soundmodem/newqpsk

NEWQPSK modem for the Usermode Soundcard Modem driver
=====================================================

This package implements the NEWQPSK protocol (also known as Q15X25) as
designed and defined by Pawel Jalocha, SP9VRC, <pawel.jalocha@cern.org>.
Pawel's original modem is written in Motorola 56k assembler for the
DSP56002EVM evaluation module or the Alef Null DSP Card 4. This code
is a port of the original code to C.

Most of the features of the original modem are implemented. The most
important feature still lacking is the support for tuning the transceiver.
The modem has been used for real life QSOs with performance comparable to
the original modem.

Soundmodem configuration
========================

There is an IMPORTANT detail in soundmodem core configuration concerning
NEWQPSK. In the "Channel Access" section one should set "TxDelay" to a
non-zero value. The actual value does not matter, the modem will ignore
it. (If your transmitter is slow and you need to compensate that then you
can increase "Tune length" as explained later.) Equally important is that
one sets "TxTail" to zero. Failing to set these will cause the NEWQPSK
modem to behave erratically.

The actual NEWQPSK modulator and demodulator are subject to five
configuration parameters that can be set with the `soundmodemconfig' tool:

  Bits per second

	This sets the modem raw bit rate. The available range is 1000 to
	5000. Rates 2500bps and 3000bps correspond to sample rates of
	8000sps and 9600sps in the original code respectively.

  Interleave depth

	Sets the interleave depth. Allowed range is 0...16. Zero
	disables interleaving.

  FEC level

	Sets the used error correcting code. Possible values are:

		0	no FEC
		1	simple (15,11) FEC
		2	BCH (15,7) FEC.
		3	Walsh (15,5) FEC

	Note that BCH (15,7) is NOT compatible with the original
	EVM56K modem.

	The FEC overhead can be calculated from the figures in
	parenthesis. For example Walsh (15,5) lowers the user data
	rate to 2500*5/15 = 833.3 bps.

  Tune length

	On modulator side this sets the length of the tune preamble
	in symbols. Allowed range is 0...64.

	On demodulator side this sets the minimum of received
	tune preamble symbols needed to make a tuning decision.
	Allowed range is 0...32. Zero means the demodulator will jump
	directly to waiting sync state.

  Sync length

	On modulator side this sets the length of the sync preamble
	in symbols. Allowed range is 16...64.

	On demodulator side this sets the minimum of received
	sync preamble symbols needed to make a symbol sync decision.
	Allowed range is 8...32.

More detailed configuration of the modem parameters can be done at compile
time with the modemconfig.h file in this directory. The parameters there
are not to be changed unless you know what you are doing... :)

NEWQPSK FEATURES
================

NEWQPSK modem was designed primarily with HF in mind. You are supposed to
connect it to a normal HF SSB rig. However NEWQPSK might also find some
use in VHF/UHF FM packet. As FM has no frequency ambiquity the tune
preamble can be disabled (tune length = 0). Also it might make sense to
shorten the sync preamble.

NEWQPSK is an AX.25 packet modem and can be used for anything regular
packet can, including TCP/IP. Here is a short list of NEWQPSK highlights:

  - AX.25 compatible

  - 15 carriers spaced by 125 Hz (150 Hz) modulated with DQPSK
    (differential 4-level phase shift keying) at 83.33 baud (100 baud)

  - Total raw data rate 2500 bps (3000 bps)

  - Two phase preamble for frequency shift correction and fast symbol
    sync. The modem can correct frequency shift up to about 100 Hz.

  - Forward Error Correction: three simple schemes with different levels
    of data redundancy

  - Time/frequency diversity (interleave) to de-localize burst errors

(The figures in parenthesis above are for sample rate of 9600 sps.)

Note that the envelope of the transmitted signal is NOT constant. The
modulation has a crest factor of about XX dB which means that with a 100W
transmitter and a power meter that reads average power you should adjust
the output reading to about YY Watts. Exceeding this will cause clipping
that will not only cause interference to other band users but also make
copy at the other end worse.

Also keep in mind that this is a WIDE mode (about 2000...2400Hz wide). You
should be very careful not to transmit over other band users and you
should probably keep away from the narrow band digital segments.

DISCLAIMER
==========

The sole purpose for me to write this modem implementation was to learn
about DSP in general and Pawel's novel modem in particular. I'm fairly new
to DSP and this code is mostly a result of countless hours of staring at
the original Motorola 56K assembler source and banging my head against the
wall... I can't claim I understand all aspects and every detail in the
modem. There are probably misunderstandings and errors in the code and if
you spot one, please tell me. I want to learn more. Also if you have ideas
for further development please share them with me. I have already some
ideas of my own.

-- 
Tomi Manninen, OH2BNS <tomi.manninen@hut.fi>