CA1231390A - Radio paging system and receiver therefor - Google Patents

Abstract

ABSTRACT

According to the present invention, there is provided a radio paging system having a paging receiver, the receiver comprising: means for receiving a first carrier wave modulated with a first plurality of preamble codes and one of first address and control codes and a second carrier wave modulated with a second plurality of preamble codes, the first plurality of preamble codes and a second address code, the first plurality of preamble codes being shorter than the second plurality of preamble codes; means for demodulating the first and second carrier waves; means for processing the output of the demodulating means into first and second plurality of preamble codes, first and second address codes and control code; means for generating first and second control signals having first and second repetition periodes, respectively, the first repetition period being shorter than the first plurality of preamble codes and shorter than the second repetition period, the second repetition period being shorter than the second plurality of preamble codes; means for supplying power to a prescribed part of the receiver in response to one of the first and second control signals; and means for supplying the second control signal from the generating means to the power supply means in response to the control code and supplying the first control signal from the generating means to the power supply means in response to the second plurality of preamble codes.

Description

~.2~3~6~

Radio Paging System and Receiver Therefore Background of the Invention The present invention relates to a radio paging system, and more particularly to a radio paging system enabling a battery saving function at each of its battery-powered receivers.
artery saving system~.of..thi.s.kind..includ~.the.~... . ...
"Digital Radio Paging Communication System" by Moscow et at., disclosed in the U.S. Patent No. 4,194,153, issued on March 18, 1980 and assigned to the present applicant.
In the proposed battery saving system, a paging receiver intermittently operates at a predetermined first repetition period, and, upon receiving and detecting a battery saving release signal set for a time duration longer than the first repetition period and transmitted from a base station,-continuously operates for a certain length of time to receive a paging signal addressed to the receiver. (The .. last-mentioned length of time is, set longer than the time duration of the sequence of a signal comprising the battery saving release signal and the paging signal transmitted from the base station.) Then, when the above-mentioned certain length of time elapses, the paging receiver again returns to the intermittent operation at the first repetition period.

I 3~3 In this system of the prior art, the firs-t repetition period of each paging receiver is usually so set as to be adequate for processing calls when the system is at its peak traffic. Even when the receivers are paged much less frequently than in the daytime and therefore the base station rarely transmits paging signals, each paging receiver repeats its intermittent operations at the f first repetition period, resulting in a disadvantage that the receivers waste battery power. This disadvantage so far has been coped with by turning off, during nighttime, power supply to the receiver with a power switch. However, the paging receiver under the turned off state unavoidably misses paging signals.

Summary of the Invention lo Therefore, an object of the present invention is to provide a radio paging system capable of reducing the power consumption at paging receivers.
Another object of the present invention is to provide a paging system capable of changing the repetition period 20 of the battery saving pulses for paging receivers depending on the call traffic.
Yet another object of the present invention is -to provide a paging receiver capable of changing the battery saving periods adapted to the paging system outlined hereinabove.

~.~23 According Jo the present invention, there is provided a radio paging system having a paging receiver, the receiver comprising: means for receiving a first carrier wave modulated with a first plurality of preamble codes and one of first address and control codes and a second carrier wave modulated with a second plurality of preamble codes, the first plurality of preamble codes and a second address code, the first plurality of preamble codes being shorter than the second plurality of preamble codes; means for demodulating the first and second carrier waves; means for processing the output of the demodulating means into first and second plurality of preamble codes, first and second address codes and control code; means for generating first and second control signals having first and second repetition periods, respectively, the first repetition period being shorter than the first plurality of preamble codes and shorter than the second repetition period, the second repetition period being shorter than the second plurality of preamble codes; means for supplying power to a prescribed part of the receiver in response to one of the first and second control signals; and means for supplying the second control signal from the generating means to the power supply means in response to the control code and supplying the first control signal from the generating means to the power supply means in response to the second plurality of pry amble codes.

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64~6-282 According to another aspect, the present invention provides a method adapted to a radio paging system for switching the repetition period of receiver energizing pulses depending on whether the number of calls exceeds a predetermined number, said method comprising the steps of:
supplying power to a prescribed part of a paging receiver at a first repetition period;
receiving a control code when said prescribed part is supplied with said receiver-energizing pulse at said first 0 repetition period;
changing in response to the control code the repetition period of said receiver energizing pulse from said first period to a second repetition period longer than said first repetition period, receiving a preamble code when said prescribed part is supplied with said receiver energizing pulse at said second repetition period; and changing in response to the preamble code the repetition period of said receiver-energiæing pulse from said second 0 repetition period to said first repetition period.

