GB2406751A - Cyclical paging in a mobile communications system - Google Patents

Abstract

The present invention relates to the field of paging in telecommunications. When the "wake up" paging channel is used for another service, e.g. Multimedia Broadcast Multicast Services (MBMS), then certain mobiles expecting a "wake up" page will be woken up unnecessarily, reducing battery life. It is likely that a small group of terminals will have regular paging cycles which coincide with the MBMS paging signals, and therefore receive a large number of unnecessary wake ups. This will considerably reduce the battery life of this group of terminals. The aim of the invention is for a large number of terminals to receive a small number of unnecessary wake ups, therefore spreading a smaller reduction in battery life over more users. Three methods are described; using different values of cycle length for regular and MBMS paging, MBMS paging having a variable cycle and a different paging indicator for the two pages.

Description

240675 1 Paging in a Mobile Communications System This invention relates

to paging in a communications system, particularly hut not exclusively to managing and controlling individual and group paging.

A long battery lifetime is important t'or most types of user equipments (lJE), such as mobile telephones or personal digital assistants (PDAs). To increase the battery lifetime of such mobile devices discontinuous reception (DRX) is used in cellular communications systems hke Universal Mobile Telecommunications Systems (UMTS). In DRX mode, the mobile device is in a sleep mode from which it awakes periodically to check if the network wants to initiate communication towards it. Discontinuous reception is applied when mobiles operate in a low activity mode i.e. when it is neither engaged in a speech call nor active in a dale transfer session. Discontinuous Reception mode is described for example in "lJser Equipment (UK) procedures in idle mode and procedures for cell reselection in connected mode (RRC)", 3GPP TS 25.304, version 3.13.0 (2003-06) and Radio Resource Control Specification (RRC) 25.331, version 3. 15.() (20()3-06) by 3GPP (A Generation Partnership).

In order to initiate communication from the network towards a UK, in UMTS the network provides a Paging Indicator Channel (PICH). This PICH is a physical channel used to carry the paging indicators (PI). The network sets, for each UK, a number of bits (i.e. the PI) on the PTCH to indicate that the network wants to initiate a communication with the particular UK.

If upon wake-up from DRX these PI are set tor a particular UK, the UE reads the Paging channel (PCH). The PCH is used to transfer the Paging message which includes further details about the communication the network wants to imitate.

MATS also provides for services such as Multimedia Broadcast Multicast Services (MBMS). MBMS services are usually designed for a large group of users and may for example be used to provide the user with stock exchange news or short video clips of a particular kind. The user might tor example be interested in obtaining information about certain soccer games, such that the interested users are provided with video clips of every soccer goal achieved In a particular game or league.

Such services like the MBMS should be available to UEs in sleep mode. Consequently, MBMS requires a mechanism to trigger a group of mobiles to acquire the MBMS information. In the following this mechanism is referred to as "MBMS paging". The MBMS paging triggers the UE to read further information on another channel like the PCH channel or the MCCH channel.

MBMS paging may be designed such that it is specific for one particular MBMS service. In this case, only mobiles interested in a particular MBMS service are woken up in order to acquire information. Service specific MBMS paging may avoid power consumption and therefore battery drain for a large number of UEs, as not all UEs are required to wake up after paging for a specific MBMS service occurs. However, service specific MBMS paging requires suf'f'icienL means to distinguish different services on the channel used to indicate paging.

One possible solution is to re- use, for the MBMS paging, the existing PICH channel, PTs and paging occasions for the regular paging. This solution means that the same paging indicators that are currently allocated to a particular UE or a particular group of UEs might also be used to trigger a group ol'MBMS interested mobiles for a particular MBMS service.

However, the above Implies that when MBMS paging is performed, UEs listening for regular paging on the PTCH used for the MBMS paging will wake- up without finding the expected paging message on PCl1. These useless wake- up occasions drain the mobile's battery. Moreover, in case an MBMS paging always collicles with the same paging indicator of a particular UE or group of UEs, the same IJE or the same group of UEs is affected every time MBMS paging is performed.

