GB2476085A

GB2476085A - Line transmission repeater with equalizer, suitable for use in wireless communication system for tunnel

Info

Publication number: GB2476085A
Application number: GB0921672A
Authority: GB
Inventors: Chris Ward
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2009-12-10
Filing date: 2009-12-10
Publication date: 2011-06-15
Also published as: GB0921672D0

Abstract

A repeater apparatus (111, 112) for distribution of radio frequency signals in a tunnel (100) or confined environment is disclosed. The repeater apparatus comprises an input for receiving an input signal from a first section of a coaxial cable (110); an equaliser (200, 300) configured to compensate for loss of signal as a function of frequency; and an output for outputting the equalised signal to a second section of the cable (110). The repeater apparatus (111, 112) may include an equaliser control circuit for controlling the equaliser (200, 300) to set a level of amplification using at least one control signal, the control signal having been generated according to a measurement indicating signal attenuation. The control signal may be based on at least one pilot signal, and preferably on two pilot signals of different frequencies. The repeater is preferably associated with one or more antennas for transmitting or/and receiving wireless communication signals. The repeater may form part of a system in which a wireless RF signal is received from a mobile device 130, the signal is transmitted along a cable 120 with spaced repeaters, and is finally wirelessly transmitted to another mobile device.

Description

Communications Repeater The present invention relates to radio communications, and in particular, to radio communications within a confined environment such as a tunnel or inside a building.

There have been a number of problems and difficulties associated with providing radio communications within tunnels or other confined environments, particularly over a wide range of frequencies that cover safety critical links, cell phone and other commercial communications applications. For example, cell phones typically use a frequency band between 900 MHz and 2 GHz, and wi-fl uses frequencies of around 2.4 GHz. Some emergency services applications are in the region of 400 MHz.

One previously known approach is to provide a number of spaced apart repeater amplifiers within the tunnel, linked together by electrically conducting leaky feeder radiating cables. These cables have slots at regular intervals to allow radiation from the cable to escape, and to allow the cable to pick up radio signals from the environment.

However, this feeder cable arrangement has many disadvantages. A lot of attenuation occurs, and relatively high power signals must be used to overcome the cumulative cable attenuation. For example, transmitter power levels of several tens of watts, e.g. between 50W and 100W, are fairly typical. Due to the high power consumption of the repeater amplifiers, the potential fire risk can be unacceptable when in the proximity of safety-critical systems. Furthermore, this arrangement is expensive to install and maintain, as a separate feeder cable is required for each service, and the cables need to be regularly dusted to prevent performance deteriorating.

An alternative known system for radio communications in tunnels uses optical fibre distribution to local transceivers that radiate relatively high power signals from antennas spaced at moderate intervals along the tunnel. However, optical links are expensive, they are limited in dynamic range and the transceivers are generally limited to the bandwidth of one specific system. In such systems, the antennas radiate signals in a straight line, but do not cope well with significant bends in tunnels. The power consumption of the links and transceivers is relatively high, and as with leaky feeders, the potential fire risk can be unacceptable in certain circumstances.

These previously known systems are not well suited to long, winding tunnels, in view of their power requirements and costs. The present inventor has developed a low power solution that overcomes these significant problems of the prior art.

One aspect of the present invention provides an apparatus and corresponding method for distribution of radio frequency signals in a tunnel or confined environment. The apparatus has an input for receiving an input signal from a first section of a cable, an equaliser configured to compensate for loss of signal as a function of frequency; and an output for outputting the equalised signal to a second section of the cable. The equaliser may also provide broadband amplification. In one example, the equaliser may provide, in addition to the frequency compensation necessary, around 12dB of wide band gain.

Thus, the apparatus functions as a repeater, reducing distortion and attenuation affects to allow the signal to be propagated further along the cable.

Embodiments of the present invention also address the problem of needing to control the level of amplification very precisely to avoid the signal incrementally increasing or decreasing in amplitude as it passes through a large number of serially connected equalisers. In some implementations, there may be 10 to 15 or more serially connected equalisers, which may not be equally spaced apart. Furthermore, amplifiers can vary in gain according to their age and condition, and embodiments of the invention provide compensation for this.

Embodiments of the invention control the level of amplification by the use of a calibration signal to set an appropriate level of amplification in the system. The calibration signal may be a single frequency tone, or it may comprise multiple frequencies. It may be transmitted at relatively frequent time intervals, e.g. once every several minutes, to allow the amplification in each repeater to be corrected on a frequent basis. In some embodiments, the interval frequency may be between once an hour and times an hour. However, in other embodiments, the tone may be sent more frequently or less frequently than this range, and it may even be a continuous tone.

