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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallYou can build a learning implementation of the MIL-STD-188-110 narrowband HF data waveform in modern C++ as a baseband library, and test it against the standard’s own channel conditions without any radio hardware. You cannot call the result compliant or interoperable until its framing, coding, interleaving, and acquisition match the full standard and it passes the specified tests. This article maps the scope, the Appendix C waveform parameters, a practical software structure, and the test conditions a build must meet.
Build against MIL-STD-188-110D with Change 1
The controlling edition is MIL-STD-188-110D with Change 1. The Defense Logistics Agency (DLA) record for the standard, reachable through its ASSIST Quick Search, lists a document date of 6 November 2024 and marks the document active. Check that record before you start, because a later revision would supersede this edition.
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The standard establishes technical requirements and design objectives for modem interoperability and performance in voice-frequency communications systems. It is a specification rather than a tutorial. The parameters below show what the standard covers, but they are not a complete set of values you can code from.
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Three different claims are often blurred together, and they should be kept apart:
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- A working modem transmits and recovers its own data in a loopback test. This shows your code is internally consistent. It says nothing about the standard.
- A standard-shaped implementation covers named parts of the standard, such as the Appendix C waveform, and uses its parameters. It has not been shown to meet the full set of test conditions.
- A conformant, interoperable modem matches the standard’s exact bit ordering, tables, polynomials, acquisition rules, and waveform timing, and meets the performance requirements under the specified tests. Only this level supports a compliance claim or interoperation with other modems.
Label your project with the level it actually reaches. A learning project has real value at the first two levels.
Choose Appendix C or Appendix D before writing code
The standard covers two distinct waveform families. A first implementation should pick one and say so in its documentation.
| Attribute | Appendix C (narrowband) | Appendix D (wideband) |
|---|---|---|
| Channel scope | Narrowband HF data modem waveforms above 2400 bps in 3 kHz channels | Contiguous bandwidths from 3 to 48 kHz |
| User data rates | 3200, 4800, 6400, 8000, and 9600 bps; uncoded 12800 bps is a design objective | 75 to 240,000 bps |
| Symbol rate | 2400 symbols per second | Not stated in this article; see Appendix D of MIL-STD-188-110D |
| Modulation | PSK and QAM | Not stated in this article; see Appendix D of MIL-STD-188-110D |
| Forward error correction | Constraint-length-7, rate-1/2 convolutional code punctured to rate 3/4 | Not stated in this article; see Appendix D of MIL-STD-188-110D |
| Interleaving | Block interleaver with six lengths, from 1 to 72 frames | Not stated in this article; see Appendix D of MIL-STD-188-110D |
If your project covers Appendix C, say “implements Appendix C” and state that Appendix D is outside its scope. Do not describe a narrowband-only project as covering MIL-STD-188-110 in general.
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The Appendix C waveform in brief
Appendix C fixes several parameters that shape your data structures. Each one needs its exact values from the standard before you write the corresponding code.
- Symbol rate: 2400 symbols per second.
- Modulation: PSK and QAM, with the constellation for each data rate defined by the standard.
- Error correction: a constraint-length-7, rate-1/2 convolutional code punctured to rate 3/4. The rate-1/2 code produces two coded bits per input bit. Puncturing removes some coded bits to reach rate 3/4, which means four transmitted bits for every three information bits.
- Interleaving: a block interleaver with six lengths, from 1 to 72 frames.
- Signaling: the data rate and interleaver setting are carried in the waveform so the receiver can acquire them.
A constraint length of 7 gives the decoder 64 states, which drives its computational cost. The standard fixes the code, not the decoding algorithm. A soft-decision Viterbi decoder is the common choice for a code of this type, provided its output matches the coded bitstream the standard defines.
Software architecture: keep the modem separate from the radio
Structure the baseband modem as a chain of independently testable stages, each a pure transform from one sequence to the next. The sequence below is an engineering decomposition for explanation. The standard’s bit-level details decide the actual behavior of each stage.
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Transmit path
- Bit framing and any scrambling the standard specifies.
- Forward error correction encoding, then puncturing to rate 3/4.
- Interleaver load and read, using the length signaled for the selected mode.
- Mapping bits to the constellation specified for the selected data rate.
- Symbol timing and phase-continuous waveform generation at 2400 symbols per second.
Receive path
- Receive filtering, then timing and frequency acquisition.
- Demodulation to soft decisions.
- Deinterleaving.
- Decoding.
- Recovery of the framed user data.
Keep the radio interface outside the modem
The standard describes downconverting an RF signal to baseband for a channel simulator, and upconverting back to RF when an embedded modem is tested through RF-only access. Radio filters in that path can change the measured result. You do not need that path to build the baseband core. Keep the modem independent of audio and RF device code so that unit tests and channel simulations stay deterministic.