It is therefore an object of the present invention to alleviate the disadvantages discussed above.

It is another aim ol the present invention to provide a system and method for managing and coordinating a first and a second paging mechanism, which both use the same paging indicator channel.

According to one aspect of the present invention, there is provided a method of paging a plurality of mobile terminals in a discontinuous reception system, wherein one paging indicator channel and a plurality of paging indicators on said paging indicator channel are used for cyclically paging a t'irst and a second service, the method comprising the steps of: i) for each mobile station, defining first paging instances; and ii) for a particular second service, defining second paging instances, wherein the first paging incidences are re-used for the second service and wherein coincidences between the pagmg instances for said t'irst and second service are spread between multiple of said mobile terminals.

It Is noted that paging instances include both paging occasions and paging indicators. Paging instances coincidence when both paging occasions and paging indicators coincidence.

In this way, when performing MOMS paging, it is not always the same mobile terminal or the same group of mobile terminals listening for regular paging, that wakes- up uselessly. instead, the useless wake-ups are distributed between dil'f'erent mobile terminals or different groups of mobile terminals.

This means that also the resulting battery drain is distributed more evenly between different mobiles.

According to another aspect of the present invention, there is provided a cellular communication system, comprising a plurality of terminals and one or more base stations, wherein said communications system is adapted to page said plurality of terminals via a paging indicator channel from said one or more base stations using a first paging mechanism, and wherein said system is further adapted to page at least some of said terminals via said paging indicator channel using a second paging mechanism in group mode, said system further comprising means for managing said first and second paging mechanisms and controlling interference between said first and second paging mechanisms so as to spread interference between multiple said terminals.

According to another aspect of the present Invention, there is provided a communications system, adapted to provide a first service using individual paging to a plurality of terminals, and to provide a second service using group paging to at least some of said terminals, said system further comprising means for managing the paging for said first and second services and for controlling interference between the paging for the first and second services, so as to spread interference between multiple said terminals.

Embodiments of the present Invention will now be described, by example only, with reference to the accompanying figures, whereby Fig. IA and 1B are schematic outlines of the mobile telecommunications network, in which the present invention can be Incorporated; Fig. 2 is a schematic illustration ol the frame structure of one radio frame of the paging indicator channel in the network shown in Figs. 1A and 1B; Fig. 3 is a schematic illustration of regular paging and MBMS paging reusing paging occasions and paging indicator according to the prior art; Fig. 4 is a schematic illustration of regular paging and MBMS paging reusing paging occasions and paging indicator according to embodiments of the present invention; Fig. 5 is a schematic illustration of the paging occasions and paging Indicators of regular paging and MBMS paging reusing paging occasions and

paging indicator according to the prior art;

Fig. 6 is a schematic illustration ol the paging occasions and paging indicators of regular paging and MBMS paging reusing paging occasions and paging indicator according to one embodiment of the present invention; and Fig. 7 is a schematic illustration of the paging occasions and paging indicators of regular paging and MBMS paging reusing paging occasions and paging indicator according to another embodiment of the present invention.

In Figure IA a schematic outline of a mobile telecommunications network according to the Universal Mobile Telecommunications System (UMTS) standard Is shown. The typical architecture of such a network comprises mobile user equipments (UEs) 8, an UMTS Terrestrial Radio Access Network (UTRAN) 3 and one or more core networks (CNs) 1. UMTS is a third generation radio network using wideband code division multiple access (W-CDMA) technology.

