Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
2000-04-28
2004-09-14
Chin, Vivian (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S436000, C370S331000
Reexamination Certificate
active
06792284
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The object of the invention is an improved method and arrangement for permitting and performing cell reselection made by a terminal of a cellular radio system. The invention relates particularly to permitting and performing cell reselection in situations where the terminal requires a special service, such as GPRS (General Packet Radio Service).
2. Brief Description of Related Developments
In order to understand the background of the invention we first describe as examples prior art solutions for changing the active cell in the GSM system (Global System for Mobile telecommunications) and in the associated GPRS data packet transmission service.
FIG. 1
shows the basic structure of a GSM network. In a mobile communications network there is at least one core network (CN) and one or more radio access networks (RAN). The core networks are formed by various exchange systems which in addition to versatile data transmission facilities can provide various value-added services. A core network comprises switching centres MSC (Mobile services Switching Centre), other network elements which in the GSM system are for instance the nodes associated with packet radio communication, the SGSN (Serving GPRS Support Node) and the GGSN (Gateway GPRS Support Node) and the associated transmission systems. The radio access networks are located between a core network and the terminals. A radio access network comprises base stations (BS) and a radio network controller (RNC). Each base station has a permanent connection to the respective radio access network controller. On the other hand the controllers of different radio access networks have a permanent connection to at least one node of the core network. One or more radio access networks can operate between the terminals and the core networks, and via a certain radio access network a terminal can communicate with several core networks.
A terminal of a cellular radio system always tries to select a certain base station and operate (camp) within its coverage area, or cell. Traditionally cell reselection was based on the measurement of strength of the received radio signal, either at the base station or at the terminal. For instance, in the GSM system each base station transmits a signal on the so called beacon frequency, which is different for neighboring base stations. In the GSM system a base station transmits on the so called BCCH channel (Broadcast Control Channel) the parameters p
1
and p
2
to the terminal, whereby these parameters are used to calculate of the so called C values. For instance the C
1
value commonly used in the cellular GSM network is calculated from the equation below:
C
1
:=(A-Max.(B,
0
)), where
A:=average level of the received signal−p
1
(dB)
B:=p
2
−maximum transmission power of the terminal (dB)
p
1
:=allowed minimum value for the received signal
p
2
:=allowed maximum transmission power for a terminal.
In addition the terminals must measure the reception levels of the broadcast signals which they have received, so that they will be able to calculate the C
1
value of each received cell. The cell which has the highest calculated C
1
value is the most advantageous regarding the radio connection. In order to optimise cell reselection the network can also transmit additional parameters which enable the use of so called C
2
values. A more detailed description is presented in the reference [11]. The base stations transmit to the terminals information about the BCCH frequencies used by the neighboring cells, so that the terminals will know on which frequencies they must listen in order to find the BCCH transmissions of the neighboring cells. In each cell the BCCH channel transmission also contains information about how the terminals can request random access in the respective cell in order to establish a connection.
The cells of the GSM system offer to its user the basic services: voice transmission, low-speed data transmission, and various short message services. The operation of the GSM system is standardised by ETSI (European Telecommunication Standardization Institute). The cells in the GSM system are interleaved regarding their coverage areas, so that at the cell borders it is necessary to decide which cell the terminal in question shall use. In non-active mode the terminal decides which cell to camp on, but in active mode the decision is influenced, in addition to the quality of the connection, also by other factors, for instance by the loading degrees of the cells of the network. In the dedicated mode the core network decides which of the two cells the terminal shall use, and the timing of the cell change, which it informs to the terminal. According to the ETSI standard, in the dedicated mode terminal can not independently access the service of another cell. In the idle mode the terminal decides, on the basis of the C
1
/C
2
values which it has calculated for a number of neighbor cells, to which cell it shall belong. The actual timing of the cell change either in the dedicated mode or in the idle mode is not very important regarding the terminal, because all cells of the system normally provide the same basic services, and the user of the terminal will not notice the change from one cell to another.
However, new services are being constructed on the platform provided by the GSM system, so that these new services are able to provide various value-added services to the user of a terminals. These services might not be provided over the whole coverage area of the GSM network, but only in some limited areas. One such service could be GPRS. In the GPRS service area the user of a terminal can choose whether he uses the basic GSM services or whether he utilises the value-added services provided by GPRS.
FIG. 2
shows the provision of a GPRS service over a limited area. Of the cells in
FIG. 2
the cells
1
,
3
,
6
and
8
offer the GPRS service in their areas, but in all other cells only basic GSM services are available. The figure also shows the path of a mobile terminal from point P
1
to point P
7
. In the active mode at the cell border regions the cell change may be controlled by the network, or it might be controlled by the terminal. Thus the following situation for a terminal could occur: the terminal obtains GPRS services on the distances P
1
→P
2
, P
3
→P
4
and P
6
→P
7
. On the distances P
2
→P
3
, and P
4
→P
5
→P
6
only the basic GSM services are available. If the user started in P
1
with an active GPRS connection, he will in the worst case notice that the GPRS services are interrupted for a shorter or longer period of time during an active session when moving towards P
7
. The user will be surprised by the service interruptions, because in the user's opinion he is moving within the GPRS coverage area made known by the service operator. Another example of an undesirable case is that the terminal can not establish a desired GPRS connection to a cell supporting the GPRS service when the terminal is located in an unsuitable place, for instance in
FIG. 2
, on the distance P
2
→P
3
. This situation could occur even when the terminal is located clearly within the GPRS coverage area defined by the network service, if the terminal, due to C
1
/C
2
values are forced to reselect to a cell not supporting GPRS service. A third inconvenient case is the point P
7
, which is located at the border of the cells
6
and
7
. In the worst case the terminal has to make repeated cell changes between a cell supporting GPRS service and a cell not supporting GPRS service. This means that the user will experience the situation where he at certain times has GPRS service available and at other times does not have GPRS service available, even though he is at the location all the time.
The GPRS standards and the proposed revisions to them have tried to solve the above presented problem in the following way. The network permits that all terminals supporting GPRS and camping in a GPRS cell c
Dalsgaard Lars
Lundell Antero
Chin Vivian
Nokia Mobile Phones Ltd.
Perman & Green LLP
West Lewis G.
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