Abstract:
In a satellite communications system, where a user terminal  34  may wish to page more than one satellite  10  or more than one spot beam  30 , and where the definition of a timeslot  38  for a paging channel  48  occurs a predetermined period after the sending of a broadcast burst  42 , the possibility of sustained temporal collision between paging channels  48  between adjacent satellites  10  or adjacent spot beams  30  is avoided by periodically, progressively, and cyclically, moving the predetermined period.

Description:
FIELD OF THE INVENTION 
     The present invention relates to satellite communications systems where a user terminal is able to communicate with an earth station via a communications satellite. In greater detail, the present invention relates to such communications systems where the user terminal utilises a paging channel in an attempt to gain communications via the satellite. In greatest particularity, the present invention relates to such systems where there may be more than one paging channel available, either from nearby spot beams from the same satellite or from more than one satellite. 
     BACKGROUND 
     In terrestrial based cellular telephone systems, the user terminal, usually in the form of a radio telephone handset, communicates with one or more spaced base stations. Each base station puts out broadcast messages on a broadcast channel (BCCH) and also provides a paging channel (PCH). The timing of the messages on each BCCH from the plurality of base stations and for each PCH is such that there is no opportunity for the BCCH from one base station to collide with the BCCH or PCH from another base station, nor any opportunity for the PCH of one base station to collide with the PCH or BCCH of any other base station. Thus, if a user terminal, in a terrestrial cellular system, wishes to decode paging from a neighbouring base station, it is required to listen to the BCCH of the neighbouring base station to learn the PCH details of the neighbouring base station, and is then able to access the PCH of the neighbouring base station without fear of collision between BCH and PCH channels. Collision, where one signal coincides in time with another, renders signals unreadable and the vital information is lost. 
     Satellite communication systems generally resemble cellular systems in that a user terminal communicates with a base station in the form of an earth station. However, the presence of an intervening communications satellite complicates the issue. The communications satellite receives messages from the earth station and relays them to the user terminal. The communications satellite also receives messages from the user terminal and relays them to the earth station. Unfortunately, while a terrestrial cellular system has virtually no propagation delay between the base station and the user terminal, the high flying altitude of the communications satellite and separation on the surface of the earth between the user terminal and the earth station means that a significant propagation delay ensues. In the example to be given in the description of the preferred embodiment, round trip (earth-satellite-earth) propagation delays can exceed one third of a second. 
     Communications satellites provide the user terminal a plurality of spot beams in a spaced array of areas of radio coverage on the surface of the earth. While these correspond, in general, to the cells of the terrestrial cellular systems, there are major differences. The spot beams are much larger and significant differences in propagation delays can result between adjacent and nearby beams. So that the coverage should have no gaps, the spot beams overlap to a considerable extent, resulting in the signals for one spot beam being audible in many others, and vice versa. Thus, even if an attempt is made, as in a terrestrial system, to synchronise the BCCH and PCH channels for each spot beam over the entire range of the array of spot beams, so that no collision occurs, there is still a significant chance that the differential propagation delays will cause collisions within the spot beams of a single satellite. 
     Even worse, the communications satellite is generally one of a plurality of communications satellites in orbit about the earth in a constellation which may include a plurality of overlapping planes. The cones of radio coverage, each divided into spot beams, are forced to display overlap so that no portion of the surface of the earth is left without satellite coverage. Thus, spot beams from different satellites in the same orbital plane overlap, and spot beams from satellites in different orbital planes overlap. A user terminal can thus find itself subject to signals from two, three, or more satellite at the same time. Again, because of differential propagation delays between the different satellites, attempts at BCCH and PCH synchronicity are doomed to fail, and if no synchronicity is attempted between satellites, there will be inevitable collisions. 
     The nature and origin of these problems is further explained in the description of the preferred embodiment, provide hereafter. 
     Thus, current art techniques are of no avail to overcome these difficulties. It is possible to provide incredibly elaborate schemes for inter-spot-beam and inter-satellite synchronicity, but these would impose a very high equipment and system time overhead, as well as increasing the system cost very considerably by rendering the user terminals, which could be numbered in millions, each considerably more elaborate than is currently necessary. What is required is a simple approach which can overcome all of these problems. 
