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MFSK Intro and Theory
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By Murray Greenman, ZL1BPU
MFSK Overview for Beginners
MFSK is a technique for transmitting digital 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 signaling 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).
MFSK uses relatively narrow tone spacing, 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 of 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).
Advantages
MFSK has several performance advantages:
- High rejection of pulse and broadband noise due to narrow receiver bandwidth per tone
- Low baud rate for sensitivity and multi-path rejection - data bit rate higher than symbol baud rate
- Constant transmitter power
- Tolerance of ionospheric effects such as doppler, fading and multi-path
Most important of all, with an MFSK system, the error rate improves as the number of tones is increased, so with as many as 32 tones the performance is unrivalled. With PSK systems the opposite is true.
Disadvantages
Let's be fair - there are disadvantages to MFSK! The main disadvantages are related to the narrow spacing and narrow bandwidth of the individual tone detectors - drift can be a problem and accurate tuning is essential. Good tuning indicators and AFC 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.
Alphabet Coding
There are many ways to encode the alphabet from the keyboard for transmission. Perhaps the most common now is ASCII (ITA-5), but ITA-2 (as used by teleprinters) is common. MFSK16, like PSK31, is based on a Varicode, which, unlike most such alphabets, assigns a different number of bits to different characters, so that more frequently used characters have fewer bits and are therefore sent faster.
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 101010110100
Thus, 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 re-synchronized in case of errors, than is possible with other systems, and so a minimum of data is lost.
Text Throughput
The user is most interested in the actual usable text throughput, which is specified in characters per second (CPS) or words per minute (WPM). Both depend on the alphabet used, and the number of words per minute depends on the average word size. In English this is taken for convenience to be five letters plus a letter space. So we can say that:
Text Throughput (CPS) = User Data Rate / Alphabet Bits per Character
Text Throughput (WPM) = CPS x 60 / letters per word
Worked Example
Say we are using an MFSK system with 16 tones (16FSK), operating at 15.625 baud with FEC Rate = 1/2, and an ASCII alphabet using 10 bits/character. Then:
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 WPM
This will take place in a bandwidth little more than 16 x 15.625 = 250 Hz.
Comparisons
Amateur Radio RTTY operating at 45.45 baud achieves 60 WPM with no error correction, and requires about 300 Hz bandwidth. 300 baud packet is error corrected, but is unsuited by its design to HF conditions, and rarely delivers better than 30 WPM, and often much less. Packet requires 1 kHz bandwidth. PSK31 operates at 31.25 baud, and in QPSK mode gives error corrected text at 31.25 WPM approximately. It has the narrowest bandwidth, less than 100 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.
MFSK16
The new 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:
- The transmitted data is bit oriented, rather than character oriented.
- The fundamental signal is a single symbol, not a symbol pair.
- Error reduction coding is built in.
- Tone spacing and baud rates are divisions of 125.
- The transmitted tones are phase synchronous CPFSK.
- No symbol phase or other AM information is transmitted.
1. The system can therefore potentially transmit text and binary files, any alphabet including varicodes, and can use error coding.
2. The tones and baud rates (15.625 Hz, 31.25 Hz etc) are chosen to allow straightforward PC sound card sampling at 8 kHz sample rate.
3. This means the transmitter need not be linear. Using the receiver FFT, the transmitted carrier phase can be extracted, and from it the symbol phase is deduced. This technique is very fast and reliable.
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.
- Accurate tuning for transceive operation using "point and click" techniques
- Convolutional coded FEC (Forward Error Correction) with interleaver for error reduction
- FFT (Fast Fourier Transform) symbol filtering and detection
- Symbol sync recovery by measuring transitions or carrier phase in the symbol detector FFT
- Two signaling speeds with differing numbers of tones (but the same bandwidth) to suit conditions
The MFSK Varicode is slightly more efficient than others, since 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.
Note: Murray has an excellent website with much more information on MFSK16 and other related modes. This is an great resource for anyone interested in learning more about this fascinating new mode:
MFSK, "The official MFSK website"Murray Greenman, ZL1BPU. http://www.qsl.net/zl1bpu/MFSK/