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

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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