T-Mobile recently filed a document with the FCC warning the agency that as much as 60% of its upload cellular traffic using 2.5 GHz spectrum for connections up to six miles from a tower will be disrupted if the FCC allows satellite cellular to share the same or nearby spectrum using unsynchronized traffic. T-Mobile says that satellite transmissions for cellular delivered as close as 15 miles from one of its cell sites could be majorly disruptive to cellular traffic.
T-Mobile is worried about FCC plans to allow 2.5-2.9 GHz for direct-to-device (D2D) satellite cellular, which sits next to T-Mobile’s workhorse 2.5 GHz spectrum it uses for cellular service. The reality is that cellular signals drift outside of the assigned spectrum bands, particularly the signals generated by cellphones, which are outside of the control of a cellular carrier.
T-Mobile is also worried about the volumes of satellite broadband that might come from an FCC approval for SpaceX to launch as many as 15,000 satellites to provide direct-to-device cell service. Even more troubling would be the announced SpaceX plan to acquire space on as many as 80,000 U.S. cell towers to bolster its satellite service with terrestrial towers. It would be devastating to T-Mobile if a carrier uses unsynchronized signals across the country and on the same towers where T-Mobile is located.
To understand T-Mobile’s complaint, it’s worth looking at the technology behind its claim. Modern cellular networks rely on synchronized cellular spectrum to function. That refers to a network management framework where the cell towers and user cellphones in a region are perfectly aligned by the use of an ultra-precise time clock. This timing precision is required to efficiently transmit data over shared or adjacent radio frequencies without causing severe network interference.
If cell towers don’t have perfect, nanosecond-level time synchronization, one tower might attempt to download data at the same time a neighboring tower is uploading data on the same frequency. These kinds of timing problems lead to what is called a destructive overlap, which results in dropped calls, lagging internet speeds, and packet loss inside the cellular network.
T-Mobile is highly concerned that satellite cellular providers are not being held to the same standards as the rest of the industry, and T-Mobile and CTIA, the cellular industry spokesman, have asked the FCC to require the same standards for satellite cellular traffic that are applied to terrestrial cellular providers. At a minimum, T-Mobile and CTIA are asking the FCC to open a docket to explore the technical issues.
When I look back at how the FCC has acted in the recent past, the agency seems to be in a big hurry to approve satellite cellular service without undertaking the detailed technical and market analysis that has always been used in the past. Whether this is being done through favoritism for the new satellite providers or is just the FCC’s way to support the new shiny ball – it is not treating the new satellite companies in the same way it treated the cellular industry or other users of spectrum in the past.
It wouldn’t be surprising if T-Mobile is exaggerating the extent to which its upload traffic from customers would be disrupted. We’ve seen carriers make exaggerated claims in the past to get the FCC’s attention. It would be a disaster if D2D providers are allowed to use unsynchronized signals and disrupted even 5-10% of terrestrial cellular traffic. Hopefully the FCC will take T-Mobile seriously and give the issue the kind of analysis that has always been routine in the wireless industry.