CA1288819C - Acknowledge back pager with frequency control apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An acknowledge back (ack-back) pager is provide for use in a paging system which includes a central station which transmits a group of message signals to a group of ack-back pagers which are addressed as a group. The users of the group of addressed ack-back pagers indicate a response to their respective pagers thus providing ack-back data. The pagers in the group of addressed ack-back pagers then simultaneously transmit back to the central station their ack-back data on different frequency sub-bands, a different frequency sub-band being allocated to each of the pagers in the group. The invention provides apparatus for controlling the frequency of the selected sub-band with very high accuracy.

Description

38~

CM~0670J
Inventor:
Xazi~i~rz Siwiak ~C~53~ P~G~

F ~SCRI~ION OFJl~A~Ilil~Q:~

~ IG. 1 is a block diagra~ of a convention~l display type radio paging ~y e~O
FIG. 2 i6 a block diagr~m o~ th~ ack-~ack paging ~y~;tem of the present invention. -~0 rIG. 3 ~ a block ~ gram 9~ th@ oentral ~tation employ~d in t~ p~giny ~y~tem o~ FIG. 2.
FIG. 4A ~ ~ ti~e v~. ev~nt represen'cation afthe tr~nsDl~ $s~olnS ~ro~a the central s~eat~on o~ the ~yst¢~ of th~ lav~ntiorl.
15 ~ ~IG. ~B ~s ~ ~epresentation o~ ~n address ~lo~k used in the paging prot~c~1 o~ the p~ging syste~D of the in~ntion.
F~G. ~ repr~entation of a me~6age ~locX
u~ed in th~ p~g~ng protocol o~ the paging ~yste~ of

2 0 the ~nverltion ~IG~, 4D i6 a ti~e v~. ~vent repr~sentation of the r~.eiv~r po~ion oX the centr~1 st~tion.
IGr 4E ~ a tiD~e ~rs. ev~n~ r~pre6entation of the activ~ty of ~ck~acX p~g~r AB-1.
FIG. ~ ne ~. ev~nt repr~Rnt~t~c~n of ~he activ~lty o~ ~ck-back pa~er A~-2.
IG. 4G i~ a ti~ ~V6- ~Yent repr~entation he a~ti~ty ~ ~ck~ ek p~ger l~-M.
3o FIG~ 41~ ~g a ti~e v~ event ropresentation of the ~ctivity o:E ~ non ~ck-b2lck pager 1n ~e paging ~y~t0~ o the irlv~ntil3n.
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~2s~ai~ , FIG. ~II la a lt~ V8. ~vent representat~c~n of the activity o~ ~n unp~g~d ~ck-back pager ~n the paging ~;y~t~m of the invention.
~ IG. 5 ~ a flowchart depicting the operation of 5 the central ~tation ~n 1:h2 paging ~;y~t~m o~ the invonti~n.
FIG. S is ~ bloc)c diagrs~ o~ one o:E the ac)c-back pager~ emplvy~d ~ n the p~ging E;y6tem of tl~e invent~on .
FIG. 7 is ~;ubchanne~ fr~uenc:y look-up table emplc~yed by *he ack~back pager6 in the 6y~tem o~ the in~ntion.
~ IG. ~ i6 a flowcll~:rt of ~he op~ra~ion ~f the ~ck-ba~k pa~er~ of the paging ~y~tem of th~
lS ~nv~ntion.

~hi~ ~nvention rel~t~s in general ~o r~dio 20 co~uni~:at~on~i 6yste~i. ~ore particularly, the ~nvention rel~s to raâio paging ~y~te~ns.
In the pa~t ~Qv~r~l ye~r~, radi~ paging technoll~gy h2~s ~dvanc~d ~ro~ ~e r~t~er ~ ple *one-only pager ~tone ~lerlt: only, no voice), to the tone : 25 ~nd voic~ pag~r (ton~ ~lert wi1:h ~ voi~e ~ne s~ge) and ~ore recen~ly to the ~alp31anu~neric ~i~play pager.
In ~ typ~c~l cc~nv~nti4nal alphanum~ric displ~y paginq ~y~telD ~uch a~ thzt ~own a~ ~y~ n 10 in FIG. 1, a t:en~rAl trarl~itter ~t~tiorl t~r paging terminal 20 is 30 used to gener~te tlh~ æad~s pag~ wh~ch ~re trans2ll~tted via ~ radio link t~ a 1~et of paging rOE~caiver~ 1, 2,. 3 . . .N, wher~n N ~ he to~l number o~ pag~r6 ~ ~;y~tQ?~D ~0. A unique dig~tal addr~ss is as~oci~ad with ~ch i~f p~ging r~c:el~r~ 1, 2, 3...N.
A pa~ ~hich ~- transmitted by paglng ter~in~l 20 .
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consi~t~ Or t21e unique digitally ~ncc-ded address of the par~icular pa~er to whic~ the p~ge is target~d, immedi~tely followed by a corresponding ~ligitally encoded numeric: or alphanumeric page ~essage which is 5 intended ~or di~play on the ~arge~ paS~er.
Typically, the numeric or ~lphanumeric paga ~oe~as~e i~ 6tor~d in a ~Demory wit~in the paging rec~iver for later recall and display hy the pager u~er. ~aging receivers are available wi~h a wide 10 range o~ message storage capabilities which range from the ability to store ju~t a ~ew rather short nu~eric page meRsages to the ability to ~;tore a relatively lars~e number of longer alphanu~eric page ~es~i;ages .
However, conventional displ~y paging systems are generally one way ~ystems. That i~, khe user receives a paging ~essage fr~m the central ten~linal but ha6 no way of respond~ ng to that mess~ge with his or her pager. Instead, the pager u~er DUSt ~eek out a telepbone or other D~eans of responding to the sriginat~r o~ paging ~essage.

BR~;-- W_ Accvrdingly, it i~ one ob~ ect oî the present invention i~ to pro~ride ~n acknowl~d~ ack (ack-,}~aokJ pag~r which il; capable o~ responding bac~c ~o the p~ging ter~in~l and th~ call~r.

3 D Another ob; ~ct of the present inventior~ iE; to provid~ an ack-back p~g~r wh~reby a group of :~ addre~ed ack-back pagers are capable o~
6i~ 1t~neou61y tr~n~mitting acXnowl~dg~ baclc signals ~n a plurality of r~spectiv~ predeter~ined ~ub-band 3 5 requ~nci~ .
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,~, 121388~9 .
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~ nother ob~ec:t o~ the invention ~ ~o pro~ide an ~cknowledge back pager wllic~l can ~electa~ly 'cransmit ac~cnowledge ~ack ~;ignal6 on ~Iny one of the ~f l~rementic~ned ~ublDant~ fr~quencie~ wit}l very high S accuracy.
In one smbodiment of the inven~ion, an ~c}cnowledge b~ck p~ger ~ provid~d or processing paSIing ~gnal~ trans~itted at ~ pa~in~ c21annel fre~uency of F~x ~ro~n a r:entr~l p~ging 6tation. The 10 invention i~ directed to cs:~ntrolling t~e frequency of the ~cknowled~ back 8i5~n~ 6 transmit'ced by t~le acknowledge ~lc pager. The acknowl2dge ~acX pager nclude~ ocal 06cillator oscillating a~ a ~requency FLo to generate an FL.~ signal. ~ first 15 ~ixer i6 C9Up~ to ~e osc~llator ~or ~ixing th~
pa~ing ~ nal~ with ~e FLo ~i~nal t~ generate 8 down-conve~a ~ignal ~t a fregu~ncy Fc. A ~reguency deter~ining ~pparatu~ 1~ coupled to th~ irst ~ixer, for d~temlinlng the frequency Fc 0~ the dswn-20 convert~d ~i~nal. The pager includ~6 a ~ b~nd~;elector for ~eI~tinq one o~ ~ plur~lity of frequ~ncy sub-~n~ on whieh ~aid pager ~ tc~
tr~nsmit ~cknowle~ge 3b~cX ~i~n~l~. The ~ band ~elecl:or deter~ines ~ ~elected su~-band i~nd a 25 re~uency o~cet FD w~th respect to ~:he ~requency F~. The of~et FD corree;pon~ to the ~;elected sub-- ~n~. The p~ger ln~lu~ De~ ry for ~tor~ng the l~ ~e fr~a#ancy FRX ~nd ~ an~ Por :: de~ermini~g the ~1UQ 0~ ~ ~Eor ~he e~ d ~ band 30 wherein ~ FC)~FD ~ Fc). The pag~r ~urther ~n~lud~ vi~er c:$~au~t *or ~l~vi~ing the FLo ~;ignal ~y N to g~ner~te ~n F~N ei~nal. In eonclu~ion, the pager ~nelu~e~ ~ ~econd ~xer, ~c~upled to the ~ivid2r ~n~ o~;c~ or, for ~nixing 35 the FL~ sigr~al w~ ald FLo~N ~;ignal to generate a c~ r ~igna~ for 3~n ac)c-~ck ~ign~l~

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1~88~9 ,.~, ~ he ~o~-ture~ of the ~nvent~on ~eli~ved to be novel ~re speoi~ ally aiet rorth in the ~pperlded c~aims. ~owester, th~ invcntion i~elf, bc~ 6 to it~ Istruc~ure ~nd method of operatiorl, m~y be~ be 5 under6tc~d by r~err~ng to the following de6criptior ~nd the ~c:co~Dpanying dr~wing~.

