Single Cell Technologies
These technologies were the precursors of today’s cellular technologies described further down.
They are generally narrow channel and require the user to change channels. Some are standardized, but many are proprietary solutions.
Several of those technologies still are used today, mainly for voice and fixed applications.
- Land mobile radio system
A land mobile radio system (LMRS) is a person-to-person voice communication system consisting of two-way radio transceivers (an audio transmitter and receiver in one unit) which can be mobile, installed in vehicles, or portable (walkie-talkies). Public land mobile radio systems are made for use exclusively by public safety organizations such as police, fire, and ambulance services, and other governmental organizations, and use special frequencies reserved for these services. Private land mobile radio systems are designed for private commercial use, by firms such as taxis or delivery services. Most systems are half-duplex, with multiple radios sharing a single radio channel, so only one radio can transmit at a time. The transceiver is normally in receiving mode so the user can hear other radios on the channel; when a user wants to talk he presses a push to talk button on his microphone, which turns on his transmitter. They use channels in the VHF or UHF bands giving them a limited range, usually 3 to 20 miles (4.8 to 32 km) depending on terrain, although repeaters installed on tall buildings, hills or mountain peaks can be used to increase the coverage area. Older systems use AM or FM modulation, while some recent systems use digital modulation allowing them to transmit data as well as sound.
- Trunked Radio
A trunked radio system is a digital two-way radio system that uses a digital control channel to automatically assign frequency channels to groups of users. In a half-duplex land mobile radio system a group of users with portable two-way radios communicate over a single shared radio channel, with one user at a time talking. These systems typically have access to multiple channels, up to 40 – 60, so multiple groups in the same area can communicate. In a conventional (non-trunked) system, channel selection is done manually; before use the group must decide on which channel to use, and manually switch all the radios to that channel. There is nothing to prevent multiple groups in the same area from choosing the same channel, causing conflicts. A trunked radio system is an advanced alternative in which the channel selection process is done automatically.
Trunking is a more automated and complex radio system, but provides the benefits of less user intervention to operate the radio and greater spectral efficiency with large numbers of users. Instead of assigning a radio channel to one particular user group at a time, users are instead assigned to a logical grouping, a “talkgroup”. When any user in that group wishes to converse with another user in the talkgroup, a vacant radio channel is found automatically by the system and the conversation takes place on that channel. Many unrelated conversations can occur on a channel, making use of the otherwise idle time between conversations. Each radio transceiver contains a microprocessor which handles the channel selection process. A control channel coordinates all the activity of the radios in the system. The control channel computer sends packets of data to enable one talkgroup to talk together, regardless of frequency.
The primary purpose of this type of system is efficiency; many people can carry many conversations over only a few distinct frequencies. Trunking is used by many government entities to provide two-way communication for fire departments, police and other municipal services, who all share spectrum allocated to a city, county, or other entity.
- Digital Mobile Radio
Digital mobile radio (DMR) is a limited open digital mobile radio standard defined in the European Telecommunications Standards Institute (ETSI) Standard TS 102 361 parts 1–4 and used in commercial products around the world. DMR, along with P25 phase II and NXDN are the main competitor technologies in achieving 6.25 kHz equivalent bandwidth using the proprietary AMBE+2 vocoder. DMR and P25 II both use two-slot TDMA in a 12.5 kHz channel, while NXDN uses discrete 6.25 kHz channels using frequency division and TETRA uses a four-slot TDMA in a 25 kHz channel.
DMR was designed with three tiers. DMR tiers I and II (conventional) were first published in 2005, and DMR III (Trunked version) was published in 2012, with manufacturers producing products within a few years of each publication.
The primary goal of the standard is to specify a digital system with low complexity, low cost and interoperability across brands, so radio communications purchasers are not locked into a proprietary solution. In practice, given the current limited scope of the DMR standard, many vendors have introduced proprietary features that make their product offerings non-interoperable with other brands.
Project 25 (P25 or APCO-25) is a suite of standards for digital mobile radio communications designed for use by public safety organizations in North America. P25 radios are a direct replacement for analog UHF (example FM) radios but add the ability to transfer data as well as voice, allowing for a more natural implementation of encryption or messaging. P25 radios are commonly implemented by dispatch organizations, such as police, fire, Ambulance and Emergency Rescue Service, using vehicle-mounted radios combined with walkie-talkie handheld use.
Starting around 2012, products became available with the newer phase 2 modulation protocol, the older protocol known as P25 became P25 phase 1. P25 phase 2 products use the more advanced AMBE2+ vocoder, which allows audio to pass through a more compressed bitstream and provides two TDMA voice channels in the same RF bandwidth (12.5 kHz), while phase 1 can provide only one voice channel. The two protocols are not compatible. However, P25 Phase 2 infrastructure can provide a “dynamic transcoder” feature that translates between Phase 1 and Phase 2 as needed. In addition to this, phase 2 radios are backwards compatible with phase 1 modulation and analog FM modulation, per the standard. On the other hand, EU area created the standard for Terrestrial Trunked Radio similar to Project 25. P25 fills a similar role as TETRA or DMR protocols.