Sensitivity
Understanding Receiver Sensitivity
Sensitivity determines the maximum range of a wireless system. A receiver that is 3 dB more sensitive can detect signals at 1.4x greater distance (since power decreases as distance squared). Improving sensitivity is one of the most effective ways to extend system range or reduce transmit power requirements.
Sensitivity Calculation
Sensitivity = Noise floor + Required SNR = -174 + 10 log10(BW) + NF + SNR_min. Each term in this equation represents a design knob: noise figure (LNA choice), bandwidth (filter design), and minimum SNR (modulation/coding choice).
Factors Affecting Sensitivity
- Noise figure: Each 1 dB increase in NF degrades sensitivity by 1 dB.
- Bandwidth: Doubling bandwidth degrades sensitivity by 3 dB (more noise).
- Required SNR: Higher-order modulation requires higher SNR, degrading sensitivity.
- Implementation loss: Real receivers have 1-3 dB additional loss from ADC quantization, filter shape factor, and imperfect synchronization.
Example: BW = 20 MHz, NF = 5 dB, QPSK (SNR_min = 10 dB):
Sensitivity = -174 + 73 + 5 + 10 = -86 dBm
Example: BW = 200 kHz, NF = 2 dB, GMSK (SNR_min = 9 dB):
Sensitivity = -174 + 53 + 2 + 9 = -110 dBm
Frequently Asked Questions
What is receiver sensitivity?
Sensitivity is the minimum signal power a receiver needs to successfully demodulate the signal with acceptable quality (BER or SNR). It is set by the noise floor plus the minimum required SNR. Lower (more negative) sensitivity numbers indicate a more sensitive receiver.
How do you improve receiver sensitivity?
Use a lower noise figure LNA at the front end, narrow the receiver bandwidth (process less noise), choose a more robust modulation scheme (lower required SNR), add coding gain (FEC), or cool the receiver (reduce thermal noise). Each 1 dB improvement in NF yields 1 dB better sensitivity.
What determines maximum communication range?
Range is determined by the link budget: EIRP minus path loss must exceed receiver sensitivity plus required link margin. Sensitivity directly determines the maximum allowable path loss, which determines range. A 6 dB improvement in sensitivity doubles the maximum range.