shallow zip-file copy from codec2 e9d726bf20

1 files changed, 38 insertions, 0 deletions

diff --git a/octave/doppler_spread.m b/octave/doppler_spread.m
new file mode 100644
index 0000000..b6fccf5
--- /dev/null
+++ b/octave/doppler_spread.m
@@ -0,0 +1,38 @@
+% doppler_spread.m
+% David Rowe Jan 2016
+%
+% Returns gausssian filtered doppler spreading function samples for HF channel
+% modelling.  Used PathSim technical guide as a reference - thanks Moe!
+
+function [spread_FsHz states] = doppler_spread(dopplerSpreadHz, FsHz, Nsam)
+
+  % start with low Fs so we can work with a reasonable filter length
+
+  sigma = dopplerSpreadHz/2;
+  lowFs = ceil(10*dopplerSpreadHz);
+  Ntaps = 100;
+  Nsam_low = ceil(Nsam*lowFs/FsHz + Ntaps); % fill filter memory
+  
+  % generate gaussian freq response and design filter
+
+  x = 0:lowFs/100:lowFs/2;
+  y = (1/(sigma*sqrt(2*pi)))*exp(-(x.^2)/(2*sigma*sigma));
+  b = fir2(Ntaps-1, x/(lowFs/2), y);
+  
+  % generate the spreading samples
+
+  spread_lowFs = filter(b,1,randn(1,Nsam_low) + j*randn(1,Nsam_low));
+
+  % resample to FsHz, scaling for desired spreadFreqHz
+
+  spread_FsHz = resample(spread_lowFs(Ntaps+1:Nsam_low), FsHz, lowFs);
+  assert(length(spread_FsHz) >= Nsam);
+  
+  % return some states for optional unit testing
+  states.x = x;
+  states.y = y;
+  states.b = b;
+
+endfunction
+
+