GB2176079A - Direction-adaptive repeaters for guided radio systems - Google Patents

Abstract

A repeater for use in a two-way guided radio system such as a leaky-feeder system is basically a simple one-way device but adapts itself automatically to suit the direction of propagation of a signal appearing at either of its terminals 1 and 2 and remains in that steady directional mode for the duration of such signal. Its main application is for direct two-way communication between mobile radio sets on a single frequency. The presence of an incoming signal is detected by detectors 8, 9 and a logic circuit 10 selects the appropriate switches 4-7 such as PIN diodes so as to connect the amplifier 3 for passing the detected signal. In a further embodiment (Figure 2) two oppositely connected amplifiers (11 and 12) are alternately switched to be operative by an oscillator (16) which also controls sampling of an incoming signal by means of a switch (14). When the presence of an incoming signal is detected the amplifier for passing that signal remains operative. <IMAGE>

Description

SPECIFICATION Direction-adaptive repeater for guided radio systems Radio waves do not propagate effectively through mines, tunnels and similar environments and so communication by such means is generally not possible using normal techniques and equipment.

This is well known and a common method of overcoming the difficulty is to install through the mine or tunnel a special type of radio-frequency transmission line known as a 'leaky feeder' or sometimes a 'radiating cable'. In its function as a transmission line it serves to guide or artificially propagate radio signals over useful distances without serious loss. At the same time the leakage fields which result from a deliberate imperfection in the screening or balance of the line enable signals to be coupled between the line and any mobile radio set in its vicinity.

In this way communication between two mobile radio sets may be achieved over useful distances in a tunnel by simply installing such a leaky feeder through the tunnel. Radio signals from the first radio set induce corresponding radio-frequency currents into the leaky feeder; these then propagate in the transmission-line mode of the feeder to the vicinity of any further similar mobile radio set, where their accompanying external leakage fields couple signals into such set; the communication path is reciprocal, so that signals transmitted by the second set can similarly be received by the first.

Generally one of the radio sets is connected galvanically to the leaky feeder as a radio 'base station', thus eliminating one of the coupling processes and allowing a greater range to be achieved. If, then, communication is still desired between two equally mobile sets the signals in each direction between such sets can be arranged to be relayed instantaneously by a fixed base station connected galvanically to the line, so allowing the improved range to be achieved in such communication between mobile sets.In such a case, the mobile sets must all be arranged to transmit on a first specified frequency but to receive on a second specified frequency; the fixed base station is arranged to receive on the first frequency and to transmit on the second frequency, thus avoiding the serious problems and complications that would arise if the base station were to attempt to receive and transmit simultaneously on the same frequency.

The range of such a system may be further improved, virtually to any required extent, by introducing amplifiers or 'repeaters' into the leaky feeder to compensate periodically for the line losses suffered by the signals propagating in the feeder. If the fixed base station is to be used to relay transmissions between mobile radio sets then such repeaters need to be able to amplify signals travelling through them in both directions simulta neously. Again, the use of different frequencies for reception and transmission by the base station al lows this to be achieved since the signals in the two directions may thereby be distinguished and separated. However, such simultaneously-two-way repeaters are complicated and expensive, and several arrangements that avoid their use have been described in the literature and widely adopted.

The present invention does not make use of simultaneous relaying by a fixed base station, but reverts instead to the arrangement of direct transmission between mobile radio sets involving two coupling processes, into and out of the line, and a single frequency of operation for both directions of communication. The additional losses associated with such simpler arrangement are partially compensated by introducing repeaters into the line in similar manner to the arrangement employing a fixed base station.

The difficulty that has hitherto prevented the use of repeaters in systems without fixed base stations in this manner is that the repeaters need to be capable of amplifying signals in both directions on the one frequency, albeit not in this case simultaneously. The feature that allows this in the arrangement to be described involves snesing circuits which detect the presence of a signal to be amplified and determine the direction from which it approaches the repeater and thus the direction in which the repeater is to operate for the duration of that particular transmission.

The operation of a repeater following this broad principle may be understood by reference to Figure 1. In this diagram, 1 is the connection to the leakyfeeder line in the one direction and 2 is the connection in the other, and 3 is the main signal- amplifying element of the repeater. The switches shown at 4, 5, 6 and 7 enable the element to amplify in the one direction or the other: if switches 4 and 5 are closed, the repeater will amplify signals entering at port 1 and leaving at port 2; if switches 6 and 7 are closed alternatively the repeater will amplify signals entering at port 2 and leaving at port 1.