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8~:L9 DETAILED DESCRIPTION OF ~ rJ~rIoN

FIG. 2 is a simplified block diagram of the acknowledga back paging system 100 o~ the present invention. Paging system 100 includes a central 6tation or paging terminal 110 which is capable of both tran~mitting outgoing paging signals and of receiving acXnowledge back (ack-back~ paging signals.
Paging sy~tem 100 include~ a plurality of ack-back pagers 121, 122... P, wherein P i~ the t~tal number o~
ack-back pagers in the pager population of system 100. Each of ack-back pag~rs 121, 122...P has the capability o~ receiving paging signals from central station 110 and of permitting the pager user to respond to such paging ~ignals. That is, pagers 121, 1220..P permit the user to reply or acknowledge back to a page from central ~tation 110. It is noted that conventional non ack-back pagers such as pager 130 are al~o includabl~ in system 100. In FIG. 2, double arrows between central station 110 and each of ack-back pager~ 121, 122...P are used to denote that two ~: way co~mu~ication ~xist~ between central ~tation 110 ~: and such ack-back pager~. A singl~ arrow denotes that only one way communication exists between ~tati~n 110 and pager 130.
FIG. 3 is a more detailed block diagram of : central station or paging terminal 110. Central station 110 includes a conventional telephone interface 140 of the type generally used for central paging terminals. Telephone interface 140 couples outside telepho~e lines 141, 142, etc. to an input 150A of a microcomputer 150. Telephone inter~ace 140 converts message signals from lines 141, 1~2, etc. to digital signals whi~h microcomputer 150 can process.
For exampl~, a caller wishing to send an alphanumeric ,;, ~`, ~2~81~L9 page to an ack-back pager user uses dual tone multi frequency (DTMF) to key in a desired message.
Telephone interface 140 then converts such analog DTMF alphanumeric message to its digital eguivalent which microcomputer 150 processes as discussed later in more detail. Central station 110 ~urther includes a keyboard 160 coupled to a data input 150B of microcompu~er 150. Keyboard 160 permits an operator to directly input messages into microcomputer 150 for transmission to pagers within the pager popula~ion.
A read only memory ~ROM) 170 i5 coupled to a memory port 150C of microcomputer 150. ROM 170 includes a control pro~ram which control~ the operation of microcomputer 150 and the circuits coupled thereto. A random access memory (RAN~ 180 is coupled to a memory port 150D microcomputer 150. RAM
180 provides temporary storage space for microcomputer 150 a~ it carries ou~ the instructions of the control program within ROM 170.
When a paging message and the identity of the particular pager to be addressed are provided to microcomputer 150; the control program causes microcomputer 150 to generat~ digital paging signals at it~ output 150E accoxding to the protocol later described~ Microcomputer output 150E is coupled via a level shifter 190 to the input o~ a transmitter ~00. ~he output of transmitter 200 is coupled to an `: , antenna 210 having di~ensions and characteristics appropriate to the particular paging frequency channel selected for:the operation of central station : 110. Level chifter 190 serves to adjust the signal : level of the paging signals generated at microcomputer output 150E to a level appropriate for the input of trAnsmitter 200.

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8~g For purposes of this example, it will be assumed that ack-back pagers 121, 122-P are acknowledging back via phase shift keyed (PSK) digital modulation.
Those skilled in the art will appreciate that other forms of modulatibn as well may be employed by acknowledge back pagers 121, 122-P to respond to the paging signals transmitted by c~ntral station 110.
In such a PSK embodiment, c~ntral ~tation 110 i~cludes a r~ceive antenna 220 for receiYing tha ack-back signals transmitted by ack-back pager~ l21, 122-P. In actual practice, antenna 210 ~ay also be employed as antenna 220. Receive antenna 220 is coupled to the input of a PSK receiver 230 ~hich includes an in-pha~e (I) output 23OA and a quadrature (Q) output 230B. Receiver output~ 230A and 230B are respectively connected to inputs 240A and 240B of digital signal processor 240. One digital 6ignal processor which may b~ ~mployed as prooessor 240 is the model DSP56000 manufactured by Motorola, Inc.
Digital signaI processor 240 includes a control input 240C which is coupled to a control output 150F of microcomputer 150 to permit microcomputer 150 to control processor 240. Digital ~ignal processor 240 further includes a data output 240D which is coupled to the data input 150G o~ microcomputer 150. Thu~, it i8 ~een that digital signal processor 240 decodes the digital data received at the I and Q inputs 240A
and Z40B thereof and transforms such information into diqital data which is provided to ~icrocomputer data input 150G.
FIG.'~ 4A-4I are timing diagrams which s~ow the signaling protocol employed by central station llO
and ack-back pagers 121, 122-P. More specifically, PIG. 4A is a simpli~ied timing diagram of the paging protocol transmitted by centrai station 110. In FIG.

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4A, ti~e is represented on the horizontal axis and respective events are denoted as they occur at designated points in time along such time axis.
Central station 110 ~irst transmits a preamble signal 300 during a time interval T1. In one embodiment, preamble symbol 300 consists of a plurality of alternating O's and l's transmitted for a duration of time T1. For example, preamble symbol is a 010101...
signal.
In accordance with the present in~ention, central station 110 groups paging addresses into groups of M wherein M is the number of paging addresses in a particular group. Far purposes of this example, and not by way of limitation, the number of paging addresses and thus the number of messages corresponding to such addresses is selected to be 20 (that ls, M=20). That is, as messages are called into central station 110 via telephone interface 140 or ~eyboard 160, 6uch paging ~essages and corre~ponding address information are held or stored in RAM ~80 until a group of up to M=20 meæsages has been provided to station 110. In alternative embodiments of the invention, non ack-back pages may be intersper~ed with ack-back pages to ; 25 incr~ase the e~ficient throughput of the paging system if desired as will be discussed later. The group of M=20 ack-back pagers is a subgroup of th~
overall population of P pagers. Once station llD has received 20 or M paging messages, microcomputer 150 sequentially transmits the 20 corresponding addresses as a group 310 during a time interval T2 subsequent to time interval Tl as shown in FIG. 4A.
FIG. 4B shows the sequential relationship of each of the addresses within group 310. The address of the fir~t pager of the group of ~ pagers to be ~X~38~

addressed is designated address 1 and is transmitted first in group 310 as shown. The pager to which addres~ 1 corresponds is designated AB-1 for reference. The address of second pager of the group of M selected ack-bacX pagers i8 desiynated address 2 and is transmitted immediately following address 1.
The pager to which address 2 corresponds is designated pager AB-2. This process o~ address transmi~sion CQntinUe~ sequentially in the same fashion until all of the addresses o~ the group of M
pager~ are tr~ns~itted ending with address ~, the address of the last or M'th pager in group 310. The pager to which address M corresponds is designated pager AB-M. A non-ack back pager AB-3 is shown addres~ed in the block of M pages as will be described later in the discussion of FIG. 4H.
In one embodiment of the invention, the duration of time during which preamble signal 300 is transmitted, namely Tl, is approximately equal to 10 msec. Those skilled in the art will appreciate that Tl may have values greater than or less than 10 msec providing Tl i~ suf~iciently long to permit the ack-back receivers 121, 122.~.P to synchronize to the ~ paging ~ignals transmitted by central station 110.
: -25 Appara~u~ ~or synchronizing paging receivers to paging 6ignals i well known to those skilled in the art and i~ included in ack-back pagers 121, 122...P.
For purpo~es of example, the ti~e duration T2 of the group 310 of addresses is ~elect~d to be ~:~ : 30 approximately equal to 1 sec. Those skilled in the : : art will appreciate that T2 may actually be greater vr less than 1 sec depending upon the number o~
paging addresses M selected to be in the group 310 ~:: and the frequency of transmission of the digital data compri6ing such paging addresses. The selection of .

,, ~'' " '-' ~12~88~13 ~2 the time period T2 in this example should not be taken as in any way limiting the invention. To reiterate, the particular pagers of the population P
which are addressed in address block 310 are designated as pagers A~-l (the first pager to be addressed), pager AB-2 (the second pager to be addressed)...pager AB-M (the last pager addressed of the group of M pagers).
After transmission of the group o~ M addresses, central station 110 transmits a reference carrier signal at a frequency FRX at 320 during a time interval T3 ~ollowing time interval T2. Subsequent to transmission of reference carrier 320, central station 110 sequentially transmits the 20 paging message~ corresponding to the 20 paging addresses of address group or block 310. More specifically, thase M or 20 data ~essages are sent a~ a group or block 330 of messages. Each o~ the M messages in block 330 bears a predeter~ined relationship to the order of ; 20 the pager addresse~ in block 310. For axample, in one embodiment of tha invention and as ~hown more ~; clearly in FIG. 4C, message ~lock 330 includes message 1 data followed in time by an end o~ message (EOM) field. The EOM field of me~sage 1 i5 followed :: 25 sequçntially in time by the message 2 data which is in turn followed by another ~OM ~ield. The process of-sending the respective messages 3, 4, etc. within , messag~ block 330 continues until ~essage M is : ~ tran mitted followed by a raspective EO~ field as shown in FIG. 4C.
In the embodiment of the invention described above, the predetermined relationship between the sequence of message~ transmitted in message block 330 : and the sequence o~ pager addresses transmitted in address block 310, i~ conveniently selected such that , , `' .

