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Pls clarify concept of carrier recovery on receiver chain

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maolh

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Hi all,

My application is a satellite receiver with only one channel (not multi-channel application). Bit rate of digital message is 400bps. The specification says: "The data are encoded biphase-L or Manchester. The carrier is phase-modulated positive and negative 0.8 +/-0.025 radians peak, referenced to an unmodulated carrier. " Could you classify this modulation for me? It's BPSK (I don't think so), partial QPSK, CPM, or PAM, or something else? The specification gives me the impression that the carrier is directly digital-modulated by digital message without any pulse shaping, do you agree with me?

I also have some confusion on the concept of carrier recovery. Pls allow me to ask the following questions to make it clear for me:

1. For my application, do I need implement carrier recovery to demodulate signal?Why?
2. Carrier recovery and carrier synchronization are the same thing, right?
3. Which stage is the carrier recovery accomplished? RF, IF, or baseband, or needs be done on all three stages?
4. There is no preamble CW in my application, however, the transmitter always send HDLC frame starting with "7E" in hex. The maxium message frame length (including "7E") is 200bits. How can the carrier recovery be done for this case?

Thank you for your patience to read. I appreciate any comments and answers.
 

maolh

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Re: Pls clarify concept of carrier recovery on receiver chai

Nobody can give a hint?
 

biff44

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Re: Pls clarify concept of carrier recovery on receiver chai

Well, it is BPSK because it is "binary" (ie two phases). But usually BPSK is +/- Π radians! With only +/- 0.8 radians, it will look like it is a 4PSK, but only using 2 of the 4 phase states.

This, of couse, makes it harder to detect the bits, so you will have performance like it is a 4PSK (or 4 QAM if you prefer) system.

The only benefit over a full 4PSK system, is that the modulation trajectory does not go thru the origin, so the power amplifier up on the satellite does not have to operate with as much signal envelope peaking. Otherwise, I do not know why they would do things this way!

Yeah, you will have to recover the carrier for your demod.
 

    maolh

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teteamigo

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Re: Pls clarify concept of carrier recovery on receiver chai

Syncronization concepts:

Carrier Recovery -- the demodulator needs a signal inphase with the carrier.

Ex: s(t)*cos(wc*t) -->demodulate with generated carrier (don't matter the method like obtain it) gc(t)=cos(wc*t+phase)

When s(t)*cos(wc*t)*cos(wc*t+phase)=s(t)*[cos(wc*t)^2*cos(phase) - sen(2wc*t)*sen(phase)]

If phase=0 then SNR at entry of Digital Decision is maximum.


Clock recovery -- the digital decision needs a signal to sample the digital signal at discrete time events to minimize BER.
 

    maolh

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