A bigger amplifier is the solution to that. It might bring some other
amplifier technologies to the ham bands. Even with existing equipment,
if you have a 1 KW linear, that translates to 125 watts of digital which
is not bad at all. The actual addition of carriers is also phase dependent.
R
Patrick Lindecker wrote:
Hello to all,
For me, the main problem, for Hams, of the multi-carriers modulation (OFDM...)
is that the power is drastically limited (if you want to, legitimally, keep
linear):
If you have two carriers in parallel, the mean power/max power ratio is equal
to 1/2
If you have three carriers in parallel, the mean power/max power ratio is
equal to 1/3
when n becomes big, the ratio tends to 1/square(n) (the carriers phases being
independant, with application of the big numbers law)
For example, for MT63 where you have 64 carriers in parallel, the ratio is
1/8. You transmit only 12.5 watts with a 100 watts maximum XCVR.
73
Patrick
- Original Message -
From: Bill McLaughlin
To: digitalradio@yahoogroups.com
Sent: Saturday, March 17, 2007 3:26 AM
Subject: [digitalradio] Re: MPSK vs OFDM vs MFSK for HF High Speed Data
Hi Bonnie,
Thanks for initiating this discussion:
Throw the prospect of incremental frequency shift keying into the mix
for discussion; know a few are working on this mode(s)also.
The OFDM (AM-QPSK)+6dB better SNR may or may not be an issue...it
depends on usagethe usual HF near LUF versus nearer MUF or
VHF/UHF question.
The key may well be your comment later, all other factors being
equal. Greater raw throughput seems very dependant upon S/N (we all
know this intuatively). You are correct, PSK overall is a known
quantity...QPSK abit less so.
In a sense you have hit upon the crux of the issueam simple so
bear with me. If the SNR is high enough, then higher raw throughput
is available. Question (well one of them) for discussion; where is
the threshold? Also some mitigating factors such as robustness
(never sure that has been defined) and the ever-lovable bandwidth.
73,
Bill N9DSJ
--- In digitalradio@yahoogroups.com, expeditionradio
[EMAIL PROTECTED] wrote:
Since there is work presently being done to advance HF data
communications, I thought it would be good to start a dialogue
here about the advantages of PSK signals on HF, over some of
the other choices. I'm not a world expert on these particular
systems, however, I have used them and have an understanding. I
have also been involved in design engineering of commercial
radio communications using high speed data OFDM, AM-QPSK, and
MPSK on DSP platforms. I hope that some of the individuals who
are working on new HF data systems and data modes will engage
in this discussion.
Background.
PSK signals have been long proven for HF communications.
The MIL STD 188-110 type PSK signals have been in constant use by
government and other entities for HF data and email, and they are
now being adopted by hams.
The standard 188-110 serial tone modem is an example of a
Multi-Phase PSK signal (8PSK) running at a phase shift symbol rate
of
2400 symbols per second. That means it shifts a constant carrier's
phase between 8 different polar degree positions 2400 times per
second. This raw bit speed is modified by software to get a data
channel at various selectable levels from 75 baud to 4800 baud.
The lower baud rates such as 75 baud, provide more robust comms,
capable of low SNR, and operation in weak signal conditions.
The highest baud rates such as 2400 baud provide faster data
throughput but require a somewhat better quality channel, not
weak signals.
This -110 MIL Standard 8PSK signal is about 3kHz wide.
It has an audio baseband signal approximately 300Hz to 3300Hz
with a center frequency of 1800Hz. Some of the newer ham radios
have adequate passband width for this signal.
Since most ham radio and commercial SSB transceivers have a more
narrow passband (~2.5kHz), at least 2 modified non-standard
versions of the -110 PSK signal were independently developed
(MARS-ALE and RFSM2400) to fit within the narrower SSB passband
of ham transceivers.
The RFSM2400 uses a 6PSK signal at 2000 symbols per second for its
narrow non-MIL-standard mode rather than the 8PSK MIL-standard
signal.
It is centered on 1500Hz, and provides an audio baseband signal
that is approximately 300Hz to 2700Hz. It also uses a short burst
of BPSK signal for sync/control.
Why Multi-Phase PSK?
Phase detection is inherently faster than tone frequency detection
such as used with FSK or MFSK signals. In the present state of the
art
for Frequency Shift Keying demodulation, the tone is present for
several cycles to be detected reliably at audio baseband, so this
