Documentation
DVB C/N Cheat Sheet
Quick reference guide to DVB-S2 and S2X thresholds, with the key ratios, roll-off penalties and symbol-rate-to-bitrate arithmetic.
- 2026-10-09 12:32
- 2026-10-13 12:14
- dB, weakest S2 mode−2,35
- dB, S2X VL-SNR floor−9,9
- PER at QEF (S2)10⁻⁷
- dB short-frame penalty0,2–0,3
Three ratios, not one
Es/N0
Per symbol. What every MODCOD table gives you.
Eb/N0
Per information bit. Eb/N0 = Es/N0 − 10·log₁₀(η)
C/N
Carrier over noise in a stated bandwidth. Meaningless without that bandwidth.
In symbol-rate bandwidth, C/N = Es/N0. In occupied bandwidth it is lower by 10·log₁₀(1+α) — so “C/N = 7 dB” means Es/N0 = 7 dB in one reading and 8,3 dB in the other at α = 0,35. Establish the reference bandwidth before comparing anything.
QPSK 1/4 has Es/N0 = −2,35 dB and Eb/N0 = +0,75 dB. Same link. The carrier really can sit below the noise floor — anyone who says otherwise is thinking in Eb/N0 while reading an Es/N0 number.
DVB-S2 — the modes you actually use
Ideal Es/N0 at QEF, PER = 10⁻⁷, AWGN, normal FECFRAME. Full 28-mode table in the in-depth version.
| Mode | η bit/sym | Es/N0 dB |
|---|---|---|
| QPSK 1/2 | 0,988858 | 1,00 |
| QPSK 2/3 | 1,322253 | 3,10 |
| QPSK 3/4 | 1,487473 | 4,03 |
| QPSK 5/6 | 1,654663 | 5,18 |
| 8PSK 3/5 | 1,779991 | 5,50 |
| 8PSK 2/3 | 1,980636 | 6,62 |
| 8PSK 3/4 | 2,228124 | 7,91 |
| 8PSK 5/6 | 2,478562 | 9,35 |
| 16APSK 2/3 | 2,637201 | 8,97 |
| 16APSK 3/4 | 2,966728 | 10,21 |
| 16APSK 5/6 | 3,300184 | 11,61 |
| 32APSK 3/4 | 3,703295 | 12,73 |
| 32APSK 5/6 | 4,119540 | 14,28 |
| 32APSK 9/10 | 4,453027 | 16,05 |
Read the overlaps. 8PSK 3/5 delivers 1,780 bit/symbol at 5,50 dB; QPSK 9/10 delivers 1,789 at 6,42 dB — same throughput, 0,92 dB cheaper on 8PSK. And 16APSK 2/3 beats 8PSK 8/9 by 1,72 dB at equal efficiency. “Lower modulation order is more robust” holds only at equal code rate, which is rarely the real comparison.
These are ideal figures
| The table assumes | Reality |
|---|---|
| Computer simulation | Not hardware measurement |
| 50 LDPC iterations | A receiver stopping earlier does worse |
| Perfect carrier & sync recovery | Real demods lose a fraction of a dB |
| No phase noise | Every LNB adds some; high-order APSK is very sensitive |
| AWGN channel | Real links add rain fade, interference, non-linearity |
| Normal FECFRAME, no pilots | Short frames cost 0,2–0,3 dB more |
The standard says it plainly: “For calculating link budgets, specific satellite channel impairments should be taken into account.” The table is a floor, not a design threshold.
QEF is not “no errors”. It is defined as under one uncorrected error-event per transmission hour for a 5 Mbit/s service — so a higher-bitrate service sees more events at the same PER. And LDPC has a cliff: half a dB above threshold is perfect, half a dB below fails almost completely.