- pa -Brief Description of Drawings the above and other objects, features and advantages of the present invention will become more apparent from the detailed description hereunder taken in conjunction with the accompanying drawings, wherein:
FIG. 1 is a block diagram illustrating an embodiment of the base station of a paging system according to the present invention;
FIGS. PA to OH are time charts for describing the operation of the base station illustrated in FIG. 1;
FIG. 3 is a block diagram illustrating an embodiment of a paging receiver for use in the radio paging system according to the present invention;
FIGS. PA to OF are time charts for describing the operation of the paging receiver illustrated in FIG. 3; and FIGS. PA to YE are time charts for describing the repetition period switching operation for battery saving by the paging receiver illustrated in FIG. 3.

Detailed Description of the Preferred Embodiments Referring to FIG. 1, a subscriber's telephone set is connected to a trunk 31 in an encoding unit 3 through a telephone exchange 2. A register 33 counts the dial pulses inputted from the trunk 31, and converts the call number of a personal paging receiver transmitted from the calling Z5 parties telephone set 1 into a binary-coded decimal (BUD) number. A set call switch 32 permits the entry of the 3~:3 receiver's number into the register 33 by an operator 75 manual procedure. A memory circuit 35 stores the BUD
number coming from the register 33. A code converter 36 converts into a binary code the BUD number supplies from the memory circuit 35. An encoding circuit 41 adds parity check bits to the output signal from the code converter 36 to provide a cyclic code.
A preamble code generator I repeatedly generates a unique word for a predetermined first duration of time (for a length of time to repeat the generation 9 times in this instance) or second duration of time (for a length of time to repeat the generation 1,025 times). A sync code generator 38 generates a sync code to follow the unique word. A battery saving period switching (ASPS) code generator 39 generates a ASPS code to follow the sync code.
An end code generator 40 generates an end code. A timing signal generator circuit 34 individually controls the circuits 35, 36, 37, 38, 39, 40 and 41, OR gates 42 and 43, and RAND gates 44 and 45, and supplies an encoding unit output through another RAND gate 46, to a transmitter 5.
The transmitter 5 comprises an FISK modulator 51, a frequency converter 52, a power amplifier 53 and an antenna 54.
When trying to call paging receiver (to be described in further detail below), an ordinary telephone subscriber dials on telephone set 1 the call number assigned for the paging receiver. The dial signal is inputted to the trunk 31 via the exchange 2, and further to the register 33. It also is possible for an operator to manually operate the set call switch 32 to enter the call number into the register 33.
The register 33, having received a predetermined number of calls (for example four), transfers to the memory circuit 35 all the call numbers converted into BUD numbers. The memory circuit 35 keeps the call numbers until a read signal comes from the timing circuit 34. The capacity of the memory circuit 35 is 80 calls in this embodiment.
As call numbers are supplied to the memory circuit 35, the timing circuit 34 actuates the preamble code generator 37 to supply the preamble code to the transmitter 5 via the OR gate 42 and RAND gates 44 and 46. The length of time during which the preamble code P see FIG. PA or 2B) is supplied is equal to nine words Leo 155 msec x 9 = 1,395 msec), and each word (hereinafter called the unique word) consists of a Betty code pattern as shown in FIG. ED. The preamble code generator 37 has a 31-bit counter actuated by the output of the timing circuit 34, a unique word supply , counter and a read-only-memory (ROM, for instance ~PD501D
manufactured and marketed by NEW Corporation in which the code pattern of FIG ED is set in advance. This preamble code generator 37 reads out the contents of the ROM in response to the output of the 31-bit counter, and further repeats the reading of the Russ contents the number of times set by the unique word supply counter. Upon completion I

