DW1000:carrier sense with RSSI

Hi, all.
I want to make a carrier sense with RSSI.
Before transmitting data, we need to check whether other terminal is transmitting data with the same preamble.
The method of calculating RSSI is written in 4.7.2 “Estimating the receive signal power” in DW1000 User manual.
And, in this, “C=channel impulse response Power value reported in the CIR_PWR field of Register file: 0x12 – Rx Frame Quality Information,” is described.

But, I think that this CIR power value is updated in the case of the success of preamble received.
I want to get RSSI in the case of no preamble detected.
How can I get RSSI when no preamble detected?