taysml
Member
P25 Simulcast Multipath Interference
Gee I can't stay out of this thread. bailey2, here's my 2 cents.....
When you say "LSM" I'm going to assume that you mean linear simulcast in general, which, according to the P25 standards, also includes WCQPSK.
See my point 2 of post 09-05-2015, 11:39 AM for the reason that simulcast is used.
Simulcast systems have existed for several years, even long before the existence of linear simulcast. So a simulcast system doesn't have to be linear, and in fact there are still many older fielded systems that use FM transmissions from the towers such as P25 Phase 1 C4FM and proprietary modulations that the manufacturers used even before that. See my point 5 of post 09-05-2015, 11:39 AM for the reason why linear simulcast is usually used these days instead of C4FM simulcast.
For some cases, you are right about not needing as many TX sites with linear simulcast ..... but not necessarily always. Better delay spread tolerance afforded by linear simulcast sometimes means that the transmit sites can be placed further apart and thus reduce the quantity needed for a bounded coverage area such as a city limit However, the ability to spread the sites further can be counteracted by a customer's inbuilding coverage requirements, which tends to force the sites closer together to enable achieving the requisite signal level impinging on the buildings (in a large area) from the transmit sites. Cities, in particular, usually have quite onerous requirements for so-called "general" coverage in buildings, often requiring penetration of a 30 dB or greater assumed building loss and achieve a requisite voice quality inside the building (so-called DAQ rating).
I personally know of no customer, standards or Industry Forum bodies that I ever been involved with that wanted linear simulcast for enhanced security from scanners. If properly designed, scanners should work fine with linear simulcast even for severe delay spread conditions. Security needs drive customers towards added encryption of the digital signal, which commercial scanners won't be able to decode.
By the way, I'm very surprised that your scanner doesn't decode voice when you use a Yagi for the geometry that you describe . When between two sites, your Yagi antenna should offer enough angular resolution to capture one of the sites and attenuate the other. Off the top of my head, the following possibilities come to mind.... 1) your Yagi's directional pattern is being corrupted if you're attempting to use it near physical obstructions such as indoors, 2) there are actually more than two sites in the system (usually the case), you have near-equal signal levels from at least two that your Yagi can't separate, and the overlapping signals are from sites with large differences in path length to your scanner 3) The system is encrypting everything (not likely) so your scanner can't decode it . Good luck.
Gee I can't stay out of this thread. bailey2, here's my 2 cents.....
When you say "LSM" I'm going to assume that you mean linear simulcast in general, which, according to the P25 standards, also includes WCQPSK.
See my point 2 of post 09-05-2015, 11:39 AM for the reason that simulcast is used.
Simulcast systems have existed for several years, even long before the existence of linear simulcast. So a simulcast system doesn't have to be linear, and in fact there are still many older fielded systems that use FM transmissions from the towers such as P25 Phase 1 C4FM and proprietary modulations that the manufacturers used even before that. See my point 5 of post 09-05-2015, 11:39 AM for the reason why linear simulcast is usually used these days instead of C4FM simulcast.
For some cases, you are right about not needing as many TX sites with linear simulcast ..... but not necessarily always. Better delay spread tolerance afforded by linear simulcast sometimes means that the transmit sites can be placed further apart and thus reduce the quantity needed for a bounded coverage area such as a city limit However, the ability to spread the sites further can be counteracted by a customer's inbuilding coverage requirements, which tends to force the sites closer together to enable achieving the requisite signal level impinging on the buildings (in a large area) from the transmit sites. Cities, in particular, usually have quite onerous requirements for so-called "general" coverage in buildings, often requiring penetration of a 30 dB or greater assumed building loss and achieve a requisite voice quality inside the building (so-called DAQ rating).
I personally know of no customer, standards or Industry Forum bodies that I ever been involved with that wanted linear simulcast for enhanced security from scanners. If properly designed, scanners should work fine with linear simulcast even for severe delay spread conditions. Security needs drive customers towards added encryption of the digital signal, which commercial scanners won't be able to decode.
By the way, I'm very surprised that your scanner doesn't decode voice when you use a Yagi for the geometry that you describe . When between two sites, your Yagi antenna should offer enough angular resolution to capture one of the sites and attenuate the other. Off the top of my head, the following possibilities come to mind.... 1) your Yagi's directional pattern is being corrupted if you're attempting to use it near physical obstructions such as indoors, 2) there are actually more than two sites in the system (usually the case), you have near-equal signal levels from at least two that your Yagi can't separate, and the overlapping signals are from sites with large differences in path length to your scanner 3) The system is encrypting everything (not likely) so your scanner can't decode it . Good luck.