     The present invention seeks to provide such a solution. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the present invention consists in a method for use in a satellite communications system, employing a concatenation of timeslots for transmission or reception, a user terminal being operative to select an individual timeslots from a each of a repetitive plurality of spaced patterns of timeslots as a paging channel, said method being characterised by said user terminal periodically, with a first period, altering the selected timeslot in each pattern for the selected timeslot to execute a cycle through each of the timeslots in each pattern. 
     According to a second aspect, the present invention consists in a user terminal for use in a satellite communications system employing a concatenation of timeslots for transmission or reception, said user terminal being operative to select an individual timeslots from a each of a repetitive plurality of spaced patterns of timeslots as a paging channel, said user terminal being characterised by being operative, periodically, with a first period, to alter the selected timeslot in each pattern for the selected timeslot to execute a cycle through each of the timeslots in each pattern. 
     According to a third aspect, the present invention consists in a satellite communications system a including a user terminal and employing a concatenation of timeslots for transmission or reception, a user terminal being operative to select an individual timeslots from a each of a repetitive plurality of spaced patterns of timeslots as a paging channel, said system being characterised by said user terminal being operative, periodically, with a first period, to alter the selected timeslot in each pattern for the selected timeslot to execute a cycle through each of the timeslots in each pattern. 
     The first, second and third aspects also provide for the periodic altering of the selected timeslot including alteration after each pattern. 
     The first, second and third aspects also provide that the concatenation of timeslots includes a plurality of equispaced timeslots bearing broadcast bursts, the broadcast bursts separating and internally dividing the patterns of timeslots, the broadcast bursts having a repetition rate which is an integral multiple of the repetition rate of the patterns, and each of the broadcast bursts defining a timeslot which is not in any of the patterns of timeslots, consecutive integral numbers of bursts carrying a message structure which has a length of consecutive bursts which is selected from a set of predetermined numbers of consecutive broadcast bursts, and that user terminal, periodically, with a second period, alters the selection of the selected timeslot one place further in the cycle, the second period being a consecutive repetition number of broadcast bursts, the repetition number not being in the set of predetermined numbers and being neither divisible by nor divisible into any member of the set. 
     The first, second and third aspects, yet further, provide that the user terminal calculates a paging channel group, counts the timeslots to calculate an incremental paging group number, uses the incremental paging group number to calculate a timeslot allocation number, and counts the timeslots in each pattern and employs the timeslot allocation number to select the timeslot for said paging channel. 
     According to a fourth aspect, the present invention consists in a method for use in a satellite communications system including the steps of employing a user terminal to receive broadcast message frames and to select a paging channel at a predetermined time after commencement of said broadcast message frames, said method being characterised by including the step of said user terminal periodically, progressively and cyclically, altering said predetermined time. 
     According to a fifth aspect, the present invention consists in a user terminal for use in a satellite communications system, operative to receive broadcast message frames and to select a paging channel at a predetermined time after commencement of said broadcast message frames, said user terminal being characterised by being further operative, periodically, progressively and cyclically, to alter said predetermined time. 
     According to a sixth aspect, the present invention consists in a satellite communications system including a user terminal, operative to receive broadcast message frames and to select a paging channel at a predetermined time after commencement of said broadcast message frames, said system being characterised by said user terminal being operative, periodically, progressively and cyclically, to alter said predetermined time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further explained, by way of an example, by the following description, taken in conjunction with the appended drawings, in which: 
     FIG. 1 shows one plane of a constellation of communications satellites, in orbit about the earth, and shows the extent of radio coverage; 
     FIG. 2 shows how two orthogonal orbits of communications satellites can be used to give radio coverage to the full earth; 
     FIG. 3 shows a typical communications satellite footprint on the surface of the earth; 
     FIG. 4 illustrates how the footprints of FIG. 3 can overlap; 
     FIG. 5 shows a typical environment for a satellite communications system, with an earth station and a user terminal on the surface of the earth, in mutual communications via one or more satellites; 
     FIG. 6 shows the TDMA (Time Division Multiple Access) timeslots as they are used to provide communications between the earth station and the user terminal, via the satellite(s) of FIG. 5; 
     FIG. 7 shows the relationship between the concatenated timeslots of FIG.  6  and broadcast message bursts; and 
     FIG. 8 shows the broadcast message frame structure and the relationship therewith of a user-terminal selected paging channel. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a planar constellation of communications satellites  10  disposed about the earth  14 . The plurality of communications satellites  10  are evenly disposed around a circular orbit  12  above the surface of the earth  14 . Each of the communications satellites  10  is designed to provide radio communications with apparatus on the surface to the earth  14  when the individual communications satellite  10  is more than 10 degrees above the horizon. Each communications satellite  10  therefore provides a cone  16  of radio coverage which intersects with the surface of the earth  14 . 
