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Wide Area Time Service with
Extended Features
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Abstract
The Digital Radio Mondiale
standard for long / medium / short-wave digital audio broadcasts
(freely available for downloading as ETSI TS 101980) includes time
data, but like with RDS and DVB, the data format specification is not
really optimized towards high-precision clock synchronization and the
DRM COFDM demodulator needed is significantly more complex than the AM
receivers that decode the time signals listed above. This Wide Area
Time Service is meant to fix the many myriad problems of this
technological design shortcoming. |
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Frequency Band Allocation
- Longwave
- Medumwave
- Shortwave
- Bandwidth (LW): 35 Hz
(32.5
Hz + Guard band)
- Bandwidth (SW): 500 hz
/
1000 hz / 2000 hz
Transmission
Layer (Waveform & Coding Specification)
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Longwave
Transmission
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Brief history of QPSK transmission modes
In December 1997, the creators of the PSK31 transmission mode (a BPSK
waveform) introduced the QPSK transmission mode.
- In this mode, instead of just keying by
phase reversals (0-deg, 180-deg), an additional pair of 90 and 270
degree phase-shifts were introduced creating (0-deg, 90-deg,
180-deg, 270-deg).
- If you thought of BPSK as reversing the
polarity of the signal, then QPSK can be thought of as two BPSK
transmitters on the same frequency but 90 degrees out of phase with
each other.
- By thinking of the receiver as being two
BPSK demodulators at 90 degrees, we have two channels sharing the same
frequency, but of course, with only half the transmitter power in each.
- Therefore we have twice the bit-rate but
at 3dB less overall signal-to-noise ratio.
- It thus becomes possible to use the QPSK
feature to transmit data at twice the speed with 3dB less noise margin.
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QPSK and (π/4)–QPSK in this case have essentally the same
spectral
properties |
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QPSK31: an improvment on PSK31, with better error correction
The error-reduction code chosen is one of a type known as convolutional
codes. The code systems used in the past have been block codes, where
each character is a fixed-length code, and a fixed number of extra bits
are added to make a longer block, and this longer block is capable of
correcting errors within itself. These extended blocks are then
transmitted as a serial bitstream. In a convolutional code, the
characters are converted to a bitstream and then this bitstream is
itself processed to add the error-reduction qualities. There is no
relationship between the boundaries between characters and the
error-reduction process. Since the channel errors are also not related
in any way to the character boundaries, convolutional codes are better
suited to serial links than block codes, which were originally designed
for protecting errors in memory banks and similar structures.
π / 4–QPSK
This final variant of QPSK uses two
identical constellations which are
rotated by 45° (45° = [π / 4] radians, hence the name)
with respect to
one another.
- Usually, either the even or odd data bits are used to
select points from one of the constellations and the other bits select
points from the other constellation.
- In practice the first transmitted symbol (1 1) is taken
from the
1st constellation and the second transmitted symbol (0 0) is taken from
the 2nd
constellation.
- This also reduces the phase-shifts
from a maximum of 180°, but only to a maximum of 135°.
- The amplitude fluctuations of (π / 4)–QPSK are in between
OQPSK and
non-offset QPSK.
In comparison to traditional QPSK, (π / 4)–QPSK lends
itself to easy demodulation using less expensive equipment.
- This special form of QPSK has been
adopted for use in TDMA cellular telephone systems in the
Microwave range.
- (π / 4)–QPSK demodulation
circuits
are readily available for longer wavelengths
- It is expected that (π / 4)–QPSK
and QPSK and PSK can be used and should be used by this transmission
system so as to allow it to cope with the vagueularities of the
ionosphere at longer wavelengths.
- Note that magnitudes of the two component
waves change as they switch
between constellations, but the total signal's magnitude remains
constant.
- One property this modulation scheme
possesses is that if the modulated signal is represented in the complex
domain, it does not have any zero crossings.
- The (π / 4)–QPSK signal
waveform does not pass through the origin. As the signal does not pass
thru the origin -- this lowers the dynamic range of
fluctuations (with)in the signal.
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Shortwave Transmission |
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By encoding the data to transmit (what you type on the
keyboard) in a complex way, using 64 different modulated tones, the
MT63 developer Pawel Jalocha SP9VRC has been able to include a large
amount of extra data in the transmission of each character, so that the
receiving equipment can work out, without any doubt, which character
was sent, even if 25% of the character is obliterated.
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.
- I have not decided what information, if any should be used
in this
channel for this telecommunications service, however it would be
acceptable to transmit an idle stream of {...01010101...} until a
proper use can be found for this low complexity low bandwidth channel.
- The secondary channel is not a prime function within the
current MT63 specification -- therefore some software provides for its
use while other software does not.
- The option
to transfer binary files in MT63 (such as higher-level documents or
spreadsheets) is at the whim of the programmer with respect to the
current MT63 specificaiton.
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MT63 sounds unusual, (it sounds
like a roaring noise) but the
performance is spectacular. Some users maintain that under poor
propagation conditions (namely excessive fading and multipath) find
that
MT63 works better than the current PACTOR waveform or Clover. Under
good conditions the performance
advantages of MT63 are less obvious. |
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Jamming immunity
within the
existing MT63 specification
MT63 is also far more immune to unintentional man made interference
(QRM), jamming (QRJ) as well as natural interference (QRN) than the
majority of existing conventional radiotelegraphy 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.
- Using "short interleave" MT63, the signal is still not
viable for sending time signals. The time delay is still too large to
be of any use for public use.
MT63 bandwidth guidelines
- below 5 mhz, only use 500 hz mode
- between 5 mhz and 15 mhz use 1 Khz mode
- from 15 mhz to 30 mhz use 2 Khz mode
MT63's Walsh coding: a design flaw?
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 principal only short length interleaving
should be used with turbo codes.
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2.54
cm Service (11802.85 MHz), for wristwatches and appliances
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This service must also be able
to operate at 11802.85 MHz or 11.80285 GHz. However, at this time the
emission format has not be settled upon.
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Physical Layer Coding
The current Walsh + Convolusional coding of
MT63
(for ECC) must be completely abandoned for this application. Error
Correction with the raw bitstream at the
Physical
Layer should be done via (low complexity) Hamming codes, specifically a
low complexity
Hamming (6
, 2) Code
These Hamming parameters
should
provide adequate
error correction with minimal decoder complexity. Most codewords in
this
codebook have a Hamming Distance of either 2 or 4. Many codewords
have a Hamming Distance of 6.
The
average
Hamming Distance in this case is around 3. As this transmission system
is only
best designed for 1 hop shortwave service (under 1500 km), and medium
power LW service
-- this
ECC provision [although imperfect] is adequate. |
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Packet
Coding |
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The MPEG elementary stream
packets, and most MPEG packets in general are too huge for the
application desired here. A typical MPEG packet's payload is 255 bytes.
NICAM packets are equally huge, and not worth considering here. Many
parts of the NICAM packet structure are open or unallocated leaving the
exact interpretation of the packet unclear.
HLDC packets can be as huge as NICAM packets, but have a workable less
complex structure.
ATM packets just don't have the right structure, and are typically have
payloads of 56 bytes.
Overall, it may be best for this transmission system to use the same
coding as the CCSDS packet format as has been standardized for Uplink
and Downlink communications in the solar system. However, only the
smallest packet and frame sizes can be supported with respect to the
limited bandwidth of the communications system. Also, the lowest
complexity form of the CCSDS format must be used.
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Related Links
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Document created
29 April 2002
Document
last revised
- 27 May 2007
- 01 May 2008
- 24 October 2009
Author
Max Power
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