GB2304494A

GB2304494A - Alarm responsive multisite radio system

Info

Publication number: GB2304494A
Application number: GB9516717A
Authority: GB
Inventors: Reinhard Seiler
Original assignee: Motorola GmbH
Current assignee: Motorola Solutions Germany GmbH
Priority date: 1995-08-15
Filing date: 1995-08-15
Publication date: 1997-03-19
Anticipated expiration: 2015-08-15
Also published as: GB9516717D0; GB2304494B; DE19632841A1; RU2160964C2

Description

2304494 TISITE RADIO S"T" BASE STATION AND METHOD OF OPERATING A RADIO

SYSTEM

Field of the Invention

This invention relates to a multisite radio system, such as a private mobile radio system and separately and in addition it relates to a radio base station and a method of operation of a multisite radio system.

Backeround to the Invention A simulcast system is a radio communication system having a number of base stations and synchronising features to synchronise the base stations such that their transmissions are sufficiently closely simultaneous such that signals from different base stations to a given mobile radio unit do not interfere with each other, but arrive at the mobile radio unit practically simultaneously.

Wide area coverage radio frequency communication networks typically comprise a number of distant receivers or transceivers combined 2D into a single system, with their audio signals routed through different input stages of a comparator, such as a SpectraTac (trade mark) comparator. One or a subset of the difiFerent audio signals routed through the different input status of the comparator is selected by a signal quality dependent voting process. A received signal quality indication (typically a received signal strength indicator or RSSI) is provided at each of the base stations and passed to the comparator to indicate to the comparator the base station or site at which a received signal is strongest. The comparator if votes" the signal with the strongest RSSI.

UK Patent Application No. 9401727.4 to be published on 23 August 1995 under serial number GB2286753 describes a method of providing an RSSI indication from a base station to a comparator over a limited bandwidth audio telephone line.

A problem arises when a mobile radio generates an alarm status message and transmits this to a base station where it is received at a relatively low signal strength. Such an alarm status message could be disregarded (i.e. not voted) in favour of a stronger signal from another mobile radio at a difFerent site.

There is a need for an improved multisite radio system with alarm status messages and voting.

CM00357M 2 Summary of the Invention

According to the invention, a base station is provided having traffic circuitry for receiving radio signals from a mobile radio and sending traffic information to a comparator, a received signal quality measurement circuit for measuring the quality of radio signals received, a signalling circuit for sending to the comparator an indication of the quality of signals received, and alarm circuitry coupled to the signalling circuit for causing the signalling circuit to override the indication of the quality of signals received from that base station with an indication of a signal quality of at least a predetermined magnitude.

The invention provides a simple manner of communication alarm conditions to the comparator and allows for improved alarm handling by the comparator.

The arrangement is particularly suited to use over low-bandwidth long distance telephone lines where extensive digital signalling is not practical.

Other aspects of the invention are defined in the claims.

A preferred embodiment of the invention is now described, by way of example only, with reference to the drawings.

Brief Description of the Drawinas

FIG. 1 is a block diagram of a base station according to the preferred embodiment of the invention.

FIG. 2 shows an example of a low frequency signal represented as being transmitted through a medium having band pass characteristics.

FIG.3 is a block diagram of a wide area system in accordance with the preferred embodiment of the invention.

Detailed Description of the Drawings

Referring to FIG. 1, a radio system site is shown in the form of a base station 10 having an antenna 11 and traffic circuitry 12. In its simplest form, the traffic circuitry 12 is a receiver, but typically it also CM00357M 3 includes transmitter circuitry which is not shown. The traffic circuitry 12 has a received signal quality measurement circuit 13 for measuring an indication of the quality of the signals received, such as the RSSI. As an alternative, other measurements such as bit error rate could be used as indication of the quality of the signal received. The base station 10 has signalling circuitry 14, an example of which is described in GB Patent Application No. 9401727.4 in the form of a reference signal generator and a variable amplitude signal generator. The traffic circuitry 12 has an output 15 and the signalling circuitry 14 and the output 15 of the traffic circuitry 12 are connected to a combining element 16 which is connected to an output 17 of the base station 10. The traffic circuitry 12 also has alarm circuitry 18, the operation of which is described below.

Signals received at the antenna 11 are converted to base-band in the traffic circuitry 12 to provide a traffic signal at the output 15, for example an audio signal. The received signal quality measurement circuit 13 receives from the traffic circuitry 12 an indication of the strength of the received signal, which the circuit 13 passes to signalling circuitry 14. In the normal course of operation, signalling circuitry 14 generates a signal indicative of the RSSI and this signal is added to the output from the traffic 2D circuitry 12 at combining element 16. The combined signal is passed to the output 17.

Referring to FIG. 2 the pass band characteristics of a typical telephone transmission line connected to output 17 are shown. A first signal of a first frequency F1 and a second modulated signal of a second frequency F2 are generated by signalling circuitry 14 and indicated at F l and F2 respectively. It is also indicated schematicallY how a RSSI signal (shown to the left in FIG. 2) modulates the amplitude of the second signal. Preferably, the difference between the first and the second frequencies is small comparatively with the frequency range of the band pass characteristics of the transmission line, to minimise the band width used by the first and second signals on the transmission line. The first and second frequencies are preferably placed close to either the lower cut off frequency of the band pass characteristics of the transmission line or the higher cut off frequency of the band pass characteristics of the transmission line. The generation of these signals is described in more detail in UK Patent Application No. 9401727.4 to be published on 23 August CM00357M 4 1995 under serial number GB2286753. It is to be understood that other methods of indication the RSSI over a transmission line can be devised.

