DominoEX - an IFK Mode for HF

A highly robust digital mode for HF, designed especially for NVIS propagation


What is DominoEX?

DominoEX is a computer-based radio mode intended for simplex chat-mode operation on the lower HF bands, although the performance is excellent from LF to VHF. You can use any SSB transmitter, even an old "steam radio" type. DominoEX is an ideal mode for beginners, since it is easy to tune. DominoEX has been designed to get around several problems which exist with other digital modes, such as difficult tuning, slow response, and poor performance on the 160 - 40m bands.

  • DominoEX is so robust that FEC is not required for perfect print.
  • The mode is twice as fast as PSK31, as well as more sensitive.
  • The mode is very 'slick' (no turn-around delay between overs).

DominoEX is a digital mode with MFSK (Multi-Frequency Shift Keying), used to send data (for example, hand-typed text) by radio. MFSK sends data using many different tones, sent one at a time. Each tone element ('symbol') can carry several bits of data, unlike most other modes, where each element represents only one bit. Thus the symbol rate is much lower for the same data rate when MFSK is used. This is beneficial, since it leads to high sensitivity with good data rate and modest bandwidth. Even more important, low symbol rates are less effected by multi-path reception timing effects.

Thus MFSK is ideal for HF operation, since it has good noise rejection and good immunity to most propagation distortion effects which adversely affect reception of other modes. MFSK is already used on HF by modes such as MFSK16, ALE, THROB and Olivia, but DominoEX is the only mode to use an Incremental Frequency Keying strategy. DominoEX is a reasonably narrow-band mode (more like MFSK16 or RTTY).

A narrow-band application of MFSK presents some challenges. The main problem is that radio transceivers with high stability and tuning accuracy are required, since very small frequency steps are used (say compared with RTTY). MFSK is also prone to interference from data arriving from different ionospheric paths (although less so than other modes), and like many modes, prone to interference from fixed carriers within the data passband. Forward Error Correction (FEC) can be deployed to reduce errors, but such modes can become slow and clumsy to operate, or consume excessive bandwidth. With DominoEX, we have taken a different approach, concentrating on perfecting the design for best NVIS (Near Vertical Incidence Signal) reception without requiring FEC. All the inherent MFSK problems are also avoided or much reduced.

DominoEX uses a series of new techniques to counter the general limitations of MFSK. To avoid tuning problems, IFK (Incremental Frequency Keying) is used, where the data is represented not by the frequency of each tone, but by the frequency difference between one tone and the next, an equivalent idea to differential PSK. An additional technique, called Offset Incremental Keying (IFK+) is used to manage the tone sequence in order to counter inter-symbol interference caused by multi-path reception. This gives the mode a great improvement in robustness.

DominoEX offers:

- A simple and easy to use QSO mode
- Incredible tuning tolerance - up to 200Hz offset
- Incredible drift tolerance - up to 200Hz/minute!
- High typing speed with low bandwidth
- No need for a highly linear transmitter
- Designed specifically for 160/80/40/30m
- Secondary (ID) channel for full-time transmission identification
- Varicoded data for higher throughput - up to 140WPM!
- High multi-path tolerance for NVIS conditions
- Low uncorrected error rate - no FEC required!
- Full extended ASCII character set
- FEC option offered for secure copy under the poorest conditions

DominoEX will handle TX/RX offsets and mistuning of up to 200Hz with ease. It also has better (by design) multi-path performance than other modes, and this means low-error copy without requiring FEC, which in turn means faster typing and quicker response. FEC can be switched on when required, and gives improved copy under marginal or interference conditions. FEC detection and switching can also be automatic.

The combination of IFK+, soft-decision decoding, and FEC with code puncturing substantially eliminates carrier interference and dramatically reduces the effect of lightning static. The FEC option is similar to MFSK16 (same FEC coder, same varicode alphabet), but the IFK+ modulator is used. The best implementation of this option is THOR by Dave W1HKJ. Be aware that in some early versions of the DominoEX FEC option the interleaver is shorter than desirable (L=4 instead of L=10), which considerably weakens the FEC performance.

Multiple Modes/Speeds

No one mode can be perfect for all conditions. When propagation is stable (line of sight, VHF tropo, or single hop on higher bands) high speeds are possible, offering enhanced performance with good tuning tolerance and reasonable robustness (rejection of interference). When conditions deteriorate, multi-path propagation can cause excessive timing errors, and a slower mode is indicated. The slower modes also have better sensitivity. This approach can be likened to 'changing gears' in a car when the going becomes tough.

