Signal-to-Noise Ratio
Understanding SNR
SNR is the single most important parameter in communications theory. It determines the theoretical maximum data rate (Shannon capacity), the achievable modulation order, and the resulting bit error rate. All receiver design ultimately aims to maximize SNR.
SNR Requirements
| Application | Min SNR Required |
|---|---|
| Voice (BPSK, BER=10^-3) | 7 dB |
| Data (QPSK, BER=10^-6) | 10.5 dB |
| LTE 64-QAM | 22 dB |
| 5G NR 256-QAM | 29 dB |
| Wi-Fi 6 1024-QAM | 35 dB |
Improving SNR
- Reduce receiver noise figure (better LNA).
- Increase signal power (higher TX power or antenna gain).
- Reduce bandwidth (but also reduces data rate).
- Use coding gain (FEC improves effective SNR by 3-10 dB).
Frequently Asked Questions
What is SNR?
SNR is the ratio of signal power to noise power. Higher SNR = better signal quality. SNR determines achievable modulation order and BER. 10 dB SNR for QPSK, 22 dB for 64-QAM, 29 dB for 256-QAM.
How does SNR relate to Shannon capacity?
Shannon capacity C = B*log2(1+SNR) gives the theoretical maximum data rate for a given bandwidth B and SNR. Doubling bandwidth doubles capacity. Doubling SNR (3 dB) adds only ~1 bit/Hz of capacity.
What is the difference between SNR and Eb/No?
SNR = signal/noise in a given bandwidth. Eb/N0 = energy per bit / noise density (bandwidth-independent). They are related: Eb/N0 = SNR x B/R (bandwidth/data rate). Eb/N0 is more fundamental for comparing modulation schemes.