The core network I may comprise Mobile Switching Centre (MSC) or Serving GPRS (General Packet Radio Services) Support Nodes (SGSN). The core network is connected via communication links to a number of Radio Network Controllers (RNCs) 4. The RNCs are dispersed geographically across areas served by the core network 2. Each RNC 4 controls one Radio Network Subsystems (RNSs), including one or more base stations 6 such as "Nodes B" located remote from, and connected by further communication links to, the RNC 4. Each base station 6 transmits radio signals to, and receives signals from, user equipment or terminal 8 which is in an area served by that base station 6. The area is referred to as a "cell". A UMTS network is provided with a large number of such cells, which are ideally contiguous to provide continuous coverage over the whole network territory. See the UTRAN Overall Description, 3GPP TS 25.401, version 3. 10.0 (2003-06) by 3GPP for more details.

As explained above, the UMTS systems may apply discontinous reception mode in order to increase the battery lifetime of UEs. Regular paging on the paging indicator channel is used to wake up UEs in sleep mode in order to initiate communication with the UK. Regular paging is used to wake-up a specific UK. The period between successive wake- ups is called the DRX cycle. Within the DRX cycle, each UE wakes up at a certain system frame number (SFN): the UE's paging occasion. At this specific paging occasion, the UE monitors a number of bits within the PICH frame: the UE's paging indicator (Pi). More information about paging in UMTS systems may be found in "Physical channels and mapping of transport channels onto physical channels (FDD)", 3GPP TS 25.21 1, version 3.

Referring now to Figure 2, the structure of a PICH frame is schematically illustrated.

One P1CH radio frame lasts 1() ms and consists of 300 bits. Of those, 288 bits are used to carry paging Indicators. The system can be configured to carry 18, 36, 72, or 144 paging indicators. The remaining 12 hits are reserved lor future use and are currently not be transmitted.

In Frequency Division Duplex (FDD) mode, the paging occasion for a mobile is defined as follows: Paging occasion= (IMSI div K) mod (DRX cycle length) + n * DRX cycle length ( 1) whereby K Is equal to the number of Seconday Common Control Physical Channels (SCCPCHs) carrying a paging channel, as included in System Information Block type 5, and n is a natural number n = 0,1,2... as long as SFN Is below its maximum value as determined by the DRX cycle length.

This formula illustrates that parameter DRX cycle length corresponds to the duration between successive paging occasions. The DRX cycle length is a parameter the network can configure. The above formula also illustrates that the mobile specific offset within this period is determined by 0JJ.set = (IMSI Lid K) mod (DRX cycle length) (2) The paging indicator for a mobile is defined as follows Paging indicator= DRX Index mod Np (3) whereby the DRX Index is given by DRX index = IMSl div 8192, and Np is the number of paging indicators per l'rame.

In this regard it is noted that if the UE has no IMSI, for instance when a user makes an emergency call without USIM, the UE shall use IMSI = 0 and DRX cycle length = 256 (2.56 s) as default numbers In the previous formulas ( I), (2) and (3).

The UE may he attached to dii't'erent Core Network (CN) domains, whereby each core network uses a different DRX cycle lengths. In this case the UE applies the shortest of those DRX cycle lengths.

In case the UE is in UTRAN connected mode, the DRX cycle length is the shortest of the UTRAN DRX cycle length and the CN domain specific 1() DRX cycle lengths of the CN domains the UE is only attached to with no signalling connection established.

Recently, a new service is discussed to be included in the UMTS network, net the Multimedia Broadcast Multicast Services (MBMS), as described above.

The embodiments described in the following apply to the case that the MBMS paging is specific t'or one particular MBMS service and that the PCH used for regular paging is re-used for MBMS paging. In this case, only mobiles interested in a particular MBMS service are woken up in order to acquire information and that the same paging indicators that are currently allocated to a particular UE or a particular group of UEs will also be used to trigger a group of MBMS interested mobiles for a particular MBMS service.

However, as described above, in this case, when MBMS paging is performed, UEs listening t'or regular paging at the PI used for the MBMS paging will wake- up without finding the expected paging message on the PCH.

MBMS paging occasions may for example be defined by MBMS pagings 'ccasioz= (MBMS ID motel MBMS DRX cycle length) + n * MBMS DRX cycle length (4) whereby the MBMS ID is an identification number that is specific for each MBMS service. The MBMS DRX cycle length can either be different or the same for different MBMS services.