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address 1 is first transmitted in block 310 and the message 1 corresponding to such address 1 is transmitted first in the later following message block 330 occurring during time slot T4. To S illustrate this predetermined relationship further, address 2 is transmitted second, that is immediately after address 1 in address block 310.
Correspondingly, in the later following time slot T4, message 2 is transmitted second, that i~, immediately following ~essage 1~8 E~M ~ield. The same relationship exists between the remaining addresses in block 310 and messages in block 330.
The invention, however, is not limited to the particular predetermined relationship described above between the seguence of pager addres~es in address block 310 and corresponding messages in message block 330. For example, in another embodiment of the invention, the se~uence of pager addresse-c would remain as illustrated in FIG. 4B with address 1 b~ing 20 sent first ~ollowed by address 2 and 80 forth until address M i8 transmi~t~d completing the block.
~owever, the s~quential order in which the messages in message block 330 are transmitted in ~uch embodiment ~ay commence with transmission of message M first ~ollowed by message M 1 (or message 19~
~ollowed by message M-2 ( 18 ~ and G0 forth until ~ ~ message 1 is finally transmitted at the end of ; ~essag~ ~look 310. (EOM fields are ~till situated between messages.~ What is important here is that a predetermined relationship exists between the order in which the paging addresses are transmitted in addr~ss block 310 to the order i~ which the paginq : : messages are transmitted in message block 330 so as to permit ~cknowledge back pagers AB-l, AB-2,...AB-M
to match a particular message within block 330 to a respective paging address of block 310. This enables a particular pager to determine which of the 20 paging messages in block 330 i6 intended for it, as will be discussed subsequently in more detail.
Although examples have been discussed above wherein the predetermined relationship between the order of the pager addresses of addre~s block 310 and the paging mes~ages of message block 330 are both asce~ding, and in the other example 10 ascending~descending, those ~killed ln the ar~ will appreciate that an arbitrary relat~onship between the paging addresses on block 310 and the paging messages of block 330 may al80 be oelected as long as this predetermined known relationship is programmed into 15acknowledge back pagers 121, 122 . . . P.
A reference carrier exhibiting a frequency of FRX is generated during a period o~ time T3 subsequent to the end of transmission o~ the pager addresse~ in address block 310. In one embodiment of the invention, T3 i. qual to approximately 70 msec.
Those skill~d in the art will appr~ciate that T3 may be longer or shorter than 70 ~sec providing the reference carrier ~hown at 320 exhi~it~ a time duration ~ufficiently long to enable frequency ~5 determininq circuitry, later described, in ack-back pagers 121, 122~..P to determine the frequency of reference carrier 320. ~ ::
,FIG. 4D is a ti~e vs. event diagram of the status of receiver 230 in central station 110.
Subsequent to time period T~, receiver 230 at central station 110 is turned on to receive ack-back ~ignals from the 20 pager~ in the group of M during a time :` period T5. Each of the group of N ack-back message signals transmitted by the respective ack-back pagers in the group of M are on a dif~erent respective .
, , , , ' , : .
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338~
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frequenry sub-band within a common frequency channel as will be discussed in more detail subsequently.
Receiver 230 is thus capable of distinguishing and decoding message signals on each o~ the 20 or M
S different sub-band frequencies. The con~iguration and operation of receiver 230 is discussed in more detail later.
FIG. 4E i~ a time versus event diagram for the status of ack-back pager AB-1, that is, the first addressed pager o~ the group of ~ pagers. FIG. 4E is drawn to the same time scale as FIG. 4A. During the T1 time interval, pager AB-l receives the preamble at 340. During the following time period T2, pager AB-l receives and decodes address 1, which in thi~ example is the address of pager AB-l. It is noted that prior to reception of t~e pr~a~ble at 340, pager AB-l is in a ~sleep~ or ~battery saver~ stateO That is, prior to such Tl time period, pager AB-1 and the other pagers of the pop~lation o~ P pagers, have se~eral o~
their power consuming circuit6 turned off or placed in low power consumption ~tates. Those skilled in the art are already familiar with the powering down of radio pager c~rcuits in ~rder to achieve ~attery ~aving and thus exactly which circuits in the pager are powered down, and the degree to which they are powered down, are not discussed here in detail. ~hat is important, however, is that the ack-back pagers of the population of P pagers are placed in a ~battery saving~ state or ~sleep state~ during prescribed periods of time such as that mentioned above and which will be later speci~ied.
; When pager AB-1 receives the preamble 340 during time period Tl, pager AB-l is switched from a battery saving ~tate to a fully operational ~tate such that pager AB-l is capable of receiving :~

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information transmitted thereto. That is, subsequent to reception of the preamble at 340, pager AB-1 is fully turned on ~uch that pager AB-l receives and decodes its address at 350 at the beginning of the T2 time period. In one embodiment of the invention, pager AB-1 conveniently returns to the ~sleep state~
~or the remainder o~ the T2 time period during which pager addresses are transmitted. Prior to receiring the reference carrier FRX ~t time period T3, pager AB-l is raturne~ from the ~sleep state~ to the ~ully operational state. Upon reception of the reference carrier, FR at 360, pager AB-l determines the frequency of such carrier in a manner described in more detail subsequently.
Re~erring to FIG. 4E, in conjunction with 4CJ
it is seen that the message l transmitted during time period T4 at 370 is received by pa~er AB-l at 380 as shown in FIG. 4E. Pager AB-1 receives me~sage 1 at 380 and ~atches message l to address l. That is, by means later described in more detail, pager AB-1 is programmed to determine that messa~e 1 i the particular message of the group of ~ messages which is intended ~or pager AB-l. Subsequent to reception and display of message 1 at 380 as shown in FIG. 4E, the user o~ pager AB 1 indicates his or her response to message 1 during a time period T6 at 385. Time period T6 i~ not drawn to æcale with respect to the ~ther time periods discussedO Time period T6 is sufficiently long to per~it indication of a respon~e 3 0 by the pager user. Subsequent to time period T6, :~ pagers AB 1, AB-2 ... AB-M simultaneously transmit acXnowledge back signals on respective ~requency sub-bands (subchannels) back to central station 110 as at 390 during a time period T5. Subsequent to the ack-back transmission at 390, pa~ers AB-1, AB-2 ... AB-M

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-are placed in the ~sleep state~ until awake~ed again by a preamble as at 340. In an alternative embodiment of the inve~tion, ack-bac~ pagers AB-l...~B-20 reply back automatically without action by the pager user. In such an embodiment, prior to being paged, the user preselects a reply already stored in the pager or key~ into the pager a predetermined mes~age which the pager uses ac the ack back reply when it i~ later addre~sed by central station 110. For example, the ack-back pager user selects a 'not availabl~ response or otherwise keys into the pager ~ ~not availabl~ response when the pager user wishes to inform callers into central station 110 that the pager user is not taking any calls curr~ntly. Clearly, the reply data may be provided to the ack-back pagers in many different ways. In the ca6e of a user selectable response already programmed into the pager, time period T6 can be arbitrarily short, that is just sufficiently long enough to permit transmi~sion o~ such a selectable respon6e whQs~ length is predetermined and known to the microcomputer 150 in central s~ation 110.
: FIG. 4F is a time versus event diagram of the status of ack-~acX pager AB-2, that i~, the second pager addxessed of the group of M ack-back pagers.
Pager AB-2 receives the prea~ble at 340 and then switches from a ~sleep state" to a fully turned on : state. Pager AB-2 receives address 1 (the address of pager AB-1) at 350. Pager AB-2 decodes such address 1 at 350 and determineæ that the dec~ded address is not its own ~ddres~. At 400, pager AB-2 receives its own address, nam21y address 2. Pager AB-2 decodes and determines that address 2 is its own address. As with pager AB-l of FIG. 4E, pager AB-2 of FIG. 4F
- 35 goes to the ~sleep state~ for the remaind~r of the T2 , ' : , , .
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12l388~

/~
time period. Pager AB-2 ~wakes up~ in time for reception of the reference carrier FRX at 360 during time period T3. As seen by examining FIG. 4F in conjunction with FIG. 4C, pager AB-2 receives the AB-l page data transmitted at 370 within time period T4. As explained in more detail subseguently, paqer AB-2 det~rmines that the AB-1 message data is not a match. That i~, pager AB-2 detarmines that the pager AB-1 message data (message 1) is ~ot intended ~or pager AB-2. After the end of ~essage (EOM~ mark~r following message 1, pager A9-2 receives the AB-2 messag~ data (message 2) at 410 within time period T4. Pager AB-2 determines that the message 2 data at 410 is a match and that such messaqe 2 data is intended for AB-2. The message 2 data is then displayed to the user of pager AB-2 who indicates an acknowledge back response to pager AB-2 during time period T6 at 4 ~5 . During the subsequent time period T5, the acknowledge back message is sent to central : 20 station 110 on a second ~requency sub-band differ~nt : from the ~irst frequency sub-band on which pager AB-l transmits. Subsequent t~ transmission of the ;~ acknowledge b~ck response at time period T5, pager A~-2 is caused t~ g~ tc sleep.
FIG. 4G is a time versus~event diagram of the status of ack-back pager AB-M, the last of the group of M pagexs to be addressed. Pager AB-M receives the preamble at 340 to switch it from a ~battery saver state~ to a fully operational state. Pager AB-M then ; 30 receiv~s the }9 addresses of the other pagers in tha ~ group of M, sucb as at 350 and 400 until finally :~ pager AB M receives and decodes its ~wn address at420. Pager AB-~ is thus signaled that a message for ~; ; it will be transmitted m~mentarily. Pager AB-M
receives the re~renca carrier signal ~RX at 360.

