USRP Receive problem. (20MHz sample rate)

I want to use USRP to receive real 802.11a/g wifi signal, so I set
sample rate 20MHz and appropriate centre frequency. But frequency
spectrum of the receiving OFDM signal is strange. (see picture:

Besides that, I did another experiment. using a USRP generate a 300kHz
bandwith OFDM signal, anther receiving end also using 300kHz sample
rate, and we can see a perfect OFDM frequency spectrum wave.(see
picture: fft300.jpg)

Does anyone know why sample rate increase, OFDM frequency in receiving
end turn into this shape?

USRP: N210
Daughter board: xcvr2450