Mission Critical Communications Magazine Article
Written by: Jim Holthaus, V.P. of Product Marketing
• Improved performance in background noise. P25 equipment achieves 10 to 25 decibel (dB) improvements in background noise reduction.
• Tone signaling. Dual-mode multifrequency (DTMF), Knox and single tone are now supported.
• Paging. P25 paging receivers are available.
• Improved coverage. P25 Phase 1 technology is about +7 dB better than 25-kilohertz (kHz) analog.
• Enhanced signaling. Talking party ID, group calls, unit-to-unit calls, all calls, emergency alerts, emergency calls, call alerts, radio check, radio unit monitoring and others are offered.
• Location services. Integrated GPS receivers provide location information.
A Little History
Early deployments of P25 radio products highlighted the challenges of digital voice compression in high background noise. Equipment noise including fire apparatus, personal alert safety system (PASS) alarms and self-contained breathing apparatus (SCBA) presented unique challenges for the P25 vocoder.
Another limitation of early P25 radio products was compatibility with widely deployed paging and tone-signaling systems. The lack of availability of P25 paging receivers was of primary concern with volunteer departments. Given that the original P25 vocoder is optimized for voice, many tone signals such as DTMF, Knox Box or paging tones were highly distorted over P25 radios.
Coverage was another area of concern. While the P25 Common Air Interface (CAI) was designed to provide equivalent or better radio coverage footprints to analog, differences in real-world operation were noted. The digital air interface maintained excellent intelligibility almost all the way to the limits of coverage, but then dropped off rapidly. Analog maintained some level of intelligibility even beyond the edge of coverage, and the gradual degradation was noticeable.
Testing conducted by the National Telecommunications and Information Administration (NTIA) in 2008 reinforced the anecdotal reports of public-safety practitioners with respect to performance in high background noise and reported range issues.
Digital Voice Systems Inc. (DVSI), working with NTIA and P25 equipment manufacturers, went to work to develop methods to test vocoder and air interface performance, determine baseline performance and then set out to implement critical improvements. This effort was conducted under the standards development process of the Telecommunications Industry Association (TIA).
New Developments
Technology improvements within the P25 vocoder, including noise reduction, automatic gain control and advanced error correction coding, were developed, explored and tested to verify that the changes would translate to improvements in real-world fireground scenarios.
The P25 vocoder was also modified to detect most tone signaling common to public-safety systems including DTMF, Knox Box and paging tones. These signals are now detected and encoded by the vocoder for transmission. At the receiver, the subsequent tones are then regenerated to preserve fidelity of the original tone.
The P25 standard now includes a vocoder performance test that measures noise reduction for 15 noises including vehicles (car, boat, helicopter and firetruck), sirens, alarms (PASS and low air), crowds, saws, water pumps, fog nozzle and pink noise.
In parallel, half-rate vocoder technology was developed to support P25 Phase 2 operation, which delivers double the spectrum efficiency of P25 Phase 1 systems. To support interoperability of Phase 1 and Phase 2 systems, a half-rate/full-rate conversion capability is also available.
Available P25 Products
These vocoder improvements, along with other technology to improve performance in high-noise environments typical to public-safety communications such as enhanced noise-cancellation methods, are now available in products using both the full-rate vocoder (P25 Phase 1) and half-rate vocoder (P25 Phase 2).
In addition to vocoder improvements to two-way radio products, a P25 paging receiver was introduced, allowing many public-safety agencies the ability to use existing P25 infrastructure for paging services.
Integration of GPS receivers into handheld subscriber radio equipment coupled with updates to the P25 location services standards provide opportunity to obtain location data during critical incidents. This provides robust operation for units operating outdoors, for example during wildland fire incidents.
2013 Narrowbanding Effects
One additional development affecting radio users in VHF and UHF spectrum was the requirement to narrowband by January 2013. Analog systems are now required to operate on 12.5-kilohertz (kHz) channels. Testing has shown that narrowbanding can have pronounced effects when RF channel impairments and background noise are considered.
P25 offers better range for the fireground in the required narrowband world. P25 Phase 1 technology is about +7 dB better than 25-kilohertz analog and close to +10 dB better than the newly required 12.5-kilohertz analog for the same delivered audio quality (DAQ).
Although P25 has improved coverage over analog and significantly over narrowband analog, concerns with the difference in real-world operation cannot be ignored. A digital receiver generally will provide a good-quality signal to a greater range than an analog receiver and then effectively stop receiving once the range limits are reached.
Typically, in these conditions, the corresponding analog audio is still detectable but is virtually incomprehensible because of the poor signal-to-noise ratio, and thus, it is of little practical use. Technically, the analog signal may have greater range but may have a significantly smaller useful range. When considering a possible conversion to using P25 for the fireground, it is strongly encouraged to do a detailed evaluation of RF coverage performance on your system to ensure that buildings will be properly covered, especially on the fringe of a system’s operational area.
P25 radios are just one of many communications technologies available to meet the demanding needs for fireground communications. The latest P25 radios are well equipped to address mission-critical communication challenges for fireground operations. If you have not evaluated P25 technology for use on the fireground in the past few years, now may be a good time to take a comprehensive look and see if the performance and features of current P25 equipment are an improvement compared with your current solution.
More information on P25 vocoder and range improvements can be found here.
The Project 25 Interest Group (PTIG) original white paper is here. Visit the PTIG website for more P25 information including a list of P25 systems around the globe.
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Jim Holthaus is vice president Project 25 (P25) solutions for Relm Wireless. He has been active in the development of LMR products and P25 digital radio standards since 1993. Holthaus is a senior member of the Institute of Electrical and Electronics Engineers (IEEE), a member of the Association of Public-Safety Communications Officials (APCO) International and vice chair of both Project 25 Interest Group (PTIG) and the Telecommunications Industry Association (TIA) private radio section. He’s also a chair of the TIA-TR8.25 Compliance Assessment Formulating Group TIA/APCO P25 Interface Committee (APIC) Vocoder Task Group and has chaired the TIA/TR8.4 Vocoder and TIA/TR8.10 Trunking Subcommittees.