Comtech EF Data MIDAS Version 4.X NMS Configuration - Manual (Page 137)

MIDAS 4.X NMS Configuration, Rev. 6
Frequency Translation A-5
Ensure RX Center is above the Transponder Center it would be:
11.7325 GHz – 11.72 GHz = 12.5 MHz above Transponder Center
Add (or subtract) this offset from the IF Center (70 MHz) to find the RX Center IF fre-
quency:
70 MHz + 12.5 MHz = 82.5 MHz
Ensure 2.4 MHz of bandwidth, the beginning and end of the space segment is determined
by the following formula:
RX IF Center – 1/2 bandwidth = Space Segment Beginning IF
(82.5 MHz – 1.2 MHz = 81.3 MHz)
RX IF Center + 1/2 bandwidth = Space Segment Ending IF
(82.5 MHz + 1.2 MHz = 83.7 MHz)
Select Space Segment, view IF. Right click on any area in Free Space (YELLOW), and
select Add Bandwidth. Select IF, and add Start 81.300000 and End 83.700000. Close
Space Segment window and Select Control Channel window will appear.
Control Channel
Setup
The Outbound and Inbound Control Channels are both 19.2 kbps QPSK 1/2 modulated
carriers. Modulated carriers have a symbol rate, which is the amount of bandwidth that the
transmitted data and forward error correction (FEC) will occupy. An FEC of 1/2 means
that for every bit of data there is an additional FEC bit added, so 1 data bit in will produce
2 symbol bits out. QPSK modulation has the effect of transmitting 2 symbol bits per each
Hz of bandwidth occupied. So, at QPSK 1/2, the data rate of the carrier is equal to the sym-
bol rate.
It is best to place the Control Channel at the beginning of the Public Pool to achieve the
highest amount of bandwidth efficiency. The MIDAS system will automatically use a fre-
quency resolution of 5 kHz when determining the placement of the Control Channels. If
the carriers were spaced 25 kHz apart, the carrier spacing would be:
Carrier Spacing = Distance between Center frequencies in kHz/symbol rate kHz
(25 kHz/ 19.2 kHz)
Carrier Spacing = 1.302
If the carriers were spaced 30 kHz apart, the carrier spacing would be:
Carrier Spacing = 30 kHz/ 19.2 kHz
Carrier Spacing = 1.5625
Since the Control Channels are critical to the MIDAS system, it is recommended to space
the carriers 30 kHz apart. Now that the Control Channel spacing is known, the only thing
that needs to be determined is where to place the center of the Outbound Channel at the