|
The TETRA Enhanced Data Service (TEDS) is a next-generation TETRA enhancement which can bring data-centric field operations to life. But, as Tero Pesonen explains, its new extension, Direct Access, has features which will offer even more benefits to users The importance of data communication is ever increasing. In field operations, the requirement is quickly changing from better coverage to more, better and faster data. After all, when there is a connection (coverage), the next logical need is for improving the operations. People in the field, as well as field commanders, need a better and more precise picture of the situation. But at the same time, there must not be information overflow. Today, public safety budgets are flat – yet operations may be more demanding, and more costly. It is imperative that daily field routines become more efficient, and this can be done with data. However, not just any data. Before field operations can be founded on data in addition to voice, data must become absolutely reliable. Data has to be as secure, available and dependable as the mission-critical voice link that TETRA systems provide. When field operatives can trust the ready availability of task-related data as much as they trust the ability to communicate with their group, operational models can be changed. And then they can fully take advantage of the information-sharing possibilities that data communications bring. The answer: TEDS The TETRA Enhanced Data Service (TEDS) is the TETRA feature that can bring data-centric field operations to life. TEDS is the high speed data part of the ETSI TETRA Release 2 standard. It is a mission-critical data bearer service that is fully integrated into TETRA standard, and so it can be introduced into some vendors’ TETRA systems as a software upgrade. From a technical viewpoint, TEDS is an always-on data service which provides both Short Data Service (SDS) and IP packet data services. It has quite a clever design: it offers the possibility to use variable channel bandwidths ranging from the current TETRA 25?kHz channels up to 50, 100 and even 150 kHz. In this way it can make use of the entire available spectrum – and even discontinuous spectrum. In practice, a combination of 25 and 50?kHz channels may be the most common choice, because coverage for TEDS will then be the same as for TETRA voice. The new high-speed data services can thus be provided over a wide area, without the need to add more base station sites to the network. TEDS has another remarkably clever characteristic: it strives to maintain the connection as long as possible. To achieve this, it changes its modulation and error coding schemes automatically to more and more robust ones as the link between the TETRA base station and TEDS-capable radio becomes weaker. TEDS uses QAM (Quadrature Amplitude Modulation) 64, 16 and 4, where QAM 4 is the most robust mode. Because WiMAX and LTE use the same modulation, TEDS can be regarded as a member of the same generation. QAM modulation is very-spectrum efficient, which is important for public safety radio where frequencies are a scarce resource. For radio users, TEDS brings two significant benefits: more capacity for data and faster data. On 50?kHz TEDS channels, TEDS yields almost eight times the SDS traffic compared to the TETRA Main Control Channel (MCCH). The IP data throughput is up to 100?kbit/s – quite an increase over the speed of single-slot TETRA packet data. For owners of TETRA networks, TEDS also brings benefits. The ability to increase data capacity and speed without the need for a complete change of technology or hardware in the network is an obvious advantage. To take an example, let us imagine that one cell needs to deliver 100?kbit/s of traffic. A traditional TETRA 1 cell would need from eight to ten TETRA transceiver radios – together with support such as electricity supply, battery back-ups, cooling, and the space that these transceivers need – plus, of course, 200–250?kHz of spectrum. In contrast, the same traffic could be delivered by a single TEDS 50?kHz carrier on QAM 64 modulation. TEDS would also require just one-fifth of the transmission and spectrum, one-tenth of the hardware and space, and even less electricity. Significant savings! TEDS Direct Access is a new extension to the TEDS part of the ETSI TETRA Release 2 standard. Simply put, it is a mechanism that enables dedicated data devices to register directly on the TEDS data carrier. Without TEDS Direct Access, every TETRA radio terminal registers on the Main Control Channel. According to the service it requires, it is directed to traffic channels (TCH) for voice calls, to packet data channels (PDCH) for packet data transfer, or to the TEDS channel for high speed data. This mechanism is called TETRA Conventional Access. TEDS Direct Access and Conventional Access are complementary methods. Each method answers the needs of different operational scenarios. TEDS Conventional Access is necessary for TEDS-capable handportables, for example, which would be used mainly for voice. Direct Access, in contrast, is ideal for dedicated TEDS data radios.
|