CA2104555A1 - Pager with reverse paging facility - Google Patents
Pager with reverse paging facilityInfo
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- CA2104555A1 CA2104555A1 CA 2104555 CA2104555A CA2104555A1 CA 2104555 A1 CA2104555 A1 CA 2104555A1 CA 2104555 CA2104555 CA 2104555 CA 2104555 A CA2104555 A CA 2104555A CA 2104555 A1 CA2104555 A1 CA 2104555A1
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- frequency
- remote unit
- base station
- message
- signal
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- Position Fixing By Use Of Radio Waves (AREA)
Abstract
ABSTRACT OF THE INVENTION
Position determining apparatus including at least one base station including a base transmitter which periodically transmits timing signals and a plurality of mobile remote units. Each remote unit includes a mobile receiver operative to receive timing signals transmitted by at least one base transmitter; synchronization circuitry receiving the timing signals and providing a spread-spectrum frequency-hopping sequence characteristic of the particular remote unit; a mobile transmitter producing a radiated frequency-hopped signal responsive to the frequency-hopping sequence and the timing signal; and message generator operative to modify the radiated frequency-hopped signal to transmit a message to the at least one base station.
Position determining apparatus including at least one base station including a base transmitter which periodically transmits timing signals and a plurality of mobile remote units. Each remote unit includes a mobile receiver operative to receive timing signals transmitted by at least one base transmitter; synchronization circuitry receiving the timing signals and providing a spread-spectrum frequency-hopping sequence characteristic of the particular remote unit; a mobile transmitter producing a radiated frequency-hopped signal responsive to the frequency-hopping sequence and the timing signal; and message generator operative to modify the radiated frequency-hopped signal to transmit a message to the at least one base station.
Description
~ 7423CAN 12~/93 ~ 5 ` PAGER WITH REVER~ PAGING FACILITY
2 FIELD OF THE INV~NTION
3 This invention relates to the field of personal pagers 4 and in particular to the pagers having a reverse paging provision.
6 ~ACRGROUND OF THE INVENTION
7 Paging systems in which pocket sized remote units are 8 carried by a individuals have become a ubiquitous feature of 9 modern life. Such units allow for individuals to be paged from a base station and for the transmission of messages to 11 the remote unit. Generally such messages are displayed on a 12 low power display such as a liquid cry~tal display.
13 Alternat~vely the message is transmitted to the remote units 14 by voice. In response to the message, the individual generally communicates with the sender by telephone.
16 Position location systems are well known. In 17 particular, systems for the location o~ ships based on 18 triangulation of fixed or moving transmitters by the ship 19 are well known. Furthermore, it is known to transmit a signal from a movable object and to find the position of the 21 object by triangulation from fixed receivers or from a 22 single moving receiver.
23 In the TRANSIT system, six polar transit satellites, 24 whose instantaneous positions are well known, transmit a fixed tone. A receiver on a ship receives this signal and 26 determines both the time at which the Doppler shift gradient 27 is a maximum and the magnitude of the Doppler shift. From 28 the time of shift and the known position of the satellite at 29 that time, the latitude is determined. From the magnitude of the shift, the longitude can be determined.
31 In a variant of the TRANSIT system, called WARK, the 32 ground basecl ob~ect, such as a shlp, transmlts the signal 33 which ls received, with its Doppler shift, by the satellite.
34 The satellite rebroadcasts the signal to a ground station which computes the object's position and transmits it to the 36 object.
37 Automatic personal locators (APL) and Automatic 38 Vehicle Locators (AVL) are also known in the art. In 17423CAN 12/8/93 - 2 ~
` general, these devices are relatively large and transmit 2 substantial power or are small and have a limited range. If 3 the system is small, attempts to increase its range will 4 result in its batteries being consumed too quickly for the system to be practical.
6 Spread spectrum techniques and, in particular, 7 frequency-hopping techniques for communication are also well 8 known. See for example, Di~lt~l Communications by, John G.
9 Proakis, McGraw Hill, NY pp. 845-860. In general, these systems have a bandwidth of several kilohertz, which ls 11 suitable for information transmission. In view of this 12 requirement, such devices have either a large size or shor~
13 range or both.
14 The use of a wide band receiver for the reception of narrow band, frequency-hopped signals is known ln the art.
16 In such systems, signals are received from one or more 17 sources by a wide band receiver and the slgnals are 18 processed by a fast Fourier transform to provide signals in 19 the frequency domain. A computer divides this spectrum into relatively narrow preset communication channels, which 21 correspond ts the channels which are used by the 22 transmitting stations. When a plurality of transmitting 23 stations are used, the sy~tem is capable of separating 24 between them and can thus receive more than one message, although the problem of de-interleaving of the signals 26 becomes difficult when large numbers of potential or actual 27 transmitters are involved.
28 Location systems for vehicles based on GP5 or Loran-C
29 are also known. In these systems a mobile receiver receives the GPS or Loran-C signals, determines its own position and 31 broadcasts this position to a central station.
32 Two-way radio systems are also known. However, no 33 pocket-sized system no system has been proposed which will 34 provide the combination of low-power (for long battery life and small size) and wide area of response necessary for a 36 paging system which can operate over a substantial area such 37 as a city.
17423CAN 12/8/93 - 3 - ~ 5 SU~IMARY OF T~IE INVENTION
2 The present invention seeks to fill the need which 3 exists for a paging system in which any one of a large 4 number of remote pocket sized receivers (also referred to as "remote units") can be paged from a base statlon and in 6 which the remote units can also page, and preferably send 7 messages to, other remote uni~s or to telephones specified 8 by the paging remote unit (referred to herein as a "reverse-9 paging~ provision).
Preferably, the system is also provided with the 11 facility to determine the location of the remote unit wh~n 12 it is paged (or at other times) such that the remote unit 13 can also be used to request assistance ln emergency 14 situations.
None of the prior art methods fully solves the problem 16 of determining the position of large numbers of remote units 17 over a large area and certainly none of the methods provides 18 for a system which can provide for reverse paging.
19 In general, the pr sent invention provides for a system of paging in which the remote unit has a small ~ize 21 and weight and an acceptable battery life. The present 22 invention allows for a large number of remote units which 23 can be independently paged and which can respond by sending 24 any one of a number of messayes to any telephone number specified by the carrier of the individual remote unit. In 26 order to avoid the problems of battery drain in the remote 27 unit, the amount of information transferred from the remote ~8 unit must be kept low, and to allow for a large number of 29 remote units, and low power the transmission bandwidth must bP narrow. Finally, it is desirable for the system to 31 operate under spread-spectrum techniques and within the 32 watt power level allowed by FCC Regulation 15-247, which is 33 incorporated herein by reference.
34 In general, such paging systems have large numbers of remote units which are only sporadically activated.
36 In the present invention, the system operates in a 37 spread-spectrum mode using frequency-hopping in order to 38 provide maximum range and minimum power. In addition to a 17423CAN 12/8/93 - 4 - ~ ~ V ~ 5 ~ ~
large number of remote units, the system comprises one or a 2 small number of base systems. The base system~s) comprises a 3 very wide band receivsr, which receives frequency-hopped 4 signals from the remote units, and whose output signal is Fourier transformed and segmented into a very large number 6 of narrow frequency channels. In order to accommodate a 7 large number of users, these ch~nnels are very narrow, 8 resulting in a very low noise for the system and thus a wide 9 range, with low power for the portable remote units.
This type of system, by its very nature, transmits 11 information very slowly; therefore, it is preferable that 12 the information transmitted be precoded. Furthermore, since 13 a delay of several seconds in receiving a message from the 14 remote pager is generally acceptable, this time is available for sending simple messages on a very narrow-band system.
16 Therefore, in a preferred embodiment of the inven~ion 17 the base station includes a computer which stores a 18 plurality of coded messages which can be chosen by the 19 remo~e unit, and which the remote unit can thsn speci~y be sent to another remote unit or to a specified telephone 21 number. This telephone may be either the telephone which has 22 sent a message to the remote unit, or a number which is 23 stored in the computer and retrieved by a code sent by the 24 remote unit or to any other telephone number specified by the remote unit. In an alternative preferred embodiment of 26 the invention, the reverse paging message is translated into 27 a facsimile message which is sent to a remote facsimile 28 machine.
