Forward link text compression in satellite messaging
View Patent ↗A method of transmitting communication information employing a coding scheme being defined by a character set. The characters of the character set are encoded into codewords of a sequence of bits, the codewords being of varying length. The characters are divided into at least two categories. The categories may be a first category of shifted characters and a second category of unshifted characters. Where a shifted character is encountered during encoding a shift codeword is inserted in the sequence of bits. This allows each category to use overlapping codewords. Then the sequence is converted to ASCII characters to be transmitted.
1. A method of transmitting information represented by characters of a defined character set over a communications channel, comprising the steps of:
dividing said character set into subsets;
encoding the characters in each subset with codewords of varying length dependent on the probability of occurrence of the encoded character, said subsets employing overlapping codewords, and each codeword within each said subset comprising a unique sequence of bits; and
transmitting a unique indicator to identify a said subset to which transmitted characters belong.
2. The method of claim 1 , wherein said unique indicator is a shift character indicating that subsequently transmitted characters are drawn from a different subset.
3. The method of claim 2 , wherein one of said subsets is a default subset, and said unique indicator is only transmitted prior to initial transmission of a character from a said subset different from the default.
4. The method of claim 2 , wherein the unique indicator is a high probability codeword.
5. The method of claim 4 , wherein said high probability codeword has four bits.
6. The method of claim 1 , wherein each codeword is limited to eight bits.
7. The method of claim 6 , further comprising the step of padding the sequence of bits with zeros to ensure the sequence contains an integer number of bytes.
8. The method of claim 5 , further comprising the step of converting the sequence of bits to ASCII characters for transmission over the communication channel.
9. The method as claimed in claim 1 , wherein the communications channel is a satellite forward link.
10. The method as claimed in claim 1 , wherein said characters are represented by the following sequences of bits:
Unshifted character
Shifted character
ASCII dec
ASCII hex
Character
Codeword
# bits
ASCII dec
ASCII hex
Character
69
45
E
000
3
48
30
0
32
20
Space
110
3
32
20
Space
65
41
A
1001
4
50
32
2
73
49
I
0110
4
52
34
4
78
4E
N
0101
4
53
35
5
79
4F
O
0111
4
51
33
3
83
53
S
0010
4
54
36
6
84
54
T
1011
4
49
31
1
NA
NA
Shift
0011
4
NA
NA
Un-shifted
67
43
C
01000
5
36
24
$
68
44
D
10100
5
57
39
9
72
48
H
11111
5
55
37
7
76
4C
L
10001
5
35
23
#
82
52
R
11110
5
56
38
8
70
46
F
101011
6
58
3A
:
71
47
G
100001
6
60
3C
<
77
4D
M
111001
6
44
2C
,
80
50
P
010011
6
33
21
!
85
55
U
111011
6
46
2E
.
87
57
W
111000
6
47
2F
/
89
59
Y
100000
6
62
3E
>
66
42
B
1110101
7
43
2B
+
86
56
V
0100101
7
61
3D
=
63
3F
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0100100
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45
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J
11101000
8
41
29
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75
4B
K
11101001
8
40
28
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81
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10101001
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88
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10101000
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38
26
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10101011
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10101010
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Control G.
11. A method of compressing textual data defined by characters forming part of a character set, wherein the characters are encoded by binary codewords of variable length, comprising the steps of:
dividing said character set into subsets;
encoding the characters in each subset with codewords of varying length dependent on the probability of occurrence of the encoded character, said subsets employing overlapping codewords, and each codeword within each said subset comprising a unique sequence of bits; and
inserting a unique indicator in the compressed data to identify the subset to which said encoded characters belong.
12. The method of claim 11 , wherein said unique indicator is a shift character indicating that subsequently encoded characters are drawn from a different subset.
13. The method of claim 12 , wherein one of said subsets is a default subset, and said unique indicator is only inserted prior to a string of one or more characters from a said subset different from the default.
14. The method of claim 12 , wherein the unique indicator is a high probability codeword.
15. The method of claim 14 , wherein said high probability codeword has four bits.
16. The method of claim 11 , wherein each codeword is limited to eight bits.
17. The method of claim 16 , further comprising the step of padding the sequence of bits with zeros to ensure the sequence contains an integer number of bytes.
18. The method of claim 16 , further comprising the step of converting the sequence of bits to ASCII characters for transmission over the communication channel.
