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Modulation Scheme |
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The satellite-to-satellite link uses an Offset Quaternary Phase Shift Keying (OQPSK) and uses
a spread sprectrum system.
OQPSK is a good choice for satellite communications because it has a good spectral efficiency
of 2bit/s/Hz and resistant to phase shifts. Spread spectrum is used for this link for its important
advantage: the processing gain. The processing gain is:
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This extra gain allows to reduce the transmit power.
This communication technique is also very useful for its ability to allow multiple satellites to
communicate at the same frequency and during the same time.
The spread spectrum parameters are the followings:
- Carrier frequency: 2.16GHz
- Chip rate: 10.23Mchip/s
- Sequence length: 1023bits
- Bandwidth: 20.5MHz
- Bit rate: 1kbps
- Transmit Power: 10W(10dBW)
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Figure 3: Transmitter diagram
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Based on these parameters, the intra-satellite link will experience a C/N ratio of 4.2dB and the WAAS system will have a worst-case C/N ratio of 1.2dB. The discrepancy between the two values is due to a lesser worst-case antenna gain on the WAAS receivers. This occurs because the WAAS antenna may have to track satellites that are as far as 80° off-axis, which corresponds to a gain of -3dB. |
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