How is C/N calculated in a satellite link?

The satellite link budget calculates C/N as: C/N(dB) = EIRP(dBW) + G/T(dB/K) - FSPL(dB) - k(dBW/K/Hz) - B(dBHz) - additional_losses(dB). EIRP = transmit power + antenna gain. G/T = receive antenna gain / system noise temperature (figure of merit). FSPL = free-space path loss. k = Boltzmann constant = -228.6 dBW/K/Hz. B = noise bandwidth. Additional losses: atmospheric attenuation, rain fade, pointing loss, polarization mismatch. For a typical Ku-band DTH link: EIRP = 52 dBW, G/T = 12 dB/K, FSPL = 205.8 dB, k = -228.6. C/N = 52 + 12 - 205.8 - (-228.6) - 10*log10(27e6) - 1 = ~12 dB. This must exceed the C/N threshold for the chosen modulation + FEC (e.g., DVB-S2 QPSK 3/4: C/N threshold = 4.0 dB).

What is the difference between C/N and SNR?

C/N is measured at the RF or IF stage BEFORE demodulation and filtering. It is the carrier power relative to the total noise in the measurement bandwidth. SNR is measured AFTER demodulation, representing the ratio of the desired signal to noise. For linear modulation (AM, SSB): SNR approximately equals C/N. For FM: SNR is much higher than C/N above the FM threshold due to FM's noise improvement (FM gain). For digital: C/N determines Eb/N0 which determines BER. The relationship: Eb/N0 = C/N + 10*log10(B/Rb) where B = noise bandwidth and Rb = bit rate. For matched filtering: B approximately equals Rb, so Eb/N0 approximately equals C/N. In spread-spectrum systems: C/N can be negative (signal below noise floor) while Eb/N0 is positive after despreading gain.

How do cascaded elements affect C/N?

In a cascaded system (uplink + satellite transponder + downlink), the overall C/N is: 1/(C/N_total) = 1/(C/N_up) + 1/(C/N_down) + 1/(C/N_intermod). All ratios in linear (not dB). The weakest link dominates. Example: C/N_up = 20 dB (100 linear), C/N_down = 12 dB (15.8 linear). C/N_total = 1/(1/100 + 1/15.8) = 1/0.0733 = 13.6 linear = 11.3 dB. The downlink dominates. Even with a 20 dB uplink, the total is only 0.7 dB better than the downlink alone. Implication: improving the stronger link provides diminishing returns. System design must balance uplink and downlink C/N for efficiency.

Link Budget

C/N Ratio

/see-over-en/ Carrier-to-Noise

C/N(dB) = C(dBm) - N(dBm). N = kTB. Measured before demodulation. Satellite: C/N = EIRP + G/T - FSPL - k - B. Cascaded: 1/C/N_total = Σ(1/C/N_i). Weakest link dominates. C/N ≠ SNR: for FM, SNR >> C/N (FM gain). For digital: E_b/N₀ = C/N + 10log(B/R_b).

Before: Demodulation

Noise: N = kTB

Cascade: Reciprocal sum

Understanding C/N Ratio

C/N is the fundamental metric for link quality before demodulation. It tells you how much the carrier signal exceeds the noise floor within the receiver bandwidth. Combined with the modulation scheme and error correction coding, C/N directly determines whether a link will work or not. In satellite communications, the link budget calculation centers on computing the end-to-end C/N and verifying it exceeds the threshold for the chosen modulation.

C/N Calculations

C/N Ratio:
C/N(dB) = C(dBm) - N(dBm). N = kTB. Measured before demodulation. Satellite: C/N = EIRP + G/T - FSPL - k - B. Cascaded: 1/C/N...

Key specifications:
-228.6 dB | -198.6 dB | 1.0 dB | 4.0 dB | 8 PS | 6.6 dB

Path loss: FSPL = 20log(d)+20log(f)+32.44

C/N Requirements by Modulation

Modulation + FEC	C/N Threshold	Spectral Eff	Standard	Application
QPSK 1/2	1.0 dB	1.0 bps/Hz	DVB-S2	Low C/N links
QPSK 3/4	4.0 dB	1.5 bps/Hz	DVB-S2	Standard DTH
8PSK 2/3	6.6 dB	2.0 bps/Hz	DVB-S2	HD broadcast
16APSK 3/4	10.2 dB	3.0 bps/Hz	DVB-S2	VSAT
32APSK 4/5	13.6 dB	4.0 bps/Hz	DVB-S2	High C/N

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

Aspect	C/N Ratio Spec	Typical Range	Impact	Design Note
Primary function	C/N(dB) = C(dBm) - N(dBm)...	Application-dep.	Critical	Verify in sim
Operating range	Measured before demodulation...	Application-dep.	Critical	Verify in sim
Performance	Satellite: C/N = EIRP + G/T - FSPL - k -...	Application-dep.	Critical	Verify in sim
Integration	Cascaded: 1/C/N total = Σ(1/C/N i...	Application-dep.	Critical	Verify in sim
Trade-off	Weakest link dominates...	Application-dep.	Critical	Verify in sim

Common Questions

Frequently Asked Questions

Satellite link budget?

C/N = EIRP + G/T - FSPL - k - 10log(B) - losses. Ku-band DTH example: 52 + 12 - 205.8 + 228.6 - 74.3 - 1 = ~12 dB. Must exceed modulation threshold (QPSK 3/4: 4.0 dB).

C/N vs SNR?

C/N: before demod, RF/IF. SNR: after demod. Linear mod: C/N ≈ SNR. FM: SNR >> C/N (FM improvement). Digital: Eb/N0 = C/N + 10log(B/Rb). Spread-spectrum: C/N can be negative.

Cascaded?

1/(C/N_total) = Σ(1/C/N_i). Weakest dominates. Up 20 dB + down 12 dB = total 11.3 dB. Improving strong link gives diminishing returns. Must balance uplink and downlink.

Related Terms

← C/I Ratio C/N0 →