19. The method of claim 11 , wherein said characters are represented by the following sequences of bits:
Unshifted character
Shifted character
ASCII dec
ASCII hex
Character
Codeword
# bits
ASCII dec
ASCII hex
Character
69
45
E
000
3
48
30
0
32
20
Space
110
3
32
20
Space
65
41
A
1001
4
50
32
2
73
49
I
0110
4
52
34
4
78
4E
N
0101
4
53
35
5
79
4F
O
0111
4
51
33
3
83
53
S
0010
4
54
36
6
84
54
T
1011
4
49
31
1
NA
NA
Shift
0011
4
NA
NA
Un-shifted
67
43
C
01000
5
36
24
$
68
44
D
10100
5
57
39
9
72
48
H
11111
5
55
37
7
76
4C
L
10001
5
35
23
#
82
52
R
11110
5
56
38
8
70
46
F
101011
6
58
3A
:
71
47
G
100001
6
60
3C
<
77
4D
M
111001
6
44
2C
,
80
50
P
010011
6
33
21
!
85
55
U
111011
6
46
2E
.
87
57
W
111000
6
47
2F
/
89
59
Y
100000
6
62
3E
>
66
42
B
1110101
7
43
2B
+
86
56
V
0100101
7
61
3D
=
63
3F
?
0100100
7
45
2D
−
74
4A
J
11101000
8
41
29
)
75
4B
K
11101001
8
40
28
(
81
51
Q
10101001
8
64
40
@
88
58
X
10101000
8
38
26
&
90
5A
Z
10101011
8
37
25
%
39
27
‘
10101010
8
7
07
Control G.
20. An apparatus for compressing textual data defined by characters forming part of a character set, wherein the characters are encoded by binary codewords of variable length, comprising:
an input device for receiving said characters;
an output device for outputting compressed data; and
a processor for encoding the characters in each subset with codewords of varying length dependent on the probability of occurrence of the encoded character, said subsets employing overlapping codewords, each codeword within each said subset comprising a unique sequence of bits, and inserting a unique indicator in the compressed data to identify the subset to which said encoded characters belong.
21. The apparatus of claim 20 , wherein said unique indicator is a shift character indicating that subsequently encoded characters are drawn from a different subset.
22. The apparatus of claim 21 , wherein one of said subsets is a default subset, and said unique indicator is only inserted prior to a string of one or more characters from a said subset different from the default.
23. The apparatus of claim 22 , wherein said processor pads the sequence of bits with zeros to ensure the sequence contains an integer number of bytes.
24. The apparatus of claim 22 , wherein said processor converts the sequence of bits to ASCII characters for transmission over the communication channel.
25. The apparatus of claim 20 , wherein said characters are represented by the following sequences of bits stored in memory:
Unshifted character
Shifted character
ASCII dec
ASCII hex
Character
Codeword
# bits
ASCII dec
ASCII hex
Character
69
45
E
000
3
48
30
0
32
20
Space
110
3
32
20
Space
65
41
A
1001
4
50
32
2
73
49
I
0110
4
52
34
4
78
4E
N
0101
4
53
35
5
79
4F
O
0111
4
51
33
3
83
53
S
0010
4
54
36
6
84
54
T
1011
4
49
31
1
NA
NA
Shift
0011
4
NA
NA
Un-shifted
67
43
C
01000
5
36
24
$
68
44
D
10100
5
57
39
9
72
48
H
11111
5
55
37
7
76
4C
L
10001
5
35
23
#
82
52
R
11110
5
56
38
8
70
46
F
101011
6
58
3A
:
71
47
G
100001
6
60
3C
<
77
4D
M
111001
6
44
2C
,
80
50
P
010011
6
33
21
!
85
55
U
111011
6
46
2E
.
87
57
W
111000
6
47
2F
/
89
59
Y
100000
6
62
3E
>
66
42
B
1110101
7
43
2B
+
86
56
V
0100101
7
61
3D
=
63
3F
?
0100100
7
45
2D
−
74
4A
J
11101000
8
41
29
)
75
4B
K
11101001
8
40
28
(
81
51
Q
10101001
8
64
40
@
88
58
X
10101000
8
38
26
&
90
5A
Z
10101011
8
37
25
%
39
27
‘
10101010
8
7
07
Control G.