DX ModeA new weak signal DX mode for Radio Amateurs, employing M-ary FSK, phase continous tones and convolutional coded FEC.
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Rather than use a glossary of the dozens of technical terms used here, you will
find the definitions spread through the text. Whenever you come across the little
symbol, hold the mouse over it to see
the definition of the preceding term.
or unidirectional (simplex or broadcast)
rather than one way at a time (half duplex).
data communication has limited bandwidth
available,
and usually involves half duplex operation, while the available transmitter power is
definitely limited, both by legislation and by cost. Fortunately, Amateurs
have lesser expectations of reliability, and can always postpone operation
until conditions improve, or simply talk to someone else! Amateur Radio operators also
have an almost unique interest in real-time
communications, now abandoned by almost
all other HF services in favour of automated message techniques.
Radio Amateurs wishing to transmit data or text rather than voice (digital modes) are often interested in very robust transmissions over very long distances, such as from one side of the world to the other. Bandwidth needs to be kept to a minimum, since all the Amateurs in the world share the same limited space, and power requirements should be modest. Fortunately transmission speeds can often also be modest, which is helpful since speed can be traded off for improved reliability, lower power, or narrower bandwidth. This concept is the logical concept of early work in communications theory by Claude Shannon (1947).
was used in this way for many years. Hellschreiber
.
has recently been revived for the same purpose, and has proved to be very
effective. New techniques such as PSK31
.
by Peter G3PLX and STREAM
.
by Nino
IZ8BLY, have extended the performance of narrow-band links considerably.
The recent trend in these new modes, as illustrated in the last two examples,
has been to use differential PSK (DPSK) transmissions,
since DPSK offers very high sensitivity and rejection of noise. Such modes
are therefore ideal for low power. However, the biggest problems facing
very long distance (DX) communication on HF are generally selective fading
and ionospheric modulation of the signal, rather than sensitivity,
and the PSK modes do not handle these problems very well.
Option can be very effective.
In the past, MFSK was used successfully by the British Foreign Office, the
Belgian and French military and others, using such systems as
Piccolo and
Coquelet.
Such systems were designed for high communications reliability in the days of electromechanical equipment. These old MFSK systems provided very good performance for the time - robust, sensitive and reliable, with good results in fading and poor ionospheric conditions without requiring error correction. There are some modern military systems of a broadly similar nature used for similar reasons.
The opportunity has now arrived to modernise the MFSK technique, creating a
new high performance yet inexpensive mode that will benefit from the advantages
of MFSK, plus the simplicity of the PC and sound card,
and the advantages of many associated DSP techniques,
since PCs are now fast enough to perform this type of processing.
data using multiple tones,
extending the RTTY two-tone technique to many tones,
usually, but not always, one tone at a time.
MFSK means Multi - Frequency Shift Keying, and should not be
confused with MSK (Minimum Shift Keying). There are a number of different
techniques, using concurrent (or parallel) tones, sequential
(one after another) tones, and combinations of tones. MT-Hell
can be either concurrent or sequential, DTMF
tones used for telephone signalling are concurrent tone pairs, while
Piccolo and Coquelet, although using tone pairs, are decidedly sequential.
MFSK transmissions have a unique sound, almost musical, which is why Piccolo and Coquelet received their names (Coquelet means rooster).
Piccolo Mk 6 sample (277kB)
IZ8BLY FSK8 sample (325kB)
FSK16 FEC sample (184kB)
MFSK uses relatively narrow tone spacings, so remarkable data rates are achieved
for a given bandwidth - 64 bps in a signal bandwidth of 316 Hz is typical.
The following picture is a spectrogram
of an MFSK16 signal (16 carriers)
with a spacing of 15.625 Hz and operating at 15.625 baud. The transmission operates
at 62.5 bps (about 80 words per minute!) and occupies about 316 Hz bandwidth.
