The head of the FCC's office of engineering and technology is asking the wireless industry for more information on exactly how LTE operations in unlicensed spectrum will work. Julius Knapp, chief of the FCC's office of engineering and technology, took special aim at Verizon Wireless' LTE-U Forum, which has proposed a standard for the technology called LTE-U that uses Carrier Sense Adaptive Transmission (CSAT) to check for other users in unlicensed spectrum.
"Though the record reflects significant testing of CSAT sharing protocol with Wi-Fi, commenters did not provide information regarding the rationale behind the selection of certain key parameters for CSAT. Specifically we would like to know, what was the basis for selecting the maximum listening protocols? Some specifications seem to suggest that these parameters are implementation-dependent and may be set by operators. Please explain the decision to have CSAT transmit on a channel even if it appears to be occupied." Knapp wrote in a letter on the FCC's investigation into LTE Unlicensed technology. The agency is taking a look at exactly what Verizon, T-Mobile, Qualcomm and others plan to do with LTE in unlicensed spectrum, and how they plan to do it.
Knapp issued a range of other questions about CSAT, including how the system would decide how much data to transmit through unlicensed spectrum.
Verizon Wireless representatives did not immediately respond to a request for comment on Knapp's questions. The LTE-U Forum was formed in 2014 by Verizon in cooperation with Alcatel-Lucent, Ericsson, Qualcomm and Samsung. Verizon has said it plans to deploy LTE-U technology sometime next year. T-Mobile US too has said it will deploy the technology, though the company hasn't provided many details on its plans.
In his letter to the industry, Knapp also noted that there appear to be four separate standards in the works for transmitting LTE signals in unlicensed spectrum:
- Verizon's LTE-U Forum's technology, dubbed LTE-U
- A technology from the 3GPP called Licensed Assisted Access (LAA) that would be incorporated into its Release 13
- A second option from the 3GPP called LTE-WLAN Radio Level Integration and Interworking Enhancement (LWA)
- And Qualcomm's proprietary MuLTEfire technology
Knapp also addressed the sudden and intense interest in the technology from the wireless industry. Due to the skyrocketing costs of licensed spectrum (the FCC's recent AWS-3 auction raised a total of $41 billion in winning bids), wireless carriers are keen to deploy LTE into unlicensed spectrum in order to inexpensively add capacity into their networks.
"In the past, industry has generally cited the benefits of international harmonization and reliance on the private sector to develop standards in order to gain economies of scale and minimize development costs. Given this historical view, we seek to understand the reasons behind the strong interest in implementing the LTE U specification in the near future that would be unique to the United States," Knapp said. "One concern is the claim by some commentators that the technology is being introduced in the United States because systems are not required to implement spectrum sharing etiquettes as mandated in other parts of the world."
Most other countries in the world have more stringent requirements for unlicensed operations, including a "listen before talk" (LBT) rule that requires unlicensed spectrum users to first check whether other users are active in the bands they intend to use.
Indeed, the Wi-Fi industry has raised a number of concerns about LTE Unlicensed technologies. They are worried that LTE transmissions could affect Wi-Fi users.
After Knapp issued his letter, WifiForward tweeted that it appreciates the FCC's "continued vigilance on LTE-U & its efforts to ensure that the tech doesn't disrupt millions of Wi-Fi users." WifiForward is an advocacy group that is working to free up more spectrum for unlicensed use.
- see this FCC letter
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