How do I determine whether my link is thermal noise limited or interference limited?

A communication link is thermal noise limited when the dominant factor degrading signal quality is the receiver thermal noise floor (kTB). It is interference limited when signals from other transmitters (co-channel interference, adjacent channel interference, or self-interference) exceed the thermal noise and become the primary limitation. Determining which regime applies: (1) Calculate the thermal noise floor: N = kTB = -174 dBm/Hz + 10×log10(BW_Hz) + NF. For a 10 MHz channel with 5 dB NF: N = -174 + 70 + 5 = -99 dBm. (2) Estimate the total interference power at the receiver: I = sum of all interfering signals reaching the receiver (after antenna pattern, propagation, and filtering). (3) Compare: if I < N - 10 dB: thermal noise limited (interference negligible). If I > N + 10 dB: interference limited (thermal noise negligible). If |I - N| < 10 dB: both contribute; SINR = S / (N + I). The regime determines the design strategy: Thermal noise limited: improve sensitivity by reducing NF, increasing antenna gain, or increasing transmit power. Every dB of improvement translates directly to better performance. Interference limited: improving receiver sensitivity does not help (the interference rises with the signal). Instead: use directional antennas to reject interference, frequency planning to avoid co-channel conflicts, interference cancellation (SIC, MIMO nulling), and tighter filters to reject adjacent-channel energy. Modern cellular systems (LTE, 5G) are typically interference limited in urban areas (many nearby base stations on the same frequency) and thermal noise limited in rural areas (few interferers, signals are weak).