dl-fldigi/fldigi_doxygen/user_src_docs/psk.txt

/**
\page psk_page PSK Single and Multi-Channel Modems

\tableofcontents

\section bpsk_modem  BPSK, QPSK, 8PSK modems

PSK are narrow band low to moderate symbol rate modes using either single or
multiple carrier differential <b>P</b>hase <b>S</b>hift <b>K</b>eying.

Current supported variants:

BPSK: Binary, 2 constellations (1) <br>
QPSK: Quadrature, 4 constellations <br>
8PSK: Octal, 8 constellations <br>
\par (1) 
	A convenient way to represent PSK schemes is on a constellation diagram.
This shows the points in the complex plane where, in this context, the real and 
imaginary axes are termed the in-phase and quadrature axes respectively due to 
their 90° separation.<br>

\par NOTE:
	Multi-Channel PSK modems use BPSK modulation.

PSK63FEC and the PSKxxxR modes are forward error correcting modes. PSK63FEC
is compatible with the MultiPsk mode of the same name. The PSKxxxR, or
robust, modes use both forward error correction and interleaving to achieve
about 4 dB s/n improvement over standard PSK. These modes are used primarily
by the PskMail user community. They are the invention of John Douyere,
VK2ETA, a member of the fldigi development team.
<br>

In addition to the binary phase shift keying the signal is 100%
raised-cosine amplitude modulated at the symbol rate. This reduces the power
to zero at the phase change. Because of this amplitude modulation, the
signal bandwidth is relatively narrow. Synchronization at the receiver
is straight forward because it can be recovered from the amplitude
information. Differential PSK is used to provide continuous phase changes
when idle (to maintain sync), and by allowing the receiver to measure phase
difference from symbol to symbol, to reduce the effects of ionospheric
Doppler phase changes which modulate the signal. The slower modes are
more affected by Doppler, and the QPSK and 8PSK modes are particularly affected.
<br>

With no interleaver and limited coding length, the QPSK mode Forward Error
Correction coding gain is limited, and under burst noise conditions on
HF the performance is usually worse than the BPSK option at the same
baud rate. In general the narrow-band BPSK modes work well on a quiet
single-hop path, but give poor performance in most other conditions.
<br>

Many of the multi-carrier and 8PSK modes exceed the baud and bandwidth limitations
imposed by the FCC (US operators only).  These modes are intended for use on VHF/UHF
and have proven to be very robust on FM even when used with repeaters.
<br>

\image html psk-signal-oscope.png "PSK63 signal transmitting text data - oscilloscope view"
\image latex psk-signal-oscope.png "PSK63 signal transmitting text data - oscilloscope view" width=5.0in
<br>

\image html psk-signal-waterfall.png "PSK63 signal transmitting text data - waterfall view"
\image latex psk-signal-waterfall.png "PSK63 signal transmitting text data - waterfall view" width=1.0in
<br>

\image html qpsk-signal-oscope.png "QPSK63 signal transmitting text data - oscilloscope - waterfall view"
\image latex qpsk-signal-oscope.png "QPSK63 signal transmitting text data - oscilloscope - waterfall view" width=5.0in
<br>

\image html qpsk-signal-waterfall.png "QPSK63 signal transmitting text data - waterfall view"
\image latex qpsk-signal-waterfall.png "QPSK63 signal transmitting text data - waterfall view" width=1.0in
<br>

\section multi_bpsk_modem  Multi-Channel BPSK modems

\image html MultiChannelBPSKWaterfall.png "PSK63R20C signal transmitting text data - waterfall view"
\image latex MultiChannelBPSKWaterfall.png "PSK63R20C signal transmitting text data - waterfall view" width=4.0in
<br>

\image html MultiChannelBPSKOScope.png "PSK63R20C signal transmitting text data - oscilloscope view"
\image latex MultiChannelBPSKOScope.png "PSK63R20C signal transmitting text data - oscilloscope view" width=4.0in
<br>

\section eight_psk_modems  8PSK modems

The 8 PSK modes are intended for use on VHF/UHF FM systems.  They provide a very 
high data rate suitable for use with both flmsg and flamp and the transfer of 
digital data.
<br>

<center>
Mode      | Baud  | WPM  | Mode-FEC  | Baud  | WPM
:--------:|:-----:|:----:|:---------:|:-----:|:----
8PSK125   | 125   | 635  | 8PSK125F  | 125   | 317
8PSK250   | 250   | 1270 | 8PSK250F  | 250   | 635
8PSK500   | 500   | 2540 | 8PSK500F  | 500   | 1690
8PSK1000  | 1000  | 5080 | 8PSK1000F | 1000  | 3386
----------|-------|------| 8PSK1200F | 1200  | 4170
</center>
<br>

The FEC modes do not all use the same Viterbi polynomials to achieve the forward 
error correction.  That is why the WPM rates for the FEC modes are not multiples 
of the base 125 baud.  The WPM rates are only an indication of relative rates.  
The actual transfer rate is highly dependent on the data content.  This is always 
true for modes which use a VARICODE.
<br>

