What this tool is
MODCOD Beta 10.1 Stabilized is a modem MODCOD analysis and trade-study companion. It separates TX waveform statistics, RX demodulator/FEC requirements, and end-to-end modem/link efficiency.
It sweeps Eb/N0 and produces analytic/parametric estimates of BER, SER, PER,
envelope CCDF/PAPR, and the implied throughput and goodput.
Important: results are engineering estimates intended for comparison and intuition.
They are not a substitute for a bit-accurate implementation simulation. DVB-S2X, CCSDS, LTE and NR buttons are parameter presets in this build; where a preset uses a generic FEC estimate it is labelled as an estimate and must be replaced with standards, simulation or measured decoder data for design release. Channel-mismatched FEC curves are disabled rather than silently re-anchored.
Key definitions
- Eb/N0 (dB): energy per information bit to noise spectral density. For AWGN, it is a primary performance axis.
- BER: bit error probability at the detector/decoder output.
- SER: symbol error probability (post‑detector / post‑decoder proxy).
- PER / FER: uncoded and BER-modelled modes use the independent-bit packet expression; analytical block codes and standards profiles use their codeword/frame error probability directly where available.
- Throughput: raw delivered information bit‑rate (PHY rate) for the chosen parameters; fixed MCS → independent of Eb/N0.
- Goodput: successful payload bit‑rate; decreases with PER (and payload/header/CRC overhead).
- CCDF/PAPR: complementary CDF of instantaneous power relative to mean; used for PA back‑off design.
Typical workflow
- Use User-Defined for a clean QPSK / no-FEC / AWGN starting point, then enable any advanced features you need; or select a DVB‑S2X/CCSDS/LTE/NR parameter preset.
- Set the channel model and any implementation margin to reflect real‑world losses.
- Adjust packet size and overhead (header/CRC, CP/pilots for OFDM).
- Run a sweep and inspect BER/PER and the required Eb/N0 at the target error rate.
- Use the Trade‑off summary to compare candidates: spectral efficiency, Shannon gap, PAPR, goodput.
Inputs
Stabilized coding scope
The FEC block intentionally does not infer an LDPC/Turbo/Polar waterfall from rate or iteration count. Use the built-in family estimate only for exploration, then replace it with standards/reference, simulation or measured decoder data. Code rate, packetisation, throughput and interleaver geometry are handled separately.
Every input affects either (a) the error‑rate curves, (b) the throughput/goodput scaling, (c) PAPR/CCDF, or (d) the summary metrics.
Modulation
| Control | Meaning | Effect in the tool |
|---|
| Modulation |
Selects the signaling family (PSK/QAM/APSK/FSK/GMSK/OFDM). |
Sets constellation size (bits/symbol), the uncoded BER/SER model, and which UI fields are relevant/enabled. |
| Roll‑off α |
RRC excess bandwidth (raised cosine). α=0 → Nyquist, α>0 increases occupied bandwidth. |
Industry-standard treatment: does not alter intrinsic Eb/N0 or Es/N0 in AWGN. It sets occupied bandwidth B = Rs(1+α), therefore enters spectral efficiency, C/N conversion and the occupied-bandwidth Shannon comparison. |
| Symbol rate (Msym/s) |
Absolute symbol clock. |
Scales throughput and goodput in Mb/s. Error rates vs Eb/N0 are normalized and largely independent of symbol rate. |
| RRC span (symbols) |
Filter length (impulse response truncation) for pulse shaping. |
Used in the waveform assumptions; longer spans imply “cleaner” shaping. (Does not attempt full ISI optimisation.) |
| OFDM sub‑mod |
Constellation on each active subcarrier (e.g. QPSK/64QAM/256QAM). |
Determines bits/symbol on subcarriers; drives BER/SER and rate metrics while the overall modulation is OFDM. |
| OFDM Nfft / Used |
FFT size and number of occupied tones. |
Used to compute occupancy/guard‑band efficiency and to parameterise PAPR statistics (more tones → higher PAPR tendency). |
| OFDM CP / pilots (%) |
Cyclic prefix and reference/pilot overhead as percentages of the time‑frequency resources. |
Reduces throughput/goodput and η. It is reported as resource overhead and is not applied as a hidden Eb/N0 penalty to the subcarrier BER model. |
| FSK detection / tone spacing |
Selects coherent or non-coherent orthogonal FSK. Tone spacing is expressed relative to symbol rate. |
BER/SER use orthogonal FSK theory; tone spacing sets the occupied-bandwidth/spectral-efficiency estimate rather than changing BER. The minimum is 0.5×Rs for coherent and 1×Rs for non-coherent orthogonality. |
| GMSK BT |
Gaussian filter bandwidth‑time product (typ. 0.3 in GSM‑like systems). |
Controls the GMSK spectrum/ISI trade and the waveform characterisation; envelope is treated as near constant. |
Decoding and coding chain
| Control | Meaning | Effect in the tool |
|---|
| Enable FEC | Enables the complete receive FEC performance contract. | A coded curve is evaluated only from the selected explicit error-rate table. Generic built-ins remain engineering estimates. |
