE-Unlicensed (LTE-U) is a standard being developed by 3GPP to allow cell phone operators like Verizon and T-Mobile to use unlicensed spectrum around 5 GHz.

LTE-U allows cell phone operators to offload capacity from the licensed channels when they become congested and divert it to unlicensed spectrum. It uses a 3GPP capability called Carrier Aggregation, which allows a cell phone operator to use spectrum capacity at several different frequencies at the same time. These frequencies may be in contiguous bands of the spectrum or they may be in different bands.

3GPP is now using the phrase Licensed Assisted Access using LTE (LAA-LTE) rather than LTE-U to make it clear that this is intended for use by licensed cell phone operators. CableLabs gave a presentation to 3GPP in June last year, touting

LTE-U Standalone (http://www.3gpp.org/ftp/workshop/ 2014-06-13_LTE-U/Docs/RWS-140004.zip). LTE-U Standalone could be used by cable operators, who are aggressively deploying Carrier Grade Wi-Fi hotspots, to improve Wi-Fi spectral efficiency and quality of service. But this proposal didn’t gain much traction at 3GPP.

The main issue now being considered is co-existence between LAA-LTE and Wi-Fi. LTE-U proponents recognize that they must assure that Wi-Fi still has access to a fair share of the unlicensed spectrum. Companies like Qualcomm and Nokia have posted white papers online which say, in effect, “Hey, no problem.” But as CableLabs has found in several studies, there are ways to “game” the fairness requirement.

Wi-Fi at 2.5 GHz is a “polite” technology, designed to allow fair access to spectrum, even when there are many users in an area sharing the band. It uses a protocol that is generically known as Listen Before Talk (LBT). But even with LBT, collisions do occur, and Wi-Fi has a procedure to wait a specified time (a “backoff” period) before trying to resend. But if a collision occurs on the second try, the Wi-Fi device doubles the backoff period until the next try, and doubles it again on the third try. This is called exponential backoff.

In Europe, ETSI has adopted standards that include LBT requirements for unlicensed operation at 5 GHz (called Broadband Radio Access Networks or BRAN). Those requirements, however, allow the use of a fixed backoff period. And LTE-U proponents are proposing this method to 3GPP.

CableLabs has done simulations showing that the fixed backoff gives LTE-U unfair access to shared spectrum and leads to much lower throughput and greater latency for Wi-Fi users. See http://www.cablelabs.com/wi-fi-vs-eu-lbthouston-we have-a-problem/ And other co-existence plans proposed by LTE-U proponents are similarly unfair to Wi-Fi users.

In the United States, there are specific rules for transmitter behavior for sharing unlicensed frequency bands. They are included in the FCC Rules. They already require specific LBT behavior for 5 GHz devices because parts of the 5 GHz band are shared with military radars. It isn’t clear whether cell phones with LTE-U capability are already allowed in the 5 GHz unlicensed bands under existing FCC Rules, or whether (more likely) additional rules would be demanded by the WiFi industry to assure fairness.

But there is another consideration. LTE-U standards are planned to be included in 3GPP Release 13, which is due to be published in March 2016. However, Release 12 might be delayed three to six months, which might then delay Release13. It takes about a year for the first products to hit the market after a 3GPP Release is published.

The LTE-U proponents aren’t interested in using the 2.5 GHz Wi-Fi band because it is already too congested. For now, there are relatively few 5 GHz Wi-Fi devices deployed compared with 2.5 GHz. But if there is a concerted effort by laptop and tablet manufacturers, Wi-Fi access point manufacturers and cable MSOs, there could be enough 5 GHz unlicensed devices by 2017 or 2018 to also deter interest in 5 GHz LTE-U. A lot of spectrum policy is determined by who gets there first.

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