ASI C/N
Understanding Adjacent Satellite Interference C/N
In the crowded geostationary arc, dozens of satellites broadcast on overlapping frequencies separated by just 2–3 degrees of orbital spacing. Your ground antenna must receive the signal from your assigned satellite while the antennas on either side blast interference into your receiver's sidelobes. ASI C/N quantifies this interference.
How ASI Enters the Link Budget
The total C/N at the receiver input is the combination of all noise and interference sources:
- Thermal noise (C/N₀): from the receiver's own noise temperature.
- Uplink interference: from other ground terminals transmitting on the same frequencies.
- Adjacent satellite interference (ASI): from neighboring satellites' downlink transmissions.
The total C/N is computed by combining all contributions: (C/N)total = 1 / (1/(C/N)thermal + 1/(C/N)uplink + 1/(C/N)ASI). A low ASI C/N degrades the total link margin, potentially forcing a reduction in modulation order (from 16-APSK to QPSK) to maintain an acceptable bit error rate.
Antenna Sidelobe Control
The primary defense against ASI is a high-quality ground antenna with tight sidelobe control. ITU-R Rec. S.465 defines a reference radiation pattern envelope (29 – 25·log(θ) dBi for θ > 1°) that all earth station antennas must meet. Larger antennas have narrower beamwidths and lower sidelobes, providing better ASI rejection — one reason why professional VSAT terminals use 1.2–2.4 meter dishes rather than the 0.6 meter dishes common in consumer satellite TV.
Key Equations
Adjacent Satellite Interference (ASI) C/N is a critical satellite link budget parameter that quantifies the ratio of desired carrier power (C) to interference noise power...
Key specifications:
2.4 m | 0.6 m | 0 dB | 1 mW | 30 dB | 1 W
Link budget: C/N = EIRP−FSPL+G/T−10log(kB)
Comparison
| Aspect | ASI C/N Spec | Typical Range | Impact | Design Note |
|---|---|---|---|---|
| Primary function | In the geostationary orbit (GEO), commun... | Application-dep. | Critical | Verify in sim |
| Operating range | When a ground terminal points its antenn... | Application-dep. | Critical | Verify in sim |
| Performance | This interference power, normalized to t... | Application-dep. | Critical | Verify in sim |
| Integration | Regulatory frameworks (ITU-R Rec... | Application-dep. | Critical | Verify in sim |
| Trade-off | S.580, S.465) define minimum antenna sid... | Application-dep. | Critical | Verify in sim |
Frequently Asked Questions
How is ASI C/N calculated?
ASI C/N = (G_main - G_sidelobe) + (EIRP_desired - EIRP_adjacent) in dB, where G_main is the ground antenna gain toward the desired satellite, G_sidelobe is the gain toward the adjacent satellite (determined by the off-axis angle), EIRP_desired is the desired satellite's power spectral density at the ground terminal, and EIRP_adjacent is the adjacent satellite's power spectral density. This calculation is performed for each adjacent satellite and combined to determine the aggregate ASI.
Why is ASI worse at C-band than Ku-band?
At C-band (4 GHz), a 2° orbital separation corresponds to a relatively small angular offset for a ground antenna's sidelobe pattern. The same physical antenna has a wider beamwidth at lower frequencies, meaning the sidelobes are closer to the main beam and intercept more adjacent satellite power. At Ku-band (12 GHz), the same antenna has a narrower beamwidth and steeper sidelobe rolloff, providing better rejection. This is one reason why orbital spacing at C-band requires larger earth station antennas.
Can ASI be mitigated by the satellite operator?
Yes, through frequency coordination and power spectral density management. Adjacent satellite operators coordinate their frequency plans to minimize overlap. They may also reduce transmit power density in frequency bands shared with neighboring satellites. The ITU Radio Regulations provide the framework for this coordination process, balancing each operator's right to use their assigned spectrum with the obligation to protect adjacent satellite services.