Bringing broadband to TETRA
If anybody really needs mobile broadband, it’s the emergency services. But upgrading the tried-and-trusted TETRA radio system raises questions about spectrum allocation.
TETRA, or Terrestrial Trunked Radio, has for many years offered a resilient and secure digital radio for voice communications and short messaging. Europe’s emergency services have found it well suited to their work: if you cut the wiring to a TETRA basestation, local handsets can still communicate with each other directly; if you blow up a TETRA exchange, there will be another one on standby to take its place.
Terrorist incidents in cities are a major worry, but accidents and emergencies also happen on moors and mountain ranges, which is why TETRA coverage reaches 99 per cent of the total landmass in the UK.
Where TETRA is beginning to fall short, however, is in data transfer, which is limited to a maximum multi-slot packet data-rate of just 12-15kbit/s. It’s no wonder, therefore, that many UK police forces give their officers PDAs to carry alongside their TETRA radios, so they can use the public cellular networks to access databases and fill in forms.
Public network limits
While 3G cellular networks can usually handle data rates of several megabits per second, during major incidents, such as the terrorist bombings in London and Madrid, they have become congested or even unavailable. This is why there is unease in many quarters about the growing reliance on these networks for day-to-day public safety operations. But what are the alternatives?
In cities and around strategically important locations, commercial mobile networks can be bolstered with backup basestations and extra power supplies. Support for video and high-speed data can be provided for major incidents in remote areas by bringing in satellite communications as needed.
There remain two issues for these ad hoc approaches to emergency communications: matching TETRA’s coverage, and being able to prioritise access by specific user groups or individuals during an emergency.
“With TETRA, you can press an emergency button on a terminal and make a priority call,” explains Mark Hunter, senior project consultant of the wireless design and consultancy firm Plextek. “And there are more nuanced hierarchical mechanisms where commanders can press ‘transmit’ and their signal will go ahead of others because they are trying to issue commands.” Commercial cellular networks offer no such facilities.
“When a phone requests a channel from the nearest basestation to make a call, it transmits a small burst on a random-access channel. If it doesn’t get through, the burst is sent again. In the extreme case of too many users trying to make a call simultaneously, without prioritisation their requests will all collide and none will successfully be able to make a call or set up a data channel,” he adds.
The immediate contender for supporting higher-speed public safety data in Europe is TETRA’s data extension, TETRA Enhanced Data Service (TEDS). TEDS is a wideband service based on dedicated carriers. It can support 50kHz, 100kHz and 150kHz channel widths, to deliver user bit-rates of 100 to 500kbit/s. Of course, for a given power, the broader the band the signals use, the smaller the cell range, so measures such as higher power transmitters or better antennas are needed to avoid coverage gaps between basestations.
TEDS uses the same air-interface techniques as wireless standards such as WiMax and LTE (3GPP Long-Term Evolution) with the same modulation scheme (quadrature amplitude modulation) and multiple subcarriers. “But TEDS has very narrow channels in comparison [LTE channels, for example, are 10 to 20MHz] which means users can deploy high data rates in small amounts of spectrum. It makes the use of high-speed data in a privately owned system possible,” says David Chater-Lea, senior technologist working for Motorola, who is chairing the working group standardising data for TETRA in ETSI, the European Telecommunications Standards Institute.
An alternative to TEDS is to use a different modulation scheme on the existing TETRA 1 infrastructure, according to Tim Fowler, commercial director of Cambridge Consultants’ wireless division.
“TETRA 1 is a four-level, 2-bit-per-symbol differential phase-shift key system, but there is a proposal to change it to 8-PSK, which with three bits per symbol adds 50 per cent to the capacity of existing basestations,” he explains. “It’s simple to do and has only a moderate impact on coverage but it hasn’t gained much interest from the infrastructure providers because, I guess, they can’t sell more basestations on the back of it.”
For broadband data, Chater-Lea’s ETSI group is starting to look at how a ready-made standard such as LTE could be adapted for public safety use, to provide broadband access around cities and motorways, with wideband and narrowband technologies as a fallback in rural areas.
“We would need to add efficient support for group communications so a number of people would be able to simultaneously view, say, video from an incident. Today’s broadband standards send the same information separately to everyone, whereas with TETRA, information is put on one channel and broadcast,” he says.
Since May 2009, there has been a working example of a dedicated broadband public safety wireless data network in New York (see box, above). Built by Northrop Grumman using IP Wireless’ 3GPP TD-CDMA MIMO technology, the network is designed to deliver high-speed data and video (with burst data rates of 7Mbit/s) to emergency-service vehicles driving around the city.
It also provides communications for VoIP emergency call-boxes and connectivity to traffic-signal controllers (to monitor and provide updates to programs for a given traffic-light signal), as well as following the location of the sanitation fleet in the city.
A version of the network (operating on spectrum between 872 to 876MHz and 918 to 921MHz) was successfully tried in 2007 in the UK for three months by the National Police Improvement Agency (NPIA), which, according to Tom Afferton, director of wireless engineering for Northrop Grumman, shared the same concerns as the New York authorities about the poor resilience and availability of commercial networks during emergencies.