The 8psk signal is similar to both bpsk and qpsk, but with 8 possible phase states 
instead of the 2 and 4 associated with bpsk, qpsk.  The format of the signal does 
not lend itself easily to a conventional AFC.  Instead, the modes should be used 
with RsID enabled.  The RsID signal will both determine the mode and the mode 
center frequency (to the nearest 2.6 Hz).  A finer resolution of the mode center 
frequency can be made using the optional pilot carrier.  This pilot carrier is 
placed at the frequency
<br>

f0 – samplerate / symbollen, f0 is modem center frequency
<br>

samplerate / symbollen is the same as the mode bandwidth.  The waterfall and 
spectrum signature for the 8psk500 mode is shown here with and without the 
pilot carrier:
<br>

\image html eight-psk-signal-oscope.png "8PSK125 signal transmitting text data - oscilloscope - waterfall view"
\image latex eight-psk-signal-oscope.png "8PSK125 signal transmitting text data - oscilloscope - waterfall view" width=5.0in
<br>

\image html eight-psk-signal-waterfall.png "8PSK125 signal transmitting text data - waterfall view"
\image latex eight-psk-signal-waterfall.png "8PSK125 signal transmitting text data - waterfall view" width=1.5in
<br>

The two oscilloscope views above clearly show the combined phase and
amplitude modulation of these modes.
<br>

\image html 8psk500f.png "8PSK500F idle signal without pilot"
\image latex 8psk500f.png "8PSK500F idle signal without pilot" width=3.0in
<br>

\image html 8psk500f-pilot.png "8PSK500F idle signal with pilot"
\image latex 8psk500f-pilot.png "8PSK500F idle signal with pilot" width=3.0in
<br>

\image html 8psk500f-spectrum.png "8PSK500F idle signal without pilot"
\image latex 8psk500f-spectrum.png "8PSK500F idle signal without pilot" width=1.5in
<br>

\image html 8psk500f-spectrum-pilot.png "8PSK500F idle signal with pilot"
\image latex 8psk500f-spectrum-pilot.png "8PSK500F idle signal with pilot" width=1.5in
<br>

Decoding errors are reduced as the tracking point nears the actual transmit center 
frequency.  The loss of signal power is more than offset by the decoder improvement.  
Field testing has shown that the pilot tone needs to be at -40 dB or greater 
(less negative).
<br>

Pilot tone detection does require a bit more cpu power.  The pilot tone is detected 
using a sliding fast Fourier transform, sfft, which computes the frequency of 
the pilot to approximately 1 Hz resolution.   The sfft only evaluates the signal 
at 11 discrete frequencies, so it is necessary that either the RsID, or manual 
tuning is used for the initial signal acquisition.  The detector is set to provide 
lock when the pilot s/n is 2:1 or better.  The signal tracking point is then 
adjusted to place the pilot tone at the correct frequency location.  For example, 
if the RsID put the tracking point at 1502 Hz, the pilot would then adjust for 
1500 (if that is the correct tracking frequency).  The adjustment is made once 
per second.  Unlike AFC, which is continuous, the pilot adjustment is discrete 
and occurs in 1 Hz steps.  If the pilot s/n is less than 2:1 then no adjustment 
is made to the tracking point.  The pilot tone is transmitted during the 8psk 
preamble as well as during the data transmission.  You should see the tracking 
point adjust once at the beginning of the transmission and then stay fixed.
<br>

With these modes, a very linear transmitter is required. Over-driven
operation results in excessive bandwidth, poorer reception and
difficult tuning. Overdrive usually occurs by having the audio
signal much too large. These are very sensitive modes and usually very little
power is required. QRP operation of 80, 40, 30 and 20 meters can provide
nearly 100% copy over multi-hop paths. In many instances PSK can provide
better decoding than CW.
<br>

Setting up for a good clean on air signal that will receive the accolades
of your QSO partners is easy. Follow the instructions on using the
\ref audio_adjust_page "Tune" button</a> and you will have a clean on signal.
<br>

Good reception of PSK signals requires that the demodulator be phase locked
to the incoming signal. Fldigi has both a fast acquire / slow tracking AFC
system. Place the red bandwidth bar (see above) so that it overlies
the desired signal and then press the left mouse button. The signal
should quickly lock on a decoding should commence immediately. It is
almost impossible to visually tell whether a BPSK or QPSK signal is
being received. Under very high s/n you might be able to hear the
difference, but that is even difficult for most operators. If you are not
able to decode a signal that looks like a BPSK and the bandwidth of the
signal matches the baud rate then it might be a QPSK signal. Just change
mode a try reacquiring the signal.
<br>

For further information about <a href="http://en.wikipedia.org/wiki/Phase-shift_keying">Phase Shift Keying</a>
<br>

\ref psk_page "Return to Top of Page"
<br>
\ref main_page "Return to Main Page"


*/