How the channel impulse response obtained mathematically

I am using the Decawave Trek toolkit in my experiments. I use one board as the transmitter to send a packet. In the receiver, I get CIR by reading data from accumulator.

I don’t understand why there is only one impulse after plotting the CIR figure. I read the UWB protocol in 802.15.4 and there should be multiple impulses based on its preamble. So how is this CIR obtained?