:' ' ~ ' ` '', ' ' , ~2~881~t -,~
Referring to FIG. 4G in conjunctio~ with FIG. 4C, it is seen that pager AB-M receives message 1, message 2 ...message M-l and determines that all of these messages are not mat~hes. That is, such page data message6 are not intended for AB-M. Pager AB-M
receives the page data message M transmitted at 430 rFI~. 4C) and received at 440 (FIG. 4G) within time period ~4. Pager AB-~ determines that ~uch message M
at 440 i5 intended for pager AB-M and displays the contents as such message M to the pager user. During time period T6 at 415, the pager user s~pplies ack-back pager AB-M with an acknowledge back response.
During the subs2guent time period T5, pager AB-X
sends such acknowled~ back response back to the central station 110 on a frequency sub-band M at 45 different fro~ the frequency sub-bands on whi~h the remaining ack-back pagers AB-ll AB-2 ... AB-(M-l) ~ransmit. Subseguent to the transmission of the ack-back response at 450 during time period T5, pager AB-~ switche~ to the ~sleep ~tate~.
one e~bodiment of the invention accommodates the ~ituation where one or ~ore of pagers within the group of M pagers are not ack-back pagers. For example, it will be as umed that pagar AB-3 is not a pa~er with acknowledge back capability, but rather is an alphanu~eric display pager which operates as shown in the time versus status diagram of FIG. 4H. Non : ~ acX-ba k pager AB-3 recei~es a prea~ble at 340 which causes pager AB-3 to switch from a ~sleep state~ to a fully operational state.~ Subsequent to receptio~ of the preamble at 340, non ack-back pager AB-3 receives address 1 at 350 and address 2 at 400 during time interval T2. In this par~icular example, it is assumed that pager AB-3 is the third pager addressed within time interval T2. That i8, address 3 is the ~38~3~9 --2~
address which corresponds to pager AB-3. Pager AB-3 receiv~s address 3 within time interval T2 at 460 as shown in FIG. 4~. Pager AB-3 decodes addres~ 3 and determines that pager AB-3 has been paged a~d that a page data message will be transmitted to ~t shortly.
Non ack-back pager AB-3 is activated to an ~awake state~ during time interval T4. Pagar AB-3 then locates the particular AB-3 page me-~sage which i~
intended for it within ti~e period T4. That is.
since the predetermined relationship ~etween the order of the page messages transmitted within time period ~4 is known by pager AB-3 with respect to the order of the addresses transmitted within a time period T2, pager AB-3 locates or ~at~hes the AB-3 page data message at 470 in a manner æimilar to that employed by the remaining pagers within the group of M. For example, in this e~bodiment of the in~ention, since pager AB-3 was the third pager to be addressed in the group o~ M pager~, pager AB-3 will expect its mes~age to likewise be third in the sequence of messases with ~essage block 330 (FIG. 4A) or more specifically at 470 of ~Ig. 4N. Once message 3 is so selected, pager AB-3 displays ~essage 3 ~o the pager user. In thi~ particul~r embodiment, the pager user does not have the option to transmit a response back : to the central station 11~. Thu~, non ack-back pager AB-3 i~ switched to a ~sleep state~ after the AB-3 : message corresponding thereto has been r~ceived.
FIG. 4I is a ti~e versus event diagram of the status o~ an unpaged ack back pager of the population ` of ack-back pagers 121, 122,.. P. That i , FIG. 4I
illustrate~ what occur~ when an ack-back pager receive~ and decodes addresses which do not correspond to the unique address of such unpaged pager~ ~ore ~pecifically, the unpaged pager, which ' ~ ', ' `

' ~ -: ' ' ';
, ' " ' ' iL2~38819 ~r is referred to as pager AB-U, receives the preamble signal at 340 and switches from a ~sleep ~tate~ to a fully operational ~tate. Pager AB-U then proceeds to receive a group of M or 20 pager addresses at 480 during time interval T2. Pager AB-U fail~ to find its address within that group of M addresses. Thus, after time p~riod T2, pager AB-U return~ to the ~slaep state~ where it will remain where it will remained ~or a predetermi~ed period of time.
Altern~tively, at the end of address block 480, a ~go to sleep# signal can be transmitted to all pagers which did not receive a valid address to cause such pagar~ to enter the sleep ~tate. FIG, 4I also represents the time versus event ~tatus of an unpagad non ack-back pager.
FIG. 5 is a flow chart of the control program which is re~ident in ROM 170 of central ~tation 110.
This control proqram controls the operation of microcomputer 150 in the manner which follow~. The flow chart of FIG. 5 6ummarizes the operation of ~: central BtatiOn 110 which was described above in th~
di~cu~sion of the ignalling protocol illustrated in : FIGts. 4A - 4I. In accordance with block 500 of the : : :flow chart o~ FIG. 5, microcomputer 150 is subjected to a power-on reset when it i~:turned on. That is, : system Yariables are initialized at that point in : ~ ti~e, For example, M, which is the number of ack-back pager~ in a particular group is initialized at a predetermined number, for example 20. Additionally, a mescage counter variable, I, is initialized at a : vAlue of O in block 500. Once initialized, central station 110 is ready to accept messages ~rom telephone callers into interface 140 or from a system operator at keyboard 160 as per block 510. When a me~sage for a particular pager u6er is inpu~ into ..
.' ' ~L2~388~L9 -~/

central station llO, such message is stored in RAM
180 together with indicia of the particular pager for which such message i6 intended as per block S20.
Such message is counted by incrementing the message counter variabl~ I by the quantity l as per ~lock 530. Microcomputer 150 then makes a determination as to whether the number o~ ~essage~ which have been collected and stored in memory is equal to M or 20 in this example. That i8, as per decision block 540, microcomputer 150 determines whether ~essage counter I equals M. If the ~essage counter I does not equal M, ~hich signifies tAat a group of M messages have not yat been fully collected, then flow continues to block 545 where a determination is mad~ whether or lS not a time out of T0, for example T0 = 10 ~ec, has been exceeded. I~ the time out has not been exceeded, then flow continues back to input block 510 to await input o~ yet another me~sage. If in blocX
545 it deter~ined that the time out has been exc~eded, then a preamble ~ignal i5 transmitted at block 550. This time out feature is provided such the ~icroco~puter 150 will not have to wait for long periods o~ time for a gueue of ~ message~ to be collect~d pri~r t~ transmitting ~uch messages. I~
prior to expiration of the time out, it is determined that ~eGsage counter I doss e~ual M at block 540, then trans~ission of th~ preambl~ ~ignal i8 commenced : at block 550.
~icrocomputer lSO then looks up and retrieves from memory the addresses which correspond to each of the group of M pager~ as seen at block 560. The addresses within such group of ~ pagers are sequentially *ransmitted in a predetermined order, ~or example, ~first in last out' or ~first in first ~ 35 out~, as per eubseguent blocks 57Q through 610. More : , .
- : ' :
.. . .

,.

~2~

2~3-specifically, counter I is reset t~ 1 and now functions as an address counter as per blocX 570.
Address I is retrieved from memory as per block 580.
That is, in the first time through the loop ~tarting 5 at 580, since I = l, address 1 is retrieved from memory. That is, microcomputer lS0 looks up the particular pager address which correspond~ to the pager for which message l i8 intended. Address 1 is then transmitted as per block 590. At decision block 600, micro~omputer 15Q makes a determination of whether or not all M addresses of the group of ~
addresses corresponding to the M messages have been transmitted. ~his is determined by microcomputer 150 calculating whether or not I is equal to ~. If address counter I i8 not equal to M, then all 20 addresses have not been transmitted and I is then incremented by 1 a~ per block 6100 Flow then continues back to block 580 at which the next address of the group of M=20 addresses i8 retrieved from : 20 memory. Thi~ proces continues unti:l I = M at block 600 which ~ignifie~ that all 20 ad~re~ses have be~n retrieved and sequentially transmitted as a group.
Flow then continues to block 620 at which reference ~; carrier ~C is transmitted.
: 25 Counter I is then reset to I = 1 as per block 630. Counter ~ is now employed as a m ssa~e counter : again in the ~ubsequent portion of the flow ch~rt of FIG. 5. Nessage I is retrieved from memory a~ block 640. ~he ~irst time through the loop starting at ~: 30 block 640, I is e~ual to l and thus message numker 1 is retrieved at blo¢k 640 the first time through such loop. Message I, or in thi~ case message ~, is then ~: transmitted by central ~t~tion 110 as per block 650.
~n end o~ message (E0~) marker is transmitted : 35 immediately æubsequent to message 1 to mark t~e end ." ~
.
` ` ' .~' . `
.: . :' ' , ~

, , .
, ~2~388i~3 .~
of such message as per block 660. A determination is then made at decision blocX 670 as to whether or not all of th~ messaqes in the group of M messages have been r~trie~ed from memory and ~ransmitted. This is acco~plished by microco~puter 150 making a determination as to whether I is presently equal to M. If microcomput~r 150 finds that I i~ not yet equal to M, then I is incremented by 1 as per block 680 and flow continues back to retrieve message block lo 640. The next message, for example message 2, i~
then retrieved ~ro~ ~emory as per block 640. Message 2 is then trans~itted a6 per block 65Q and followed by an end o~ message ~EO~) marker as per block 660.
This process csntinues until ~inally all M mPssages have been transmitted follow2d by respective EOM
marker~. It is thus seen that the M mes~ages are transmitted as message group.
From the flow chart o~ FIGo 5 ~ it will be observed that the group of messages trans~itted as per block 640 through 680 bear~ a predetermined order relationship with respect to ~he order o~ tha transmis5ion o~ the addre~ses ~f the corresponding : group of ~ adre~ses a~ per blocks 570 through 610.
That is, in thi~ partioul~r example addr~ss 1 was ~irst transmitted, ~ollQwed by addre c 2 and so ~orth up to addre~s ~ thi~ example, the transmission of the group of ~ mes~age ccur~ in the same srder as the ~roup of adresses. That is, mes~age 1 corresponding to the first address i~ f irst transmittad followed by message 2 which corresponds to the second address and 80 ~orth up to message M
which corresponds to the ~'th addressed pager. Other predetermined ralationship orders are possible between the order of transmission o~ the messages o~
the group of ~ messages and the order of the group of , ~ .
' , .

