CBRS usage highlights the strengths and weaknesses of C-band spectrum in the US

U.S. carriers have spent over $80 billion on C-band licenses to improve their users’ 5G experience. To understand why the carriers have spent so much on this new spectrum capacity, Opensignal has analyzed the experience on CBRS and other similar types of spectrum already in use. This new analysis highlights how the CBRS band can boost download speeds, but also how lower power levels dramatically affect users’ experience if they are not close to a cell tower. 

We compared average 4G download speeds in the cities between the three main U.S. mobile operators — AT&T, T-Mobile and Verizon — with and without the supporting use of mid-band spectrum such as CBRS (3.5 GHz, band 48). For AT&T, we considered the WCS band (2.3 GHz, band 30) and for T-Mobile — the EBS/BRS band (2.5 GHz, band 41). To account for the varying bandwidth sizes, we analyzed only the connections when the same amount of 40 MHz was used — for the main and supporting spectrum bands combined. 

Looking at Verizon’s average 4G download speeds with and without the support of the CBRS band in the urban areas, we observed that the use of this band significantly boosted 4G download speeds. On the connections with the CBRS band involved, 4G download speeds experienced by Opensignal users were 78.8% faster than those without the CBRS band — clocking in with an impressive score of 74.4 Mbps. 

This was faster than what AT&T and T-Mobile achieved with the support of their mid-band spectrum in the cities — 47.3 and 61.6 Mbps, respectively. Our users on AT&T saw an uplift of 8% in their average 4G download speeds when connected to the WCS band, while our users on T-Mobile observed an improvement of 42.9% in their 4G download speeds with the 2.5 GHz band.

Citizens Broadband Radio Service  (CBRS, 3GPP band 48) is a spectrum band in the 3.55-3.7 GHz range assigned by the Federal Communications Commission (FCC) to share between different tiers of users. While CBRS frequencies are commonly used for 5G service globally, in the US current CBRS usage is limited to 4G. These tiers include incumbent (naval radars, Fixed Satellite Service, Broadband Wireless Access), Priority Access License (PAL), and General Authorized Access (GAA) users. The GAA tier provides an unlicensed spectrum that users can access for free, to support their existing or build their new private networks. Verizon is one of the companies that has been using the GAA tier, along with the PAL tier licenses acquired in August 2020.

Interestingly, we saw Verizon users connect to CBRS in more cities in October 2021 than 12 months earlier. We detected the use of the CBRS bands on 4G devices in 36.3% of U.S. Metropolitan Statistical Areas (MSAs) analyzed in October 2021, compared to 24.8% of MSAs in October 2020.

While CBRS and other mid-band spectrum help improve speeds, due to physics, higher frequencies tend to have less good reach than lower frequency bands. To understand this, we looked at how power levels (RSRP, or Reference Signal Received Power) changed with greater distance from cell towers for three mid-bands — 2.3 GHz on AT&T, 2.5 GHz using T-Mobile’s 4G network and CBRS usage by Verizon users. We also analyzed two lower bands used by Verizon — 700 MHz and 850 MHz bands. We used LTE Timing Advance values to indicate distance — one unit of Timing Advance corresponds to a distance of approximately 256 feet (78 meters).

We observed that as users’ distance from base stations increased, power levels seen for the CBRS band gradually deteriorated, going below the -120 dBm threshold where the signal strength would be considered poor. By contrast, users saw that RSRP values remained above -110 dBm for lower bands used by Verizon like 700 or 850 MHz bands, even for devices significantly farther from towers, like 15-18 Timing Advance units (approximately 3840-4610 feet from a base station).

The power level drop on the CBRS band is significantly worse than the fall seen by users on T-Mobile’s 4G network using the 2.5 GHz band, or by AT&T users connecting with the WCS band — when smartphones were farther from base stations in cities, RSRP values stayed above the -120 dBm threshold, placing the signal strength in the Fair category. However, 5G new radio (NR) is designed to improve mid-band spectrum reach, and 5G users will likely see a smaller fall in power levels when connecting on C-bands.

Power levels correlated with the speeds users experienced on each band. While the CBRS band users saw an impressive average download speed of 136.7 Mbps close to the tower (under 770 feet, approximately, or 0-3 timing advance), their speeds dropped significantly farther away from towers. Users’ average 4G download speeds nearly halved to 76.9 Mbps at approximately 770-1540 feet (3-6 timing advance) and dropped by nearly 80% over 2300 feet (over 9 timing advance). There were similar, although not as dramatic, falls in users’ speed on all bands when users were connected at a greater distance from the cell tower.

These differences in speed by distance mean that the use of the CBRS band by Verizon for supporting 4G connections in densely populated urban areas helps make a difference to users but is not a long-term solution, as it would require a denser network to provide the best experience possible with this band — which is likely less financially feasible, especially outside of urban areas. Long-term network quality improvements can be achieved with further 5G roll-outs, as the way 5G technology is designed, it is likely to use the mid-band spectrum range more efficiently and to its full potential.

The use of C-band is key for future 5G roll-outs

Opensignal’s analysis shows that Verizon successfully used the CBRS band to boost its users’ 4G download speeds. But this band will also remain strategic for future 5G roll-outs, especially as more devices that support C-band become available.

With the CBRS Priority Access Licenses purchased in 2020 during Auction 105 and the C-band spectrum secured in February 2021 during Auction 107, both Verizon and AT&T intend to use C-band to step up their 5G game and compete more strongly with T-Mobile. As we showed in our recent analysis, T-Mobile has already used its 2.5 GHz mid-band spectrum assets — acquired through the merger with Sprint — to boost its users’ 5G experience.

Mid-band spectrum frequencies — especially the C-band — have proven to be an essential part of 5G deployments worldwide, and will play an even bigger role in the U.S. soon. Opensignal’s findings on the relationship between power levels, distance and users’ speeds are especially relevant in the U.S. now given the delay in C-band rollout and discussion around lowering power levels to minimize the risk of interference in other services.