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What is Project 25
Project 25 (P25) is a set of standards developed jointly by the Association of Communications and Public Safety (Association of Public Safety Communications Officials International, APCO), the National Telecommunications Association (National Association of State Telecommunications Directors, NASTD), the Federal Agency for National Communications Systems (Federal Agencies and the National Communications System, NCS) and the Telecommunications Industry Association (TIA). P25 is an open architecture communication system aimed at meeting the needs of government, governmental and other organizations for operational communications for security purposes. The P25 standard defines the interfaces, interactions and capabilities of any system compatible with the P25 standard. The P25 radio station can operate both in analog mode for compatibility with existing analog FM radio stations, and in analog or digital mode with other P25 radio stations. The P25 standard is not closed, so any manufacturer can create their own digital P25 radios.
The main objectives of the P25 project:
Communication systems of the P25 standard are divided into several generations (phases) for a gradual and smooth transition from outdated analog systems to digital ones.
Phase 1
Phase 1 radios operate in the 12.5kHz band in analog, mixed and digital modes. For digital transmission, radio stations of this generation use C4FM (continuous 4 level FM, continuous 4-level FM) nonlinear modulation and are backward compatible with existing FM radio stations. In addition, the P25 standard describes an open interface to external radio equipment to simplify the docking of radio communication systems from different manufacturers.
Phase 2
Communication systems of this generation are currently under development in order to find a way to transfer data at a speed of 4800bit/seconds in the 6.25kHz band. Phase 2 P25 radios use time and frequency multiplexed modulation schemes (TDMA and FDMA) in order to efficiently utilize the spectrum. In addition, issues of interaction with existing equipment, interfaces with base stations and repeaters, radio frequency distribution, etc. are still being resolved.
Phase 3
The purpose of the Phase 3 equipment will be to ensure high-speed data transmission for public safety purposes. The areas of activity will focus on aerial and terrestrial broadband radio communications, through which it will be possible to transmit and receive speech, video and data at high speed over a large area. European Telecommunications Standards Institute (ETSI) and TIA are working together on Phase 3, known as the MESA (Mobility for Emergency and Safety Applications, Mobility for Security and Emergency Applications) project
How does the P25 radio station work?
The R25 radio station works almost the same as a regular analog FM radio station. According to the P25 standard, the radio station must operate in normal analog mode, ensuring backward compatibility with existing radio equipment. When the P25 radio station is operating in digital mode, the carrier frequency is shifted to one of four possible positions, thus encoding four possible two-bit combinations. This is a modified 4-level frequency manipulation (4FSK) used in analog systems. In analog mode, the P25 radio station works exactly like a conventional FM radio station, with support for CTCSS, DCS signaling, pre- and post-distortions, wide- and narrow-band operation, and so on. In digital mode, the P25 transmitter converts analog speech into a sequence of packets using an IMBE vocoder, then decodes digital data back into analog speech in the receiver. In addition, service information is added to the encoded speech (primarily for multiple access to the radio channel) and noise-tolerant coding procedures are applied. The analog signaling of CTCSS and DCS is replaced by their digital equivalents - NAC (network access code) codes. Encryption of voice traffic can be used to protect against eavesdropping. There is also a special channel for low-speed data transmission. To interact with each other, P25 radio stations use the Common Air Interface (CAI, common radio interface). This standard interface describes the types and contents of radio signals transmitted. A P25 radio station using CAI must work towards another P25 radio station using CAI, regardless of the manufacturer. The existing P25 radio stations operate in the 12.5kHz band. Thus, 2 channels can be placed in the 25kHz band, instead of one as with traditional analog FM transmission. In Phase 2, the R25 radio stations will operate in the 6.25kHz band, which will allow 4 subscribers to be placed in the same band. The P25 radio station must operate in analog mode on channels with bands of 25kHz and 12.5kHz, which allows users to gradually switch from analog to digital radio stations. Voice and data transmission in the P25 radio station can be encrypted. The P25 standards describe the use of AES and DES algorithms, as well as some others. There is also a separate specification for the OTAR (over the air rekeying) encryption key exchange algorithm. In the P25 communication channel, voice or data is transmitted at a speed of 9600bps, protected by an FEC (Forward Error Correction, direct error correction code) code. This allows the receiver to combat over-the-air interference and increase the communication range. The P25 supports low-speed data transmission when data is mixed with the transmitted voice, as well as transmission at full speed of 9600 bits/sec.
The architecture of the R25 radio station
Analog-to-digital, digital-to-analog and speech encoding/decoding
The P25 uses a specific voice digitization method using an IMBE vocoder (Improved Multi-Band Excitement). The vocoder analyzes the voice at the microphone input and, instead of transmitting the samples themselves, transmits some characteristics representing the voice. The receiver uses these characteristics and synthesizes the synthetic equivalent of the transmitted voice. The IMBE vocoder is highly optimized for the human voice and does not transmit other types of signals very well, including DTMF. The IMBE vocoder samples the signal from the microphone input, generating 88 bits every 20 msec. Thus, the vocoder transmits speech at a speed of 4400 bits/sec.