of the supply of the (nine-word) preamble code P, the timing circuit 34 actuated the sync code generator 38 to supply the sync code/ consisting of the code pattern of FIG. YE, in the position of word #l in FIG. 2B. The timing circuit 34, following the completion of the sync code supply, further supplies a read signal to the memory circuit 35, and at the same time actuates the code converter 36, encoding circuit 41 and RAND gate 45. The duration of the actuation is equal to 80 words a the maximum (155 msec x 80 = not more than 12.4 sea) as shown in FIG. us.
If call numbers are stored in the memory circuit 35, the stored numbers are transferred to the code converter 36 one by one in the order of their storage in response to the read signal from the timing circuit 34 until the memory circuit 35 is cleared. The code converter 36 converts BUD
numbers into 21-bit binary codes. The encoder 41 adds 10 parity check bits to the 21-bit information codes to supply an address number word representative of the call number and consisting of the Bose-Chandhuri Hocquenghen BACH
(31, 21) cyclic code, to the transmitter 5 via the RAND
gates 45 and 46. An example of code pattern of address number words is shown in FIG. 2G. As illustrated in FIG. 2B, 8C address number words at the maximum from #2 to #81 in that order are consecutively supplied. This encoding circuit 41 may be composed of shift registers and adders as described in Wesley Peterson, "Error-Correcting Codes,"

l~2~3~ I
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pp. 149 -152~ 1962, The MUTT. Press.
The memory circuit 35, when it is cleared, supplies its output to the timing circuit 34. In response to this output, the timing circuit 34 suspends the operation of the circuits 35, 36 and 41, and at the same time actuates the end code generator 40 to send the end code E to the transmitter 5 via the OR gates 42 and 43 and RAND gates 44 and 46 as shown in FIGS. PA and 2B. The one-word code pattern of the end code E is shown in FIG. OH. This code pattern is a Sudanese (PUN) pattern consisting of 31 bits.
The end code generator 40 which may be composed similarly to the aforementioned preamble signal generator 37, completes code supply in two words (155 msec x 2 = 310 msec).
The timing circuit 34, upon completion of code supply my the end code generator 40, starts a timer built into it.
The length of time at which this timer is set is 2 minutes and 38.72 seconds (1,024 words x 155 msec) in this embodiment.
In response to a new cull number, if any, from the memory circuit 35 within this period of time, the timing circuit 34 repeats the foregoing series of actions. The sequence of signals at this time is shown in FIG. PA.
Meanwhile the timing circuit 34, when the time of its button timer has lapsed, shifts to actions to change the battery saving repetition period. Thus the timing circuit 34 starts the preamble code generator 37 to supply nine unique words similar to code P in FIG. 2B, and then enables f~3~ I

the sync code generator 38 to supply the one-word sync code shown in FIG. YE. Further, the timing circuit 34 starts the ASPS code generator 39 to supply at least one word of ASPS code consisting of the 31-bit pattern shown in FIG. OF, and then starts the end code generator 40 to supply two words of the end code of FIG. OH. The timing circuit 34 generates no output unless a new call number is supplied to the memory circuit 35.
When a new call number is supplied to the memory circuit 35, the timing circuit 34 starts the preamble code generator 37, and at the same time changes from 9 to 1,025 the count of the unique word supply counter within the preamble code generator 37 by supplying a unique word supply counter switching signal to the preamble code generator 37 through a connecting line 47, so that 1,025 unique words (P' in FIG. 2C) are supplied from -the preamble code generator 37.
After that, the timing circuit 34 suspends signal supply for a nine-word length of time, restarts the preamble code generator 37, and at the same time changes from 1,025 Tao 9 the count of the unique word supply counter within the preamble code generator 37 by supplying the unique word supply counter switching signal, so that the signal sequence of FIG. 2B is supplied. The aforementioned signal sequence is illustrated in FIG. YE. The transmitter 5 transmits through the antenna 54 a carrier wave modulated with an output signal sequence provided from the encoder unit 3.

