What is a Link Budget?
A link budget is like a financial budget, but for radio signals. It tracks all the power gains and losses between a transmitter (cell tower) and receiver (your phone). The basic equation is:
Rx Power = Transmit Power + All Gains - All Losses
Complete Signal Path Analysis
-
Transmitter (Cell Tower)
- Base Power: +43 dBm (About 20 watts)
- Antenna Gain: +18 dBi (Focuses signal like a spotlight)
- Cable Losses: Varies with length (-0.5 dB per meter)
-
Signal Path Losses
- Free Space Loss: Calculated using Okumura Model
- Building Penetration: -15 dB (Walls, windows, etc.)
- Fast Fading: -10 dB (Rapid signal fluctuations)
- Slow Fading: -8 dB (Environmental changes)
- Interference: -3 dB (Other signals)
-
Receiver (Mobile Phone)
- Antenna Gain :+2 dBi
- Body Loss:-4 dB (Human absorption)
- Cable Loss: -0.5 dB (Internal connections)
Real-World Example:
Calculation:
(+) Transmit Power: +43 dBm (Tower)
(+) Tower Antenna: +18 dB (Focused beam)
(-) Free Space Loss: -100 dB (Distance & Frequency)
(-) Building Loss: -15 dB (Coffee shop walls)
(-) Fast Fading: -10 dB (Movement)
(-) Slow Fading: -8 dB (Environmental)
(-) Interference: -3 dB (Other signals)
(+) Phone Antenna: +2 dB (Reception)
(-) Body Loss: -4 dB (Hand holding phone)
(-) Cable Loss: -0.5 dB (Internal)
(+) Total Gains= 20 dB
(-) Total Losses=140.5 dB
Rx Power = Transmit Power + All Gains - All Losses
Final Signal: -77.5 dBm ( Fair 3 bars)
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