The calibration signal may comprise a single pilot tone. Alternatively, it may comprise more than one pilot tone. The pilot tones may be detected remotely, after passing through a plurality of repeater units. The detected level may be used to generate a control signal for controlling the amplification of one or more of the equalisers.

In some embodiments, the equaliser in the repeater apparatus may include a first amplifier with a flat frequency response connected in parallel with a second amplifier with a frequency dependent response. One of these amplifiers may be configured to be controlled according to an attenuation of the first pilot tone, and the other amplifier may be configured to be controlled according to an attenuation of the second pilot tone.

In some embodiments, the equaliser in the repeater apparatus may include a single amplifier with variable time constants. The amplifier may have a first control input configured to be controlled according to an attenuation of the first pilot tone, and a second control input configured to be controlled according to an attenuation of the second pilot tone.

In other embodiments, a plurality of repeater apparatuses may be provided, and all of these may use the same type of equaliser circuit. Alternatively, at least some of these repeater apparatuses may use different types of equaliser circuit, e.g. including the configurations described above or alternative configurations.

If one of the amplifiers failed completely, the failure could be detected by the calibration process, allowing the faulty amplifier to be pinpointed, in order to simplif' the maintenance of the system.

In a repeater apparatus according to an embodiment of the invention, a direct electrical connection between the input and the output of the repeater apparatus may be provided, to allow signals to continuously be transmitted along the cable, even in the event of an individual repeater failure. In addition, a first coupler may be used at the input and a second coupler may be used at the output, to couple the signal from the cable to an equaliser connected in parallel with the direct electrical connection between the input and the output. Thus, the input power can be split by the first coupler between a first path comprising the direct electrical connection and a second path comprising the equaliser. The second coupler can be configured to combine transmission power from these first and second paths and couple it to the output.

The couplers may be passive devices, such as power dividers or directional couplers. In one embodiment of the invention, (nominally) 6dB couplers are used, which results in a 1.25dB power drop between the input signal and the first path, and a 6dB power drop between the input signal and the second path. However, the present invention is not limited by the type or value of any such coupling devices. In some further examples, couplers may be selected in the range of 4dB to 8dB or in the range of 1dB to 10dB. In other examples, power splitters (e.g. Wilkinson splitters) may be used instead of couplers.

The first path may include a delay stage that is configured for creating a signal delay to match the delay caused by the equaliser circuitry on the second path. The delay stage may comprise a time delay transmission line. Thus, the time delay transmission line can be positioned between the through ports of the two couplers. The delay may be first order matched to the delay of the active stages. The first coupled port may drive the equaliser, which may consist of active devices and a series of time constants selected to increase gain with frequency by the compliment of the cable attenuation characteristics used to distribute the signals. The equaliser may amplify the signals in addition to the equalisation, and the equaliser output may feed the second directional coupler, returning signals to the distribution cable at a level similar to the original launch power.

The repeater apparatus may include an antenna coupled to the output of the equaliser, for transmitting the equalised signal as a radio signal. For example, output from the second directional coupler port may be fed to a wideband antenna positioned for radiation along the tunnel. In another configuration, the repeater apparatus may include an antenna coupled to the input of the equaliser, for receiving radio signals to be combined with the signal from the cable, and amplified at the equaliser, before being transferred back to the cable. Thus, a separate receive repeater with the termination of the first coupler and the antenna connections swapped, may be deployed such that received signals are fed along the distribution cable in the reverse direction to signals being transmitted. The antenna may be a broadband antenna, and in some embodiments it may be provided in printed form as part of an active electronics circuit board.

The cable may be 50 ohm coaxial cable. Power for the active stages may be fed along the cable. The power may be transmitted along the inner core of a coaxial cable as an AC waveform. Some cables may include DC Galvanic breaks, and the AC power distribution may use transformer isolation across the Galvanic breaks.

A further aspect of the invention provides a system for signal transmission in a tunnel, comprising a number of repeaters according to the invention, and a cable serially connecting the repeaters. The spacing between each adjacent pair of repeaters is preferably greater than or equal to a predetermined minimum value, where the spacing is selected to prevent signal loss in the cable exceeding a threshold. The spacing apart may be kept to below about 100 metres, to prevent excess signal loss, although in other embodiments, a different maximum spacing may be used, e.g. 120 metres, or 150 metres, or a larger or smaller value, e.g. 80 metres, or 50 metres. As trains in a tunnel are typically longer than 100 metres, one or two repeater units would be constantly feeding a signal into the gap between the train and the tunnel wall, allowing it to propagate into the carriages.

