GB2182470A - Paging system - Google Patents

Abstract

A paging system includes one or more base stations (Fig. 1) which transmit paging codewords to a number of pagers. When a pager identifies its codeword it transmits, after a predetermined time interval, a chirp signal which is received by a digital matched filter (64) at the base station to acknowledge receipt of the paging call. A series of chirps may be transmitted, each an up-chirp or a down-chirp to represent a bit, to indicate message quality. <IMAGE>

Description

SPECIFICATION Radiopaging systems The present invention relates to radiopaging systems.

Such systems comprise one or more base stations which are capable of sending out radio transmissions carrying identifying codewords to small pagers carried by individuals in order to alert the carrier of the pager with the transmitted codeword. Systems which provide alert paging simply cause the required individual's pager to emit an audible signal. The carrier must then go to a telephone and make an appropriate telephone call. Message paging systems allow a brief message, which is displayed on a small screen, to be transmitted to the pager.

For various reasons, a paging signal transmitted by the base station to an individual pager may not be received. For example, the pager carrier could be in a location which is in radio shadow.

Alternatively, he may have switched off his pager or its batteries may have failed. Since paging systems are frequently used to alert people such as doctors who are required urgently, it is desirable that the person originating the paging cell should be aware if a paging transmission has not been received so that the transmission can be repeated or alternative arrangements made.

Existing paging systems do not provide this facility.

The base station for the paging system can radiate relatively high power but the individual pagers have to be portable and operate on battery power, so that there is generally insufficient power available for a pager to transmit a signal which will be received by the base station.

Moreover, because of the requirement for low cost and portability, the frequency standards employed in the individual pagers are not particularly accurate. For example the frequency may vary by up to +10 ppm from the normal carrier frequency. Therefore, at the present VHF paging frequencies in the 150 MHz band this can result in a possible frequency offset of + 1.5kHz. Where the paging channel is in the newly proposed 450 MHz band for paging services in the U.K., the possible frequency offset is +4.5kHz.

In order that a low power transmission be received by a radio receiver it is desirable that the transmission should have a low bandwidth so that the noise bandwidth of the receiver can be sufficiently small for the received signal to be distinguished over the background noise level.

The paging system of the present invention is intended to solve the technical problem of providing a paging system such that an individual pager can acknowledge a transmission made to it, with a high probability of its acknowledgement signal being received.

Accordingly, the present invention provides a paging system comprising a base station having a transmitter and a plurality of individual pagers, the base station transmitter being adapted to selectively transmit a paging codeword for each individual pager, each pager comprising a receiver, and means for emitting an alert signal in response to a received codeword being identical to the pager's codeword, characterised in that each pager further comprises means for transmitting a reply signal in the form of a frequency chirp, a predetermined interval after receiving its codeword, and the base station comprises a receiver comprising matched filter means for detecting such reply signals.

By using a chirp signal as the response and detecting it with a matched filter, it is possible to reduce the noise bandwidth of the base station receiver and, at the same time, accommodate the variations in the carrier frequency of the indiviual pagers. Preferably the matched filter means are adapted to respond to a chirp having a greater frequency spread and duration than the chirp signals transmitted by the pagers so that any chirp signal transmitted forms part of the chirp of the matched filter in order to take account of carrier frequency offsets in the pagers.If the carrier frequency of the pager is below the nominal carrier frequency, the response chirp appears to be an earlier part of the larger matched filter chirp, whereas if the carrier frequency of the pager is above the nominal carrier frequency of the response chirp appears to be a later part of the matched filter chirp.

A further advantage of the present system is that by arranging for each reply signal to be transmitted after a predetermined interval from receipt of the paging codeword, only one pager reply signal will be transmitted at any one time, therefore avoiding clashes of incoming signals to the base station receiver, and also providing for identification of the pager transmitting the reply signal.

In one embodiment of the invention a single channel is provided for transmission of the paging codewords and the reply signals. In such an embodiment the channel is time-division-multiplexed with the base station transmitter transmitting for a period substantially equivalent to the predetermined time interval and the pagers transmitting their reply signals in sequence in the succeeding period.

In an alternative embodiment, a separate channel is provided for the reply signals so that the base station transmitter can operate continuously.

In a further embodiment of the invention, the reply chirp can be selectively an up chirp or a down chirp in order to transmit one bit of information back to the base station transmitter. In message paging systems where each pager receives its paging codeword followed by a number of message codewords, a reply signal consisting of a number of chirps may be transmitted a said predetermined interval after the paging codeword and each message codeword, each chirp reply signal may be selectively an up-chirp or a down-chirp in order to transmit one bit of information for each reply signal. The information transmitted back to the base station may advantageously represent an indication of the message quality received.

