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Sat-Earth Link
 
- Downlink
 
 
 
 
 
   
   
   

 
     
  Downlink  
 


In selecting equipment for the link between the Mars GPS satellites and the DSN, the downlink was the first to be analyzed. This was necessary because transmit power is highly limited on satellites, and the resulting received S/N is typically lower on the downlink. For this reason, a higher carrier frequency was chosen for the downlink (32GHz vs 34.5GHz). The following design choices were made for the downlink:

  • Center Frequency: 34.5Ghz
  • Bandwidth: 100MHz
  • Satellite Antenna: 3m (diameter), Gain: 60.5dB
  • Transmit Power: 20W(13.0dB)
  • Rain Attenuation: 3dB adjustment (99.9% of the time)
  • Design Margin: 2dB
  • Bit Rate: 12 Mbit/s

The result of the Link budget calculations was a received C/N ratio of -8 dB. Conducting communications with under such a low C/R would be impossible with an uncoded link, and thus it was necessary to add turbo coding.

Turbo Coding

For a bitrate (H) of 12Mbit/s, and a Bandwidth of 100MHz, we should have no problem filling the spectrum with a 1/8 rate Turbo Coder (coding factor: 8) and a filter rolloff factor of 0.1.  The following formulas can be used to prove this.

  
 

 
 
Next Eb/No can be determined based on the following formula:		  
 

 
 

Since the value is greater than 0.9dB, this link will be capable of maintaining a BER that approaches zero.

  
 
Block Diagram 		 
 

Figure 5: Receiver Diagram
Figure 5: Receiver Diagram
  
     
  Error Correcting Code  
 
The turbo decoder is the hardest decoder to implement. The decoder is based on the Viterbi algorithm. This Turbo decoder is a soft decoder because the bit decisions are made knowing the probability of 0’s and 1’s in the previous received bit stream. We can use a Turbo decoder as following:		  
 

Figure 6: Base Station Decoder
Figure 6: Base Station Decoder