TETRA is a mobile radio technology that is being used for a proposed new police communications system, offered throughout the UK by the Home Office. The system is called Airwave, and in England and Wales the network of masts is being installed by a part of mmO2 plc, known as O2 Airwave. TETRA, like mobile phone systems, uses masts (or ‘base stations’) and handsets that look very like a chunky mobile phone. The masts usually have three poles, and each pole usually has four flattened loops. Some antennae are straight. TETRA offers the user fairly secure communications, that, like all other digital radio technologies, delivers clear speech. It allows users with the radios the ability to talk in a group, and to talk to other users with similar digital systems elsewhere. It has a ‘help’ button that enables the user to call for assistance very quickly. Other facilities, such as looking up police computer information, whilst said to be possible in the future, will not be delivered as promised. TETRA is a microwave system, like ordinary mobile phones, but the masts ‘talk’ to each other directly. Unlike mobile phone masts, they transmit constantly, 24 hours a day. TETRA messages are sent in pulses or bursts that are transmitted at a frequency coinciding with the electrical frequency of the human brain in its waking state. In a report to the Government, this was declared unsafe, and to be avoided. There are many reasons why it is unsafe, but this message was ignored when the Home Office decided only to consider TETRA systems
cost effective mobile communications for private networks many specialised facilities not available in public mobile networks usually stand-alone networks advanced systems allow interconnection with fixed network (voice/data) use of VHF/UHF allow good coverage with small antennas Target users police forces fire brigades emergency health military forces transportation fleets: goods, taxi, buses utilities: electricity, gas, water
Basic mobile radio systems simple two-way radio without base stations users within range of each other fixed frequency assignment (sometimes manually selectable) usually no privacy Advanced mobile radio systems cellular network architecture - trunked systems secure communications efficient use of frequency spectrum possibility of shared resources - PAMR: Public Access Mobile Radio examples of technologies: TETRA / TETRAPOL
Reliability high service availability minimum service still possible in case of large breakdown Centralised / Decentralised operation central dispatch point to organise users possibility of direct mode operation (without central control or any infrastructure al all) Various types of call point-to-point (single user to single user) group (pre-defined number of users) broadcast (all users in the network; all users in a selected area)
Call set-up sub-second call set-up time PTT - Press To Talk operation usually no need for action to answer Security increased security possibility of end to end encryption Call priorities classes of users with higher priority urgent calls may be able to pre-empt other call types
Network architecture Network functions Establishing service Location registration Register/de-register Connection restoration Call re-establishment Security features Services supported at the defined system interfaces Services supported at the defined system gateways Trunking methods Message trunking Transmission trunking Quasi-transmission trunking
Introduction Static Rural Area (RAx Hilly Terrain (HTx) Typical Urban (TUx) and Bad Urban (BUx) Equalizer testing model (EQx) Sensitivity, interference rejection and nominal performance Definition of sensitivity Reference sensitivity levels and reference sensitivity performance Equivalent signal-to-noise ratio TETRA Interference rejection Nominal performance Sensitivity comparison of TETRA with analogue FM systems Comparison of co-channel interference rejection of TETRA with analogue FM Path loss model Rural Area Suburban area Link budget Signal variability Area coverage Noise limited Coverage for stationary operation Coverage for moving MSs Interference limited Quasi-synchronous operation Direct mode
OSI reference model of a TETRA system Testable boundaries Description of MS/BS air interface protocol architecture Mapping of higher layer data on to the physical layer Physical layer Physical resources TDMA frame structure Slot structure Radio transmission burst structure Slot structure formats within the physical layer Organization of layer 2 - Data Link Control (DLC) Logical channels in the lower MAC Logical channels in the upper MAC Use of the logical channels and mapping between layers Physical layer functionality of TETRA System Lower MAC functionality How it all hangs together Broadcast channel Signalling channel Traffic channels Logical channels supporting operation of the physical layer MAC functionality Random access procedures Reserved access procedures Call set-up procedures Individual calls Group calls Layer 3 functionality Cell selection and re-selection Establishing service Acquiring cell synchronization Acquiring network information Cell selection and re-selection in TETRA systems System modes of operation Normal mode Extended mode TETRA systems minimum mode Discontinuous downlink transmissions - time sharing mode MCCH sharing mode Carrier sharing mode MS modes of operation Idle mode Common signalling and packet mode Traffic mode Stealing mechanism Uplink stealing Criteria for uplink stealing Downlink stealing Reception of downlink transmissions Energy economy mode Independent allocation of uplink and downlinks Classes of MS MS power classes MS receiver classes MS duplex capability Frequency half duplex operation Frequency full duplex operation Support of air-interface encryption Support of concurrent calls Data services in Voice plus Data Short data service Circuit mode data Packet mode data X.25 connection mode packet data Connectionless mode Network interconnection
Area coverage techniques Single site wide area coverage Cellular channel re-use Quasi-synchronous transmission Time sharing transmission Antenna diversity Site diversity Direct Mode (DM) / trunked gateway On-frequency repeaters Comparison of some area coverage techniques Small active cells versus passive receive only cells Cellular versus quasi-synchronous Quasi-synchronous versus time-shared transmission DM / trunked gateway versus on-frequency repeaters Radio parameters Frequency allocation
Introduction Impact of unwanted emissions near the carrier Impact of unwanted emissions far from the carrier Military systems
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