Satellite Telemetry Data Transmission Immunity from the ASI and Jamming Using DSSS Optimized PN Codes in DS-CDMA Systems

In the satellite telemetry command, controlling the power of the uplink signal, radiated away from the Earth to the spacecraft and downlink signal, radiated toward the Earth form the spacecraft is very important. The Uplink is transmitted at high power because of the unlimited power, while downlink should be controlled in power because of limited spacecraft power resources. This results in a lower Power Spectral Density (PSD) for the downlink and higher PSD for the uplink, which in many cases the uplink power due to be strong enough to reach the satellites at GEO would often also violate the PSD limits in the other orbits satellite telemetry signals like LEO. In addition due to long distance between the Earth and satellite, the received signals at the both terrestrial and transponder receivers are extremely week and strongly influenced by intentional interference (like jamming) or inadvertent interferences (such as the ASI). Spreading spectrum can solve the PSD problems for both directions but consumes a wide bandwidth. Fortunately, signals can be distinguished by using different spreading codes, allowing for CDMA. The Pseudo Noise (PN) spreading codes allow using many satellites or users (located in one satellite) the same frequency with overlapping signals simultaneously and also range measurement as useful ability to track the spacecraft in the space by the expanding value of the PN codes (Doppler Shift).[1] This paper specially considers to satellite telemetry data transmission immunity from the interference using Direct Sequence Spread Spectrum (DSSS) based on CDMA namely DS-CDMA (Direct Sequence-CDMA) by the unique properties correlation functions of the Gold and M-Sequence codes. Also the codes are examined in the fading channels, Reyleigh and AWGN, in terms of the BER vs. Eb/No to compare the practical results with theoretical values.