| FEC curve preset / effective code rate | Selects the performance table and complete-chain information/coded-bit ratio. | The rate controls throughput and spectral efficiency. It does not create an LDPC/Turbo/Polar waterfall. |
| Curve metric / values / channel / source | Declares whether the supplied table is BER, FER/BLER or PER, whether values are coding gain or absolute required Eb/N0, and its provenance/channel. | Channel metadata is enforced: a curve calibrated for a different selected channel is shown as a warning and the coded result is disabled. |
| Optional FEC k / n | Exact FEC block information/code bits when known. | Overrides the scalar rate for block/padding arithmetic; the rate field is visually disabled while a valid k/n pair is active. |
| Interleaver type / geometry | Block, convolutional, random or S-row geometry. | Reports memory, latency and nominal error separation where defined. No dB coding gain is assigned. |
Packet (bits)
| Control | Meaning | Effect in the tool |
|---|
| Packet length (bits) |
Payload size. |
Sets payload fraction and PER sensitivity. Larger packets: PER rises for the same BER. |
| Header / CRC (bits) |
Non‑payload bits per packet. |
Reduces payload fraction and increases total bits used in the PER approximation. |
| Target BER |
Design threshold for “acceptable” BER. |
Used to compute required Eb/N0 where the selected FEC model provides a post-decoder BER curve. |
| Target FER / PER | Independent packet/frame error target used to derive the modem Eb/N0 requirement. | Used directly for FER-reference/analytical block-code models and for uncoded PER inversion. Reference/interpolation/extrapolation status is reported with the required Eb/N0. |
Channel
| Control | Meaning | Effect in the tool |
|---|
| Fading type |
Selects AWGN, Rayleigh, Rician, Nakagami‑m, shadowing, burst, or frequency‑selective severity. |
Rayleigh and Nakagami-m are averaged over their fading distributions; Rician uses a Nakagami-m moment-match approximation. Burst/frequency-selective/shadowed modes remain engineering estimates. |
| K‑factor (Rician) |
Ratio of LOS to scattered power (dB‑like parameter but entered linearly here). |
Higher K increases the moment-matched Nakagami m and approaches the AWGN limit. |
| Nakagami m |
Fading severity parameter: m=1 Rayleigh, m>1 milder, m<1 more severe. |
Sets the Nakagami-m fading distribution used for average BER/SER. |
| Shadowing σ (dB) |
Lognormal slow fading standard deviation. |
Acts as an additional margin/penalty on the required Eb/N0. |
| Burst len / depth |
Parameters for bursty error processes (correlated fades). |
Used with the interleaver model to determine how much burst‑mitigation gain is realised. |
| Freq‑selective severity |
Proxy for multipath delay spread / selectivity / channel estimation stress. |
Applied as an implementation penalty term (especially relevant for OFDM presets). |
| Implementation margin (dB) |
Catch‑all margin for non‑idealities (phase noise, quantisation, PA distortion, channel estimation loss, etc.). |
Applied as a receiver/radio implementation loss to both uncoded and coded curves. |
Sweep and display
| Control | Meaning | Effect in the tool |
|---|
| Eb/N0 sweep (dB) |
Min / max / step for the x‑axis sweep. |
Determines resolution and the ability to read required Eb/N0 accurately (smaller step → more precise). |
| Operating Eb/N0 (dB) |
Point operating condition used for the RX scatter and point metrics. |
Higher values show tighter clusters and lower point error rates; the sweep curves remain unchanged. |
| Simulation symbols |
Number of seeded modem symbols used by the client-side waveform simulation. The same setting drives the RX constellation cloud and the single-carrier RRC CCDF/PAPR simulation. |
The same count is used for both simulations. The constellation renders a representative subset so large runs remain responsive. The CCDF uses all simulated symbols and reports its empirical 1/N floor; tail values with fewer than about 10 expected exceedances are labelled low confidence. |
| CCDF oversample |
Oversampling factor for single-carrier waveform peak capture and a parameter of the analytic OFDM CCDF approximation. |
Higher single-carrier oversampling improves peak capture; OFDM uses one server-side analytic estimator so the displayed PAPR and exported PAPR are identical. |
| BER / SER / coded / PER trace controls |
Checkboxes in the RX error-performance plot header. |
Pre-FEC BER/SER remain available as references; when channel fading or implementation margin is selected, the BER plot also shows a dashed intrinsic-AWGN reference so the physical modulation threshold and selected-channel crossing are both explicit. Coded traces appear only when the active FEC table is valid for the selected channel. |
Plots
RX error performance vs Eb/N0
Shows analytic BER and SER estimates versus Eb/N0 for the selected effective modulation (for OFDM: the sub‑carrier modulation).
