The comment period for the Federal Communications Commission’s query on the use of LTE over unlicensed spectrum ends this Friday, but companies are heatedly dueling over the technology in the industry comments submitted thus far.
The debate falls along unsurprising fault lines: The companies that have worked on the development of LTE-U, particularly those involved in the LTE-U Forum formed by Verizon Communications, Qualcomm, Samsung, Alcatel-Lucent, Ericsson and LG – said that the technology is just fine to be used with Wi-Fi and as good a neighbor to Wi-Fi as Wi-Fi is to itself.
Other companies in the Wi-Fi space, from vendors to the cable industry, are more wary. While many make the point to say they welcome new players in the unlicensed space, they still have considerable concerns about coexistence. The National Cable and Telecommunications Association filed comments on behalf of the cable industry and its 10 million Wi-Fi hot spots saying that “[license-assisted access] and LTE-U will cause debilitating interference to other unlicensed services unless they incorporate effective sharing mechanisms.”
Keep in mind that LTE-U isn’t just an umbrella term for the use of LTE in unlicensed bands. LTE-U is also the name for the technical specifications that have been put forth by the LTE-U Forum – presented as a standard – for implementing LTE over unlicensed spectrum in markets such as the U.S., where listen-before-talk mechanisms are not required (such as in Europe). LTE-U is based on 3GPP’s LTE releases 10-12, but relies on proprietary mechanisms such as Qualcomm’s Carrier Sensing Adaptive Transmission to mitigate impact on Wi-Fi.
Meanwhile, License-Assisted Access is the form of LTE-U that is working its way through standardization at 3GPP. Designed for use in Europe and Japan, which have instituted requirements that LTE take a listen-before-talk approach to operating in unlicensed spectrum, LAA requires some changes to the LTE interface and is expected to be standardized early next year.
Both technologies are expected to meet the letter of the law on the FCC’s requirements for unlicensed use. The question is whether LTE in unlicensed spectrum could be a sufficiently bad neighbor that the FCC would have to step in, either preemptively or after the fact, to protect the Wi-Fi ecosystem.
Drawn from the comments put to the FCC so far, the primary issues are:
Testing results: The testing of LTE-A and LAA thus far has been extremely mixed, depending on who is doing the testing. Qualcomm’s tests are cited in many comments as evidence that Wi-Fi and LTE can coexist.
“Adding a neighboring LTE-Unlicensed node does not impact an existing Wi-Fi node any more than would adding another Wi-Fi node. In fact, Qualcomm’s demos showed that replacing a Wi-Fi node with LTE-U improves the average throughput for nearby Wi-Fi users,” Qualcomm said.
Meanwhile, Aruba Networks said that it has seen demonstrations of LTE-U that showed it could coexist nicely with Wi-Fi, but called the demos “somewhat friendly implementations of LTE-U and test conditions, and there are inadequate controls in the proposed LTE-U Forum standards to preclude individual device and operator implementations from differing significantly from … the demonstrations.”
Both Broadcom and Google submitted their own testing results that showed significant impact to Wi-Fi. Broadcom’s testing, outlined in its comment to the FCC, concluded that LTE-U can indeed be a good neighbor to Wi-Fi in terms of throughput, latency and the ability to sustain VoIP calls, but only if “robust” coexistence mechanisms are implemented that include the ability to detect weak Wi-Fi signals (-82 dBm), an initial wait time of at least 43 microseconds, and exponential back-off.
“Broadcom has considerable concerns about LTE-U’s coexistence with Wi-Fi and believes that it is unlikely to be a good neighbor because the coexistence algorithms are proprietary, because of variations in equipment, and ultimately because coexistence is controlled at time of operation,” the company said.
Regulation: The companies that submitted comments generally lauded the idea of a regulatory light touch and reminded the FCC of its track record of keeping its requirements technology-neutral, leaving the work of standards bodies to set technological requirements. Proponents of LTE-U/LAA don’t want further intervention by the FCC. Others want to see the FCC mandate or encourage cooperation, or at the very least, as Microsoft put it, play a role “as a convener, a facilitator and a regulatory backstop” if industry cooperation doesn’t end up being sufficient.
Coexistence: The interpretation of “fair sharing” of the spectrum, and the extent to which Wi-Fi is impacted, is in the eye of the beholder, some commenters said. Although LTE-U proponents cite channel selection and duty cycling among its mechanisms for ensuring that Wi-Fi continues to work well in its presence, others say that the performance gains from hogging resources will be too attractive for operators to resist simply out of the goodness of their hearts.
“Due to differences in the understanding of ‘fairness’ or ‘fair use,’ we believe there are significant risks in the near-term for the coexistence of LTE-U as defined by the proprietary LTE-U Forum standards,” said Aruba Networks. “There is no framework or agreed upon minimum definition of coexistence in place that have been agreed upon outside of the proprietary LTE-U forum.”
Standards development: Many comments noted that the companies supporting LTE-U have basically formed their own standards group outside of 3GPP, the Wi-Fi Alliance or the Institute of Electrical and Electronics Engineers. Although the wireless operators and participants in the group cite overlapping membership in at least one or more of the official standards groups as being sufficient evidence of cooperation and their own best interests, the very nature of the LTE-U Forum seems to make many companies suspicious of the specifications it is putting out.
Broadcom said that it “does not believe that LTE-U as envisioned by the LTE-U Forum meets the normal criteria of a standard. Participation in this group is restricted; detailed specifications are not provided; and sharing algorithms are proprietary.” AT&T reiterated its support for the use of LTE in unlicensed spectrum, but sniffed that LTE-U would not be a global standard like LAA.
Cablevision attacked the group more directly when it said that “licensed carriers have made the calculated decision not to develop the LTE-U standard through, or in cooperation with, the [IEEE], the typical body for the development of standards for unlicensed communications technologies. Instead, they have diverted the standards work, in the case of LAA, to the 3GPP, where the licensed carriers are dominant. Moreover, to the extent that 3GPP might require too much sharing or proceed too slowly, the carriers have created a new standards group, the LTE-U Forum, which they control entirely.”
In a comment from IEEE’s chairman of the 802 group that handles WAN/LAN standards, Paul Nikolich, said flatly that “there has been no coordination between IEEE 802 and any standards body associated with LTE-U, because LTE-U was not developed by a standards body. It is the understanding of IEEE 802 that LTE-U is a proprietary solution that implements a duty cycle approach to medium sharing that does not use appropriate sharing mechanisms to ensure coexistence with IEEE 802.11 family of standards.”
Nikolich categorized the conversations between 3GPP and IEEE 802 as “communications” rather than “coordination” and added that IEEE 802 believes that “3GPP should engage with the relevant stakeholders, such as IEEE 802, in a joint forum such as a workshop, or series of workshops, to facilitate understanding of the potential spectrum sharing issues for the IEEE 802.11 family of standards and LAA, and come to agreement on appropriate sharing characteristics to ensure fair coexistence.”
The deadline for comments is Friday. Even as the FCC examines the issues around LTE over unlicensed spectrum, technology development is continuing quickly. Alcatel-Lucent said it expects to see field trials of LTE-U small cells in the third quarter, with commercial deployment early next year.