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#include <complex>
#include <cstdint>
#include <span>
#include <vector>
using Sample = std::complex<float>;
// Each stage is a pure transform so it can be tested in isolation.
struct Encoder {
// Rate-1/2, constraint-length-7 mother code, punctured to rate 3/4 for Appendix C.
// Generator polynomials and puncture pattern must come from the standard.
std::vector<std::uint8_t> encode(std::span<const std::uint8_t> bits);
};
struct Modulator {
// Symbol rate 2400 symbols per second; constellation selected by data rate.
std::vector<Sample> modulate(std::span<const std::uint8_t> coded_bits);
};
The sketch shows interface boundaries only. It contains no bit ordering, polynomial, or constellation table, so it should not be treated as a conformant implementation.
What this introduction leaves to the standard
- The exact bit ordering and any scrambling sequence.
- The generator polynomials and the puncturing pattern for rate 3/4.
- The constellation mapping for each data rate.
- Acquisition and frame-timing rules.
- Everything in Appendix D.
- Operating conditions beyond the fixed-frequency case named in the performance requirement. Check the standard before claiming any other operating mode.
Performance requirements to test against
For Appendix C narrowband operation in fixed-frequency mode with the maximum interleaver, the standard sets a coded bit error rate target of 1.0 × 10-5 or better under each condition in Table C-XVII. The standard states the requirement this way:
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“The measured performance of the narrowband MDR waveform, using fixed-frequency operation and employing the maximum interleaving period (the 72-frame ‘Very Long’ interleaver), shall achieve coded BER of no more than 1.0E-5 under each of the conditions listed in Table C-XVII.”
— MIL-STD-188-110D with Change 1, Appendix C, U.S. Department of Defense
This requirement is tied to the 72-frame setting. This article does not establish a requirement for the shorter interleaver lengths, so check the standard before making any claim about them.
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- Turn your Raspberry Pi to SDR Transceiver. Many amateurs have done a lot with Raspberry Pi ranging from cat control to converting it to a full fledged SDR transceiver. The latest kid in the block of SDR Transceivers is Radioberry. It’s a combination of Raspberry Pi with a radio board developed using AD9866 12-bit microcontroller
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SNR thresholds by data rate and channel
The standard lists rate-dependent SNR thresholds for two channel conditions: additive white Gaussian noise (AWGN), and the ITU-R Poor fading channel described below.
| User data rate | AWGN SNR threshold | ITU-R Poor SNR threshold |
|---|---|---|
| 3200 bps | 9 dB | 14 dB |
| 4800 bps | 13 dB | 19 dB |
| 6400 bps | 16 dB | 23 dB |
| 8000 bps | 19 dB | 27 dB |
| 9600 bps | 21 dB | 31 dB |
| 12800 bps (uncoded design objective) | 27 dB | No ITU-R Poor threshold listed |
These values are the standard’s listed test conditions for the Appendix C waveform. They are not guaranteed performance on a real HF path.
The two channel models
The AWGN condition is a baseline with additive Gaussian noise and no fading. The ITU-R Poor channel, as the standard describes it, has two independent Rayleigh fading paths of equal average power. The second path arrives with a fixed 2 ms delay, which produces an echo, and the fading has a 1 Hz two-sigma bandwidth. The standard describes the baseband HF simulator as patterned after the Watterson model, a widely used HF channel model built from tapped, Rayleigh-fading paths.
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The standard sets minimum test durations of 60 minutes for each AWGN condition and 5 hours for each ITU-R Poor condition. Plan your regression runs around those times.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A test plan that climbs in difficulty
- Unit tests per stage. Check each stage against test vectors taken from the complete standard. Vectors produced by your own encoder prove only that the code agrees with itself.
- Noise-free loopback. Transmit a known bit pattern and recover it exactly, for every data rate and interleaver length you implement. Passing this is necessary but not sufficient.
- AWGN runs. Test at each rate’s listed SNR, for at least 60 minutes per condition, measuring coded BER as the standard defines it.
- ITU-R Poor runs. Test with the two-path Rayleigh model at each rate’s listed SNR, for at least 5 hours per condition.
- Optional RF stage. Add an upconversion and downconversion path through real radio hardware once the baseband results hold. Record how radio filters affect the outcome.
Trade-offs in a first design
| Decision | What you gain | What you pay |
|---|---|---|
| Use the 72-frame “Very Long” interleaver | It is the setting tied to the BER requirement above and spreads errors across the most frames | It has the longest buffering and delay of the six lengths |
| Implement Appendix C only | A smaller scope with one waveform family to verify | Appendix D is not covered, so the project cannot claim full MIL-STD-188-110 coverage |
| Baseband simulation first | Deterministic, repeatable tests with no radio required | Radio filters, the RF chain, and real propagation are not exercised |
| AWGN before ITU-R Poor | Short runs that isolate coding and modulation bugs | Fading behavior is not tested until the 5-hour ITU-R Poor runs |
Build the Appendix C baseband chain and its tests first. Treat any radio work as a later stage that depends on those results.
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