     The surface of the earth has three types of areas. A first type of area  18  is one which has radio coverage from only one communications satellite  10 . A second type of area  20  is an area where there is radio coverage from more than one communications satellite  10 . Finally, a third type of area  22  receives radio coverage from none of the communications satellites  10  in the orbit  12  shown. 
     FIG. 2 illustrates how the communications satellites  10  are disposed in orthogonal orbital planes. 
     The first orbit  12  of FIG. 1 is supplemented by a second orbit  12 ′ having communications satellites  10  disposed there about in a similar manner to that shown in FIG.  1 . The orbits  12 ′ are orthogonal to one another, each being inclined at 45 degrees to the equator  24  and having planes which are orthogonal (at 90 degrees) to each other. 
     In the example shown, the communications satellites  10  orbit above the surface of the earth  14  at an altitude of  10  355 km. Those skilled in the art will be aware that other orbital heights and numbers of communications satellites  10  may be used in each orbit  12 ,  12 ′. This configuration is preferred because the example provides global radio coverage of the earth  14 , even to the north  26  and south  28  poles, with a minimum number of communications satellites  10 . In particular, the orthogonality of the orbits ensures that the communications satellites  10  of the second orbit  12 ′ provides radio coverage for the third types of area  22  of no radio coverage for the communications satellites in the first orbit  12 , and the communications satellites  10  in the first orbit  12  provide radio coverage for those areas  22  of the third type where the communications satellites  10  of the second orbit  12 ′ provide no radio coverage. By such an arrangement, it is ensured that every point, on the surface of the earth  14 , has, at least, one communications satellite  10   10 ′, visible at all times. 
     It will become clear that, although the two orbits  12 ,  12 ′ are here shown to be of the same radius, the invention as hereinbefore and hereinafter described will function with orbits  12 ,  12 ′ of different radii. Equally, there may be more than two orbits  12 ,  12 ′. So far as the present invention is concerned, the only requirement is that every part of the surface of the earth  14  is in receipt of radio coverage from at least one communications satellite  10  at all times. 
     FIG. 3 shows the structure of the cone  16  of radio coverage provided by each communications satellite  10 . For convenience, the radio coverage cone  16  is shown centred, on a map of the earth, at latitude 0 degrees at longitude 0 degrees. The cone  16  of radio coverage is divided into a plurality of spot beams  30 , by means of a corresponding plurality of directional antennae on the communications satellite  10 . The communications satellite  10  is intended for mobile radio telephone communications and each of the spot beams  30  corresponds, roughly, to the equivalent of a cell in a cellular radio telephone network. In FIG. 3, the cone of radio coverage  16  is distorted due to the geometry of the map of the earth&#39;s surface provided. FIG. 3 also shows the extent of interaction of the cone  16  of radio coverage down to the edges of the cone  16  being tangential to the earth&#39;s surface, that is, to the point where the cone  16  represents a horizontal incidence at its edges, with the surface of the earth. By contrast, FIG. 1 shows the cone  16  at a minimum of 10 degrees elevation to the surface of the earth. 
     It is to be observed, that because of the curvature of the earth, the spot beams  30  are of near uniform, slightly overlapping circular shape at the centre whereas, at the edges, the oblique incidences of the spot beams  30  onto the surface of the earth  14  causes considerable distortion of shape. 
     FIG. 4 shows how the cones  16  of radio coverage may interact with the surface of the earth to produce many types of different regions. 
     As discussed with reference to FIG. 1, numerous cones or radio coverage  16  may overlap to produce first areas  18  where there is radio coverage by only one communications satellite, second areas  20  where there is radio coverage by two communications satellites, and even fourth areas  32  where coverage is provided by three or more communications satellites. It is to be understood that each of the cones  16  of radio coverage represented in FIG. 4 is divided, as shown in FIG. 3, into its own independent set of spot beams  30 . 
