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[Discuss-gnuradio] a real world testing using USRP in a free space envir

From: Bill Stevenson
Subject: [Discuss-gnuradio] a real world testing using USRP in a free space environment, need help on some questions
Date: Fri, 13 Feb 2009 09:19:52 -0800 (PST)

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!!!


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