GB2278761A - Leaky feeder communication system - Google Patents

Abstract

A leaky feeder communication system has a leaky feeder cable network 6 made up of at least three spaced apart cable portions 2, 3, 4, 5 which define a plurality of loops. The cable portions are coupled together at their ends by way of two splitter/combiners SC, with at least one base station A, B coupled to a respective splitter/combiner. The base stations A, B are arranged to be in RF communication with stand alone transceiver means TR in proximity to the network via the network. A further cable arrangement is described (Fig 3). Uses of the system are off-shore installations, ships and buildings. <IMAGE>

Description

COMMUNICATION SYSTEM The present invention relates to a radio frequency (RF) communications system using leaky feeder technology suitable for use in areas with difficult RF reception such as off-shore installations, ships, buildings and the like having enclosed or confined areas to permit radio communication therein.

In leaky feeder communications systems, leaky feeder cables are used to carry radio signals around enclosed areas where it would otherwise be difficult or impossible to propagate RF signals, for instance inside the steel walled modules of an oil rig. Conventional leaky feeder cable systems allow communication between a number of wireless portable transceivers and a base station transceiver and between portable transceivers.

A signal transmitted from one transceiver leaks into a portion of the leaky feeder cable network in the vicinity of that particular transceiver and is carried along the cable to a remote base station. The base station amplifies the signal and re-transmits it, usually on a different frequency, along the leaky feeder cable network. The re-transmission then leaks out of the cable to be received by other transceivers in the vicinity of other parts of the cable network. In order to maintain contact between all transceivers, irrespective of their position within the area to be covered by the communications system, the leaky feeder cable must pass through every part of that area.

Conventional design practice uses a tree-network of cabling radiating out from a base station. The signals pass through splitter/combiners at each node of the tree-network eventually terminating in loads at the ends of the network which absorb any remaining power arriving thereat. Alternatively the cable ends may be left unterminated or antennas may be added.

It will be understood that even one break in the tree-network, for instance due to fire or explosion on an oil-rig, will cut communications to all other parts of the network below the break with potentially catastrophic results for instance in emergency evacuation procedures on an oil rig installation.

There is moreover always the risk that the base station or some other part of the redundant system might also be damaged resulting in complete loss of communications with an affected area.

In order to avoid the problems of such a break in the cable network or of a fault in the base station, separate overlapping redundant or duplicate systems are employed so that if one system is damaged the other, hopefully undamaged, system may still be usable. For example, Figure 1 shows a layout employing two separate, simultaneously used (on different channels), overlapping tree-network systems, A and B, wherein one system (B-shown in long dotted line) is redundant (for emergency use), in use in an installation having seven separate zones and including one fully enclosed sub-zone within the communications area to allow communication therebetween. It will be appreciated that installing such a second redundant system, particularly in off-shore installations can be extremely expensive often as much as twice the cost of installing a single system, due to the large amount of cabling required and the high cost of installation.

It is an object of the present invention to avoid or minimise one or more of the foregoing disadvantages.

According to a first aspect of the present invention there is provided a leaky feeder communication system for transmitting and receiving RF signals to and from a stand alone RF transceiver means in proximity to a part of the system, the system comprising: at least two splitter/combiners; a leaky feeder waveguide network operable at radio frequencies, the network comprising at least three spaced apart paths each of which is coupled at a first end to a first of the splitter/combiners and at a second end to a second of the splitter/combiners so that the network can provide at least two closed loops in an area to be provided with communications; and at least one base station coupled to the waveguide network via one of said splitter/combiners for RF communication with the network, wherein RF signals can travel in either direction around said loops between the or each base station and any point on the waveguide network.

Thus with a leaky feeder communications system embodying the present invention, communication either between a base station and a stand alone RF transceiver means or between individual stand alone RF transceivers means may be maintained even with a break in the leaky feeder network or in the event of damage to or a fault in the base station where two or more base stations are provided. Moreover a leaky feeder communications system may be installed in a relatively simple and economic manner.

Preferably the or each base station is provided with a plurality of channels each formed and arranged for transmitting and receiving RF signals on different frequencies for selectably communicating with different ones of a plurality of stand alone RF transceiver means.