     FIG. 5 shows the elements of the communications system wherein the present invention operates. A user terminal  34 , optionally but not necessarily in the form of a portable telephone handset, is located on the surface of the earth  14 . The user terminal  34  seeks to establish communications with a first communications satellite  10 A by accessing the paging channel (PCH) of the first satellite  10 A. The user terminal  34  also wishes to access the paging channel PCH of a second communications satellite  10 B, which may or may not be in the same orbit  12  as the first communications satellite  10 A. The first and second communications satellites  10 A  10 B are both in communications contact with a satellite earth station  36 . Alternatively, each of the satellites  10 A  10 B can be in communication with their own earth stations  36 , rather than sharing a single earth station. 
     By being able to access the paging channel PCH of more than one satellite  10 A  10 B, the user terminal  34  gains a greater reliability of access to the system. An example of the advantage is where, temporarily, the radio path to one of the satellites  10 A  10 B, may be cut off. The user terminal may then gain access to the system through the other satellite  10 B  10 A. The first  10 A and second  10 B satellites, once accessed and processing a call from the user terminal  34 , can pass messages and voice communications from the user terminal  34  to the earth station  36  and from the earth station  36  to the user terminal  10 , both potential paths being via a satellite  10 A  10 B. 
     FIG. 6 is a diagrammatic representation of the structure of the TDMA burst and data block structure. 
     First satellite burst  38 A originates from the first satellite  10 A and is divided into six data blocks  40 A. The earth station  36  arranges the timing of transmission and reception between the first satellite  10 A and the user terminal  34  to compensate for the propagation delay (propagation delay  1 ) between the user terminal  34  and the first satellite  10 A such that there is coincidence with a second six data blocks  38 B which represents the set of timings used by the user terminal  10 . It is to be understood that the first satellite  10 A has separate sending and receiving antennae, so that each of the data blocks  38 A,  38 B can be used both for transmission or reception. The user terminal  34  has but one antenna, so that each data block  38 B is selectable usable for transmission or reception, but not, simultaneously, for both. 
     At the user terminal  34 , on call setup or registration, a transmission data block TX is allocated for transmission to the first satellite  10 A. A first reception data block RX is allocated for reception from the first satellite  10 A. At the same time, supplementary information is sent between the first satellite  10 A and the user terminal  34  through a Slow Associated Control Channel SACCH 1 . The information sent from the satellite  10 A to the user terminal  34  includes an indication of the transmission power that the satellite  10 A is using to send signals to the user terminal  34 . 
     It is to be appreciated that each burst  38 A  38 B is merely a time window to be used in whichever way that the system requires, for transmission or reception. Those data blocks  40 , not allocated to the user terminal  34  in the example given, may be used to communicate between the first satellite  10 A and any other user terminal  34 . Each data burst  40 , TX RX comprises an encrypted and encoded digital representation of analog signals, such as speech or facsimile, to be sent either from the user terminal  34  the first satellite  10 A or from the first satellite  10 A to the user terminal  34 . By the exchange of data blocks  38 , so the progress of a telecommunications operation is achieved. 
     FIG. 7 shows the relationship between data bursts  38  and the broadcast signals BCCH from the first satellite  10 A of the present example. 
     At the user terminal  34 , the bursts  38  form into a continuous string  39 . However, the broadcast channel (BCCH) from the first satellite  10 A, provides a BCCH burst  42  which, while being as long (or thereabouts) as a data block  40 , is not corrected for propagation delays between the first satellite  10 A and the user terminal  34  and so can straddle the time of two adjacent data blocks  40 . The BCCH burst is repeated every 25 TDMA burst periods  38  and, in addition, “walks” along each TDMA burst so that on subsequent appearances of the BCCH burst  42  the BCCH burst  42  moves from data block  40  to data block  40 , so that no one data block  40  is monopolised by the BCCH burst  42 . This is achieved by the repeat period of the BCCH burst  42  not having a common divisor with the period of six data blocks  38 . The BCCH burst  42  returns to the original data block  40  after one second, and then recycles through them. Other timings are possible, within the spirit of the invention. The BCCH burst  42  carries information concerning the amount of power being used to transmit the BCCH burst  42  from the first satellite  10 A. Alternatively, the BCCH  42  power information can be carried on SACCH 1 . 
