GB2183969A - Network equalisation - Google Patents

Abstract

In a quasi-synchronous radio system, e.g. for use with mobile subscribers, transmitters are coupled by links (land-line or radio) to a control station, and each such link has a programmable equaliser (PEQR) to ensure that the modulation applied to all transmitters is in phase; etc. To adjust these equalisers, the control station has a reference equaliser (RP) and a modulation, varying over the modulation band-width, which is applied to the reference equaliser (RP) and the link equaliser (PEQR) to be adjusted. The output of the reference equaliser and a monitor of the signal from the transmitter are both applied to a comparator (COMP) whose output is used to suitably adjust the coefficients of the link equaliser. This can be done singly for the transmitters, as required, or to all transmitters. <IMAGE>

Description

SPECIFICATION Network equalisation The present invention relates to an equalisation arrangement for use in a radio network using quasi-synchronous radio transmitters.

To explain the requirements for equalising the transmission links between a control point location and a number of quasi-synchronous radio transmitters in an area coverage system, a typical arrangement is shown in Fig. 1. Such an arrangement is most likely to be used in mobile and personal radio systems operating at VHF and UHF frequencies.

Fig. 1 shows the essential elements of an area coverage radio system using close tolerance quasi-synchronous radio transmitters.

The term "quasi-synchronous" means that all of the radio transmitters operate on what is norminally the same frequency. Such transmitters are close-tolerance transmitters. An area of territory is covered by N radio transmitters BS1-BSN, each at one of N base stations. All transmitters operate simultaneously with common modulation essentially on the same carrier frequency, fco. Typically each transmitter carrier is controlled within + 1 Hz from a substandard frequency generator (E.G. rubidium clock). This is so that a single radio transmitter frequency is used to cover a large area rather than assigning N discrete carrier frequencies, one per transmitter, which would mean that a mobile receiver would need to change frequency as it traversed the area to maintain contact with the transmitter sites.In a quasi-synchronous arrangement of this type, a mobile receiver transversing the area of coverage does not have to retune to another frequency as it leaves the service area of one transmitter and enters the service area of another.

So that system can operate satisfactorily, it is important that the transmission links TL1-TLN between the point at which the moduration originates, i.e. the control point location, and the individual transmitter sites is equalised in terms of bandwidth, phase, and overall delay such that the modulation from each transmitter is as closely identical in bandwidth, phase, and timing as is practically possible. To achieve this equalisation of the links from the control point location to the individual radio transmitter sites, equalisers, EQ1-EQ4 at the control point location, are introduced into the transmission links from the control point to the transmitter sites. In current practice these equalisers are manually set to compensate for the characteristics of the transmission links to the radio transmitter sites.

The modulation applied to the area coverage system originates at the control point location, and is typically voice modulation conveying instructions from a controlling operator or despatcher, and ''pip tone" as an alternative when the operator or despatcher is listening to an incoming transmission from a mobile.

"Pip tone" is a combination of audio tones giving a highly disctinctive audio indication which is modulated onto the transmitters, and when received by a mobile, is interpreted thereat as an indication that the operator or despatcher is busy.

The nature of the links between the control point and the individual base stations is variable; they may be physical line, derived (carrier and digital) circuits, or radio links. Thus the propagation characteristics of the transmission links to the radio transmitter sites may vary considerably in nature, and with time, and temperature. Thus the manual setting of the individual equalisers is a highly skilled operation and requires an observer within the area coverage system to report back to the control point regarding the performance of the system and the need to further adjust the equalisers.

Furthrr, if one or more of the links between the control point and the transmitter sites fails and an alternative link has to be commissioned or switched into use, the equalisers need to be checked and possibly reset to ensure that synchronous modulation from the base stations is maintained. This again is a laborious and time consuming process.

An object of the present invention is to overcome or minmise the above disadvantages.

