Comm Notes
Satellite link budget, EIRP, G/T ratio, path loss calculation, rain attenuation, and link margin
Satellite Link Design: Engineering Reliable Space Communication
Satellite link design is the process of calculating whether a communication link between a ground station and a satellite will deliver adequate signal quality. The link budget — a systematic accounting of all gains and losses — determines whether your signal arrives with enough strength to be decoded reliably. Getting this calculation right is critical: too little margin and the link fails in rain; too much margin wastes expensive satellite power.
The Link Budget Equation
The fundamental equation for received signal power:
Pr = Pt + Gt + Gr - PL - Lother
All values in dB/dBm:
- Pr = received power (dBm or dBW)
- Pt = transmitted power (dBm or dBW)
- Gt = transmit antenna gain (dBi)
- Gr = receive antenna gain (dBi)
- PL = free-space path loss (dB)
- Lother = additional losses (atmospheric, rain, pointing, etc.)
Free-Space Path Loss
The dominant loss in any satellite link:
PL(dB) = 20×log₁₀(4πd/λ) = 20×log₁₀(d) + 20×log₁₀(f) + 92.45
Where d = distance in km, f = frequency in GHz.
Example for GEO at 12 GHz:
- Distance: 35,786 km (minimum), ~40,000 km (typical with elevation angle)
- PL = 20×log₁₀(38,000) + 20×log₁₀(12) + 92.45 = 91.6 + 21.6 + 92.45 = 205.6 dB
This enormous path loss (~200+ dB) is why satellite communication requires high transmit power, large antennas, and sensitive receivers.
EIRP: Effective Isotropic Radiated Power
EIRP combines transmitter power and antenna gain into a single figure of merit:
EIRP(dBW) = Pt(dBW) + Gt(dBi)
Typical satellite EIRP values:
- C-band transponder: 36-40 dBW
- Ku-band (DTH TV): 48-56 dBW
- Ka-band spot beam: 55-65 dBW
Higher EIRP means the satellite delivers more signal to the ground — enabling smaller receive antennas.
G/T: Receive System Figure of Merit
The receive system is characterized by G/T (antenna gain divided by system noise temperature):
G/T(dB/K) = Gr(dBi) - 10×log₁₀(Tsys)
Where Tsys = antenna noise temperature + LNA noise temperature + cable losses.
Typical G/T values:
- Large earth station (4.5m dish): +35 to +40 dB/K
- VSAT terminal (1.2m): +20 to +25 dB/K
- DTH receive system (60cm): +12 to +15 dB/K
- Satellite receive (from ground): -5 to +5 dB/K
C/N Ratio: The Bottom Line
The carrier-to-noise ratio at the receiver input determines link quality:
C/N(dB) = EIRP + G/T - PL - k - B - Lother
Where:
- k = Boltzmann's constant = -228.6 dBW/Hz/K
- B = noise bandwidth (dBHz)
Equivalently: C/N₀(dBHz) = EIRP + G/T - PL + 228.6 - Lother
And: Eb/N₀(dB) = C/N₀ - 10×log₁₀(Rb)
Rain Attenuation
Rain is the primary variable loss factor at Ku-band and above:
| Frequency | Rain attenuation (0.01% exceedance, heavy rain) |
|---|---|
| C-band (4/6 GHz) | 0.5-1 dB (negligible) |
| Ku-band (12/14 GHz) | 5-15 dB |
| Ka-band (20/30 GHz) | 10-30 dB |
| V-band (40/50 GHz) | 20-50 dB |
Rain margin must be included in link budget for the required availability:
- 99.5% availability (44 hours/year outage): moderate rain margin
- 99.9% availability (8.8 hours/year outage): large rain margin
- 99.99% availability (53 minutes/year): very large margin
Techniques to combat rain fade:
- Uplink power control (increase power during rain)
- Adaptive coding and modulation (ACM): reduce rate during fade
- Site diversity: Switch to geographically separated ground station
Complete Link Budget Example
DTH Television at Ku-band:
- Satellite EIRP: 52 dBW
- Free-space path loss (12 GHz, 38,000 km): -205.6 dB
- Atmospheric loss: -0.3 dB
- Rain margin (99.7% availability): -3.0 dB
- Receive antenna gain (60 cm): 33.7 dBi
- System noise temperature: 150 K → 10×log₁₀(150) = 21.8 dB
- G/T = 33.7 - 21.8 = 11.9 dB/K
- Boltzmann's constant: +228.6 dB
- C/N₀ = 52 + 11.9 - 205.6 - 0.3 - 3.0 + 228.6 = 83.6 dBHz
- For DVB-S2 QPSK rate 3/4 at 30 Msps: Required C/N = 6.5 dB
- Available C/N = C/N₀ - 10×log₁₀(30×10⁶) = 83.6 - 74.8 = 8.8 dB
- Link margin = 8.8 - 6.5 = 2.3 dB ✓ (adequate)
Overall Link: Uplink + Downlink
For a bent-pipe satellite, the overall C/N combines uplink and downlink noise:
(C/N)total⁻¹ = (C/N)up⁻¹ + (C/N)down⁻¹ + (C/I)intermod⁻¹
The total C/N is always worse than the weakest individual link. Typically the downlink limits performance because satellite transmit power is constrained.
Key Takeaways
- The link budget systematically accounts for all gains (EIRP, G/T) and losses (path loss, rain, atmosphere) to determine received signal quality.
- Free-space path loss exceeds 200 dB for GEO links — the dominant loss requiring high EIRP and sensitive receivers to overcome.
- EIRP characterizes the transmitter, G/T characterizes the receiver — these two figures of merit determine link performance.
- Rain attenuation is the primary variable impairment at Ku/Ka bands, requiring 5-30 dB margin depending on frequency and availability target.
- The overall bent-pipe link C/N is limited by the weaker of uplink or downlink — both must be designed with adequate margin.
- Adaptive Coding and Modulation (ACM) dynamically adjusts throughput to match instantaneous channel conditions, maximizing average capacity.
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