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! 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 > -- - 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
