From: drowe67 <drowe67@01035d8c-6547-0410-b346-abe4f91aad63>
Date: Thu, 31 Jul 2014 06:41:15 +0000 (+0000)
Subject: re-arranged mod and demod so shift up and down from carrier freq is a sep step. ... 
X-Git-Url: http://git.whiteaudio.com/gitweb/?a=commitdiff_plain;h=77daaaaa330bf9d76209c47ea386b0e1838a2c4f;p=freetel-svn-tracking.git

re-arranged mod and demod so shift up and down from carrier freq is a sep step.  fdmdv_ut runs OK.  Now to add LPF.  En route to reducing CPU load of rx filter

git-svn-id: https://svn.code.sf.net/p/freetel/code@1773 01035d8c-6547-0410-b346-abe4f91aad63
---

diff --git a/codec2-dev/octave/fdmdv.m b/codec2-dev/octave/fdmdv.m
index 9c67df4f..e70ec427 100644
--- a/codec2-dev/octave/fdmdv.m
+++ b/codec2-dev/octave/fdmdv.m
@@ -52,6 +52,23 @@ global sync_uw = [1 -1 1 -1 1 -1];
 global gt_alpha5_root;
 gt_alpha5_root = gen_rn_coeffs(alpha, T, Rs, Nsym, M);
 
+% rx decimation filter
+
+global Nrxdec;
+       Nrxdec=31;
+global rxec;
+       rxdec = fir1(Nrxdec, 0.25);
+if 0
+  % tmp code to plot freq resp.  20dB attn of any aliases should be fine
+  % not real sensitive to in-band attn, e.g. outer tones a dB down should be OK
+  % in terms of BER
+  figure(1)
+  [h,f]=freqz(rxdec,1,4000);
+  hdB=20*log10(abs(h));
+  plot(hdB(1:1200))
+  grid
+end
+
 % Converts gray code to natural binary
 
 global m4_gray_to_binary = [
@@ -185,10 +202,12 @@ function tx_fdm = fdm_upconvert(tx_filt)
   global Fsep;
   global phase_tx;
   global freq;
+  global fbb_rect;
+  global fbb_phase_tx;
 
   tx_fdm = zeros(1,M);
 
-  % Nc/2 tones below centre freq
+  % Nc/2 tones below zero
   
   for c=1:Nc/2
       for i=1:M
@@ -197,7 +216,7 @@ function tx_fdm = fdm_upconvert(tx_filt)
       end
   end
   
-  % Nc/2 tones above centre freq
+  % Nc/2 tones above zero
 
   for c=Nc/2+1:Nc
       for i=1:M
@@ -215,6 +234,13 @@ function tx_fdm = fdm_upconvert(tx_filt)
     tx_fdm(i) = tx_fdm(i) + pilot(i);  
   end
  
+  % shift up to carrier freq
+
+  for i=1:M
+    fbb_phase_tx *= fbb_rect;
+    tx_fdm(i)     = tx_fdm(i) * fbb_phase_tx;  
+  end
+
   % Scale such that total Carrier power C of real(tx_fdm) = Nc.  This
   % excludes the power of the pilot tone.
   % We return the complex (single sided) signal to make frequency
@@ -222,7 +248,7 @@ function tx_fdm = fdm_upconvert(tx_filt)
 
   tx_fdm = 2*tx_fdm;
 
-  % normalise digital oscilators as the magnitude can drift over time
+  % normalise digital oscillators as the magnitude can drift over time
 
   for c=1:Nc+1
     mag = abs(phase_tx(c));
@@ -998,6 +1024,23 @@ function rn_file(filename)
   fclose(f);
 endfunction
 
