Extending LTE Advanced to Unlicensed Spectrum
29. Mai 2014
Unlicensed spectrum is the set of airwaves that anyone, using any technology that complies with a minimal set of technical regulations, can utilize to develop and deploy a wireless communications technology. This light-touch policy framework supports the permission-less innovation that is crucial to the successful evolution of wireless, leading not only to the development of technologies such as Wi-Fi and Bluetooth, but also new devices, applications, products and services associated with them.
The extension of the benefits of 4G LTE Advanced technologies to unlicensed spectrum, a concept first announced by Qualcomm in 2013, is poised to drive another wave of innovation. LTE Advanced in unlicensed spectrum, modified to fairly share spectrum with all other unlicensed users of the band including Wi-Fi, can help double the capacity and coverage compared with Wi-Fi deployments today. It will also deliver seamless mobility, all while maintaining fair coexistence with neighbor Wi-Fi deployments. In essence, users are provided with a greatly enhanced mobile broadband experience.
Operators such as Verizon and T-Mobile have announced plans to deploy a variant of LTE in unlicensed spectrum called LTE-U next year. But organizations opposed to LTE-U are trying to stall these developments, arguing that LTE-U will not coexist fairly with Wi-Fi and even asking the U.S. Federal Communications Commission (FCC) to delay approvals for LTE-U equipment.
While debate is a staple of the wireless industry, the public discussion is reaching a point in which misinformation and some plainly wrong arguments threaten a slow-down in the pace of innovation we all need in wireless. To set the record straight, here is a list of the top myths that opponents of LTE-U are spreading.
Myth #1: LTE-U is not good for consumers; it’s only good for operators
Truth: LTE-U delivers ~2x capacity and coverage compared to carrier Wi-Fi solutions which will benefit both operators and end-users. For the same amount of data, LTE-U users will finish their use of unlicensed channel faster leaving more time for Wi-Fi users to utilize the unlicensed channel. And thanks to the aggregation with licensed spectrum, LTE-U provides the same seamless mobility experience as LTE today. The higher performance combined with seamless mobility means a better user experience for subscribers. Furthermore, tests show that LTE-U is a better neighbor to Wi-Fi than Wi-Fi itself, so users of adjacent Wi-Fi access points are better off when operators deploy LTE-U instead of carrier Wi-Fi.
Myth #2: LTE-U is a rude neighbor, lacks etiquette protocols to ensure fair coexistence with Wi-Fi
Truth: LTE-U is actually extremely polite, particularly to Wi-Fi. LTE-U will use only certain portions of 5 GHz unlicensed bands, leaving the entire 2.4 GHz bands and a substantial amount of 5 GHz spectrum for Wi-Fi and other unlicensed technologies.
LTE-U listens for clear channels before initiating transmissions. If no clear channel is available, LTE-U will listen for other LTE-U and Wi-Fi access points and limit its channel utilization to a fair fraction. If a total of 10 access points are sharing the same channel – a scenario that will only occur in a highly dense environment – LTE-U will restrict its transmissions to 1/10th of the time.
Furthermore, LTE-U limits continuous transmissions to no more than 50ms, allowing neighbor networks to deliver on latency sensitive applications such as voice over Wi-Fi. Finally, in low traffic conditions, LTE-U will release the 5 GHz unlicensed channel and operate exclusively in the licensed bands using traditional LTE operation.
Myth #3: Operators will manipulate coexistence mechanisms in LTE-U infrastructure to access more than fair share in unlicensed bands
Truth: Operators with plans for LTE-U deployments have already expressed their public endorsement for fair sharing practices. A letter filed with the FCC last August, signed by Verizon and T-Mobile among industry stakeholders, says: “To be clear, we are strong supporters of Wi-Fi. And we recognize and appreciate that new technologies must share unlicensed spectrum fairly. That concern was the very reason why the LTE-U Forum developed and released extensive etiquette protocols and coexistence testing requirements that ensure that LTE-U operates alongside Wi-Fi and other technologies as well as, if not better than, Wi-Fi does today”.
