Wireless Communication Basics
Take the thermal noise power density at 290 K as −174 dBm/Hz. For a receiver with a bandwidth of 1 MHz and a noise figure of 6 dB that needs an S/N of 10 dB to demodulate, which is closest to the receiver sensitivity (the minimum signal power that can be received)?
- A−114 dBm
- B−108 dBm
- C−98 dBm
Show answer and explanation
Answer: C. −98 dBm
The thermal noise power in a 1 MHz bandwidth is −174 + 10·log₁₀(10⁶) = −174 + 60 = −114 dBm. Treating the receiver's noise figure as increasing the noise by that amount, the input-referred noise power is −114 + 6 = −108 dBm. A signal 10 dB above that is needed for the required S/N, so the sensitivity is −108 + 10 = −98 dBm. Narrowing the bandwidth or lowering the noise figure or required S/N improves (lowers) the sensitivity.
- A. This is the thermal noise power in 1 MHz itself, without adding the noise figure and the required S/N.
- B. This is the noise power including the noise figure, without adding the required 10 dB S/N.
- C. Correct. −174 + 60 + 6 + 10 = −98 dBm.