What is the difference between C/N, Eb/No, and SNR and when do I use each one?
C/N vs Eb/No Explained
Understanding the distinction between C/N and Eb/No is fundamental to communication system design. They answer different questions: C/N asks "how strong is my signal relative to noise?" while Eb/No asks "is each bit received reliably?"
| Parameter | Free Space | Urban | Indoor |
|---|---|---|---|
| Path Loss Model | Friis (1/r²) | Okumura-Hata | IEEE 802.11 |
| Fading Margin | 0 dB | 10-30 dB | 5-15 dB |
| Multipath | None | Severe | Moderate-severe |
| Typical Range | Line of sight | 1-30 km | 10-100 m |
| Shadow Fading (σ) | 0 dB | 6-12 dB | 3-8 dB |
Frequently Asked Questions
What is the difference between Eb/No and SNR?
SNR (signal-to-noise ratio) = C/N measured in the receiver bandwidth. It is the same as C/N for a single carrier. Eb/No = SNR × (B/R_b). For a system where B = R_b: Eb/No = SNR numerically. For OFDM (5G NR): the "SNR" reported by the receiver is typically the per-subcarrier SNR (the signal power on one subcarrier divided by the noise on that subcarrier). This per-subcarrier SNR ≈ Es/No (because each subcarrier carries one symbol, and the noise bandwidth equals the subcarrier spacing). So: per-subcarrier SNR ≈ Es/No = Eb/No + 10×log10(bits per symbol). The relationship between SNR and Eb/No depends on the modulation and system bandwidth definition.
How is C/N measured in practice?
For CW signals: use a spectrum analyzer. Measure the carrier power (C) as the peak power in the signal bandwidth. Measure the noise floor (N) in the same bandwidth. C/N = C - N (in dB). For modulated signals: use a modulation analyzer or wideband power meter. Measure the total signal power in the channel bandwidth. Measure the noise power in an adjacent empty channel (of the same bandwidth). C/N ≈ signal power - adjacent noise power (in dB). For digital signals: the receiver typically computes EVM, which is related to C/N by: C/N ≈ -20×log10(EVM). This is the most accurate method because it accounts for all impairments (noise, distortion, interference).
How much Eb/No margin do I need?
The link margin is the difference between the available Eb/No and the required Eb/No: margin = Eb/No_available - Eb/No_required. Recommended margins: clear-sky (nominal conditions): 3-6 dB. This accounts for: implementation losses (real vs ideal receiver: 1-2 dB), component tolerances (0.5-1 dB), temperature variation (0.5-1 dB), and miscellaneous losses. Rain fade (satellite links): additional 3-15 dB depending on the frequency (20+ GHz) and the availability requirement (99.5% vs 99.99%). Multipath fading (terrestrial mobile): additional 10-40 dB (depending on the environment and the diversity techniques used). A link budget must include all margin allocations to ensure reliable operation under the worst-case conditions.