GB2284288A

GB2284288A - Wide area paging system

Info

Publication number: GB2284288A
Application number: GB9324427A
Authority: GB
Inventors: Alfred Caspers; Manfred Mueller
Original assignee: Motorola GmbH
Current assignee: Motorola Solutions Germany GmbH
Priority date: 1993-11-27
Filing date: 1993-11-27
Publication date: 1995-05-31
Anticipated expiration: 2013-11-27
Also published as: GB2284288B; HK1008767A1; GB9324427D0

Abstract

A paging system comprises a central transmitting station arranged to transmit paging signals using a predetermined signalling scheme, e.g. POCSAG, having a finite number of addresses and at least one paging receiver arranged to receive the signals and to operate in response to at least a selected one of the addresses. The central transmitting station is arranged to modulate the signals with a selected tone and the paging receiver is arranged to operate in response to the at least one selected address and the selected tone. This avoids inadvertent reception of paging messages between adjacent systems. Store-and-forward stations may be provided.

Description

WIDE AREA PAGING SYSTEM Field of the Invention This invention relates to a paging system, such as a system in accordance with the POCSAG specification and it is particularly related to such a system which operates over a wide area. The invention further relates to a transmitting station of a paging system and, separately, a paging receiver.

Summary of the Prior Art To fit alarms to tone only receivers or text receivers in the field of fire and rescue services, a digital alarm method in accordance with the POCSAG specification has been used. In order to increase the range and to cover larger areas, several sender/receiver stations are being operated as "digital alarm converters" (DAC), which work in accordance with the "store and forward" process. In this process, varied expansion processes are possible e.g. token ring systems, concentric expansion from a central station via one or more concentrically placed DAC rings, path control etc.

As the frequencies available in most countries for this application are restricted (Germany: 8 frequencies, France: countrywide 1 frequency etc.), an overlapping of the fields of transmission of various systems could occur. These would most certainly occur with overextended ranges or poor transmission range planning. Such conditions could lead to a fail condition in the respective alarm systems, as the POCSAG specification allows no possibility for accommodating the call sign of a transmission network within the code. The use of a frame within a batch of the POCSAG code to install such a call sign represents a restriction in the full use of the address field as shown in the POCSAG specification.

In the field of conventional two way private radio, it is known to provide private line (PL) subaudible tone, otherwise known as control tone carrier subaudible squelch (CTCSS) in which a subaudible tone is continuously transmitted throughout a voice transmission. A particular mobile radio will only open its squelch and thereby allow transmissions to be heard through its loudspeaker, where the subaudible tone matches a predetermined allocated tone. This allows different user groups to be distinguished within a single system. Different user groups are provided with different PL tones and security of conversation is provided between the groups. The arrangement also prevents extraneous noise from activating the squelch and can be used to prevent adjacent systems from activating the squelch.

Summarv of the Invention According to a first aspect of the invention, a paging system is provided comprising a central transmitting station arranged to transmit paging signals using a predetermined signalling scheme having a finite number of addresses and at least one pager receiver arranged to receive set signals and to operate in response to at least a selected one of said addresses. The transmitting station is arranged to modulate the signals with a selected tone and the paging receiver is arranged to operate in response to the at least one selected address and the selected tone.

In this manner, a call sign can be transmitted at the same time as the paging signal, for example the POCSAG signal.

In another aspect, the invention provides a paging system comprising a central transmitting station arranged to transmit paging signals using a predetermined signalling scheme and at least one store-and-forward base station arranged to receive said signals, characterised in that the central transmitting station is arranged to modulate said signals with a selected tone and the store-and-forward base station is arranged to operate in response to the selected tone by retransmitting the signals.

In a further aspect, a transmitting station of a paging system is provided, arranged to transmit paging signals using a predetermined signalling scheme having a finite number of addresses, characterised in that the transmitting station is arranged to modulate said signals with a selected tone.

A paging receiver is also provided for receiving paging signals conforming to a predetermined signalling scheme having a finite number of addresses, said paging receiver being arranged to operate in response to receipt of a paging message comprising at least a selected one of said addresses, characterised in that the receiver is additionally arranged to operate in response to receipt of the selected tone.

