GB2073552A - Telecommunication system - Google Patents

Abstract

A radiating line telecommunication system has a pair of lines respectively connected to a base transmitter (Tx) and receiver (Rx). Unidirectional amplifiers (A) or repeaters along the lines interconnect them so that signals are conducted alternately in opposite directions. The signal in each amplifier is varied in phase cyclically with respect to the phase of the signal in the previous or next amplifier, and a carrier wave of the transmitted signal is split in frequency into two components so spaced in frequency that the spacing frequency is resonant with the length of transmission line between successive repeaters. <IMAGE>

Description

SPECIFICATION Telecommunication system This invention relates to telecommunication systems of the kind comprising a pair of parallel radiating transmission lines in which signals are initially transmitted on one of the radiating transmission lines from a transmitter base station to a mobile station, and signals from the mobile station are transmitted on the other radiating transmission line at its connection to a receiver base station and unidirectional repeaters are connected at intervals between the transmission lines in a manner so as to conduct alternately in opposite directions.

Such a system is described in British Patent Specification No. 1371291 where an open braid coaxial cable is disclosed as the radiating transmission line. This type of line is known as a 'leaky feeder' and although the braided form of cable is the most popularly used other forms such as coaxial cable having a cylindrical outer sheath with a longitudinal slit along it can be used for particular applications. In the specification above referred to the communication system disclosed employs a series of unidirectional repeaters of amplifiers to amplify the signals and to compensate for attenuation caused principally by the 'lossy' nature of the radiating transmission line.

This type of telecommunication system is now used successfully for communication purposes in mines and tunnels where conventional radio systems would be useless over any material distances due to the walls of the tunnel or mine absorbing the signals propagated from a transmitter. In practice it has been found necessary to improve the basic system to compensate for defects which have become apparent in use. One significant improvement has been the application of a 'tail-back' technique where an additional length of radiating cable has been connected to one side of each repeater and then led back parallel to the main radiating cable on the other side of the repeater in a direction towards the next adjacent repeater for a distance usually equivalent to about half the distance separating the repeaters.

This has enabled a boost to be given to the radiated signal strength in the region adjacent the repeater and to compensate to some degree for attenuation of the radiated signal. Such an improvement is illustrated in the description of British Patent Specification No. 1485156.

Afurther modification is described in British Patent Specification No. 1497288 which discloses a system in which a pair of parallel leaky feeder lines are used with one of the lines being connected to the base receiver station and the other to the base transmitter station. The lines are connected at spaced intervals by unidirectional amplifier elements which are arranged so that alternate elements conduct in the opposite direction to adjacent elements. In addition routing elements are connected in each line between each adjacent pair of amplifier elements in such a mannerthatthe elements route signals first along part of one of the lines and then along part of the other line with the transfer between lines beiing effected through one of the amplifier elements.

In co-pending application No. 79/30511 there is disclosed an improvement to the system of patent specification No. 1497288 in which the lines are interconnected at intervals by stations each inciuding a pair of unidirectional amplifier elements connected to conduct between the lines, and filter means arranged to direct a signal having a first frequency through one of the elements and to direct a signal having a second different frequency through the other of the elements, the elements in alternate stations along the line being connected to conduct in the opposite direction to adjacent stations.

In British Patent Specification No. 1491468 there is a further disclosure which describes a tail back feature which can be used advantageously to provide directional diversity, realised by arranging switching at a suitably high rate so that signals in the mid-span regions of weakest signal are received alternately from a single source or a resultant of two sources.

It is an object of the present invention to show how the principles of directional diversity, as described in British Patent Specification No.1491468, may be applied with advantage to the improved bidirectional routing system described in co-pending patent application No. 79/30511. Further improvements are also claimed in our co-pending application No. 80 26205 from which this application is divided. Figure 1 of the accompanying drawings, to which reference is now made, shows schematically a simplified representation of the improved bidirectional routing arrangement described in copending patent application No.79/30511. Figure 2 indicates a typical phase shifting circuit which can be use.Referring first to Figure 1 circuit elements denoted by triangles A are one way amplifiers or repeaters, capable of amplifying, if necessary simultaneously, signals from base transmitter Tx to a mobile station M and from a mobile station M to the base receiver Rx such signals being sufficiently spaced in frequency to enable discrimination betwees them in the circuits of the base receiver Rx and in the routing filters F denoted by circles.

