# Energy Efficiency Perspectives of PMR Networks +
## 2 Present and Future PMR Systems
### TETRA and TEDS for Speech and Data Communications  
TETRA is a multiple access digital system for secure private radio communications. It allows the transmission of high quality voice and low-speed data, and it has been proposed mainly for  
Figure 2: Terrestrial Trunked Radio (TETRA) evolution. emergency services, public safety, and, in general, for all the scenarios where a bounded secure area for communications is needed. The TETRA system has some unique functionalities that cannot be obtained by commercial cellular systems: these features are motivated by the specific purposes of the secure private radio systems with respect to those pursued by widespread cellular technologies. In particular, TETRA offers:  
* group calls;
* reduced call set-up time (below 300 ms) with respect to 2G technologies;
* direct mode of operation using other mobile devices as repeaters;
* secure data transmission by end-to-end encryption;
* push-to-talk mode. As depicted in Figure 3, the TETRA architecture is similar to a generic cellular network and includes the following standard interfaces:  
* _Air Interface (AIR I/F)_, which ensures the interoperability of terminal equipment of different manufacturers;
* _Terminal Equipment Interface (TEI)_ facilitating the independent development of mobile data applications;
* _Inter Systems Interface (ISI)_, which allows the interconnection between TETRA networks of different manufacturers;
* _Direct Mode Operation (DMO)_ guaranteeing the communication between terminals also beyond network coverage. A Time Division Multiple Access (TDMA) is used to allocate four time slots, each of which has a duration of 14.167 ms, into a 25 kHz bandwidth carrier. Each time slot represents a full rate channel or, optionally, two half rate channels in order to increase the random access. Frequency division duplexing is used to associate to a time slot the uplink and downlink channels. Data and control information is mapped within each channel and the transmission is done through \(\pi/4\)-PSK modulation. The physical content of a time slot is organized as a burst, as described in Figure 4. At each base station, the first slot of each TDMA frame of one carrier is occupied by the Broadcast Common Channel (BCCH). The remaining time slots of such carriers and, eventually, all the time slots of other available carriers can be assigned to a Traffic Channel (TCH). When a channel is idle because it is not assigned to any TCH, a dummy burst is transmitted in order to maintain a continuous bit flow. Figure 3: TETRA architecture. The first release of TETRA allows a maximum uncoded data rate of 28.8 kbps. In order to improve this standard to support IP-based multimedia services, TEDS has been introduced taking care of the maximum backward compatibility [17]. TEDS can be considered as an evolution of TETRA system, with improvements focusing on transmission and bandwidth extension, in order to meet the need for higher speed data services. In particular, as reported in Figure 5, channels with different bandwidths are available. Allowed bandwidths are 25 kHz, as in the previous release, 50 kHz, 100 kHz and 150 kHz. Moreover the adoption of spectral-efficient multilevel modulation schemes enables the possibility of link adaptation: 4-QAM, 16-QAM and 64-QAM are added to \(\pi/4\)-PSK scheme, used in the first release of TETRA. In order to detect errors and protect the information, different channel coding schemes have been introduced. By adaptively selecting the opportune modulation, taking into account the needs and the link quality, a throughput beyond 500 kbps can be obtained, as shown in Table 1. The TEDS improvements are not only related to physical layer. Classifying data flows into classes allows for negotiation of the opportune quality of service for each flow in terms of throughput, delay, precedence and reliability. Allowed classes are real-time class for live audio and video transmission, and telemetry class for bursted low capacity transmissions, and background class for file transfer and web applications.