Received Power Is Only Half the Story
- The desired signal (received power ) and the noise (density in W/Hz) both arrive at the receiver together
- The receiver can only work with their comparison:
- S/N₀ compares received signal power with noise per hertz
- Units: dB-Hz (W ÷ W/Hz = Hz) — re-emphasized in the Oct. 19 clarifications, since S/N₀ is not a dimensionless SNR
Signal-to-Noise Density
- in dBW, in dBW/Hz, in dB-Hz
- Noise density comes from the system noise temperature: (Thermal Noise)
- Where the received power comes from: Received Power
- Full end-to-end form (complete link calculation):
Same Ratio at Every Reference Plane
- Signal and noise must be evaluated at the same reference plane (reference planes)
- Moving from the antenna terminal (A) to the receiver input (B) through a 1 dB feed: and both fall 1.0 dB, so is unchanged
When Bandwidth Is Needed
- tells us the noise power per hertz
- A bandwidth tells us how much noise a receiver or instrument admits
- Total noise is needed to compare measured powers as S/N:
Lunar Link in a 1 MHz Bandwidth
- (from the lunar link dB-Hz)
WARNING
- Narrowing helps only while it still passes the needed signal
- Use the equivalent noise bandwidth
- S/N₀ is the calculated quantity; modulation determines what value is sufficient → Energy per Bit