collect 400 samples from a LFM (5MHz bandwidth) signal modulated on 18GHz

Your sampling rate is set at fs, which is about 20 MHz, this means your Nyquist is about 10 MHz, regardless what your carrier is, i.e., your  signal is aliased. If you want to see 18 GHz at the center, you will have to sample at 36 GHz, this is obviously very expensive. On the other hand, the first step in receiver is normally mix it down so the carrier frequency is really used for transmission, and does not affect the signal processing much (except we need to compensate for RF distortions). This is why we often simply simulate at base band because it captures all the band that interests us.