5. Communication Links : Page 1
The link is designed basically for the relay of information from the observatory on the far side of the moon to the Earth. The measurements of the observatory will be sent to the Earth via the relay satellite to earth link continuously. The information from the Earth to the observatory will also be sent via the system. The uplink frequency is 37.750 GHz (from the observatory to the satellite) and the downlink frequency is 33.250 GHz (from the satellite to the earth).
The modulation scheme that will be used in the system is Quadrature Phase Shift Keying (QPSK) which modulates the carrier signal with two data bits per symbol through the use of In phase and Quadrature channels.
Figure 9 : Signal Constellation for π/4 QPSK
Since the link will operate a high bit rate, a very low BER is required in order to provide the relay of measurements continously without any loss. The BER of the system will be 10-15 . Turbo Codes will be used in the system and the C/N in the satellite transponder and earth station input will be set to 5 dB. The BER is to be provided by using Turbo Product Codes [ref]. A margin of 3 dB for QPSK and an implementation margin is added to the C/N.
The link budget is calculated by using (1)
The antenna gain of the system is calculated by using (2)
Noise power is calculated by using (3)
The transmitted power is calculated by using (4)
Page 2