29 In a preferred embodiment of the invention, the message can be sent as a synthesized voice message, a 31 printed message, a displayed message for another pager or in 32 any other form. Alternatively or additionally, an 33 acknowledgement is sent by the remote unit to the base 34 station (and preferably by the base station to the sender of the message) in response to activation of an acknowledgement 36 code by the carrier of the remote unit. Preferably, the 37 remste unit acknowledgement message includes the telephone 38 number of the caller, so that the acknowledg ment can be 17423CAN 12/8/93 - 5 ~ S ~
made by a separate system from the pagin~ system.
2 Alternatively, the same system is used fox paging and 3 acknowledgement.
4 When using the frequency-hopping spread-spectrum technique, the remote units must be accurately time locked 6 with the base stations. In a preferred embodiment of the 7 invention, synchronization signals are transmitted 8 periodically from a central location and are received by the 9 remote units. These periodic signals define a synchronizing ime to the remote units and, preferably, an internal memory 11 therein determines the correct frequencies for the frequency 1~ hopping transmission, so that they transmit at the correct 13 frequencies at the correct times. Alternatively, the 14 frequencies can be generated according to an algorithm stored in the remote unit.
16 Addltionally, the base stations, if more than one `are 17 used in the system, must be accurately time locked.
18 The receivers have a greatly reduced de-interleaving 19 problem, since the receivers in the base station will be able to identify each of the transmitters from its initial 21 frequency or after a very few ~requency hops.
22 It is not necessary that the remote units' radiators 23 have very high frequency stability. In a preferred 24 embodiment of the invention, ordinary crystal-controlled transmitters having an accuracy of one part in 100,000 (zlO
26 kHz) are sufficient, even thou~h the frequency channels are 27 made much narrower (of the order of 50 Hz or less) in order 28 to allow for identification at large distances. The total 29 band-width of the system is preferably of the order of 1.5 Mhz, which means that there are abou~ 30 thousand available 31 channels. However, to assure that there is ~inimal cross-32 talX or interference between adjacent transmitters or 33 between different adjoining frequencies of the same 34 transmitter (as described below), only about 3000 of these available channels are used~ Broader or narrower total 36 system bandwidth could al50 be used depending on the 37 requirements of the particular application. Since only 38 several remote units will be broadcasting at a particular time at or near the same frequency, one, or at most a few, 2 frequency hops will be sufficient to determine which unit is 3 transmitting, since the system need only search for a 4 limited number of possible frequencies and combinations. The deviation of the remote units can then be determined. All 6 subsequent received signals are corrected for the same, or a 7 proportional, deviation.
8 In order to further aid in identiflcation of the 9 remote unit and the rejection of background noise, the frequency of the transmit~er during a particular hop is 11 preferably varied in a predetermined manner (~twiddling") 12 which is recognized by the computer. Such twiddling may 13 include a small repetitive frequency hop of, for example, 14 about 450 Hz, i.e., about 9 channels, during the transmission at a given frequency-hopped frequency.
16 In one preferred embodiment of the invention the 17 sequence of twiddled frequencies de~ermines the in~ormation 18 which is transmitted in that one of the frequencies 19 represents a "zero" ~0) and the other represents a "one" (1) in blnary code. In a second preferred embodiment of the 21 invention, transmission is in pulse form at a given pulse 22 repetition rate; a "one" is represented by transmission at a 23 given phase of the carrier and a "zero" by transmission at a 24 carrier phase shift of 180 degrees from the given phase.
Position detection, if prPsent, is preferably 26 performed by using any appropriate method such as 27 triangulation.
28 Triangulation direction finding is preferred and, in a 29 preferred embodiment of the invention, it is based on an interferometric system. Position locating systems in which 31 the position is determined by the time of arrival of signals 32 requires a wide bandwidth. The present syst~m uses a very 33 narrow bandwidth to increase the number of channels and 34 reduce noise. As is well known, the resolution of position determining systems is inversely proportional to the 36 bandwidth of the system; since the bandwidth o~ the signals 37 in the present system is very low, the system has 38 essentially no range resolution based on time o~ arrival.
17423C~N 12/8/93 - 7 ~
The present system thus preferably uses an interferometric 2 direction finding system which is as accurate as the time of 3 arrival systems, but generally requires either a moving 4 receiver or more than one receiver for locating the transmitter. For systems without position detection, only 6 one base station is required.
7 By using these techniques, the system accommodates a 8 large number of users and allows for synchronism of the 9 spread spectrum/frequency hopping by the users.
There is, therefore, provided, in a preferred 11 embodiment of the invention, paging apparatus including at 12 least one base station including a base transmitter which 13 periodically transmits timing signals; and a plurality of 14 remote units including a mobile receiver operatlve to receive timing signals transmitted by at least one base transmitter; synchronization circuitry receiving the ti~ing 17 signals and providing a spread spectrum frequency-hopping 18 sequence characteristic sf the particular remote unit; a 19 mobile transmitter producing a radiated frequency hopped signal responsive to the frequency-hopping sequence and the 21 timing signal; and a message generator operative to modify 22 the radiated frequency-hopped signal to transit a message to 23 the at least one base station.
24 In a preferred embodiment of the invention the base station includes a base receiver which receives the radiated 26 frequency hopped signal; and signal identification circuitry 27 operative for identifying the remote unit based on the 28 frequency sequence of received frequency-hopped signal.
29 Preferably, the base station also includes means for determining direction of the remote unit.
31 In a preferred embodiment of the invention, the remote 32 unit includes means for providing an activating signal to 33 activate the mobile transmitter in response to activatlon 34 signals. The activation signal may be produced by a carrier of the remote unit or may be generated by the base station 36 and received by the remote unit.
37 Preferably, the remote unit includes a frequency 38 synthesizer which produces a frequency responsive to the 17423CAN 12/8/93 - 8 - ~ 5~
resonant frequency of a resonant circuit. In a preferred 2 embodiment of the invention, the resonant circuit includss a 3 fixed, high-Q, element, preferably a crystal and a 4 perturbating element which is periodically switched into the re~onant circuit, whereby the resonant frequency is 6 periodically switched.
7 Preferably, the perturbating element is a capacitor 8 switched by a diode.
9 In a preferred embodiment of the invention, the message generator includes a frequency generator which 11 qenerates a sequence of pulses each having one of two nearby 12 frequencies in response to an input; and an encoder which 13 chooses a particular pul~e sequence responsive to an lnput 14 by a carrier of the remote unit, whereby the particular sequence represents a message chosen by the carrier.
16 Preferably, the base station includes a decoder w~ich 17 receives and decodes the sequence; and a message dispatcher 18 which dlspatches the message to an intended destination.
19 There is further provided, in a preferred embodiment of the invention a reverse-paging ~ransmission system 21 including a plurality of portable remote units; and a base 22 station operative to transmit messages to any one of the 23 remote units, wherein the xemote units are operative, when 24 activated by a carrier of the remote unit, to transmit coded messages to the base station and the base station is 26 operative to transmit the messages to a destination 27 indicated by the portable unit.
28 BRIE~ DESCRIPTION OF THE DRAWINGS
29 The invention will be better understood in conjunction with the following description of the non-limiting preferred 31 embodiments of the invention which are descri~ed in 32 conjunction with the drawings in which: ~
33 Fig. 1 is a schematic block diagram of a preferred 34 embodiment of a paging/reverse-paging system in accordance with a preferred embodiment of the invention;
36 Fig. 2A is a schematic block diagram of a preferred 37 embodiment of a remote unit in accordance with a preferred 38 embodiment of the invention;
17423CAN 12/8/93 - 9 ~
Fig. ~B is a view of the remote unit showing the 2 display and the keypad;
3 Fig. 3 is a schematic diagram of a crystal controller 4 in accordance with a preferred embodiment of the invention;
s Fig. 4 is a schematic block diagram of a preferred 6 embodiment of a base station in accordance with a preferred 7 embodiment of the invention;
8 Fig. 5 is a drawing showing a preferred configuration 9 of antennas used in the base station of the prese~t in~ention and which is especially sui~able for location of 11 the remote units; and 12 Fig. 6 is a more detailed block diagram of a portion 13 of a preferred base receiver in accordance with a preferred 14 embodiment of the invention.
D~TAILED DESCRIPTION OF THE: PREFlæl~Rl2D EMBODI~ENTS
16 As shown in Fig. 1, a pager~reverse pager system~ in 17 accordance with a preferred embodiment of the invention, 18 comprises one or more base systems 10 and a plurality, 19 generally a large number, of mobile remote units 12, which can be carried by people or vehicles which are to ~e paged.
21 In general, communication with a remote unit 12 is requlred 22 only intermittently, and the remote unit is astivated, as 23 described below, only during suc~ periods.
