Band n77 (C-Band, 3.7 GHz)
Understanding C-Band
The C-band auction was a watershed moment for US 5G. Before Auction 107, Verizon and AT&T had no significant mid-band spectrum for 5G, relying on mmWave (very limited coverage) and low-band DSS (very limited speed). The C-band acquisition gave both carriers the spectrum needed to compete with T-Mobile's n41 deployments. The $81.2 billion total price reflected the strategic importance of mid-band spectrum for credible 5G performance.
Deployment was complicated by adjacency to the radar altimeter band (4200 to 4400 MHz), requiring reduced power near airports and a phased rollout. The first 100 MHz (A block, 3700 to 3800 MHz) became available in January 2022; the remaining B block cleared by December 2023. Both Verizon and AT&T deploy 64T64R massive MIMO panels, achieving spectral efficiency comparable to T-Mobile's n41. The key difference is coverage: at 3.7 GHz, each cell covers roughly 30% less area than at 2.5 GHz.
C-Band Technical Parameters
US C-Band: 3700 – 3980 MHz (280 MHz)
A block: 3700–3800 MHz | B block: 3800–3980 MHz
NR Configuration:
SCS: 30 kHz | Max channel BW: 100 MHz
Peak DL (100 MHz, 256-QAM, 4-layer): ~2.3 Gbps
Path Loss at 3.7 GHz (urban macro):
PL = 132.4 + 37.6 log(dkm) dB
1 km: 132.4 dB | 2 km: 143.7 dB
vs. 2.5 GHz (n41): +2.8 dB more loss
US C-Band Auction Winners
| Operator | Spend | Avg Spectrum | Deployment |
|---|---|---|---|
| Verizon | $45.5B | 161 MHz nationwide | Primary mid-band 5G |
| AT&T | $23.4B | ~80 MHz | Primary mid-band 5G |
| T-Mobile | $9.3B | ~40 MHz | Supplement to n41 |
| US Cellular | $1.3B | Variable | Rural/regional |
Frequently Asked Questions
What happened in the C-band auction?
Auction 107 (Dec 2020 to Jan 2021) sold 280 MHz at 3700 to 3980 MHz for $81.2B. Verizon: $45.5B (161 MHz). AT&T: $23.4B (~80 MHz). T-Mobile: $9.3B (~40 MHz). Deployment phased: A block (3700 to 3800 MHz) available Jan 2022, B block cleared Dec 2023. Existing satellite users relocated to upper C-band.
How does C-band compare to Band 41 (2.5 GHz)?
C-band has 2.8 to 3.0 dB more path loss (~30% less coverage area per cell). But C-band offers more total bandwidth (280 MHz vs. 194 MHz) and global harmonization for broader device support. Building penetration is 3 to 5 dB worse. Both support massive MIMO with 64T64R panels and similar spectral efficiency.
What caused the aviation interference concerns?
C-band (3700 to 3980 MHz) is adjacent to radar altimeters (4200 to 4400 MHz). The FAA raised concerns about receiver desensitization in older altimeters with poor filtering. Solution: reduced power near airports, exclusion zones, and airline altimeter upgrades. Largely resolved by mid-2023 with most aircraft certified compatible.