Embodiments of the present invention thus use radiators placed at relatively close intervals and compensate for cable loss, to allow a single system to cover a wide frequency range with low overall power consumption and low cost components. In one embodiment, at a distance where the cable loss is in the order of 6dB at the highest frequency in use (e.g. 50m for 1 8mm outer diameter 75ohm coax at 2GHz), an active equaliser is placed that splits off the signals, provides frequency compensation, and returns the amplified signals to the cable. For transmission, it also radiates the signals locally via a wide bandwidth antenna at a total power in the region of OdBm per signal or below. For reception, a second similarly configured system is located nearby with the antenna connection moved to the input of the equaliser, such that signals fed into each repeater are maintained through all successive repeaters until terminated at a base station.

Under fault conditions in the active components, the signals will pass through without equalisation, in the region of up to about 10dB lower. The locally radiated signal will fall by up to about 20dB, but still will be available for communications purposes. The system is therefore relatively tolerant of fault conditions.

Cumulative equalisation errors from a large number of repeaters in series would compromise the system dynamic range. Errors can be controlled by transmitting two unmodulated pilot tones from the base unit, one below the lowest frequency used and the other above the highest frequency. The levels of these tones are sensed at the line termination, and two control signals are fed back to all repeaters in parallel, via the feeder cable, to alter the degree of equalisation each provides and establish the correct pilot tone amplitudes at the termination. Two-frequency correction is sufficient to correct cable loss equalisation over more than an octave as the cable loss follows a well-behaved curve of attenuation with frequency and with temperature.

A wide variety of different formats may be used for the feedback signals, and the format is not limited to any particular standard or form. The wide frequency range of unoccupied spectrum the cable can convey at low loss can be use to allow a number of different services to be used, or to allow services that require large bandwidths to be used, for example video images.

The feedback also gives a degree of correction for local repeater fault conditions; with the signal levels from units before the failure all transmitting progressively higher power, and those after transmitting lower power, in a stepped saw tooth pattern.

In some embodiments of the invention, a 50 ohm cable may be used with the transmitters, and a 75 ohm cable may be used with the receivers, to give a lower loss of the received signals, but a higher power handling capability for the transmitted signals.

However, the invention is not limited by any particular cable impedance values. In another example, the cable impedance could be 50 ohms for both receiving and transmitting.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an interior of a tunnel, having a radio apparatus according to an embodiment of the invention; Figure 2 is a schematic diagram of a pilot tone calibration system in an embodiment of the invention; Figure 3 is a circuit diagram of a transmitting repeater apparatus, in an embodiment of the invention; and Figure 4 is a graph showing the frequency response of the power signal; Figure 5 is a flow chart of a signal processing method performed by a transmitting repeater apparatus in an embodiment of the invention.

Figure 6 is a circuit diagram of a receiving repeater apparatus, in an embodiment of the invention; and Figure 7 is a flow chart of a signal processing method performed by a receiving repeater apparatus in an embodiment of the invention.

Figure 8 is a circuit diagram of an equaliser circuit used in an embodiment of the invention; Figure 9 is a circuit diagram of an equaliser circuit used in an alternative embodiment of the invention; Figure 10 is a flow chart showing a method of generating and using pilot tones to control the level of amplification in an embodiment of the invention; An embodiment of the invention, installed within a tunnel, is shown in figure 1. Figure 1 is a schematic diagram of section of a tunnel wall, where the tunnel ceiling 101 is indicated by the upper horizontal line, the tunnel floor 102 is indicated by the lower horizontal line, and the tunnel sidewall 100 is the area between the upper and lower horizontal lines.

A first cable 110, for carrying signals for transmission to mobile stations in the tunnel, is shown along the sidewall 100 of the tunnel. At intervals along this cable 110, repeaters 111 are positioned. These repeaters 111 are also configured to transmit radio frequency signals to mobile stations in the tunnel, and each repeater has an antenna 112 for this purpose. Two repeaters 111 are shown, but more may be present in areas of the tunnel not covered by the drawing.

A second cable 120, for carrying signals received from mobile stations in the tunnel, is also shown along the sidewall 100 of the tunnel, just below the first cable. Repeaters 121 are positioned at intervals along this second cable 120. These repeaters 121 are configured to receive radio frequency signals from mobile stations in the tunnel, and each repeater has an antenna 122 for this purpose. Three repeaters 121 are shown, but more may be present in areas of the tunnel not included in the thawing.

A mobile station 130 is shown in the tunnel. This may be built-in communications apparatus within a train or other vehicle, or it may be a passenger's mobile phone, or equipment belonging to the emergency services, or any other kind of movable radio apparatus within the tunnel.

In this example, the mobile station 130 is shown by dotted arrows as receiving transmissions from the right hand side transmitter 111, and having its radio transmissions picked up and processed by the middle receiver 121. In practice, in this embodiment, a mobile station 130 selects the strongest signal from one of the transmitters in the tunnel.