24 Fig. 2A shows the details of a singl2 remote unit.
Remote unit ~2 comprises an antenna 14, for receiving 26 signals from base system 10 and an antenna 15 for 27 transmittiny signals to the base system(s). Alternatively, 28 the same antenna can be used for both sending and receiving 29 signals from the base system. Antenna 14 feeds a paging receiver 16 which sends synchronization si~nals sent by base 31 system 10 to an input port 18 of a microcontroller 20.
32 Microcontroller 20 receives the signals from the receiver 33 and resets an internal clock (not shown, but generally part 34 of microcontroller 20) based on the time of arrlval of the coded signals received by the remote unit. Microcontrsller 36 20 also includes at least one look-up table which stores a 37 frequency-hopping sequence (or algorithm for computing such 38 sequence) which is characteristic of the particular remote ~ unit.
2 Base system 10 also sends messages to the remote unit 3 which are received by the antenna/microcontroller and are 4 displayed on a display 21 which ls preferably an LCD or other low-power display and which is preferably part of the 6 pocket-sized remote unit ~oftell referred to as a "pager").
7 Voice messages and other means of paging as are known in the 8 art may also be used.
9 A second input port 22 receives "transmit" information from a keypad 24 which is also preferably an integral part 11 of the pager and which is activated by a user.
12 Alternatively, if the position of the remote unit ls 13 desired, a transmit (locate) command may be recelved from a 14 base system via the pager receiver.
An output port 26 of microcontroller 20 supplies 16 frequency control signals to an RP synthesizer 28 w~ich 17 drives antenna 15 via an amplifier 30. Amplifier 30 is 18 preferably a C-class amplifier, producin~ up to 1 watt of 19 power. Amplifier 30 is activated in response to signals received from a second output port 32 of microcontroller 20.
21 The nature of these signals will be dPscribed below.
22 Preferably, the frequency of transmission of remote 23 unit 12 is determined by frequency synthes~,er 28. The 24 reference frequency for synthesizer 28 is provided by a 2s crystal controller 34 which is typically a resonant circuitO
26 Crystal controller 34 is attached to cryst~l input terminals 27 of RF synthesizer 28, such that the output frequency of the 28 RF synthesizer is the resonant frequency multiplied by a 29 factor derived from the signal at port 26.
Fig. 2B shows a preferred embodiment of a remote unit 31 in accordance with the present invention. Display 21 is 32 preferably a liquid crystal display or other low power 33 display. Keypad 24 includes the capability of acknowledging 34 receipt of a message by pressing a single acknowledgement key 25. The entire remote unit is packaged ln a minlature 36 case ~7.
37 A preferred embodiment of crystal controller 34 is 38 shown in Fig~ 3. Crystal controller 34 preferably includes a crystal 36, such as a high Q quartz crystal which oscillates 2 at, for example, 9.6 Mhz, a switching circuit 37, which i~
3 activated from a third output port 38 of microcontroller 20, 4 and a small perturbating capac:Ltance which is connected in parallel to crystal 36 when switching c~rcuit 36 ls 6 activated.
~ In the circuit of Fig. 3, when a diode 40 is back-8 biased, crystal 36 has the series combi~ation of the diode 9 capacitance and the 3.3 pf capacitance 41 across it. When the diode is forward biased, the capacitance across the 11 crystal is 3.3 p~. The effect of the change of capacitance 12 is to perturbate the effectiYe frequency of the crystal ~y 13 about one part in 50,000 and thus to twiddle the transmitted 14 frequency by a like percentage.
If only the position of the remote unit ls desired, 16 then the signals which are broadcast by the remote unit will 17 be a preset identification code, preferably, alternating 18 frequencies of the twiddle.
19 In the message transmit mode, the carrier uses keypad 24 to key in a code which represents a message and a further 21 code which represents the message destination. In a 22 preferred embodiment of the invention, a series of messages, 23 which may be tailored to the particular user, are each 24 assigned a code, for example a three digit code. When the user wishes to send one of these messages, he activates his 26 remote unit, types in the three digit code and a further 27 code wh1ch indicates the destination of the message. This 28 latter code could be a telephone number or may represent a 29 particular remote unit. One such code would represent a command to send a return message to the sender of the la~t 31 message.
32 Additionally, the user may type in a, short message 33 which is not stored in the base station together with a 34 destination code. In this case the typed message will be sent.
36 In a practical situation, if the remote unit receives 37 a message to go to a particular address, the carrier could 38 acknowledge that he has received the message and ls 17423CAN 12/8/93 ~ 5~
` responding by typing in a code which represents the message 2 ~Message received, am responding~ together with a code which 3 directs that the message be forwarded to the sender of the 4 original message. This message would be forwarded by the base station as described below.
6 Figs. 4, S and 6 show the details of one base station.
7 Base station 10 includes a CPU 54 which generates timing 8 signals for synchronizing the remote units and which feeds 9 these signals to a transmitter 56 for transmission to the remote units by transmitting antenna 50. Generally, antenna 11 50 is mounted together with a receiving antenna system 52 on 12 a tower 60 or on the top of a tall building. While separate 13 antennas are shown for receiving and transmitting, one 14 shared antenna can be used, as is well known in the art.
In a preferred embodiment of the invention CPU 54 also 16 activates the transmitter to transmit activation signals to 17 one or more remote units when activation of these units is 18 desired from the base unit, as for example when the location 19 of the remote unlt is to be determined.
Antenna 52 receives signals from remote units and 21 passes these signals to RF unit 58, which down~converts the 22 signals to IF band and passes them on IF unit 62. I~ unit 62 23 provides base-band signals representative of the amplitude 24 and ~hase of the received signals within a plurality of frequency hands. Digital Signal Processing (DSP3 receiver 64 26 receives these signals and determines the fra~uency and 27 phase of the signals received by the elements of antenna 28 system 52. The frequency and phase information is fed into 29 CPU 54 which then identifies the remote unit and determines its direction, if desired. More detailed operation of the 31 receiver system is described in the following paragraphs 32 with reference to Figs. 5 and 6. I
33 Fig. 5 shows a typical antenna system used for 34 transmission and for the optional direction finding function of the system. Antenna 50 which transmits the signals to the 36 remote units is typically a vertical dipole antenna array 37 which radiates toward the horizon and toward the ground 38 uniformly for all azimuthal angles. Antenna system 52 17423CAN lZ/8/93 - 13 ~
` comprises a plurality of sets o~ three back-fire dipole 2 antennas 66 which are not quite evenly spaced. In operation, 3 each of the antennas 66, which face in the general direction 4 of a remote unit, receives signals transmitted by the remote unit with different amplitucle and phase. The phase 6 information is used to determine the precise direction of 7 the transmitting remote unit, however, there exists a phase 8 ambiguity since antennas 66 are relatively widely spaced.
9 The amplitude differences are therefore used to determine the general direction of the remote unit and eliminate this 11 ambiguity. The spacing of antennas 66 is chosen to lncrease 12 the phase sensitivity of the system without unduly 13 increasing the ambiguity to the point where it cannot be 14 resolved based on the amplitude differences~
The outputs of antennas are transmitted to RP unit 58 16 (shown on Fig. 6) by cables 68.
17 The outputs of antennas 66 are fed into a 5PMT switch 18 70 which sequentially feeds the outputs into an RF to IP
19 down~convertor 72. An input from antenna 50 is also down-converted in down-convertor 73 and provides a reference 21 signal for the separate signals dsrived from antennas 66.
22 The output of the two down-convertors is fed into a power 23 divider 74 which produces a plurality of, typically thirty-24 six, substantially identical signals which are normalized for amplitude by the signal from antenna 50. Each of these 26 thirty-six signals is base-band converted by base-band 27 convertors 76 to produce a different segment of the 28 frequency range converted to base band. For example, for an 29 overall system bandwidth of 1.5 Mhz, the segments are typically 40 Khz wide and overlap to assure continuous 31 coverage. The outputs of the base-band convertors are 32 typically the in-phase and quadrature components of the IF
33 signals. These components are digitized by a dual ADC 78 a~d 34 the digitized signals are fed to a Digital FFT 80, typically having 1024 points, each of which represents one channel.
36 Depending on the number of remote units and other 37 design factors, the overall bandwidth can be greater than 38 1.5 Mhz and the number of identical signals can be greater 17423CAN 12/8/93 - 14 ~ L15~ 5 than thirty-six.
2 The individual spectra are then fed to energy and 3 phase detectors 82, which determine the relative amplitude 4 and phase of the signals at the individual antennas ~6.
