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From: | Nikita Airee |
Subject: | Re: [Discuss-gnuradio] Sync short in gr-ieee802-11 |
Date: | Wed, 12 Apr 2017 19:19:36 +0530 |
Hi Nikita,
I'll jump into the general_work method here, directly:
switch(d_state) { case SEARCH: { int i; for(i = 0; i < ninput; i++) { if(in_cor[i] > d_threshold) { if(d_plateau < MIN_PLATEAU) { d_plateau++; } else { d_state = COPY; … break; } … } consume_each(i); return 0; } case COPY: { int o = 0; while( o < ninput && o < noutput && d_copied < MAX_SAMPLES) { if(in_cor[o] > d_threshold) { if(d_plateau < MIN_PLATEAU) { d_plateau++; // there's another frame } else if(d_copied > MIN_GAP) { d_copied = 0; d_plateau = 0; … break; } … out[o] = in[o] * exp(gr_complex(0, -d_freq_offset * d_copied)); o++; d_copied++; } if(d_copied == MAX_SAMPLES) { d_state = SEARCH; } … consume_each(o); return o; } }So, this is a pretty classical state machine approach: at every call of general_work, the block is in either the COPY or the SEARCH state. We only care about whether COPY also does a search operation, and, lo:
if(d_plateau < MIN_PLATEAU) { d_plateau++; // there's another frame } else if(d_copied > MIN_GAP) { d_copied = 0; d_plateau = 0;is exactly that.
Best regards,
Marcus
On 12.04.2017 12:12, Nikita Airee wrote:
Hello everyone,
I was trying to understand how the synchronization in gr-ieee802-11 actually works and went through:
Bastian Bloessl, Michele Segata, Christoph Sommer and Falko Dressler, "An IEEE 802.11a/g/p OFDM Receiver for GNU Radio"
However, it seems that the Short Sync and many other blocks have developed further since then.
From what I understand, the current sync_short.cc does the following:
1) searches for the peak normalized correlator o/p and compares with the set threshold2) as enough number of simultaneous peaks are detected in correlator o/p, it moves onto the copy stage
Here is where my doubt lies. Does the block keep looking for new frames while the old one is being copied? Also, what is MIN_GAP's function in the code and how is its value decided upon?
Could you please clear up these doubts a little?
Bests,Nikita Airee
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