1. A few words about OFDM and serial tone modem.
Let's find out how the fight between ISI and Doppler shift
takes place in these systems. OFDM uses the great number of low
speed channels so the symbol duration increases. While the
duration of ISI is much smaller than symbol duration everything
goes well. Consequently there is an aim to increase the number
of channels ad infinitum BUT at the same time natural limitation
takes place. It's just a Doppler shift effect. Hence there is
always a compromise between ISI and Doppler shift. Moreover we
should take into consideration a big peak factor which results
in non-effective usage of power of transceiver. There are
methods directed at improvement of peak-factor, but the most
part of them makes the system characteristics worse.
In case of serial tone modulation the fight ISI with Doppler
is provided with adaptive algorithms. The more effective and
faster they are the larger number of Doppler and ISI the modem
can manage.
As for RFSM it should be mentioned that now it includes
rather efficient adaptive algorithms that work properly at a
speed of 600(500) up to 4800(4000) bps (wide/narrow mode). To
work at a speed 6400(5333) - 8000(6666) much more compound
algorithms are needed. In particular using turbo-equalization
will improve noise proof feature at all rates.
Therefore OFDM and serial tone modem can be more efficient
in dependence on channel statement. In my opinion serial tone
modem with effective adaptive algorithms is the most effective.
We'd like to mention that under certain circumstances either
serial tone or OFDM modem can fail to provide connection, for
example, when the Doppler shift is extremely high (polar
communications). In that case one should use the methods of
"spectrum spread" that extending the symbol in time and
frequency. Unfortunately the speed would not be high in this
case.
So the best way out is to measure the channel
characteristics and choose the speed of transmission and
modulation method according to them. The full adaptation of the
all characteristics is required.
2. About our users.
The project RFSM-2400/8000 was initially aimed at
organizations (not for HAMs)! (First version had no 0,3-2,7
band, which is adapted for HAMs).
Its prime value is that high-performance algorithm is used
in it. Consequently only technical specialists of organizations
where data (files, mail etc.) transmission through HF is needed
can estimate the program at its true worth. They need the
following: high speed of connection and data transmission. They
are the FIRS GROUP OF OUR USERS. For example there are
organizations (our users at the moment) who even haven't looked
upon HAM -modems (little speed, instability, absence of files
transmission in spite of excellent chat-exchange).
If you are interested in RFSM as in a program for chat-
exchange (or even for file transmitting but you do not need a
high speed) and runner is not important for you:. You are the
SECOND GROUP OF OUR USERS. $60 may be a pretty penny for this
product for you.
There is also not numerous GROUP OF USERS - THE THIRD ONE
The representatives of this group are specialists in HF-
radiocommunications and radioamateurs at the same time who is
interested in algorithms of a high efficiency - the runner of
the program. May be $60 is rather expensive for them but they
can trial versions for free. They communicate with us suggesting
interesting and moreover useful ideas. We really appreciate
their advices and suggestions. Due to the THIRD GROUP the first
version of RFSM has transformed in the product adopted for HAM.
3 . There are several remarks on the open source codes.
a) RFSM-2400 (and all the more RFSM-8000) is not just a
"dumb" modem though such a rate is also possible (it was used in
PSKMail). Our product is an accomplished system of communication
that provides different types of services including
receiving/transmitting e-mail on Internet.
b) Speaking about OFDM it should be pointed out that we have
got experience in such a kind of modulation and can remark that
to construct this modem is incommensurably easier than Serial
Tone Modem. But the modem of this kind doesn't compare with RFSM
characteristics. If we were not be able to realize Mil-STD
correctly and use OFDM in RFSM, we would not be sorry to
distribute source codes.
c) Philosophy. Professional free software is possible
because qualified developer has been grown up by certain
company. The buyers have already paid for software and
progressive developer as well. Then at the same time free
software appears (like RFSM-2400) - like an ad, to create an
image or ease consumers' tasks. The fact that software is free
is a result of successful sales of developer. However free
software is not possible in fact. The bigger the quantity of it
the poorer it's quality. So said "Write on C++ for food ;)"
There is also rather INTERESTING free software that contains
ads in bulk - for example mobile phones or cars or pasta ads.
You don't have to pay for this software!!! BUT you have already
paid for this software as an end-user of these mobile phones,
cars, pasta: As the prices of these goods include the price of
software.
d) You insist that the program should distribute for free.
Ok. RFSM-2400 - a result of years of investigation - is
absolutely free. Now you say that you are not going to use it as
it is not widespread. To sum up what's been said you do not need
this program for free or ... whatever! So we are just making
a conversation...
Dmitry.
