Simulcast issues with commercial radios

[scanmanmi]

I understand there are issues like terrain and distances that dictate a simulcast system but am I correct to assume the main reason is minimizing frequency consumption? I don't see why you would want all the negative issues when you could just put up several normal trunking towers. My question is are commercial radios, at least to some degree, succeptable to the simulcast demod isssues scanners have? Has there ever been a study on how many calls are dropped due to simulcast? Is one brand generally accepted as better or worse?

 

[iMONITOR]

The idea behind simulcast is so communications can be seamless and uninterrupted as a vehicle travels away from one tower and towards another, one city to another, etc. I think cellphones operate in a similar manner. A few years back while monitoring Macomb County Sheriff I would hear dispatch tell the officer "repeat, you just went digital". The officers voice would break up with the infamous Donald Duck effect. Macomb added about 5 new towers and that seemed to resolve the problem about 99% from what I can tell. Commercial P25 radio are designed for this enviroment and work better than most scanners. However I do admit the Uniden SDS series appear to have overcome the problem.

 

[mmckenna]

I understand there are issues like terrain and distances that dictate a simulcast system but am I correct to assume the main reason is minimizing frequency consumption?

Primary reason is so there is no user input required. The subscriber radio will hear which ever tower, and the system will pick the receiver that has the best signal.

I don't see why you would want all the negative issues when you could just put up several normal trunking towers.

Multi-site trunking systems are very expensive. For small agencies, trunking may not be needed. A small department that only needs one or two channels would save money by using a conventional simulcast system.

My question is are commercial radios, at least to some degree, succeptable to the simulcast demod issues scanners have? Has there ever been a study on how many calls are dropped due to simulcast? Is one brand generally accepted as better or worse?

Commercial radios usually have better receivers than a scanner will.
Usually issues with simulcast are on the system end. Transmitters off frequency or signals improperly timed.

I don't know of any study that would show how many calls are lost, but I suspect it would be very low if a system is designed correctly. There are a lot of simulcast systems out there. If they didn't work, we wouldn't be seeing them.

 

[iMONITOR]

Usually issues with simulcast are on the system end. Transmitters off frequency or signals improperly timed.

When they were rolling out MPSCS in Michigan I think the biggest problem was related to timing of signals. It seem like is was cleaned up quickly as I recall.

 

[MTS2000des]

Frequency reuse is why we do it, and to minimize number of RF subsites. In many regions, getting multiple 700/800 "clean" frequency pairs over a large area is a real issue. Linear simulcast solves this problem. When engineered and optimized, it works very well, unlike 98 percent of consumer scanners which are turds when it comes to receiving LSM.

There can be issues with receiving LSM on actual subscriber radios with delay spread, but usually engineers design such nulls so they are at the edge of a coverage area or immediately outside it.

 

[GTR8000]

I don't see why you would want all the negative issues when you could just put up several normal trunking towers.

What "all" negative issues would those be?

You're starting off with a false premise that simulcast trunking is problematic with the subscriber radios operating on the system. It is not, it works very well across all brands of "real" radios, as they are engineered and built with these simulcasts in mind. Also, unless the system is a real turd, the overwhelming majority of these systems are designed and constructed properly to greatly minimize or eliminate any minor issues that a simulcast may inherently have.

Do not use the poor experience that most scanner users have with these systems as the basis for forming a judgment as to how well they function with proper subscriber radios. It would be akin to claiming that the Daytona Speedway is a lousy track because your economy car has bad performance when running laps. Run the same track in a high end sports car and it's a different story entirely.

 

[RFI-EMI-GUY]

As far as commercial simulcast and limitations of tower seperation. The practical limitation is determined by the modulation , the relative distance between the subscriber receiver and each of the transmitting towers. The term is Time Differential Interference. It is caused by delay in the modulation received from two or more sources. When the receiver sees a delay in a digital 1 or 0 bit, it cannot distinguish one bit state from another, Having more towers is not necessarily a benefit as the error and jitter becomes additive like interference. The latest technology uses a form of four level modulation so that the baud rate allows twice the bits in same time period allowing a slower baud rate. And an amplitude timing element to help define when a bit starts. This is called linear simulcast. APCO 25 uses 9600 bits in 4800 baud phase 1 and 12000 bits in 6000 baud phase 2. Most of the consumer scanners cannot utilize the amplitude component of linear simulcast and so do a mediocre job of demodulating the data.

 

[PriorMike]

Are conventional simulcast systems handled much the same way?

 

[RFI-EMI-GUY]

Are conventional simulcast systems handled much the same way?

exactly the same principal. Though you won't see phase 2 modulation in conventional P25.

Analog simulcast has similar concerns, though it becomes an audio distortion issue.

 

[GTR8000]

Conventional P25 simulcasts can use QPSK/LSM modulation instead of the traditional C4FM, so the same principals of simulcast trunked system traffic channels would apply.