Actual Overhead Rate Calculation - Radyne DMD20 Installation And Operation Manual

DMD20/DMD20 LBST Universal Satellite Modem
User Data
Rate
512,000
1,024,000
2,048,000
3,072,000
4,096,000
6,312,000
6,312,000
6,312,000

3.20.4 Actual Overhead Rate Calculation

The following is the actual calculation the modem does to calculate the overhead ratio:
1. The modem calculates the minimum in-band rate to limit the size of the user data
slots to 2,500 bits (the result is truncated to an integer).
Minimum In-Band = (User Data Rate * Control Ratio)/((Control Ratio + 1) * 250)
2. Using the bigger of Minimum In-Band or the selected In-Band, the modem
calculates the number of bits for each user data slot (result is truncated to an
integer).
Slot Bits = (User Data Rate * (Control Ratio * 10))/(In-band Rate * (Control Ratio + 1))
The actual ratio the modem uses is:
Actual Ratio = (Slot Bits + 10)/Slot Bits
Example 1:
User Data Rate:
In-Band Rate:
Control Ratio:
Minimum In-Band = (1,024,000 * 7)/((7 + 1) * 250) = 3,584
Slot Bits = (1,024,000 * (7 * 10))/(19,200 * (7 + 1)) = 466
Actual Ratio = (466 + 10)/466 = 1.021
Example 2:
User Data Rate:
In-Band Rate:
Control Ratio:
Minimum In-Band = (6,312,000 * 7)/((7 + 1) * 250) = 22,092 (
Slot Bits = (6,312,000 * (7 * 10))/(22,092 * (7 + 1)) = 2,500
TM103 – Rev. 2.5
In-Band Control
Rate Ratio
19,200 1/7
19,200 1/7
19,200 1/7
19,200 1/7
19,200 1/7
19,200 1/7
19,200 1/3
19,200 1/1
1,024,000 bps
19,200 bps
1 to 7
6,312,000 bps
19,200 bps
1 to 7
Theory of Operation
Aggregate Data Overhead
Rate
533,974
1,045,974
2,069,951
3,093,943
4,117,951
6,337,248
6,337,606
6,350,418
(less than In-Band Rate)
more than In-Band Rate)
Ratio
1.043
1.021
1.011
1.007
1.005
1.004
1.004
1.006
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