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FIG. 3 is a circuit diagram illustrating a paging receiver according to the present invention. The operation of this receiver will be described below with reference to time charts of FIG. 4.
A modulated carrier wave transmitted from the transmitter 5 on the base station side is picked up by an antenna Lowe and received and demodulated by a receiving section 200 to be converted into a base band signal. This base band signal is shaped by a waveform shaping circuit 300 into a rectangular wave, which is supplied to a signal selecting circuit 400. These operations are, of course, performed with battery saving function -turned off, i.e., while every part of the receiver is supplied with power.
This period lasts for it followed by a period of 5_ of "on"
state of battery saving (where t is one-word length of time) as shown in FIG. 4B.
Now, when the paging receiver receives the modulated carrier wave shown in FIG. PA, a bit-sync circuit 410 regenerates a clock signal bit-synchronlzed with the demodulated signal, and supplies it by way of a line 900 to a preamble code detector 420, a sync code detector 430, an end code detector 440 and an address code detector 450.
The output of the waveform shaping circuit 300 is supplied to one input of the preamble code detector 420 through an 25 AND gate 486. The other input of this AND gate 436 is connected to the Q output of a flip-flop OF 730 within a purser circuit 700. The Q output of the F/F 730 is "H' only when the battery saving is on state, but it is 'Ill when it is off state. Therefore, the signal to the preamble code detector 420 is given only under the on state The detector 420, upon detection of the preamble code, provides a detection pulse of FIG. 4C at a connection line 901. By this detection pulse is set the F/F 730 of the I- purser circuit 700, and the Q output of-the F/F 73~, through a NOR gate 740, keeps a switching transistor 750 on.
Meanwhile, the Q output of the F/F 730 becomes 'Ill to turn the AND gate 486 into a forbidden state. The pulse of the line 901 shown in FIG. 4C further starts a timer 460, which, in response to the clock signal from the line 900, begins counting the time. The length of time at which this timer 460 is set (the third duration of time) is it as shown in FIG. 4B~
Now supposing that no sync code (#1 in FIG. PA) is detected within this it length of time, a time-out signal is outputted to a line 902 and resets the F/F 730 -through 20 an OR gate 490. As a result, the Q output of the F/F 730 becomes "L", and this "L" signal turns off the switching transistor 750 via the NOR gate 740 to return the receiver to the on state of battery saving. Meanwhile, when the Q output of the F/F 730 becomes "H", the AND gate 486 turns into a connected state.

3 3 Jo If the sync code I in FIG. PA) is detected within it, a detection pulse is outputted to a line 903 as shown in FIG. ED. This detection pulse resets the timer 460 via the line 903, and at the same time causes a timer 470 to start counting the time. The timer 470 is set so that the fourth duration of time is 80t as shown in FIG. 4B.
At the end of the period 30t set at this timer 470, the switching transistor 750 is turned off as in the case of the timer 460.
Here the circuit structures of the preamble code detector, sync code detector 430 and end code detector 440 are similar and therefore that of the sync code detector 430 will be now described as representative of the three.
The sync code of FIG. YE is provided to a 31-bit shift 15 register 434. The 31-bit output of the shift register 434 is supplied to an AND gate 435 directly when the corresponding bits of the sync code are at "H" level and, through inventors 431, 432, 433 and so on when the corresponding bits of the same are at "L" level Only when 20 the 31 bits supplied to the shift register 434 are respectively identical to the 31 bits of the sync code, the AND gate 435 provides a detection pulse of "HO' level (FIG. ED) at the line 903. The preamble code detector 420, sync code detector 430 and end code detector 440 differ from one another only in the positions of inventors arranged to match the "L" level of the code patterns of FIGS. ED, YE and OH.

