Cell selection in a packet radio network

Details Cell selection in a packet radio network Cell selection in a packet radio network

2001-01-12

2004-06-15

Banks-Harold, Marsha D. (Department: 2686)

Telecommunications

Transmitter and receiver at same station

Radiotelephone equipment detail

C455S432100, C455S435100, C370S352000, C370S329000, C370S465000

Reexamination Certificate

active

06751472

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to cell selection in a mobile telecommunications network which comprises a packet radio network. An example of such networks is a combined GSM/GPRS network.
A mobile station (MS) in a cellular system, when switched on, selects the most suitable cell. Later, when the MS is in idle mode (i.e. not having a call in progress) and it is camped normally on a cell of the selected PLMN, it monitors its neighbouring cells for a possible handover, in case one of the neighbouring cells provides better service. This process is called cell reselection. The GSM/GPRS cell reselection chapters are based on ETSI recommendations GSM 03.22, GSM 03.60, GSM 03.64 and GSM 05.08. These recommendations are incorporated herein by reference, but for convenience, the parts of those documents which are essential for understanding the invention are repeated in this application.
Cell reselection in GSM
A cell reselection can be triggered if a path loss criterion parameter C
1
indicates that the path loss to the cell has become too high; or the current serving cell has become barred; or there is a better cell in terms of the cell reselection parameter C
2
in the same LA, or a much better cell in another LA of the selected PLMN. There are other reasons for a cell reselection but they are not essential for understanding the invention (such as a downlink signalling failure, or a random access attempt fails after a certain number of repetitions).
The MS will reselect a new cell in order to achieve the following conditions which are commonly referred to as a “normal service state”: a) The registered PLMN is the selected PLMN; b) The MS is camped on a suitable cell of the LA on which the last successful LU was performed, and that cell has the highest value of C
2
for any unbarred cell in that LA; c) The value of C
2
for the MS is not lower than the value of C
2
for any suitable cell in another LA of the selected PLMN by more than the hysteresis value CRH (Cell Reselect Hysteresis).
There may be temporary conditions under which not all these are fully satisfied, e.g., during cell selection on a PLMN or while in the process of reselecting another cell. However, if the above cannot be satisfied for a certain period of time, the MS will normally enter a limited service state in which it will try to camp on an acceptable cell. In this state, only emergency calls are possible.
Parameters C
1
and C
2
are used to ensure that the MS is camped on the cell with which it has the highest probability of successful communication on uplink and downlink. The MS uses C
1
to determine whether a cell is suitable to camp on, C
1
depends on 4 parameters: i) RLA (Received Level Average) is the received signal level suitably averaged; ii) RXLev_Access_Min which is broadcast on the BCCH, and is related to the minimum signal that the operator wants the network to receive when being initially accessed by an MS; iii) MS_TXPWR_MAX_CCH which is also broadcast on the BCCH, and is the maximum power that an MS may use when initially accessing the network; and iv) P which is the maximum power of the MS.
Use of the parameter C
1
enables the MS to determine whether communication is possible with the network in the absence of interference. However, an additional safeguard against interference is used. This is to monitor the success rate of the MS in decoding signalling blocks of the paging subchannel. If there is a downlink signalling failure, i.e. the success rate drops too low, this indicates probable interference on the downlink, and the MS attempts to find another suitable cell. The path loss criterion parameter C
1
used for cell selection and reselection is defined by:
C
1
=(A−Max(B,
0
)),
where
A=RLA−RXLev_Access_Min
B=MS_TXPWR_MAX_CCH−P
All values are expressed in dBm. The path loss criterion is satisfied if C
1
>0.
In order to optimize cell reselection, additional cell reselection parameters can be broadcast on the BCCH of each cell. The cell reselection process employs a parameter C
2
that depends on these parameters. The parameters used to calculate C
2
are as follows:
Cell_Reselect_Offset which applies an offset to the C
2
reselection criterion for that cell.
Penalty_Time: When the MS places the cell on the list of the strongest carriers, it starts a timer, which expires after the Penalty_Time. This timer will be reset when the cell is taken off the list. For the duration of this timer, C
2
is given a negative offset (Temporary_Offset). This will tend to prevent fast moving MSs from selecting the cell. The all ones bit pattern on the Penalty_Time parameter is reserved to change the sign of Cell_Reselect_Offset and the value of Temporary_Offset is ignored as indicated by the equation defining C
2
.
Temporary_Offset which applies a negative offset to C
2
for the duration of Penalty_Time after the timer T has started for that cell. An infinite value can be applied, but a number of finite values are also possible.
Timer T is implemented for each cell in the list of strongest carriers. T is started from zero at the time the cell is placed by the MS on the list of strongest carriers, except when the previous serving cell is placed on the list of strongest carriers at cell reselection. In this, case, T is set to the value of Penalty_Time (i.e. expired).
The reselection criterion C
2
is used for cell reselection only and is defined by:
C
2
=C
1
+Cell_Reselect_Offset_Temporary_Offset*H(Penalty_Time−T) for Penalty−Time< >11111
C
2
=C
1
−Cell_Reselect_OffsetforPenalty_Time=11111
for non-serving cells:
H(x)=0 for x<0; and H(x)=1 for x>=0; and for serving cells: H(x)=0
Cell reselection in GPRS
In order to receive GPRS service, a cell reselection may be required. When the MS is in GPRS Idle state and it wishes to initiate a GPRS attach procedure, the following applies. If the currently camped-on cell supports GPRS then no cell reselection is required. If the currently camped-on cell does not support GPRS, then reselection of a cell supporting GPRS is required before execution of the attach procedure. If, however, the MS is in GPRS Standby or Ready state, cell selection and reselection procedures specific to GPRS are used (GPRS mode). Instead of the parameters C
1
and C
2
, a GPRS MS applies the corresponding GPRS parameters C
31
and C
32
, if provided. If the MS is in active mode, then the changes from one cell to another is performed according to the GSM handover procedures.
In GPRS Standby and Ready states, the MS performs cell reselection, except for a class A MS while in a circuit switched connection, in which case the cell is determined by the network according to the GSM handover procedures. The GPRS mode cell reselection procedures apply to the MSs attached to GPRS if a PBCCH (Packet Broadcast Control Channel) exists in the serving cell. Otherwise the MS shall perform cell reselection according to the GSM idle mode procedures. Optionally, the network may control the cell selection.
The cell reselection criteria C
31
and C
32
are provided as a complement to the current GSM cell reselection criteria. This provides a more general tool to make cell planning in GPRS as similar as possible to existing cell planning in GSM. The GPRS specific cell reselection algorithm is only applicable if the PBCCH is allocated. If the PBCCH is not allocated, the MS shall perform cell reselection according to the C
2
criteria. The following cell reselection steps shall be followed, wherein (s) and (n) denote serving cell and neighbor cell respectively: 1) Path loss criterion (C
1
). The path loss criterion C
1
>=0, is used as a minimum signal strength criterion for cell selection for GPRS in the same way as for GSM in Idle mode. 2) Signal strength threshold criterion (C
31
) for hierarchical cell structures (HCS). 3) Cell ranking (C
32
). 4) Cell reselection rules.
The MS selects the cell having the highest C
32
value among those having the highest priority clas

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