With advanced satellite modems closely approaching the limits of spectral efficiency and configuration flexibility, industry efforts have more recently focused on reducing the transmit and receive filter roll-off factor (carrier spacing) by as much as possible without impacting modems error correction capability. The combination of shaper roll-off filters and advanced FEC coding (like granular LDPC) can now bring faster replacement ROIs for service providers, even towards low link data rates.
The (downloadable) attached new comparison calculator developed by Datum Systems for its latest product line of M7 modems has added the capability of finely reducing the SCPC carrier spacing all the way down to 1.08 (only 8 % carrier spacing).
The calculation of 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” vs “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 can be used to iterate between a number of modem configurations towards the goal of maximizing spectrum savings. The ability to now play with the modem roll-off factor while keeping carrier C/N constant is a new capability incorporated into the comparison.
The following 5-step procedure can be applied to any SCPC scenario and 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 E10 and E15 – this forces the calculator to take the values inputted in rows 6 and 7 for each link direction. Select modulation in D10 and D15 based on current scenario. Also input the roll-off factor of the legacy modems (such as 1.35) for each direction in cells C10 and C15.
5- Iterate to Find Best Configuration for Scenario “B”: Iterate with various Datum configurations to achieve maximum savings using same spectral power (see hints below). We recommend evaluating advanced options such as LDPC 16k-block (cells E/F11 and E/F16) and sharp roll-off factors like 1.08. Throughout the iterations, seek to keep the carrier C/N constant (with respect to configuration A), so that capacity vacated goes with its share of spectral power (true savings).
Hints to determine best configuration for maximum savings:
- Start Scenario “B” with a sharp roll-off factor (such as 1.08) and 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, then 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 very close.
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”.
iv. Reducing the roll-off factor has an impact on the required C/N (the smaller the roll-off, the higher the C/N) and thus, in order to keep the C/N constant, it is usually needed to switch to a more robust FEC code rate.Thus, if the C/N of scenario “B” continues being higher than that of scenario A when using the most robust FEC rate (½), then increase the roll-off factor slightly (see manufacturer options) and reiterate.
This iterative process will output the exact modem configuration (combination of ModCod, Coding block size and roll-off factor) that maximizes the number of bits per hertz of spectrum, without an increase in C/N (real savings). The comparison will provide a calculation of savings in J12 (outbound), J17 (inbound) and J22 (total).
Calculation of end-to-end latency
- 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 latency is not acceptable, the block size can be reduced until the desired latency is found. The larger the block size, the more efficient the error correction process is 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 by 16 Kbit blocks is acceptable and negligible towards high bit rates. End-to end delay calculations are found in column M.
Note that by changing the LDPC block size, you will affect the coding efficiency and, thus, you may nee to re-iterate to find best ModCod configuration constrained by the required end-to-end latency.
In addition to use of advanced LDPC coding with configuration flexibility, the incorporation of granular roll-off filters as another configuration option can substantially reduce the required satellite bandwidth without an increase in spectral power. Bandwidth savings can then be used to calculate the payback period when evaluating the economics of replacing legacy modems.
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