Also connected to the two ports 1 and 2 are respective detector circuits 8 and 9 which detect the presence of incoming signals at those ports and indicate suitably to the logic circuit shown at 10. This determines the direction from which the signal is approaching and controls the switches 4, 5, 6 and 7 accordingly so as to provide the correct polarity of connection for the amplifying element 3. In the absence of any incoming signal all switches 4, 5, 6 and 7 will remain open; once an incoming signal has been detected the appropriate switches will close and remain in that condition until the signal ceases, whereupon they will open again.

Although the switches shown at 4 - 7 are indicated as mechanical contacts, it would be possible and normally preferable to employ solid-state switches such as PIN diodes. Further, the actual switching arrangements may be simplified by duplicating the amplifying element 3; the two counterparts of this are permanently connected back-toback, that is in parallel but in opposite directions, and activated alternatively by connection of the power supply to one or the other.

The preferred and more detailed arrangement shownin figure 2 incorporates this and further im provements. Here, the elements 11 and 12 are the two counterparts of the main amplifying element, either of which may be activated through connection of the appropriate DC supply or control voltage as shown at DC-A and DC-B. At 13 is a subsidiary radio-frequency amplifier which is connected alternately through switch 14 to the two ports of the repeater, 1 and 2. The tuned circuit or filter 15 ensures that only those signals in the frequency band of interest are passed to the amplifier 13.

In the absence of any signal being detected the switch 14 connects alternately to the input ports 1 and 2 at a rate of at least 10 alternations per second under the control of the oscillator 16 (again, Ithe switch 14 will preferably be of solid-state form). At the same time, the DC supply is automatically switched between amplifier 11 and amplifier 12 in sympathy, and the alternations are so related that the amplifier 13 is connected to the output of the energized amplifier element 11 or 12 at any time rather than to the input. Thus, if a signal should appear at either port 1 or 2 it will be amplified by the respective amplifying element 11 or 12 before being applied through switch 14 to the amplifier 13. Any radio-frequency signal amplified by the amplifier 13 is then demodulated in a demodulating circuit denoted schematically at 17.

If, then, a continuous radio-frequency signal within the frequency range of interest is presented at port 1 then that signal will be amplified by amplifier 11 during that part of the time-cylce that amplifier 11 is energized and appear in amplified form at port 2; from there it will be passed out to the leaky feeder but will also be passed through switch 14 and filter 15 to the radio-frequency amplifier 13 and thence be demodulated at 17. If as will normally be the case during a radio conversation in simplex mode there is no similar signal being received from the leaky feeder at port 2 at that same time then there will correspondingly be no radiofrequency signal passed by switch 14 and filter 15 to amplifier 13 during the part of the time-cycle that amplifier 12 is energized.In these circumstances the output signal from the demodulating circuit 17 will take the form of a square wave having a periodicity equal to that of the switching time-cycle as governed by the oscillator 16. Similarly, if a continuous incoming signal should be present at port 2 but not at port 1 then a corresponding square wave will be derived at 17 but in this case in opposite phase with respect to the oscillator 16.

Any such square wave derived at 17 is amplified by amplifier 18 and passed to a phase-sensitive detector 19 in addition to a steady DC bias voltage.

Also passed to the phase-sensitive detector 19 is a reference signal from the oscillator 16. Two output signals from the phase-sensitive detector 19 in mutual antiphase are applied respectively to the amplifiers 11 and 12 at DC-A and DC-B as the supply or control voltages causing one or the other of them to be energized at any one time.

It will be clear to those skilled in the art that with the appropriate senses of signal phasing and polarity set at the phase-sensitive detector 19 the arrangement depicted in Figure 2will thus provide the necessary operation that in the-absence of any signal being received either at port 1 or port 2 the corresponding output voltages from the phase-sensitive detector 19 will cause the two amplifiers 10 and 11 to be energized alternately at the rate determined by the frequency of the oscillator 16.

If, now, an incoming signal should be presented by the leaky feeder at either port 1 or port 2, causing on subsequent demodulation and amplification a square-wave signal sufficient in amplitude to over-ride the steady DC bias applied to the phasesensitive detector 19, then the supply or control voltages DC-A and DC-B will no longer alternate in sympathy with the oscillator time-cycle but will latch in a condition that steadily energizes the appropriate amplifier 11 or 12 for the duration of the incoming signal.

It will be understood that during such steady energization of one or other of amplifiers 11 and 12 the switch 14 continues to alternate in sympa -thy with the oscillator time-cycle so as to maintain the square-wave signal to the phase-sensitive detector 19 and thus in turn to maintain the steady energization.

