[Lf] [Fwd: LF: PSK08 Tests / CPM]

[Andre' Kesteloot]

Klaus von der Heide wrote:

> Andy, G4JNT wrote:
>
> ...
> <There is another modulation scheme suitable for this band that
> <does allow constant amplitude, and hence highly efficient class D
> <amplifiers.  That is Continuous Phase Modulation (CPM), of which
> <Minimum Shift Keying is an extreme example.   Listen to the
> <transmission from Rugby on 16kHz or 22.1 kHz for an excellent
> <example of a 100 Baud version of MSK.
> <
> <CPM is a very processor and memory intensive mode hence it is only
> <now becoming popular, but at the speeds used at LF my feeling is
> <that a PC could cope even if a 56002 EVM does not have the necessary
> <memory capacity.  CPM is theoretically a few dB better than PSK in
> <S/N terms, and in fact with the right coding is said to be able to
> <get to within 0.7db of the Shannon limit. Take this together with
> <the constant envelope waveform and it could prove very exciting for
> <very weak signal LF dxing.
> <
> <If anyone reading this has a more intimate knowledge of CPM please
> <get in touch.  As far as I 'm concerned its all squiggles in a text
> <book at the moment, and as for turning these into DSP code....
>
>
> CPM ist the general class of continuous phase modulation.
> All modulation schemes resulting in constant amplitude
> and continuous phase belong to this class. Especially
> in conjunction with the Partial Response technique high
> data rates at low bandwidth can be achieved.
>
> Although the 136 kHz band seems to be very narrow,
> amateur LF-transmissions are limited in power, not in
> bandwidth, i.e. the extremely low power forces the
> information bit rate into the region of 1 bit per second.
> A bandwidth of less than 1 Hz would be possible. But,
> on the other hand, Shannons Formula says that LARGER
> bandwidth is better if you are limited in power.
> Indeed, as Andy, G4JNT, mentioned, the bitrates are so
> low, that a PC could do the job of decoding.
>
> I therefore propose to use PSK (and possibly FSK for
> transatlantic distances because control of the phase
> may be lost), at a baudrate several times the
> information bitrate. A good example is the interleaved
> deep space code used by Pactor 2. Since the bit rate on
> LF is much lower, the same, i.e. soft-Viterbi-decoding
> of whole packets, could be done at lower code rate
> and longer constraint length. The best code for that
> purpose known to me is the (56,14,36)-convolutional
> code. I am searching for even longer constraint lengths
> than 14. (Pactor 2 uses contraint length 9 at rate 1/2).
>
> As a textbook I use K.D.Kammeyer: Nachrichtenuebertragung
> (Communication Theory), Teubner,1992.  CPM and Partial
> Response is well written - may be because the book is
> some 8 years old.
>
> At the moment, I am very busy in building up a practical
> course on communication technique for students of computer
> science at the University of Hamburg based on simulations
> in MATLAB including coding and Viterbi-decoding.
> Unfortunately, there is no time left for amateur radio
> except from writing an e-mail.
>
> 72 de Klaus, DJ5HG