Changing the standard for public transport

In the November/December issue of Radio Comms Asia Pacific, David Cox, operations director of Pacific Wireless Communications, took issue with the DMR bid to ‘take over the world’ of mobile radio in his article “DMR - is it the road to nowhere?” He argued that Tetra has largely already achieved what DMR is wanting to do so, following on from that argument, this article looks more closely at what the DMR standard is all about.

Over the last 30 years, mobile radio communications have matured, leading to the availability of high-performance analog radio networks. The features and functionality of these networks have been honed to deliver both speech and data in a form that addresses the vast majority of public transport operators’ needs.

The challenge is for a suitable digital mobile radio communications standard to replace the analog that has served a wide range of industries including transport.

To address this, ETSI (the European standards body) created the ERMTGDMR group and tasked it with producing a suitable standard.

The DMR standard provides a progressive range of communications that start with basic, low-cost business radio through conventional radio systems to wide-area (even national) trunked infrastructure-based networks.

Public transport has used mobile radio for many years. In earlier times, the systems were mainly voice-centric and offered limited data services. However as technology progressed, data services delivered from a common subscriber unit have become commonplace.

Data has become the predominant message load on the radio system. Standards-based solutions such as MPT1327 still continue to deliver the vast majority of transport voice and data needs but do not offer digital speech or improvements in spectral efficiency.

The DMR standard offers some real benefits to those considering a move to digital.

One of the aims of DMR is to address the need for a standard that would not use very specific frequency bands or require wide channels and therefore suffer from the many issues of reallocation and scarcity of resource.

Typically, public transport operations have not featured among the highest tiers of the mobile radio user community and often government and public safety organisations get the best allocations.

DMR delivers a solution that will allow continued mobile radio use in allocations from 66 to 900 MHz. The channel bandwidth required is also only 12.5 kHz which aligns well with the directives of most of the world’s frequency regulators.

From the public transport operator’s viewpoint, this means they may be able to continue to use their existing radio channels and benefit from doing this in three ways.

• DMR delivers a two-slot TDMA solution. This delivers, in effect, two voice streams within the 12.5 kHz radio channel bandwidth (see Figure 1). DMR offers such voice features as broadcast (all fleet) and group voice calls. This results in more voice message capacity and improved operations;
• DMR offers a range of data services that can be tailored to specific needs. The over-the-air rate is 9600 bits per second. This stream can be divided in two to deliver either two voice paths or a combination of voice and data or be combined to produce a dual-slot, data-only service.

• Generally transport networks tend to follow a hub-and-spokes model, with the city centre or CBD as the hub, with the routes and density of transport activity thinning out as you reach the edge of the city. This model is best suited to the most scalable networks.

Figure 1: TDMA (DMR) - two time slots across 12.5 kHz give 6l.25 kHz equivalence.

Although much can be said about the technology, the effectiveness of any standard is in its ability to deliver a viable business solution. DMR, with its wide range of data service options, does not rely on the one-size-fits-all model and can be adapted to meet the wide range of public transport industry needs.

In most ITS solutions that deliver improved traffic flows, better operations information and downstream user real-time services, a range of data messages is required to be transported over the radio service.

The primary requirement and starting point for most ITS solutions is accurate, low-latency vehicle location messages (AVL). These are normally short, typically 10-12 bytes of data sent at regular intervals. Transmission interval might be as short as every 15 seconds although most operators work on a 30 second fleet update.

Just consider the numbers involved, where a fleet of 500 vehicles, transmitting location messages every 30 seconds creates 1000 messages a minute. Using some DMR-based techniques, this demand can be reduced and easily handled.

Similarly, short messages in the other direction are also catered for. Good AVL delivers information that can determine timetable adherence, predict issues, traffic flows, route compliance, prioritise traffic lights and be used for real-time passenger information (RTPI) systems.

Within the AVL message other parameters can be delivered, such as passenger count or alarms. A part of this design is a method of sending messages back to the vehicle that can alert the driver to whether they are ahead or behind schedule.

From an operations point of view, longer messages are required in both directions between the vehicle and the control centre. Drivers often need to request the right to speak on the radio system, thus creating a level of discipline and reducing driver distractions.

These requests to talk messages are normally sent over the radio system as data. Other messages over the system include: driver log on/log off duty and route. Automated messages might include separate automatic passenger counting, as well as delaying or advancing services to maintain correct interservice or intermodal timings (connection protection).

Another form of these messages is for onboard signage or text-to-speech messages. In general these messages have to be sent immediately and are longer than the typical AVL message.

These services are mainly used to enhance operations and save time and effort at the control centre and reduce the work involved by the driver.

DMR infrastructure needs to support other services that flow from the AVL and operational messages. Where a prediction engine is fed from this data, both operational and real-time information can be delivered.

The standard allows for messages to deliver arrival predictions to street-mounted signs. Suitable small radio modules with ultra-low power consumption can be deployed in signs. Solar-powered rural signs can be served by DMR-capable radios.

The DMR format allows a high update rate that meets the needs of public transport organisations. For example, during times of disruption, hundreds of signs may need to be updated.

The Tait DMR Prototype image.

A typical system might involve as many as 1000 signs with updates every 30 seconds. Non-exclusion requires both visual indications and voice is delivered to the signs. Again, there are practical solutions to meet these needs.

Users now expect to have choice of the services that they use for transport. Reliable information delivered to signs can help increase the patronage of the service, reducing user churn and defection to other forms of transport.

Communication is critical, especially during emergencies. The DMR standard allows emergencies to be handled in terms of voice but also in the data realm. Emergencies are given priority and resources cleared to deliver a fast call set-up.

Data messages can be configured to provide regular location updates if it is a moving incident. During an incident it may be useful to listen to the activity in the vehicle.

If the emergency is an accident, being able to coach the driver through the process of dealing with the situation can be highly valuable, as well as being able to talk to the passengers.

The inherent benefits of digital speech and vocoding reduces the noise at the limits of the system range and has a background noise-reduction feature, which helps to improve audio intelligibility during emergencies.

Transport operators face weather events and crises of varying scale. Even with the most advanced planning, infrastructure can become overwhelmed. This is often the case with public mobile phone systems due to overloading or waterlogging. Although less likely, even mobile radio infrastructure can succumb.

Public transport vehicles are often part of the local evacuation plan and form the most efficient way of moving large numbers of people away from the danger area. DMR offers an effective long-range, direct vehicle-to-vehicle service for both voice and data.

Even when not connected to its infrastructure, it can still be used in an open net service to provide vital communications. Typical direct vehicle-to-vehicle ranges are into the tens of kilometres. This can also provide radio communications for groups of vehicles beyond the normal radio network range.

Mobile radio still offers the most flexibility, resilience and control of all the available technologies. Transport networks are required to deliver a wide range of services. Figure 2 shows a sample of the services that the radio solution needs to interface with or support.

Figure 2: The Tait DMR TM8300 prototype image subject to change.

The DMR standard provides a very good fit as it provides an over-the-air protocol that can be used to support all these services. But it does not just end there.

The standard will also support or help deliver:

• IP connected infrastructure;
• Cost-comparable terminals and infrastructure to MPT1327;
• Potential re-use of vehicle aerials and site elements such as aerials and combining, reducing the cost of the migration as no frequency change may be required;
• Terminals that support access to legacy analogue systems if required in emergencies.

The case for DMR to become the standard for mobile radio communications in public transport is very strong, with many of the features and functionality pointing to a more practical approach to the needs of the transport market.

Customisation and adoption are not inhibited by DMR but enhanced. When considering new solutions or projects it must be given serious consideration.