Wide Area Time Service with Extended Features

Frequency Band Allocation

• Longwave
• Medumwave
• Shortwave
• Bandwidth (LW): 35 Hz (32.5 Hz + Guard band)
• Bandwidth (SW): 500 hz / 1000 hz / 2000 hz

Longwave Transmission

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.

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.

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.

Jamming immunity within the existing MT63 specification

• 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.

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

• http://www.ee.unb.ca/cgi-bin/tervo/hamming.pl?X=+Generate+&L=6&D=2&T=000000
• It would be assumed that using 5 of the 6 Hamming Code bits is appropriate, pending further research.

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.

Related Links

• ntp.org (Network Time Protocol Organization; NTP FAQ)
• Precision_time_protocol (NTP alternate)
  

• OSI_model (OSI Model Specification)
  
• Error_correction (Error Correction) 
• Cyclic_redundancy_check (CRCs)
  
• Magic_number_(programming)
  
• Varicode (Varicode)
  
• Covert_channel (Covert Channel) 
• PSK31 (PSK31)

• Hamming_code (Hamming Codes)
• Convolutional_code (Convolusional Codes)
  
• Reed-Solomon_code (Covolusional Coding)
  
• Viterbi_algorithm (Viterbi Decoding) 
  

• http://www.decafbad.com/twiki/bin/view/Main/HexOddities (humor, real world used of odd hex numbers)
  
• http://www.vlf.it/trond2/list.html (VLF frequency allocation)
• http://www.unitconversion.org/unit_converter/frequency-wavelength.html (a handy tool)