The transmitters and receivers may be staggered in position, so that the signal transmitted by a transmitter 111 does not cause unnecessary interference to the receiver 121. In one embodiment, the receivers on the receive cable may be located half way between the positions of transmitter on the transmit cable.

In some embodiments, the transmitter 111 and receiver 121 units may be housed together in a common casing, with two cables entering and leaving the casing, to carry the signals for transmission and the received signals. Separate antennas may be used, or a single antenna may be used. The antennas may be designed with directional properties, to maximise the power transmitted in a useful direction, towards the interior of the tunnel, and to minimise the power transmitted into the tunnel wall, thus reducing problems of wasted power.

The cable used may be standard co-axial cable. The distance between repeaters may be selected to keep the signal strength from dropping below an acceptable minimum level.

Figure 2 is a schematic diagram of a pilot tone calibration system in an embodiment of the invention, including modules or circuits for generating a pilot (or calibration) tone, and modules or circuits for sensing the pilot tone, in order to allow an optimum level of amplification to be set. At the left hand side of the figure, a base station 400 is shown, which is located at the end of the tunnel, e.g. it may be positioned within the first few metres inside the tunnel or it may be positioned within a few metres outside of the tunnel. The base station 400 includes both a transmitter and a receiver for transmitting data to mobile stations in the tunnel via coaxial cable 110 and receiving data from mobile stations in the tunnel via coaxial cable 120. A pilot tone generator 401 is located at the end of the coaxial cable 110, and this may be located within the base station, or close to the base station.

The pilot tone signal from the pilot tone generator 401 and the transmissions from the base station are combined and sent along the inner core 115 of the coaxial cable 110.

The pilot tone passes through a number of amplifiers 200, each located within one of the transmitter units 111 shown in figure 1.

A pilot tone sensor 402 is provided in the middle of the tunnel, to detect the pilot tone and send appropriate control signals back to the repeaters if adjustments are necessary.

The control signal format is preferably selected for good reliability, robustness and simplicity, because the cascaded mean-time-between-failures in pipe-lined systems is critical. It is desirable that any local failure of the control signal mechanism at one repeater should not compromise the RF performance of that unit if it still functions, and should not interfere with other units downstream. Thus, the control signal format may indicate whether the gain should be increased or decreased from its present setting, allowing a repeater unit to simply remain frozen at its last setting in the event of a failure. In some embodiments, this indication of an increase or decrease of gain may simply be a selection of one of two possible states of the control signal, where a control signal with a first of two possible states indicates an increase in gain by a predetermined amount or proportion, and a control signal with a second of two possible states indicates a decrease in gain by a predetermined amount or proportion.

The control signal may use a carrier frequency located away from the power distribution frequency and its harmonics. It can be advantageous for this carrier frequency to be chosen as a frequency where off-the-shelf filters pre-exist. In one example, the carrier frequency is selected as 455kHz, which is a common Intermediate Frequency used for LW, MW, HF radios. Cable loss at this frequency would be very low.

In one example, the modulation scheme for the control signal is AM (amplitude modulation), and may also use automatic gain control (AGC) to cope with uncertain signal levels. However, the invention is not limited to any particular modulation scheme or type. For example, narrow deviation frequency or phase modulation may be used.

These modulation types are insensitive to signal level above threshold, and more robust in the presence of interference.

The control signal receiver may comprise circuits for performing the following functions, in series: * splitting off the low frequencies from the coax, e.g. in some embodiments by sharing the power distribution route initially; * a narrow bandpass filter straddling the modulation sidebands; * a limiting amplifier; * a frequency discriminator; * a low pass filter; and * a data slicing threshold comparator.

The data transmitted by the modulation may be a packet serial digital stream comprising a synchronisation sequence, and a series of bits indicating which correction to apply and whether to go up or down in gain. The information on whether to go up or down in gain may be provided as single bits with no error correctionldetection. If further adjustments are needed, further control signals are sent. The absence of a control signal may indicate that no change is required.

In embodiments where the control signal provides information about both a high frequency pilot tone and a low frequency pilot tone, then a minimum of two bits would be required in the control signal to provide correction information based on the two pilot tones.

In alternative embodiments, a magnitude of the gain increase or gain decrease may also be transmitted in the control signal, e.g. as a multi-bit binary value. This could allow a larger increment or decrement in the gain to be made more quickly, at the repeater units.

While in the above described embodiments, the control signal comes from the line termination units, in alternative embodiments it is possible for the control signal to originate elsewhere. The data stream may also include a set of bits instructing a particular unit by its unique identifier to perform certain functions in isolation from the others, for example built-in-test-procedures (BITE) such as turning up to full gain or switching off transmissionlreception completely. In this way, under the control of a supervisory routine, the health of a complete system can be checked by monitoring the coupling between a transmitter and its two nearest receivers, and progressively implementing this testing procedure in steps along the tunnel.