CPU 54 which receives these amplitude and phase 6 signals also comprises a generator for generating the 7 control signals for sequentially-switching swltch 70. Thus 8 computer 54 effectively sequentially receives the amplitude 9 and phase of all signals which are received by each of antennas 66. Based on this information, a determination is 11 made of the direction of the remote unit as described above.
12 CPV 54 also includes the frequency-hopping tables for 13 all the remote units. Thus, when the computer receives a 14 signal which is identified as belonging to one of the remote units, it determines which remote units belong to the group 16 of remote units which would transmit at a frequency near the 17 received frequency.
18 Clearly, if the frequency generator in the remote un1t 19 were perfect, then the CPU could determine lmmediately which remote unit is transmitting. However, since the accuracy o~
21 transmission of the remote units is only about 10 kHz, the 22 CPU waits for a number of hops to determine a pattern of 23 transmission. W~th this pattern in hand and the rough values 24 of the frequency known, the system then determines which remote unit is actually transmitting and its deviation from 26 the norm. This deviation will no~ change substantially over 27 the short term and is used for the entire measurement.
28 Each base station also includes a modem for 29 transmltting information to a central station, In particular, for position determination, one of the CPUs will 31 generally act as a central station which will send timing 32 signals to the other stations and receive dlrection 33 information regarding the transmittiny remote units from all 34 the base stations. The central CPU will use this information to compute the position of the radiating remote unit, if 36 desired.
37 Alternatively, a single mobile base station may be 38 used which tracks the target while it moves, thus effecting 17423CAN i~8/93 - 15 - ~ 5 the triangulation.
2 In the above explanation, the system both identifies 3 and fin~s the direction of the remote system. In an 4 alternate preferred embodiment of the invention, only identification and message transmission is required and the 6 system can be simplified by using only a single base 7 station, a simpler receiving antenna (i.e., only a single 8 antenna and no switch 703 for the base station and omitting 9 the energy and phase detection of the signals in the channels.
11 In the preferred embodiment o~ the invention having 12 both a paging and a reverse-paging function, CPU 54 receives 13 the signals which are transmitted by remote unit 12 and 14 determines ths codes which have been transmitted ~as described above). It then sends the messages indicated by 16 these codes to the desired destination via modem 84.
17 In a preferred embodiment of the invention, the 18 message can be sent as a synthesized voice messags, a 19 printed message, a displayed message for another pager or in any other form.
21 Alternatively or additionally, an acknowledgement is 22 s~nt by the remote unit to the base station ~and preferably 23 by the base station to the sender of the message) in 24 response to activation of an acknowledge code by the carrier of the remote unit.
26 I~ an alternate preferred embodiment of the inven~ion, 27 the remote unit acknowledgement message includes the 28 telephone number of the caller, so that the acknowledgement 29 can be made by a separate computer system from the paging system.
31 Due to the extremely low bandwidth of the system, the 32 information transmission rate is very low, nevertheless, a 33 message code or several such codes and a telephone number 34 can be transmitted within a few seconds. Since the major time factor involved is the sending of the message by CPU 54 36 over the telephone lines, this type of delay is hardly 37 noticeable to the user.
38 The data transmission rate is about 50 baud. Each hop 17~23CAN 12/~/93 - 16 -` is active for approximately 180 msec and there is a dead 2 time of about 10 msec between hops. At least 50 hops or 10 3 seconds is therefore required for the reverse paglng 4 messages.
In the preferred embodiment of the invention in which 6 the fre~uencies transmitted by the remote units have been 7 twiddled, the computer rejects any signals which are not 8 associated with a "twin" at the correct frequency 9 difference. This improves the rlejection of the noise by the system and th~s improves the range of detection of the 11 remote units.
12 Use of twiddling also allows for a positlve message 13 transmission mode in which a pulsed signal is transmitted at 14 a given pulse repetition rate by the remote unit at one of lS the two twiddled frequencies. The choice of which of the 16 frequencies is to be transmitted would depend on the 17 message, with one of the frequencies representing a one and 18 the other a zero. This type of positive transmission mode in 19 which a signal is always sent, greatly improves the reliability of the transmission.
21 In summary, a particular remote unit, when activated, 22 transmits a frequency~hopped spread-spectrum signal. The 23 exact frequencies used by the particular remote unit are 24 stored in a memory associated with microcontroller 22 (Fig.
2) and the frequency transmitted at a particular time will 26 depend on the identity of the remote unit and on the time of 27 transmission.
28 In order to overcome the inherent inaccuracies in the 29 timing circuits of the remote units, synchronizing signals are periodically transmitted by one of the base stations to 31 all of the remote units. These timing signals reset the time 32 base in the remote units so that it transmits using a 33 frequency-hopping regime which is recognized by the base 34 stations.
3S Since each remote unit has its own distinctive 36 frequency hopping "fingerprint" which is stored both in 37 microcontroller 20 of the remote unit and CPU 54 of base 38 stations 10; when a remote unit transmits its fingerprint, 17423CAN 12/8/93 - 17 ~ 3~
the base station translates the received signal $nto a 2 frequency value. Within a small number of frequency hops, 3 the computer is able to identi~y the particular remote unit 4 in spite of a frequency deviation in the transmission (or the receiver).
6 Alternatively or additionally, the synchronizing 7 signals may also include a high frequency burst which is 8 used by controller ~0 to correct the base frequency 9 generated by the remote unit's base crystal oscillator. In this case the number of hops required to identify the remots 11 unit is substantially reduced, at the cost of a somewhat 12 more complicated remote unit.
13 Preferably, the signals transmitted by the remote 14 stations include coded messages and destination information lS which are uncoded by the base station which sends the 16 messages to the desired destination.
17 Each base station may comprise antennas and circuitry 18 which enable the CPU to determine the direction of the 19 remote unit. Direction information from a plurality of base stations enables a central station to determine the position 21 of the remote unit.
2~ Additional variations of the invention will occur to a 23 person versed in the art. In particular, power levels and 24 frequency bands will vary from ~urisdiction to ~urisdiction, depending on the local regulations. The above preferred 26 e~bodiment is not exclusive but is meant to be illustrative 27 only. The scope of the invention is defined only by the 28 claim~ which follow.
6 ~ACRGROUND OF THE INVENTION
7 Paging systems in which pocket sized remote units are 8 carried by a individuals have become a ubiquitous feature of 9 modern life. Such units allow for individuals to be paged from a base station and for the transmission of messages to 11 the remote unit. Generally such messages are displayed on a 12 low power display such as a liquid cry~tal display.
13 Alternat~vely the message is transmitted to the remote units 14 by voice. In response to the message, the individual generally communicates with the sender by telephone.
16 Position location systems are well known. In 17 particular, systems for the location o~ ships based on 18 triangulation of fixed or moving transmitters by the ship 19 are well known. Furthermore, it is known to transmit a signal from a movable object and to find the position of the 21 object by triangulation from fixed receivers or from a 22 single moving receiver.
23 In the TRANSIT system, six polar transit satellites, 24 whose instantaneous positions are well known, transmit a fixed tone. A receiver on a ship receives this signal and 26 determines both the time at which the Doppler shift gradient 27 is a maximum and the magnitude of the Doppler shift. From 28 the time of shift and the known position of the satellite at 29 that time, the latitude is determined. From the magnitude of the shift, the longitude can be determined.
31 In a variant of the TRANSIT system, called WARK, the 32 ground basecl ob~ect, such as a shlp, transmlts the signal 33 which ls received, with its Doppler shift, by the satellite.
34 The satellite rebroadcasts the signal to a ground station which computes the object's position and transmits it to the 36 object.
37 Automatic personal locators (APL) and Automatic 38 Vehicle Locators (AVL) are also known in the art. In 17423CAN 12/8/93 - 2 ~
` general, these devices are relatively large and transmit 2 substantial power or are small and have a limited range. If 3 the system is small, attempts to increase its range will 4 result in its batteries being consumed too quickly for the system to be practical.
6 Spread spectrum techniques and, in particular, 7 frequency-hopping techniques for communication are also well 8 known. See for example, Di~lt~l Communications by, John G.
9 Proakis, McGraw Hill, NY pp. 845-860. In general, these systems have a bandwidth of several kilohertz, which ls 11 suitable for information transmission. In view of this 12 requirement, such devices have either a large size or shor~
13 range or both.
