[hpsdr] The problem with digital modes

[David Holman]

Personally I don't think that TDMA or CDMA have a good place on the HF bands. The military uses those techniques with varying degrees of success on HF. I do think that digital voice is something that we need to do more of. There are waveforms that are compatible with the HF bands and work great.  Are they perfect, no, but developing new things has been the mantra of the Ham for decades. And we need to develop more HF compatible waveforms. And they don't need to consume more bandwidth than SSB or AM does now. 

However the excuse that "a problem with trying to make that switch on HF is that such modes would be incompatible with the existing users of the bands" is is a crock of crap. You don't have to allocate special sections of of the band.  The FCC decided some time ago that digital voice is still voice and can exist in the voice sections of the band.  I hear that excuse all the time when I am trying to convince public service folks to use use DStar on their events. DStar and D-RATS are much more efficient at handling event traffic than FM Voice.  Just because someone might not have it, is is a crock of crap excuse for not having it as an option or using it. For those who find it incompatible there is nothing stopping them from joining in the fun.  

Just my 2 cents

David, AC7DS




-------- Original message --------
From: Shirley Márquez Dúlcey <mark at buttery.org> 
Date:05/12/2014  11:24  (GMT-07:00) 
To: hpsdr at lists.openhpsdr.org 
Subject: Re: [hpsdr] The problem with digital modes 

I agree that, in that case, we really could use better CODECs. Nearly everything we do in ham radio is based on narrow-band frequency-division multiplexing. We could certainly stand a bit more time-division multiplexing or, better still, frequency-hopping or CDMA spread-spectrum multiplexing. I'm not going to hold my breath tho'. That would ensure that the spectrum was used more efficiently. 

A problem with trying to make that switch on HF is that such modes would be incompatible with the existing users of the bands; you would have to set aside a band segment for TDMA or spread spectrum use. (Spread spectrum that used an entire HF band would raise the noise floor of the band and would be considered undesirable by anybody trying to do weak signal work. FCC rules don't allow spread spectrum on HF in any case, and nobody has yet proposed making it legal.) We are seeing some experiments with TDMA on VHF and UHF frequencies. DMR (and its gussied-up even more proprietary cousin, MOTOTRBO) uses TDMA to put two voice channels into the space of one. But it might be better to take an entire band segment and set it aside for something that worked more like GSM or CDMA rather than intermixing DMR and analog FM channels.

CDMA historically had the problem of patent encumbrance. Some of the relevant patents expired recently and others will expire soon, so it may be time for some new ham work on that mode. One catch is that the spreading standard would have to be made widely available; otherwise it would constitute an unauthorized cipher. It's also not clear how the ID requirement could be met in a CDMA system.

I am not pleased by the proliferation of ham radio modes that use proprietary technologies - things like PACTOR, or the various digital voice modes that use the proprietary AMBE codec. The need for that codec spoils the otherwise basically open DMR standard, and now that the free software Codec2 is available there is no good reason for hams to be putting their efforts there. 
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