For the purpose of explanation signals in the direction of base transmitter Tx to mobile station M will be considered primarily, though an exactly corresponding treatment applies for signals in the direction from mobile station M to base receiver Rx. On account of the transmission losses within the radiating coaxial lines themselves the strongest signals will be received by a mobile station in the vicinity of the repeaters A, while the weakest signals will be received in the regions mid-way between successive repeaters; at such points, itwill be understood, the signals are being received equally from the two physical directions, that is, from the repeater on each side.

At a typical position of a mobile station situated in a mid-span region of the line therefore, signals received from the two directions will nominally or potentially be equal in magnitude. However it is known that the random multi-path propagation effects arising from reflections in the environment and by nearby objects, or the ground, or tunnel walls, will be uncorrelated for the two directions of signal propagation. The two signal components will accordingly vary substantially in a random and uncorrelated manner.Likewise the resultant or sum of the two signal components will itself vary in a similar random manner and be uncorrelated with either component If now it is arranged that the phase of one of the signal components is changed the random pattern of variation of that component experienced in the mid-span region will not change since it is not itself a function of simple phase. However the resultant or sum of the two signal components will change since that resultant is a function of the relative phase of the two components, and that has been changed.If furthermore the phase of that one component is changed substantially, approaching or reaching a complete phase reversal and in a cyclic manner at a rate which is high in comparison with the highest modulation frequency being transmitted, then the resultant signal will undergo a corresponding cyclic variation in amplitude as the two components are successively brought into and out of phase with one another. The result is that the combination signal at the mobile receiver will at its periodic maximum amplitude always reach at least the value of either particular component, and will in fact equal the sum of the two individual amplitudes at one particular point in the cycle of phase variation.Since the likelihood of either component being too weak is a prob abilityfunctionend is small (say, lessthan 1 in 10), the probability has now been reduced to that of two uncorrelated signals being simultaneouslytoo small and so the separate probabilities are multiplied, for example in the case postulated to 1 in 100.

It is a simple matter, as has been explained in the application of other but related types of diversity systems, to arrange that the receiving detector cir- cuit responds in fact to the maximum amplitude of such a cyclical variation of signal strength. To realise the potential improvement described it is therefore necessary to arrange that the signal transmitted by each repeater varies in phase in a cyclic manner to the extent of a complete phase reversal in each cycle, with respect to the phase of the signal transmitted by the preceding or following repeater. This can be achieved by introducing suitable phaseshifting means into the signal path at each repeater.

Atypical circuit is shown in Figure 2 where the input is fed to a transformer 1 which has a current controlled resistor such as a PIN diode 2 across its secondary winding in series with a capacitor 3. The secondary winding is also tapped at 4 by line 5 and another tapping is taken at the junction of the resistor2 and capacitor 3. An oscillator7 is connected across the lines 5 at 6 which constitute the output.

More than one such circuit can be connected in cascade for better effect.

It is advantageous if the cyclic variations in phase introduced by the repeaters are themselves sync hronized in frequency throughout the system. This can be achieved bytransmitting such frequency information in the form of a separate signal over the leaky feeder at the appropriate frequency which mighttypically be 8KHz. Alternatively the sync hronizing signal can be carried on the radio carrier itself.

In this embodiment of the invention no other mod ifications to the system are necessary except to ensure that the receivers used are capable of accommodating the increased bandwidth which results from the phase variations of the signals, and that the detectors are of a suitable type to sense the maxima of the cyclic variations.

In an alternative embodimentthe repeaters themselves may be left unmodified, provided that they are capable of accommodating the increased bandwidth which will be shown to be necessary. In this alternative embodiment the radio carrier is split in frequency into two components, so spaced in frequency that the spacing frequency is resonant with the length of transmission line between successive repeaters. Both carrier components carry the modulation. Again considering the operation of the arrangement in terms of a transmission from base to mobile, but accepting that the path is reciprocal with the discussion equally valid for transmission from mobile to base, the transmitter and receiver must both be capable of operating simultaneously on the two frequencies concerned or else be duplicated; in fact the former condition can be easily met with little extra complication.

The mobile receiver M situated in the same general critical region midway between repeaters A will again experience the resultant effects of signals, at both frequencies, coming from two directions simultaneously. At either one frequency the signals will again tend to reinforce orto cancel according to the precise spacial position of the receiver. But in comparing the effects at the two frequencies it will be found thatthe components at the one frequency will reinforce while the components at the other frequency cancel and vice versa. This results from the specified frequency difference between the two carriers, which ensures that the relative phases between components are exactly reversed for the two carrierfrequencies in the mid-span region.By arranging that the receiver can respond to either frequency, therefore, it is always ensured of a signal equal at least to the quadrature sum of the two components at each frequency. This embodiment of the invention does not invoke probability concepts since the relationships between the resultants at the two frequencies are actually negatively correlated rather than simply uncorrelated.

