Re: [Discuss-gnuradio] Sync short in gr-ieee802-11

[Marcus Müller]

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 threshold

2) 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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