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MT63 is a digital radio modulation mode for transmission in high-noise situations developed by Pawel Jalocha SP9VRC. MT63 is designed for keyboard-to-keyboard conversation modes, on HF HAM bands.

Contents

Features and Attributes

MT63 distributes the encoding of each character over a long time period, and over several tones. This implementation is key to its robustness under less than ideal conditions.

The data encoding is performed utilizing 64 different modulated tones. By doing this, it is able to include a large amount of extra data in the transmission of each character. This allows the receiving equipment to accurately identify and verify the transmitted character even when 25% of the character data is obliterated.

MT63 implements an elaborate set of error correction techniques. It uses a Walsh function that spreads the data bits of each character across all 64 of the tones of the signal spectrum and simultaneously repeats the information over a period of 64 symbols within any one tone. This takes between 3.2 and 12.8 seconds, depending on the error correction mode chosen.

The combination results in superb impulse noise rejection. At the same time, in the frequency domain, significant portions of the signal can be masked by unwanted noise or other transmissions without any noticeable effect on successful reception.

Transmission Speed

Transmission speed is good because there are so many individual tones to describe the information. At the same time, the individual symbol rate per tone remains slow (which is good protection against ionospheric disturbances).

Secondary Channel

In addition, MT63 has the facility for a secondary channel running simultaneously alongside the main channel. This can be put to a variety of uses, such as the generation of a continuous identification or beacon.

The secondary channel is not a prime function of the mode and therefore some implementations of the protocol provide for it, others do not.

Easy to Tune

Tuning of MT63 modes is not all that critical. This is because the mode uses Forward Error Correction techniques to examine different combinations of the 64 tones that calculate the correct location within the spectrum. As an example, MT63-1K will still work if the decoder is off tune by as much as 100Hz. MT63-2K is even less exacting, with an error of 250Hz being tolerated.



When to use this mode

When conditions are good and audio bandwidth is not an issue, MT63 (particularly the 2 kHz version) using the "long interleave" setting an excellent mode to use. The data rate is very rapid and the use of multiple error correction techniques results in the most robust broadcast mode readily available to the amateur.

Although it can be a little tricky to identify by ear or eye, the mode has a generous tolerance to tuning inaccuracies and its immunity to impulse noise is excellent- typically among the best of the popularly available radio modulation modes.

When signals are weak or unstable

A disadvantage to MT63 is that when trying to receive a weak signal, it becomes difficult to discern from the noise. Additionally, the wider pass-band leaves the mode susceptible to admitting too many unwanted audio products. Under these conditions, MFSK16 hugely out-performs the other modes while still maintaining a respectable data rate.

While only occupying a modest amount of spectrum, the error correction and time interleaving combine to recover the data from the most marginal signals, a full 10dB lower in the noise than MT63 can manage.

One of the compromises with MFSK16 is the heavy burden of 100% power demand on the transmitter. It is also notoriously difficult to tune when barely visible on the waterfall display and very stable equipment is a prerequisite for successful operation.

When the channel is heavily congested with other traffic

MT63 and MFSK16 both cope well up to a point. However, if it is simply a matter of finding a small enough gap in the activity, then the minimal bandwidth of BPSK31 and QPSK31 are to be preferred. QPSK31 does offer better error correction QPSK31 has a 5 to 10% improvement versus BPSK31 in certain types of propagation conidtions.

This often means that little improvement is noticed in practice. The PSK modes are very easy to tune in and work reliably on modest computers.

Another great advantage of BPSK31 in particular is that it is the most well known and easily understood of the data modes.

For the future

A 3kHz version of MT63 would be very interesting to assess. This would be a good bandwidth to choose for optimum use of the space available under current licensing which continues to think of a single speech channel allocation as being this wide. In the broadcast application considered here, there would not be an issue if the arrival of the data were to be delayed by a few more seconds. Therefore, to improve accuracy without increasing bandwidth, it would be beneficial to add more time-based interleaving.

Not much exploration has yet taken place into some of the newer or more obscure variants of these data modes. For example, PSK125 is similar to PSK31 but by using two sidebands of 125Hz the data rate is dramatically increased. The greater bandwidth also allows the introduction of improved error correction. There is also PSK250, operating at a faster rate again.