According to the invention there is provided a quasi-synchronous radio system, which includes radio transmitters operating on nominally identical frequencies and serving an area, a central control station coupled to all of said transmitters for supplying modulation to be applied to the signals emitted from the transmitters, each said transmitter being coupled to the central station via a telecommunications link, a programmable equaliser associated with each said transmitter and its said link and which is adjustable to suitably set the parameters of that link, a reference equaliser at the central station which, when a said transmitter's equaliser is to be adjusted, is set to a condition appropriate to the required setting of that transmitter's equal .iser, connection to the reference equaliser and to the link for the transmitter equaliser to be adjusted via which a test signal is applied to said equalisers, a comparator to which the outputs from the reference equaliser and from a monitor of the transmitter are applied, and connections from the comparator to the transmitter's equaliser by which the equaliser is adjusted as required in response to the comparator's output, which adjustment involves applying to the equaliser coefficients appropriate to set it to the desired condition.

An embodiment of the invention will now be described with reference to Fig. 2 of the accompanying drawings, which is a simplified representation of the common equipment used in a quasi-synchronous radio system.

Referring to Fig. 2, it is to be noted that the common equipment at the control point includes a scanning tone generator (STG), a reference programmable equaliser (RP), a comparator (COMP) and a controller (CON). The per transmitter equipment, shown for the Rth transmitter of a quasi-synchronous area coverage scheme, includes a programmable- equaliser PEOR and switches S 1 RZ S2R, 53R and 54R The switches are controlled over a selection bus (SB) from the common controller (CON) and the delay and gain, "D" and "G", coefficients for the programmable equaliser PEQR are provided by the common comparator.

The programmable equalisers for the per transmitter equipment and the reference equaliser in the common equipment use digital filtering techniques, and as is well-known the characteristics of such equalisers with respest to bandwidth, phasing, and overall delay, may be set to a specific set of characteristics by supplying delay and gain coefficients to them.

The comparator (COMP) in the common equipment is an analogue comparator which compares the modulation from the reference equaliser (RP) with the modulation received from the monitor of a particular transmitter, and by examining relative delay, gain, and phasing across the audio band, derives delay and gain coefficients to adjust the per transmitter programmable equaliser such that the monitored modulation from the transmitter in question matches the modulation from the reference programmable equaliser.

The operation of the system will now be described. In normal operation, the switches S1R, S2", S3R, and S4R are unoperated, and modulation (normally speech) and "pip tone" (indicating operator/despatcher busy) is applied simultaneously to each per transmitter equaliser and thus to each transmitter in the system. When it is desired to check, or change, the equalisation of a link to a transmitter, for exampie, the Rth transmitter, an external trigger is given to the controller (CON). The controller then operates to a preset sequence. First, switches S1RÇ S2R, S3RW and 54R are operated. Next reference delay and gain coefficients are supplied to the reference equaliser (RP).These coefficients compensate for the nature of the monitoring link from the transmitter TXR, which may take one of a number of forms such as a local monitor at the transmitter site connected by physical line, derived line, or radio link from the TXR base station, or by direct "off-the-air" reception at the control point of the transmitter Tx, transmission by directional aerials. It may also be necessary to locate this directional receiving monitor point some distance away from the control point such that it can receive all transmitters within the quasi-synchronous area coverage scheme on a directional basis.

Not only do the reference coefficients fed to the reference programmable equaliser compensate for both the characteristics of this monitoring arrangement but they also compensate for the "worst case" situation requiring equalisation on the transmission link to any particular transmitter site. Thus by comparing the modulation from the monitor and the modulation derived via the reference equaliser, the comparator determines the delay and gain coefficients required by the per transmitter equaliser to enable the reference modulation and the monitored modulation to be identical.

Having loaded the reference delay and gain coefficients into the reference equaliser, the scanning tone generator (STG) is turned on. It provides a standard sinusoidal tone that slowly scans the audio pass band of the system. This provides a standard reference of modulation which is applied to the transmitter under test via the per transmitter equaliser PEQR, and also provides the standard input into the reference equaliser (RP). The comparator (COMP) is then activated by the controller (CON) and the locally generated scanning tone via the reference equaliser (RP) is compared with the signal arriving from the monitor of the transmitter. The comparator (COMP) then derives the appropriate delay and gain coefficients to be fed to the per transmitter equaliser PEQR such that the reference and received signals correspond.