14 The use of a wide band receiver for the reception of narrow band, frequency-hopped signals is known ln the art.
16 In such systems, signals are received from one or more 17 sources by a wide band receiver and the slgnals are 18 processed by a fast Fourier transform to provide signals in 19 the frequency domain. A computer divides this spectrum into relatively narrow preset communication channels, which 21 correspond ts the channels which are used by the 22 transmitting stations. When a plurality of transmitting 23 stations are used, the sy~tem is capable of separating 24 between them and can thus receive more than one message, although the problem of de-interleaving of the signals 26 becomes difficult when large numbers of potential or actual 27 transmitters are involved.
28 Location systems for vehicles based on GP5 or Loran-C
29 are also known. In these systems a mobile receiver receives the GPS or Loran-C signals, determines its own position and 31 broadcasts this position to a central station.
32 Two-way radio systems are also known. However, no 33 pocket-sized system no system has been proposed which will 34 provide the combination of low-power (for long battery life and small size) and wide area of response necessary for a 36 paging system which can operate over a substantial area such 37 as a city.
17423CAN 12/8/93 - 3 - ~ 5 SU~IMARY OF T~IE INVENTION
2 The present invention seeks to fill the need which 3 exists for a paging system in which any one of a large 4 number of remote pocket sized receivers (also referred to as "remote units") can be paged from a base statlon and in 6 which the remote units can also page, and preferably send 7 messages to, other remote uni~s or to telephones specified 8 by the paging remote unit (referred to herein as a "reverse-9 paging~ provision).
Preferably, the system is also provided with the 11 facility to determine the location of the remote unit wh~n 12 it is paged (or at other times) such that the remote unit 13 can also be used to request assistance ln emergency 14 situations.
None of the prior art methods fully solves the problem 16 of determining the position of large numbers of remote units 17 over a large area and certainly none of the methods provides 18 for a system which can provide for reverse paging.
19 In general, the pr sent invention provides for a system of paging in which the remote unit has a small ~ize 21 and weight and an acceptable battery life. The present 22 invention allows for a large number of remote units which 23 can be independently paged and which can respond by sending 24 any one of a number of messayes to any telephone number specified by the carrier of the individual remote unit. In 26 order to avoid the problems of battery drain in the remote 27 unit, the amount of information transferred from the remote ~8 unit must be kept low, and to allow for a large number of 29 remote units, and low power the transmission bandwidth must bP narrow. Finally, it is desirable for the system to 31 operate under spread-spectrum techniques and within the 32 watt power level allowed by FCC Regulation 15-247, which is 33 incorporated herein by reference.
34 In general, such paging systems have large numbers of remote units which are only sporadically activated.
36 In the present invention, the system operates in a 37 spread-spectrum mode using frequency-hopping in order to 38 provide maximum range and minimum power. In addition to a 17423CAN 12/8/93 - 4 - ~ ~ V ~ 5 ~ ~
large number of remote units, the system comprises one or a 2 small number of base systems. The base system~s) comprises a 3 very wide band receivsr, which receives frequency-hopped 4 signals from the remote units, and whose output signal is Fourier transformed and segmented into a very large number 6 of narrow frequency channels. In order to accommodate a 7 large number of users, these ch~nnels are very narrow, 8 resulting in a very low noise for the system and thus a wide 9 range, with low power for the portable remote units.
This type of system, by its very nature, transmits 11 information very slowly; therefore, it is preferable that 12 the information transmitted be precoded. Furthermore, since 13 a delay of several seconds in receiving a message from the 14 remote pager is generally acceptable, this time is available for sending simple messages on a very narrow-band system.
16 Therefore, in a preferred embodiment of the inven~ion 17 the base station includes a computer which stores a 18 plurality of coded messages which can be chosen by the 19 remo~e unit, and which the remote unit can thsn speci~y be sent to another remote unit or to a specified telephone 21 number. This telephone may be either the telephone which has 22 sent a message to the remote unit, or a number which is 23 stored in the computer and retrieved by a code sent by the 24 remote unit or to any other telephone number specified by the remote unit. In an alternative preferred embodiment of 26 the invention, the reverse paging message is translated into 27 a facsimile message which is sent to a remote facsimile 28 machine.
29 In a preferred embodiment of the invention, the message can be sent as a synthesized voice message, a 31 printed message, a displayed message for another pager or in 32 any other form. Alternatively or additionally, an 33 acknowledgement is sent by the remote unit to the base 34 station (and preferably by the base station to the sender of the message) in response to activation of an acknowledgement 36 code by the carrier of the remote unit. Preferably, the 37 remste unit acknowledgement message includes the telephone 38 number of the caller, so that the acknowledg ment can be 17423CAN 12/8/93 - 5 ~ S ~
made by a separate system from the pagin~ system.
2 Alternatively, the same system is used fox paging and 3 acknowledgement.
4 When using the frequency-hopping spread-spectrum technique, the remote units must be accurately time locked 6 with the base stations. In a preferred embodiment of the 7 invention, synchronization signals are transmitted 8 periodically from a central location and are received by the 9 remote units. These periodic signals define a synchronizing ime to the remote units and, preferably, an internal memory 11 therein determines the correct frequencies for the frequency 1~ hopping transmission, so that they transmit at the correct 13 frequencies at the correct times. Alternatively, the 14 frequencies can be generated according to an algorithm stored in the remote unit.
16 Addltionally, the base stations, if more than one `are 17 used in the system, must be accurately time locked.
18 The receivers have a greatly reduced de-interleaving 19 problem, since the receivers in the base station will be able to identify each of the transmitters from its initial 21 frequency or after a very few ~requency hops.
22 It is not necessary that the remote units' radiators 23 have very high frequency stability. In a preferred 24 embodiment of the invention, ordinary crystal-controlled transmitters having an accuracy of one part in 100,000 (zlO
26 kHz) are sufficient, even thou~h the frequency channels are 27 made much narrower (of the order of 50 Hz or less) in order 28 to allow for identification at large distances. The total 29 band-width of the system is preferably of the order of 1.5 Mhz, which means that there are abou~ 30 thousand available 31 channels. However, to assure that there is ~inimal cross-32 talX or interference between adjacent transmitters or 33 between different adjoining frequencies of the same 34 transmitter (as described below), only about 3000 of these available channels are used~ Broader or narrower total 36 system bandwidth could al50 be used depending on the 37 requirements of the particular application. Since only 38 several remote units will be broadcasting at a particular time at or near the same frequency, one, or at most a few, 2 frequency hops will be sufficient to determine which unit is 3 transmitting, since the system need only search for a 4 limited number of possible frequencies and combinations. The deviation of the remote units can then be determined. All 6 subsequent received signals are corrected for the same, or a 7 proportional, deviation.
8 In order to further aid in identiflcation of the 9 remote unit and the rejection of background noise, the frequency of the transmit~er during a particular hop is 11 preferably varied in a predetermined manner (~twiddling") 12 which is recognized by the computer. Such twiddling may 13 include a small repetitive frequency hop of, for example, 14 about 450 Hz, i.e., about 9 channels, during the transmission at a given frequency-hopped frequency.
16 In one preferred embodiment of the invention the 17 sequence of twiddled frequencies de~ermines the in~ormation 18 which is transmitted in that one of the frequencies 19 represents a "zero" ~0) and the other represents a "one" (1) in blnary code. In a second preferred embodiment of the 21 invention, transmission is in pulse form at a given pulse 22 repetition rate; a "one" is represented by transmission at a 23 given phase of the carrier and a "zero" by transmission at a 24 carrier phase shift of 180 degrees from the given phase.
Position detection, if prPsent, is preferably 26 performed by using any appropriate method such as 27 triangulation.
28 Triangulation direction finding is preferred and, in a 29 preferred embodiment of the invention, it is based on an interferometric system. Position locating systems in which 31 the position is determined by the time of arrival of signals 32 requires a wide bandwidth. The present syst~m uses a very 33 narrow bandwidth to increase the number of channels and 34 reduce noise. As is well known, the resolution of position determining systems is inversely proportional to the 36 bandwidth of the system; since the bandwidth o~ the signals 37 in the present system is very low, the system has 38 essentially no range resolution based on time o~ arrival.
17423C~N 12/8/93 - 7 ~
The present system thus preferably uses an interferometric 2 direction finding system which is as accurate as the time of 3 arrival systems, but generally requires either a moving 4 receiver or more than one receiver for locating the transmitter. For systems without position detection, only 6 one base station is required.
7 By using these techniques, the system accommodates a 8 large number of users and allows for synchronism of the 9 spread spectrum/frequency hopping by the users.
