Broadcom engineers explain why they’re still concerned about LTE-U

Broadcom HQ
Image: Coolcaesar/CC BY-SA 3.0

While the Wi-Fi Alliance is marching forward with an LTE-U coexistence test plan due to be released Sept. 21, not everyone is convinced it’s the best plan.

Qualcomm made that clear last week, saying the draft plan lacks technical merit, is fundamentally biased against LTE-U and rejects all the input that Qualcomm provided over the past year. WiFiForward said neither side received its preferred outcome, and the Evolve coalition, which represents mobile carriers and associations like CTIA, said the Wi-Fi Alliance buckled under political pressure from the cable lobby.

Others said the compromises on the test plan could leave half of all outdoor Wi-Fi connections vulnerable to LTE-U degradation.

As engineers at Broadcom described it, the compromises that led to progress on the LTE-U coexistence test plan were substantial, including some that they’re still concerned about. But, cognizant that operators like T-Mobile and Verizon are eager to deploy the technology, they said they’re determined to press ahead and continue conversations with the cellular community.

“Broadcom is really focused on seeing this executed,” Christopher Szymanski, director of product marketing and government affairs at Broadcom, told FierceWirelessTech. Szymanski was among those who attended one of the Wi-Fi Alliance’s regularly scheduled LTE-U coexistence workshops in San Jose last week.

Noting how some operators are upset that the plan is taking too long to establish, he added that he and his colleagues believe it’s going faster than any other program the Wi-Fi Alliance has undertaken. The fact is, he said, there’s been only one slip in the timeline – the original target was to have the plan finalized in August, and that slipped into September.

“We are not necessarily a cheerleader” for the current test plan, Szymanski said. “We think the current draft is sufficient to demonstrate that LTE-U will significantly harm the user experience in many Wi-Fi scenarios,” and it’s going to meet certain objectives, but the company is also concerned that critical parameters related to proving coexistence were removed in order to reach some form of consensus. These parameters would have revealed the true extent of LTE-U’s harm to community and consumer Wi-Fi access, he asserted.

One of the concerns Broadcom brought up during the process had to do with signal levels. While the IEEE 802.11 minimum specification requires that Wi-Fi devices listen for Wi-Fi signals at -82 dBm in conducting a clear channel assessment, chip manufacturers and OEMs commonly design Wi-Fi products to listen for signal levels well below -82 dBm, even as low as -90 dBm.

Others, like CableLabs, have said that about half of all Wi-Fi connections operate at a received signal strength (RSSI) of less than -82 dBm, so using a Wi-Fi RSSI of -82 dBm in the Wi-Fi Alliance test plan is an extraordinary compromise because it puts half of all outdoor Wi-Fi connections at risk of LTE-U degradation.

Another concern related to the fact that there will be many scenarios where the LTE-U interference signal is stronger than the Wi-Fi signal received by the consumer, i.e., a person connecting to the internet is closer to an LTE-U evolved NodeB (eNB) than the Wi-Fi access point. This is called a negative SINR scenario, and this scenario was removed in order to move the process forward, according to Szymanski.

One of the tenets from the very beginning of the work between LTE-U and Wi-Fi revolves around determining whether an LTE network impacts a Wi-Fi network any more than a Wi-Fi network impacts another Wi-Fi network. 

And while experienced engineers are working on how all of this will be tested in labs, “in the real world, we don’t know exactly how this is going to work out,” said Clint Brown, director of business development at Broadcom.

Living up to the definition of compromise, none of the stakeholders appear happy with all the details in the run-up to the final test plan for LTE-U and Wi-Fi coexistence. But Szymanski said they intend to continue working with the cellular community, including in 3GPP and other bodies.

“Our goal is to ultimately see that devices that use Wi-Fi are not harmed by this technology," he said.

Related articles:
Evolve coalition blames Wi-Fi Alliance for buckling under cable lobby pressure
Wi-Fi group claims ‘extraordinary compromise’ on LTE-U, but Qualcomm begs to differ
Broadcom: LTE-U program proceeding faster than typical Wi-Fi Alliance plan