Can two modulations be used in the same frame
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.