MFSK 2000
HF propagation and the Ionosphere
Design of digital modes for HF must take into account the effect the Ionosphere
has, both in terms of propagation, and the way it distorts the signals.
Introduction
The ionosphere is a force to be reckoned with by all comms engineers and Radio Amateurs. It of course
affects HF most of all, but the effects are noticed from LF to VHF. At VHF and
beyond other effects occur, such as tropospheric ducting, meteor and aircraft
scatter, canyon effects, picket fencing, and a host of other effects. VHF and UHF
are also the domain of satellite and moon bounce communications, with rotation
fading, libration and motion doppler effects.
This is not intended to be a treatise on the ionosphere, but some of the effects which
have a significant impact on digital mode design will be discussed. First, the various
frequency ranges and their characteristics will be described, followed by a discussion
of each of the more significant effects and how they can be overcome - especially by MFSK 2000 of course!
LF Propagation
On LF,
reception of signals is characterised by slow
diurnal
and seasonal variations of signal strength, and
very stable phase.
The signals follow the line of the earth, so are not to any
great extent affected by ionospheric reflections. At times, late at night for
example, fading (interaction of ground wave and sky wave) can occur.
LF is plagued by man-made noise, especially TV, motor and power noise.
MF and Low HF
MF
and HF
up to about 5 MHz have short (ground wave) range during the day, as a result of
sky wave absorption in the D layer. This area of low ionisation levels but high ion density
is quickly activated at sunrise, and blankets the arriving sky wave (from the E and F layers).
At night, the D layer fades quickly, allowing the sky waves to penetrate, giving a range
of several thousand km, and sometimes more, as the signal is now able to
bounce off the higher ionised layers. At night signal strengths are very high, but
so too is the strength of the lightning noise, which travels long distances. These frequencies are
a real challenge for digital transmission and reception.
This area is affected by man-made noise, especially during the day, and by lightning
burst noise at night. Because the sky wave propagation modes can be numerous, interaction
between them, and with the ground wave, leads to very strong fading and significant variations
in signal arrival time.
Reception is also seriously affected by interference from other
radio signals.
HF DX Bands
The area from 5 to 30 MHz is generally considered to be the area where long distance
communications is best.
This area is strongly affected by ionospheric reflection, so
multiple "bounces" and both short and long paths can be effective. Short path signals
generally follow the obvious "great circle" route between stations. Long path generally
takes place in the evening, travelling on a reciprocal path, through the night-time ionosphere
which is generally more stable.
HF is strongly affected by the activity of the sun, ionising the many reflective
layers on a daily basis. This ionisation varies with short term solar activity, and the 11 year
solar cycle. When the sun is especially active (during a solar storm) HF propagation
can disappear for days. During a solar activity minimum, ionospheric propagation
is generally limited to single hop range unless unusual propagation paths occur.
The ionization of the reflecting layers varies with altitude, time of day and latitude.
A strong concentration of ions at the poles and the variation in concentration
during the day leads to relatively fast changes in refractive index, modulating the
phase, frequency and amplitude of signals in both random (noise) and predictable
ways (diurnal rising or lowering reflective height).
Lightning noise is a problem at the lower end of the HF range. Very weak signals
are a bigger problem at the upper end, and all the HF area is affected by fading and
doppler effects. Reception of the more popular HF frequencies is also seriously affected by
interference from other radio signals.
VHF and UHF
Apart from rare occurences of sporadic E and effects such as Trans-equatorial ducting,
ionospheric effects are few on VHF, and non-existent on UHF. Ducting through tropospheric temperature inversion,
is common, but unlike ionospheric reflection, this is a "lens"
effect, like a light pipe, rather than a reflective effect. VHF and UHF signals
are characterised by stable phase when ground wave communications are used.
Satellite communications suffers from gross (movement induced) doppler frequency shifts.
Moon-bounce suffers from "libration fading" which affects the phase and amplitude
of signals arriving from the moon. Fading and polarisation rotation effects are
noticed on satellite and terrestrial signals. Reflection from other objects
causes random and often very deep fades. When the transmitter or receiver is in motion,
or a large moving object (like an aircraft) creates a reflective path, this fading is rhythmical
at a rate dependent on the speed and the signal wavelength (sometimes called the picket fence effect").
Designing a Digital Mode
Having discussed the properties of the various frequencies on which Amateurs wish
to communicate we need to consider the problems that face attempts to communicate,
which are of course part of the design strategy for digital modes.
Some are the properties of the ionospheric medium just discussed, while others
are man-made or limitations of the technology:
- Man-made noise, such as TV buzz, power noise (continuous noise)
- Lightning noise, man-made clicks and splats (pulse or burst noise)
- Fading and very weak signals
- Signal arrival time (multi-path reception)
- Selective fading
- Ionospheric doppler modulation effects
- Interference from other signals, carrier interference
- Frequency drift, satellite doppler shift
To design a digital mode to address all these problems is a tall order!
Since it is not possible to create a data mode to do everything under all conditions,
these things tend to be designed for specific applications, under the most likely
conditions. Here are some of the applications used by Amateurs:
- HF image transmission (SSTV, FAX)
- VHF high speed packet data transmission
- LF weak signal slow narrow band transmission
- HF message forwarding (bulletin board systems)
- HF person-to-person conversation at typing speed
It is this last "Rag-Chew" category that we will concentrate on.