In a preferred embodiment of the invention the leaky feeder network comprises a cable network for laying in the area to be provided with communication. The system may be provided with at least two, and preferably two, base stations coupled to the network via respective splitter/combiners with said at least three paths extending between these two splitter/combiners.

Thus the system provides a multiplicity of alternative routes in different directions around said loops or paths of said network between a given part of the network and the or each base station.

It will be understood that on the one hand the power and sensitivity of the base station(s) and RF transceiver means and on the other hand the signal losses in the network (due to inter alia attenuation by the cable and any splitter/combiners used in the cable network and the output isolation or secondary port isolation thereof, used to interconnect loops or paths and the coupling factor along the longest route with the most splitter/combiners) will generally need to be matched to each other to a greater or lesser extent in order to ensure transmission of signals of a strength sufficient to be detected at each end of substantially any given communications link within the area to be covered by the system, with blind spots or reception nulls reduced to a minimum. Thus for example if there is used in the network cable with high signal attenuation, then, especially in the case of longer loops, it will be necessary to use larger cables which have lower losses for a system with base stations and transceivers of a given power output and sensitivity.

Preferably a safety margin is included over and above the minimum sufficient power and sensitivity levels of said base stations and said RF transceiver means.

Conveniently sub-loops may be incorporated in parts of said loops where it is desired locally to extend the area covered by the communications systems.

Alternatively or in addition, spurs may be connected to the loops to extend locally the area covered. Such spurs may moreover extend in a tree like structure to cover other sub-zones to be provided with communications though, as indicated above, the use of tree-structures has some disadvantages and is less preferred.

Alternatively the loops may be interconnected by cross links or chords to cover additional zones.

According to a second aspect of the present invention there is provided a leaky feeder communication method of transmitting and receiving RF signals to and from a stand alone RF transceiver means in proximity to a leaky feeder waveguide network, the method comprising: laying at least three spaced apart waveguide paths in an area to be provided with communications; coupling each of said paths at a first end to a first splitter/combiner and at a second end to a second splitter/combiner to provide at least two closed loops; and coupling at least one base station to the network via a splitter/combiner, wherein RF signals can travel in either direction around said loops between the or each base station and any point on the waveguide network.

According to a further aspect of the present invention there is provided a leaky feeder communications system comprising at least two spaced apart base stations for transmitting and receiving RF signals to and from a common leaky feeder cable network connected thereto for communication with stand alone RF transceiver means, in proximity to part of said cable network, wherein in use of the system said cable network defines at least one closed loop extending at least partly through a zone to be provided with communications by said system so as to provide two substantially spaced apart alternative routes in different directions around said loop between a said part of the network and each of said at least two base stations, said base stations and said RF transceiver means having sufficient power output and reception sensitivity to allow communication therewith at substantially all parts of said loop by both said alternative routes.

For a better understanding of the present invention and in order to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which; Fig. 1 is a block diagram of a conventional leaky feeder system; Fig. 2 is a schematic diagram of a leaky feeder communications system according to a first embodiment of the invention; and Fig. 3 is a block diagram illustrating use of a system according to a second embodiment of the invention in an installation having a communications area with seven separate zones and a fully enclosed sub zone.

As shown in Fig. 2, a leaky feeder communications system, generally indicated by reference number 1, comprises two base stations transceivers A, B, which operate both as transmitters and receivers, interconnected by a leaky feeder coaxial cable 6 network 7 defining two loops made up of first and second, and third and fourth cable portions 2, 3 and 4, 5 respectively. (It will of course be appreciated that the cable network 7 shown also defines other loops made up of different combinations of these cable portions e.g. 2, 4 and 3, 5; and 2, 5 and 3, 4. It will accordingly be understood that the present invention is not restricted to the use of any discretely formed and identifiable loops as such, but only requires a network defining alternative routes in different directions around each one of one or more loop circuits).

In use of the system, a signal Si from a portable transceiver TR, again operating both as transmitter and receiver, leaks into part of the cable 6 proximal the transceiver TR and may pass around any or all of the routes defined by the two network loops of cable to either base station A, B. Splitter/combiners SC are provided at nodes 8, 9 where the cable portions 2-5 are coupled to short base station link cable portions 10, 11.

