What is the difference between C/N and SNR?

C/N (Carrier-to-Noise) is measured at the RF carrier before demodulation, in the RF bandwidth. SNR (Signal-to-Noise Ratio) is measured after demodulation, in the baseband. For simple modulation schemes without processing gain, C/N approximately equals SNR. For spread spectrum systems (CDMA, GPS), the processing gain provides SNR improvement over C/N: SNR = C/N + Processing Gain. GPS C/A code has about 43 dB processing gain, so it can demodulate signals with C/N = -30 dB. C/N is the more fundamental quantity for RF link design because it includes only the RF link parameters.

How is C/N calculated from a link budget?

C/N = EIRP - Path_Loss + G_rx/T - k - B (all in dB). Where: EIRP = transmit power + antenna gain (dBW). Path_Loss = free space or actual propagation loss (dB). G_rx/T = receiver antenna gain divided by system noise temperature (dB/K). k = Boltzmann constant = -228.6 dBW/K/Hz. B = noise bandwidth (dB-Hz). Example satellite link: EIRP = 50 dBW, path loss = 200 dB, G/T = 20 dB/K, B = 73 dB-Hz (20 MHz): C/N = 50 - 200 + 20 - (-228.6) - 73 = 25.6 dB.

How does C/N relate to Eb/No?

Eb/No = C/N - 10*log10(Rb/B), where Rb is the data rate and B is the noise bandwidth. For a 1 Msps QPSK signal in 1 MHz bandwidth: Eb/No = C/N - 10*log10(2e6/1e6) = C/N - 3 dB (since QPSK carries 2 bits/symbol, Rb = 2*symbol_rate). For BPSK in matched bandwidth (Rb = B): Eb/No = C/N. The relationship matters because BER curves are plotted vs. Eb/No, while link budgets calculate C/N. You convert between them using the spectral efficiency of the modulation scheme.

Link Analysis

Carrier-to-Noise Ratio (C/N)

/kar-ee-er too noyz ray-shoh/ (CNR)

Carrier-to-Noise Ratio (C/N) is the ratio of received carrier power to noise power in a specified bandwidth, in dB. It is the primary RF link quality metric: C/N = EIRP - Path Loss + G/T - k - B. Related to E_b/N₀ by the spectral efficiency of the modulation. Determines BER, available MCS, and link availability for every communication system.

Category: Link Analysis

Units: dB

Relation: C/N = E_b/N₀ + 10log(R_b/B)

Understanding Carrier-to-Noise Ratio

C/N is the bottom line of any RF link budget. After accounting for transmit power, antenna gains, path loss, and receiver noise, C/N tells you whether the link will work. Every dB of C/N above the minimum required enables higher-order modulation (more throughput) or provides fade margin (more reliability). The link budget is fundamentally an exercise in maximizing C/N within the constraints of power, bandwidth, and regulatory limits.

C/N Calculation

Carrier-to-Noise Ratio (C/N):
Carrier-to-Noise Ratio (C/N) is the ratio of received carrier power to noise power in a specified bandwidth, in dB. It is the primary RF link...

Key specifications:
-228.6 dB | 3 dB | 50 dB | 200 dB | 20 dB | 73 dB

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

Required C/N by Application

System	Modulation	Required C/N	Target BER	Margin
GPS L1 C/A	BPSK	-30 dB (pre-corr)	N/A (code)	43 dB proc. gain
DVB-S2 (QPSK 1/2)	QPSK	1.0 dB	10^-7	Rain fade margin
LTE (QPSK)	QPSK	-1 to 3 dB	10^-1 (BLER)	FEC corrects
Wi-Fi 6 (256-QAM)	256-QAM	~30 dB	10^-2 (PER)	Indoor only
Point-to-point MW	128-QAM	~28 dB	10^-6	Rain, multipath

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	Carrier-to-Noise Ratio (C/N) Spec	Typical Range	Impact	Design Note
Primary function	Carrier-to-Noise Ratio (C/N) is the rati...	Application-dep.	Critical	Verify in sim
Operating range	It is the primary RF link quality metric...	Application-dep.	Critical	Verify in sim
Performance	Related to E b /N 0 by the spectral effi...	Application-dep.	Critical	Verify in sim
Integration	Determines BER, available MCS, and link...	Application-dep.	Critical	Verify in sim
Trade-off	Understanding Carrier-to-Noise Ratio C/N...	Application-dep.	Critical	Verify in sim

Common Questions

Frequently Asked Questions

C/N vs. SNR?

C/N: measured at RF, before demodulation, in RF bandwidth. SNR: measured after demodulation, at baseband. For simple modulation: C/N ~ SNR. Spread spectrum: SNR = C/N + processing gain. GPS: 43 dB processing gain allows -30 dB C/N. C/N is the fundamental RF link parameter.

Link budget calculation?

C/N = EIRP - Path_Loss + G/T - k - B. All in dB. k = -228.6 dBW/K/Hz. Satellite example: 50 - 200 + 20 + 228.6 - 73 = 25.6 dB. Each link budget parameter directly adds or subtracts from C/N.

C/N to Eb/No?

Eb/No = C/N - 10*log10(Rb/B). BPSK: Eb/No = C/N. QPSK: Eb/No = C/N - 3 dB. Conversion uses spectral efficiency. BER curves plotted vs. Eb/No; link budgets calculate C/N.

Related Terms

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