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  • Can two modulations be used in the same frame?
  • For qam/qpsk/psk/ modulation scheme for the data bits can i have a fixed preamble with psk modulation?
  • does this help in achieving faster carrier freq lock and timing lock?
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  • $\begingroup$ Hi Steve! Can you ask the second question separately? It's really got little to do with the first, and also, 10ppm alone tells us nothing – you need to tell us way, way, way more about your system. $\endgroup$ – Marcus Müller Jun 1 at 20:28
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Can two modulations be used in the same frame.

Yes. Nobody's stopping you....

For qam/qpsk/psk/ modulation scheme for the data bits can i have a fixed preamble with psk modulation?

If you want, why not? It's actually pretty common that systems with a flexible modulation scheme (e.g. IEEE 802.11, "wlan") have a header symbol that's in a fixed modulation that tells the receiver what the rest of the frame is.

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  • $\begingroup$ Ok I will ask the second question seperately. Does the fixed modulation has advantages of fast lock and traxking?. So if the variable mod is used as preamble just like data does it need more preamble than the fixed mod preamble? $\endgroup$ – Steve Jun 1 at 21:02
  • $\begingroup$ that depends on the modulation, and how you intend to lock, no general statement can be given. As a logical rule: if something has a header, that header must be intact at the receiver, or else the receiver can do nothing with the whole frame. So, the header typically uses the most robust modulation, and in my WLAN example, that's BPSK. $\endgroup$ – Marcus Müller Jun 1 at 21:04
  • $\begingroup$ And can I use the same preamble length irrespective of the bandwidth I.e may be 10 kbps to 1 Mbps. Or does narrowband needs large preamble that wideband as carrier offsets affects narrowband more? $\endgroup$ – Steve Jun 1 at 21:17
  • $\begingroup$ Steve, you're constantly missing the point here: a preamble has a specific purpose. If your preamble fulfills the requirements that your transmission imposes on the preamble, it works. If it doesn't fulfill them, it doesn't work. I can't tell you need to do for your specific system – define your needs for the preamble, and you'll see that the constellation used is not usually part of these requirements. In fact, there's a lot of systems that don't use any modulation scheme for the preamble, just intentionally send a pseudo-white sequence, for example. $\endgroup$ – Marcus Müller Jun 1 at 21:28
  • $\begingroup$ Ok I will define for a particular system and post it seperately. So whole idea of preamble is to get the carrier and timing recovery done Which we can use a robust psk mod. So whatz left is phase ambiquity removal which can be used for a specific mod qam/qpsk etc. Is that right? Thanks for your valuable input Marcus -- Steve $\endgroup$ – Steve Jun 1 at 21:58
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It happens all the time in LTE, there are various channels within a frame and some channels have a fixed modulation scheme while others have varying. In a simpler system there could be only one physical channel and it would include a reference signal part to help the Receiver with channel estimation, frequency or time tracking etc. This part of the channel would have fixed modulation. The data part would have a varying modulation scheme based on channel quality conditions etc.

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