~L~

M addresses as has been discus~ed earli~r~ What is important, is that such predetermined relationship between the message order and th~ address order is known and is programmed into the acX-back pagers as i8 discussed later in more detail.
After it i~ determined that the transmission of the group sf M messages is complete as per ~lock 670, flow continues to block 690 at which central station llo pauses to permit the ack-back pager users which lo have received ~essages to key an appropriate r~sponse into their acX-back pager~ for trans~i~sion subsequently back to central station 110. For example, such ack-back pager~ may include a keyboard or a switch that is toggled by the message recipient to signify a yes or a no. It will be appreciated that it will take ~ignificantly less time for a user to toggle one key to indicate ~ predetermined response, for example a yes or a ~canned message~
(for example, I w.ill call you back), than it would take for a user to kQy in a respon e on a keyboard or keypad situated on the pager. However, 6uch keyboard or keypad e~bodiment~ of the ack-back pager herein are considered:to be within the ~cope o~ the invention in that t~ey provide alterna~ive ways of : 25 ind~cating the user's response to the ack-back pager.
After pausiny to permit the addressed~pager user to key in their responses, central sta~ion 110 : simultaneously receives M acX-back 6ignal~ from a group of M addressed pagers as per block 700. These ack-back responses are then provided~to the ~ appropriate corresponding callers via telephone :: interface 140. Flow then continues back to block 510 to p~rmit other paging messages to be input into central station llo.

, ., ~38~9 FIG. 6 is a bl~ck diagram of one of ack-back pagers 121,122...P., namely ack-back pager 121. In one embodiment of the invention, ack-back pagers 121, 122...P transmit ac~nowledge back signals on the same radio frequency a~ that on which central 6tation 110 transmit~ although this is not necessarily a requirement of the system. That is, other embodiment~ of the invention ar~ contemplated wherein t~e ack-back pa~er~ trans~it ack-back ~ignals at ~requencie~ other than within the spectrum of the paging channel employed by central station 110.
However, in the present embvdimentj circuitry is included within such ack-back pagers to enable the pagers to accurately tune to and transmit ack-back signals ~t di~ferent sub-bands within the same paging channel spectrum as that empl~yed by central station 110 for transmission of paging signals. Mare specifically, each of ack-back pagers 121, 122...P is capable of transmitting ack-back signals on a plurality o~ ~ different sub-bands within the paging frequency channel on which central ~tation 110 and : the ack-back pagar& trans~it and r2ceive. All of the ~: ack-back pa~ers wi~hin a particular ~roup of M
addre~sed ack back pagers simultaneously transmit acknowledge back signals back to central station 110 during a time period occurring after ~uch gr~up ~f M
~: : ack-back pagers are addreæsed and are ~ent respective messaqes. To:per~it such simultaneous transmission of ack-back signals on:~ different frequency sub-: 30 bands via frequency division multiplexing (FDN), it : has been found that pagers 121, 122.. ~P must be able to tune to each of the ~ di~erent sub-bands with extreme accuracy in frequency. The subsequ~ntly : : described g~equency control circuitry within ack-back : pager 121 permits ~uch accuracy in Rub-band frequency ::
~ ~ .
.: .
: ' . ' :, .
:~ ' ' ;
,~ .

38~

,'~
tuning. An example of one 6ingle conver6ion receiver whlch ~6 adapt~ble to accommodate the a~orementioned freguency control ~ircuitry $n accordance wit~ the present inve~t~on 1~ ~e ~otorola Sen~ serie6 display pager a~ described in the publication 'Sen6ar~ Sexies - Di~pl~y GSC Radio Paqer~, Motorola ~ublie~tion No. 6~P81Q38C75-A.

Ack-~ck pager 121 include~ ~ tran~mitO~eceive ~nte~na 800 ~xhlbiting ~n appr~pr~ate ~i~e and geometry t~ per~it tr~n~mi~sion ~nd r~c~ption ~f radiD ~reguency 6i~nal~ on the radio ~r~guency paging channel on which c~ntr~l 6tation 110 tranGmit~ and recei~e~. Antenn~ ~00 i~ ~oupled to a ¢ommon port 810A o~ a tran~mit r~ceiv~ ~w~tc~ 810. Transmit~
rece~ve ~witch 810 include~ ~ r~ceive p~rt 810B and 8 transmit port QlOC ln ~ddition to the ~ve ~entioned ~nten~a lnput port ~lOAo Switch SlO ~nclude~ a I control lnput 810D ~ show~ in FIC. 6. ~he~ an ¦ 20 ~ppropr~ate control ~nput ~ign~ upplied tc ~ontrol ~nput 810D, tr~n~mit/r~ceive ~wit h ~10 coupl~ ~nt~nn~ port 810A to rQcei~ port ~10~ ~o ~l~ce p~g~r ~2~ in the r~c~ive ~d~. Altarnative~y, pager ~21 ~ pl~ n ~he tr~ns~it ~o~ when an ~ 25 ~ppr~pr~te e~ntrol ~ign~l ~6 ~uppl~ed to ~ntrol : input 810D such th~t trsnsmit rece~v~ ~witch ~10 c~upl~ t~ ~ntenn~ ~np~t por~ ~lOA tD tran~mit port 81VCo ~h~e control ~gn~ls aro ~uppl~d to eontrol input 810D by microcD~puter ~20. One ~icroprocessor whic~ ~ay b~ ~mployed ~ ~icrocompu~er 820 ls the ~del ~C146~70562 manufaeture~ by ~otorola, Inc.
Rec~ive p~rt 810B ~ ~wi~h ~10 ~ ~oupl~d to the input Df ~ radio freguency a~pli~i~r ~30. ~t is noted th~t t~e ~requency of ~he r~dio paging channel on which central 6tation llO tr~n~it6 i~ defined to * Trademark , . .

~ 2~88~g be FRX, for example, 150 ~Hz. Thus, the radio frequency paging signals which reach ack-back pager 121 and which are provided to amplifier 830 exhibits a frequency of F~x or 150 ~Hz. Amplifier B30 amplifies the radio paging ~ignals from central paging station 110 and provides such ampli~ied signals to the input ~f a bandpa~ filter 840.
Filter 840 is typically ~f the preselector type which filters off any undesired signals adjacent the paging channel frequency.
The output of ~ilter 840 i6 coupled to`an input 850A of a two input mixer 850~ ~ixer 850 includes inputs 850A and 850B and an output 850C. A local oscillator 860 which oscillates at a frequency of FL~
is coupled via an amplifier 870 to mixer input 850B.
Mixer 850 down-converts the ~iltered RF paging ~ignal at frequency ~RX thereto by mixing ~uch signal FLo signal. In this ~anner, the down converted ~F signal generated at the output 350C o~ mixer 850 is at an inter~ediate frequency o~ FRX-FLo which is d~fined to equal Fc.
Mixer output 85~C i~ coupled to the input of an : intermedi~te freguency (IF) amplifier:890 which amplifie~ the down-converted RF paging ~ignals. The : 25 ~utput o~ IF amplifier 890 is coupled to a count input 8aOA of microcomputer 820 to determine the down-converted re~erence carrier frequency Fc as , later ~escribed. The~output of IF ampli~ier 890 is : also coupled to the input a demodulator 900 which demodulates the down-converted RF paging signals provid~d there~o. That is, demodulator 90~ separates ; the preamble, address~ and message signals from the carrier wave on which they were transmitted by central station 110. The data signals thus resulting are provided to ~icrocomput~r input 820B via a . `.
.' ,' '.
., , 12~ 9 . ,~g connection to demodulator 900 as shown in FIG. 6.
Such data signals include preamble, address, and me~sage signal~. Microcomputer 820 of pager 121 decodes the addres~ signals provided at data input 820B and compares the incoming deGoded page addresses with the predetexmined unique addres~ of such pager 121 which i6 stored in a code memory 910. Code memory 910 i5 typically an electronically erasable program~able read only memory (EEPROM3 such that unique pager addres6 code~ are easily a6signed and programmed into each of ack-back pager~ 121,122...P.
As seen in FIG. 6, memory 910 is coupled via a buss to a memory port 820C o~ microcomputer 820. When microco~puter 820 determines that one of the addresses in a received group o~ M pager addresses correspond6 to the unique addres~ o~ 6uch pager 121, then ~icrocomputer 820 decodes the following group of M messags. Miçrocomputer 820 selects which of such messages is intended for pag~r 121.
In a known fashion, microcomputer 820 generates appropriate output signals which are supplied via linear 6upport ~odule 920 to ~udio module 930 ~nd speaker 9~0 to alert the pager user that a messag : ha~ be~n received. The selected ~e~sage i~ ~tored in a random access memory (R~M) 950 which is coupled via a bus to micrGcomputer memo~y port 820D. A liquid cryst~l di~play modul~ 960 i~ coupled to the display ou~put 820E Or microcomputer 8~0 ~uch that the selected message re eived by pager 121 can be displayed for viewing by the pager user.
Alternatively, the pager u~er can recall the page message from memory 950 subsequent to the alert signal for viewing later at a more convenient time.
A clock circuit 970 i~ coupled to a clock input 820F

12~381~

3~
of microcomputer 820. Clock 970 provides microcomputer 820 with a reference time base.
A user reply input device 980 i~ coupled to a data input port 8ZOG of microcomputer 820 a~ shown in FIG. 6. In one embodiment of the invention, the user reply input device 980 i~ a four po~ition switch, the positions of which are respectively designa~ed as choices A, B, C, and D. By preagreement between the pager user and the pager caller, each of choices A, B, C, and D is agreed to have a predetermined meaning. For example, choice A when selected by the pager user could be a ~Yes~ response to the caller's message. Choice ~ could be aNo~ response. Choice C
is a ~Maybe~ response and Choice D is a ~Cannot Reply Now~ response~ Those skilled in the art readily appreciate that the output o~ such a four position switch when used in input device 980 i~ readily converted to a digital signal which i~ ~upplied to data input port 820G for processing~by microco~puter 820. Altarnatively, a 2 position or YES/NO switch could be employed in user-input device 980.
It is noted that user reply input device 980 is not limited to the ~ulti-position switch which was discus6ed above. Rather, other input devices, for example~ a keyboard or other key entry ~evices may be : employed as user input device 980 in other embodiments o~ the invention to generate reply data.
The reply data i8 then transmitt~d back to central station 110 by pager 12I during acknowledge back reply field 390 as shown in the acknowledge back protocol 6hown in FIG. 4E. The paging channel centered around frequency FRX i~ ~ivid~d into ~
different sub-channels. Bach pager of the group of M
ack-back pager~ which were addressed now respond back si~ultaneously as ~ group during the appropriate .
:
'~ , . .
' ' : .'' ' ' ' '. ~

