GMSK Modem Data Pump
Page 23 of 37
MX909A PRELIMINARY INFORMATION
4.5.3 Interleaving
The 240 bits of a Mobitex¥ Data Block are interleaved by the modem before transmission to give protection
against noise bursts and short fades. Interleaving is not performed on any bits in the Mobitex¥ Frame Head.
Considering the 240 bits to be numbered sequentially before interleaving as 0 to 239
('0' = bit 7 of byte 0, '11' = bit 0 of FEC for byte 0, ... ,'239' = bit 0 of FEC for byte 19 - see Figure 6), then they
will be transmitted as shown in Figure 13.
The modem performs the inverse operation (de-interleaving) in receive mode on Mobitex¥ Data Blocks.
INPUT DATA
DATA BLOCK
INPUT DATA
RESULTING FEC
Byte
0
7
0
6
1
5
2
4
3
3
4
2
5
1
6
0
7
3
8
2
9
1
0
loaded first
10 11
1
2
12 13 14 15 16 17 18 19
24 25 26 27 28 29 30 31
36 37 38 39 40 41 42 43
48 49 50 51 52 53 54 55
60 61 62 63 64 65 66 67
72 73 74 75 76 77 78 79
84 85 86 87 88 89 90 91
96 97 98 99 100 101 102 103
108 109 110 111 112 113 114 115
120 121 122 123 124 125 126 127
132 133 134 135 136 137 138 139
144 145 146 147 148 149 150 151
156 157 158 159 160 161 162 163
168 169 170 171 172 173 174 175
180 181 182 183 184 185 186 187
192 193 194 195 196 197 198 199
204 205 206 207 208 209 210 211
216 217 218 219 220 221 222 223
228 229 230 231 232 233 234 235
20 21 22 23
32 33 34 35
44 45 46 47
56 57 58 59
68 69 70 71
80 81 82 83
92 93 94 95
104 105 106 107
116 117 118 119
128 129 130 131
140 141 142 143
152 153 154 155
164 165 166 167
176 177 178 179
188 189 190 191
200 201 202 203
212 213 214 215
224 225 226 227
236 237 238 239
3
4
5
6
7
8
9
10
11
12
13
14
15
16
INTERLEAVED OUTPUT TO
LOW PASS FILTER
loaded last 17
CRC 18
CRC 19
FIRST OUT
0
1
2
3
4
5
6
7
8
9
10
22
11
23
12
13
14
15
16
17
18
19
20
21
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
112
124
136
148
160
172
184
196
208
220
232
101
113
125
137
149
161
173
185
197
209
221
233
102
114
126
138
150
162
174
186
198
210
222
234
103
115
127
139
151
163
175
187
199
211
223
235
104
116
128
140
152
164
176
188
200
212
224
236
105
117
129
141
153
165
177
189
201
213
225
237
106
118
130
142
154
166
178
190
202
214
226
238
107
119
131
143
155
167
179
191
203
215
227
239
108
120
132
144
156
168
180
192
204
216
228
109
121
133
145
157
169
181
193
205
217
229
110
122
134
146
158
170
182
194
206
218
230
111
123
135
147
159
171
183
195
207
219
231
LAST OUT
Figure 13: Interleaving - Input / Output
4.5.4 Scrambling
All formatted bits of a Mobitex¥ Data Block are passed through a 9-bit scrambler. This scrambler is initialized
at the beginning of the first Data Block in every Frame. The 511-bit sequence is generated with a 9-bit shift
register with the output of the 5th and 9th stages XOR'ed and fed back to the input of the first stage. The
scrambler is disabled during all other tasks, apart for TSO.
¤2001 MX-COM, Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480134.005
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