Thanks Glen - great to have independent confirmation of our 10dB-ish system gain results.
I started with the DStar GMSK pulse shaping coeffs which I think is BT=0.5? Yes I think it's theortically about 1dB off PSK but I'm not sure what "alpha" to put in the theoretical BER equation. It's all in gmsk.m if you can read octave/matlab code. Yes I spent a Merry Xmas coding up a digital PLL, used the design from the original GMSK for telephony paper back from 1981. refs and code in gmsk.m Re better than SSB, lemme run BW*CNR on the scheme in this blog post: http://www.rowetel.com/blog/?p=3700 That works at -8dB AWGN SNR (3000 Hz), but the QPSK signal is actually about 500 Hz wide. So the SNR (500 Hz) is -8+10*log10(3000/500)= -0.218 or linear 0.95. So is that CNR*BW = 0.95*500? Not sure if I did that right. Sorry to other readers for all the comms math! Then again ... this stuff will matter for Ham's in the 21st century. Cheers, David On 06/02/15 12:16, glen english wrote: > Hi David > Coherent demod! no wonder it does so well. So you are demodulating and I > and Q with locked carrier like genuine MSK. > In that case, little benefit with QPSK I suspect. Maybe a dB ..maybe a > bit more with the more open eye on QPSK . What BT are you using for the > G in GMSK ? I gather there is no squeeze for bandwidth so quite broad.. > > OK so it is a couple of dB better than my numbers assumed. so , more > like 9 to 10dB advantage.. Yep I'd believe that . > > Actually I prefer to compare in terms of temperature, or Janskys, or > (bandwidth * CNR) > ...bandwidth*CNR is nice and easy. > FM 3/5 25k wideband = 2.5*16000 = 41000. > > I disagree that hiss in the background is an issue. > In fact, I strongly disagree. > In fact, I think it is a right pain not to know 'if the channel is open' > because there is no background noise. > But of course, we must quantity this. Perhaps less than 16dB SNR is > annoying for armchair copy. > But if you are an HFDX SSBer, then you'd be used to 0dB SNR. > > I reckon the real bar to do better then is SSB- (CNR*BW) of 2400 is > quite usable. That woudl require the codec to work down around a couple > of dB CNR. > > On 6/02/2015 11:47 AM, David Rowe wrote: >> Hi Glen, >> >> The Codec 2/GMSK combination sounds error free at Eb/No = 6dB, 1% BER. >> I think that's a SNR of 6dB if we assume a noise BW of 1200 Hz and 1200 >> bit/s. It's a coherent demod. You can maybe drop it 2dB for less than >> 5/5 readability. >> >> For example: the 35.5dB C/No test point on the blog post (which I would >> say is still 5/5) works out to 35.5 + 10*log10(1200) = 4.7dB Eb/No (and >> SNR if you use a 1200 Hz noise BW). >> >> Thanks for the calculations, great to have another way of working it >> out, especially to actual levels at the antenna. Your real world >> experience is very valuable. Sound's like we are close to each other. >> >> After SNR=Eb/No=6dB, the DV system is noise free. That's an important >> gain as well, constant hiss is very fatiguing. But then so is Codec 2 >> as well for some people I guess ;) >> >> Re the SSB radio. I'm not proposing everybody uses SSB radios to send >> Codec 2 over GMSK. It was just something we had laying around so we >> could quickly test this concept. It's just a mixer. We are just 2 guys >> working part time to test an audacious goal - so we improvised. >> >> For people to use this for real, we need a new generation of VHF radio >> hardware. A radio you and I can buy, power up, and start using. That's >> what Daniel and I plan to build in 2015. Like the SM1000, but a real >> TDMA capable, "open" VHF radio people can use - and hack. For a couple >> of hundred bucks. >> >> QPSK is an option but needs a linear(ish) PA. GMSK can be interpreted as >> offset QPSK at half the symbol rate (so the papers say). It would save >> some bandwidth but even GMSK is only using 1200Hz which is a huge >> improvement over current VHF systems. I'm not sure PSK will give us 3dB >> Eb/No versus BER gains? My understanding (and simulations) show GMSK >> pretty close to PSK. >> >> Cheers, >> >> David >> >> On 06/02/15 10:08, glen english wrote: >>> Yea, nice work David. >>> What SNR do your simulations say the modem will work to, BER =1/50, BER >>> = 1/1000 ? >>> >>> I reckon, for a readability of 3/5 you will get the following on a >>> decent radio >>> T sky=290k, NF RX = 627k (a reasonable VHF receiver without any thing >>> fancy) Ttot = 917k >>> >>> analog