Myth #4: LTE-U disregards Wi-Fi standards on fair use of spectrum
Truth: Wi-Fi standards do not define what is fair use of unlicensed spectrum. In other words, currently, there is no baseline for Wi-Fi-to-Wi-Fi coexistence. Nonetheless, LTE-U always senses the other nearby Wi-Fi access points and will share spectrum fairly will these nearby users, as explained above and in the documents linked above.
The chart below shows some interesting insights from one of the LTE-U tests with several Wi-Fi access points operating concurrently in the same channel. Red lines show throughput for 4 access points from “Wi-Fi vendor A”, and blue lines show the same for “Wi-Fi vendor B”. Whether LTE-U is off or on (green line), “vendor B” access points always take a more aggressive share of the medium compared to those from “vendor A”.
LTE-U goes above and beyond any current regulations and specifications for coexistence, delivering improved performance without negative impact to the average Wi-Fi throughput (in magenta).
Myth #5: LTE-U is a proprietary, non-standard technology
Truth: The foundation of LTE-U is the 4G LTE standard, defined by 3GPP in its releases 10, 11 and 12. Detailed technical documents that define the LTE-U specification and coexistence are publicly available and any vendor can develop compliant equipment. See LTE-U Forum website. Also, new coexistence test cases were added to the LTE-U Forum specifications in response to technical comments from Wi-Fi stakeholders.
Myth #6: More testing is needed to ensure coexistence with Wi-Fi
Truth: LTE-U is one of the most tested technologies when it comes to coexistence with Wi-Fi. Results from extensive sets of tests performed by Qualcomm Research and the LTE-U Forum are documented in comments and subsequent replies filed with the FCC. Qualcomm has demonstrated coexistence in public events such as CES and MWC, as well as numerous other demonstrations for industry stakeholders, analysts and media at its San Diego campus.
Signals Research Group (SRG), led by well-regarded industry analyst Michael Thelander, just did extensive testing on its own with support from Verizon and Qualcomm. SRG’s work is based on numerous lab tests and analysis, and the results show again successful coexistence between LTE-U and Wi-Fi. SRG’s presentation of these results summarizes: “When LTE-U must share a channel with Wi-Fi, it makes fair use of the channel – the transmit time is proportional to the total number of radiating Wi-Fi sources that are using the channel”.
Myth #7: LTE-U does not comply with unlicensed spectrum regulations
Truth: Operators deploying LTE-U will comply with all applicable regulations for unlicensed spectrum. In particular in the U.S., LTE-U complies fully with FCC regulations.
Myth #8: More regulatory oversight is required to ensure coexistence
Truth: The current regulatory policy towards unlicensed technologies is working exactly as intended. There is no question that the combination of the unlicensed policy framework and the permission-less industry innovation it engenders is resounding success, producing a continuously evolving stream of new technologies, devices, apps, products, and services. This framework has fostered an environment in which Wi-Fi, Bluetooth, ZigBee, Z-Wave, Thread, DECT, LTE Unlicensed and many more technologies can be successfully deployed
Myth #9: There hasn’t been enough industry collaboration around LTE-U
Truth: Collaboration on coexistence between LTE-U and Wi-Fi has been ongoing for many months — including both one-on-one collaborations and activities within industry groups.
All founding members of the LTE-U Forum are also members of the Wi-Fi Alliance, and they all have a substantial stake in the continued success of Wi-Fi. The LTE-U coexistence specifications were published in March 2015, and were updated in June based on feedback received from the Wi-Fi industry stakeholders (e.g. adding uplink and VoIP test cases).
Qualcomm, on its own and with its collaborators in the industry, has worked closely with many other companies on bilateral and multilateral bases and through industry organizations, such as 3GPP, the LTE-U Forum, ATIS, IEEE 802 and the Wi-Fi Alliance on coexistence. There have been scores of presentations, demos, and meetings as detailed in our recent FCC filing. This work is ongoing and these collaborative sessions will continue.