A paging system in accordance with the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a block diagram of a base station for use in the present paging system; Figure 2 is a block diagram of an individual pager for use in the present paging system; Figure 3 is a diagrammatic representation of the reply signal for various carrier frequency offsets; Figure 4 is a representation of the chirp spectrum within a paging channel; and Figure 5 is a timing diagram of a time-division-multiplexed paging channel showing the transmitted paging codewords and the chirp reply signals.

The paging system comprises one or more base stations 1 and a number of individual pagers 2 distributed within the service area of the base stations. It will be appreciated that a complete paging network may include a number of base stations distributed throughout a large service area to give coverage over a wide range. Each individual pager has its own distinct codeword stored in it so that when that codeword is transmitted by each of the base stations it is recognised by the pager and causes the pager to emit an audible signal. To make a paging call to a pager of unknown location all the base stations, or those within predetermined zones, transmit the codeword but, generally the pager will only be able to receive the transmission from its nearest base station. In the following only one base station is referred to for simplicity.

The base station illustrated in Fig. 1 includes a transmitter. Information regarding which pagers are to be called is fed via an input 4 to a logic control circuit 6 of the transmitter. The logic control circuit outputs the necessary codewords in digital form to an FSK modulator 8, the output of which is amplified and transmitted from antenna 10. It will be appreciated that in order to radiate the signal at the required frequency, the output of the FSK modulator 8 must be mixed or multiplied in a known fashion in one or more stages to produce the required carrier frequency. All transmitted signals from the base station are in principle received by a receiver in each pager. However if the pager is in a radio shadow or is turned off it will not receive the transmission from the base station Each pager 2 comprises a small, receiver antenna 11.Signals received by the antenna 11 are fed via an amplifier 12 to one input of a mixer 14. The other input to mixer 14 is fed from the output of a frequency miitiplier 16 which receives a standard frequency signal from a crystal oscillator 18. The output from the mixer 14 is fed via a filter 20, which selects one of the sum and difference frequencies generated by the mixer 14, to an amplifier 22. The output from the amplifier 22 is fed to a frequency discriminator 24 which converts the received FSK signal from the base station into a digital representation of the codewords transmitted. The output from the frequency discriminator is fed to a logic control circuit 26. The logic control circuit compares the transmitted codeword with the stored identification codeword for the particular pager.If there is identity between the received codeword and the stored codeword then a signal is transmitted to an audio output 28, for example a bleeper. In the case of a message paging system, the codewords following the identical paging codeword will represent message information. These are also decoded by the logic control 26 and corresponding outputs fed to a display 30 which displays the transmitted message.

The above-described paging system is essentially conventional and it will be appreciated that various modifications can be made in its design in accordance with known practice. For this reason, further details of the base station transmitter and the pager receiver are not given.

In the present paging system provision is made for the transmission of reply signals from a pager to the base station to acknowledge receipt of the paging signal. For this purpose each pager is provided with a transmitter and the base station is provided with a receiver. The reply signal chosen for use in the present system is a chirp. A chirp is a signal which sweeps linearly in frequency from a first frequency to a second frequency over a predetermined period of time.

For an "up-chirp" the first frequency is less than the second frequency and vice versa for a "down-chirp". In the present embodiment the reply signal is selected to be a 6kHz up-chirp of 40 ms duration.

The receiver of the base station 1 comprises a receiving antenna 42 connected to an amplifier 44. The output of the amplifier 44 is fed to a mixer 46 where it is mixed with a frequency output from an oscillator 48. The output of the mixer 46 is filtered to select the required intermediate frequency and passed to an amplifier 52. The output of the amplifier 52 is fed via a further mixer 54 fed from an oscillator 56. The output of the mixer 54 is fed via a filter 58 to an amplifier 60. The output of the amplifier 60 is fed to an analogue to digital convertor 62 which samples the received signal, for example at a rate of 60kHz. The digital samples are fed from the analogue to digital convertor 62 to a digital matched filter 64.

The digital matched filter 64 is adapted to correlate the received signal with a stored digital representation of the expected frequency chirp. When the received frequency chirp coincides with the stored representation, that is when there is correlation, the output from the matched filter will peak. The matched filter may be designed in various ways. For example it may be implemented as a shift register through which the digital samples of the received signal pass.

