TETRA is an ETSI standard, first version published 1995. TETRA is endorsed by the European Radio Communications Committee (ERC) and mandated for use in Europe.
TETRA uses Time Division Multiple Access (TDMA) with four user channels on one radio carrier and 25 kHz spacing between carriers. Both point-to-point and point-to-multipoint transfer can be used. Digital data transmission is also included in the standard though at a low data rate.
TETRA Mobile Stations (MS) can communicate Direct Mode or using Trunked infrastructure (Switching and Management Infrastructure or SwMI) made of TETRA Base Stations (TBS). As well as allowing direct communications in situations where network coverage has been lost, Direct Mode or DMO also includes the possibility of using one (or a chain) of TETRA terminals as relays for a signal. This functionality is called DMO gateway (from DMO to TMO) or DMO Repeater (DMO to DMO). In rescue situations this feature could allow direct communications underground or in areas of bad coverage.
In addition to voice and dispatch services, the TETRA system supports several types of data communication. Status messages and short data services (SDS) are provided over the system's main control channel, while Packet Data or Circuit switched data communication uses specifically assigned traffic channels.
All traffic is normally encrypted. TETRA provides both over the air encryption and end-to-end encryption.
In Britain the public sector TETRA system operates under the name 'Airwave'. In Belgium, the 'A.S.T.R.I.D' system uses TETRA. In the Netherlands, the TETRA system is called 'C2000' In Sweden the RAKEL system uses TETRA. In Finland the VIRVE (short for viranomaisradioverkko, loosely translated: "officer network") network uses TETRA. VIRVE and C2000 are only two of the many TETRA networks that cover an entire country.
The base stations normally transmit continuously and (simultaneously) receive continuously from various mobiles - hence they are Frequency Division Duplex. TETRA also uses TDMA (see above). The mobiles normally only transmit on 1 slot/4 and receive on 1 slot/4 so they are both Time Division and Frequency Division Duplex.
Speech signals in TETRA are sampled at 8 kbit/s and then compressed with a vocoder using a technique called ACELP (Adaptive Code Excited Linear Prediction). This creates a data stream of 4.567 kbit/s. This data stream is error-protection encoded before transmission to allow correct decoding even in noisy (erroneous) channels. The data rate after coding is 7.2 kbit/s - the capacity of a single traffic slot when used 17/18 frames.
A single slot consists of 255 usable symbols, the remaining time is used up with synchronisation sequences and turning on/off etc. A single frame consists of 4 slots, and a multiframe (whose duration is 1.02 seconds) consists of 18 frames. Hyperframes also exist, but are mostly used for providing synchronisation to encryption algorithms.
The downlink (i.e. the output of the base station) is normally a continuous transmission consisting of either specific communications with mobile(s), synchronisation or other general broadcasts. All slots are usually filled with a burst even if idle (continuous mode). Although the system uses 18 frames per second only 17 of these are used for traffic channels, with the 18th frame reserved for signalling, Short Data Service messages (like SMS in GSM) or synchronisation. The frame structure in TETRA (17.65 frames per second = 18 frames in 1.02 seconds), consists of 18000 symbols/s / 255 symbols/slot / 4 slots/frame, and is the cause of the perceived "amplitude modulation" at 17 Hz and is especially apparent in mobiles/portables which only transmit on one slot/4. They use the remaining three slots to switch frequency to receive a burst from the base station two slots later and then return to their transmit frequency (TDMA).
This first representation demonstrates where the SRT (Slow Reselect Threshold) the FRT (Fast Reselect Threshold) and propagation Delay exceed parameters are most likely to be. These are represented in association with the decaying radio carrier as the distance increases from the TETRA Base Station.
From this illustration, these SRT and FRT triggering points are associated to the decaying radio signal strength of the respective cell carriers. The thresholds are situated so that the cell reselection procedures occur on time and assure communication continuity for on-going communication calls.
The initial cell selection is performed by procedures located in the MLE and in the MAC. When the cell selection is made, and possible registration is performed, the MS (mobile station) is said to be attached to the cell.
The mobile is allowed to initially select any suitable cell that has a positive C1 value, i .e. the received signal level is greater than the Minimum Rx Level for Access parameter.
The initial cell selection procedure shall ensure that the MS selects a cell in which it can reliably decode downlink data, i.e. on a main control channel (MCCH), and which has a high probability of uplink communication. The minimum conditions that shall have to be met are that C1 > 0. Access to the network shall be conditional on the successful selection of a cell.
At mobile switch on, the mobile makes its initial cell selection of one of the base stations, which indicates the initial exchanges at activation.
The minimum RX access level information element shall indicate the minimum received signal level required at the SwMI in a cell, either the serving cell or a neighbour cell as defined in table 18.24.
The serving cell becomes improvable when the following occurs:
A neighbour cell becomes radio usable when the cell has a downlink radio connection of sufficient quality.