When a mobile radio generates an alarm signal, which could take one of a number of forms, alarm circuitry 18 identifies the existence of an alarm condition and indicates to signalling circuitry 14 the presence of an alarm condition at the site. Signalling circuitry 14 overrides its present RSSI or other signal quality measurement with an indication of a signal quality of at least a predetermined magnitude. The predetermined magnitude is, in the preferred embodiment, greater than the aximum level of RSSI indication. For example, in a circuit where the signalling circuitry 14 provides a reference signal and a variable signal, and where the variable signal normally varies from a low level to the level of the reference signal, the variable signal is sent to combiner element 16 at a level higher (e.g. 15 - 20% higher) than the reference signal, as represented by the level of F2 labelled "Alarm" in FIG. 2. The reference level may itself be higher than the maximum RSSI level indication.

Other forms of signalling can be used by signalling circuit 14, such as signalling in the upper end of the audio band, variable frequency signalling or digital signalling.

2D Referring to FIG. 3, the base station 10 is shown as well as three other base stations 20, 30 and 40. Communicating with these base stations is a mobile radio 25. The respective base stations are connected to a control unit 50 by means of telephone lines 51, 52, 53 and 54. The comparator 50 comprises a decode and control unit 60 and a switching unit 70. The switching unit 70 comprises a number of switching elements 71 to 74. The switching circuit 70 has an output 75 coupled to a console 76 which has an audio output in the form of a loudspeaker 77. The comparator 50 has an output port 78 connected to an alarm reporting system 79.

In operation, the mobile radio 25 transmits a signal which is received at more than one base station, for example base stations 10 and 20.

Traffic signals are sent over the telephone lines 51 and 52 to the comparator 50, where they are received at the decode and control unit 60.

The decode and control unit 60 decodes the RSSI measurement indicators within the various signals received and identifies which is the strongest signal as received at the base station 10 or 20. The signal with the strongest RSSI is likely to be the signal of highest quality and the decode CM00357M and control unit 60 "votes" to receive that signal. The decode and control unit 60 causes switching elements 71 to connect the signal from the telephone line 51 to the output 75. Other arrangements are possible, for example with signals from telephone lines 51 and 52 being combined in a diversity combining arrangement in switching elements 71 and 72. The voted signal is passed to the console 76 and output at the speaker 77.

If a new signal is received from another mobile radio (not shown) at one of base stations 30 and 40, it is not "voted" by the decode and control unit 60 until the original signal terminates. If, however, an alarm message is received at one of base stations 30 and 40, it is desirable to preempt the signal from mobile radio 25 with the alarm message. This is achieved by one of the base stations 30 and 40 receiving the alarm message as described with reference to Fig. 1 and forcing its RSSI to the predetermined level. This is detected by decode and control unit 60, which identifies the high level RSSI as being an alarm message. The decode and control unit 60 disconnects the signal from base station 10 or 20 and causes switching element 73 or 74, as appropriate, to switch the new alarm signal to the output port 78 connected to the alarm reporting system 79.

As soon as an alarm occurs at the mobile unit or other remote unit 25, the RSSI level indication signal is increased to a predetermined level, e.g. higher than the corresponding reference level. This condition is detected by the decode and control unit 60 and the alarm message which follows immediately is directed to a separate output port 78 of the comparator 50, which is connected to the customers alarm reporting system 79. If the message comes on a voted channel, another input or another combination of inputs can be selected for the voice message transfer. This assures that the voice message continues without interruption.

CM00357M 6

Claims

Claims

A multisite radio system comprising:

a plurality of base stations and a comparator coupled to the plurality of base stations, each base station having traffic circuitry for receiving radio signals from a mobile radio and sending traffic information to the comparator, a received signal quality measurement circuit for measuring the quality of radio signals received and a signalling circuit for sending to the comparator an indication of the quality of signals received; the comparator having a plurality of inputs connected to the plurality of base stations, an output and comparison and switching circuitry for comparing indications of signal quality from different base stations and switching traffic from one or more selected base stations to the output dependent on a result of comparison, characterized by alarm circuitry in at least one base station, the alarm circuitry being coupled to the signalling circuit of the base station for causing the signalling circuit to override the indication of the quality of signals received from that base station with an indication of at least a predetermined magnitude.
2. A radio system according to claim 1, wherein the signalling circuit sends to the comparator indications indicative of signal quality within a range of indications extending to a maximum indication indicative of aximum signal quality and wherein the predetermined magnitude is greater than the maximum indication.
3. A base station having traffic circuitry for receiving radio signals from a mobile radio and sending traffic information to a comparator, a received signal quality measurement circuit for measuring the quality of radio signals received, a signalling circuit for sending to the comparator an indication of the quality of signals received, and CM00357M 7 alarm circuitry coupled to the signalling circuit for causing the signalling circuit to override the indication of the quality of signals received from that base station with an indication of a signal quality of at least a predetermined magnitude.
4. A base station according to claim 3, wherein the signalling circuit sends to the comparator indications indicative of signal quality within a range of indications extending to a maximum indication indicative of maximum signal quality and wherein the predetermined magnitude is 10 greater than the maximum indica-Lion.
5. A method of operating a radio system comprising at least one mobil radio, a plurality of base stations and a comparator having inputs coupled to the base stations and having an output, comprising the steps ofreceiving radio signals from the mobile radio and sending traffic information to the comparator; measuring the quality of radio signals received at each base station; sending to the comparator indications of the qualities of signals received at different base stations; 2D comparing the indications of qualities from the different base stations; switching traffic from one or more selected base stations to the output dependent on a result of comparison and overriding the indication of the quality of signals received at a given 25 base station with an indication of at least a predetermined magnitude when there is an alarm situation at the given base station.

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