HF "Rag-Chew" Mode Typical Specifications |
- Communications half duplex (one person at a time transmits), manually operated
- Data transmitted at typing speed (25 WPM), perhaps 50 WPM to allow for small files to be transferred
- Full ASCII character set, including European accented characters
- Modest to good error performance on noisy signals
- Fast turn-around from receive to transmit and vice-versa
- Easy tuning and tolerance of drift and mis-tuning
- Should operate with an SSB transceiver
- Should operate with a Pentium PC with 16 bit sound card
- Should be relatively immune to all the reception problems listed above!
|
Problems to Surmount
- 1. Continuous Noise
-
The best counter to continuous noise is to limit the bandwidth of the signal.
Another method is to employ a technique which is noise immune, such as FSK and especially PSK.
Use of error recovery systems is very helpful in reducing the effects of noise. Methods such as
FAX, SSTV and Hellschreiber are relatively noise tolerant because the eye is able to
easily recognise patterns and ignore noise.
- 2. Burst Noise
-
Unlike continuous noise, which affects the signal all the time, burst noise is characterised
by occasional bursts of energy so large that they swamp the receiver and cause data to be
missed. Asynchronous systems are seriously affected by such
noise since it causes loss of
synchronism. The simplest systems (such as RTTY) simply pass on the errors caused by
burst noise to the user, and accept the occasional serious loss of synchronism. Burst noise can be
managed very well using error recovery systems, especially if an interleaver is used to
spread the lost data bits across several characters, thus reducing the instantaneous
error correction load.
- 3. Fading and Weak Signals
-
A system with good AGC or dynamic range will be affective agains fading (slow changes in
signal strength, or faster changes due to multi-path cancelling). Systems such as PSK that
are independent of signal amplitude are also very effective.
- 4. Multi-path Reception
-
While multi-path reception can cause signal cancellation and therefore fading, the bigger
problem is that the various components of the signal can arrive at significantly different
times, because of the different path lengths. It is not unusual for two different paths
to differ in time by 5 - 10ms, which may be a significant proportion of the symbol period.
As each component fades or strengthens,
the data can be distorted and the timing changed to interfere with correct recognition
of the data bits. At 50 baud, 5ms represents 25% distortion.
One of the best counters to multi-path problems is to use a very low symbol
rate. MFSK is ideal in this regard. For example, at 10 baud, typical of MFSK systems,
5ms timing error represents only 5% distortion. PSK systems with asynchronous detection
(PSK-Hell is the only known example)
perform reasonably well, but conventional synchronous demodulation PSK modes such as PSK31
do not cope well with bad multi-path, which is very common on the lower HF bands at night.
The following diagram illustrates multi-path and other effects. The ionosphere is not a perfect
mirror, and in addition to the different path lengths and therefore time of flight,
there is considerable delay at the reflective layer as the refractive index and therefore
speed of the radio waves changes.
An illustration of multi-path, selective fading and doppler effects
- 5. Selective Fading
-
Fading is a particularly insidious problem as it has several difficult components - weak signals
and therefore noise; selective fading which causes distortion of pulse shapes and some frequencies
to be weaker than others; and large timing variations. As seen in the diagram above,
many signals with completely different timing and therefore phase can arrive at the receiver,
and cause cancellation or enhancement of each other.
The cancellation can be very sharp, eliminating or weakening a narrow range of frequencies
within the signal. This next spectrogram image is a classic example of this effect. The
signal is exactly 1 kHz wide (an MT63 transmission from VK2DSG on 14 MHz), and the diagonal
white lines through the image are the effect of multiple-path induced selective cancellation
within the signal. The horizontal time scale is about 10 seconds.
An MT63 signal with selective fading
The best counter to this problem is to
employ a very sensitive narrow band system with very slow symbol rate. PSK31 is a modern
example of such a mode. Hellschreiber (Feld-Hell and PSK-Hell) is also very effective.
MFSK is also very good in this regard, provided the detection of each carrier is independent of
the others, or the system can cope with missing symbols (such as by using an error reduction system).
- 6. Ionospheric Doppler Modulation
-
The ionosphere is always moving. This is illustrated in the diagram further up this page.
As the earth rotates, the polarised layers change in height over hundreds of kilometres, and their
ion densities and refractive index also change. Thus the effective height of the reflecting layer
can move at speeds up to hundreds of km/hr, quite sufficient speed to alter the frequency slightly,
although at least reasonably predictably.
The ionosphere, particularly in the regions near the poles, can be especially
disturbed by solar activity, and this random variation of refractive properties,
like waves in the ocean, unfortunately causes significant random modulation of the phase,
frequency and amplitude of signals. The effect is of course most noticeable on
paths across the poles, such as the long-path from New Zealand and Australia to
Europe. This is especially a problem with PSK modes, but does effect all modes
to some extent.
- 7. Interfering Signals
-
Much interference (Morse or SSB) is of a burst nature, and can be countered by systems
designed to handle burst
noise. Carrier interference and repetitive continuous interference is difficult to counter,
and best achieved by using wide band highly redundant systems such as spread spectrum or
frequency division multiplex. Error reduction coding can be very effective on this type of
interference if the signal is spread so that only part of it is interfered with. MT63 is
an example of this type of strategy, spread in time to limit the effect of burst noise,
and spread in frequency to limit carrier interference.
- 8. Drift and Doppler Shift
-
This problem essentially means that the received signal is not optimally tuned. Of course
narrow systems are more effected. PSK does not tolerate even the smallest amount of drift,
while FSK is more tolerant. MFSK is not very tolerant, since the signal quickly drifts
outside the narrow channel filters. AM and wideband modes are best. Feld-Hell, which is an AM
modulation system, copes very well with satellite links, combining its good noise immunity
with extreme tolerance of mistuning.