The paging indicator for a MBMS is then given by MBMS p'ging indicator= MBMS DRX Index Nod Np (5) Hereby the DRX index is given by DRX Index = MBMS ID div 8192, and Np is the number of paging indicators per frame.

The above implies that when MBMS paging is performed, mobiles listening t'or regular paging at the PI used for the MBMS paging will wake up without finding the expected paging message on PCH. This useless wake up drains the mobile's battery. Moreover, in case an MBMS service always collides with the same paging indicator, the same UE or the same group of UEs Is affected every time MBMS paging is performed for this service.

Referring now to Figure 5, this will be illustrated by means of an example. Consider a UE with DRX cycle length of 32, an IMSI of 123 and a single S-CCPCH carrying the PCH (i.e. K= 1). According to equation (1) above, the paging occasions for "regular paging" are then given by regular paging occasions= 27, 59, 91, 123, 155, .. The paging Indicator is 0.

Figure 5 is a schematic illustration of the frame of the paging indicator channel. The paging occasions and paging indicators in the PICH for the terminal are marked by hatching.

Furthermore, consider an MBMS service with an MBMSDRX cycle length ol 64 and an MBMSTD of 91. According to the formula above, the paging occasions lor "MBMS paging" are then given by MBMS pagmg occasions= 27, 91, 155, .. The paging indicator is 0.

Again, the MBMS pagmg occasions and paging indicators for that particular MBMS service are marked in Figure 5 As can be seen from Figure 4, the paging for a service using MBMSTD 91 will uselessly wake- up a mobile listening for regular paging with IMSI 123 at every other paging occasion. Within an SFN cycle -if 4096 frames, this UE wakes up 64 times.

This would mean that the battery lifetime of this specific UE is significantly reduced.

This is schematically illustrated in Figure 3. Figure 3 shows mobile terminals I to 15. Ol those, mobile terminals 10 to 15 are subscribers to a particular MBMS service. As explained above, the same paging indicator channel is used for the regular paging and the MBMS paging, and therefore the paging occasions of the MBMS service co-incidences with the regular paging occasions of one particular mobile terminal, here mobile terminal 2 marked by the thick circle in Figure 3.

Likewise, UEs responding to paging for MBMSID91 also wake- up uselessly when regular paging for a mobile with lMS1 123 is performed.

According to embodiments of the present invention, instead of impacting the same UE or same group of UEs with unnecessary wake-ups at every MBMS paging, every MBMS paging collides with the regular paging of a different UE or a dit't'erent group ot' UEs. Thus the battery lifetime of one specific UE or a group of IJEs is not drastically reduced as described in the example above. Instead, the battery lifetime of a large number of UEs is reduced with a small acceptable percentage.

This is schematically illustrated in Figure 4. Similar to Figure 3 described above, Figure 4 shows mobile terminals 1 to 15, and mobile terminals 10 to 15 are subscribers to a particular MBMS service. Again the same paging indicator channel is used for the regular paging and the MBMS paging. However, according to embodiments of the present invention, the paging incidences of the MBMS service co-ncidences with regular paging incidences of alternate mobile terminals, here mobile terminals 2, 3, 7 and 9.

All tour terminals are marked by circles In Figure 4. However, as the paging occasions of the MBMS service coincidence alternately with different mobile terminals, the number of unnecessary wake-up per telephone is significantly reduced compared to the situation as illustrated in Figure 3 above. Therefore, the mobile terminals which are affected by the collisions of paging occasions are considerably less affected by power drain compared to terminal 2 in Figure 3.

In the following, three dtt'ereat embodiments of the present invention are described. ]3

First Embodiment According to this embodiment, collision of paging occasions are avoided by using a different values of the DRX cycle length for regular and tor MBMS paging. According to the specifications, the DRX cycle length for regular paging is given by 2k frames (i.e. 4,8,16 etc) In FDD mode (see 3GPP TS 125 304) . If for MBMS paging a different DRX cycle length is chosen, such as for example 3k frames (i.e. 9,27,X 1 etc.) , collisions with one particular UE or a group of Ues can be significantly reduced.