     The user terminal  34 , by electing to listen on a particular slot  40 , other than its transmission slot Tx or its reception slot Rx for a long enough period, is sure, eventually, to receive the BCCH burst  42  from the first satellite  10 A. 
     The operation of the user terminal, and the timing relationships described, are identical between the user terminal  34  and the second satellite  10 B. 
     The BCCH  42  is provided on a predetermined frequency, other than that used for actual communications. The user terminal  34  can switch to the BCCH  42  frequency for the first satellite  10 A to hear the BCCH  42  from the first satellite  10 A, and, as will be apparent to those skilled in the art, the user terminal  34 , because of the timing relationships described, will also be able to tune to the frequency of the BCCH for the second satellite  10 B and be sure to receive the BCCH  42  from the second satellite  10 B. 
     FIG. 8 shows the relationship between BCCH messages and PCH messages. Whereas FIG. 7 was shown synchronised on the 6 data-block  40  TDMA bursts  40 , where the repeat period is always a multiple of six data blocks  40 , FIG. 8 is synchronised on the BCCH bursts  42  of FIG. 7, where the repeat period is always a multiple of  25  data blocks. 
     Each BCCH period  44  comprises 25 data blocks  40 . The initial data block  40  in each BCCH period is the BCCH burst  42 , otherwise shown in FIG.  7 . The four sequential BCCH bursts  42 , taken together, make up a BCCH message Frame  46 . That is, the user terminal  34  must receive and decode all four sequential BCCH bursts  42  to discover the content of the BCCH message. All of the other data blocks  40  can be allocated to the paging channel PCH. The BCCH busts  42  and the paging channel PCH are separated not only in time, but also in frequency. The satellite(s)  10 A  10 B use one frequency to send out the BCCH bursts  42 , and another to provide the paging channel PCH. The data block  40  timeslots, as shown in FIGS. 6,  7  and  8  are just that, available timeslots, and the different signals shown therein need not be on the same carrier frequency. 
     In the preferred embodiment, each BCCH period  44  can have as few as one and as many as eleven allocated paging channels  48  therein. This gives a minimum of  4  and a maximum of  44  allocated paging channels  48  in the BCCH message frame  46 . Each user terminal  34  uses just one allocated paging channel  48 . Thus, in the example, the allocated paging channel  48  for a particular user terminal  34  appears just once in each BCCH message frame  46 . 
     The paging channels are divided into paging groups. That is to say, in the example given, the user terminal  34  selects just one of the forty-four possible allocated paging channel time slots  48  as its own paging channel, but not for every BCCH message frame  46 . 
     The satellite  10 A sends the BCCH message to the user terminal  34 . The BCCH message contains the frequency of the paging channel PCH and also a number N, being the repeat number of BCCH message frame  46  between repeat paging channels  48 . To put it another way, although the user terminal  34  is allocated one of the timeslots  40  in the BCCH message frame  46  as its paging channel, this does not occur in every BCCH message frame  46 , but only in every Nth BCCH message frame  46 . 
     Now, each user terminal  34  contains a SIM card (subscriber Identity Module) (also known in GSM) which contains an IMSI (international mobile subscriber identity), being a number. The IMSI numbers are evenly distributed among user terminal  34  SIM cards. In order to determine to which paging channel group a particular user terminal  34  belongs, a relationship based on the IMSI is used. 
     IF PG=the paging group to be allocated to a particular User terminal  34   
     If N=a number, as described above and 
     PR=the number of paging channel slots  48  which are reserved for each BCCH message frame  46  and 
     IMSI is a number, as described above, 
     Then 
     PG=IMSI modulo (N×PR) 
     where the modulo operation means that the number N×PR is used as the base of a counting system for the IMSI, PG being the residue (remainder) after the modulus (N×PR)has been “rolled over” (subtracted from the IMSI a sufficient number of times to leave a residue which is less than the modulus but greater than zero) 
     In this manner, a number PG, representing the paging group of a particular user terminal  34 , which, statistically, is fairly evenly spread among the user terminals  34  which might access the system  35   10 A. It is therefore unlikely that user terminals will “clump” onto one of the total of (N×PR) paging channels available with an N BCCH message frame  46  repeat period. 
     Having allocated the paging group PG, the present invention next proceeds to allocate a timeslot  40  for the particular paging channel group PG. 
