Calculating satellite capacity savings when considering two options of SCPC modems can be done in two ways:
Link Budgets (LBs): Redo link budgets based on the new modem performance curves. The new LBs will allow the comparison of scenarios “A” and “B”.
Comparison Calculator: A very close approximation can be obtained quickly without redoing LBs. Assuming LBs were already done for scenario “A”, the comparison calculator attached with this post (courtesy of Datum Systems) can be used to iterate between a number of modem configurations towards the goal of maximizing spectrum savings.
The following 5-step procedure can be quickly applied in any SCPC scenario to calculate capacity savings. Output can then be used to determine the payback period when replacing legacy modems.
1- Gather Scenario “A” configuration: Obtain the following information about the Scenario “A” SCPC links (outbound and inbound):
- Codec: Viterbi, Turbo, R/S, etc.
- Link Data Rate (in Kbps)
- Modulation mode (MOD): QPSK, 8PSK, etc.
- FEC Rate (CODE): 1/2, ¾, 7/8, etc.
- Carrier Spacing: 1.2x, 1.35x, etc.
2- Obtain Modem Performance: Given Scenario “A” configuration, obtain the required Eb/No at 1E-8 BER from modem manual (SCPC modem manufacturers post modem manuals on their web sites).
- Also take note of the end-to-end coding latency for given MODCOD configuration (manufacturers report this at 64kbps). This can be used to also compare not just savings but also end-to-end processing latency.
3- Input Scenario “A” Data: Input required Eb/No (@BER 1E-8) and processing latency (@64Kbps) in raws 6 and 7 of the comparison calculator (outbound and inbound carriers)
4- Select “Other” in cells D10 and D15 – this forces the calculator to take the values inputted in raws 6 and 7 for each link direction. Select modulation in C10 and C15 based on current scenario.
5- Iterate to Find Best Scenario “B”: Iterate with various Datum configurations to achieve maximum savings using same spectral power. We recommend evaluating advanced options such as LDPC 16k-block (cells D/E11 and D/E16). Throughout the iterations, seek to keep the carrier C/N constant, so that capacity vacated goes with its share of spectral power. Hints to determine configuration for maximum savings:
- Start Scenario “B” with higher order modulation (e.g. 8QAM/8PSK if scenario A uses QPSK) and least robust FEC rates (e.g. 16/17).
i. If C/N is lower than in config A, jump to an even higher modulation order and repeat process.
ii. If C/N is higher than in config A, then increase FEC robustness until C/Ns are similar.
iii. If the most robust FEC code does not achieve this, then step back and use same modulation as in scenario A. Then, starting with the least robust FEC, seek the FEC rate that provides same C/N as in A. The comparison will provide a calculation of savings (I12, I17 and I22).
- End-to-end processing latency is a function of the link data rate and code block size.
i. The general rule of thumb is to start with the maximum block size (16K in the case of Datum) and see if the processing latency is acceptable.
ii. If not, the block size can be decreased until the desired latency is found. The larger the block size, the more efficient the error correction process so it is always advisable to use the block size that maximizes efficiency without latency impacting the transported application.
iii. Typically, for links at or above 500Kbps, the delay introduced 16K blocks are acceptable and negligible towards high bit rates.
Question: What other tools or shortcuts do you use to quickly calculate incremental efficiencies to determine performance for new projects or modem replacements?
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