Next, if the address code of this receiver supposed to have the code pattern of FIG. 2G) is transmitted from the base station in the position of the ~81 word of FIG. PA, the address code detector 450 will output a detection pulse shown in FIG. YE over a line 904, and this pulse is supplied to an alert tone generator 500 to actuate it. The generator 500 continuously generates an alert output as shown in FIG. OF, and drives a speaker 600 to let the bearer of the receiver know the fact of its being paged. In FIG. 4F, indicates a timing at which a reset switch 501 is pressed to cease the alert tone generation.
Now will be described in detail the operation of the address code detector 450. A sync code detection pulse (Lee FIG. ED) at the line 903 actuates a read pulse generator 454 to sequentially and cyclically generate read pulses synchronized with the clock signal from the line 900 at output terminals #l to #31. In a programmable read-only memory (PRO) 453, the address code assigned to the paging receiver is written in advance. The PROM may be made a the the so-called detachable cord plug. The read pulse generator 454 comprises a shift register. First, in response to a read pulse from #l of the read pulse venerator 454, the first bit of the address code stored in the PROM 453 is read out, and supplied to one of the input terminals of a two input exclusive NOR gate 451. To the other input terminal of the gate 451 is supplied an output from the of waveform shaping circuit 300, and this gate 451 outputs an "H" level signal tithe input are identical with each other or an "I," level signal if not. this "H" level signal is counted by a 31-bit counter 452. Since the clock signal from the line 900 is supplied to the counter 452, if the consecutive bits from the PROM 453 and the outputs of the waveform shaping circuit 300 are respectively found by the exclusive NOR gate 451 to be identical with each other, the counter 452 will count up sequentially until the #31 bit and, if all the 31 bits are found to be identical, supply a detection pulse (FIG. YE) over the line 904. Then the counter 452 is reset by the trailing edge of the read pulse of #31 to prepare itself for the next counting.
FIG. PA shows a modulated carrier wave transmitted from the base station. Unless four call number signals from the telephone exchange 2 (See FIG. 1) are supplied to the encoding unit 3 of the base station within a prescribed length of time (1,024 words' length), the base station transmits from its transmitter 5 a nine-word (it) preamble Z0 code (P in FIG. PA), followed by a one-word sync code (1 in FIG. PA), a one-word battery saving period switching ASPS) signal (2 in FIG. PA) and a two-word end code (E in FIG. PA).
After that, the modulated carrier wave emission from the transmitter 5 is suspended until four call number signals are registered in the Emory circuit 35 of the encoding unit 3. Upon registration of four address number signals, as described with reference to FIG. 2C, a modulated carrier wave is emitted from the transmitter 5 as shown in FIG. PA.
The battery saving operation of the paging receiver shown in FIG. 3, corresponding to the modulated carrier wave of FIG. PA, is represented by FIG. us. The receiving section 200 and the waveform shaping circuit 300, as shown in FIG. 5B, are repeatedly turning on (it) and off it the battery saving operation, regulated by the purser circuit 700. When a preamble code P is detected by code detector 420 while the receiving section 200 and the waveform shaping circuit 300 are within the it period of the off state of battery saving, a detection pulse is generated at point at of FIG. 5C. At this time, as described with reference to FIG. 4C, the timer 460 sets the third duration of time it as shown in FIG. 5B. And a sync code, as described with reference to FIG. ED, is detected at point do of FIG. ED, and the timer 470 sets the fourth duration of time 80t. Here is detected a battery saving period switching (ASPS) signal 2, shown in FIG. PA, by a ASPS, i 20 signal detector 480 see FIG. 3), and a detection pulse of FIG. YE is outputted at a line 905. The pulse on the line 905 resets the timer 470 by way of an OR gate 485, further resets the F/F 730 by way of the OR gate 490 and inverts its Q output to "L" level. It further turns off the switching 25 transistor 750 via the NOR gate 740.

Here, the detection pulse on the line 905 sets an F/F
760, and inverts thy outputs Q and Q. The outputs Q and Q
of the F/F 760 are connected to AND gates 780 and 770, respectively, and keep the former in forbidden state and the latter in a connected state. To the other input terminal of the AND gate 770 is repeatedly fed a control signal at a repetition period represented by By in FIG. 5B, while to the other input terminal of the AND gate 780 is repeatedly fed a control signal at a repetition period represented by By in FIG. 5B. These control signals are supplied from a control signal generator 720, which frequency-divides the output of an oscillator 710 to generate the required control signals at the repetition periods By and By. The pulse to reset and initialize the control signal generator 720 is lo obtained by inverting the output Q of the F/F 730 with an inventor 790. Consequently, the NOD gate 740 is controlled by the output of the AND gate 770, and repeatedly turns on and off the switching transistor 750 at the repetition period of By.
Now will be described the advantage of changing the repetition period of battery saving pulses. Since the average current of the receiving section 200 and the waveform shaping circuit 300 is 3 ma and that of the signal selecting circuit 400 and the purser circuit 700 is 150 PA, the average current during the period By is I

it x 3000 + it x 1500 = 964 PA

or, during the other I

it x 3000 1021t x 150 = 155 PA

Accordingly, the amperage for the paging of the present invention is only 16 percent of that involved.in.the.~. .. . ..
absence of switching from Al to By.
Then, if a preamble code of 1,025t in time length, represented by I' in FOG. I is transmitted from the base station, the preamble code detector 420 outputs the lo detection pulse to the line 901 at point c2 in FIG. 5C.
The pulse on the line 901 resets the F/F 760 and inverts the outputs Q and Q. As a result, the AND gate 780 is switched from the forbidden to the connected state, and the AND gate 770, the other way around, the battery saving period being changed from By to Al. Since the pulse on the line 902 (at point c2 in FIG. 5C) induces similar ! actions to what were described with reference to point at in FIG. 5C, battery saving is suspended for a period of it.
Because no sync code is detected within this period of it as illustrated, the receiver resumes battery saving in response to a time-out signal from the timer 460. A it pause is provided between preamble codes P' and P on the base station side to avoid impossibility to suspend battery I