When the incoming signal ceases the squarewave signal applied to 19 also ceases and the arrangement reverts to the condition where the two amplifiers 11 and 12 are energized alternately in readiness for the detection of a further incoming signal from either direction.

If at any time there should appear signals from the leaky feeder at the two ports 1 and 2 simultaneously, either because two communicating parties are inadvertently attempting to transmit simultaneously or because one communicating party is so located with respect to the repeater as to induce signals into the leaky feeder on both sides of the repeater, then the repeater will respond to the direction presenting the strongest signal providing that the excess is adequate to overcome the steady DC bias applied to the phase-sensitive detector 19.

If two such signals are substantially equal in amplitude the repeater will ignoree both and continue to alternate; if such a situation is caused by the near proximity of one of the calling parties then the induced signal into the feeder will normally be adequate without amplification at that repeater to reach and activate the next repeater in each direction.

It will be understood that in a typical system a plurality of repeaters such as has been described may be used and spaced at suitable intervals throughout a length of leaky feeder to compensate for line losses as they arise. Successive repeaters encountered by a signal induced into the leaky feeder will sense the signal and adapt their operating modes in turn. Providing each repeater employs a frequency of alternation in its sensing operation at least as high as that suggested a typical chain of such repeaters will adopt their correct operating modes within a small fraction of a second and there will be no risk of iosing the start of any transmitted message. Although all such repea ters in a system will respond and adapt to a signal, not every repeater will necessarily be involved in a particular transmission path between communicating mobile stations.

Notwithstanding that the arrangement has been described here specifically in relation to direct communication between two or mobile stations, and will generally be most beneficial in that application, it is to be understood that a base station may if desired be connected galavanically to the leaky feeder and participate in the same communication facility with any of the mobile stations as between the mobile stations themselves, and such communication will benefit from the elimination of one of the two coupling losses involved in communication between mobile stations.

Notwithstanding also that the arrangement as has been here described in detail and as an example applies specifically where the guidance medium of the radio signals is a leaky feeder, it is to be understood that the invention may be applied equally to any case where the radio signals are guided in a linear sense such as by a tunnel or mine roadway in which the communication may be taking place or by some other form of metallic conducting means other than a leaky feeder.

As a further example, it is well known that radio waves within in a certain frequency range will propagate well within a tunnel or mine roadway not containing any suitable guiding conductors but only in so far as that tunnel or mine roadway is straight and unobstructed; bends, corners and obstructions may introduce serious impediment to the propagation. In such circumstances the invention described may be applied in such a manner that the two ports 1 and 2 of the repeater are connected not to a leaky feeder but to directive antennas placed on either side of the impediment to propagation. It is also possible to apply the invention to such cases of guidance by tunnel or mine roadway in an unimpeded section of the radio path where the signal is nevertheless decaying below a satisfactory level through the natural and gradual attenuation characteristics of the tunnel or roadway, provided that in such case precautions are taken such as by the use of more highly directive antennas to preserve the stability of the repeater.

Claims (7)

1. An amplifying repeater forming part of a two-way guided radio system and capable of amplifying signals passisng in either direction but not in both directions simultaneously, being arranged to sense the presence of a signal passing in either direction through the system and to adapt itself to amplify signals passing in that direction for the duration of such signal.

2. A repeater as claimed in Claim 1 adapted specifically for use in systems where the guiding medium is a leaky feeder otherwise known as a radiating cable.

3. A repeater as claimed in Claim 1 adapted specifically for use in systems where the guiding medium is a tunnel or mine roadway.

4. A repeater as claimed in any previous claim in which the repeater contains separate amplifying elements for the two directions of transmission and activates either but not both simultaneously for the purpose of amplifying signals in the required direction.

5. A repeater as claimed in any previous claim in which the sensing circuit element is common to the two directions and at a suitable rate alternately senses for the presence of signals passing through the transmission medium in the two directions.

6. A repeater as claimed in Claim 5 in which during the absence of a signal being sensed in either direction activates the repeater to amplify alternately in the two directions at a suitable rate, the sensing operation being effected on any resulting output signal from the amplifying element so activated.

7. A repeater as claimed in Claim 6 in which the demodulated output signal from the sensing element is applied to a phase-sensitive detector together with a phase-reference signal from an oscillator controlling the alternation of the sensing operation, the output voltage from such phase-sensitive detector serving to control the direction in which the repeater is activated to operate at any one instant of time.