Amplifiers, Back-Off and Modulation, QPSK

Greetings All-

I am trying to bring myself up to speed with things like EVM, Crest Factor, etc. I get the whole peak to average which dictates how much back off one might use. I don?t yet have a feel for what is good and what is poor in terms of quantifying how much I can get away with.

Currently I am dealing with a QPSK system that has a bidirectional amp out front. I am trying to put together some sort of spec on how these amplifiers should perform, mostly the PA. I can see the shoulders coming up as I drive them harder; see the compression point, etc. What I don?t have a feel for or can?t quantify is what kind of degradation I will have on the link if I drive the amplifiers at say 1-dB compression or 2 or 3 for that matter. If one were to ignore adjacent channel requirements, how hard could you drive the amplifier before things really far apart? What the symptoms present themselves at all distances or just when you might try to extend the range?

Would appreciate any discussion on this or pointing me toward some good discussion on how/why certain criteria for back-off/linearity is arrived at.

Thanks!

See The Designer's Guide Community Forum - Two tone test for transmitter / PA