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[Discuss-gnuradio] OFDM test with USRP

From: Henry Jin
Subject: [Discuss-gnuradio] OFDM test with USRP
Date: Wed, 18 Mar 2015 17:34:25 -0700

Hi everyone,

I have built a tx and a rx grc flowchart for OFDM using USRP. I found the range of the amplitude of the input is fairly small in order to get it working (i.e., the USRP at the rx end can successfully decode the packets.)  I wound if anyone can share some thoughts about this? I've been stuck here for several days. 

To quantify the packet reception performance, I modified digital.crc32_bb so that even when the crc test fails, the wrong packet is still fed to the output. In this way, I can evaluate the packet error rate (PER). Through my test, I found the const in multiply_const in the tx part (which corresponds to amplitude adjustment) needs to be (0.05, 0.025). In this range, the PER can be 0. However, when the const value increases, for example, to 0.06. the PER is around 0.0044. When we further increase the const value to 0.08, the PER is above 10% or even higher. I guess this is probably because the input signal amplitude of the USRP needs to be in a certain working range.  However, it seems to me this range is too small. Besides, how can I find some numerical statistics about the amplitude limitation? On the other hand, if the amplitude is decreased to 0.02, many packets will be failed to synchronize. But I do observed that the packets successfully synchronized are also correctly decoded. In other words, the PER is still 0, but the number of packets correctly decoded is actually much smaller than the number of packets sent. 

I also tried to enable the two disabled blocks in the files but it is of no help.

Thanks for any comments.


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