Detailed explanation on P-25 Radio Systems
Contributed APCO Open Forum Digest on July 21, 2018
Rapid City, South Dakota
First, digital radio signals do not travel 'farther' than analog signals. Neither do higher frequency (800MHz) signals travel 'faster' than lower (150MHz) signals. Second, the lower the frequency, i.e.: 155MHz vs. 800MHz, the further the signal effectively travels because signal loss occurs primarily from reflection and absorption. The atmosphere is a good absorber and earth a good reflector. In the best of all worlds, the receiving antenna would 'see' the base station antenna.
A major change in 2-way systems occurred in 2012 to meet the FCC mandated narrowband requirements. Signal bandwidth for almost all of the VHF and UHF public safety channels was reduced from 25 to 12.5 KHz and modulation reduced from a working 3KHz to 1.5KHz. What that means is the range of modulation for human speech in analog format dropped by half. This reduction affected speech intelligibility as the range of tonality was compressed into a smaller footprint passing through the airwaves. Most VHF and UHF systems lost approximately 15-20% of 'effective' radio coverage as speech became unintelligible as the radio distance from the antenna increased. While the distance the radio signal traveled remained the same (effective radiated power), its ability to provide a usable signal changed. Digital modulation (P.25 in particular, although there are a number of other formats), converts analog voice sounds into a digital format (1s & 0s) by use of a VOCODER (voice encoder) or CODEC (coder-decoder). It is done the same as music changed from records (discs with needles) to tape and CDs. The CODEC slices the voice into 8, 16, 32 or more segments per second and records a byte (or 2 -hexidecimal) of data representing the frequency. The bytes are transmitted as a packetized (number of samples packaged together i.e.: 1/2 second worth) stream and decoded at the receiving end to reproduce the sounds. The more data samples used, the better the audio quality. The effective radio bandwidth and modulation protocol used limits the amount of data transmittable over a 2-way channel. Typically, P.25 and other digital modulation formats give a reliable 9.6 Kbs using 12.5 KHz narrowband. This results in speech comparable to what was experienced using wideband (25KHz) analog modulation and strong-signal analog narrowband. However, digital modulation has it's good news and bad news. On the good side, digital bit streams can be corrected for packet errors. The CODEC can interpret missing packets and provide acceptable, albeit interrupted, speech. This error correction process effectively increases the range of digital modulated speech over that of analog within the same narrowband constrained frequency. Also, it is simple and effective to encrypt the digital bit stream. On the not-so-good side are 2 other digital characteristics. First, at fringe boundaries of weak signal, the digital audio will drop off completely as the number of packet errors no longer support error correction. This is different than analog which may still show indication of transmit by the presence of carrier noise. In some cases, i.e.: a downed firefighter in a basement, a trained listener may be able to discern a message from the noise. Second, CODECs effectively operate within a range of frequency and incoming volume from the microphone. Loud background noise, i.e.: sirens, crowds, music, etc. can overwhelm the CODEC and the voice message is left out of the conversion. Thankfully, manufacturers have improved the CODEC algorithms and in some cases provided for secondary microphones to do noise correction. The FCC licenses for your system indicate that you use the VHF spectrum for dispatch communications. Converting from analog to digital in VHF is not a panacea. For many 2-way systems it is a very expensive ($2x analog) upgrade which may not solve the particular problems experienced. Although the FCC proposed to reduce the modulation in the VHF and UHF public safety bands a second time to 6.25 KHz, it has placed a moratorium on that effort. It is now apparent that the laws of physics and business have intervened. No modulation schemes allow a usable analog voice conversation at 6.25KHz. Few modulation schemes allow an acceptable digital bandwidth (error-corrected stream) on a single 6.25KHz modulated frequency which is not part of a trunked system. Most schemes (including Phase II P.25) require existing 12.5 KHz modulation and split the bit stream to allow for multiple conversations. While this method has proved effective at increasing channelization with available frequencies, voice quality suffers considerably. Second, there is a major unintended consequence. If the only method is to use the existing system (i.e.: a 12.5 KHz repeater) to service 2 channels, how is that going to affect different users of the spectrum? For instance, are different businesses (or public safety agencies) going to approach each other and say 'let's share a repeater?' Thankfully, the FCC recognized this debacle and postponed the second round of narrowbanding indefinitely. Also, the use of 12.5 KHz analog modulation in the 700 and some 800 MHz public safety bands for inter-operable use is mandated. So, for agencies currently on analog VHF, there is no current FCC mandate to change to digital modulation. Finally, P.25 digital 2-way radio modulation is not based on the internet protocol (IP). It is a standard, efficient digital format to pass voice over 2-way channels. While it supports IP transport, it is a bandwidth limited mechanism because of the limitations of the overall modulation available. It is apparent that LTE (Long Term Evolution) modulation which uses blocks of frequencies and supports native IP will become the future basis for radio based voice transmission (VOIP/VOLTE). Cellular companies are in the process of converting systems to strictly LTE based data transport. The FirstNet initiative is the alternative for public safety to move mobile data transport (i.e.: mobile computers). The FirstNet Committee, over a 3 year period, recognized that because the operational model of the system requires a nationwide vendor approach, it would be acceptable to franchise the process to a commercial provider who then manages the system and provides administration for the users. This model will appeal to many public safety agencies who desire not to be the operator of a 2-way radio system. Public safety users have priority and pre-emption to 'insure' communications reliability. Like changing from digital to analog modulation, FirstNet and LTE have their own news. Good, for agencies in urban, suburban and metro areas which have good AT&T radio coverage. Not so good for rural areas not commercially viable where AT&T will provide temporary solutions until nationwide coverage is improved. Also, certain liabilities such as wide-band jamming may require consideration. Finally, LTE is a paradigm which requires a sophisticated control mechanism (CORE). Under the current process, if site communication is lost with the CORE or the site itself fails, subscriber units will cease to function without another near-by site. Given current FCC rules and the laws of physics, LTE only handsets will not be able to provide 2-way communications comparable to portable radios otherwise using convention or trunked radio systems. I included this information to reassure agencies who have invested or are deciding to invest significant funds to replace or upgrade a 2-way system. First, those on VHF or UHF frequencies do not have to convert to digital from analog in the near to mid future. Typical FCC mandates incorporate practical dates which consider the usable lifetime of existing systems. I encourage metropolitan area users of T-Band systems to move quickly as you have had a 10-year warning period. For VHF and UHF users, the 2nd narrowband moratorium (which requires digital modulation) will likely be replaced by an entire spectrum reallocation and the conversion to block based LTE. Second, trunked radio systems are effective mostly for regional or metro initiatives which involve consolidation of multiple agencies requiring separate or dynamic communications paths. For regional and rural agencies which have low subscriber unit counts (less than 100), conventional 2-way analog radio is still the best and most economical alternative. Finally, the original question from Sara. Radio coverage, while affected somewhat by the modulation scheme, is primarily governed by the antenna's height above ground and secondarily by the emitted power. Where the terrain varies significantly in height, the alternative is to provide a sufficient number of base station sites in effective locations. For 2-way systems which use portable radios, the number of sites typically includes receive-only sites for fill-in. If the question is why converting to digital modulation made little change to the coverage area, the answer is likely that you need more base station sites. Sorry to say, coverage is a matter of money. Just ask AT&T. One other question I frequently encounter considers the need to cover multiple, geographically diverse agencies (particularly fire protection districts) at a single radio dispatch position. This is best addressed in a conventional (analog or digital) environment using a simulcast mechanism. While trunking provides a more elegant alternative, it is significantly more expensive and requires ongoing user administration. Although it is becoming increasingly more difficult to KISS, I love the challenge!