On 20/12/2014 1:32 PM, David Rowe wrote: > Hi Glen, > > I just played around with the timing (Toff variable) in gmsk.m. At the > 1% BER, Eb/No = 6dB point the demod BER hardly changes with +/-10% > timing error, and the BER doesn't double (1dB implementation loss) until > I plug in a timimg error of -30%, +20%. > > So I'd say we have a good shot at getting an ideal-performance timing > estimator running. That 1981 paper suggest looking at the difference > between the Re and Im arms, or we could even just try sampling at 10 > different timing offsets and look at some statistic like the eye > opening. I'm guessing that no raised root cosine filter makes life > easier than PSK. > > I wonder if phase estimation is hard for GMSK?. I'm not even sure what > DStar does for that, have to read the spec. There is some implied phase > information in that logic at the end of the coherent demod, IIRC the way > phases evolve can be modelled as OQPSK following some sort of state > machine. Only certain state changes are allowed. > > I think it might be worth pushing this simulation through to the point > where it can be tested over real VHF channels using stored files. We > could then try using a regular FM radio and an SDR and see what sort of > gain we can get over over FM. > > Not sure what to do about a transmitter ..... not sure if we can rely on > a FM modulator to generate GMSK accurately. Maybe a SSB radio using > upconverted stored files of GMSK? At 1200 bit/s that would be 1200 Hz > wide. However we couldn't reproduce FM That way using the same tx/rx. > > I like the name G-star! > > Cheers, > > David > > On 20/12/14 11:13, glen english wrote: >> Hi David >> 1- Any FM demodulated process. If it's post demod, there will be the >> threshold. >> that's right - no one cares about low CNR 'cause it is unusable. >> 2 - and you got to do carrier recovery very quickly for burst transmissions. >> 3- I think good speech quality if the system allows for it at the VHF is >> just as important as HF. Probably estimated SNR would be enough to >> decide to use the extra bits. >> It's a matter of coming up with a good scheme for codec2 to provide >> better quality with say 2 x the bits. >> 4- With a tighter rolloff, finding those clock spectral lines is always >> going to be more difficult than if there is , at the extreme, zero rolloff. >> >> For my G-star system which I am fiddling with for VHF, I have your >> codec2 @ 1000 bps , FM discriminator detection and block product turbo >> coding. You need to deal with the clicks and improve threshold >> performance for it to all be worthwhile, and the FEC satisfactory does that. >> >> I have not spent the time yet to sort your codec to provide 1) >> hierachical performance and b) unequal protection of certain bits (IE I >> have not spent the time to understand which bits are most important- >> except for the obvious which is the MSBs are always more important than >> the LSBs ! >> >> cheers >> >> >> On 20/12/2014 11:24 AM, David Rowe wrote: >>> Thanks Glan, I was hoping some one with a solid comms background would >>> take a look at those recent posts. Some questions: >>> >>> 1/ Re the post-FM scenario, do you mean using FM radios with through >>> their "data" (discriminator) port? Thanks, I had wondered what those >>> detectors look like a low CNRs. Clicks don't sound like much fun. >>> >>> That might be why the non-coherent demod I described first in the post >>> is popular - it works whenever the current FM radios do. So no one >>> notices the loss of low CNR modem performance. >>> >>> 2/ Thanks for the tip on low CNR clock recovery, not something I've had >>> to look at yet with MSK, although I've achieved good results with PSK >>> modems. (G)MSK is pretty close to OQPSK. I might try some clock >>> recovery simulations at low CNRs and see how we go. That 1981 paper had >>> near ideal results in their practical modem 30 years ago, so suspect it >>> can be done with a non-post FM (ie SDR) radio. We might need >>> differential detection for phase recovery, I haven't looked into that yet. >>> >>> 3/ Yep, the hierarchical scheme is what I'm currently looking at for HF, >>> in particular pushing the speech quality lower for poor channels. >>> Thinking the opposite for VHF. With plenty of bandwidth on VHF we