The design of any leaky feeder system as shown in Fig. 1 or Fig. 3 requires that sufficient RF power passes both from the base station to the transceiver, and from the portable transceiver to the base station and thereby between separate portable transceivers for intelligible communications. A calculation is performed with the base station transmitting and the hand-portable receiving and then vice versa. Both calculations have the same form and only if both give a positive result (adequate received power) will the system work.

Under normal conditions (i.e. with no cable breaks) the configuration shown in Fig. 1 must conform to the following mathematical criterion: P-[A x (L) ]-[(S) )-CF-R-SM > 0 wherein P is the output, transmitted power of the base station transceiver/portable transceiver (in dBm).

A is the attenuation per unit length (in dB/m) of the cables used to carry the signal to the transceiver/base station. Individual cable spur lengths may be denoted by L meters and (L) is the sum along a route.

S is the attenuation (in dB) of each splitter/combiner used to join collections of three or more cable portions or paths at nodes along the route to the transceiver/base station.

CF is the coupling factor (in dB) which describes the leakiness of the cable. This is the ratio of the power level passing through any point on the cable and the power that would be received by a standard antenna at a specified, perpendicular distance away (standard distance). In a reciprocal fashion, this is also equal to the ratio of the power transmitted by a similarly sited, standard antenna and the power induced within the cable.

The distance (in meters), that the transceiver may stray from the cable may be commonly referred to as d. In many cases, the size of the zones served by the leaky feeder cable is similar to the standard distance and so d can be ignored. The tendency of metal enclosures to confine the radiated signals also tends to reduce the significance of d. For simplicity therefore d is not considered in the above formula.

R is the sensitivity (in dBm) of the receiver of the base station/transceiver. This is the lowest received power level which can be converted into intelligible speech by base station/transceiver.

SM is the safety margin (in dB), a safety factor added to the calculation to allow for component tolerances and uncertainties in the parameters used.

Any additional attenuating components would be accounted for in a similar way.

The sumst(L) andf.(S) denote the sum of all the cable spur lengths along the most direct route between the transceiver and the base station and the sum of all the splitter/combiner attenuations (en route) respectively.

To avoid having to perform the same calculation for many different transceiver positions, the worst case position (i.e. that with the highest value of A xS (L)+i(S)) is usually deduced and the calculations carried out for this case alone.

It will be appreciated that the embodiment of the invention shown in Fig. 2 and 3 will also have to conform to the foregoing criteria. It will be, however, understood that as a plurality of different routes are available along which a signal may pass, some additional factors will have to be taken into account.

With reference to Fig. 2, if a break occurs in the cable at point P then no signal from the base stations, A, B can arrive at the far side of point P (remote from the respective base station) by the shortest route. Instead the signal will take all of the alternative routes available around the other sides" of the various loops constituted by the second, third and fourth cable portions 3, 4, 5. The power of the signal must therefore be sufficient to pass along each of these longer routes. Moreover if the signal has to pass 2 or more splitter/combiners SC, it will encounter further attenuation in the form of the secondary port isolation, ISO (in dB), of the splitter/combiner SC (i.e. the attentuation (in dB) of signals crossing between any two secondary ports of a splitter/combiner).

Thus a communications system will have to have sufficient power to overcome all these losses and attenuation. This is given by the equation P-[A x (L) )-[(S) )-ISO-CF-R-SM > 0 where (L) is the sum around a loop.

It will be appreciated that the power of the signal received by the base station/transceiver will be the sum of the signals crossing over from all the undamaged (intact) routes on the various loops and that as each signal will have a given magnitude and phase, the final signal received will be the vector sum of these individual signals. In practice the relative phases of each signal are not readily controllable. Thus it is possible that the vector sum of two out of phase signals could be zero. In order to reduce the probability of this it will be appreciated that the larger the number of "loops" (also corresponding to a larger number of possible intact routes there still being available) the greater the probability of constructive addition of the vectors of the individual signals being greater than zero.