Pre-FEC BER/SER can be toggled directly in the plot header and remain available as the demodulator reference.
Interpretation: do not expect every FEC to produce a BER curve. The app shows coded BER only for models such as the supported Viterbi bound. Analytical block codes and standards reference datasets naturally report codeword error / FER in the FER/PER panel.
PER vs Eb/N0
PER is derived from BER using a block‑error approximation over the total packet bits (payload + header + CRC):
PER ≈ 1 − (1 − BER)^{N}, where N is the total packet bit count.
Larger packets therefore require lower BER to reach the same PER.
TX / RX constellation
Orange markers are the ideal transmitted constellation; blue markers are an illustrative received/demodulated sample at the selected Operating Eb/N0. Memoryless fading views use perfect phase/channel tracking with fade-dependent noise enhancement.
It is a visual diagnostic of Euclidean distance, SNR, and modulation order.
For OFDM, the plot represents a representative sub‑carrier constellation rather than time‑domain OFDM samples.
Envelope CCDF / PAPR
Plots the complementary CDF of instantaneous power normalised to mean:
CCDF(x) = P(P/mean(P) > x). The x‑axis is in dB relative to mean (so 0 dB corresponds to mean power).
- Constant‑envelope waveforms (e.g. FSK/MSK/GMSK) should sit near the floor for x ≥ 0 dB (PAPR ~ 0 dB).
- OFDM has a heavy tail and requires PA back‑off; more tones and higher oversampling typically increase peak capture.
The summary reports PAPR @ CCDF = 1e‑6, a common PA‑sizing metric (tail probability).
Goodput / Throughput vs Eb/N0
Throughput is the raw PHY rate implied by symbol rate, bits/symbol, coding rate, and overhead factors
(for OFDM: occupancy, CP, pilots). With a fixed MCS, it is essentially independent of Eb/N0 (horizontal line).
Goodput accounts for packet success:
Goodput ≈ Throughput × payload_fraction × (1 − PER).
At high Eb/N0, PER → 0 and goodput saturates at the payload‑limited maximum.
Trade‑off summary metrics
The summary panel aggregates the most decision‑relevant quantities for MODCOD selection.
Values are computed from your current settings and the modeled curves.
| Metric | Meaning / how to use it |
|---|
| Bits / symbol | Log2(M) of the effective modulation (for OFDM: per sub‑carrier). Drives η and throughput. |
| Net code rate | Product of stage‑1 and stage‑2 rates. Lower rate → more redundancy → better sensitivity but reduced throughput. |
| Net η (b/s/Hz) | Effective spectral efficiency after overhead factors (roll‑off, OFDM occupancy/CP/pilots). Used for Shannon limit comparison. |
| Overhead (%) | Non‑payload fraction from packet header/CRC and PHY overhead (e.g. CP/pilots). High overhead reduces goodput even at high Eb/N0. |
| Req Eb/N0 @ target BER | Energy-per-bit AWGN threshold from the modulation/decoder model. Following ETSI/3GPP convention, RRC roll-off is not added to Eb/N0; it is accounted in occupied bandwidth and C/N. |
| C/N @ occupied BW | Derived from C/N = Eb/N0 + 10log10(η). For single-carrier RRC, η = bits/symbol × code-rate /(1+α), so α is explicitly included. |
| Req Eb/N0 @ target PER | Equivalent operating point for a PER target derived consistently from the Target BER and packet length. |
| Shannon limit | Theoretical minimum Eb/N0 for the occupied-bandwidth η in AWGN with ideal coding and infinite length. RRC α (or OFDM spectral occupancy) therefore enters the Shannon comparison through η. |
| Gap to Shannon | Req Eb/N0 − Shannon using occupied-bandwidth spectral efficiency (RRC α / waveform occupancy included in η; implementation margin excluded). Smaller is better under those assumptions. |
| Est coding gain | Modelled horizontal shift versus uncoded reference attributable to the selected decoders and iterations (plus any overrides). |
| PAPR @ CCDF=1e‑6 | Peak‑to‑average power ratio at a tail probability of 1e‑6. Drives PA back‑off and efficiency. Lower is better. |
| Raw throughput | Raw PHY information rate for the fixed selected MODCOD. It is independent of Eb/N0 until adaptation is introduced. |
| Goodput @ metrics | Payload success rate at the selected metrics Eb/N0, accounting for PER and packet overhead. |
Design tip: You rarely optimise one metric. Typical constraints:
(i) BER/PER at a required availability, (ii) goodput target, (iii) PA back‑off (PAPR), (iv) occupied bandwidth.
