1 //
2 // gmskmodem_example.c
3 //
4
5 #include <stdio.h>
6 #include <stdlib.h>
7 #include <getopt.h>
8 #include <math.h>
9 #include "liquid.h"
10
11 #define OUTPUT_FILENAME "gmskmodem_example.m"
12
13 // print usage/help message
usage()14 void usage()
15 {
16 printf("gmskmodem_example -- Gaussian minimum-shift keying modem example\n");
17 printf("options (default values in <>):\n");
18 printf(" u/h : print usage/help\n");
19 printf(" k : samples/symbol, default: 4\n");
20 printf(" m : filter delay [symbols], default: 3\n");
21 printf(" n : number of data symbols, default: 200\n");
22 printf(" b : bandwidth-time product, 0 <= b <= 1, default: 0.3\n");
23 printf(" s : SNR [dB], default: 30\n");
24 }
25
main(int argc,char * argv[])26 int main(int argc, char*argv[]) {
27 // options
28 unsigned int k=4; // filter samples/symbol
29 unsigned int m=3; // filter delay (symbols)
30 float BT=0.3f; // bandwidth-time product
31 unsigned int num_data_symbols=200; // number of data symbols
32 float SNRdB = 30.0f; // signal-to-noise ratio [dB]
33 float phi = 0.0f; // carrier phase offset
34 float dphi = 0.0f; // carrier frequency offset
35
36 int dopt;
37 while ((dopt = getopt(argc,argv,"uhk:m:n:b:s:")) != EOF) {
38 switch (dopt) {
39 case 'u':
40 case 'h': usage(); return 0;
41 case 'k': k = atoi(optarg); break;
42 case 'm': m = atoi(optarg); break;
43 case 'n': num_data_symbols = atoi(optarg); break;
44 case 'b': BT = atof(optarg); break;
45 case 's': SNRdB = atof(optarg); break;
46 default:
47 exit(1);
48 }
49 }
50
51 // validate input
52 if (BT <= 0.0f || BT >= 1.0f) {
53 fprintf(stderr,"error: %s, bandwidth-time product must be in (0,1)\n", argv[0]);
54 exit(1);
55 }
56
57 // derived values
58 unsigned int num_symbols = num_data_symbols + 2*m;
59 unsigned int num_samples = k*num_symbols;
60 float nstd = powf(10.0f,-SNRdB/20.0f); // noise standard deviation
61
62 // create modulator
63 gmskmod mod = gmskmod_create(k, m, BT);
64 gmskmod_print(mod);
65
66 // create demodulator
67 gmskdem demod = gmskdem_create(k, m, BT);
68 gmskdem_set_eq_bw(demod, 0.01f);
69 gmskdem_print(demod);
70
71 unsigned int i;
72 unsigned int s[num_symbols];
73 float complex x[num_samples];
74 float complex y[num_samples];
75 unsigned int sym_out[num_symbols];
76
77 // generate random data sequence
78 for (i=0; i<num_symbols; i++)
79 s[i] = rand() % 2;
80
81 // modulate signal
82 for (i=0; i<num_symbols; i++)
83 gmskmod_modulate(mod, s[i], &x[k*i]);
84
85 // add channel impairments
86 for (i=0; i<num_samples; i++) {
87 y[i] = x[i]*cexpf(_Complex_I*(phi + i*dphi));
88 y[i] += nstd*(randnf() + _Complex_I*randnf())*M_SQRT1_2;
89 }
90
91 // demodulate signal
92 for (i=0; i<num_symbols; i++)
93 gmskdem_demodulate(demod, &y[k*i], &sym_out[i]);
94
95 // destroy modem objects
96 gmskmod_destroy(mod);
97 gmskdem_destroy(demod);
98
99 // print results to screen
100 unsigned int delay = 2*m;
101 unsigned int num_errors=0;
102 for (i=delay; i<num_symbols; i++) {
103 //printf(" %4u : %2u (%2u)\n", i, s[i-delay], sym_out[i]);
104 num_errors += (s[i-delay] == sym_out[i]) ? 0 : 1;
105 }
106 printf("symbol errors : %4u / %4u\n", num_errors, num_data_symbols);
107
108 // write results to output file
109 FILE * fid = fopen(OUTPUT_FILENAME,"w");
110 fprintf(fid,"%% %s : auto-generated file\n", OUTPUT_FILENAME);
111 fprintf(fid,"clear all\n");
112 fprintf(fid,"close all\n");
113 fprintf(fid,"k = %u;\n", k);
114 fprintf(fid,"m = %u;\n", m);
115 fprintf(fid,"BT = %f;\n", BT);
116 fprintf(fid,"num_symbols = %u;\n", num_symbols);
117 fprintf(fid,"num_samples = %u;\n", num_samples);
118
119 fprintf(fid,"x = zeros(1,num_samples);\n");
120 fprintf(fid,"y = zeros(1,num_samples);\n");
121 for (i=0; i<num_samples; i++) {
122 fprintf(fid,"x(%4u) = %12.8f + j*%12.8f;\n", i+1, crealf(x[i]), cimagf(x[i]));
123 fprintf(fid,"y(%4u) = %12.8f + j*%12.8f;\n", i+1, crealf(y[i]), cimagf(y[i]));
124 }
125 fprintf(fid,"t=[0:(num_samples-1)]/k;\n");
126 fprintf(fid,"figure;\n");
127 fprintf(fid,"plot(t,real(y),t,imag(y));\n");
128
129 // artificially demodulate (generate receive filter, etc.)
130 float hr[2*k*m+1];
131 liquid_firdes_gmskrx(k,m,BT,0,hr);
132 for (i=0; i<2*k*m+1; i++)
133 fprintf(fid,"hr(%3u) = %12.8f;\n", i+1, hr[i]);
134 fprintf(fid,"z = filter(hr,1,arg( ([y(2:end) 0]).*conj(y) ))/k;\n");
135 fprintf(fid,"figure;\n");
136 fprintf(fid,"plot(t,z,t(k:k:end),z(k:k:end),'or');\n");
137 fprintf(fid,"grid on;\n");
138
139 fclose(fid);
140 printf("results written to '%s'\n", OUTPUT_FILENAME);
141
142 return 0;
143 }
144