Can two modulations be used in the same frame 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?
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.