1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
|
% qpsk.m
%
% David Rowe Sep 2015
%
% Octave functions to implement a QPSK modem
1;
% Gray coded QPSK modulation function
function symbol = qpsk_mod(two_bits)
two_bits_decimal = sum(two_bits .* [2 1]);
switch(two_bits_decimal)
case (0) symbol = 1;
case (1) symbol = j;
case (2) symbol = -j;
case (3) symbol = -1;
endswitch
endfunction
% Gray coded QPSK demodulation function
function two_bits = qpsk_demod(symbol)
bit0 = real(symbol*exp(j*pi/4)) < 0;
bit1 = imag(symbol*exp(j*pi/4)) < 0;
two_bits = [bit1 bit0];
endfunction
% Inserts pilot symbols a frame of symbols. The pilot symbols are
% spread evenly throughout the input frame.
function frameout = insert_pilots(framein, pilots, Npilotstep)
lpilots = length(pilots);
lframein = length(framein);
frameout = zeros(1, lframein + lpilots);
pin = 1; pout = 1; ppilots = 1;
while (lpilots)
%printf("pin %d pout %d ppilots %d lpilots %d\n", pin, pout, ppilots, lpilots);
frameout(pout:pout+Npilotstep-1) = framein(pin:pin+Npilotstep-1);
pin += Npilotstep;
pout += Npilotstep;
frameout(pout:pout) = pilots(ppilots);
ppilots++;
pout++;
lpilots--;
end
endfunction
% Removes the pilots symbols from a frame of symbols.
function frameout = remove_pilots(framein, pilots, Npilotstep)
frameout = [];
lpilots = length(pilots);
pin = 1; pout = 1;
while (lpilots)
%printf("pin %d pout %d lpilots %d ", pin, pout, lpilots);
%printf("pin+spacing-1 %d lvd %d lframein: %d\n", pin+spacing-1, lvd, length(framein));
frameout(pout:pout+Npilotstep-1) = framein(pin:pin+Npilotstep-1);
pin += Npilotstep+1;
pout += Npilotstep;
lpilots--;
end
endfunction
% Estimate and correct phase offset using a window of Np pilots around
% current symbol
function symbpilot_rx = correct_phase_offset(aqpsk, symbpilot_rx)
rx_pilot_buf = aqpsk.rx_pilot_buf;
Npilotstep = aqpsk.Npilotstep;
Nsymb = aqpsk.Nsymb;
for ns=1:Npilotstep+1:Nsymb
% update buffer of recent pilots, note we need past ones
rx_pilot_buf(1) = rx_pilot_buf(2);
next_pilot_index = ceil(ns/(Npilotstep+1))*(Npilotstep+1);
rx_pilot_buf(2) = symbpilot_rx(next_pilot_index);
% average pilot symbols to get estimate of phase
phase_est = angle(sum(rx_pilot_buf));
%printf("next_pilot_index: %d phase_est: %f\n", next_pilot_index, phase_est);
% now correct the phase of each symbol
for s=ns:ns+Npilotstep
symbpilot_rx(s) *= exp(-j*phase_est);
end
end
aqpsk.rx_pilot_buf = rx_pilot_buf;
endfunction
% builds up a sparse QPSK modulated version version of the UW for use
% in UW sync at the rx
function mod_uw = build_mod_uw(uw, spacing)
luw = length(uw);
mod_uw = [];
pout = 1; puw = 1;
while (luw)
%printf("pin %d pout %d puw %d luw %d\n", pin, pout, puw, luw);
pout += spacing/2;
mod_uw(pout) = qpsk_mod(uw(puw:puw+1));
puw += 2;
pout += 1;
luw -= 2;
end
endfunction
% Uses the UW to determine when we have a full codeword ready for decoding
function [found_uw corr] = look_for_uw(mem_rx_symbols, mod_uw)
sparse_mem_rx_symbols = mem_rx_symbols(find(mod_uw));
% correlate with ref UW
num = (mem_rx_symbols * mod_uw') .^ 2;
den = (sparse_mem_rx_symbols * sparse_mem_rx_symbols') * (mod_uw * mod_uw');
corr = abs(num/(den+1E-6));
found_uw = corr > 0.8;
endfunction
|