Each location of the shift register is connected to a summer. Inverters are connected between some of the locations of the shift register and the summer. The inverters are positioned so that when the digital samples of an expected signal are at particular positions in the shift register all the negative samples are connected via inverters to the summer. The positions of the inverters thus constitute a stored representation of the expected signal. The output of the filter is taken from the summer. Therefore, when the expected signal is received and when its samples have reached a particular point in the shift register determined by the first sample having reached the end of the register, the output from the summer will reach a peak. The matched filter has the effect of concentrating all the energy receiver over the duration of the chirp into a single pulse output of short duration.The effective receiver noise bandwidth for a chirp detected in this way is inversely proportional to the duration of the chirp. The peak output of the filter will be at a significantly higher level than the output at other times (e.g. 30dB higher). However its level relative to background noise may be significantly less. The embodiment of Fig. 1 is applicable where the noise or interference can be regarded as always less than the level of the peak output from the filter. In an alternative embodiment discussed below account is taken of potentially higher noise or interference levels at the receiver which could cause detection of false replies or failure to detect genuine replies.

The output of the digital matched filter 64 is connected to a threshold circuit 66 supplied with a reference voltage input 68. The reference voltage 68 is set to a level so that the probability of the threshold being exceeded due to receiver noise is low. A signal is output from the threshold circuit only when a chirp is received and when the digital matched filter reaches its peak. The output from the threshold circuit is fed to the logic control circuit 6 of the base station.

In the pagers 2, the logic control circuit 26 also controls the transmitter. At a predetermined time interval after establishing identity of a received codeword with the pager codeword, a signal is fed from the logic control circuit to a ramp generator 32. The ramp generator 32 generates a ramp voltage which is fed to a voltage controlled crystal oscillator 34 and causes the output frequency of that oscillator to sweep upwardly or downwardly in accordance with the slope of the ramp generated. The oscillator 34 may be the same oscillator as the receiver oscillator 18.

The oscillator 34 outputs an up- or a down-chirp which is fed via a frequency multiplier 36 to an amplifier 38. The output of the amplifier 38 is connected to the pager antenna 11. The amplifer 38 also receives an input from the logic control circuit 26 which turns the amplifier on only while a reply signal is to be transmitted by the pager. At all other times the amplifier 38 is turned off in order to prevent interference to replies from other pagers and to prevent an excessive drain on the pager power supply which is typically a battery (not shown).

Operation of the paging system will now be described with particular reference to Figs. 3-5 of the accompanying drawings.

In order to page a particular person the base station transmits the paging codeword of the required person's pager. On receipt of its own paging codeword, the logic control circuit of the pager 2 causes a frequency chirp to be generated and transmitted back to the base station after a predetermined time interval, typically of the order of one minute. A typical frequency chirp is illustrated in Fig. 3. The illustrated chirp is an up-chirp. It will be appreciated that down-chirps can also be used either consistently or selectively in order to transmit one bit of information back to the base station. The chirp has a 6kHz frequency spread centred on the pager carrier frequency. In the present example the chirp has a duration of 40 ms. In the present example it is assumed that a paging channel 25kHz wide in the 450 MHz UHF band is used.Ideally the spectrum of the chirp will appear centrally within this channel as illustrated in Fig. 4. However, the crystal oscillator 34 used in the pager will not normally have an accuracy better than 10 parts per million. Thus the carrier frequency of the pager may be offset to either side of the nominal value by up to 4.5kHz. Such maximum offsets would result in the frequency spectrum of the chirp being as indicated by the chain lines in Fig. 4. By using a 6kHz bandwidth chirp it is clear from Fig. 4 that the chirp will always remain within the allocated paging channel or 25kHz despite this frequency offset.

The logic control circuit 26 of the pager 2 causes the ramp generator 32 to pull the voltage controlled crystal oscillator 34 to produce the required frequency chirp. The logic control circuit 26 produces its start signal to the ramp generator 32 after a delay so that the reply signal will start a predetermined time interval after the start of transmission of the codeword by the base station. Thus a chirp will always start at time t, regardless of any frequency offsets. The effect of the frequency offset is shown in Fig. 3A.Thus, if the carrier frequency of the pager is precisely the nominal frequency, the chirp will start at time t1 with a frequency of f1 which is 3kHz below the nominal carrier frequency and sweep to a frequency f2 3kHz above the nominal carrier frequency at time t2, where t2-t1=40 ms. If the carrier frequency is the maximum 4.5kHz below the nominal carrier frequency the chirp will be as illustrated by the dotted line in Fig. 3A.