The necessary split of the carrier frequency into two components at the transmitter Tx may be achieved conveniently by use of a conventional single-frequency transmitter operating at a frequency equal to the mean frequency of the two desired components in conjunction with a doublebalanced modulator and a local oscillator operating at a frequency equal to half the required spacing between the two components, that is equal to half the frequency at which the span of leaky feeder between successive repeaters resonates at a halfwavelength, due consideration being given to the fact that the velocity of propagation within the leaky feederwill probably be less than in vacuo.In such arrangement the single frequency carrier provided by the conventional transmitter will conveniently be already modulated normally by either amplitude or frequency or phase modulation, this is by frequency or phase modulation then the resulting two components produced by the double-balance modulator will carry the same modulation, but in mutually opposite senses. The arrangement described would apply at either the fixed base transmitter or at the mobile station or at both, according to which it were desired to bring the benefit of diversity to the base to-mobile transmission orto both.

Atthe receiving station, again whether base Rx or mobile M, it is possible simply to duplicate the receiver for the two frequencies, and to combine their audio-frequency output signals. Alternatively a single receiver of the superheterodynetype may be employed so arranged that the local oscillator within the receiver has its frequency set at the mean value of the two incoming frequency components so that it is equally responsive to the two components.It would be further possible to simplify such an arrangement by offsetting the local oscillator from the true mean frequency, typically by 4 kHz so that the resulting intermediate frequencies from the two carrier components were no longerthe same but staggered in relation to one another by an amount equal to twice that offset, typically 8 kHz, and the detector in the receiver arranged to be of a type which is capable of simultaneously demodulating such spaced components and summing their demodulated signals. In such an arrangement the only special features of the receiver in comparison with a standard receiver would be the ability to accept both incoming signals by the simple omission of the normal second-channel or image rejection filter, a suitable enlarged bandwidth for the intermediate-frequency circuits, and specification of the appropriate type of detector.

In a typical system using the principles described the spacing between repeaters in the leaky feeder system might conveniently be 500 metres, and the velocity ratio of the leaky feeder might be 0.87. In such a case the frequency at which the span between successive repeaters becomes resonant at a half wavelength is 260 kHz, and so for best effect it should be arranged that the spacing between the two carrier components is close to that frequency. If the suggested method of obtaining the two carrier components is used then the frequency of the local oscillator associated with the double-balancedmodulator should be close to 130 kHz. If the suggested method of receiving the signals is used then the intermediate frequency in the receiver will also be that same frequency close to 130 kHz.

Claims (8)

1. A telecommunication system ofthe kind comprising a pair of parallel radiating transmission lines in which signals are initially transmitted on one of the radiating transmission lines from a transmitter base station to a mobile station and signals from the mobile station are transmitted to a receiver base station on the other of the radiating transmission lines at its connection to the receiver base station and unidirectional repeaters are connected at intervals between the transmission lines in a manner so as to conduct alternately in opposite directions, is characterized in that each repeater is arranged to vary in phase in a cyclic manner the signal passing therethrough with respect to the phase of the signal transmitted by the preceding or following repeater and in that a carrier wave of the transmitted signal is split in frequency into two components so spaced in frequency that the spacing frequency is resonant with the length of transmission line between successive repeaters.

2. A system as claimed in Claim 1 in which both parts of the carrier wave carry a modulation.

3. A system as claimed in Claim 1 or Claim 2 in which the transmitter is arranged to operate a frequency equal to the mean frequency of the two desired components of frequency in conjunction with a double-balance modulator and a local oscillator operating at a frequency equal to half the required spacing between the two components.

4. A system as claimed in Claim 3 in which the modulation is frequency or phase modulation and the two components produced by the balance modulator carries the same modulation but in mutually opposite senses.

5. A system as claimed in any preceding claim in which the receiver is of the superheterodyne type and a local oscillator within the receiver has its frequency set at a mean value of the two incoming frequency components.

6. A system as claimed in any one of Claims 1 to 4 in which the receiver is of the superheterodyne type and a local oscillator within the receiver is offset from the true mean frequency to stagger the two carrier wave frequency components by an amount equivalent to twice the offset.

7. A system as claimed in Claim 6 in which a detector in the receiver is arranged to be capable of simultaneously demodulating the spaced frequency components and summing their demodulated signals.

8. A system as claimed in Claim 6 or 7 in which the offset is of the order of 4KHz.