There is, therefore, provided, in a preferred 11 embodiment of the invention, paging apparatus including at 12 least one base station including a base transmitter which 13 periodically transmits timing signals; and a plurality of 14 remote units including a mobile receiver operatlve to receive timing signals transmitted by at least one base transmitter; synchronization circuitry receiving the ti~ing 17 signals and providing a spread spectrum frequency-hopping 18 sequence characteristic sf the particular remote unit; a 19 mobile transmitter producing a radiated frequency hopped signal responsive to the frequency-hopping sequence and the 21 timing signal; and a message generator operative to modify 22 the radiated frequency-hopped signal to transit a message to 23 the at least one base station.
24 In a preferred embodiment of the invention the base station includes a base receiver which receives the radiated 26 frequency hopped signal; and signal identification circuitry 27 operative for identifying the remote unit based on the 28 frequency sequence of received frequency-hopped signal.
29 Preferably, the base station also includes means for determining direction of the remote unit.
31 In a preferred embodiment of the invention, the remote 32 unit includes means for providing an activating signal to 33 activate the mobile transmitter in response to activatlon 34 signals. The activation signal may be produced by a carrier of the remote unit or may be generated by the base station 36 and received by the remote unit.
37 Preferably, the remote unit includes a frequency 38 synthesizer which produces a frequency responsive to the 17423CAN 12/8/93 - 8 - ~ 5~
resonant frequency of a resonant circuit. In a preferred 2 embodiment of the invention, the resonant circuit includss a 3 fixed, high-Q, element, preferably a crystal and a 4 perturbating element which is periodically switched into the re~onant circuit, whereby the resonant frequency is 6 periodically switched.
7 Preferably, the perturbating element is a capacitor 8 switched by a diode.
9 In a preferred embodiment of the invention, the message generator includes a frequency generator which 11 qenerates a sequence of pulses each having one of two nearby 12 frequencies in response to an input; and an encoder which 13 chooses a particular pul~e sequence responsive to an lnput 14 by a carrier of the remote unit, whereby the particular sequence represents a message chosen by the carrier.
16 Preferably, the base station includes a decoder w~ich 17 receives and decodes the sequence; and a message dispatcher 18 which dlspatches the message to an intended destination.
19 There is further provided, in a preferred embodiment of the invention a reverse-paging ~ransmission system 21 including a plurality of portable remote units; and a base 22 station operative to transmit messages to any one of the 23 remote units, wherein the xemote units are operative, when 24 activated by a carrier of the remote unit, to transmit coded messages to the base station and the base station is 26 operative to transmit the messages to a destination 27 indicated by the portable unit.
28 BRIE~ DESCRIPTION OF THE DRAWINGS
29 The invention will be better understood in conjunction with the following description of the non-limiting preferred 31 embodiments of the invention which are descri~ed in 32 conjunction with the drawings in which: ~
33 Fig. 1 is a schematic block diagram of a preferred 34 embodiment of a paging/reverse-paging system in accordance with a preferred embodiment of the invention;
36 Fig. 2A is a schematic block diagram of a preferred 37 embodiment of a remote unit in accordance with a preferred 38 embodiment of the invention;
17423CAN 12/8/93 - 9 ~
Fig. ~B is a view of the remote unit showing the 2 display and the keypad;
3 Fig. 3 is a schematic diagram of a crystal controller 4 in accordance with a preferred embodiment of the invention;
s Fig. 4 is a schematic block diagram of a preferred 6 embodiment of a base station in accordance with a preferred 7 embodiment of the invention;
8 Fig. 5 is a drawing showing a preferred configuration 9 of antennas used in the base station of the prese~t in~ention and which is especially sui~able for location of 11 the remote units; and 12 Fig. 6 is a more detailed block diagram of a portion 13 of a preferred base receiver in accordance with a preferred 14 embodiment of the invention.
D~TAILED DESCRIPTION OF THE: PREFlæl~Rl2D EMBODI~ENTS
16 As shown in Fig. 1, a pager~reverse pager system~ in 17 accordance with a preferred embodiment of the invention, 18 comprises one or more base systems 10 and a plurality, 19 generally a large number, of mobile remote units 12, which can be carried by people or vehicles which are to ~e paged.
21 In general, communication with a remote unit 12 is requlred 22 only intermittently, and the remote unit is astivated, as 23 described below, only during suc~ periods.
24 Fig. 2A shows the details of a singl2 remote unit.
Remote unit ~2 comprises an antenna 14, for receiving 26 signals from base system 10 and an antenna 15 for 27 transmittiny signals to the base system(s). Alternatively, 28 the same antenna can be used for both sending and receiving 29 signals from the base system. Antenna 14 feeds a paging receiver 16 which sends synchronization si~nals sent by base 31 system 10 to an input port 18 of a microcontroller 20.
32 Microcontroller 20 receives the signals from the receiver 33 and resets an internal clock (not shown, but generally part 34 of microcontroller 20) based on the time of arrlval of the coded signals received by the remote unit. Microcontrsller 36 20 also includes at least one look-up table which stores a 37 frequency-hopping sequence (or algorithm for computing such 38 sequence) which is characteristic of the particular remote ~ unit.
2 Base system 10 also sends messages to the remote unit 3 which are received by the antenna/microcontroller and are 4 displayed on a display 21 which ls preferably an LCD or other low-power display and which is preferably part of the 6 pocket-sized remote unit ~oftell referred to as a "pager").
7 Voice messages and other means of paging as are known in the 8 art may also be used.
9 A second input port 22 receives "transmit" information from a keypad 24 which is also preferably an integral part 11 of the pager and which is activated by a user.
12 Alternatively, if the position of the remote unit ls 13 desired, a transmit (locate) command may be recelved from a 14 base system via the pager receiver.
An output port 26 of microcontroller 20 supplies 16 frequency control signals to an RP synthesizer 28 w~ich 17 drives antenna 15 via an amplifier 30. Amplifier 30 is 18 preferably a C-class amplifier, producin~ up to 1 watt of 19 power. Amplifier 30 is activated in response to signals received from a second output port 32 of microcontroller 20.
21 The nature of these signals will be dPscribed below.
22 Preferably, the frequency of transmission of remote 23 unit 12 is determined by frequency synthes~,er 28. The 24 reference frequency for synthesizer 28 is provided by a 2s crystal controller 34 which is typically a resonant circuitO
26 Crystal controller 34 is attached to cryst~l input terminals 27 of RF synthesizer 28, such that the output frequency of the 28 RF synthesizer is the resonant frequency multiplied by a 29 factor derived from the signal at port 26.
Fig. 2B shows a preferred embodiment of a remote unit 31 in accordance with the present invention. Display 21 is 32 preferably a liquid crystal display or other low power 33 display. Keypad 24 includes the capability of acknowledging 34 receipt of a message by pressing a single acknowledgement key 25. The entire remote unit is packaged ln a minlature 36 case ~7.
37 A preferred embodiment of crystal controller 34 is 38 shown in Fig~ 3. Crystal controller 34 preferably includes a crystal 36, such as a high Q quartz crystal which oscillates 2 at, for example, 9.6 Mhz, a switching circuit 37, which i~
3 activated from a third output port 38 of microcontroller 20, 4 and a small perturbating capac:Ltance which is connected in parallel to crystal 36 when switching c~rcuit 36 ls 6 activated.
~ In the circuit of Fig. 3, when a diode 40 is back-8 biased, crystal 36 has the series combi~ation of the diode 9 capacitance and the 3.3 pf capacitance 41 across it. When the diode is forward biased, the capacitance across the 11 crystal is 3.3 p~. The effect of the change of capacitance 12 is to perturbate the effectiYe frequency of the crystal ~y 13 about one part in 50,000 and thus to twiddle the transmitted 14 frequency by a like percentage.
If only the position of the remote unit ls desired, 16 then the signals which are broadcast by the remote unit will 17 be a preset identification code, preferably, alternating 18 frequencies of the twiddle.
19 In the message transmit mode, the carrier uses keypad 24 to key in a code which represents a message and a further 21 code which represents the message destination. In a 22 preferred embodiment of the invention, a series of messages, 23 which may be tailored to the particular user, are each 24 assigned a code, for example a three digit code. When the user wishes to send one of these messages, he activates his 26 remote unit, types in the three digit code and a further 27 code wh1ch indicates the destination of the message. This 28 latter code could be a telephone number or may represent a 29 particular remote unit. One such code would represent a command to send a return message to the sender of the la~t 31 message.
32 Additionally, the user may type in a, short message 33 which is not stored in the base station together with a 34 destination code. In this case the typed message will be sent.
