Method and arrangement for changing cells

Details Method and arrangement for changing cells Method and arrangement for changing cells

1999-09-15

2003-04-08

Le, Thanh Cong (Department: 2749)

Telecommunications

Radiotelephone system

Zoned or cellular telephone system

C436S169000, C436S169000

Reexamination Certificate

active

06546251

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to an improved method and arrangement for cell reselection by a mobile station in a cellular radio system. In particular the invention relates to cell reselection in a situation where the mobile station is attached to a special service such as the General Packet Radio Service (GPRS), for example, offered by the current cell.
To provide background for the invention it will be next described by way of example prior-art arrangements for changing the active cell in the GSM (Global System for Mobile telecommunications) system and in a GPRS service used in the GSM.
FIG. 1
shows the basic structure of digital cellular systems. Typically, a mobile communications network comprises at least one core network CN and one or more radio access networks RAN. The core networks CN consist of various central systems which may offer various intelligent network services in addition to versatile communications possibilities. A core network comprises mobile services switching centers MSC, other network elements (which in the GSM include the serving GPRS support node, SGSN, and gateway GPRS support node, GGSN, for example), and the associated transmission systems. The radio access networks are located between the core network and mobile stations. A radio access network comprises base transceiver stations BTS and a radio network controller RNC. Each base station BS has a fixed connection to the radio network controller RNC. The radio network controllers in turn have fixed connections to at least one core network node. Between the mobile stations MS and core networks CN there may operate one or more radio access networks, and through a given radio access network a mobile station MS can be connected with several core networks CN.
FIG. 2
shows the coverage areas of services offered by a cellular system. Area
20
shows the coverage of basic GSM services, and areas
21
,
28
and
29
are GPRS coverage areas within area
20
. Typically, area
21
may be the center of a city, area
28
may be e.g. an airport, and area
29
may be a research and development or other office complex where GPRS service is needed.
FIG. 3
shows in more detail the GPRS base stations
23
,
24
,
25
and
26
and their coverage areas
23
a
,
24
a
,
25
a
and
26
a
in a city center
21
.
FIG. 3
does not show the basic-service GSM base stations the coverage areas of which are within area
21
.
FIG. 3
shows that even an area that offers GPRS services may have locations, here location
27
, which are not within the coverage area of any GPRS base station. In such a location a mobile station has to connect to a base station which only offers the basic GSM service. In addition, it can be seen from
FIG. 3
that when a mobile station is moving in an area where the base station coverage areas are small, the serving base station has to be changed at short intervals.
A mobile station in a cellular radio system always tries to choose a base station coverage area to camp on such that the quality of the radio connection is adequate. Traditionally, the cell selection has been based on the measurement of the received radio signal level either at the mobile station or at the base station For example, in the GSM system each base station transmits a signal on a so-called broadcast control channel BCCH which has different frequencies at adjacent base stations. Mobile stations measure the strengths of the BCCH signals they receive and decide on the basis of the measurements which cell is the most advantageous from the radio connection quality perspective. Base stations also inform mobile stations about the BCCH frequencies used in the neighbouring cells so that the mobile stations will know what frequencies they have to listen to in order to find the BCCH transmissions of the neighbouring cells. In each cell the BCCH channel transmission also includes information about how the mobile stations can make so-called random access requests in that particular cell in order to set up a connection.
FIG. 4
shows the GSM broadcast frequency channels in a so-called
51
multiframe. In addition to the BCCH, there are included the frequency correction channel FCCH, synchronization channel SCH, and the common control channel CCCH.
FIGS. 5
a
,
5
b
and
5
c
depict the GPRS
51
and
52
multiframe structures. In the GPRS broadcast channels the GSM BCCH and CCCH channels are replaced by their GPRS counterparts, namely the PBCCH and PCCCH channels. In the
51
multiframe structure,
FIG. 5
a
, the FCCH and SCH channels are illustrated, even though they are not specified in the current GPRS specification. In the GPRS
52
multiframe structure no FCCH or SCH channels not shown, due to the fact that they are not specified in the
52
multiframe structure of the current specification. In the
52
multiframe structure shown in
FIG. 5
b
there is PBCCH (PSI) only in one radio block per multiframe, while in the
52
multiframe structure shown in
FIG. 5
c
there is PBCCH (PSI) information in four radio blocks per multiframe. Even though
51
multiframe structure is used as an example in this text, the ideas illustrated here are also valid for the
52
multiframe structures. This is because of the fact that there are no essential differences between the two frame structures In the GPRS system, the list of the base stations' neighbour cells is sent in a so-called Packet Information type
3
(PSI
3
) and Packet Information type 3bis (PSI3bis) messages transmitted on the PBCCH channel.
In the GSM system, each base station includes in its BCCH signal the data required for connection requests. In addition, each base station may regularly transmit at the ECCH frequency so-called system information (SI) messages. These messages contain information about the ability of the base station to provide advanced services, especially GPRS network service.
If a mobile station camped on a cell that offers basic GSM service needs to change to another cell, it listens to the neighbouring cells' BCCH channels and receives SI messages from these. On behalf of this information the mobile station can determine whether a cell supports GPRS service or not, and if it does, the mobile station can start operating on the GPRS channel immediately. For example, an idling mobile station can at regular intervals update its information about the neighbouring cells' characteristics by receiving SI messages so that the information can be quickly retrieved from the mobile station's memory at the moment of possible cell reselection.
FIG. 6
shows the prior-art process that takes place when a mobile station already in the GPRS service needs to change serving cell. The mobile station has already received a neighbour cell list from the serving base station. This list is used in order to continuously examine whether or not there is a need to change the serving cell, phase
60
. If the mobile station finds that cell reselection is necessary, it tunes to the neighbour cell and starts receiving the system information of the BCCH of the chosen cell, phase
61
. On the basis of these data the mobile station makes a preliminary selection of the cell and examines that the cell can offer a minimum of necessary service, phase
62
. When the mobile station has chosen a suitable cell to get service from, the mobile station starts receiving the necessary SI or PSI from this cell in order to obtain GPRS service, phase
63
. The chosen cell may or may not offer GPRS service.
A disadvantage of the method described above is that it is not sensible, as regards power consumption and communications resources, for the mobile station, while camped on a GPRS cell, to receive basic-GSM SI messages, but in practice the mobile station starts receiving SI messages only after the cell reselection procedure has started. Thus the mobile station does not keep an up-to-date list of the neighbour cell characteristics. The arrangement described above puts a load on the mobile station connected with the GPRS service because the mobile station camped on a cell first has

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