Channel encoding-decoding
Channel encoding is a method in which data transmission systems use noise–tolerant encoding and other data protection methods to ensure their correct recovery on the receiving side. Noise-tolerant coding and data protection are designed to improve the performance of the entire system operating in conditions of channel interference such as noise, fading and interference. The P25 standard includes a wide range of different types of encoding: interleaving, linear block coding, Hamming codes, Gaulois codes, Reed-Solomon codes, BCH, shortened cyclic codes.
Modulation/demodulation and filtering
In Phase 1, work is carried out in the 12.5kHz band using C4FM modulation, which is a kind of QPSK in which each character is shifted in phase by 45 degrees relative to the previous character. Despite the fact that the information is encoded by the phase of the C4FM signal, its carrier amplitude remains constant, thus generating a FM modulated signal with a constant envelope. This feature allows the FM radio paths of analog radio stations to be used for Phase 1 almost unchanged, which simplifies the transition of analog equipment to the digital standard. In Phase 2, work is carried out in the 6.25kHz band using CQPSK modulation. The phase and frequency of the modulated CQPSK signal are simultaneously modulated in such a way as to minimize the width of the occupied spectrum. The result is an amplitude-phase modulated signal. The transmission is carried out at a speed of 4800bit/sec, in which each simole transmits 2 bits of information. The table of correspondence between symbols and bits for both types of modulation is shown below
Information bits From : 11, 10, 00, 01
Symbol: -3, -1, +1, +3
C4FM Modulation (Phase 1): -1.8 kHz, -0.5 kHz, +0.5 kHz, +1.8 kHz
C4FM modulation (Phase 2): -135 degrees, -45 degrees, +45 degrees, +135 degrees
The C4FM signal modulator includes a Nyquist filter of the "raised cosine" type, a forming filter and an FM modulator. The C4FM modulator is based on a quadrature modulator that modulates two carriers simultaneously. The phase of the signal in the quadrature channel is delayed relative to the phase of the signal in the common-mode channel by 90 degrees. The filtered 5-level signal received from the output of the conversion table is used as the input signal of the quadrature modulator.
The CQPSK demodulator is capable of receiving signals with both C4FM and CQPSK modulation. The FM signal detector at the input of the demodulator allows you to receive analog FM, C4FM and CQPSK signals. This allows you to keep the receiving path of the P25 radio station unchanged during the transition from Phase 1 to Phase 2 with a 6.25kHz operating band. The alteration is only needed for the transmitter. In addition, such a demodulator will receive analog FM and C4FM signals equally well. At the moment, the Phase 2 equipment is not yet in production. It requires the linearity of the output amplifiers to transmit a CQPSK signal sensitive to nonlinear distortion.
The main objectives of the P25 project:
- Based on the principles of open architecture, to provide additional competition between different communication systems
- To provide effective, reliable and cheap communication to government and other organizations
- Provide improved functionality and additional features with a focus on public safety needs
- To increase the efficiency of using the radio frequency spectrum
Communication systems of the P25 standard are divided into several generations (phases) for a gradual and smooth transition from outdated analog systems to digital ones.
Phase 1
Phase 1 radios operate in the 12.5kHz band in analog, mixed and digital modes. For digital transmission, radio stations of this generation use C4FM (continuous 4 level FM, continuous 4-level FM) nonlinear modulation and are backward compatible with existing FM radio stations. In addition, the P25 standard describes an open interface to external radio equipment to simplify the docking of radio communication systems from different manufacturers.
Phase 2
Communication systems of this generation are currently under development in order to find a way to transfer data at a speed of 4800bit/seconds in the 6.25kHz band. Phase 2 P25 radios use time and frequency multiplexed modulation schemes (TDMA and FDMA) in order to efficiently utilize the spectrum. In addition, issues of interaction with existing equipment, interfaces with base stations and repeaters, radio frequency distribution, etc. are still being resolved.
Phase 3
The purpose of the Phase 3 equipment will be to ensure high-speed data transmission for public safety purposes. The areas of activity will focus on aerial and terrestrial broadband radio communications, through which it will be possible to transmit and receive speech, video and data at high speed over a large area. European Telecommunications Standards Institute (ETSI) and TIA are working together on Phase 3, known as the MESA (Mobility for Emergency and Safety Applications, Mobility for Security and Emergency Applications) project
How does the P25 radio station work?