Furthermore it will be appreciated that it will generally be desirable to provide for sufficient power for communications; in the worst case scenario that is a break in the cable in an area relatively close to the area in which the base station is situated on the longest path cable between the base stations. It will be appreciated though that in practice the cost of providing systems which are capable of communication in the event of a worst possible case scenario may be prohibitive and not commercially justifiable and it should accordingly be understood that the present invention includes within its scope systems which maintain communications in most or substantially all cases but not necessarily the worst possible scenarios.

It will be appreciated that various modifications may be made to the above described embodiments without departing from the scope of the present invention. Thus for example local or sub loops 10 or spurs 12 could be connected to or across main loops 4, 6 (see Fig. 3) to extend coverage to sub zones 14 of the seven main zones 8.

Other modifications may include the use of base stations which are remotely controllable by way of the leaky feeder network whereby it is possible to switch one off remotely or switch the channels on which it is operating remotely, from the other base station, particularly in the case of an emergency where one base station cannot be manned. The provision of multiple connections between base stations allows these same cables to be utilised for remote control with high resistance to damage.

In environments where RF fields pose a potential explosion hazard, the maximum power level allowed within each unprotected cable may be set by safety considerations. By incorporating the splitter/combiners SC inside a flameproof enclosure, preferably together with the base station and other electrical apparatus, the power level in those cables exposed to risk may be minimised, due to the division of energy between the cables.

While the maximum damage resistance of the system may only be achieved while all the two or more stations are present, it should be noted that the remaining system will continue to operate if one or more base stations are detached from the system, either permanently or temporarily for maintenance. Ideally, a coaxial short circuiting device should be attached in place of the missing base station to retain all useful signal energy within the remaining system as far as practicable.

Claims (13)

1. A leaky feeder communication system for transmitting and receiving RF signals to and from a stand alone RF transceiver means in proximity to a part of the system, the system comprising: at least two splitter/combiners; a leaky feeder waveguide network operable at radio frequencies, the network comprising at least three spaced apart paths each of which is coupled at a first end to a first of the splitter/combiners and at a second end to a second of the splitter/combiners so that the network can provide at least two closed loops in an area to be provided with communications; and at least one base station coupled to the waveguide network via one of said splitter/combiners for RF communication with the network, wherein RF signals can travel in either direction around said loops between the or each base station and any point on the waveguide network.

2. A system according to claim 1, wherein the or at least one said base station is coupled to the network via said first splitter/combiner.

3. A system according to claim 1 or 2 and comprising at least two base stations coupled to the network via respective ones of the splitter/combiners.

4. A system according to claim 3 when appended to claim 2, wherein a second base station is coupled to the network via said second splitter/combiner.

5. A system according to any one of the preceding claims, wherein the or each base station comprises means for receiving signals from a stand alone RF transceiver means via the network, means for amplifying received signals, and means for retransmitting the amplified signals back onto the network.

6. A system according to any one of the preceding claims, wherein the transmission power and detection sensitivity of the or at least one of the base stations are sufficient to enable communication via that base station and all points on the network via at least two different paths.

7. A system according to claim 6, wherein the or at least one said base station can communicate with all points on the network via all possible paths.

8. A system according to any one of the preceding claims, wherein the leaky feeder network is provided by a leaky feeder cable network.

9. A system according to any one of the preceding claims which includes at least one sub-loop, coupled to the network at intermediate points of said paths for extending the communication coverage of the system.

10. A system according to any one of the preceding claims and comprising a spur coupled to the network at an intermediate point of one of said paths for extending the communication coverage of the system.

11. A leaky feeder communication system substantially as hereinbefore described with reference to Figure 2 or Figure 3 of the accompanying drawings.

12. A leaky feeder communication method of transmitting and receiving RF signals to and from a stand alone RF transceiver means in proximity to a leaky feeder waveguide network, the method comprising: laying at least three spaced apart waveguide paths in an area to be provided with communications; coupling each of said paths at a first end to a first splitter/combiner and at a second end to a second splitter/combiner to provide at least two closed loops; and coupling at least one base station to the network via a splitter/combiner, wherein RF signals can travel in either direction around said loops between the or each base station and any point on the waveguide network.

13. A leaky feeder communication method substantially as hereinbefore described with reference to Figure 2 or Figure 3 of the accompanying drawings.