The NPIA worked with Sussex Police to test the system on a police car fitted with video cameras and automatic number-plate recognition. Live video was streamed from the vehicle to fixed and mobile devices throughout the coverage area to check on the police national computer any vehicle whose number plate was read. “We were able to read all number plates of passing vehicles from the front and the rear and check them all against the live database in real time,” said Guy Kenyon, a systems engineer who works for Northrop Grumman in the UK.
Before any such network can be built in Europe, however, there needs to be spectrum in which to operate it. North America has had the foresight to allocate two 10MHz slots (split into uplink and downlink) at around 700MHz for LTE-based mobile public-safety broadband and there is a further 10MHz slice (the D-Block) up for discussion. [The New York set-up uses a licensed 2.5GHz band but there are plans to build a 700MHz overlay network for hand-held applications.]
So far EU public-safety users have agreed that they need at least two 1.5MHz bands for extra TETRA voice channels, two 3MHz bands for TEDS wideband data, and another two 10MHz bands for broadband data over the next five to 10 years. Where these slots will be found is yet to be resolved. Another oddity is that while TEDS can provide wideband data in 150kHz channels, there is insufficient spectrum put aside to fully support the extra space such channels would require, limiting any early deployments to 50kHz channels, which will offer data rates of around 100kbit/s.
Currently, most European TETRA systems occupy two 5MHz slots within the 380 to 400MHz band, which is reserved for European public safety and security forces. Ideally, they would extend into the 410 to 430MHz band with TEDS, or failing that 450 to 470MHz.
“410 to 430MHz is a harmonised band, used by the PMR community across Europe and the MoD in the UK. If 80 per cent of the EU countries can agree on it for TETRA-TEDS, it will go ahead. In the UK, it might be more tricky because Ofcom has already auctioned off two 2MHz blocks in that band,” says Jeppe Jepsen, director of international business relations for Motorola, based in Brussels, who is a member of the general assembly at ETSI, and a board member of the TETRA association. There is also spectrum below 380MHz for military and NATO use, adds Jepsen, which TETRA would like to use, but it seems unlikely to become available without a fight.
Frequencies between 790 and 860MHz released by broadcasters as part of the digital dividend would be ideal for future broadband public-safety networks but the bands are being auctioned off for commercial 4G (LTE) across Europe.
One further option is to use spectrum from a possible second digital dividend. The band from 698 to 790MHz, for instance, would align Europe with North America, which would be handy for economies of scale and equipment availability.
Bands above 1GHz could also be used, but public-safety users prefer spectrum at lower frequencies because building penetration is better and you get the best coverage for the least number of basestations.
No plans in the UK
Currently, there are no concrete plans for spectrum allocation in the UK to support high-speed data for emergency services. Speculation that commercial licensees of the UK’s digital dividend spectrum might be obliged to support public-safety data as part of the deal seems over-optimistic.
The 2005 UK audit on spectrum holdings, known as the Cave report, recommended that market mechanisms should be used “in all but exceptional cases”. As a spokesman from Ofcom commented by email: “The UK user community has not developed a business case or socialised a user requirement with Ofcom.”
There is frustration amongst equipment suppliers, developers and users about this impasse on harmonising spectrum in Europe.
“There is a lot of talk about public safety and national security and how we need secure, reliable communications, but no sign of any action beyond proposals to extend the TETRA standard to boost its data-rate,” says Northrop Grumman’s
A study carried out for the German government by Wik-Consult said that demand for spectrum to support video and high-speed data for public safety has been recognised for years, and the costs associated with providing such capabilities can be justified by any combination of lives saved, crimes deterred, gains in operational efficiency, and avoidance of injury or loss of life on the part of emergency services personnel.
What is required is the will to take the steps to meet that demand. It is, after all, safety on the line.
New York’s safety network
New York City’s broadband public-safety wireless data network, NYCWiN, was commissioned in 2006 by the city’s Department of IT and Telecommunications in response to a series of events – the 11 September 2001 attacks, an oil refinery fire, a power blackout and a fatal accident involving the Staten Island commuter ferry – during which crucial public networks stopped working.
The test that won Northrop Grumman the $500m contract was an incident scenario set up at the World Trade Center site, involving command vehicles and the actual individuals who would direct operations in such circumstances.
“Each network was loaded with extra background users to simulate the escalation of an incident. We then failed different network elements and asked the users about the performance. In the case of our solution, the critical users could still continue to work with their mission-critical applications such as mapping and despatch,” explains Tom Afferton, director of wireless engineering for Northrop Grumman.
NYCWiN, which uses the 2.5GHz licensed band, covers 95 per cent of the city from 400 rooftop basestations, each divided into three 120° sectors to provide 360° of coverage from a site. The network can support up to around 20Mbit/s of total bandwidth uplink and downlink from a 120° sector shared across all users in that area. For a given user, it can burst up to around 7Mbits/s for either the uplink or downlink, but more typical speeds would be 4Mbit/s on the downlink and 2Mbits/s on the uplink.
NYCWiN’s architecture is like Airwave’s TETRA digital radio network in the UK in so far as security, resilience and redundancy were designed in from the start. Features include end-to-end encryption, and built-in intrusion protection with inspection of all packets. Devices and users both have to be identified to get access to the network, and the network establishes different priorities for different classes of users and applications, giving priority access to higher grades of traffic such as VoIP calls from emergency call boxes.
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