Example Results - Comtech EF Data CDM-760 Installation And Operation Manual

Appendix L
CDM-760 Advanced High-Speed Trunking Modem
5. Basic performance validation:
a. Measure/record the DPD compensation at Site A.
b. Measure/record the Es/No at Site B.
c. Turn DPD function OFF at both sites and compare the measured Es/No and BER,
assuming the demodulator is able to achieve lock.
6. Initiate service
a. Turn DPD back to REMOTE mode at both sites. If the demodulator cannot relock, you
must repeat this procedure starting from Step 1, without returning to Step 5.
b. Disable BER test mode.
L.4.1.3

Example Results

As mentioned, DPD can provide a significant link margin increase. The increase comes in the
form of two separate link budget categories:
• First, the link margin required to accommodate transponder nonlinear effects may be
reduced.
•
Second, the transponder output back off (OBO) may be reduced.
Example results for a C-band, 36 MHz transponder are described here. Note the performance
improvement will vary from transponder to transponder and link to link depending on the exact
characteristics.
Figure L-10 and Figure L-11 display the total degradation reduction provided by digital
predistortion for 16-APSK and 32-APSK, respectively, with DVB-S2 EB1. The tests used a 5%
square-root raised cosine rolloff factor and 34.28 Msps. Additionally, the OBO was optimized for
each configuration, which varied between 1.2 dB and 3.2 dB. The balance between the reduced
margin for nonlinear distortion versus the reduction in the OBO is shown by the stacked bar
graph format.
Figure L-10. Link Margin Increase for 16-APSK DVB-S2 EB-1 ModCods, C-Band, 36MHz
Transponder
L–10
Revision 4
MN-CDM760