381~

_3O

acknowledge back field. Each o~ the M pagers of the group responds on a different frequency sub-band wîthin the group o~ N sub-bands. In one embodiment of the invention wherein M=20, the paging channel is divided into 20 different frequency sub-channels or s~b~bands which are center~d around a frequency FRX
and are separated by sub-channel spacings of approximately 1 kXz. That i~, each of the 20 ~ub-bands, designated sub-bands 1-20, is of~et 1 kHz with respect to each other a~ shown in the table of FIG. 7~ The table of ~IG. 7 ~hows each of pagers AB-1, AB-2 ... AB-20 of a group of M addressed pagers and frequency information with respect to the respective sub-channels or sub-band~ on which such pagers acknowledge back or respond. For example, in one embodiment sf the invention wherein the centsr of the paging channel is at a freguency FRX equal to 150 M~z, pager AB-l of the group of M addressed pagers acknowledgeæ back on a freguency of 149.990S MHz which csrresponds to an off6et, FD, of .0095 MHz with respect to the FRX center channel fraquency. In a 6i~ilar fashion, the pager of the group of M
addressed pager6 which ~ designated as pager A8-2 acknowledges back on a second 6ub-band having a freguency o~ 149.9915 ~Xz whlch corresponds to an offset, FD, of -.0085 M~z with respect to the FRX
center channel frequency. Continuing on with pagers AB-3, A~-4 ... AB-20, 6uch remaining pagers respond back on the different subchannels specified by the0 frequencies and offsets shown in the tsble of FIG. 7.
Each of the group of M pagers designated AB-l, AB-2 ... AB- 20, and in fact all of the pagers of the population of P ac~nowledge back pagers are capable of acknowledging b~cX on any one of the M different frequency ~ub-bands. That i8, the control program 12~38819 , .~/
,.3~
stored within memory 910 i8 capable of directing microcomputer ~20 and associated frequency synthesis circuitry later described to transmit acknowledge back signàls on a selected one of the ~ or 20 different sub-bands.
In more detail, the output of amplifier 870 is coupled to the input of an amplifier 990. Thus, an amplified replica of the FLo signal from local os~illator 860 ap~ear~ at the ~utput of amplifier 990. ~he output ~f ampli~ier 990 i~ coupled to input lOOOA of a mixer 1000 having inputs lOOOA and lOOOB.
In thi~ manner, the ampli~ied local oscillator signal i6 provided to mixer input lOOOA~ The output of amplifier ~90 is also coupled to the input of a divide by N circuit 1010. Divide by N circuit 1010 is a progra~mable prescaler which digitally divides the FLo signal by an integer value N. Such divider circuits are well known to those ~killed in the art and are readily available ~rom ~ny commercial sources. The divider circuit ~elected for divider circuit 1010 exhibits an allowable range of programmable divider~, N, between 2048 and 8192 and can accommodate an input ~requency near 150 MHæ for thiæ particular embodiment of the invention. Divide ~ 25 by N circuit 101~ is coupled to an input 820H of :; : microcomputer 820 ~u~h that the value selected for N
~ is provided to divider 1010. ~he frequency of the : ~ignal which is generated at the output of divider ~ circuit 110 is equsl ~o FL~/N. The output o~ divider circuit 1010 is coupled to an input 1020A of a two : input exclu~lve OR gate 1020. Exclusi~e OR gate 1020 i6 employed aæ a pha6e modulator and includes inputs 1020A and 1020~. The remaining input 1020~ of exclu~ive OR gate 1020 i~ coupled to the reply data ; 35 output 820I of microcomputer 820. In this manner, .

reply data which includes indicia of the acknowledge back response provided by the pager user at user reply input 980 i8 provided to exclusive OR gate 1020 for phase modulation onto to FL~/N signal provided at input 1020A. A phase modulated ack-back signal is thus generated at the output of exclusive OR gate 1020.
The output of exclusive OR gate 1020 is coupled to input 1000B of mixer 1000~ In thi~ manner the phase modulated ack-back signal exhibiting a frequency of FL~/N i8 mixed with the FLo ~ignal euch that the ack-back transmit fre~uency of FTX at the output of mixer 1000 is equal to FLo plus FLo/N. The output of mixer 1000 is coupled via a bandpass filter 1030 to a power amplifier 1040. Bandpass filter 1030 filters any undesired signal components from the FTX
signal. ~mplifier 1040 amplifies the filtered FTX
signal up to a signal level sufficient for transmission back to central station 110. The output of amplifier 1040 i5 coupled to the trans~it input 810C of transmit/receive ewitch 810. It is noted that there i8 a pr~determined relation~hip between the particular sub-band frequency on which each of ack-b~ck pagers A~ AB-20 responds to either the order of each pager's particular addréss within the group of ~ pagers or the order of each pager's particular message within the group of M pagers.
From the aarlier discussion, it:will be recalled that the order of the messages within a group o~ M
messa~es bears ~ predetermined rel~tionship to the : order in which the addre ses for such message~ were transmitted in the corresponding address group. The relationship between the selection of frequency sub-bands for ack-back transmission and the order of transmission of the N addresses or M messages is ~, ~
: , : . , . .~ .

:

-established to enable microcomputer 150 in central station 110 to determine which ack-back signal sub-band transmission corresponds to which acknowledge back pager address of the group o~ M pagers.
For example, assuming that pager AB-l in the table of FIG. 7 i8 the first ack-back pager of the group of M pagers to be addressed or receive a messag , then, ack-back pager AB-l responds back on a sub-channel or sub-band freguency designated sub-band 1 which corresponds to the frequency and offset noted in Table 1. Assuming that pager AB-2 in the table of FIG. 7 is the second pager of the group of M pagers which is addressed or sent a message, then, pager AB-2 acknowledge back on sub-band number 2 which corresponds to a frequency and offset shown in the t~ble of FIG. 7. To continue this example, assuming that paqer AB-20 is the twentieth pag~r of the group of M pag~rs to be addressed or receive a message, then pager AB-20 acknowledges back on a sub-band frequency 20 which corresponds to the frequency and offset ~hown in the table of FIG. 7. Although each of pa~ers AB-1, AB~2 ... AB-20 responds back on the different respective ub-band6 1-20 noted in FIG. 7, all of such pagers respond back simultaneously in a common time 810t or field a6 already described.
It is n~tad that other predetermined relationships between the ack-back sub-band order and the order in which the addresse~ or messages were transmitted to the group of M pagers may be employed.
That i8, although i~ the example above, the order of : the M addres~es ~or M mes6ages) and the corresponding order of the N sub-bands are ~oth a~cending, in another embodiment of the i~vention in which the order of the addresses of the group of M pagers AB-l ... AB-20 is the ~ame as the prior example ~2~

(asc~nding~, the order of the acknowledge back sub-bands is reversed as compared to the prior example (descending~ That is, pager AB~l responds back on sub-band 20; pager AB-2 responds back on sub-band 19 ... and pager AB-2~ respond back on sub-band 1.
Also, as mentioned briefly earlier i~ this document, alternatively in another embodiment ~f the invention, the relationship between the order in which pager addresses or ~essages were re~eived by the group of ~ pagers and the order of assignment o~
sub-bands ~or ack-b~ck to such M pagers can be arbitrary. What is important is that a predetermined relationship exists between the order o~ assignment of sub-bands and the order in which the pager addresses or messages arrive at the group of M
pagers. Again, thi~ predetermi~ed relationship i~
program~ed into memory 170 of microcomputer 150 in central station 110 such that microcomputer 150 can determine which sub-band is being used by each of the pager~ AB-l, A~-2 ... AB-20 as ~hey ac~nowledge back.
An example i8 nDW presented showing how one of the ~B-l, AB-2 ... A~-20 pager6 ~elects a ~ub-band freguenay ~n which to reepond and generate~ an acknowled~e back signal at that ~requency. For ~: 25 purposes of thi6 example, the third pager to be : ~ addressed or recelve a message in the group of M
pagers, that i8 pager ABo3 ~ will be discussed. In , this example, unlike the example of FIG. 4H, pager AB-3 is an acknowledge back pager. After reading the message which i supplied to the display 960 o~ pag~r AB-3 (such as pager 12I of FIG. 6), the pager AB-3 user indicates a reply at input devioe 980 as already discu sed. The control program in memory 910 of pager AB-3 caus2s microcomput2r 820 therein to recognize that AB-3 i~ th third pager o~ the group M

.

~ '` ` '`' = 20 pagers to be addressed. A sub-band look up table is stored in memory 910. The sub-channel looX
up table includes the appropriate frequency offset, FD, for each of the 20 different frequency ~ub-channels as shown in FIG. 7. As mentioned,microcomputer 820 of pager AB-3 determines that it has received the third address or third message in the respective address or message group sequences.
Using this information, microcomputer 820 fetches from memsry the particular ~reguency offset, FD, from the sub-band loo~ up table in memcry 910 which corresponds to the third sub-band or sub-band 3.
In the circuit arrangement of FIGo 6 ~ the ark-back frequency FTX equal FLo plus FLo/N. FLo/N
15 varies according to the particular sub~band on which ack-back is to be transmitted and with the amounts of error between the FLo local oscillator frequency and the FRX reference frequency. It i~ noted that the FRX paging channel center frequency i8 stored as a reference number in memory 910. In the above eguati~n defining FTX~ N equal8 (FRX ~ FC)/(~D + FC) Microcomputer 820 per~orms the cal~ulation of N and provide6 the value of N which corresponds to the third sub-b~nd to divider circuit l~10~ More specific~llyt to calculate the value of N, microcomputer 820 determines the frequency o~ the dQwn-conYerted referenGe carrier signal, Fc~ by counting the ~requency of ~uch ~ignal a~
microco~puter inpu$ ~20A during reference carrier trans~i~sion time T3. Microcomputer 820 retrieves the FRX referen~e center ~raquency value from memory 910 and al o retrieves the FD ~requency of~set for the third sub-band from the sub-band lookup table also tored in memo~y ~lO. With all the variables which define the dIvisvr N thus bei~g known, . . :

,. . ~ ' .. ~

~.