The pilot tone sensor may be configured to detect two pilot signals, one arriving from each direction in the tunnel via separate sections of coaxial cable. A pilot tone generator 404 for the receive coaxial cable 120 is located in the middle of the tunnel, and may be configured to generate pilot signals for separate sections of coaxial cable 120, carrying signals in each direction towards the two ends of the tunnel.

The pilot tone sensor 402 and the pilot tone generator 404 may be provided in a terminator unit, e.g. a single box with a transmit and receive cable emerging in each direction along the tunnel. The tei-minator unit may be powered from the coax, e.g. in the same way as the equalisers are powered.

Pilot tone generator 404, and pilot tone sensor 403 provide correction in a similar manner in the reverse direction for coaxial cable 120 with coaxial core 125, and amplifiers 300.

The pilot tone may be a pure frequency with no spectral width, and may be about 10-20 dB below the signal level. Twoor more pilot tone frequencies may be used together.

These may include a first frequency at the lower end of the expected signal frequency, and a second frequency at the upper end of the expected signal frequency. This can allow frequency dependent correction for power loss in the cable. The cable loss has a predictable form per unit length of cable. Thus, using only two pilot frequencies for calibration can provide acceptable results over a wide frequency range.

Figure 3 is a circuit diagram showing a circuit within a transmitting repeater 111, according to an embodiment of the invention. The circuit input and output are connected to the coaxial cable 110 of figure 1. An input signal is received on the inner core 115 of the coaxial cable 110, from the left hand side of figure 3. This input signal on the inner core 115 then passes through a time delay transmission line 207, and is output via the inner core of the coaxial cable 110 at the right hand side of figure 3, to further transmitting repeaters. The outer sheath 116 of the coaxial cable 110 is connected to ground.

The circuit also has a first coupler 203 for coupling input signals from the coaxial cable to an equaliser 200, where frequency dependent amplification is carried out to compensate for attenuation in the coaxial cable 110, and a second coupler 205 for returning the equalised signals back to the coaxial cable 110. In this example, the couplers are 6dB couplers. However, alternative values and configurations could be used instead. The equaliser 200 may provide broadband amplification, as well as equalisation.

Some of the amplified signal output from the equaliser 200 is transferred back to coaxial cable 110 via a second coupler 205. However, a proportion of the amplified signal from the equaliser 200 is not returned to the coaxial cable, and instead is sent to a broadband antenna 112 for transmission of the signal in that region of the tunnel.

In this example, the equaliser 200 is powered via the coaxial core 115. The power is preferably provided by an AC waveform having a frequency well outside the frequency range of the RF signals being transmitted along the cable. The spectrum below 900 MHz may be used for powering the system, as well as sending data and control signals.

A power line 201 connects the coaxial cable core to the power input of the equaliser via an inductor 202, to prevent radio frequency signals being transmitted to the power input of the equaliser 200.

Figure 4 is a graph showing the frequency response of the cable to the power waveform.

For typical repeater spacings, the response has significant peaks at zero, around 1MHz, around 2MHz, etc, and losses are minimised at these peaks. This frequency response arises as a result of tapping off power for the equalisers at regular intervals along the cable, and the number of ripples or peaks corresponds to the number of taps. The power frequency may be centred on one of the low loss peaks, e.g. at the peak close to 1 MHz.

The larger the spacing between the power waveform and the radio signals the less the effects of harmonics of the waveform.

Figure 5 is a flowchart, showing the operation of the repeater apparatus in a transmitter unit such as that shown in figure 3. The process starts at step S201. At step S202, the signal is split off from the cable. At step S203, the equaliser provides frequency dependent amplification. The process then branches both to steps S204 and S205. At S204, a part of the signals are radiated into the tunnel or confined environment using a wideband antenna. At S205, the amplified signal is returned into the cable.

Figure 6 is a circuit diagram showing a circuit within a receiving repeater 121, according to an embodiment of the invention. This is similar to the transmitting repeater 111, with the antenna 122 located prior to the equalisation stage 300, instead of after the equalisation stage 300. The circuit has a coaxial input at the far left side of the figure from the cable 120 shown in figure 1. The outer sheath 126 of the coaxial input is connected to ground, and the inner core 125 is connected to a time delay transmission line 307 through to the inner core 117 of a coaxial output at the far right side of the figure. The outer sheath 118 of the coaxial output is connected to ground. The coaxial output also connects to the coaxial cable 110 shown in figure 1.