Below, we illustrate the first embodiment by means of an example.

Consider a DRX cycle length of 32, a single S-CCPCH carrying one PCH (i.e. K= 1), an IMSI of 123, and an MBMS DRX cycle length ol 81 and an MBMS ID of 108.

According to the equation (1) above, the regular paging occasions can he determined to be 27, 59, 91, 123, 155, .., whereas, according to equation (4), the MBMS paging occasions are 27, 108, IX9, Again, the paging indicators In both cases are 0.

We refer to Figure 6, which illustrates again the regular paging occasions in the PICH and the MBMS paging occasions tor that particular service.

As can be seen from Figure 6, the paging for a service using MBMS ID 108 will uselessly wake- up a mobile with IMSI 123 only at paging occasions 27 and 2619, i.e. only twice within a full SFN cycle. This means the number of useless wake- ups for this specific UE is significantly reduced i.e. by a factor 32 compared to the previous example.

Note that In this example the duration between the last paging occasion of an SFN cycle and the first of the next SFN cycle is only 46 frames rather than the regular 81 (the MBMS DRX cycle).

Second Embodiment According to the second embodiment collision of pagmg occasions can be avoided by introducing for MBMS paging a variable DRX cycle.

Below there is a described an example wherein MBMs paging is given by a variable deviation from a constant DRX cycle i.e. by adding a variable to the MBMS ID in every DRX cycle. The MBMS paging occasions is then given by MBMS paging occasion=((MBMS ID + n) mod MBMS DRX cycle length) + rz * MBMS DRX cycle length (6) Useless wake- ups can be reduced to once per SFN cycle as long as MAX(n) < MBMS DRX cycle length (both expressed in frames).

In this case the duration between successive paging occasions Is never constant while there also is a step at the end of the SFN cycle. In the example given above the duration between successive paging occasions Increases by one from paging occasion to paging occasion.

Third Embodiment According to this embodiment the paging occasions arc uncharged.

However collision is avoided by changing the paging indicator. This can tor example, be achieved by introducing a variable deviation for the paging indicator bits. One example is to add the SFN to the MBMS DRX Index when calculating the MBMS paging indicators: MBMS Magic indccator=(MBMS DRX Index + SFN) mod Np (7) For regular paging, the same PI are used at every paging occasion.

Thus, if for MBMS paging the P1 is varied for every paging occasion, the amount of useless waLe-ups is reduces by a factor ol' I/(Number of PI per paging occasion). This embodiment is illustrated in Figure 7. Figure 7 illustrates the same example as described above with rel'erence to Figure 5.

However, for MBMs paging then a different paging indicator is used for every paging occasion. This is illustrated for one paging occasion in Figure 7.

Note that this embodiment allows a collision reduction between MBMS paging and regular E'agmg t'or one specific UE or group of UE while still using an MBMS DRX cycle length which is equal to the DRX cycle length for regular paging.

Note that for the first and the second embodiments the interval between successive paging occasions Is not constant. When a different value range is used for the MBMS DRX cycle (i.e. not 2k frames), the dift'erence occurs only at the end of the SFN cycle and could result in a somewhat increased response delay. This response delay can be up to twice the DRX cycle length. In case a variable deviation is used, the Interval between successive wake- ups is never the same. Also in this case, the interval between successive paging occasions has the biggest step at the end of the SFN cycle.

This IPR proposal is mainly applicable to UMTS, although applicability on other similar systems is not excluded.

Whilst in the above described embodiment the present invention has been described in the context of FDD mode, it is appreciated that the present invention can be applied in to other modes, 1'kc for example in time division duplex (TDD) mode.