However, the error correction employed does not approach that of MT63 and, as with all PSK modes, success is limited by the need for stable propagation conditions and, as the data rate climbs, increasingly powerful computing facilities.

Error correction

MT63 utilizes Forward Error Correction. There are other modes that use FEC (for example AMTOR mode B uses a simple FEC technique), but MT63 has other advantages.

Unlike most HF modes where a character can be lost or changed into something else by a single noise burst, MT63 is inherently very robust. This is because each character is spread over many tones (to avoid interference such as other radio transmissions) and over several seconds (to avoid bursts of noise, such as lightning).

MT63's COFDM like properties

  • The MT63 signal is spread both in the time domain (temporally) and the frequency domain (spectrally). To ensure that noise bursts and other time domain interference artifacts have minimal effect, each encoded character is spread over 32 sequential symbols (3.2 sec).
  • To ensure that frequency domain effects, such as selective fading and carrier interference have minimal effect, the character is also spread spectrally by using all the tones across the width of the transmission.
  • On each of the 64 tones, the transmission data rate is fairly slow, which suits the nature of ionospheric disturbances.

Despite the low data rate, good text speed is maintained, because the text is sent on many tones at once. The system runs at several different speeds, which can be chosen to suit conditions, but 100 WPM is typical.

Sound

MT63 sounds unusual, (it sounds like a roaring noise) but the performance is spectacular. There is no connection process, as in AMTOR, Packet or PACTOR. Some users maintain that under poor propagation conditions (namly excessive fading) MT63 works better than either PACTOR II or Clover. Under good conditions the performance advantages are less obvious.

Jamming immunity

MT63 is also far more immune to interference and deliberate jamming than any of the more conventional modes.

In a "long interleave" option, the spreading is over 64 symbols (6.4 sec), with consequent improvement in resistance to impulse and periodic interference, but of course double the time taken for the data to "trickle through" the Walsh encoder and decoder pipeline.

Changeover from transmit to receive and vice-versa is however considerably slower than most modes. It therefore requires some skill and patience to "break in" on a conversation.

Criticisms of MT63

Bandwidth issues

There are disadvantages to MT63. First, the mode is broad (see below) and is quite aggressive, i.e. it causes interference to other modes, but itself is little affected by other modes.

Latency and Speed Issues

Because of the delay through the error correction and interleaving processes, it is not possible have quick turnaround conversations.

Another problem with the reception of MT63 is the fact that correctly decoded text can take up to 15 seconds to appear on the screen. A clue to synchronized reception can sometimes be gleaned if the software has a digital squelch facility. If the squelch is set up to be closed when no MT63 signal is present, it can often be seen to open as soon as it finds MT63 and ideally the text will follow quite a number of seconds later! Another clue to satisfactory reception prior to the actual appearance of the text is that the generation of large volumes of unwanted characters tends to cease when receiver synchronization has been achieved. As with other multi-tone modes where many sine waves exists side by side, the transmission chain for MT63 must be made as linear as possible.

MT63 Latency

Latency is a measure of the time taken for transmitted data to pass through the transmission and reception equipment.

  • It takes 165ms to transmit a 45 baud RTTY character via a UART.
  • RTTY has symmetrical latetency so the total latency for it is 330ms.
Mode ECC Mode Latency (sec)
MT63 500Hz short 12.8
MT63 1K short 6.4
MT63 1K long 12.8
MT63 2K short 3.2
MT63 2K long 6.4
PSK31 - <1

Walsh Coding

The current 7-bit version of MT63 uses Walsh codes, with temporal interleaving. This may not be an optimal coding scheme with respect to error correction. It must be pointed out that turbo codes could be used to accomplish the same task, with a ~25% increase in efficiency.

Justification

  • Walsh: 7 bits → 32 bits
  • Turbo: 8 bits → 24 bits (the need for varicode can be eliminated)
  • Walsh - Turbo = 8 bits, a 25% increase in system efficiency

The same temporal interleaving techniques could be used with turbo codes, but as a general principle only short length interleaving should be used with turbo codes.

Media

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See also

Related links








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