Preferably, the POCSAG signal which is transmitted in DFSK modulation (1=4kHz, 0= 4 kHz), is overlaid with a pilot tone with a low frequency and a low frequency stroke. This type of transmission call sign prevents fail conditions occurring because of interference from other networks.

The use of the pilot tone can be considered an extension of the address set of the paging signalling scheme.

The arrangement has the advantage of providing an inexpensive solution to the problem of wide area paging systems without modification of the predetermined signalling scheme.

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

Brief Description of the Drawings Fig. 1 shows two adjacent wide area paging systems, each in accordance with the present invention.

Fig. 2 shows details of a central transmitting station of Fig. 1.

Fig. 3 shows details of a paging receiver of Fig. 1.

Detailed Description of the Preferred Embodiment Referring to Fig. 1, a first POCSAG paging central station 10 is shown having a coverage area 11, and adjacent to coverage area 11 is a similar second central station 12 having a coverage area 13. It will be understood that each of these central stations 10 and 12 may be accompanied by a number of distributed base stations operating on a store-and-forward principal, for example forming a token-ring. Two such base stations 17 and 18 are shown by way of example. Each of the central stations 10 and 12 transmits over one or more frequencies but a problem arises in the area of overlap 14, between the coverage areas 11 and 13. In this area of overlap 14, there exists the problem that central station 10 and central station 12 are both transmitting into the area of overlap on a common frequency.A pager 16 moving in the coverage area 11 and having an address which is addressable from the central station 10 may move into the area of overlap 14 where, unless special measures are taken as described below, the possibility exists that central station 12 can send a message to the address identifying pager 16 and pager 16 will receive a message from central station 12, i.e. from the adjacent system, whereas it is only intended that pager 16 should receive messages from central station 10.

In accordance with the preferred embodiment of the present invention, transmissions from central station 10 are modulated with a first pilot tone, for example, a tone of 500 Hz. Transmissions from central station 12 are modulated with a second pilot tone, for example 1 kHz tone. Pager 16 includes a private line (PL) tone detector set to be activated by a 500 Hz tone and set to disregard a 1 kHz tone. Provided that a paging message received at pager 16 is received with the 500 Hz tone overlaid on the POCSAG DFSK modulated signal, pager 16 will react to a page which identifies that pager. Pager 16 will disregard any POCSAG signal which has a different PL tone, even if that POCSAG signal contains the unique address which identifies pager 16.

Referring to Fig. 2, details of the central transmitting station 10 are shown. Station 10 comprises a POCSAG address generator 20 which generates a complete POCSAG message including a unique address for a particular pager. A PL tone generator 21 is provided, which generates a dedicated 500 Hz tone. The address generator 20 generates a binary signal which is combined in differential amplifier 22 with a sinusoidal signal from PL tone generator 21 and the resulting signal is fed to a frequency modulator 23. The signal from frequency modulator 23 is amplified in a power amplifier 24 and transmitted via an antenna 25.

The transmitted signal is a DFSK signal shifting about a central carrier frequency by -4kHz to +4kHz and in addition has a 500 Hz frequency variant component.

Referring to Fig. 3, details of the pager 16 are given. A receiver circuit is provided (not shown) which has an output coupled to limiter circuit 30 and amplifier circuit 31. Limiter circuit 30 provides an output signal to inverter amplifier 32. The outputs from amplifier 31 and inverter amplifier 32 are fed to negative and positive inputs respectively of a differential amplifier 33. The output of differential amplifier 33 is fed to a PL detector 35 by means of output 34. The PL detector 35 comprises an A/D converter and a software tone detector (not shown).

Also connected to the inverter amplifier is a POCSAG decoder 36 having a data output 37 and providing a reset signal to out-of-range timer 38.

The receiver's circuitry provides a frequency demodulated signal as shown on the input to the limiter 30. Limiter 30 performs a squaring of the signal which is inverted in inverter amplifier 32 to provide the signal B as shown. Simultaneously signal A is amplified in amplifier 31 to provided signal C. Differential amplifier 33 compares signals B and C and subtracts signal B from signal C to provide the pure PL tone D as shown. The gain of amplifier 31 is selected such as the square wave portion of signal C corresponds to the limit amplitude of limiter 30.

