Implications of LTE in the 5 GHz Band

Back in December 2013, during a 3GPP Radio Access Network (RAN) plenary meeting, Ericsson and Qualcomm introduced LTE-Unlicensed, a scheme that puts LTE-Advanced signals in the 5 GHz unlicensed UNII band, in conjunction with an LTE “anchor” signal in a licensed band. The objective is supplemental downlink bandwidth (eventually both supplemental downlink and uplink bandwidth), to help meet ever-expanding mobile broadband data demands.

This proposal, and the follow-on License-Assisted Access (LAA) proposal targeted for 3GPP Release 13, has generated a great deal of discussion regarding coexistence and airtime fairness among 5 GHz LTE and 802.11n/ac signals. The FCC, as steward of both license and licensed spectrum in the United States, has been the locus of stakeholder comments, both affirmative and less sanguine. The stakes in this game are enormously high, especially for Wi-Fi services incumbents.

Wireless services providers have spent many tens of billions of dollars globally acquiring geographically exclusive spectrum licenses. With up to 500 MHz of unlicensed spectrum available between 5.17 and 5.85 GHz, the economic incentive to cellular operators for LTE-U and LAA is obvious. Qualcomm and other proponents have put forward technical methods of clear channel assessment to assure coexistence and airtime fairness based on carrier sensing and energy detection. Most countries in the world require a “Listen-Before-Talk” protocol to avoid co-channel interference, not only between LTE signals and Wi-Fi signals, but also among multiple LAA service operators.

To me, there are some interesting and perhaps more subtle open questions:

• Supplier market dynamics. It seems clear that LAA drives a tighter functional integration between the LTE transceiver and the Wi-Fi transceiver. The medium-term impact to Wi-Fi only semiconductor radio suppliers is potentially significant.
• Need for additional RF content in smartphones and small cell base stations. LAA is likely to be a boost for small cell base station suppliers, since maximum transmit power levels in unlicensed bands is much lower than licensed bands that can utilize macro cell base stations. Both LAA enabled small cells and smartphones will likely need additional 5 GHz TDD RF front-end content to meet new consumer connectivity use cases. LAA complicates the already staggering complexity introduced by 3GPP Release 10 LTE carrier aggregation, which allows up to five 20 MHz interband component carriers. Release 13 is anticipated to support up to thirty-two 20 MHz interband component carriers across licensed FDD and TDD bands, as well as the 5 GHz unlicensed TDD band. The RF front-end content required to support these new carrier aggregation combinations will no doubt have to grow substantially.
• Potential impact on the layer 2 (data link layer) architecture in next generation (“5G”) cellular and IEEE 802.11ax Wi-Fi. The admission control mechanisms defined in 3GPP LTE-A provide assured levels of Quality of Service not typically achieved in IEEE 802.11n/ac, and despite the enhancements of 802.11e, Wi-Fi is a best effort service in most consumer network implementations. This is one of the secondary technical motivations for LAA proponents. There is continued room for improvement in the MAC layer architectures of next generation wireless standards to complement expected PHY layer improvements, and it will be interesting to follow innovations in this area.

We will be publishing a report on these and other commercial and technical implications in the coming months.

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