     Now, each TDMA data block  40  is allocated an Absolute Timeslot Number (ATN). It is not important how the ATN is calculated. In general terms, a counter, modulo a very large number, continuously rolls over, so that an ATN will repeat, but only after a very long period. The TDMA timeslots (data blocks)  40  are sequentially numbered. 
     The BCCH message gives indication (let us say, the number A), to the user terminal  34 , of the ATN of the of the first BCCH burst  42 A of the BCCH message frame  46 . In the example given, the ATN of the second BCCH burst  42 B will be (A+25), the ATN of the third BCCH burst  42 C will be (A+50),the ATN of the fourth BCCH burst  42 D will be (A+75), and the ATN of the final timeslot  40 D of the BCCH message frame  46  will be (A+99). The ATN numbers just keep rolling on from BCCH message frame  46  to BCCH message frame  46 . The equivalent first BCCH burst  42  of the next subsequent BCCH message frame will be (A+100), and so on, incrementing as before. 
     The timeslot for the paging channel PCH is calculated using various equations. The first equation is used to calculate an incremental paging group number (P 1 ) which moves the paging group number by unit increment from time to time in order, ultimately, to.cause the PCH timeslots  48  to move relatively to the BCCH bursts  42 . The incremental paging group number is given by: 
     
       
           P 1=( PG+ATNdiv (100 ×N )+ ATNdiv (500 ×N ))mod( N×PR ) 
       
     
     where PG is the paging group (as before) ATNdiv (100×N) is the integral (whole number result discarding fractional parts) result of the current absolute timeslot number divided by four times the number of timeslots  40  between repeat paging opportunities, the divisor being the number of timeslots in the BCCH message frame  46  and the number of message frames  46  before a paging opportunity is repeated. The BCCH message frame  46 , in the example given, is four sets of twenty-five timeslots  40 , and the number of BCCH frames  46  between repeat paging opportunities is, as earlier states, an integer N. This second term increments by one every time the current ATN increases by (100×N). In this way the allocated paging channel number increments by 1 every N set of BCCH frames  46  so that the PCH  48  “rolls past” the BCCH positions with progression of time. 
     The third term ATNdiv(500×N) reflects the message structure of the BCCH channel  42 . While the BCCH message Frame  46  comprises four consecutive BCCH messages  42 , the actual scheduling of the BCCH message structure can involve messages which differ in length from four consecutive BCCH bursts  42 . In the preferred embodiment of the present invention, the BCCH message structure includes, for every message, a “start of tree” which corresponds to one of the BCCH bursts shown in FIG.  8 . The BCCH message structure allows “start of tree” BCCH bursts to occur every 1, 2, 3, 4, 6 or 8 BCCH bursts  42 . Please note that the number five is missing from this list. Please, also note that five, being a prime number, is not exactly divisible into any of the numbers 1, 2, 3, 4, 6 or 8. Thus, the term (500×N) represents the number of timeslots  40  in five repeats of the number of BCCH message frames  46  between repeated paging opportunities and allows ultimate synchronisation with “start of tree” even if the user terminal  34  started out on the wrong BCCH period  44 . 
     Thus P 1  represents an incremental paging group number calculated by the user terminal  34 . The user terminal keeps track of the instant ATN by means of an internal clock and occasional updates from the BCCH bursts  42 . The incremental paging group number P 1  is then user to calculate a timeslot allocation number (P 2 ). 
     The term (N×PR) represents the total number of paging channel  48  timeslots  40  reserved for each BCCH message frame  46 , multiplied by the number N of BCCH message frames  46  between repeat paging opportunities. The term ATNdiv (500×N) increments by one for every five sets of BCCH frames  46  (there being N per set) between repeat paging opportunities, and the incremental count is rolled over (counted to a moduls) with a moduls equal to the total number of paging opportunities in the set of potential paging channels  48  timeslots  40  so that the count cycles around the number of possible paging channel timeslots  40 ,  48 . 
     Thus, P 1  is a number, which cycles with the BCCH bursts  42 , P 1  starting with the paging group number, increments by one for every time the ATN becomes exactly divisible by the number of timeslots  40  in the number of BCCH message frames  46  between repeated paging opportunities, increments by one every fifth occasion of divisibility of the ATN by the number of timeslots  40  in the number of BCCH message frames  46  between repeat paging opportunities, and is counted by the modulus of the total number of allocated PCH timeslots between repeat paging opportunities. 
