A real world testing using USRP in a free space environment, need help on some questions


Hello, all!

Our team is trying to find out the relationship between PER and Received
Signal Strength as well as the relationship between Distance and
Received Signal Strength under DBPSK modulation scheme using two USRP
borads. Our test was set up in a near free space environment, but the
result was counterintuitive. Below lists the result from our test: PER =
[7.602%, 0.583%, 0.2317%, 1.095%]; RSSI = [928.7573, 1296.6, 2386.2,
2587.7] which is the value of tb.rxpath.probe.level(); distance =
[260,240,220,200]. There are two things confusing me. As can be seen,
from 240feet to 220feet, the rssi was almost doubled, but the PERs were
almost the same. For the same 20feet difference in distance, the PER
declined from 7% to 0.5% while the rssi was only increased by 300. Why
is there a huge jump in RSSI when distance changed from 240 to 220
compared with the case when distance decreased from 260 to 240,
meanwhile the improvement of PER is not obvious? Why is there a huge
plunge in PER when RSSI was increased only by 300 and distance was
changed from 260feet to 240 feet? Is there any theshold or restriction
in the USRP board? Could anyone tell me the reason for our problems?
Thanks a lot!!!