36 In a practical situation, if the remote unit receives 37 a message to go to a particular address, the carrier could 38 acknowledge that he has received the message and ls 17423CAN 12/8/93 ~ 5~
` responding by typing in a code which represents the message 2 ~Message received, am responding~ together with a code which 3 directs that the message be forwarded to the sender of the 4 original message. This message would be forwarded by the base station as described below.
6 Figs. 4, S and 6 show the details of one base station.
7 Base station 10 includes a CPU 54 which generates timing 8 signals for synchronizing the remote units and which feeds 9 these signals to a transmitter 56 for transmission to the remote units by transmitting antenna 50. Generally, antenna 11 50 is mounted together with a receiving antenna system 52 on 12 a tower 60 or on the top of a tall building. While separate 13 antennas are shown for receiving and transmitting, one 14 shared antenna can be used, as is well known in the art.
In a preferred embodiment of the invention CPU 54 also 16 activates the transmitter to transmit activation signals to 17 one or more remote units when activation of these units is 18 desired from the base unit, as for example when the location 19 of the remote unlt is to be determined.
Antenna 52 receives signals from remote units and 21 passes these signals to RF unit 58, which down~converts the 22 signals to IF band and passes them on IF unit 62. I~ unit 62 23 provides base-band signals representative of the amplitude 24 and ~hase of the received signals within a plurality of frequency hands. Digital Signal Processing (DSP3 receiver 64 26 receives these signals and determines the fra~uency and 27 phase of the signals received by the elements of antenna 28 system 52. The frequency and phase information is fed into 29 CPU 54 which then identifies the remote unit and determines its direction, if desired. More detailed operation of the 31 receiver system is described in the following paragraphs 32 with reference to Figs. 5 and 6. I
33 Fig. 5 shows a typical antenna system used for 34 transmission and for the optional direction finding function of the system. Antenna 50 which transmits the signals to the 36 remote units is typically a vertical dipole antenna array 37 which radiates toward the horizon and toward the ground 38 uniformly for all azimuthal angles. Antenna system 52 17423CAN lZ/8/93 - 13 ~
` comprises a plurality of sets o~ three back-fire dipole 2 antennas 66 which are not quite evenly spaced. In operation, 3 each of the antennas 66, which face in the general direction 4 of a remote unit, receives signals transmitted by the remote unit with different amplitucle and phase. The phase 6 information is used to determine the precise direction of 7 the transmitting remote unit, however, there exists a phase 8 ambiguity since antennas 66 are relatively widely spaced.
9 The amplitude differences are therefore used to determine the general direction of the remote unit and eliminate this 11 ambiguity. The spacing of antennas 66 is chosen to lncrease 12 the phase sensitivity of the system without unduly 13 increasing the ambiguity to the point where it cannot be 14 resolved based on the amplitude differences~
The outputs of antennas are transmitted to RP unit 58 16 (shown on Fig. 6) by cables 68.
17 The outputs of antennas 66 are fed into a 5PMT switch 18 70 which sequentially feeds the outputs into an RF to IP
19 down~convertor 72. An input from antenna 50 is also down-converted in down-convertor 73 and provides a reference 21 signal for the separate signals dsrived from antennas 66.
22 The output of the two down-convertors is fed into a power 23 divider 74 which produces a plurality of, typically thirty-24 six, substantially identical signals which are normalized for amplitude by the signal from antenna 50. Each of these 26 thirty-six signals is base-band converted by base-band 27 convertors 76 to produce a different segment of the 28 frequency range converted to base band. For example, for an 29 overall system bandwidth of 1.5 Mhz, the segments are typically 40 Khz wide and overlap to assure continuous 31 coverage. The outputs of the base-band convertors are 32 typically the in-phase and quadrature components of the IF
33 signals. These components are digitized by a dual ADC 78 a~d 34 the digitized signals are fed to a Digital FFT 80, typically having 1024 points, each of which represents one channel.
36 Depending on the number of remote units and other 37 design factors, the overall bandwidth can be greater than 38 1.5 Mhz and the number of identical signals can be greater 17423CAN 12/8/93 - 14 ~ L15~ 5 than thirty-six.
2 The individual spectra are then fed to energy and 3 phase detectors 82, which determine the relative amplitude 4 and phase of the signals at the individual antennas ~6.
CPU 54 which receives these amplitude and phase 6 signals also comprises a generator for generating the 7 control signals for sequentially-switching swltch 70. Thus 8 computer 54 effectively sequentially receives the amplitude 9 and phase of all signals which are received by each of antennas 66. Based on this information, a determination is 11 made of the direction of the remote unit as described above.
12 CPV 54 also includes the frequency-hopping tables for 13 all the remote units. Thus, when the computer receives a 14 signal which is identified as belonging to one of the remote units, it determines which remote units belong to the group 16 of remote units which would transmit at a frequency near the 17 received frequency.
18 Clearly, if the frequency generator in the remote un1t 19 were perfect, then the CPU could determine lmmediately which remote unit is transmitting. However, since the accuracy o~
21 transmission of the remote units is only about 10 kHz, the 22 CPU waits for a number of hops to determine a pattern of 23 transmission. W~th this pattern in hand and the rough values 24 of the frequency known, the system then determines which remote unit is actually transmitting and its deviation from 26 the norm. This deviation will no~ change substantially over 27 the short term and is used for the entire measurement.
28 Each base station also includes a modem for 29 transmltting information to a central station, In particular, for position determination, one of the CPUs will 31 generally act as a central station which will send timing 32 signals to the other stations and receive dlrection 33 information regarding the transmittiny remote units from all 34 the base stations. The central CPU will use this information to compute the position of the radiating remote unit, if 36 desired.
37 Alternatively, a single mobile base station may be 38 used which tracks the target while it moves, thus effecting 17423CAN i~8/93 - 15 - ~ 5 the triangulation.
2 In the above explanation, the system both identifies 3 and fin~s the direction of the remote system. In an 4 alternate preferred embodiment of the invention, only identification and message transmission is required and the 6 system can be simplified by using only a single base 7 station, a simpler receiving antenna (i.e., only a single 8 antenna and no switch 703 for the base station and omitting 9 the energy and phase detection of the signals in the channels.
11 In the preferred embodiment o~ the invention having 12 both a paging and a reverse-paging function, CPU 54 receives 13 the signals which are transmitted by remote unit 12 and 14 determines ths codes which have been transmitted ~as described above). It then sends the messages indicated by 16 these codes to the desired destination via modem 84.
17 In a preferred embodiment of the invention, the 18 message can be sent as a synthesized voice messags, a 19 printed message, a displayed message for another pager or in any other form.
21 Alternatively or additionally, an acknowledgement is 22 s~nt by the remote unit to the base station ~and preferably 23 by the base station to the sender of the message) in 24 response to activation of an acknowledge code by the carrier of the remote unit.
26 I~ an alternate preferred embodiment of the inven~ion, 27 the remote unit acknowledgement message includes the 28 telephone number of the caller, so that the acknowledgement 29 can be made by a separate computer system from the paging system.
31 Due to the extremely low bandwidth of the system, the 32 information transmission rate is very low, nevertheless, a 33 message code or several such codes and a telephone number 34 can be transmitted within a few seconds. Since the major time factor involved is the sending of the message by CPU 54 36 over the telephone lines, this type of delay is hardly 37 noticeable to the user.
38 The data transmission rate is about 50 baud. Each hop 17~23CAN 12/~/93 - 16 -` is active for approximately 180 msec and there is a dead 2 time of about 10 msec between hops. At least 50 hops or 10 3 seconds is therefore required for the reverse paglng 4 messages.
In the preferred embodiment of the invention in which 6 the fre~uencies transmitted by the remote units have been 7 twiddled, the computer rejects any signals which are not 8 associated with a "twin" at the correct frequency 9 difference. This improves the rlejection of the noise by the system and th~s improves the range of detection of the 11 remote units.
12 Use of twiddling also allows for a positlve message 13 transmission mode in which a pulsed signal is transmitted at 14 a given pulse repetition rate by the remote unit at one of lS the two twiddled frequencies. The choice of which of the 16 frequencies is to be transmitted would depend on the 17 message, with one of the frequencies representing a one and 18 the other a zero. This type of positive transmission mode in 19 which a signal is always sent, greatly improves the reliability of the transmission.
21 In summary, a particular remote unit, when activated, 22 transmits a frequency~hopped spread-spectrum signal. The 23 exact frequencies used by the particular remote unit are 24 stored in a memory associated with microcontroller 22 (Fig.