     The timeslot allocation number P 2  is calculated using the equation: 
     
       
           P 2=2+2( P 1mod PR )+(25 ×P 1 divPR ) 
       
     
     The purpose of P 2  is to distribute timeslots  40  about the BCCH message frames  46 . 
     In the preferred embodiment of the invention, the paging channel timeslots  48  must be at least one space away from the BCCH burst  42  so that the phase locked loop synthesiser, which tunes the receiver in the user terminal  34 , has time to adjust to the frequency of the paging channel PCH. The first term in the expression for P 2 , the number two, reflects this spacing. 
     The second term in the expression for P 2 , 2×(P 1 modPR), expresses the preference of the embodiment of the present invention that paging channel PCH timeslots  48  are separated by at least one non-PCH timeslot  40 . This term takes the incremental paging group number P 1 , counts it modulo the number of paging channels  48  allocated per BCCH message frame  46  (which causes unit increment for each increment of P 1 ), and multiplies the value by two (which causes the overall incrementation to be an even number. Thus, since, overall, the second term increments by two for every unit increment of the incremental paging group number P 1 , the allocated paging channel timeslots  48  will be separated by at least one timeslot period  40 . 
     The third term (25×P 1 divPR) reflects the number of timeslots  40  in a BCCH message frame  46 . Every time the incremental paging channel number P 1  increments by the number of potential paging channel allocations (PR) per BCCH frame  46 , and the result becomes integrally divisible by the total number of PCH timeslots 48 per BCCH frame, the Timeslot allocation number  25  is incremented by twenty-five, thus moving the timeslot allocation onto the next BCCH period  44 . 
     Finally, the user terminal  34  calculates which ATNp (absolute timeslot number of the paging channel) is to be used as the allocated paging channel  48  using a third equation 
     
       
         ( ATNp−BCCH   —   TN−P 2)mod(25 ×N )=0 
       
     
     That is to say, when the difference between the timeslot number of the BCCH (BCCH_TN) differs from current timeslot number (ATN or ATNp for the zero result) by the timeslot allocation number, by zero, or an integral multiple of the number of potential timeslots  40  in the number (N) of BCCH frames  46  between potential paging opportunities, that timeslot (ATNP) is usable as the paging channel  48  for that user terminal  34  on that occasion. 
     The routine for the user terminal  34  is thus 
     1) use the IMSI on the SIM card to calculate a paging group 
     2) calculate instant incremental paging group number (P 1 ) by counting the absolute timeslot numbers (ATN) of each timeslot  40  as it occurs. 
     3) Use P 1  to calculate a timeslot allocation number P 2   
     4) Use P 2  to calculate actual timeslot (ATNp) of the current paging channel (PCH,  48 ). 
     5) If desiring use of the paging channel, count timeslots (ATN) and select ATNP. 
     The purpose of each element is: 
     1) P 1  provides an incremental paging group number for the particular user terminal  34  which increments with every group of BCCH message frames  46  between paging opportunities (each N frames  46 ) and also increments to allow synchronisation with the BCCH message structure, causing the allocated PCH timeslot  48  to “walk” relative to the BCCH bursts  42  so that and collision due to simultaneity is only transient. 
     2) P 2  takes the allocated paging group (incremented) and spreads out the paging channels (PCH or  48 ) between the available non-BCCH timeslots  40  in the set of N BCCH frames  46  between possible paging opportunities. 
     3) The timeslot selection equation allows the user terminal  34  to count (ATN) timeslots as they come up and select the allocated paging channel  48  when the appropriate timeslot ATNp appears. 
     The four expressions, described above, could, of course, be combined as a single expression. The split has been made purely for ease of description. 
     The present invention would work equally well with other expressions for the elements P 1  P 2 , provided a substantially similar result occurred. In particular, while the allocated paging channel  48  is shown, in this example, to cycle around the available timeslots  38  from place to adjacent place by an incremental process, it is to be understood that the present invention will function equally well with other means of distribution of the allocated paging channel  48  in a cycle around the available timeslots  48 , such as a pseudo-random sequence. or any other function which will achieve the same cyclic effect.