Digital alternatives
The TETRA trunked radio standard was developed by the European Telecommunications Standards Institute (ETSI), as a digital alternative to analogue trunked systems. However, TETRA, with its enhanced encryption capability, has developed into a higher tier (public safety) product, currently mainly used by governments, some larger airports and government-owned utilities.
DMR (digital mobile radio), and dPMR (digital private mobile radio) are more recent ETSI-standards for digital mobile radio using two-slot TDMA and FDMA respectively. The Tier 3 standard for these systems defines a trunking protocol very similar to MPT1327 and is intended as a potential migration path for existing and perhaps future trunking customers. Tier 3 equipment is (late 2011) now becoming available, so the impact on TETRA and MPT 1327 is yet to be seen, but may well be significant. However, it is unlikely that in terms of cost that the complicated new DMR/dPMR equipment will be able to compete with the simpler MPT1327 equipment for some time, if ever.
It is worth noting that whilst many comparisons are made between Digital and Analog radio technologies, when it comes to applying these arguments to MPT1327, many of the distinctions become blurred, since MPT1327 with its digital control channel, already offers most of the features being offered by the DMR/dPMR/TETRA counterparts.
Furthermore, most MPT1327 systems are engineered to a far higher standard than conventional FM systems, partially due to the lack of CTCSS within the standard. As such arguments with regards to "noisy FM audio quality", can become misleading, since the squelch levels tend to be set rather high on MPT1327 systems, such that weak/noisy signals do not generally open the mute.
MPT1327 advantages and features
The advantage of MPT 1327 over TETRA is the increased availability, lower cost of equipment, the ease of installation, the familiarity with the equipment, and many believe that MPT 1327 is superior to TETRA, due to its uncompressed FM audio, and greater receiver sensitivity. MPT1327 control channel signalling is more resilient, since the TETRA protocol uses a complex modulation scheme that requires a far higher Signal to Noise ratio to function than 1,200 bit/s FFSK signalling.
Systems based on MPT 1327 only require one, but usually use two or more radio channels per site. Channels can be 12.5 or 25 kHz bandwidth, and can be any variety of channel spacings, with 6.25 kHz or 12.5 kHz being typical. At least one of these channels is defined as the control channel (CCH) and all other channels are traffic channels (TCs) used for speech calls.
A typical installation will have around 6–10 channels. A 7-channel trunk, correctly engineered, is capable of handling in excess of 3,000 mobile units. The capacity of the system increases dramatically with the number of traffic channels. For example, 1 traffic channel with queuing can not handle many customers, perhaps 2 minicabs with 20 mobiles. In effect this would be a CBS with queuing. However, a 7 channel trunked system can handle 40 minicabs with 20 mobiles with ease. The Erlang formulas are typically used for calculating system capacity.
Spectrum efficiency
Whilst MPT 1327 systems, unlike DMR or dPMR, do not employ digital speech compression to gain any Spectral Efficiency (voice channels per 6.25 kHz), there are several methods used that increase the Spectrum Efficiency (Erlangs per square kilometre, per 6.25 kHz).
A spectrum efficiency advantage over a 4-slot TDMA system like TETRA is in areas where a low-traffic network is required. The absolute minimum TETRA installation would require one carrier, carrying one CCH and three traffic channels, using up 25 kHz of bandwidth. The absolute minimum MPT1327 assignment is a single non-dedicated control channel, utilising 12.5 kHz, in most cases.
A non-dedicated control channel can be used as a traffic channel when all the other traffic channels are busy. This can be useful if the site is part of a multi-site network and has a very low traffic profile as the site could have a single channel rather than at least two freeing up one channel for use elsewhere. The disadvantage is loss of queuing, and data cannot be sent on the control channel whilst it is in traffic mode. A non-dedicated CCH should not be used as a "reserve tank" for a busy site as the lack of signalling will seriously affect the operation of the site.
Time-shared control channels and channel pooling
Some MPT 1327 networks can also time-share control channels, which can be useful if the network has limited frequency availability, as it frees up channels for use as traffic channels, which can also be pooled across sites so the network capacity follows the traffic. This is another advantage of MPT 1327 (and dPMR) over TDMA-based systems such as TETRA and DMR, which cannot pool traffic channels so efficiently (if at all). The disadvantage of using a time-shared CCH is that it slows down registration and calls and requires some customization of the registration process, so is only useful if the network has a patient user community!