On the lower bands there is considerable noise, and during the early evening quite severe multi-path propagation, requiring slower speeds. The tone spacing of MFSK modes must be related directly to the baud rate, and the narrowest practical spacing is numerically the same as the baud rate. Because the slower speeds imply using closer tones, which could lead to Doppler problems (well known to PSK31 users), these slower modes use double tone spacing to avoid the problem.

Six speeds are offered. The faster speeds are best used on the higher bands and when conditions are good.

  • On higher bands, start with 11 baud (the default speed) and change up if conditions are good, and down if they deteriorate.
  • 8 baud up to 16 baud are excellent for NVIS propagation on 80m. Use 8 baud when the band is especially noisy.
  • Three  s l o w  but more sensitive modes are provided for poor conditions and the lower bands.
  • DominoEX4 suits LF QSOs and HF telemetry. Sensitivity is -18dB S/N in 3kHz BW. There is a single-chip micro transmitter for DominoEX4.

    MODE BAUD BW TONES SPEED FEC TONE SPACING
    DominoEX 4 3.90625 173Hz 18 ~25 WPM ~12 WPM Baud rate x2
    DominoEX 5 5.3833 244Hz 18 ~31 WPM ~16 WPM Baud rate x2
    DominoEX 8 7.8125 346Hz 18 ~50 WPM ~25 WPM Baud rate x2
    * DominoEX 11 10.766 262Hz 18 ~70 WPM ~35 WPM Baud rate x1
    DominoEX 16 15.625 355Hz 18 ~100 WPM ~50 WPM Baud rate x1
    DominoEX 22 21.533 524Hz 18 ~140 WPM ~70 WPM Baud rate x1

    Table of DominoEX modes. * Default mode (with FEC off)

    The speeds quoted for DominoEX are approximate for typical text in English. For speed measured in Characters per Second (CPS), divide the above values by 10. Speed varies slightly with content. Speeds and parameters have been chosen following extensive on-air and simulator testing. The lowest speeds are suggested for QRP beacons - DominoEX has a full-time (secondary text) beacon built-in!

  • How does DominoEX work?

  • Before you start reading here, review the MFSK16 description, as the modes have much in common, and common features are not repeated here.

    DominoEX works by sending short transmissions of 18 different tones (called symbols), one at a time, each carrying four bits of data. Two more tones than strictly needed for 4-bit MFSK are used, in order to accomodate IFK+ code rotation. So DominoEX is an MFSK mode, meaning it is naturally sensitive, resistant to interference and ionospheric effects. However, several new techniques are used.

    • DominoEX uses an Offset Incremental Frequency Keying incremental coding algorithm called 'IFK+', developed by Murray Greenman ZL1BPU, which avoids repeated tones (inter-symbol interference strategy) and gives strong sync.
    • IFK+ means that data is carried in frequency differences rather than absolute frequency, so also gives complete independence from tuning errors and frequency drift. The codes rotate around 18 tones.
    • DominoEX has an unusual and highly efficient 'Nibble Varicode'. It also has automatic nibble sync, a large character set, and secondary channel data transmitted when the keyboard is idle. These features are designed into the character set, and add no overhead to the sending speed.
    • With FEC activated, DominoEX uses the spreading effect of IFK+ to mitigate the effects of carrier interference, and soft decision receiver techniques to reduce the effects of burst noise and Doppler.
    These features make the mode easy to use, easy to tune, convenient, and ideal for beginners.

    - Fast typing
    - Automatic ID
    - Very quick to tune
    - Needs no error correction
    - Retune without losing data
    - Faster sync than other modes
    - Designed for slick QSO exchanges
    - Full extended ASCII character set
    - With FEC, perfect copy down into the noise
    - European accented characters and simple graphics

    Nibble Varicode
    DominoEX is based on 'nibbles' of information (four bit data entities), and since exactly one nibble can be transmitted on each tone element (which we call a 'symbol'), the mode can be very efficient. The specially designed character set is defined in nibbles, marked with an identifier bit to ensure automatic nibble sync (initial nibbles have the MSB cleared, further nibbles in a character have it set). Each character transmitted consists of one, two or three nibbles (that is, one, two or three sequential tones), depending on whether the character is often or rarely used. The frequency of each symbol transmitted represents the mathematical difference between one nibble and the next (actually the increment from the last tone number to the next tone number represents the value of the next nibble).

    At the receiver, the identifier bit in each nibble (which provides character synchronism) is automatically discovered because of the bit weighting, and used to decide the nibbles in the data stream to be used to decode each character. The Varicode almost doubles the text speed (compared to ASCII), and allows a very large character set (584 characters!) to be accommodated. Because of the way the character set is defined, no extra sync bits are required - they are built into the character set.