Because of the nature of programmable equalisers which essentially operate at a number of centre frequencies of sub-bands within the audio band, the scanning tone generator (STG) could, and might better, be substituted by a sequence of reference tones at the centre of each sub-band, rather than a continuous scanning of "gliding" tone across the band.

Once the channel transmitter equaliser PEOR has been set, the controller (CON) releases, switches S1R, S2R, S3R, and S4R are released and the system reverts to normal speech and "pip tone" modulation.

In a practical application, the system may be triggered to operate either on a periodic basis, e.g. once a day or only when a per transmitter transmission link is changed, or is suspected of going out of tolerance. Alternatively, the system could be made continuously adaptive such that whenever the speech modulation from the control point was replaced by "pip tone", then the controller would be triggered at that time and the automated network equalisation system brought into operation for the duration of the period during which the controlling operator or despatcher was busy and speech modulation was not being applid to the system. Thus speech messages would not be interrupted for the equalisation to take place.

Note that to ensure that the whole of the quasi-synchronous multi-transmitter area coverage scheme exhibits "synchronous" modulation, it is necessary for the controller in the common equipment to scan each per transmitter equipment in sequence, such that, in turn, each transmission link to the radio transmission sites is checked and further equalised if required.

Claims (6)

1. A quasi-synchronous radio system, which includes radio transmitters operating on nominally identical frequencies and serving an area, a central control station coupled to all of said transmitters for supplying modulation to be applied to the signals emitted from the transmitters, each said transmitter being coupled to the central station via a telecommunications link, a programmable equaliser associated with each said transmitter and its said link and which is adjustable to suitably set the parameters of that link, a reference equaliser at the central station which, when a said transmitter's equaliser is to be adjusted, is set to a condition appropriate to the required setting of that transmitter's equaliser, connection to the reference equaliser and to the link for the transmitter equaliser to be adjusted via which a test signal is applied to said equalisers, a comparator to which the outputs from the reference equaliser and from a monitor of the transmitter are applied, and connections from the comparator to the transmitter's equaliser by which that equaliser is adjusted as required in response to the comparator's output, which adjustment involves applying to the equaliser coefficients appropriate to set it to the desired condition.

2. A system as claimed in claim 1, and in which said test signal is a signal which varies continuously over the modulation band for said transmitters.

3. A system as claimed in claim 1, and in which said test signal is a signal whose frequency varies over the transmitters' modulation band in a step-wise manner.

4. A system as claimed in claim 1, 2 or 3, and in which means is provided whereby all transmitters' equalisers are tested and adjusted in one operational sequence.

5. A quasi-synchronous radio system, substantially as described with reference to the accompanying Fig. 2.

CLAIMS Amendments to the claims have been filed, and have the following effect: A new claim 6 has been filed as follows:

6. A quasi-synchronous radio system, which includes radio transmitters operating on nominally identical frequencies and serving an area, a central control station coupled to all of said transmitters for supplying modulation to be applied to the carrier signals emitted from the transmitters, each said transmitter being coupled to the central station via a telecommunications link, a programmable equaliser associated with each said transmitter and its said link and located at the central control station, each said equaliser being adjustable to set the parameters of its said link such that the modulations on the said carrier signals have the same phase, frequency, etc.

characteristics, a reference equaliser also at the central control station which, when a said transmitter's equaliser is to be adjusted, is set to a condition appropriate to the required setting of that transmitter's equaliser, connections to the reference equaliser and to the link for the transmitter equaliser to be adjusted via which a test signal is applied to both said equalisers when the said transmitter equaliser is to be adjusted, a comparator to which the outputs from the reference equaliser and from a monitor of the transmitter whose equaliser is to be adjusted are applied, and connections from the comparator to the transmitter's equaliser by which that equaliser is adjusted as required in response to the output of the comparator, wherein the test signal used is a signal which is varied over the transmitters' modulation band, either varied continuously or varied in a step-wise manner, and wherein the adjustment effected to a said equaliser involves applying to that equaliser coefficients appropriate to set it to the desired condition.