The first coupler 303 and second coupler 305 operate in a similar manner to that described for figure 3. The equaliser 200 performs broad band amplification on the signals received via the antenna 122, as well as on the signal from the cable 120. The amplified signal leaves the output of the equaliser 300, and is returned to the co-axial cable 100 with the second coupler 305.

The antenna may be a wide bandwidth antenna. The basic equaliser configuration may cover a wide band.

Figure 7 is a flowchart, showing a similar process to figure 5 in a receiver unit. The process starts at step S301. At step S302, the signal is split off from the cable. At step S303, the equaliser provides frequency dependent amplification. At the same time, signals are received at a wideband antenna at step S204, and these are also amplified. At S205, the amplified signal, including the part received via the cable and the part received via the antenna, is returned into the cable.

Figure 8 shows a circuit diagram of an equaliser circuit used in an embodiment of the invention. The cable losses are frequency dependent so the equaliser is also required to be frequency dependent, with one tone at the top of the desired signal range and another tone at the bottom of this range being used to set the equalisation levels. The cable loss is fairly predictable, so it is not essential to calibrate over a range of intermediate frequencies, and a two point calibration has been found to be adequate.

The equaliser circuit of figure 8 includes two amplifiers connected in parallel, which are a flat frequency response amplifier 250 and a rising frequency response amplifier 251.

The graph at the top left of the circuit diagram shows the frequency response of the flat frequency response amplifier 250, which is a horizontal straight line. The graph at the bottom left of the circuit diagram shows the frequency response of the rising frequency response amplifier 251, which is an curve, matched to the coaxial cable loss.

Each of the amplifier outputs is connected to a variable attenuator. The flat frequency response amplifier 250 output is connected to a first variable attenuator 252 for which the attenuation is set by a control signal, which has been generated according to a low frequency pilot tone amplitude. The rising frequency response amplifier 251 output is connected to a second variable attenuator 253 for which the attenuation is set by a control signal, which has been generated according to a high frequency pilot tone amplitude.

If the pilot tone level drops over time, the pilot tones will be detected by the pilot tone detector as having a lower amplitude than the preferred value or range. The pilot tone detector will then generate control signals for the equaliser, to cause the amplification at the equaliser to increase, thus increasing the signal level and the pilot tone level.

Similarly, if the pilot tone increases over time, the pilot tones will be detected by the pilot tone detector as having a higher amplitude than the preferred value or range. The pilot tone detector will then generate control signals for the equaliser, to cause the amplification of the equaliser to be reduced, thus decreasing the signal level and the pilot tone level. This control mechanism allows the level of amplification at the equaliser to be maintained at a suitable value to prevent the signal building up excessively due to excessive amplification or dropping too low due to insufficient amplification.

In a further embodiment, the pilot tone amplitude may be detected at each equaliser and compared with expected values at the equalisers to determine whether to increase or decrease the equalisation levels.

Figure 9 shows a circuit diagram of an equaliser circuit used in an alternative embodiment of the invention. Frequency shaping is imposed on the equaliser using time constants that are variable, as opposed to the fixed time constants in the embodiment of figure 8. Thus, a single amplifier may be used with variable time constants. The amplifier has a signal input, a signal output, and first and second control inputs. Each of the first and second control inputs are connected to a variable resistor, connected in series to a variable capacitor that is connected to ground. The variable resistor and variable capacitor associated with the first control input are set by a control signal, which has been generated according to a low pilot feedback tone. The variable resistor and variable capacitor associated with the second control input are set by a control signal, which has been generated according to a high pilot feedback tone. The effect of this configuration on the output signal is similar to that for the embodiment of figure 8.

Figure 10 shows a flow chart for a method of generating and using pilot tones to control the level of amplification in an embodiment of the invention. The process starts at step S701. At step S702, two pilot frequencies, pilot frequency 1 and pilot frequency 2, are generated at the pilot tone generator, and these are injected into the coaxial core. The signal passes along the cable, passing through the equalisers to arrive at the pilot tone sensor. At step S703, the pilot tone sensor detects the amplitudes of pilot frequency 1 and pilot frequency 2. Then at step S 704, for each of the two pilot frequencies, a comparison of the pilot frequency with a preferred value or range is made. In some embodiments, if the amplitude is significantly greater than or less than the preferred value or range, then this may indicate a fault. Thus, at step S705, a flag at the pilot tone sensor may be set to indicate the fault, or a fault signal may be transmitted along the cable to a base station or fault detector, or some other way of marking or indicating the likely fault may be used. In this embodiment, the "outside expected range" error signal is not for the repeaters' use, but a flag to a supervisory system that something needs to be done, e.g. instigate a test procedure such as the "BITE" scheme described above the next time the tunnel is empty of trains, and locate the faulty unit or units.

