find_fcch.m

The GSMSP uses the USRP hardware device to receive data from the GSM band. This data is raw and pre

function bcch_start = find_fcch(s, start, stop)

r = s(start:stop);

OSR = 1;
BL = 142*OSR;
TB = 3*OSR;
FL = 2 * TB + BL;
PLOTEXTRA = 25;

L = length(r);

da = angle(r(1:L-1) .* conj(r(2:L)));
sa = cumsum(da);

for i = 1 : (L-BL-1)
    low  = min(da(i:i+BL-1));    
    high = max(da(i:i+BL-1));    
    df(i) = high - low;
end

subplot(2,1,1);
plot(df);
hold off;
subplot(2,1,2);
plot(sa);

[x, i] = min(df);

if x < 1
  bcch_start = i + start - TB;
  plotframe2(s(bcch_start-PLOTEXTRA:bcch_start+FL+PLOTEXTRA));
else
  bcch_start = -1;
end