,~
microcomputer 820 calculates the value o~ N and supplies the same to divider circuit 1010 as already described. The signal generated at the output of divider circuit 1010 thus exhibits a frequency of FLo/N. The FLo/N si~nal is mixed with the FLo signal at mixer 1000 to yenerate the FT~ transmit frequency of FLo + FLo/N. It is appreciated that by replacing N with the expression for N, we get F~X = FLo+FLo/N =
FL0+F~O(F~+FC)~(FRX-FC)- By ~efinition FLo = FRX-FC
since both Fc and FLo contai~ exactly th~ opposit~
frequency error. It follows that the sum FLo~FC
cancels the frequency error. It i8 noted that FTX =
FLO+FC+FD = FRX+FD exactly, The present circuit arrangement uses the local oscillator freguency FL~ as a reference for generation of the ack-back transmit ~requency FTX.
It is noted that the circuit arrangement described above, corrects f~r differences in the frequency of the local oscillator FLo.
Although, a ingle conversion embodiment of : acknowledge back pager 121 is 6hown in FIG. 6 and described above~ those skil}ed in the art will appreciate that double and other multiple conversion embodiments of the pager are readily adapted from this invention and are intended to be within its 6cope .
Each o~ pager6 121,122...P includes a threshold : ~ detector 1050 coupled between the output of amplifier 890 and input 820J of microcomputer 8~0. Threshold detector 1050 provides input ~20J a logical 0 wh~n the down-converted carrier signal at FC exhibits a voltage level less than a predetermined threshold level. However, when the signal voltage level of the FC carrier signal is equal to or greater than such selected predetermined voltaqe level, then threshold ~L2~8~
.

detector 1050 provid~s a logical 1 to microcomputer input 820J. The threshold is set, for example, so that a signal at the receiver input which is 40 dB
above mini~um usable receiver sensitivity will trigger threshold detactor 1050. Microcomputer 820 includes a power control output 820K which i~ csupled to a power level control input 1040A of varia~le output power amplifier 1040. Ampli~ier 1040 is of the type which can as~ume di~ferent power output lo levels depending upon t~e value of t~e 6ign~1 provided to 1040A. For example, in this particular embodiment, when a logical 0 is provided to input 1040A, ampli~ier 1040 opPrates or transmits at full power, for example at approximately 1.5 watts output.
However, when a logical 1 is provided to input 1040A, ampli~ier 1040 throttles back or reduces power to a ~econd lower power output level which i~
approximately 4D dB less than the full power output level. In summary, in this embodiment o~ th2 invention, when a logical 0 is prcvided by threshold detector 1050 to microcomputer input 820J indicating that a relatively low l~vel 6ignal i8 being received, ~ then microco~puter 820 generate~ a logical 0 at its : output 820K. This cause amplifier 1040 to ampli~y at the first or full output power. However, w~en threshold detector 1050 provide~ a logical l to microcomputer input 820J, indicating that a relatively high level si~nal is being r~ceived, microcomputer 820 then yenerate~ a logical 1 at output 820~. ~his in turn causes amplifier 1040 to throttle back to the second lower output power level.
The above de~cribed varia~le output power level circuit arrangement aide in avoiding the situation :when any one o~ the group o~ M pagers AB-l... AB-20 generates such a strong acX-ba~k ~ignal at central , : ' ' ' "

~2t~88~ -station 110 that such ~ignal exceeds the dynamic range o the receiver of station 110 and masks the ack-back siynals from the other pagers of the group of M.
Although in this particular ~mbodiment of the invention, a two power level amplifier 1040 is employed in conjunction with a single level threshold detector 1050, the invention may al~o be practiced using thre~hold detectors with more t~an one threshold and variable output power amplifiers with more than two selectable output power~. For example, in an alternative embodiment of the invention, threshold detector 1050 is a three range threshold detector which determines if the Fc sign~l exhibits a low, medium or high signal level. Such a threshold detector conveniently employs fir~t and second threshold That i~, when threshold detector 1050 determines that the rec~ived ~ignal lev~l at the pager i5 within a first predetermined low signal level range (1e86 than the fir6t threshold), then microco~puter 820 cause~ a three output power level amplifier, ~mployed a~ amplifier 1040, to amplify at a high output first power lavel. When the three ran~e detector 1050 detects ~hat the received signal level is within a medium ~ignal level range (between : the fir~t and second thresholds), then microcomputer 820 would cau~es amplifier 1040 to amplify at a ' medium ou~put:second pvwer level. When detector 1050 determines that the recelved signal level i5 within a third high level range (above the second threshold level), then microcomputer 820 causes amplifier 1040 to fully throttle back to a third and lowest power output level. Thus, a power control circuit is provided in which the transmit~ed output power of the ~5 ack-back pager varies inversely with the RF signal .
.

~8~319 lev~l of the paging signals it receives from central station 110.
Microcomputer 820 is programmed to generat~ a logical 1 at port 820L during the period o~ time at which pager 12~ is to transmit an acknowledge back signal back to central station 110, for example, ack-back time periQd 390 as ~hown in FIG. 4E. During all other periods of time for which pager 121 should be in the recei~e mode, microcomputer 820 i8 program~ed to generate a logical 0 at port 820L. When a logical 1 is generated at DUtpUt 820L, indicatl~g transmit mode, transmit/receive switch 810 connect~ antenna port 810A to port 810C thus connecting the transmit amplifier 1040 to antenn~ 800~ However, when a logical 0 is provided to mierocomputer port 820L, transmit/receive swikch 810 couples antenna port 810A
to port 810~ and receiver amplifi~r 830.
FIG. 8 is a flow chart o~ the control program stored in ~emory 910 which controls the operation of microcomputer 820 and pager 121. A pow~r-on-reset step is shown in block 1100. Pr~gram variables are initialized ~t this time. The r~ceiver portion of pager 121 is turned c~n ~nd be!comes ~ynchronized with respec:t to the p~ging signal~ transmi~ted on th~
paging channel by cen~ral sta~ion 110. A~ter becoming initially synchroniz~d, pager 121 goes into a ~sleep mode~ or battery saving mode as desc:ribed earlier. When pager 121 receives a preamble ~ignal, as in block 1110, pager 121 wakes up as per block 1120. An address oount variable, ADRCOUNT, i~ then initialized with a value of ~ as per block 1130. A
variable ADR~AX which repr2sents the maximum number of ack-back pagers in an acX-back group is set to have a value of M as per block 1130. Pager 121 listens to ~ach OI khe addresses within a group of M

. ' `, ~ , .

"` ' :
',, ' ''', ' .

~2~38~
-s~
addre~ses to determine if its particular address is received as per block 1140. For example, at block 1140, the fir~t address of a group of M addresses is checked to determine if it is the valid address for the particular payer 121. If the first address is not the addres~ of pager 121, then the ADRCOUNT
variable is incremented by 1 to count the number of pager addresses already received a~ per block 1150.
A deter~ination i~ then ~ade ~ to whether all of the addresse6 of the group of ~ addresse~ have been processed, block 1160. lf the variable ADRCOUNT is equal to M, then the addre~s of the particular pager 121 has not been received and ~uch pager 121 reenters - the battery saver mnde a~ per block 1170 after which pager 121 again power~ down and look8 to deternine if a preamble signal i~ received. If however in block 1160 ADRCOUNT is not equal to ~, that i6 le~s than M
signifying that all o~ the M addresses of a group of M addresse6 have not been received as in the present example with respect to the first address of such group~ then flow continues back to block 1140 where pager 121 checks the next address in the group of M
addresseG for validity. I~ any addre~ within the group of M addresses i~ determined to be the address ~or the particular pager 121, then ~low continues ~rom block 1140 to block 1180 at which the variable ADRCO~NT is incremented by 1 6uch that ADRCOUNT i8 a number which represents the order of the valid addres within the sequencing or group o~ N
addresses.
A~ter the group of M addresses is received by pager I21, pager 121 receives and detarmines the : frequency of the down-conve~ted reference carrier Fc ao per block 1190. The signal strength of the , lZ8B819 ' .~
carrier Fc is then determined by micro processor 820 as p~r block 1200.
In the ~ollowing steps, the particular message within the group of M messages which i~ intended for a particular pager within the group ~f M addressed pagers ie matched with such pager and display~d thereon. More particularly, prior to commencing to count the num~er o message~ withln the group of M
messages as such ~essages are received, a message count variable MSGCOUNT is initialized at a value of 0 as per block 1210. The receiving of the individual messages of the group of M message~ commence~ as per block 1220 at which the next message o~ such group is received. Initially, the ~irst message of the group o~ M messages is the ~next message~ received. Upon reception of a message, the ~SGCOUNT variable is incremented by 1 to count the number o~ messages that have been received a~ per block 1230. A
determination ifi then ~ade as to whether MSGCOUNT
equals ADRCOUNT at block 1240. If it i6 determined that MSGCOUNT does not equal ADRCOUNT, then more : mes~ages remain to be received in the group of M
messages and flow continues back to block 1220, at which the next messaye is received. In this example, wherein the ~irst message was received the first time around the loop formed between block 1220 and 1240, the second ~essage is received the second time around , such loop and the message counter MSGCOUNT is incremented at 1230 accordingly. When a determination is made that MSGCOUNT equals ADRCOUNT
then, the current messaye i~ displayed at block 1250.
In thie ~anner, the particular mes~age which was intended for a pager within the qroup of M pagers is di~played by matching the order of the occurrence of 3 5 such mes~age in the gr~up OI M ~Dessages with respect ~ ,' ', :