Control channel cycling
The control channel can also be set to cycle, so each channel in turn becomes a control channel, say every ten minutes it will change. The advantage of this is that the base does not have to be rated at 100% duty cycle and does not get so hot. However, this can cause problems with queuing, affects registration and control channel acquisition times. Most base stations have a 100% duty-cycle rating, but this function can be useful if local Regulatory rules do not permit Continuous CCH operation. In countries where dedicated channels for trunked systems are not possible or availability is limited it is also possible to actively manage the use of traffic channels based on co-channel use, i.e. to avoid interfering with other users by using "friendly" channel acquisition protocols.
Speech and data
Speech is sent as narrowband frequency modulation. Data messages between mobiles and the network are exchanged on the control channel at 1,200 bits per second using FFSK signalling, or a specific "modem call", known as "non-prescribed data" can be established, whereby free-form 1,200 baud data can be exchanged on a traffic channel without tying up the control channel. With the use of special modems, the speed can be 19,200 bit/s. This, along with Short Data Messaging and Status Messaging via the control channel makes an MPT1327 network ideal for managing AVL for asset management, meter reading and SCADA networks, the advantage being that the network can be used for this sort of application whilst still carrying voice traffic.
Numbering
Each subscriber in an MPT-1327 trunked radio network has a unique call number. This call number (address) is a compound number consisting of a prefix (three digits), the fleet number and the subscriber's call number within the fleet. Different numbering schemes work differently, for example Zetron uses the first two digits of the Ident as the fleet number, and the last two digits as the unit number. Idents in the 6,000–6,999 range are typically used to establish group calls. After it has been entered the call number will be converted in the mobile to a 20-bit address. The numbering rules are clarified and expanded by supplementary standards such as MPT-1343.
For the duration of his call a subscriber is exclusively allocated a traffic channel from the available pool. If all channels are occupied, the call will be queued. If the control channel has become a traffic channel, like in the case of a non-dedicated control channel, the call will be queued in the radio, although radio queuing loses the first come, first served effect, so if there are seven units queuing, the last unit to queue may get a traffic channel first.
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies. To permit this without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme.
Time-division multiple access (TDMA) is a channel access method for shared-medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium while using only a part of its channel capacity. Dynamic TDMA is a TDMA variant that dynamically reserves a variable number of time slots in each frame to variable bit-rate data streams, based on the traffic demand of each data stream.
Interim Standard 95 (IS-95) was the first ever CDMA-based digital cellular technology. It was developed by Qualcomm and later adopted as a standard by the Telecommunications Industry Association in TIA/EIA/IS-95 release published in 1995. The proprietary name for IS-95 is cdmaOne.
Terrestrial Trunked Radio, a European standard for a trunked radio system, is a professional mobile radio and two-way transceiver specification. TETRA was specifically designed for use by government agencies, emergency services, for public safety networks, rail transport staff for train radios, transport services and the military. TETRA is the European version of trunked radio, similar to Project 25.
IS-54 and IS-136 are second-generation (2G) mobile phone systems, known as Digital AMPS (D-AMPS), and a further development of the North American 1G mobile system Advanced Mobile Phone System (AMPS). It was once prevalent throughout the Americas, particularly in the United States and Canada since the first commercial network was deployed in 1993. D-AMPS is considered end-of-life, and existing networks have mostly been replaced by GSM/GPRS or CDMA2000 technologies.
Professional mobile radio are person-to-person two-way radio voice communications systems which use portable, mobile, base station, and dispatch console radios. PMR radio systems are based on such standards as MPT-1327, TETRA, APCO 25, and DMR which are designed for dedicated use by specific organizations, or standards such as NXDN intended for general commercial use. These systems are used by police, fire, ambulance, and emergency services, and by commercial firms such as taxis and delivery services. Most systems are half-duplex, in which multiple radios share a common radio channel, and only one can transmit at a time. Transceivers are normally in receive mode, the user presses a push-to-talk button on his microphone when he wants to talk, which turns on his transmitter and turns off his receiver. They use channels in the VHF and UHF bands, giving them a limited range, usually 3 to 20 miles depending on terrain. Output power is typically limited to 4 watts. Repeaters installed on tall buildings, hills or mountain peaks are used to increase the range of systems.
A cellular network or mobile network is a communication network where the link to and from end nodes is wireless. The network is distributed over land areas called "cells", each served by at least one fixed-location transceiver. These base stations provide the cell with the network coverage which can be used for transmission of voice, data, and other types of content. A cell typically uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed service quality within each cell.
