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Average is sort of a misnomer- the pkt detector model just adds together the last N samples (an average without the division). N is programmed with the ofdm_pktDetector_mimo_WriteReg_pktDet_avgLen() function from the ofdm_pktDetector_opbw driver. The various RSSI-based thresholds in the pkt detector are set relative to this sum. So in the reference design, where we set N to 16 and the pkt detection threshold to 7000, this corresponds to a threshold of 438 (7000/16) of actual average RSSI, which is a little shy of half of the RSSI's peak value of 1023.

The detection thresholds are programmed via registers. You need to call ofdm_pktDetector_mimo_WriteReg_pktDet_avgThresh and ofdm_pktDetector_mimo_WriteReg_csma_avgThresh when you change the sum length (using ofdm_pktDetector_mimo_WriteReg_pktDet_avgLen). See warpmac.c for how we use each of the functions to initialize the packet detector for the reference design.