~28~

to the order of the corresponding addres6 within the group of M addresses.
Ack-back data i6 ~upplied to microcomputer 820 by the pager user as per block 1260. The ack-back pager wait~ a~ per block 1270 for an ack-back field - (time interval) be~ore responding back to the central station 100 with the ack-back data provided by the pager user. It wa6 discu#6ed earlier that M
dirferent sub-band6 are avail~ble ~ the pager of the invent~on for transmission oP ack-back ~ignals. Each ack-back pager within a group of ~ addressed pagers respond~ back to the central station 110 on a different respective sub-band based on the value of the ADRCOUNT variable determined above ~or such pager as per block 1280. For example, in one embodiment o~
tha invention, i~ a particular pager within the qroup of M pagers i8 the ~ifth pager of the group to be addressed, then such pager has an ~DRCOUNT value of 5. As per the above di cussion, the fifth message in the group of M messages corresponds to the fifth pager addressed and is appropriately provided to the display of such ~ifth pag~r for viewing by the pager u~er. In thi6 particulax pager wherein ADRCOUN~
equals 5, ~ub-band number 5 i8 selerted ~rom tha table of FIG. 7 for use by such pager for transmittinq its acX-back signal. T~a~ iæ, the value of ADRCOUNT determines the particular sub-band which is elected for ack-back. Since in this particular exa~ple ~ub band 5 ~s selected, ~icrocomputer 820 ~ 30 accesses the ~ub-~and chart o~ the table of FIG. 7 ; and looks up the ~requency off~et FD corresponding to sub-band number 5 as per ~lock 1290. Microcomputer 820 then looks up the value o~ the pager channel center ~requency F~x in memory as per block 1300.
The value of Fc, the down-converted Farrier :

~ ~;

~L2~
-~ 3 f~4 frequency, is then retrieved from memory or is otherwise acquired as per block 1310. Using the retriev~d value~ of FRX, FC and the offset FD, the value o~ the divisor N is determined according to the equation N = ~FRX ~ FC)/(FD + F~) as per block 1320.
Divider 1110 in FIG. 6 is then ~et to have a N value as determined above in order to drive the frequency of the ack-back pager to the de~ired value which in thi~ example for ~ub-band 5 i~ 149.9945 ~Hz a~ 6et in block 1330. In thi example, FRX = 150 MHz, FC =
0.0350 MHz, FD = ~0-~055 MHz and the nearest integer value for N is N = 5084. The rasulting ack back transmit freguency FTX is ~here~ore 149.9944975 MH7 which is 2.5 Hz from the desired transmitter ~requency and well within the 30 Hz frequency tolerance required in this ~articular embodiment of the invention.
A determination i~ then made by microcomputer 820 as to whether the signal l~vel of the Fc referenco carrier i~ greater than the afore~entioned predetermined threshold lsvel. If the FC ~ignal level i8 gr~at~r than a predetermined threshold~level as deter~ined at block 1340, then the transmitter : circuit~ of pager 121 are turned on, a~ At block 1350. The ack-back data i6 then transmitted back to central station 110 at a low power level on the already ~elected frequency ~ub-band via frequency division multiplexing a~ per block 1360. After tra~miG~ion of the ack-back data, the transmitter circuit~ are turned off at block 1~70 and the battery saver mode i5 reentered a at block 1170. If, : hswever, it is determinad at block 1340 that the Fc carrier reference 6ignal does not exhibit a signal ; level greater than the pred~termined threshold, then the transmitter circuits of pager 121 are turned on .,,, ~ .

:
': ' ' ,' :. ' ' . .. . .

~2~38~
I

at block 1390 and the ack-back d ta i6 transmitted back to central station llO at a high powex level on the selected ~requency sub-band via frequency division multiplexing as per block 1400. After such transmission of the ack-back data, the transmitter circuit are turned off at block 1370 and the battery saver mode i6 reentered at block 1170.
From the above descriptlon, it i~ clear that the invention involves a ~ethod controlling the frequency o~ trans~iss~on i~ an ~ck-back pager. Suc~ pager i6 used in a radio paging ~ystem includi~g a central paging 6tation for transmitting paging signals on a paging channel frequency FRX to a plurality of remotely located acknowledge back radio pagers. Such pager includes a memory for storing the value of the frequency F~x. The method of controlling the frequency on which the acknowledge back pager transmits acknowledge back signals includes the step of generating a local oscillator signal at a frequency FLo, the local oscillakor 6ignal being designated the FL~ ~ignal. The method further include6 the step of mixing the paging signals with the FL~ 6ignal tG generate a down-converted signal at a fr~quency Fc. The method includes determining the frequency FC ~f the down~converted ignal and selecting which ~ne of a plurality of fr~uency sub-bands th~ pag~r is to transmit acknowledge back signals on. A elected ~ub:-band is thus determined.
The ~el~cting step includes selecting a frequency of~set FD corresponding to the ~elected sub-band, the offset fre~uency FD being with respect to the frequency F~x. Tha ~ethod ~urthar includes the step of determining the value of N for the selected sub-band wherein N = (FRX - FC3J~FD ~ F~3. The method concludes with dividing the FLD signal by N to , 1~8~ L9 " s'~
~6 generate an FLo/N signal and mixing the FLo signal with the FLoJN signal to generate a carrier signal for the ack-back signal.
In su~mary, the foregoin~ describes an apparatus and method or controlling the frequency of the sub-bands on which an acknowledge back pager is capable of transmitting acknowledge back signal~. The acknowledge back pager is this capa~le of tran~mitting acknowledge back signals on a plurality of re6pective predetermined ~ub-band freguencies with very high accuracy.
While only certain preferred ~eatures of the invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art. It is, there~ore, to be ~nderstood that the present claims are intended to cover all ~uch modifications and changes which ~all within the true ~pirit o~ the invention.

, ' ~.
,

Claims (7)

we claim:

1. An acknowledge back pager for processing paging signals transmitted at a paging channel frequency of FRX from a central paging station, said pager comprising;
a local oscillator oscillating at a frequency FLO to generate an FLO signal;
first mixing means, coupled to said oscillator, for mixing said paging signals with said FLO signal to generate a down-converted signal at a frequency FC;
frequency determining means, coupled to said first mixing means, for determining the frequency FC
the down-converted signal;
sub-band selection means for selecting one of a plurality of frequency sub-bands on which said pager is to transmit an acknowledge back signal, said selection means determining a selected sub-band and a frequency offset FD corresponding to said selected sub-band, said offset FD being with respect to the frequency FRX;
memory means for storing the value of the frequency FRX;
determining means for determining the value of N
for the selected sub-band wherein N = (FRX - FC)/(FD + FC) dividing means, coupled to said determining means, for dividing the FLO signal by N to generate an FLO/N signal, and second mixing means, coupled to said dividing means and said oscillator means, for mixing said FLO
signal with said FLO/N signal to generate a carrier signal for said acknowledge back signal.

2. The acknowledge back pager of claim 1 including a response indicating means wherein a user of said pager indicates a response to said pager for transmission in said acknowledge back signal, indicia of said response being modulated on said carrier signal to form aid ack-back signal.

3. The acknowledge back pager of claim 1 including means for receiving and displaying message signals transmitted by said central station.

4. An acknowledge back pager for processing paging signals transmitted at a paging channel frequency of FRX from a central paging station, said pager comprising;
a local oscillator oscillating at a frequency FLO to generate an FLO signal:
mixing means, coupled to said oscillator, for mixing said paging signals with said FLO signal to generate a down-converted signal at a frequency FC;
frequency determining means, coupled to said mixing means for determining the frequency FC of the down-converted signal;
memory means for storing a look-up table including indicia of a plurality of frequency offsets, FD, with respect to said frequency FRX, said offsets FD corresponding respectively to a plurality of frequency sub-bands, and for storing the value of FRX, and further for storing the frequency FC
determined by said frequency determining means;
sub-band selection means for selecting which one of said sub-bands in which said pager is to transmit an acknowledge back signal, said selection means thus determining a selected sub-band:
microcomputer means for determining the value of N for the selected sub band wherein N = (FRX - FC)/(FD + FC) dividing means coupled to said microcomputer means, for dividing the FLO signal by N to generate an FLO/N signal mixing means, coupled to said dividing means and said oscillator means, for mixing said FLO signal with said FLO/N signal to generate a carrier signal for said acknowledge back signal.

5. The acknowledge back pager of claim 4 including a response indicating means wherein a user of said pager indicates a response to said pager for transmission in said acknowledge back signal, indicia of said response being modulated on said carrier signal to form said ack-back signal.

6. The acknowledge back pager of claim 4 including means for receiving and displaying message signals tran6mitted by said central station,

7. In a radio paging system including a central paging station for transmitting paging signals on a paging channel frequency FRX to a plurality of remotely located acknowledge back radio pagers, said pager including a memory for storing the value of the frequency FRX, a method of controlling the frequency on which said acknowledge back pager transmits an acknowledge back signal comprising the steps of:
generating a local oscillator signal at a frequency FLO, said local oscillator signal being designated the FLO signal;
mixing said paging signals with said FLO signal to generate a down-converted signal at a frequency FC;
determining the frequency FC of the down-converted signal;
selecting which one of a plurality of frequency sub-bands said pager is to transmit acknowledge back signals on, thus determining a selected sub-band, said selecting step including selecting a frequency offset FD corresponding to aid selected sub-band, said offset frequency FD being with respect to the frequency FRX;
determining the value of N for the selected sub-band wherein N = (FRX - FC)/(FD + FC) dividing the FLO signal by N to generate an FLO/N signal mixing said FLO signal with said FLO/N signal to generate a carrier signal for said ack-back signal.