What is SNR?
SNR (Signal-to-Noise Ratio) is the ratio of the desired signal power to the background noise power, typically expressed in decibels (dB). In a wireless system, noise includes thermal noise (kTB), receiver noise figure contributions, and any other non-signal energy. SNR is a fundamental metric that determines the maximum achievable data rate and the lowest detectable signal level. While SINR is more practically relevant in cellular networks (where interference dominates), SNR is the key parameter in conducted testing and in isolated environments.
How Does SNR Work?
SNR is calculated as: SNR (dB) = 10·log₁₀(P_signal/P_noise). Thermal noise power is given by kTB, where k is Boltzmann’s constant, T is temperature (typically 290K), and B is bandwidth. For a 100 MHz 5G NR channel, the thermal noise floor is approximately -94 dBm. A receiver with a 5 dB noise figure raises this to -89 dBm. If the received signal is -70 dBm, the SNR is 19 dB. Higher-order modulation schemes require higher SNR — QPSK works at SNR > 3 dB, 16-QAM at > 10 dB, 64-QAM at > 17 dB, and 256-QAM at > 24 dB. The relationship between SNR and spectral efficiency is bounded by Shannon’s capacity theorem: C = B·log₂(1 + SNR).
Use Cases
Receiver sensitivity specification, link budget analysis for 5G NR, conducted RF testing (where interference is absent), modulation scheme selection, and noise figure characterisation of RF components.
3GPP / Standards Reference
3GPP TS 38.101 (NR UE sensitivity requirements), 3GPP TS 38.104 (NR BS sensitivity requirements), ITU-R M.2134 (IMT performance requirements)
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