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I am wondering what are the big advantages and disadvantages using QPSK, FSK,and ASK in satellite communication. In our project, which is in phase 3, are we supposed to work with QPSK modulation for our cubesat? I don't really know why they choose to work with QPSK instead of FSK. Can anybody state the big difference between these two type of modulation?

Second question, in a cubesat since the link is very costly we should use FEC instead of retransmitting the data. In our case we used convolution encoding. I would like to know what the common code rate for this encoding is ( 1/2,3/2,..).

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    $\begingroup$ Hi mmmm123. I edited the title of your question because you have a better chance of a good answer if people know what you need from that alone, and the question's text provides details. For the same reason, i'd recommend you ask the second part as a separate question. It will get more attention, and it really is a different topic. A good answer to it will allow later visitors to find that information easily, if they need it. Welcome to Space Exploration. $\endgroup$ – kim holder May 27 '16 at 23:13
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Question 1 :

The three modulation schemes you mention are all forms of digital modulation. As you allude to, information can be encode in the amplitude, frequency, or phase of a signal (frequency is the time derivative of phase). For digital communication in general:

  • ASK is very inefficient and susceptible to interference for wireless (but can be useful over fiber optics or copper).
  • FSK is less susceptible to errors and interference than ASK because interference is often confined to a specific frequency.
  • PSK is the least susceptible to error of these three. In terms of bit error rate (BER), binary PSK is about twice as good as ASK and FSK in the presence of noise, and the BER can be significantly improved through other techniques.

QPSK has the very significant advantage that it can be used to double the data rate over the same bandwidth, or half the bandwidth for the same data rate. Further, as the number of signal states increases from 4 (quadrature) to N-ary, the trade-off can be furthered. As N increases the complexity of the receiver increases.

Question 2:

You will want to use a combination of FEC and re-transmission. FEC, or at the least error detection, should be used to know if a packet of data has been received correctly. Re-transmission WILL be required because it is inevitable that data is lost at some point, so the system should be able to request a packet be resent if it is incorrectly received.

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    $\begingroup$ thank you very much for the information. well right now my concentration is deciding the FEC, other people in my time are working in AX.25 which will do the Re-transmission.however, i still dont know how to decide which code rate i will use for convolutional encoder, is 1/2 most commonly in satellite communication or cubesat? $\endgroup$ – mmmm123 May 27 '16 at 22:43

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