If the carrier frequency is the maximum 4.5kHz above the nominal value the frequency chirp will be as illustrated by the dotted chain line in Fig. 3A. These offset frequency chirps can be regarded as parts of a longer frequency chirp illustrated in Fig. 3B which ranges from the nominal carrier frequency minus 7.5kHz to the nominal carrier frequency plus 7.5kHz. Thus, the individual chirps of Fig. 3A may be regarded as parts of this 15kHz frequency chirp of 100 ms duration.

The digital matched filter 64 is designed to correlate the received chirp with the 15kHz stored chirp as illustrated in Fig. 3B. If the whole chirp were received and because of the concentrating effect of the matched filter 64 the receiver noise bandwidth would be 1/T where T is the duration of the chirp, i.e. 100 ms, so that the bandwidth is 10Hz. Since only a 6kHz portion of this total 15kHz chirp will ever be received by the digital matched filter, the peak output from the filter 64 will be consequently reduced and therefore the noise bandwidth of the receiver increased. The effective bandwidth becomes: (15/6)2x 10Hz=62.5 Hz This is considerably less than the noise bandwidth which would be required for the receiver if the pager transmitted an unmodulated reply signal.

A noise calculation for a typical example of the described paging system illustrates that the reply signal has a high probability of receipt by the base station when the pager has been able to receive the original paging codeword. It will be assumed that the Effective isotropic radiated power (EIPR) is 20dBm provided by 1W into a - l0dBi antenna. The receiver antenna gain is 5dBi, the receiver Noise figure is 10dB, and the required signal to noise ratio (C/N) at the receiver is 10dB.Because the effective receiver noise bandwidth is 62.5Hz the thermal noise level is --156dBm. Thus the minimum required signal level at the receiver is: -156 + 10 + 10 - 5 = 141dBm Thermal Noise C/N Noise figure Antenna gain Because the transmitted EIRP is 20dBm this implies a maximum allowable isotropic path loss of 20-(-141)=161dB. Since the maximum allowable path loss for reception by a pager of a transmission by the base station is typically less than the figure e.g. 158dB, if the pager receives its codeword there is a high probability that the base station will receive the reply signal.

Fig. 5 illustrates a typical paging format where a single channel between the base station 1 and the pagers 2 is provided. Paging codewords each having a 62.5 ms duration are transmitted by the base station 1 for the predetermined time interval, t, which may be up to 1 minute. In the following period, t, the reply signals from the pagers are received in the same order as the paging codewords were transmitted. Thus, a pager starts to transmit its chirp reply a period t-62.5 ms, after completion of receipt of its codeword (i.e. time t after the start of transmission of its codeword). This avoids any clashes and ensures identification of the particular pager transmitting the reply signal.The base station receiver effectively receives the reply signal as a peak output from the filter 64 when the reply chirp has reached alignment with the stored representation of the chirp in the digital matched filter 64. As will be apparent from Fig. 3B a reply signal from a pager with a carrier frequency having a maximum offset below the carrier frequency will reach alignment as soon as all the samples of the chirp are in the shift register of the filter so that the output will be received 40 ms after the start of transmission as shown for reply signal 1 in Fig. 5. If the carrier frequency is exactly at the nominal value, alignment in the matched filter will be reached 70 ms after the start of transmission of the reply chirp as shown for reply 2.If the carrier frequency offset is the maximum amount above the nominal value alignment in the filter will not be reached until the first sample has reached the end of the shift register 100 ms after the start of transmission as shown for reply N-1. However this is still less than 40 ms after the start of transmission of reply N so that there is no ambiguity. Any reply signal received by the base station in the time interval t+40 to t+100 after start of transmission of a codeword must be from the pager paged at that time.

In an improved system the logic control circuit 26 of the pager is able to select whether to send an up-chirp or a down-chirp to transmit one bit of information back to the base station. In this case the control circuit 26 sends appropriate signals to the ramp generator in order to generate an increasing or decreasing voltage input to the VCXO 34. At the base station two matched filters receive the reply signal in parallel. In one matched filter an up-chirp is stored and in the other a down-chirp. The outputs of the filters are connected to a processing circuit, in place of the threshold circuit of Fig. 1. The processing circuit is adapted to detect a received chirp, to determine which type of chirp it is, and also to ensure that a received chirp is not falsely detected if a large noise spike or powerful interference occurs.The processing circuit is also capable of correctly detecting reply chirps which come in at high power so that the output of the filter would exceed the threshold before the peak is reached. It should be noted firstiy that a valid reply chirp will have the property that there will be a peak signal of duration of the order of 1/15 kHz (=66 microseconds) out of one of the matched filters just once in the 60 ms period following the end of the chirp reply transmission, and no peak signal out of the other matched filter. Secondly the level of the received chirp reply signal could vary considerably.The presence of a valid reply chirp is declared by the processing circuit if either of the following occur in the 60 ms time slot of an expected reply: (i) there is just one peak out of one of the matched filters which exceeds a preset threshold; or (ii) the largest peak out of either matched filter exceeds the next largest peak out of either matched filter in the 60 ms period; by a predetermined ratio.

