OFDM Tranceiver

I am trying to make an OFDM transceiver using my USRP1.
I used the existing examples of GRC. (tx_ofdm.grc and rx_ofdm.grc)

I am having problem in the communication while i transmit in HF band
using LFTX and LFRX daughterboards.

The model is working fine when i transmit at 700 MHz using flex900

Thanking in advance.

Usman Khalil