Carrier frequency for QPSK Modulator?

In order to implement a QPSK modulator what can be the minimum local Oscillator frequency for 400 Mbit/sec data rate.

It is hard to tell what is the min LO power. This will depend on the modulator. But it can be predicted if you know the output power. It is normally +/- 5dBm of the output power.

QPSK modulation scheme breaks the data stream in two parallel data streams with a 1/2 data rate. If you are regarding 400MB as the main data stream, you will have applied to the modulator I and Q 200MB, what means a maximum modulating frequency equal to 100MHz. Therefore, theoretically, you would be able to use a 100MHz carrier.
However by using this minimum frequency you will have the following problems:
1- The amplifier chain in the transmission end would have to be very linear over several octaves, what is a hard to achieve;
2- To demodulate such signal you would have to have an ultralinear demodulator from DC to 100MHz what again is very hard, if not impossible, to get.

I think if you use OSQPSK, you would be better off to work with this low carrier frequency, once that is a modulation scheme with constant envelop.

NandoPG

th LO power depend your choise of the MODULATOR,or the mixer