The TETRA radio system offers many features that enable it to have more flexibility than many other systems.
The TETRA 1 radio systems provide the major features, and although further development to TETRA 2 and TEDS have enabled further facilities to be added, they still retain the TETRA 1 capabilities and are backward compatible. In this way, TETRA 1 forms the basis for all TETRA radio systems.
There are three different modes in which TETRA radio systems can be run:
- Voice plus Data (V+D)
- Direct Mode Operation (DMO)
- Packet Data Optimised (PDO)
The most commonly used mode is V+D. This mode allows switching between speech and data transmissions, and can even carry both by using different slots in the same channel. Full duplex is supported with base station and mobile radio units frequencies normally being offset by about 10 MHz to enable interference levels between the transmitter and receiver in the station to be reduced to an acceptable level.
DMO is used for direct communication between two mobile units and supports both voice and data, however full duplex is not supported in this mode. Only simplex is used. This is particularly useful as it allows the mobile stations to communicate with each other even when they are outside the range of the base station.
The third mode, PDO is optimised for data only transmissions. It has been devised with the idea that much higher volumes of data will be needed in the future and it is anticipated that further developments will be built upon the TETRA mobile radio communications standard.
TETRA radio uses TDMA techniques. This enables much greater spectrum efficiency than was possible with previous PMR systems because it allows several users to share a single frequency. As the speech is digitised, both voice and data are transmitted digitally and multiplexed into the four slots on each channel. Digitisation of the speech is accomplished using a system that enables the data to be transmitted at a rate of only 4.567 kbits/second. This low data rate can be achieved because the process that is used takes into account the fact that the waveform is human speech rather than any varying waveform. The digitisation process also has the advantage that it renders the transmission secure from casual listeners. For greater levels of security that might be required by the police or other similar organisations it is possible to encrypt the data. This would be achieved by using an additional security or encryption module.
The data transmitted by the base station has to allow room for the control data. This is achieved by splitting what is termed a multiframe lasting 1.02 seconds into 18 frames and allowing the control data to be transmitted every 18th frame. Each frame is then split into four time slots. A frame lasts 56.667 ms. Each time slot then takes up 14.167 ms. Of the 14.167mS only 14 ms is used. The remaining time is required for the transmitter to ramp up and down. The data structure has a length of 255 symbols or 510 modulation bits. It consists of a start sequence that is followed by 216 bits of scrambled data, a sequence of 52 bits of what is termed a training sequence. A further 216 bits of scrambled data follows and then the stream is completed by a stop sequence. The training sequence in the middle of the data is required to allow the receiver to adjust its equaliser for optimum reception of the whole message.
The data is modulated onto the carrier using differential quaternary phase shift keying. This modulation method shifts the phase of the RF carrier in steps of ± π /4 or ± 3 π /4 depending upon the data to be transmitted. Once generated the RF signal is filtered to remove any sidebands that extend out beyond the allotted bandwidth. These are generated by the sharp transitions in the digital data. A form of filter with a root raised cosine response and a roll off factor of 0.35 is used. Similarly the incoming signal is filtered in the same way to aid recovery of the data.
Additionally, TETRA radio uses error tolerant modulation and encoding formats. The data is prepared with redundant information that can be used to provide error detection and correction. The transmitter of each mobile station is only active during the time slot that the system assigns it to use. As a result the data is transmitted in bursts. The fact that the transmitter is only active for part of the time has the advantage that the drain on the battery of the mobile station is not as great as if the transmitter was radiating a signal continuously. The base station however normally radiates continuously as it has many mobile stations to service.
One important feature of TETRA is that the call set up time is short. It occurs in less than 300 mS and can be as little as 150 mS when operating in DMO. This is much shorter than the time it takes for a standard cellular telecommunications system to connect. This is very important for the emergency services where time delays can be very critical.
Further TETRA radio developments
While TETRA radio is a major improvement over the previous PMR systems in operation, additional data capacity is always needed. In view of the higher data capabilities now being offered by the cellular services, the TETRA radio standard is being updated to enable it to keep pace with other comparable technologies. In this way, TETRA mobile radio communications will be able to offer commercial users the advantages of a PMR service alongside the data capabilities of a cellular network.
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