At step S706, assuming that the difference between the pilot tone amplitude and the preferred value is not so great as to indicate a fault, if the amplitude is greater than the preferred value or range, the gain is decreased by a decrement z, which is generally selected as a small fraction of a dB. At step S707, if the amplitude is less than the preferred value or range, the gain is increased by an increment If required, at step S708, an updated control signal is then generated and sent through the cable to the equalisers, to indicate the amount of amplification that is needed. Preferably, the control signal will provide an indication of the frequency dependent amplification needed, determined using the two different pilot frequencies. The equalisers use the control signal to re-adjust their gain.

In some embodiments, the control signal may be a continuous signal, constantly being adjusted according to the detected level of the pilot tones. In other embodiments, the control signal may only be send when an update to the current amplification level is needed. In some embodiments, the control signal may indicate an actual level of amplification to be used. In other embodiments, it may indicate simply whether the amplification level is to be increased or decreased.

The process in figure 10 then goes back to step S703 and continues in a loop to constantly detect, raise or lower the gain to keep it close to a preferred value or range, and to detect if this is not successfully happening. In some embodiments, even if a fault is detected, steps S706 and S707 will still be performed, to attempt to bring the amplitude back towards the preferred value or range.

While the invention has been described in terms of what are at present its preferred embodiments, it will be apparent to those skilled in the art that various changes can be made to the preferred embodiments without departing from the scope of the invention, which is defined by the claims.