The presence of PL tone D on output 34 effectively activates the POCSAG decoder 36 for receipt of the unique address of the pager within the received signal (e.g. as provided by signal B).

The PL tone detector 35 coupled to output 34 is arranged to detect a 500 Hz tone and to disregard tones substantially higher or substantially lower than that tone (for example + or - 200 Hz).

In this manner the pager 16 is protected from inadvertently receiving messages from the central station 12. The same applies to pagers associated with the central station 12 which operate on a different PL tone.

The out of range timer 38 is reset by receipt of any paging message (whether or not addressed to the specific pager 16), provided that the correct PL tone is received. This results from the fact that the decoder 36 is only enabled by the correct PL tone from PL detector 35.

After a period of time of absence of any messages having the correct PL tone, the timer 38 times-out and an out-of-range indication is given (e.g.

on a display).

The circuit of Fig. 3 is also used in the store-and-forward stations 17 and 18 of Fig. 1, with the exception that these fixed base stations do not have any out-of-range timer 38. In this way, the store-and-forward stations are protected from inadvertently forwarding messages from different systems. The store-and-forward base stations do not respond to the specific addresses of the individual received paging messages, but are selectively activated by other information in the leading frame (frame zero) of a paging telegram. For example in a token-ring system, this frame will contain the address of the first base station in the ring and that base station will increment that address before retransmitting the entire telegram.

The new features of Fig. 3 can be incorporated in a paging receiver, or a store-and-forward paging base station, or both.

Where a PL tone detector is incorporated in the base stations, the advantage is achieved of protecting one system from storing and forwarding messages from the other system. In such a system, a standard pager can be used, but the pager will not be protected from receiving messages intended for pagers of identical addresses in an adjacent system. In such an arrangement, careful allocation of addresses in the two systems may be necessary. In such a system, a mixture of standard pagers and paging receivers according to the present invention can be used side-by-side.

A standard paging system, for example a Motorola CAS100 system, can be simply and inexpensively fitted with an additional PL encoder and decoder, the invention provides an inexpensive solution to the problem of prior art systems. The signal preparation for the PL decoder needs to be modified with only the relatively simple switching of Fig. 3. By using different PL codings, more systems can coexist side by side without inadvertent cross transmission of messages.

Claims

1. A paging system comprising a central transmitting station (10, 12) arranged to transmit paging signals using a predetermined signalling scheme having a finite number of addresses and at least one paging receiver (16) arranged to receive said signals and to operate in response to at least a selected one of said addresses, characterised in that the central transmitting station is arranged to modulate said signals with a selected tone and the paging receiver is arranged to operate in response to the at least one selected address and the selected tone.

2. A paging system according to claim 1 wherein the transmitting station (10, 12) is arranged to transmit the signals in differential frequency shift keying modulation.

3. A paging system according to claim 2, wherein the differential frequency shift keying modulation utilizes two frequencies separated by a predetermined frequency shift and wherein the selected tone is low relative to the predetermined frequency shift.

4. A paging system according to any one of the preceding claims, wherein the predetermined signalling scheme is in accordance with the POCSAG specification.

5. A paging system comprising a central transmitting station (10, 12) arranged to transmit paging signals using a predetermined signalling scheme and at least one store-and-forward base station (17, 18) arranged to receive said signals, characterised in that the central transmitting station is arranged to modulate said signals with a selected tone and the store-andforward base station (17, 18) is arranged to operate in response to the selected tone by retransmitting the signals.

6. A transmitting station of a paging system, arranged to transmit paging signals using a predetermined signalling scheme having a finite number of addresses, characterised in that the transmitting station is arranged to modulate said signals with a selected tone.

7. A paging receiver for receiving paging signals conforming to a predetermined signalling scheme having a finite number of addresses, said paging receiver being arranged to operate in response to receipt of a paging message comprising at least a selected one of said addresses, characterised in that the receiver is additionally arranged to operate in response to receipt of the selected tone.

8. A paging receiver according to claim 7, further comprising out-ofrange timing means responsive to receipt by the receiver of a paging message and the selected tone, whereby absence of a paging message and the selected tone for a period of time causes activation of the out-of-range timing means.