16 Feb. 2017 Information
Shopping centres, industrial plants, and ports are just a few of the applications for two-way radio, and digital mobile radio (DMR) is a modern and efficient solution for many of them.
Choosing a DMR base station largely depends on the choice of manufacturer, as some premium features may be proprietary and only work on matched handsets and base stations/repeaters. DMR re-uses 12.5 kHz spectrum channels to carry two TDMA timeslots, doubling the voice channels compared to analogue systems and reducing radio power consumption. It provides better voice quality and noise rejection over greater ranges. Many suppliers of DMR equipment include dual mode analogue/digital capability to give users an easy migration path with reuse of frequencies, sites, and equipment. An organisation using analogue radios can upgrade to DMR to add capacity and improved voice quality. DMR also has features that can boost the efficiency of user organisations. Data, telemetry, and location services are some of the features DMR Tier II and Tier III offer.
DMR’s ability to cater for a wide range of users is made possible by its three-tier approach, and this is the first thing to consider when choosing base stations. DMR Tier I, which is mainly intended for consumers and simple commercial end users, prohibits the use of repeaters or telephone interconnects. Therefore it won’t be considered further in this article.
Tier II operates in licensed bands at high power and covers larger areas. It is supported in any licensed band from 66 to 960 MHz. Tier II systems can include repeaters to extend range further, telephone interconnect, advanced call centres, data services, dispatch connection, and secure connections. In Tier II channels can only be used for one conversation and so the network can become congested. For this reason Tier II DMR is typically used where there are less than 200 users.
Tier III is a trunked solution; this means a pool of radio traffic channels is available and users take the next available channel when they press talk. A larger number of users have access to the network, the premise being that calls are generally short and the channel is available quickly after the call. Users can number from small groups to many thousands, covering a single building or several thousand square kilometres in a wide area network.
Clarify needs and select DMR tier
Tait Communications’ solutions marketing manager John Graham is the chairman of the DMR Association’s marketing group and points to the requirement for good receiver performance in DMR base stations. This is needed to maintain good voice quality when there is interference. Digital systems like DMR cannot give an indication when the signal is degrading like analogue can, so the sensitivity, blocking and interference rejection performance of the receiver is important to maintain calls where frequencies are congested.
He adds that the DMR Association is driving the standardisation of several proprietary features, such as dialling schemes, text messaging, and the AIS protocol for connection to dispatch consoles. Work in this area is still underway and potential users need to check whether the features they need are now standardised if they wish to operate a multi-vendor network.
John Grant, a senior engineer for solutions provider Digital Angel, agrees that interoperability of Tier II systems is limited – the company has experience of Hytera and Motorola Solutions base stations and radios working together but this is for single sites and voice only.
Mick Davies, EARS’s technical director, says customers generally understand radios more easily than base stations. The choice of system supplier follows from the preferred radio; choice is driven by look, feel and price. EARS’s customers tend to want simple solutions with minimum training required. A typical customer such as a shopping centre would take two base stations, providing four channels for 15 groups.
Grant says that Tier III is a still an emergent technology in the UK with a limited number of installer companies active in the market.
The choice between DMR Tier II and III is largely a question of scale and features. Ian Gudger, radio comms product and service provider Servicom’s joint managing director, says his company, which has been installing and commissioning DMR systems since 2008, begins with a site visit to determine the physical and RF situation. Gudger focuses his customers’ capacity requirements when he’s specifying the base station configuration. For example, a stack of eight repeaters with a software switch is a typical installation supporting 16 simultaneous calls.
In the UK obtaining frequencies from Ofcom is a major factor when choosing which band to use and it is vital that the DMR base stations and repeaters being considered support the bands that are available.
Servicom notes that London is a particularly difficult area, with frequency sharing being mandatory in some bands. In one case on the South Bank it found 12 licensed repeaters within three kilometres of one of its customers on the same frequency.
Frequency availability and service in the VHF bands may be available but its propagation might not be suited to the type of installation required. In the UK parts of Band III are available and manufacturers are developing solutions for this. Servicom’s Gudger uses the Ofcom spectrum information site to assess the likelihood of interference
on new installations, so he can design the system to mitigate this as much as possible. The company limits its radio systems’ coverage to essential areas only, thereby reducing exposure to interference.
Davies from EARS agrees that frequency availability is a major consideration, especially in London. EARS tends to favour UHF because interference is lower and its customers are usually in restricted areas such as on-site systems.
If the DMR system is replacing an analogue system the current sites and frequency assets come into play in the decision-making. It will also be clearer what the expectations of the end users are regarding services as their evolving needs may be driving the migration. Many DMR products also support analogue mode, making migration easy. Existing sites are likely to have power and possibly data links so installing DMR base stations is made simpler.
Gudger notes that migrating away from analogue can be done gradually and that some DMR repeaters work in both digital and analogue mode and so support a phased evolution to full digital service.