saving, which might otherwise arise if the preamble code P arrives immediately following the code P' within the it period during which the receiver is waiting for a sync code.
Next, the process in which a preamble code P is detected at point c3 in FIG. 5C and a sync code is detected at point do in FIG. ED are the same as in FIGS. 4C and ED, respectively.
An end code E (See FIG. OH) is transmitted following an address code for the purpose of letting the paging receiver resume battery saving by detecting the end code with the end code detector 440 and resetting the F/F 730 via the OR gates 485 and 490 in response to the end code detection pulse, so that the paging receiver my take no unnecessary receiving action when the address code transmission from the vase station is less than 80_.
Although only the use of the code pattern of FIG. ED
for the preamble code is referred to in the foregoing description of the preferred embodiment, the preamble code can obviously be replaced with any other code different from the sync code, battery saving period switching code, end code and address code.
It will be readily understood that the period By can be extended by grouping the paging receivers. The call 25 number copiously is the sty power of 2 (equal to 2097152) because the call number code has 21 information bits, as shown in FIG. 2G. These 2097152 different call numbers can be grouped into, for example, 200 groups each of which has 10,000 call numbers and is headed with a preamble code unique thereto. In such a grouped number system, the encoding unit comprises a sorter provided between the trunk 31 and the register 33 (See FIG. 1) with which the call numbers are sorted into prefixed groups. For each group, the encoding unit includes the register 33, the decimal-binary converter 36 and the encoder 41 (See Fig. I
The encoding unit also comprises a transmission sequence arranging circuit following the RAND gate 46 to arrange the encoded group paging codes from the encoders 41 to a paging code.
On the other hand, each paging receiver which is sorted in a given group, has to have a preamble code detector unique to the given group. Although this unique preamble code detector makes the paging receiver somewhat complex, this complexity can be eased by composing the decoder of a PROM of the code-plug type like the address ! 20 code detector 450 (FIG. 3) is and, if these preamble and r address code detectors are accommodated into a single PROM, the complexity will be more decreased.
As hitherto described, the radio paging system according to the present invention sets more than one duration and 25 repetition period for battery saving pulses for the paging receiver, resulting in the reduction of the paying receiver' 5 power consumption. In addition, the power switch of the paging receiver can be eliminated that makes the receiver more compact and easier to operate.

Claims (6)

What is claimed is:

1. A radio paging system having a paging receiver, said receiver comprising:
means for receiving a first carrier wave modulated with a first plurality of preamble codes and one of first address and control codes and a second carrier wave modulated with a second plurality of preamble codes, said first plurality of preamble codes and a second address code, said first plurality of preamble codes being shorter than said second plurality of preamble codes;
means for demodulating said first and second carrier waves;
means for processing the output of said demodulating means into first and second plurality of preamble codes, first and second address codes and control code;
means for generating first and second control signals having first and second repetition periodes, respectively, said first repetition period being shorter than said first plurality of preamble codes and shorter than said second repetition period, said second repletion period being shorter than said second plurality of preamble codes;
means for supplying power to a prescribed part of said receiver in response to one of said first and second control signals; and means for supplying said second control signal from said generating means to said power supply means in response to said control code and supplying said first control signal from said generating means to said power supply means in response to said second plurality of preamble codes.

2. A radio paging system as claimed in claim 1 further having means for selectively transmitting one of said first carrier wave and second carrier wave depending on whether the number of paging calls within a given period of time exceeds a predetermined number.

3. A radio paging system as claimed claim 2, wherein said predetermined number is one.

4. A radio paging system as claimed in claim 1, wherein said prescribed part of the receiver comprises said receiving and demodulating means.

5. A radio paging system as claimed in claim 1, wherein said receiver further comprises means for generating an alert tone in response to one of said processed first and second address codes.

6. A method adapted to a radio paging system for switching the repetition period of receiver-energizing pulses depending on whether the number of calls exceeds a predetermined number, said method comprising the steps of:

supplying power to a prescribed part of a paging receiver at a first repetition period;
receiving a control code when said prescribed part is supplied with said receiver-energizing pulse at said first repetition period;
changing in response to the control code the repetition period of said receiver-energizing pulse from said first period to a second repetition period longer than said first repetition period;
receiving a preamble code when said prescribed part is supplied with said receiver energizing pulse at said second repetition period; and changing in response to the preamble code the repetition period of said receiver-energizing pulse from said second repetition period to said first repetition period.