Receiver Sensitivity
• Sensitivity describes the weakest signal power level
that the receiver is able to detect and decode
– Sensitivity is dependent on the lowest signal-to-noise ratio
at which the signal can be recovered
– Different modulation and coding schemes have different
minimum SNRs
• Range: <0 dB to 60 dB
• Sensitivity is determined by adding the required SNR to
the noise present at the receiver
• Noise Sources
– Thermal noise
– Noise introduced by the receiver’s pre-amplifier
Thermal Noise
• Thermal Noise = N = kTB (Watts)
• k=1.3803 x 10-23 J/K
• T = temperature in Kelvin
• B=receiver bandwidth
– So Thermal Noise can be expressed by following
formula
N = -228,6 dBw + 10 log T + 10 log B
– Thermal noise is usually very small for reasonable
bandwidths
• Noise introduced by the receiver pre-amplifier
– Noise Factor = Noise Figure = SNRin/SNRout (positive
because amplifiers always generate noise)
Receiver Sensitivity Calculation
• The smaller the sensitivity, the better the
receiver
• Sensitivity (W) =
kTB * NF(linear) * minimum SNR required
(linear)
• Sensitivity (dBm) =
10log10(kTB*1000) + NF(dB) + minimum SNR
required (dB)
Sensitivity Example
• Example parameters
– Signal with 200KHz bandwidth at 290K
– NF for amplifier is 1.2dB or 1.318 (linear)
– Modulation scheme requires SNR of 15dB or 31.62 (linear)
• Sensitivity = Thermal Noise + NF + Required SNR
– Thermal Noise = kTB =
(1.3803 x 10-23 J/K) (290K)(200KHz)
= 8.006 x 10-16 W = -151dBW or -121dBm
– Sensitivity (W) = (8.006 x 10-16 W )(1.318)(31.62) = 3.33 x 10-14 W
– Sensitivity (dBm) = -121dBm + 1.2dB + 15dB = -104.8dBm
• Sensitivity decreases when:
– Bandwidth increases
– Temperature increases
– Amplifier introduces more noise