The first condition eliminates noise or interference levels above the threshold which will occur equally in the outputs of both filters. The second condition ensures detection in the case where the normal level is above the preset threshold. A peak may be identified for the purposes of the second condition as a signal having a duration of less than, for example 100 microseconds, since the expected peak from the matched filter will have a duration of 66 microseconds. It should be noted that this improved system at the base station receiver can be used even if only one type of chirp, either up or down, is ever sent by the pagers.

In a message paging system where a number of message codewords follow the paging codeword several bits of information can be transmitted back, one for each codeword received for the pager. This return information may relate to the quality of the received transmission.

The logic control circuit 6 of the base station may be programmed to retransmit a paging codeword to a pager which does not send a reply signal. After a number of unsuccessful paging calls the person who placed the paging call may be informed of the failure. In a system where information is contained in the reply signal(s) relating to message quality the paging call can be retransmitted if the message quality is below a threshold level.

Claims (14)

1. A paging system comprising a base station having a transmitter and a plurality of individual pagers, the base station transmitter being adapted to selectively transmit a paging codeword for each individual pager, each pager comprising a receiver, and means for emitting an alert signal in response to a received codeword being identical to the pager's codeword, characterised in that each pager further comprises means for transmitting a reply signal in the form of a frequency chirp, a predetermined interval after receiving its codeword, and the base station comprises a receiver comprising matched filter means for detecting such reply signals.

2. A paging system as claimed in claim 1, wherein the matched filter means are adapted to respond to a chirp having a greater frequency spread and duration than the chirp signals transmitted by the pagers so that any chirp signal transmitted forms part of the chirp of the matched filter in order to take account of carrier frequency offsets in the pagers.

3. A paging system as claimed in claim 1 or 2, wherein a single paging channel is provided between the base station and the pagers, the base station transmitting paging codewords for a first period substantially equivalent to said predetermined interval, and the pagers transmitting their reply signals in sequence in the succeeding period.

4. A paging system as claimed in claim 1 or 2, wherein two channels are provided between the base station and the pagers, the paging codewords being transmitted on one channel and the reply signals on the other.

5. A paging system as claimed in any one of the preceding claims, wherein the matched filter means comprises parallel matched filters one of which is adapted to detect an up-chirp and the other of which is adapted to detect a down-chirp.

6. A paging system as claimed in claim 5, wherein the pager reply signal selectively comprises a chirp which sweeps upwardly or downwardly in frequency, and wherein the base station transmits a plurality of message codewords to a pager after transmitting a paging codeword, the pager transmitting a sequence of chirps as reply signals a predetermined interval after each message codeword so that, in dependence on whether the reply signals are up or down chirps, a number of information bits equal to the number of message codewords transmitted to that pager are transmitted by the pager to the base station.

7. A system as claimed in claim 5 or 6, wherein the outputs of the parallel matched filters are connected to a processing circuit which declares a reply signal to have been received in a predetermined time slot in response to either of the following conditions: (i) there is just one peak out of one of the matched filters which exceeds a preset threshold; or (ii) the largest peak out of either matched filter exceeds the next largest peak out of either matched filter by a predetermined ratio.

8. A system as claimed in any one of the preceding claims, wherein the pager transmitter comprises a voltage controlled crystal oscillator and means for generating a ramp voltage which is applied to a control input of the oscillator to generate a chirp.

9. A system as claimed in any one of the preceding claims, wherein an amplifier of the pager transmitter is switched off except when a reply signal is to be transmitted.

10. A paging system substantially as herein described with reference to the accompanying drawings.

11. A pager for use in a system as claimed in any one of the preceding claims comprising an antenna, a receiver control means for comparing received codewords with a stored codeword, alert means, chirp generating means, and a transmitter, said control means being adapted to activate said alert means and said chirp generating means if the stored codeword is received so that a chirp is transmitted by said transmitter a predetermined interval after said stored codeword is received.

12. A pager as claimed in claim 11, wherein the control means is further adapted to turn on the transmitter only in response to the stored codeword being received to allow transmission of the chirp.

13. A pager substantially as herein described with reference to Fig. 2 of the accompanying drawings.

14. A base station for a paging system substantially as herein described with reference to Fig. 1 of the accompanying drawings.