"Why yes, and that us why P25 using 12.5 kHz channels can run at 9600 BPS and NXDN using a 6.25 kHz channel only runs at 4800 BPS, even though both are 4 level modulation.
When you get to a digital system you have more parameters to "play" with in the design than with simple analog FM.
So when you design a digital voice protocol like P25 or NXDN (or OpenSky, or iDEN, or TETRA) you trade off the modulation type, the modulation rate, the amount of Forward Error Correction, against the amount of payload data you require and the minimum signal level, minimum signal to noise, or minimum carrier to interference ratio you anticipate.
A digital system can then be tailored to work at the a path loss value similar to FM, greater then FM or less than FM. (Really any value the protocol designer wants to set as the margin the system designer has to provide in the system design)
Note, the design of the protocol and the design of a system using the protocol are somewhat at odds.
P25 was designed intentionally to provide the same or just slightly better range than 25 KHz FM (the standard of the day, when it was first designed). Hence it has slightly better performance than 12.5 kHz FM (when all else is the same)
Yes, it would be nice if every system could be designed like a cellular network, where you can expect to always be above the minimum threshold, and hand off to another site before you reach any fringe areas.
Unfortunately, most public safety design is site and cost limited, and both are traded off against coverage. In any large area system you are likely to find some areas where you are working at the edges of your designed signal level margin. In these areas, small reductions, such as the one created when you transition from a 25 KHz FM system to a 12.5 kHz FM system can cause issues, if not addressed elsewhere in the design. (Adding power, better antennas, lower loss line, additional sites, etc.)"
So while I agree that a digital carrier (C4FM) occupies the same space it can indeed yield a better S/N than analog, but on the interference limited design, does not digital modulation present another issue of interference or high BER being transparent to the user UNTIL the signal has deteriorated to the point of no return? For example, at work I am in a pretty tough RF environment being inside a building, we use an Astro 25 SZOL 6.xx simulcast system, originally on an analog Smartnet II prior. The digital radios suffer from robotic sounds, dropouts, and poor audio quality that are from a either weak RF, or strong inbound interference. There is no indication of SQE, BER or anything to let a user know you are in a marginal coverage area or strong interference. The indication and tone options in CPS don't work well enough in my experience and often times not at all.
Problem is, we have no way of knowing we are in a weak or marginal area UNTIL the failure in the link occurs whereas with analog FM, the user is generally aware of the deteriorating conditions (i.e, the decrease in S/N that is very apparent, static, Rayleigh fading, etc) and this allows for the operator to correct (relocating, identifying the source of the interference and eliminating it "by ear") for it. With digital this all or nothing reality can prove quite dangerous for users who generally have little to no indication their radio is operating at it's fringe or in an area high with RFI, Nextel interference, or other source because the decay isn't apparent until it's already "too late". We've all experienced this on our digital cellphones where a perfectly "good" call is dropped due to strong inbound interference (e.g. being on a mountaintop with a CDMA phone and a rouge PN causes your handset to bump you, etc) with no "warning" where as in the analog days there usually no doubt you were in a marginal area or being interfered with (e.g. crosstalk, static, fading, distortion, etc) and it was easy to predict the link was going to fail.
So while yes, a 1.5 to 3dB is significant enough on the outskirts of a coverage area with narrowband versus wideband, is it any different than a high BER on a digital system in the same scenario? At least with the analog carrier, the users will be highly aware he/she is approaching the edges of the system or encountering interference, with digital it's all or nothing, and sometimes nothing comes too soon with little or no warning to the user who's life depends on the radio to function when needed most. I am sorry, I just don't see this as acceptable and don't see how public safety benefits from this. At least with analog FM (narrow or wide) the user is usually very well aware when his/her radio is working poorly or in a poor coverage area or getting interference. That isn't the case with digital, unless you are so advanced you ears can decode the raw packets themselves, for the rest of us, we are 100 percent reliant on the vocoder, DSP and combination of so many other things to come together to produce the intelligence from the modulated carrier. Our brains have been removed from the equation, at least with analog FM we can still use them to pull that signal out of the noise "manually". We (the users) are provided with much more to work with, and when lives are in our hands, we should expect nothing less.
A poorly designed radio system with inadequate coverage is a poorly designed radio system, regardless of what kind of modulation, topology, or whatever is used to implement it. Digital will not solve these problems nor make it go away. It just presents a different set of issues altogether and they aren't any less perplexing than the limitations of analog. Just more expensive to implement and fix, and more evasive to track down in many cases.
"That is a shame, because I have a good friend who is just a Georgia Boy with no degree hanging on the wall and he is one of the best RF engineers I know."
see we are not all idiots N_Jay, thank you for coming clean for a change.
Only kidding. Lets keep it civil here before we get this thread shut down! Lets adjourn this one to the WasteLand? That way we can make German specticals out of ourselves!
