Posted: 7/30/2016 1:22:18 AM EDT
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Well, long story short I started asking myself some questions about the following article…No Ham Narrowband…and how some of the conclusions there were drawn. So I began doing my research on modulation indices, deviation and how it all relates. Drew some interesting conclusions.
To save some time, the modulation index as pointed out in the article is simply maximum deviation over maximum frequency modulated (3 kHz for analog). Where for digital signals the maximum frequency modulated can be replaced by the data rate divided by 2*symbols per bit. (i.e. C4FM uses 9600 bps at 2 symbols per bit to get 4800 b/s which is then divided by two for a modulated frequency of 2.4 kHz). After tracking down the deviation for different digital modes (more challenging than one would think) I was able to compare the differences to obtain a dB value relative to regular wideband analog (system gain). From that, a little finessing of the free space path loss formula to utilize a change in loss and we can see roughly how those dB values affect system coverage. And the results are as follows: Narrowband Analog – -3.01 dB, -29.3% coverage (I think ARS also accounted for early narrow capable radios not having narrow filters adding an extra 3 dB of signal loss) P25 Phase 1(C4FM) – -3.05 dB, -29.6% coverage (Confirms the claims of those who transitioned form analog 800 MHz trunking to P25 trunking in the same band about reduced coverage) P25 Phase 2 (TDMA) – +1.92 dB, +24.7% coverage DMR (TDMA) – +2.88 dB, +39.3% coverage (Real world I've seen ~36% coverage increase with DMR compared to wideband analog) NXDN (6.25 kHz) – +1.46 dB, +18.3% coverage D-Star (GMSK) – -2.22 dB, -22.5% coverage Fusion (C4FM) – +1.3 dB, +16.2% coverage Now some things to note. NXDN should perform more effectively in narrowband (12.5 kHz) operation according to the modulation index compared to ultra narrow. Just because some of these present loss instead of gain doesn't mean they won't work out as far…they will just be more heavily reliant forward error correction and/or risk "going digital"/R2D2ing to the point where transmission quality begins to suffer (more than what FEC can keep up with). It should also be noted that even though Yaesu markets YSF as narrowband…it will not actually fit inside a 12.5 kHz channel (nor will P25 Phase 1 for that matter) and allow an adjacent channel to be co-located next to it. Also worth noting…the new Codec2 radios in the works should feature a gain of about 9 dB as they are actually sending a relatively small amount of data over a fairly large channel (5 kHz). So it'll be interesting to see how that works out. In the scheme of things, 3 dB isn't really a lot and in the end the pathloss doesn't change any and the ground is still the biggest attenuator that has to be overcome. As a result, these mathematical calculations most likely won't pan out very often in the real world as there are only so many things that we can actually account for when it comes to system design. |
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I can tell you the DMR figure you presented seems accurate to me. We have a FM analog and DMR 70 cm repeater systems deployed on the same tower. The DMR has lower EIRP due to feed line and antenna gain plus it's 75 feet lower on the tower.
We get the same coverage out of both machines. |
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Quoted:
I can tell you the DMR figure you presented seems accurate to me. We have a FM analog and DMR 70 cm repeater systems deployed on the same tower. The DMR has lower EIRP due to feed line and antenna gain plus it's 75 feet lower on the tower. We get the same coverage out of both machines. I've seen the same with DMR in my area. |
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I've seen the same with DMR in my area. Quoted:
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I can tell you the DMR figure you presented seems accurate to me. We have a FM analog and DMR 70 cm repeater systems deployed on the same tower. The DMR has lower EIRP due to feed line and antenna gain plus it's 75 feet lower on the tower. We get the same coverage out of both machines. I've seen the same with DMR in my area. Yea…I saw a 36% coverage increase. The tested repeaters were a Motorola GR500 (freshly aligned) and an XPR8300. Both located on what has become a county annex building in Lubbock, TX. One is the 700 GMRS machine, the other the DMR-MARC associated repeater. Both have their own DB420's with less than 30 feet of 1/2" heliax into the same model of Telewave duplexer. The GR500 puts out 35W into the duplexer, the XPR 25W. |
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I can tell you it was a noticeable difference when our county sar system went narrow . a loss of coverage for sure. So there is another story behind that. Many users complained of system coverage losses when upgrading from something like Motorola SmartNet to Motorola Astro 25 trunking. The data in my OP actually follows this. Motorola SmartNet systems were wideband 800 MHz analog trunk systems (800 MHz didn't have to narrow band and the channel steps are still built for 25 kHz where 700 and 900 MHz is built for 12.5 kHz). They upgraded to P25 (narrowband) 800 MHz systems. As shown in the data, P25 is no better (or really any worse) than narrowband FM it just happens to have the ability to send data alongside (position, status, talk group and user ID) the voice where analog had to either send it at the beginning or end of the transmission or occasionally as a sub-audible tone. So there was some gain by transitioning to P25 if the options were either P25 (C4FM) or narrowband analog in just what the system could do. Now Phase 2 will change some of that and not only allow the system to recover what it did back when it was analog wideband but also improve coverage slightly more as we'll as double the system capacity while still being able to switch back to Phase 1 if needed for interoperability. Phase 2 only works trunked so it really isn't a practical solution for conventional systems. |