2) and the frequency transmitted at a particular time will 26 depend on the identity of the remote unit and on the time of 27 transmission.
28 In order to overcome the inherent inaccuracies in the 29 timing circuits of the remote units, synchronizing signals are periodically transmitted by one of the base stations to 31 all of the remote units. These timing signals reset the time 32 base in the remote units so that it transmits using a 33 frequency-hopping regime which is recognized by the base 34 stations.
3S Since each remote unit has its own distinctive 36 frequency hopping "fingerprint" which is stored both in 37 microcontroller 20 of the remote unit and CPU 54 of base 38 stations 10; when a remote unit transmits its fingerprint, 17423CAN 12/8/93 - 17 ~ 3~
the base station translates the received signal $nto a 2 frequency value. Within a small number of frequency hops, 3 the computer is able to identi~y the particular remote unit 4 in spite of a frequency deviation in the transmission (or the receiver).
6 Alternatively or additionally, the synchronizing 7 signals may also include a high frequency burst which is 8 used by controller ~0 to correct the base frequency 9 generated by the remote unit's base crystal oscillator. In this case the number of hops required to identify the remots 11 unit is substantially reduced, at the cost of a somewhat 12 more complicated remote unit.
13 Preferably, the signals transmitted by the remote 14 stations include coded messages and destination information lS which are uncoded by the base station which sends the 16 messages to the desired destination.
17 Each base station may comprise antennas and circuitry 18 which enable the CPU to determine the direction of the 19 remote unit. Direction information from a plurality of base stations enables a central station to determine the position 21 of the remote unit.
2~ Additional variations of the invention will occur to a 23 person versed in the art. In particular, power levels and 24 frequency bands will vary from ~urisdiction to ~urisdiction, depending on the local regulations. The above preferred 26 e~bodiment is not exclusive but is meant to be illustrative 27 only. The scope of the invention is defined only by the 28 claim~ which follow.
Claims (20)
1. Paging apparatus comprising:
at least one base station including a base transmitter which periodically transmits timing signals; and a plurality of remote units, each including:
a mobile receiver operative to receive timing signals transmitted by at least one base transmitter;
synchronization circuitry receiving the timing signals and providing a spread spectrum frequency-hopping sequence characteristic of the particular remote unit;
a mobile transmitter producing a radiated frequency-hopped signal responsive to the frequency-hopping sequence and the timing signal; and a message generator operative to modify the radiated frequency-hopped signal to transit a message to the at least one base station.
at least one base station including a base transmitter which periodically transmits timing signals; and a plurality of remote units, each including:
a mobile receiver operative to receive timing signals transmitted by at least one base transmitter;
synchronization circuitry receiving the timing signals and providing a spread spectrum frequency-hopping sequence characteristic of the particular remote unit;
a mobile transmitter producing a radiated frequency-hopped signal responsive to the frequency-hopping sequence and the timing signal; and a message generator operative to modify the radiated frequency-hopped signal to transit a message to the at least one base station.
2. Apparatus according to claim 1 wherein the base station includes:
a base receiver which receives the radiated frequency hopped signal; and signal identification circuitry operative for identifying the remote unit based on the frequency sequence of received frequency-hopped signal.
a base receiver which receives the radiated frequency hopped signal; and signal identification circuitry operative for identifying the remote unit based on the frequency sequence of received frequency-hopped signal.
3. Apparatus according to claim 2 wherein the base station also includes means for determining direction of the remote unit.
4. Apparatus according to claim 1 wherein the remote unit includes means for providing an activating signal to activate the mobile transmitter in response to activation signals.
5. Apparatus according to claim 4 wherein the activation signal is produced by a carrier of the remote unit.
6. Apparatus according to claim 4 wherein the activation signal is transmitted by the bases station and received by the remote unit.
7. Apparatus according to claim 1 wherein the remote unit further comprises a frequency synthesizer which produces a frequency responsive to the resonant frequency of a resonant circuit.
8. Apparatus according to claim 7, wherein the resonant circuit includes a fixed high-Q element and a perturbating element which is periodically switched into the resonant circuit whereby the resonant frequency is periodically switched.
9. Apparatus according to claim 8 wherein the fixed high-Q element is a crystal.
10. Apparatus according to claim 9 wherein the perturbating element is a capacitor switched by a diode.
11. Apparatus according to claim 1 wherein the message generator comprises:
a frequency generator which generates a sequence of pulses each having one of two nearby frequencies in response to an input; and an encoder which chooses a particular pulse sequence responsive to an input by a carrier of the remote unit, whereby the particular sequence represents a message chosen by the carrier.
a frequency generator which generates a sequence of pulses each having one of two nearby frequencies in response to an input; and an encoder which chooses a particular pulse sequence responsive to an input by a carrier of the remote unit, whereby the particular sequence represents a message chosen by the carrier.
12. Apparatus according to claim 11 wherein the base station comprises:
a decoder which receives and decodes the sequence; and a message dispatcher which dispatches the message to an intended destination.
a decoder which receives and decodes the sequence; and a message dispatcher which dispatches the message to an intended destination.
13. Apparatus according to claim 11 wherein the remote unit includes means for providing an activating signal to activate the mobile transmitter in response to activation signals.
14. Apparatus according to claim 13 wherein the activation signal is produced by a carrier of the remote unit.
15. Apparatus according to claim 13 wherein the activation signal is transmitted by the base station and received by the remote unit.
16. Apparatus according to claim 11 wherein the remote unit further comprises a frequency synthesizer which produces a frequency responsive to the resonant frequency of a resonant circuit.
17. Apparatus according to claim 16, wherein the resonant circuit includes a fixed high-Q element and a perturbating element which is periodically switched into the resonant circuit whereby the resonant frequency is periodically switched.
18. Apparatus according to claim 17 wherein the fixed high-Q element is a crystal.
19. Apparatus according to claim 18 wherein the perturbating element is a capacitor switched by a diode.
20. A reverse paging transmission system comprising:
a plurality of portable remote units; and a base station operative to transmit messages to any one of the remote units, wherein the remote unit is operative, when activated by a carrier of the remote unit, to transmit coded messages to the base station and the base station is operative to transmit the messages to a destination indicated by the remote unit.
a plurality of portable remote units; and a base station operative to transmit messages to any one of the remote units, wherein the remote unit is operative, when activated by a carrier of the remote unit, to transmit coded messages to the base station and the base station is operative to transmit the messages to a destination indicated by the remote unit.
Applications Claiming Priority (8)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US93261092A | 1992-08-20 | 1992-08-20 | |
| US07/932,610 | 1992-08-20 | ||
| US07/961,776 | 1992-10-15 | ||
| US07/961,776 US5335246A (en) | 1992-08-20 | 1992-10-15 | Pager with reverse paging facility |
| EP92311872.3 | 1992-12-30 | ||
| EP19920311872 EP0583523A3 (en) | 1992-08-20 | 1992-12-30 | Pager with reverse paging facility |
| EP92311871A EP0583522B1 (en) | 1992-08-20 | 1992-12-30 | Remote position determination system |
| EP92311871.5 | 1992-12-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2104555A1 true CA2104555A1 (en) | 1994-02-21 |
Family
ID=27442462
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2104555 Abandoned CA2104555A1 (en) | 1992-08-20 | 1993-08-20 | Pager with reverse paging facility |
| CA 2104544 Abandoned CA2104544A1 (en) | 1992-08-20 | 1993-08-20 | Remote position determination system |
Family Applications After (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2104544 Abandoned CA2104544A1 (en) | 1992-08-20 | 1993-08-20 | Remote position determination system |
Country Status (3)
| Country | Link |
|---|---|
| AU (2) | AU4476093A (en) |
| CA (2) | CA2104555A1 (en) |
| IL (1) | IL104265A (en) |
-
1992
- 1992-12-29 IL IL104265A patent/IL104265A/en not_active IP Right Cessation
-
1993
- 1993-08-19 AU AU44760/93A patent/AU4476093A/en not_active Abandoned
- 1993-08-19 AU AU44762/93A patent/AU4476293A/en not_active Abandoned
- 1993-08-20 CA CA 2104555 patent/CA2104555A1/en not_active Abandoned
- 1993-08-20 CA CA 2104544 patent/CA2104544A1/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| IL104265A (en) | 1997-04-15 |
| CA2104544A1 (en) | 1994-02-21 |
| AU4476093A (en) | 1994-02-24 |
| AU4476293A (en) | 1994-03-03 |
| IL104265A0 (en) | 1993-05-13 |
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| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Dead |