The two black horizontal lines in the picture
are at 1000 Hz and 1300 Hz, and the horizontal scale is about 20 seconds. This
short transmission contains about 120 letters. MFSK16 is always operated with FEC, so the
text throughput is actually only about 42 WPM (31.25 bps).
for sensitivity and multi-path rejection - data bit rate
higher than symbol baud rate
are necessary at the slower speeds. It is important that the radio transceiver
be very stable, and also that it has very small frequency offset between transmit
and receive (preferably less than 5 Hz).
MFSK also uses more bandwidth for a given text speed than a 2FSK or PSK system, but by the same token it is therefore more robust.
MFSK systems generally use non-coherent detection,
and space the many tones as closely as
possible, to restrict the transmitted bandwidth. The transmitted tones
must be spaced at a separation equivalent to the baud rate, or a multiple of the
baud rate - the rate at which the "dots"
are sent, otherwise it is difficult to separate one tone from another. This allows
the signalling to be orthogonal,
as will be explained over the next few paragraphs.
For example, the tone carriers can be spaced by 20 Hz when keyed at 20 baud.
MFSK signals are traditionally "hard keyed", i.e. each tone starts and stops suddenly, as in the following example.
This gives the signal its characteristic frequency domain
sin(x)/x shape, just the same as a Morse Code (CW) dot:
The shape of the transmitted signal has a main peak, with nulls spaced either side of the carrier
frequency.
The first nulls occur at the carrier frequency � the baud rate. The humps and nulls are clearly
visible on either side in Fig. 2. If you look carefully
at the spectrogram in Fig. 1, you will see these sidelobes as grey streaks above and below the
individual dots. The big hump in the centre of Fig. 2 is the wanted signal, and it is these
that cause the black dots in Fig. 1.
Of course the dots or tone bursts are not isolated, but preceded and followed immediately by other dots at the same or different close frequencies. Imagine then that we need to superimpose the sin(x)/x shape of each one to see what happens. We can arrange the spacing of the tones to achieve the best results.
Fig. 3 shows seven of these hard keyed tones superimposed, so that the nulls of
each carrier coincide with the peak of the next, to minimise cross-talk
between channels in the receiver and therefore
allow orthogonal signalling. This occurs when the baud rate and tone spacing are
numerically the same, or at multiples of the baud rate. Fig. 3 is a spreadsheet
simulation, where the spacing is X=pi radians, and would be identical to the result
of seven tones transmitted in turn at the baud rate.
The vertical scale in this graph is linear, 0 - 1, and the horizontal scale is in radians, from -20 to +20, or about �12 Hz. The baud rate is assumed to be 1 Hz.
When the transmission consists of multiple tones spaced as described, the signal broadens out across the peak, but retains the characteristic shape, as illustrated above. When random data is transmitted, the broad peak "fills out", but the side lobes remain obvious. The following image shows the spectrum of a real 8FSK signal transmitting at 31.25 baud with a tone spacing of 31.25 Hz. The vertical axis in this image is logarithmic, so the side lobes are more obvious than in the simulations above. Note that the sidelobes are spaced 31.25 Hz because of the 31.25 Hz baud rate.
The spectrogram was taken with 0dB set at the level of a single constant tone.
A standard method of calculating the necessary bandwidth of radio transmissions is laid out by the
CCIR,
and for the above transmission is 331.25 Hz (�166 Hz). Looking at the spectrum, the signal
is well below -30dB from the single tone carrier at this bandwidth (indicated by the vertical red lines),
easily exceeding the CCIR definition of 0.5% of the total transmitter power (about -20dB).
The performance (indicated by the horizontal blue line) is this good because with modern
DSP technology the signal measured above transmitted phase-synchronous tones (CPFSK).
It so happens that this occurs naturally and easily using DSP
when the duration of each tone (the symbol period) is the reciprocal of the tone spacing
Here's another spectrogram,
this time of an eight tone MFSK transmission,
received over 18,000 km on 18 MHz. Note the characteristic appearance!