Whilst in the above embodiments the present invention has been described in the context of UMTS, it is appreciated that the present Invention can be applied in the same manner to other systems.

Claims (1)

1. Claims 1. method of pagmg a plurality of mobile terminals in a

   discontinuous reception system, wherein one paging indicator channel and a plurality of paging indicators on said paging indicator channel are used for cyclically paging a first and a second service, the method comprising the steps of: i) for each mobile station, defining first paging instances; and it) t'or a particular second service, deeming second paging instances, wherein the first paging incidences are re-used for the second service and wherein coincidences between the paging instances for said first and second service are spread between multiple of' said mobile terminals.

   I 5 2. A method according to claim I, whereby eoineiclenees between paging instances are avoided by defining clff'erenl paging occasions for said first and second service.

   3. A method according to claim I or claim 2, whereby 2() coincidences between paging instances are avoided by defining dit'ferent paging indicators for said first and second service.

   4. A method according to claims 1, 2 or 3, said paging for said second service coincides alternately with different terminals or different groups of terminals.

   5. A method according to any preceding claim, wherein a paging occasion corresponds to a predetermined system frame number within said cycle.

   6. A method according to any preceding claim, wherein the first paging occasion for a particular mobile terminal is deLermmed by the t'ollowing equation: paging occasion = (IMSI do K) mod (cycle length) + n* cycle length wherein IMSI is the international mobile subscriber identifier; K Is the lS number of secondary common control physical channels (SCCPH); and n = 0,1 2... as long as the system frame number of the determined paging occasion is below its maximal number.

   7. A method according to any preceding claim, wherein the second paging occasion for a said second service is determined by the following equation: (service ID mod) second service cycle length) = n* second service cycle length, wherein n = 0,1 2... as long as the system frame number of the determined paging occasion is below its maximal number.

   8. A method according to claim 7, wherein for said first and second paging a difl'erent value for said cycle length is used.

   9. A method according to claim 6, 7 or 8, wherein for said first paging the cycle length is 2k.

   to. A method according to claim 7, 8 or 9, wherein for said second paging the cycle is 3k.

   l 1. A method according to any of claims l to 6, wherein the l S duration between successive paging occasions within a cycle is varied.

   12. A method according to claim 11, wherein the second paging occasion is determined according to the following equation: second service paging occasion = second (service lD + n) mod second service cycle length) + n* second service cycle length.

   13. A method according to any of claims l to 6, wherein for said fist paging the same paging indicator Is used within every cycle, and wherein the paging indicators used for said second service are varied withm said cycle compared to said first paging.

   14. A method according to any preceding claim, applied in discontinuous reception mode.

   15. A method according to any preceding claim, wherein said second service comprises multimedia broadcast multcast services.

   16. A method according to any preceding claim, wherein paging tor first service requires individual paging.

   17. A method according to any preceding claim, wherein second service requires group paging.

   ]8. A cellular communication system, comprising a plurality of terminals and one or more base stations, wherein said communications system is adapted to page said plurality ol terminals via a paging indicator channel from said one or more base stations using a first paging mechanism, and wherein said system is further adapted to page at least some of said terminals via said paging indicator channel using a second paging mechanism in group mode, said system further comprising means for managing said first and second paging mechanisms and controlling interference between said first and second paging mechanisms so as to spread interference between multiple said terminal s. s

   19. A method according to claim 18, wherein paging occasions of said first paging mechamsm are re-used for said second paging mechanism.

   20. A method according to claim 18 or 19, wherein said paging occasions for said second service co-incidences alternately with dt't'erent terminals or cliff'erent groups of terminals.

   21. A method according to claim 18, 19 or 20, wherem said first paging mechanism applies a discontinuous reception system.

   22. A communications system, adapted to provide a first service using indviclual paging to a plurality of terminals, and to provide a second service using group paging to at least some of said terminals, said system further comprising means for managing the paging for said first and second services and for controlling interference between the paging for the first and second services, so as to spread interference between multiple said terminals.