+
+% Saves rx decimation filter coeffs to a text file in the form of a C array
+
+function rxdec_file(filename)
+  global rxdec;
+  global Nrxdec;
+
+  f=fopen(filename,"wt");
+  fprintf(f,"/* Generated by rxdec_file() Octave function */\n\n");
+  fprintf(f,"const float rxdec[]={\n");
+  for m=1:Nfilter-1
+    fprintf(f,"  %g,\n",rxdec(m));
+  endfor
+  fprintf(f,"  %g\n};\n",rxdec(Nrxdecr));
+  fclose(f);
+endfunction
+
 function pilot_coeff_file(filename)
   global pilot_coeff;
   global Npilotcoeff;
@@ -1086,18 +1129,28 @@ global freq;
 global freq_pol;
        freq_pol = zeros(Nc+1,1);
 for c=1:Nc/2
-  carrier_freq = (-Nc/2 - 1 + c)*Fsep + Fcentre;
+  %carrier_freq = (-Nc/2 - 1 + c)*Fsep + Fcentre;
+  carrier_freq = (-Nc/2 - 1 + c)*Fsep;
   freq_pol(c)  = 2*pi*carrier_freq/Fs;
   freq(c)      = exp(j*freq_pol(c));
 end
 for c=Nc/2+1:Nc
-  carrier_freq = (-Nc/2 + c)*Fsep + Fcentre;
+  %carrier_freq = (-Nc/2 + c)*Fsep + Fcentre;
+  carrier_freq = (-Nc/2 + c)*Fsep;
   freq_pol(c)  = 2*pi*carrier_freq/Fs;
   freq(c)      = exp(j*freq_pol(c));
 end
 
-freq_pol(c)  = 2*pi*Fcentre/Fs;
-freq(Nc+1) = exp(j*freq_pol(c));
+%freq_pol(Nc+1)  = 2*pi*Fcentre/Fs;
+freq_pol(Nc+1)  = 2*pi*0/Fs;
+freq(Nc+1) = exp(j*freq_pol(Nc+1));
+
+global fbb_rect;
+       fbb_rect = exp(j*2*pi*Fcentre/Fs);
+global fbb_phase_tx;
+       fbb_phase_tx = 1;
+global fbb_phase_rx;
+       fbb_phase_rx = 1;
 
 % Spread initial FDM carrier phase out as far as possible.  This
 % helped PAPR for a few dB.  We don't need to adjust rx phase as DQPSK
diff --git a/codec2-dev/octave/fdmdv_ut.m b/codec2-dev/octave/fdmdv_ut.m
index 398c8223..b6d685ba 100644
--- a/codec2-dev/octave/fdmdv_ut.m
+++ b/codec2-dev/octave/fdmdv_ut.m
@@ -12,8 +12,8 @@ fdmdv;               % load modem code
  
 % Simulation Parameters --------------------------------------
 
-frames = 100;
-EbNo_dB = 7.3;
+frames = 20;
+EbNo_dB = 73;
 Foff_hz = -100;
 modulation = 'dqpsk';
 hpa_clip = 150;
@@ -169,6 +169,13 @@ for f=1:frames
   % Demodulator
   % -------------------
 
+  % shift down to complex baseband
+
+  for i=1:M
+    fbb_phase_rx = fbb_phase_rx*fbb_rect';
+    rx_fdm(i) = rx_fdm(i)*fbb_phase_rx;
+  end
+
   % frequency offset estimation and correction, need to call rx_est_freq_offset even in sync
   % mode to keep states updated
   
@@ -187,14 +194,10 @@ for f=1:frames
     rx_fdm(i) = rx_fdm(i)*foff_phase;
   end
 
-if 1
-  % more memory efficient but more complex
+  % LP filter
+  % Decimate to rate M/4
+
   rx_filt = down_convert_and_rx_filter(rx_fdm, M);
-else
-  rx_baseband = fdm_downconvert(rx_fdm, M);
-  rx_baseband_log = [rx_baseband_log rx_baseband];
-  rx_filt = rx_filter(rx_baseband, M);
-end
 
   [rx_symbols rx_timing] = rx_est_timing(rx_filt, M);
   rx_timing_log = [rx_timing_log rx_timing];