Claims

CLAIMS: 1. A repeater apparatus for distribution of radio frequency signals in a tunnel or confined environment, the repeater apparatus comprising: an input for receiving an input signal from a first section of a cable; an equaliser configured to compensate for loss of signal as a function of frequency; and an output for outputting the equalised signal to a second section of the cable.
2. The repeater apparatus of claim 1, further comprising: an equaliser control circuit for controlling the equaliser to set a level of amplification using at least one control signal, the control signal having been generated according to a measurement indicating signal attenuation.
3. The repeater apparatus of claim 2, wherein the input signal comprises a single frequency pilot tone.
4. The repeater apparatus of claim 3, wherein the control signal is derived from the single frequency pilot tone.
5. The repeater apparatus of claim 4, wherein the pilot tone is a first pilot tone and the input signal further comprises a second pilot tone, wherein the first pilot tone has a frequency at the lower end of the desired input signal frequency and the second pilot tone has a frequency at the upper end of the desired input signal frequency, and wherein the equaliser control circuit is configured to set a frequency dependent amplification according to a level of attenuation of the first and second pilot tones.
6. The repeater apparatus of claim 5, wherein the equaliser comprises a first amplifier with a flat frequency response in parallel with a second amplifier with a frequency dependent response, and wherein one of the amplifiers is configured to be controlled according to an attenuation of the first pilot tone, and the other amplifier is configured to be controlled according to an attenuation of the second pilot tone.
7. The repeater apparatus of claim 5, wherein the equaliser comprises a single amplifier with variable time constants, wherein the amplifier has a first control input configured to be controlled according to an attenuation of the first pilot tone, and a second control input configured to be controlled according to an attenuation of the second pilot tone.
8. The repeater apparatus of any one of claims 3 to 7, wherein the pilot tone or tones is a continuous signal.
9. The repeater apparatus of any previous claim, further comprising ID code storage circuitry for storing a unique ID code to enable the repeater apparatus to be remotely distinguished from other similar repeater apparatuses via control signals on the cable.
10. The repeater apparatus of any previous claim, further comprising switching circuitry for remotely switching the equaliser on or off in response to a control signal transmitted along the cable, to facilitate remote fault detection.
11. The repeater apparatus of any previous claim, further comprising fault registration circuitry which is activated if a detected pilot tone amplitude is outside a predetermined set of values.
12. The repeater apparatus of any previous claim, comprising a direct electrical path from the input to the output, wherein the equaliser is connected in parallel to said direct electrical path, to allow the input signal to pass through the repeater apparatus even in the event of a failure of the equaliser.
13. The repeater apparatus of claim 12, wherein the signal path through the equaliser is isolated from the signal path through the cable, the repeater apparatus further comprising a first coupler for coupling said input signal to the equaliser and a second coupler for coupling an output from the equaliser to the second section of the cable.
14. The repeater apparatus of any previous claim, wherein the equaliser is configured to provide amplification of said input signal in addition to the equalisation.
15. The repeater apparatus of any previous claim, wherein the cable is a coaxial cable and the core of the coaxial cable is configured for use as a power line to provide power to the equaliser.
16. The repeater apparatus of any previous claim, further comprising an antenna coupled to the output of the equaliser, for transmitting the equaliser output signal as a radio signal.
17. The repeater apparatus of any one of claims 1 to 16, further comprising an antenna coupled to the input of the equaliser, configured for receiving radio signals and coupling the received radio signals into the signal on the cable.
18. A system for signal transmission in a tunnel, the system comprising a plurality of said repeater apparatuses, arid a cable serially connecting said repeater apparatuses.
19. The system of claim 18, wherein the spacing between each adjacent pair of repeater apparatuses is at least a predetermined value, selected to prevent signal loss in the cable exceeding 6dB.
20. The system of claim 18 or claim 19, wherein the spacing between each adjacent pair of repeater apparatuses is selected to prevent the signal in the cable dropping by more than 10% before being amplified, in the absence of any faults.
21. The system of any one of claims 18 to 20, further comprising at least one pilot tone generator and at least one pilot tone detector, for generating said pilot tone to be passed through the equalisers.
22. The system of any one of claims 18 to 21, wherein the pilot tone detector is configured to generate a control signal using the detected pilot tone, and at least one repeater apparatus is configured to use said control signal to set a level of amplification for the equaliser.
23. A method for distributing radio frequency signals along a cable in a tunnel or confined environment, the method comprising: receiving an input signal from a first section of the cable; at an equaliser, compensating for loss of signal as a function of frequency; and outputting the equalised signal to a second section of the cable.
24. The method of claim 1, further comprising: setting a level of amplification at the equaliser using at least one control signal, the control signal having been generated according to a measurement indicating signal attenuation.
25. The method of claim 23 or claim 24, wherein the input signal comprises a single frequency pilot tone.
26. The method of claim 25, wherein the control signal is derived from the single frequency pilot tone.
27. The method of claim 26, wherein the pilot one is a first pilot tone arid the input signal further comprises a second pilot tone, wherein the first pilot tone has a frequency at the lower end of the desired input signal frequency and the second pilot tone has a frequency at the upper end of the desired input signal frequency, the method further comprising setting a frequency dependent level of amplification according to a level of attenuation of the first and second pilot tones.
28. The method of claim 27, wherein the equaliser comprises a first amplifier with a flat frequency response in parallel with a second amplifier with a frequency dependent response, the method further comprising controlling one of the amplifiers using a control signal generated according to an attenuation of the first pilot tone, and controlling the other amplifier using a control signal generated according to an attenuation of the second pilot tone.
29. The method of claim 27, wherein the equaliser comprises a single amplifier with variable time constants, the method further comprising controlling the amplifier at a first control input using a control signal generated according to an attenuation of the first pilot tone, and controlling the amplifier at a second control input using a control signal generated according to an attenuation of the second pilot tone.
30. The method of any one of claims 24 to 29, further comprising receiving the pilot tone or tones intermittently at least once per hour and not more than 30 times per hour.
31. The method of any one of claims 24 to 29, further comprising receiving the pilot tone or tones signal continuously.
32. The method of any one of claims 23 to 31, further comprising storing a unique ID code to enable the repeater apparatus to be remotely distinguished from other similar repeater apparatuses via control signals on the cable.
33. The method of any one of claims 23 to 32, further comprising switching the equaliser on or off in response to a remotely generated control signal transmitted along the cable, to facilitate remote fault detection.
34. The method of any one of claims 23 to 33, further detecting if a detected pilot tone amplitude is outside a predetermined set of values, and if so then activating a fault notification.
35. The method of any one of claims 23 to 34, wherein the cable is a coaxial cable, the method further comprising receiving power for the equaliser via a core of the coaxial cable.
36. The method of any one of claims 23 to 35, wherein an antenna is coupled to the output of the equaliser, the method further comprising transmitting the equaiiser output signal from the antenna as a radio signal.
37. The method of any one of claims 23 to 36, wherein an antenna is coupled to the input of the equaliser, the method further comprising receiving radio signals at the antenna and coupling the received radio signals into the signal on the cable.
38. A method of installing a system for signal transmission in a tunnel, the method comprising positioning a plurality of said repeater apparatuses according to any one of claims 1 to 22 in the tunnel, with a cable serially connecting said repeater apparatuses.
39. The method of claim 38, further comprising positioning the plurality of repeater apparatuses wherein the spacing between each adjacent pair of repeater apparatuses is at least a predetermined value, to prevent signal loss in the cable exceeding 6dB.
40. The method of claim 38, further comprising positioning the plurality of repeater apparatuses wherein the spacing between each adjacent paid of repeater apparatuses is selected to prevent the signal in the cable dropping by more than 10% before being amplified, in the absence of any faults.
41. The method of any one of claims 38 to 40, further comprising installing at least one pilot tone generator and at least one pilot tone detector, for generating said pilot tone to be passed through the equalisers.