    FEC Option
    With FEC activated, the transmission changes to a bit-based system similar to MFSK16, and the MFSK16 varicode is used (identical except for the extension to provide secondary text). At present no practical convolutional decoder exists for four-bit data, so a bit-based system is used, the same as MFSK16. Robust convolutional coding and error correction result, although the typing speed is halved. IFK+ continues to provide the same advantages, and the combination of IFK+ and FEC working together (soft decisions, code puncturing, stuck code spreading) add further advantages.

    Offset IFK IFK+
    The receiver measures the frequency difference between successive symbols and calculates the nibble value to be decoded into characters or groups of bits. Because frequency measurement is made with high resolution ( of the tone spacing), differential tuning errors (such as drift and frequency offset) can be rounded out. Because measurements are made synchronously, i.e. exactly over the whole duration of each symbol, and on nothing else, there is high sensitivity and very good rejection of multi-path distortion and other inter-symbol interference.

    The Offset Incremental Frequency Keying (IFK+) algorithm also ensures that no symbol is followed by another on the same or an adjacent frequency, and so no overlap can occur between tones arriving via different paths, to degrade reception. A fixed offset or increment is added to each symbol, causing the tones to rotate and avoid previously used tones. This considerably enhances the already good multi-path performance. Since symbol sync is recovered by studying the response to the tones in the receiver (no sync is transmitted), the sync is also enhanced because there can never be a missed sync edge caused by two or more sequential tones the same.

    The algorithm also spreads the effects of carrier interference. Although errors will still occur, copy can often be maintained rather than stopping dead, as happens in MFSK16 under similar circumstances. When FEC is turned on, the errors caused by carrier interference are spread by the IFK decoder and again by the FEC de-interleaver, and may be completely corrected. The results are impressive, and much improved over MFSK16 and other MFSK modes, which cannot spread these 'stuck bit' errors. The DominoEX Tuning Displays

    Other Tricks
    You can tune the signal while receiving, without losing copy! You can also do this to push an interfering carrier (but not one between the tones) out of the active receive region.

    You can set the secondary text message to a simple ID message, a net cordinators message - or anything you wish. It will be sent whenever you slow down or stop typing. It will also mask your slow keyboard skills!

    The design uses overlapping DFT techniques which provide symbol sync (as in MFSK16), and also provide a remarkable synchronous waterfall tuning display, far more sensitive than other MFSK tuning aids (example on right). Unlike conventional asynchronous tuning displays, this design shows the signal virtually noise-free until sync is lost. It also clearly shows propagation effects on the signal.

    There are also sync and sync history displays, Doppler and signal strength meters available.

  • Performance

    MFSK was first developed in the 1950s, and most subsequent commercial development went into wider band high speed systems. The first important narrow-band development was Amateur mode MFSK16, developed in 1999. Some theoretical work had been done on tone field management for high speed applications, but until the work undertaken by the author ZL1BPU and Con ZL2AFP in 2004, very little had been done to develop simple, robust MFSK techniques with high tuning tolerance and ISI enhancement strategies for narrow-band use.

    Other developers have concentrated on getting around the problems of conventional MFSK by using wider, faster transmissions with strong FEC (for example ALE and Olivia). The FEC tends to mask the underlying problems in the modem, but at the expense of wider bandwidth, unacceptable latency, or both. Virtually nothing was known about the performance of IFK modes on HF until the experimental work using DominoF and subsequent development of DominoEX in 2004.

    With no error correction, copy at -10dB S/N in 3kHz bandwidth (a signal you can barely hear) is nearly as good with DominoEX16 as MFSK16, (which is error corrected, but half the typing speed), and better than PSK31 and MT63. With FEC on, at this signal level you will copy 100% for hours at a time. Simulations have shown that it is also significantly more robust than these modes under poor propagation conditions. Unlike MFSK16 and many other modes, it is easy to tune. The mode has special properties, especially suited to the lower bands.

    Screen shot - ZL2AFP DominoEX
    (Click on image for larger view)

    How many other modes - with or without FEC - will deliver 77 WPM with 100% copy at -10dB S/N, while using only 260Hz bandwidth?
    And are they as easy to tune? Easy to use?

    Documentation

    ZL2AFP DominoEXFEC beta version 2.0d, 4 to 22 baud with FEC (4 Dec 2006, includes HELP, 252kB)
    Technical details
    DominoEX Varicode Alphabet
    Frequently asked questions

    There is a 'DominoEX Beginners' Powerpoint presentation and associated commentary available on application.

    This program and associated documentation are respectively © Copyright Con Wassilieff ZL2AFP and Murray Greenman ZL1BPU 2003-2005. Please do not copy, alter or publish without permission. All Rights are Reserved.


    Copyright Murray Greenman 1997-2009. All rights reserved. Contact the author before using any of this material.