Interactive virtual laboratory for designing and analyzing point-to-point microwave communication links. Master the fundamentals of RF link budget calculations including EIRP, Free Space Path Loss, and Fade Margin.
Understand the concept of link budget and its importance in microwave communication system design
Calculate Free Space Path Loss (FSPL) for different frequencies and distances
Determine Effective Isotropic Radiated Power (EIRP) and Received Signal Level (RSL)
Analyze fade margin requirements for different reliability targets
Evaluate link availability and outage time calculations
Design a complete microwave link meeting specified performance criteria
A link budget is an accounting of all the gains and losses from the transmitter, through the medium (free space), to the receiver in a telecommunication system. It is used to predict the received signal power and ensure that the communication link meets the required performance criteria. The link budget calculation is fundamental to microwave communication engineering and determines whether a link will close successfully.
Free Space Path Loss is the attenuation of the electromagnetic signal as it propagates through free space (a vacuum or air). It depends on the frequency of the signal and the distance between transmitter and receiver. FSPL increases with both frequency and distance.
where d = distance in km, f = frequency in GHz
The 20·log₁₀ terms indicate that FSPL increases by 6 dB for every doubling of distance or frequency. This is because:
EIRP represents the total power that would have to be radiated by an isotropic antenna (radiating equally in all directions) to produce the same power density observed in the direction of maximum antenna gain. It combines transmitter output power, antenna gain, and transmission line losses.
where Pt = transmit power, Gt = antenna gain, Lt = cable loss
The power that would be received by an isotropic antenna (0 dBi gain) at the receiver location:
The actual power received at the receiver input after accounting for receive antenna gain and cable losses:
Fade margin is the additional signal power included in the link budget to compensate for signal fading due to multipath propagation, rain attenuation, and other time-varying channel impairments. It ensures the link maintains acceptable performance during adverse conditions.
or FM = RSL − Threshold Level
Caused by atmospheric refractivity variations creating multiple signal paths. Most significant for links over water or flat terrain.
Signal absorption and scattering by raindrops. Critical for frequencies above 10 GHz.
Signal bending around obstacles when the Fresnel zone is obstructed.
Attenuation by oxygen and water vapor molecules. Significant above 10 GHz.
| Link Type | Distance | Fade Margin |
|---|---|---|
| Short haul | < 5 km | 10-20 dB |
| Medium haul | 5-15 km | 20-30 dB |
| Long haul | > 15 km | 30-40 dB |
| High availability | Any | 40+ dB |
Link availability is the percentage of time that the link operates above the required performance threshold. It is directly related to the fade margin - higher fade margins provide higher availability but require more powerful transmitters or larger antennas.
| Availability | Outage/Year | Fade Margin |
|---|---|---|
| 90% | 36.5 days | 8 dB |
| 99% | 3.65 days | 18 dB |
| 99.9% | 8.76 hours | 28 dB |
| 99.99% | 52.6 min | 38 dB |
| 99.999% | 5.26 min | 48 dB |
| 99.9999% | 31.5 sec | 58 dB |
Annual outage time can be calculated from availability percentage:
For 99.99% availability: (1 − 0.9999) × 8760 hours = 0.876 hours = 52.6 minutes per year
For frequencies above 10 GHz, rain attenuation becomes the dominant fading mechanism. The specific attenuation γ (dB/km) depends on rain rate and frequency:
where d = path length, reff = effective path length factor (typically 0.5-0.8 for microwave links)
Rain, foliage, etc.
Typically -70 to -90 dBm
Formula: r = 17.3 × √(d1 × d2 / (f × (d1 + d2)))
Set transmitter power to 30 dBm, antenna gains to 30 dBi each, cable losses to 2 dB each, frequency to 6 GHz, and distance to 10 km.
Record the Effective Isotropic Radiated Power. Verify using the formula: EIRP = Pt + Gt − Lt
Calculate Free Space Path Loss using: FSPL = 92.45 + 20·log₁₀(d) + 20·log₁₀(f)
Calculate Received Signal Level and compare with receiver sensitivity.
Determine if the fade margin is sufficient for 99.99% availability (requires ~38 dB).
Keep all other parameters constant. Vary distance from 1 km to 50 km in steps of 5 km.
For each distance, record FSPL, RSL, and Fade Margin in a table.
Create a graph showing RSL and Fade Margin vs. Distance. Identify the maximum distance for 20 dB fade margin.
Set distance to 20 km. Vary frequency from 2 GHz to 40 GHz.
Observe how FSPL increases with frequency. Calculate the additional loss when moving from 6 GHz to 18 GHz.
Calculate Fresnel zone radius at different frequencies. Discuss implications for antenna height requirements.
Determine required fade margin for 99.99% availability (~38 dB).
Select appropriate antenna gains and verify that RSL provides sufficient fade margin above receiver sensitivity.
Document your design choices and provide a complete link budget table.
| Component | Weight |
|---|---|
| Theory Understanding | 20% |
| Calculation Accuracy | 25% |
| Data Presentation | 20% |
| Analysis & Discussion | 25% |
| Format & References | 10% |
| Parameter | Symbol | Value | Unit | Notes |
|---|---|---|---|---|
| Transmit Power | Pt | 30 | dBm | RF Output |
| Tx Antenna Gain | Gt | 30 | dBi | Parabolic dish |
| Tx Cable Loss | Lt | 2 | dB | Waveguide |
| EIRP | EIRP | 58 | dBm | Pt + Gt - Lt |