Symbol rate → bitrate → bandwidth
Useful bitrate = Rs × η Occupied BW = Rs × (1 + α)
| MODCOD | Rs Msym/s | α | BW MHz | Mb/s | Es/N0 dB |
|---|---|---|---|---|---|
| 8PSK 2/3 | 35,3 | 0,20 | 42,36 | 69,92 | 6,62 |
| QPSK 3/4 | 27,5 | 0,35 | 37,12 | 40,91 | 4,03 |
| 16APSK 3/4 | 30,0 | 0,15 | 34,50 | 89,00 | 10,21 |
| 32APSK 5/6 | 33,0 | 0,10 | 36,30 | 135,94 | 14,28 |
Roll-off penalties: α = 0,35 → 1,30 dB · 0,25 → 0,97 · 0,20 → 0,79 · 0,15 → 0,61 · 0,10 → 0,41 · 0,05 → 0,21. DVB-S2 offers the first three; S2X adds the rest.
The η column assumes no pilots and is normalised to symbol-rate bandwidth. For efficiency per unit of occupied spectrum, divide by (1+α): 32APSK 5/6 at α = 0,10 is 3,745 bit/s/Hz, not 4,12.
What DVB-S2X adds
| Addition | Why it matters |
|---|---|
| Finer code rates | S2 jumps 8PSK 2/3 (6,62) → 3/4 (7,91), a 1,3 dB gap you waste or fall through. S2X fills it at 6,12 / 7,02 / 7,49 |
| 8APSK, 64/128/256APSK | Up to 5,90 bit/symbol at 19,57 dB |
| VL-SNR π/2 BPSK | Down to −9,9 dB — links DVB-S2 cannot close at all |
| Roll-off 0,15 / 0,10 / 0,05 | More payload in the same allocated bandwidth |
-L MODCOD variants | Constellations optimised for linear channels |
The -L trade is real: 16APSK 3/5-L needs 0,39 dB less than 3/5 on a linear channel and 0,56 dB more through a hard limiter. Choose on whether your transponder runs near saturation.
Non-linear penalty grows with order: QPSK 2/9 loses 0,40 dB through a hard limiter, 16APSK 1/2-L loses 2,43 dB, 256APSK 3/4 loses 4,45 dB. High-order APSK belongs on a lightly-loaded or linearised chain.
Note the metric change: S2 tables are PER = 10⁻⁷, S2X tables are FER = 10⁻⁵. Different quantities on different objects — close enough to tabulate together, not close enough to compare at the third decimal.
Margin, and why more is not simply safer
| Add for | Notes |
|---|---|
| Implementation margin | From the receiver datasheet, not from any standard |
| Phase noise | Negligible on QPSK, significant at 32APSK and above |
| Short FECFRAME | 0,2–0,3 dB — quantified by the standard itself |
| Transponder non-linearity | Up to 4,45 dB; see the S2X hard-limiter column |
| Interference | The threshold is against C/N+I, not C/N alone |
| Rain fade | Usually the dominant term at Ku and Ka |
Margin is bought with bandwidth. Picking a mode 3 dB more robust than needed means 8PSK 2/3 instead of 16APSK 3/4 — roughly a third of the capacity gone. The question is not “how much margin can I add” but “what availability do I need, and what is the cheapest mode that delivers it”.
The structural alternative: receive the same transponder at more than one site and fail over. Rain fade is local, so geographic diversity addresses it in a way no amount of coding gain at one site can.
Not on this sheet: DVB-T2 C/N
EN 302 755, the DVB-T2 standard, specifies the system but contains no required-C/N table — those figures are in the implementation guidelines. And unlike satellite, one number per mode is not very meaningful: the same modulation and code rate can differ by several decibels between fixed-rooftop, portable-outdoor and portable-indoor channel models.
So: ask which channel model any quoted DVB-T2 C/N assumes. Without that, the figure is not comparable to anything — including the satellite numbers above, which are AWGN.
Gleb Sazanov
Chief Technology Officer
Gleb Sazanov is an accomplished Chief Technology Officer (CTO) with over 20 years of experience in software development, system architecture, and cloud-based solutions. As the CTO of SATLINE, a leading provider of virtual and colocation services tailored to SATCOM businesses, Gleb drives the company’s technological strategy, fostering innovation and efficiency in data center services. His expertise spans various domains, including DevOps, system scaling, and high-performance infrastructure management. With a deep passion for cutting-edge technologies, Gleb plays a pivotal role in shaping the future of the SATCOM industry.