FM '25k' : -123dBm (assumption - 4dB CNR requirement) >>> codec2 GMSK , linear radio, SSB passband : -130dBm (assumption - 6dB >>> CNR requirement) >>> 3/5 difference = 7dB. >>> >>> 5/5 readibility : >>> analog FM '25k' : -119dBm (assumption - 8dB CNR requirement) >>> codec2 GMSK , linear radio, SSB passband : -127dBm (assumption - 8dB >>> CNR requirement) >>> 5/5 difference - 8dB. >>> >>> note- my assumptions of 6 or 8 dB CNR req for codec2 gmsk. I'm assuming >>> you are not coherently detecting >>> >>> But, David, if you are going to use a SSB radio, then why not QPSK ? >>> >>> That will be good for another 3dB, perhaps, depending on how hard you >>> want to push your amplifier. The advantage of GSMK is the use of a non >>> linear amplifier, given. >>> >>> >>> g >>> >>> >>> >>> >>> >>> On 6/02/2015 9:46 AM, David Rowe wrote: >>>> Thanks Glen - I have replied to your comment on the blog post. >>>> >>>> Cheers, >>>> >>>> David >>>> >>>> On 06/02/15 08:40, glen english wrote: >>>>> You can't compare apples for apples this way, at least not as these >>>>> things are used in the REAL WORLD >>>>> >>>>> It is IMPROPER to compare an analog and digital system on SNR because >>>>> they are very different beasts. Of course the digital will outperform >>>>> the analog system one it get sa lower BER. >>>>> >>>>> The test should be , at what power level can you achieve a readability 5 >>>>> conversation ? , at what power level can you achieve a readability 3 >>>>> conversation ? >>>>> The power level requird should be normalized for a 0dB noise figure (so >>>>> the front end of the receiver system is taken out of the equation) >>>>> >>>>> Why ? because that is how we use our radios. >>>>> >>>>> however >>>>> >>>>> I have no doubt that with a modem operating with a SSB radio, it will >>>>> outperform the FM radio hands down. >>>>> >>>>> >>>>> On 6/02/2015 7:04 AM, David Rowe wrote: >>>>>> Daniel, VA7DRM, and I have been doing some tests using GMSK, Codec 2, >>>>>> and real radios. It appears we are getting at 10dB gain over Analog FM >>>>>> and 1sts gen DV systems: >>>>>> >>>>>> http://www.rowetel.com/blog/?p=3856 >>>>>> >>>>>> - David >>>>>> >>>>>> >>>>>> >>>>>> ------------------------------------------------------------------------------ >>>>>> Dive into the World of Parallel Programming. The Go Parallel Website, >>>>>> sponsored by Intel and developed in partnership with Slashdot Media, is >>>>>> your >>>>>> hub for all things parallel software development, from weekly thought >>>>>> leadership blogs to news, videos, case studies, tutorials and more. Take >>>>>> a >>>>>> look and join the conversation now. http://goparallel.sourceforge.net/ >>>>>> _______________________________________________ >>>>>> Freetel-codec2 mailing list >>>>>> Freetel-codec2@lists.sourceforge.net >>>>>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>>>>> >>>> ------------------------------------------------------------------------------ >>>> Dive into the World of Parallel Programming. The Go Parallel Website, >>>> sponsored by Intel and developed in partnership with Slashdot Media, is >>>> your >>>> hub for all things parallel software development, from weekly thought >>>> leadership blogs to news, videos, case studies, tutorials and more. Take a >>>> look and join the conversation now. http://goparallel.sourceforge.net/ >>>> _______________________________________________ >>>> Freetel-codec2 mailing list >>>> Freetel-codec2@lists.sourceforge.net >>>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>>> >> ------------------------------------------------------------------------------ >> Dive into the World of Parallel Programming. The Go Parallel Website, >> sponsored by Intel and developed in partnership with Slashdot Media, is your >> hub for all things parallel software development, from weekly thought >> leadership blogs to news, videos, case studies, tutorials and more. Take a >> look and join the conversation now. http://goparallel.sourceforge.net/ >> _______________________________________________ >> Freetel-codec2 mailing list >> Freetel-codec2@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >> > ------------------------------------------------------------------------------ Dive into the World of Parallel Programming. The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ _______________________________________________ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
