For complex 1x mode, DFE applies IQ imbalance correction and then downsample the signal. So it allows 80%-90% of the signal bandwidth compare to the sampling rate.
For complex 2x mode, only half of the bandwidth (0~fs/2) is occupied by the signal, and the other half (the image band) can be occupied by other noise source like interference. So complex 2x mode is mainly designed for applications that needs interference cancellation.
