CDMA Satellite
Understanding CDMA Satellite
Satellite Transponder Sharing and Bent-Pipe Architecture
In satellite communications, the transponder is the payload component that receives uplink signals from the ground, frequency-translates them, and amplifies them for downlink retransmission. While commercial systems historically favored FDMA or TDMA, CDMA satellite architectures (used in the Globalstar LEO constellation and GPS) offer unique advantages. Because all earth stations transmit on the same carrier frequency, CDMA eliminates the need for strict frequency coordination and timing guard bands.
Operating CDMA through a bent-pipe transponder introduces challenges. CDMA signals have a high Peak-to-Average Power Ratio (PAPR). To prevent the satellite's High-Power Amplifier (HPA), typically a Traveling Wave Tube Amplifier (TWTA), from saturating and generating severe intermodulation distortion, the transponder must be operated with a significant output backoff (OBO) of 3 to 5 dB. This reduces the downlink EIRP compared to single-carrier FDMA operations.
Satellite Diversity and Anti-Jam Capabilities
A key advantage of CDMA in Low Earth Orbit (LEO) satellite networks is the implementation of satellite diversity. A mobile handset can transmit to, and receive from, multiple satellites in view simultaneously. The ground gateway combines these paths using RAKE receivers, achieving 3 to 4 dB of diversity gain. This diversity helps overcome the path loss and shadowing caused by terrain, buildings, or vegetation, which is critical for low-power handheld devices.
In military MILSATCOM systems, CDMA is utilized for its inherent anti-jam (AJ) and low probability of intercept (LPI) properties. The processing gain of the spread-spectrum waveform suppresses narrowband and broadband noise jamming signals. A military receiver can recover the signal even when the jammer-to-signal (J/S) ratio exceeds 20 to 30 dB, ensuring secure, reliable tactical communications.
Key Mathematical Relations
Technical Specifications Comparison
| Access Method | Anti-Jam Protection | Intermodulation Sensitivity | Power Control Dependency | Path Diversity Support | Primary Satellite Application |
|---|---|---|---|---|---|
| FDMA (Satellite) | None (susceptible to single tone jammer) | High (requires transponder backoff) | Low | No | VSAT networks, television broadcast |
| TDMA (Satellite) | None | Low (single carrier per time slot) | Medium | No | High-capacity trunking networks |
| CDMA (Satellite) | High (protected by processing gain) | High (requires backoff due to multi-user envelope) | High (LEO loop delay challenges) | Yes (RAKE combining from multiple satellites) | LEO voice networks, military systems, GPS |
Frequently Asked Questions
How does Globalstar implement CDMA satellite communications?
Globalstar utilizes a constellation of 48 LEO satellites. Handheld terminals transmit at L-band (1.6 GHz) uplink, which is received by the satellites and translated to C-band (7.0 GHz) downlink to ground gateways. The gateway uses RAKE receivers to combine signals from up to three satellites simultaneously, achieving path diversity.
Why is power control more difficult in CDMA satellite links than in terrestrial networks?
Terrestrial CDMA networks update power control commands 800 times per second to track fast fading. For satellites, the propagation delay (5 to 6 ms for LEO, 250 ms for GEO) makes fast closed-loop power control impossible. Satellite networks must rely on open-loop power control and slower closed-loop updates (50 to 100 Hz), which reduces capacity.
How does GPS use CDMA?
GPS satellites transmit navigation signals on the same frequency (e.g., L1 at 1575.42 MHz). Each satellite is assigned a unique Gold code (a pseudo-random sequence of 1023 chips). The GPS receiver correlates the incoming composite signal with the local copy of a satellite's Gold code to extract the navigation message and measure the range.