The following conditions must be met in order to declare a neighbour cell radio usable:
The neighbour cell has a path loss parameter C1 or C2 that is greater than the following: (FAST_RESELECT_THRESHOLD+FAST_RESELECT_HYSTERISIS) for a period of 5 seconds, and the service level provided by the neighbour cell is higher than that of the serving cell. No successful cell reselection shall have taken place within the previous 15 seconds unless MM requests a cell reselection. The MS-MLE shall check the criterion for serving cell relinquishment as often as one neighbour cell is scanned or monitored.
The following conditions will cause the MS to rate the neighbour cell to have higher service level than the current serving cell:
The serving cell becomes relinquishable when the following occurs: The C1 of the serving cell is below the value defined in the radio network parameter cell reselection parameters, fast reselect threshold, for a period of 5 seconds, and the C1 or C2 of a neighbour cell exceeds the C1 of the serving cell by the value defined in the radio network parameter cell reselection parameters, fast reselect hysteresis, for a period of 5 seconds.
No successful cell reselection shall have taken place within the previous 15 seconds unless MM (Mobility Management) requests a cell reselection. The MS-MLE shall check the criterion for serving cell relinquishment as often as one neighbour cell is scanned or monitored.
When the FRT threshold is breached, the MS is in a situation where it is essential to relinquish (or abandon) the serving cell and obtain another of at least Usable quality. That is to say, the mobile station is aware that the radio signal is decaying rapidly, and must cell reselect rapidly, before communications are terminated because of radio link failure. When the mobile station radio-signal breaches the Rx Lev minimum the radio is no longer in a position to maintain acceptable communications for the user, and the radio link is broken.
Radio link failure: (C1 < 0). Using the suggested values, this would be satisfied with the Serving Cell Level below -105 dBm. Cell reselection procedures are then activated in order to find a suitable radio base station.
|×Type of Radio Cover||Parametre||Distance in km''||Type of Communication|
|City||< 4||< 8||Pedestrian / Metro|
|Sub-Urban||10 to 18||20 to 36||Bus / Train|
|Countryside||18 to 31||36 to 62||Inter Regional Train|
|In Air||> 32||> 64||In Flight|
Any given TETRA radio terminal using Java (J2ME /CLDC) based technology, provides the end user with the communication rights necessary to fulfill his or her work roll on any short duration assignment.
For Dexterity, flexibility, and evolution ability, the RATP radio engineering department, have chosen to use the open sources, Java language specification administered by Sun and the associated work groups in order to produce a transport application tool kit.
Service acquisition admits different authorised agents to establish communication channels between different services by calling the service identity, and without possessing the complete knowledge of the ISSI, GSSI, or any other TETRA related communication establishment numbering plan. Service acquisition is administered through a communication rights centralised service or roll allocation server, interfaced into the TETRA core network.
In summary, the RATP TETRA MMI aims are to:
This transport application tool-kit has been produced successfully and with TETRA communication technology and assures for the RATP transport application requirements for the future mentioned hereafter.
The Home (Main) menu presents the end user with three possibilities:
Service Acquisition provides a means of virtually personalising the end user to any given radio terminal and onto TETRA network for the duration the end user conserves the terminal under his / her possession.
Status SDS provides the end user with a mechanism for generating a 440Hz repeating tone that signals a fraud occurrence to colleagues who are currently within the same (dynamic or static) Group Short Subscriber Identity GSSI or to a specific Individual Short Subscriber Identity, ISSI for the duration of the assignment (an hour, a morning patrol or a given short period allocated to the assignment). The advantage being that each of the end users may attach themselves to any given terminal, and group for short durations without requiring any major reconfiguration by means of radio software programming tools. Similarly, the aggression feature functions, but with a higher tone frequency (880Hz), and with a quicker repetitious nature, so to highlight the urgency of the alert.
The Parameters tab provides an essential means to the terminal end-user allowing them to pre-configure the target (preprogrammed ISSI or GSSI ) destination communication number. With this pre-programmed destination number, the end-user shall liaise with the destination radio terminal or Roll Allocation Server, and may communicate, in the group, or into a dedicated server to which the service acquisition requests are received, preprocessed, and ultimately dispatched though the RATP TETRA core network. This simplifies the reconfiguration or recycling configuration process allowing flexibility on short assignments.
The Parameters tab also provides a means of selecting pre selected tones to match the work group requirements for the purposes of Fraud and Aggression Alerts. A possibility of selecting any given key available from the keypad to serve as an aggression or fraud quick key is also made possible though the transport application software tool kit. The RATP recommend using the Asterisk and the Hash keys for the Fraud and Aggression quick keys respectively. For the fraud and aggression tones the RATP also recommend using 440Hz slow repeating tone (blank space 500 milli-seconds) and 880Hz fast repeating tone (blank space 250 milli seconds) respectively.
The tone options are as follows: 440 Hz 620 Hz, 880Hz, 1060Hz.
The Parameters page on yet another tab provides an Aid or Help menu and finally, the last tab within parameters describes briefly the tool kit the version and the history of the transport application tool kit to date.
|Manifest-Version 1.0||Operational TETRA MIDlet|
|Transport Operation Software|
|MIDlet Name||RATP TETRA|
for similar purposes.