COMMUNICATIONS SYSTEMS

This invention relates to communications systems, and in particular systems for communication between a fixed installation and a mobile one, e.g. on a vehicle.

In recent decades numerous communications systems have been developed and many different ones commercially used. A distinction can be drawn between communications systems adapted to effect communication between two fixed points, and systems where one or both of a transmitter and receiver are mobile. In the former type, substantial investment in the infra-structure, for example a telephone line network, may be necessary, but once that infrastructure is installed, communication is direct from one party to another and there is no problem with "overhearing". In terms of the amount of communication which can be effected, this depends on the means used to connect the two points. Conventional twisted pair copper cable does not allow very much information to be sent along it, though advances in communications technology are improving the quantities of information that can be sent. Coaxial cable allows better communication, for example of television quality signals with their

associated bandwidth, while optical fibre communication offers even greater communications possibilities.

In contrast to such "hard connected" systems, communications systems where one or both of transmitter and receiver are mobile have the major disadvantage that the signal is distributed. This is not a problem where transmission is in one direction only e.g. a television broadcast, but in terms of mobile communications systems it requires that, for example, mobile telephone systems operate with computer controlled signalling systems of extreme complexity to avoid one signal interfering with another e.g. from a different telephone conversation, and even today communication can be lost without too much difficulty, particularly if one of the parties doing the communication e.g. is doing so from a car which drives into a tunnel or goes behind a large building.

Theoretically, optical communication should enable relative security of communication to be achieved so long as a "line of sight" is present between transmitter and receiver, and where the receiver is arranged to "look at" the transmitter. Such line of sight systems have been used in various applications in the past. Thus for example European published patent specification number 0034859A discloses a communication system between a computer controlled load moving vehicle and a plurality of transmitters e.g. set in the floor of a warehouse. When the load moving vehicle passes into the range of vision of one of the transmitters, information can be transmitted to it. Such a discontinuous transmission method is of limited usefulness.

According to the present invention there is provided a

communications system for continuous transmission of information between a fixed installation and a moving vehicle or the like, wherein the fixed installation has a plurality of transmitter units each adapted to transmit information by modulated light radiation, the ranges of vision of each transmitter unit being arranged to overlap and to include within the combined range part of the path or intended path of a vehicle equipped with a receiver for the signal emitted by the transmitter.

Such a system enables continuous communication to be maintained between a fixed station and a moving vehicle over an extended part of the path of the moving vehicle. It is of particular value in connection with vehicles whose path of movement is predetermined, for example trains, trams and buses which travel over fixed routes. Because the transmitter and receiver system operates at optical wavelengths i.e. on a line of sight, spacial separation may be obtained e.g. between two tracks or opposite sides of a road along which buses or trams pass.

This can be achieved because of the major advantage of optical radiation compared with e.g. microwave radiation. Microwaves spread out and permeate an entire area, and it is therefore very difficult to use more than one microwave signal. Complex computer control systems of the type required for operating mobile telephone systems which work with microwave signals are thus not needed when setting up systems in accordance with the present invention.

By modulated light radiation we mean light of wavelengths within the range from the far infra-red to the far ultra¬ violet, though the visible and ultra-violet end of the

range are less easy to exploit using current technology than the infra-red region. Infra-red transmitters including a suitable light source and modulation means are now cheaply available as articles of commerce. In particular, a beam of laser-generated infra-red light may be modulated with ease using one of a variety of modulation systems. Likewise infra-red sensitive receiver apparatus and demodulation apparatus is commercially available.

Particularly in the application of communication systems according to the present invention to transport undertakings, i.e. to enable communication between a public service vehicle such as a train, tram or bus and e.g. safety controllers, it is of substantial importance that the communication system is itself designed to the relevant safety standards. Thus in particular, it is highly desirable to build into the system a certain amount of redundancy, and in accordance with a specific feature of the present invention, each fixed transmitter unit consists of a plurality of transmitter modules adapted to transmit the same signal. This accordingly builds redundancy into the system. If one transmitter module fails, signal strength may drop but the overall security of the system is not compromised.

The invention is illustrated by way of example with reference to the application of a communication system between a subway train and a subway station. The system is indicated diagrammatically in the drawings which shows diagrammatically a plan view of a subway station.

Referring to the drawing, a subway station is generally indicated by the area enclosed by the dashed line 1. The

subway station has a generally eastbound track 2 and a generally westbound track 3, both curved as shown. Either side of the station are approach tunnels 5, 6 and exit tunnels 7, 8 through which subway trains respectively approach and leave the station. Passengers embark and disembark via a platform 10 between the tracks.

As can be seen, the platform 10 is curved. This means that the driver of the subway train has relatively limited vision of what is going on on the platform. However, it is customary on subway systems, for security reasons, to instal a number of television cameras viewing various portions of the platform and to feed the signals from the various cameras to a station control room.

In accordance with the present invention, several infra¬ red transmitter units 12 are installed in the approach tunnel, on the edge of the platform, and in the exit tunnel. These are supplied with appropriate signals to transmit pictures of the platform, to be received in the cab of the train. Thus, well before the driver can see the platform, he can look at a television screen in the cab to determine the amount of congestion and, clearly, if any incident were to take place the driver would have more advanced warning thereof. Likewise on leaving the station following the discharge of disembarking passages and taking on embarking ones, the driver may see in the cab the view down the side of the train, ensuring for example, that no one is trapped between doors or, for example, attempting to cling to the side of the train.

Such a system is particularly valuable in promoting one person operation of underground trains without unduly

compromising safety.

The transmitter units 12 located along the platform and in the tunnels are preferably ones with duplication of actual transmitter modules so as to ensure that a signal is received. Likewise the receiver mounted on the front of the train is preferably one containing a plurality of receiver modules so that if one of them becomes unserviceable for some reason, signals can still be received. The transmitter units can be fed with a signa to be transmitted via suitable cable, preferably, fairly heavy duty coaxial cable, which may simultaneously serve as a power supply lead for the transmitter units, e.g. a 12v supply.

If desired, the communications systems may include transmitter modules on the train and one or more receiver modules by the track adapted to receive data from the train, for example a unique train identifier signal or, e.g. voice communication from the driver.

In either case, the field of vision of the transmitter receiver combinations is such that a train on track 2 received signals only from transmitters relevant to track 2, while one on track 3 receives signals relevant only to its side of the platform. The general areas of coverage of the transmitter units are shown by the dashed lines 14 of it can be seen each transmitter's area overlaps that of the transmitters to either side. Thus, the use of a multi-head system, i.e. a plurality of spaced transmitters, provides coherent beam overlap along the required receiving path, while avoiding interference between the two sets of transmitters/receiver systems.

In a further development of the system illustrated in the drawings, the transmitters arranged in the proximity of the station may be programmed to transmit data to the train which is in turn then displayed to the passengers in the train. The display may be visual or, for example, a voice synthesizer may tell the passengers which station they are approaching, what other lines run through it if it is an inter-change station, and what local attractions such as stores, museums and places of interest are to be found within easy walking distance.