The coding of the data for FEC is very simple, but the decoding is more than a beginner can be expected to understand. Chip Fleming has an interesting Tutorial on Convolutional Coding if would like to learn more.
designed to reduce errors that occur due to the transmission path. For MFSK systems
the most appropriate type of FEC is the sequential type, where every user data bit is represented
in the transmission by two or more coded data bits. This ratio is the Coding Rate of the coder.
For example, if there are two coded bits for every one data bit, the Coding Rate = 1/2.
Thus the User Data Rate is the Channel Data Rate x Coding Rate.
The number of bits per alphabet character therefore depends on the character frequency, just like Morse. For example:
Character Varicode space 100 a 101100 e 1100 E 111011100 Z 101010110100Thus, the alphabet coding performance depends on the chosen code, and with a Varicode, even on the text sent:
| Alphabet | Bits/Char |
| ITA-5 ASCII | 10 |
| ITA-2 | 7.5 |
| Varicode | ~ 7-8 |
The strength of the varicode is that the alphabet is essentially infinitely expandable. For example, all the European accented characters are defined, and others have been added for control purposes, that are outside the character set. The MFSK16 varicode is not the same as the PSK31 varicode, although the technique is similar.
Another important advantage of using a varicode is that the stream of data can be much more quickly resynchronised in case of errors, than is possible with other systems, and so a minimum of data is lost.
Text Throughput (WPM) = CPS x 60 / letters per word
Symbol Rate = 15.625 baud Channel Data Rate = 15.625 x log216 = 15.625 x 4 = 62.5 bps User Data Rate = 62.5 x 1/2 (FEC RATE) = 31.25 bps Text Throughput (CPS) = 31.25 / 10 CPS = 3.125 CPS Text Throughput (WPM) = 31.25 x 60 / (10 x 6) = 31.25 WPMThis will take place in a bandwidth little more than 16 x 15.625 = 250 Hz.
In terms of performance, of the examples given, only MFSK16 and PSK31 are considered practical for DX QSOs. PSK31 often performs poorly on long path, and provides no improvement when the FEC is used, so is usually used without it. MFSK is virtually as sensitive as PSK31 in practice and is unaffected by doppler. It is also less affected by interference, and offers effective FEC. These results are supported by ionospheric simulation tests.
The software is currently undergoing an extensive series of on-air tests. These tests build up operating experience and provide feedback to software designers on how best to control and operate the MFSK16 mode.
Ionospheric simulation tests by Johan KC7WW on his sophisticated equipment have shown excellent results. These can be seen on the Documents page, and compared with PSK31. Further tests will determine which combination of parameters should be offered in later release versions, for example special modes for weak signal or LF, maybe even modes optimised for MF and HF.
This new millenium MFSK16 mode includes continuous phase tones and many other improvements, especially to the receiver. The mode is loosely based on Piccolo, but differs in a few important ways:
Of course MFSK16 is computer oriented, rather than an electromechanical system, so will be easy and inexpensive to install, and easy to operate, with no performance compromises. The same setup is used as for PSK31 or IZ8BLY Hellschreiber. All you need is a Pentium class computer, a Soundblaster™ type 16-bit sound card and Windows 95™ or newer operating system. A LINUX version and other Windows versions should follow. The first release software to meet the specification will probably be from IZ8BLY, for Pentium PC with Windows 95, 98 or NT™. The specification includes:
The MFSK Varicode is slightly more efficient than others, since more smaller codes are available. This in turn is because the combinations "000", "0000" etc do not need to be reserved for idle and can be used inside character bit streams. Only the combination "001" is forbidden, as this signals the end of one character and the start of the next. The speed on plain language text is almost 20% faster than using the G3PLX varicode. The average number of bits per character for plain text has been measured at 7.44, giving MFSK16 a text throughput of 42 WPM at 31.25 baud user data rate.