Why Transmitted and Received Power are Calculated in dBm
What is dBm?
dBm means:
- dB relative to
- 1 milliwatt (mW)
P(dBm) = 10 log₁₀(P(mW))
Examples:
| Power | dBm |
|---|---|
| 1 mW | 0 dBm |
| 10 mW | 10 dBm |
| 100 mW | 20 dBm |
| 1000 mW (1 W) | 30 dBm |
Why Wireless Engineers Use dBm
In RF communication, signals experience:
- Transmitter power
- Antenna gain
- Cable loss
- Free-space path loss
- Fading losses
Using linear watts would require lots of multiplication and division.
In dB form:
- Multiplication becomes addition
- Division becomes subtraction
Example Link Budget
Suppose:
- Transmit power = 30 dBm
- TX antenna gain = 15 dBi
- Path loss = 103 dB
- RX antenna gain = 10 dBi
Received Power = 30 + 15 - 103 + 10
= -48 dBm
This is much easier than multiplying and dividing very large or very small numbers.
Another Example
For a single-mode optical fiber cable with an attenuation of 0.25 dB/km, the optical power (in dBm) at a distance of 100 km from a 0.1 mW light source is:
Solution:
Power of the light source:
0.1 mW = 10 log10(0.1) = -10 dBm
Optical power loss over 100 km:
Loss = 0.25 × 100 = 25 dB
Received optical power:
Preceiver = -10 - 25 = -35 dBm
Why Not Use Watts Directly?
Wireless signals can vary across extremely large ranges:
| Signal | Power |
|---|---|
| Cell tower | Tens of watts |
| Phone receiver sensitivity | Billionths of a watt |
dBm compresses these huge ranges into manageable values.
| Power | dBm |
|---|---|
| 20 W | 43 dBm |
| 0.000001 W | -30 dBm |
| 0.000000000001 W | -90 dBm |
Important Distinction Between dB Units
| Unit | Meaning |
|---|---|
| dB | Ratio only |
| dBm | Absolute power referenced to 1 mW |
| dBi | Antenna gain relative to an isotropic antenna |
This allows simple RF link-budget calculations:
- Path loss → dB
- Antenna gain → dBi
- Transmit power → dBm