What's the limit for the data rate of QPSK modulation?

[D2004]

I have to design a mixer-signal QPSK demodulator for a wireless data link targetting max data rate.
I am just wondering what the limit for the data rate of QPSK is. Is it possible to achieve 5Mbps data rate on 13.5MHz carrier frequency with reasonable BER?

 

[flatulent]

As long as the spectrum does not go down past zero frequency you can do it. It is common practice to low pass filter the baseband data going into the modulator. The real limit is the legal one of what frequencies you can radiate and the practical limit of all of the other signals on the air that will interfere with your reception.

 

[D2004]

Hi flatulent,

I was also told that the carrier frequency must be greater than half the bandwidth, to avoid having the spectrum wrap around 0Hz.

Is it right?

How to understand "avoid having the spectrum wrap around 0Hz" or in your words "As long as the spectrum does not go down past zero frequency"?