A two-way radio is a radio that can both transmit and receive radio waves, unlike a broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and receiver in one unit, used for bidirectional person-to-person voice communication with other users with similar radios. Two-way radios are available in stationary, mobile, and hand-held portable models. Hand-held two-way radios are often called walkie-talkies, handie-talkies or hand-helds. Two-way radios are used by groups of geographically separated people who need to keep in continuous voice communication, such as aircraft pilots and air traffic controllers, ship captains and harbormasters, emergency services personnel like firemen, policemen, and ambulance paramedics, taxi and delivery services, soldiers and military units, fast food and warehouse employees, and radio amateurs.
PMR446 is a licence exempt service in the UHF radio frequency band and is available for business and personal use in most countries throughout the European Union.
Project 25 is a suite of standards for interoperable digital two-way radio products. P25 was developed by public safety professionals in North America and has gained acceptance for public safety, security, public service, and commercial applications worldwide. P25 radios are a direct replacement for analog UHF radios, but add the ability to transfer data as well as voice, allowing for more natural implementations of encryption and text messaging. P25 radios are commonly implemented by dispatch organizations, such as police, fire, ambulance and emergency rescue service, using vehicle-mounted radios combined with handheld walkie-talkie use.
Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system. It is a measure of how efficiently a limited frequency spectrum is utilized by the physical layer protocol, and sometimes by the medium access control.
A trunked radio system is a two-way radio system that uses a control channel to automatically assign frequency channels to groups of user radios. In a traditional half-duplex land mobile radio system a group of users with mobile and 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 simultaneously. In a conventional (non-trunked) system, channel selection is done manually; before use the group must decide which channel to use, and manually switch all the radios to that channel. This is an inefficient use of scarce radio channel resources because the user group must have exclusive use of their channel regardless of how much or how little they are transmitting. There is also nothing to prevent multiple groups in the same area from choosing the same channel, causing conflicts and 'cross-talk'. A trunked radio system is an advanced alternative in which the channel selection process is done automatically by a central controller (computer).
TETRAPOL is a digital professional mobile radio standard, as defined by the Tetrapol Publicly Available Specification (PAS), in use by professional user groups, such as public safety, military, industry and transportation organizations throughout the world. Airbus Defence and Space is the main supplier of this technology.
In radio resource management for wireless and cellular networks, channel allocation schemes allocate bandwidth and communication channels to base stations, access points and terminal equipment. The objective is to achieve maximum system spectral efficiency in bit/s/Hz/site by means of frequency reuse, but still assure a certain grade of service by avoiding co-channel interference and adjacent channel interference among nearby cells or networks that share the bandwidth.
In GSM cellular networks, an absolute radio-frequency channel number (ARFCN) is a code that specifies a pair of physical radio carriers used for transmission and reception in a land mobile radio system, one for the uplink signal and one for the downlink signal. ARFCNs for GSM are defined in Specification 45.005 Section 2. There are also other variants of the ARFCN numbering scheme that are in use for other systems that are not GSM. One such example is the TETRA system that has 25 kHz channel spacing and uses different base frequencies for numbering.
The Um interface is the air interface for the GSM mobile telephone standard. It is the interface between the mobile station (MS) and the Base transceiver station (BTS). It is called Um because it is the mobile analog to the U interface of ISDN. Um is defined in the GSM 04.xx and 05.xx series of specifications. Um can also support GPRS packet-oriented communication.
NXDN is an open standard for public land mobile radio systems, that is systems of two way radios (transceivers) for bidirectional person-to-person voice communication. It was developed jointly by Icom Incorporated and Kenwood Corporation. It is an advanced digital system using FSK modulation that supports encrypted transmission and data as well as voice transmission. Like other land mobile systems, NXDN systems use the VHF and UHF frequency bands. It is also used as a niche mode in amateur 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.
dPMR or digital private mobile radio, is a common air interface for digital mobile communications. dPMR is an open, non-proprietary standard that was developed by the European Telecommunications Standards Institute (ETSI) and published under the reference ETSI TS 102 658.
PDT is an open industry standard for trunked radio system in China servicing police wireless communications and professional mobile radio. The standard is being maintained by the Professional Digital Trunking System Industry Association, an association of major vendors of wireless communications equipment in China. The association was formed with the help from Information and Communications Bureau of the Ministry of Public Security. The standard was used to facilitate the digital transformation of the Ministry's existing MPT analogue trunking system. Government policy supporting this domestic standard had led to the abolishment of previous GA/T industry standards based on the European TETRA standard.