MODCOD Beta 10.1 Stabilized is intended to make these trade‑offs visible in one place.
Models & assumptions
MODCOD Beta 10.1 Stabilized uses a protected calculation service with verified uncoded error-rate expressions plus a separate FEC performance-table library. Rayleigh/Nakagami fading is averaged over the fading distribution, Rician uses a moment-matched Nakagami approximation, and the remaining burst/shadow/frequency-selective modes use explicit engineering estimates.
Uncoded BER/SER
- Memoryless PSK/PAM/QAM use Gray-labelled AWGN theory. 32/128-QAM use the conventional cross-QAM lattice; because perfect Gray labelling is impossible for cross-QAM, a Smith-style pseudo-Gray nearest-neighbour penalty is used.
- APSK uses the actual normalized ring geometry shown by the constellation and a nearest-neighbour Euclidean-distance spectrum; the selected radius ratios are part of the physical modulation definition. BER assumes Gray-neighbour behaviour because a perfect APSK Gray map is not generally available.
- Orthogonal FSK uses coherent/non-coherent theory; coherent MSK has the BPSK-equivalent BER; GMSK uses the selected BT in an ideal coherent-MLSE minimum-distance model.
- Intrinsic Eb/N0 and Es/N0 are energy/noise-density quantities. RRC α does not change them in ideal AWGN; α changes occupied bandwidth and therefore C/N and bit/s/Hz. OFDM resource occupancy is handled in the corresponding bandwidth/spectral-efficiency accounting.
Coding performance
- Coded curves are generated only from explicit performance tables: built-in engineering estimates or user/reference/simulation/measurement data.
- Built-in family tables are explicitly heuristic and are labelled as estimates; they are not code-, block-length- or decoder-specific.
- No waterfall is inferred from rate or iteration count. If a valid table is absent, coded Eb/N0 is unavailable.
- A curve/channel mismatch disables coded results. Use a matched curve, or declare channel ANY only when the supplied data already includes the selected channel effects.
Channel penalties
- Rayleigh and Nakagami-m change the BER curve shape through statistical averaging; Rician is approximated by matched Nakagami-m. Shadowing remains context-only here; BURST/FREQSEL use explicit first-order fitted dB placeholders and should be replaced by measured/simulated channel curves for design release.
- Implementation margin is reported separately from the intrinsic uncoded and physical coded Eb/N0 thresholds. A convenience “including implementation” value is provided for link-budget closure.
PER model
PER is derived from BER using the independent-bit expression when the active FEC model supplies post-FEC BER. Correlated decoder failures are not inferred; for burst/frequency-selective channels use a channel-specific FER/PER curve. Interleaver geometry is reported separately and does not create a synthetic gain term.
CCDF/PAPR model
- OFDM CCDF uses one server-side analytic approximation with log-domain tail interpolation. PAPR is an envelope/PA statistic and is not charged as an Eb/N0 penalty.
- Single-carrier RRC CCDF uses seeded waveform simulation. Tail values are reported only where the sample count resolves them; the plot states N and the conservative reporting floor.
- Constant‑envelope families are represented as near‑constant envelope (PAPR ~ 0 dB).
Calibration: If you have a reference curve (e.g. 3GPP link‑level BLER), tune
Implementation margin or, preferably, replace the generic FEC estimate with the measured/simulated curve until the modeled required Eb/N0 matches your reference,
then use the tool comparatively across candidate MODCODs.