It seems like the real infomation only occupies part of the bandwidth and there are guard bands at each side of the band. So I think the alising into the infomation band is not effective at all. Any comments?
There is another thought here.
Since the bandwidth is PASSBAND bandwidth. It is RF bandwidth. If it is down converted to baseband then there is half bandwidth at each side of zero frequency. So the sampling rate is is enough and nyquist law is followed.