can >>> just send a sub-carrier at say 6dB lower (even with GMS) for >>> supplementary info. It could have a simple checksum or we could >>> estimate SNR. >>> >>> 4/ What technical problem does MSK/RRC MSK make easier than GSMK? >>> Agreed we've got plenty of bandwidth. >>> >>> 5/ I'm thinking of a couple of modems. One that gets through standard >>> FM or data port radios, and a high perf SDR based mode. They could have >>> the same bit rate and protocol, and the rx could auto-detect the >>> waveform in use. >>> >>> Cheers, >>> >>> David >>> >>> On 20/12/14 10:04, glen english wrote: >>>> Hi David >>>> >>>> Good analysis. I think though there needs to be some clarifications >>>> (having done a few of these in my professional capacity) >>>> >>>> 1) 10dB over analog FM .?Not so fast ! I reckon 3dB worse at best. >>>> You'll need to clarify whether you are using a a post FM detected >>>> output, because the threshold effect will knock you over. >>>> >>>> 12dB SINAD wont occur much before 6dB CNR, depending on the limiter and >>>> demodulator characteristics.. The clicks will be numerous and this will >>>> knock out many bits, especially as you narrow the post detection >>>> bandwidth, the click length will cover more symbols. >>>> So the best you might do is on par with analog FM isf ising post >>>> detected output. >>>> >>>> My guess is it will be 3dB worse than 25kHz FM for the first 6dB, and >>>> then of course much much better. >>>> >>>> Even if you do demod pre the FM demod and limiter, clock recovery of >>>> GMSK for coherent detection, especially down in the noise is HARD. In my >>>> experience, this is the undoing. >>>> >>>> So GMSK is generally accepted as simple with a tradeoff. No free lunch. >>>> Most engnieers come to the same conclusion. YOu limit and FM demod, >>>> which is a simple receiver, you give away the low signal low CNR operation. >>>> >>>> HOWEVER ! >>>> We've got plenty of bandwidth,so why not just MSK, or RRC MSK ? There >>>> is no need to close the eye with the tight filter, we've got no need for >>>> narrow bandwidth >>>> >>>> That is unless you want to get it through a standard FM radio. >>>> >>>> I've long been a proponent (and in my private communicationss with you) >>>> I've always been in favour of a hierarchical codec and modulation. >>>> >>>> That is the lower bit rate codec is encoded and transmitted very robust, >>>> so when the goign is really tough, it still works. >>>> Bits that provide the codec more precision are transmitted in a layer >>>> that requires a higher SNR, and are used when available. I expect that >>>> because of the no FEC nature, there will need to be some sanity check >>>> on whether those higher order bits are useful. >>>> >>>> IE when the going is really tough, the quality is low but it works. >>>> But then the signals are good, the quality improves. >>>> This is the current advanatage of wideband FM (25kHz) over digital >>>> systems. For most digital systems, we get stuck with the LCD. >>>> >>>> But you are correct, the GMSK does not easily lend itself to >>>> hierarchical schemes. >>>> >>>> >>>> glen >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> On 20/12/2014 7:22 AM, David Rowe wrote: >>>>> This week I've been working on GMSK modem simulations: >>>>> >>>>> http://www.rowetel.com/blog/?p=3824 >>>>> >>>>> - David >>>>> >>>>> ------------------------------------------------------------------------------ >>>>> Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server >>>>> from Actuate! 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-- - Glen English RF Communications and Electronics Engineer CORTEX RF & Pacific Media Technologies Pty Ltd ABN 40 075 532 008 PO Box 5231 Lyneham ACT 2602, Australia. au mobile : +61 (0)418 975077 ------------------------------------------------------------------------------ Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server from Actuate! Instantly Supercharge Your Business Reports and Dashboards with Interactivity, Sharing, Native Excel Exports, App Integration & more Get technology previously reserved for billion-dollar corporations, FREE http://pubads.g.doubleclick.net/gampad/clk?id=164703151&iu=/4140/ostg.clktrk _______________________________________________ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
