Added script that finds bursts, and extracts them from files

matlab/updated_scripts/extract_bursts_from_file.m

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+% Searches through the specified file for the first ZC sequence, and extracts the full bursts (in time)
+%
+% Because of the way that the ZC sequence works, you can be quite a long way off in frequency (signal not centered at 0
+% Hz in the recording) and still get a good correlation score.  So, the `frequency_offset` parameter can safely be set
+% to 0 even if there is an offset.  If you choose to do this, please lower the correlation threshold as the frequency
+% offset will have an effect on the correlation score.
+%
+% !!!NOTE!!! If the center frequency is off, then not only will the correlation score be lower, but the starting sample
+%            will also be incorrect.  So, if you know that your center frequency might be off by a lot, then make sure
+%            to add at least on OFDM symbol's worth of padding so that you can make sure that you see the entire burst
+%
+% @param input_path File containing complex 32-bit floating point samples (interleaved I,Q,I,Q,...)
+% @param sample_rate Sample rate that the file was recorded at.  Must be an integer multiple of 15.36 MSPS (the minimum 
+%                    sample rate for the DroneID downlink)
+% @param frequency_offset How far off from DC the signal is in the recording (set to 0 for no frequency adjustment)
+% @param correlation_threshold Score on a scale from 0.0 to 1.0 where 1.0 is a perfect match with the ZC sequence.  This
+%                              will determine how closely the recorded ZC sequence must match in order to be extracted
+%                              as a burst.  Usually anywhere from 0.2 to 0.9 are usable values.
+% @param chunk_size How many samples to process at one time.  This depends on how much RAM your system has.  This value
+%                   should likely be set > 1e6 but < 20e6.  But you do you.
+% @param padding How many additional samples before and after the burst to extract.  Must be >= 0
+% @return bursts A matrix where each row contains one burst
+function [bursts] = extract_bursts_from_file(input_path, sample_rate, frequency_offset, correlation_threshold,...
+    chunk_size, padding)
+    
+    lte_carrier_spacing = 15e3;                       % OFDM carrier spacing
+    fft_size = sample_rate / lte_carrier_spacing;     % Number of samples per OFDM symbol (minus cyclic prefix)
+    long_cp_len = round(1/192000 * sample_rate);      % Number of samples in the long cyclic prefix
+    short_cp_len = round(0.0000046875 * sample_rate); % Number of samples in the short cyclic prefix
+
+    % The first ZC sequence is the 4th symbol, and the `find_zc_indices_by_file` function will (assuming no major
+    % frequency offset) return the sample index of the first sample of the 5th OFDM symbol cyclic prefix.  So, back the
+    % index off by the number of samples in the first 4 OFDM symbols and their cyclic prefixes
+    zc_seq_offset = (fft_size * 4) + long_cp_len + (short_cp_len * 3);
+    
+    % Find all instances of the first ZC sequence
+    indices = find_zc_indices_by_file(input_path, sample_rate, frequency_offset, correlation_threshold, chunk_size);
+
+    % Number of samples that need to be extracted.  There are 9 OFDM symbols, 2 long and 7 short cyclic prefixes
+    burst_sample_count = (padding * 2) + (long_cp_len * 2) + (short_cp_len * 7) + (fft_size * 9);
+
+    bursts = zeros(length(indices), burst_sample_count); % Allocate storage for the bursts and padding for each
+
+    for idx=1:length(indices)
+        start_index = indices(idx);
+        
+        % Back the start index off by the padding and take `burst_sample_count` samples
+        burst = read_complex_floats(input_path, start_index - padding - zc_seq_offset, burst_sample_count);
+        bursts(idx,:) = burst;
+    end
+end
+