The R25 radio station works almost the same as a regular analog FM radio station. According to the P25 standard, the radio station must operate in normal analog mode, ensuring backward compatibility with existing radio equipment. When the P25 radio station is operating in digital mode, the carrier frequency is shifted to one of four possible positions, thus encoding four possible two-bit combinations. This is a modified 4-level frequency manipulation (4FSK) used in analog systems. In analog mode, the P25 radio station works exactly like a conventional FM radio station, with support for CTCSS, DCS signaling, pre- and post-distortions, wide- and narrow-band operation, and so on. In digital mode, the P25 transmitter converts analog speech into a sequence of packets using an IMBE vocoder, then decodes digital data back into analog speech in the receiver. In addition, service information is added to the encoded speech (primarily for multiple access to the radio channel) and noise-tolerant coding procedures are applied. The analog signaling of CTCSS and DCS is replaced by their digital equivalents - NAC (network access code) codes. Encryption of voice traffic can be used to protect against eavesdropping. There is also a special channel for low-speed data transmission. To interact with each other, P25 radio stations use the Common Air Interface (CAI, common radio interface). This standard interface describes the types and contents of radio signals transmitted. A P25 radio station using CAI must work towards another P25 radio station using CAI, regardless of the manufacturer. The existing P25 radio stations operate in the 12.5kHz band. Thus, 2 channels can be placed in the 25kHz band, instead of one as with traditional analog FM transmission. In Phase 2, the R25 radio stations will operate in the 6.25kHz band, which will allow 4 subscribers to be placed in the same band. The P25 radio station must operate in analog mode on channels with bands of 25kHz and 12.5kHz, which allows users to gradually switch from analog to digital radio stations. Voice and data transmission in the P25 radio station can be encrypted. The P25 standards describe the use of AES and DES algorithms, as well as some others. There is also a separate specification for the OTAR (over the air rekeying) encryption key exchange algorithm. In the P25 communication channel, voice or data is transmitted at a speed of 9600bps, protected by an FEC (Forward Error Correction, direct error correction code) code. This allows the receiver to combat over-the-air interference and increase the communication range. The P25 supports low-speed data transmission when data is mixed with the transmitted voice, as well as transmission at full speed of 9600 bits/sec.
The architecture of the R25 radio station
Analog-to-digital, digital-to-analog and speech encoding/decoding
The P25 uses a specific voice digitization method using an IMBE vocoder (Improved Multi-Band Excitement). The vocoder analyzes the voice at the microphone input and, instead of transmitting the samples themselves, transmits some characteristics representing the voice. The receiver uses these characteristics and synthesizes the synthetic equivalent of the transmitted voice. The IMBE vocoder is highly optimized for the human voice and does not transmit other types of signals very well, including DTMF. The IMBE vocoder samples the signal from the microphone input, generating 88 bits every 20 msec. Thus, the vocoder transmits speech at a speed of 4400 bits/sec.
Channel encoding-decoding
Channel encoding is a method in which data transmission systems use noise–tolerant encoding and other data protection methods to ensure their correct recovery on the receiving side. Noise-tolerant coding and data protection are designed to improve the performance of the entire system operating in conditions of channel interference such as noise, fading and interference. The P25 standard includes a wide range of different types of encoding: interleaving, linear block coding, Hamming codes, Gaulois codes, Reed-Solomon codes, BCH, shortened cyclic codes.
Modulation/demodulation and filtering
In Phase 1, work is carried out in the 12.5kHz band using C4FM modulation, which is a kind of QPSK in which each character is shifted in phase by 45 degrees relative to the previous character. Despite the fact that the information is encoded by the phase of the C4FM signal, its carrier amplitude remains constant, thus generating a FM modulated signal with a constant envelope. This feature allows the FM radio paths of analog radio stations to be used for Phase 1 almost unchanged, which simplifies the transition of analog equipment to the digital standard. In Phase 2, work is carried out in the 6.25kHz band using CQPSK modulation. The phase and frequency of the modulated CQPSK signal are simultaneously modulated in such a way as to minimize the width of the occupied spectrum. The result is an amplitude-phase modulated signal. The transmission is carried out at a speed of 4800bit/sec, in which each simole transmits 2 bits of information. The table of correspondence between symbols and bits for both types of modulation is shown below
Information bits From : 11, 10, 00, 01
Symbol: -3, -1, +1, +3
C4FM Modulation (Phase 1): -1.8 kHz, -0.5 kHz, +0.5 kHz, +1.8 kHz
C4FM modulation (Phase 2): -135 degrees, -45 degrees, +45 degrees, +135 degrees
The C4FM signal modulator includes a Nyquist filter of the "raised cosine" type, a forming filter and an FM modulator. The C4FM modulator is based on a quadrature modulator that modulates two carriers simultaneously. The phase of the signal in the quadrature channel is delayed relative to the phase of the signal in the common-mode channel by 90 degrees. The filtered 5-level signal received from the output of the conversion table is used as the input signal of the quadrature modulator.
The CQPSK demodulator is capable of receiving signals with both C4FM and CQPSK modulation. The FM signal detector at the input of the demodulator allows you to receive analog FM, C4FM and CQPSK signals. This allows you to keep the receiving path of the P25 radio station unchanged during the transition from Phase 1 to Phase 2 with a 6.25kHz operating band. The alteration is only needed for the transmitter. In addition, such a demodulator will receive analog FM and C4FM signals equally well. At the moment, the Phase 2 equipment is not yet in production. It requires the linearity of the output amplifiers to transmit a CQPSK signal sensitive to nonlinear distortion.
