Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-11-25
2002-12-10
Chin, Vivian (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S525000, C455S436000
Reexamination Certificate
active
06493555
ABSTRACT:
The present invention relates generally to cellular mobile radiocommunications systems, more particularly to managing radio resources in such systems, and even more particularly to managing radio resources in such systems while handing over a call between cells (referred to as “inter-cell call handover”).
BACKGROUND OF THE INVENTION
With reference to
FIG. 1
, the general architecture is recalled of a cellular mobile radiocommunications system such as, in particular, the Global System for Mobile communications (GSM). Such a system essentially comprises:
a plurality base stations or “Base Transceiver Stations” (“BTSs”), such as those referenced BTS
1
, BTS
2
, BTS
3
, and BTS
4
; in the example shown, each of the base stations is assigned to a respective cell, the cells being referenced C
1
, C
2
, C
3
, and C
4
(other examples naturally being possible in which the same BTS is assigned to a plurality of cells), and the base stations co-operate with mobile stations, such as MS
1
, MS
2
, MS
3
, MS
4
, situated in the cells;
a plurality of “Base Station Controllers” (“BSCs”), such as those referenced BSC
1
and BSC
2
, each of the BSCs controlling a respective subset of cells from the point of view of managing the radio resources; in the example shown, the Base Station Controller BSC
1
controls the cells Cl and C
2
, and the Base Station Controller BSC
2
controls the cells C
3
and C
4
; and
a plurality of “Mobile Switching Centers” (“MSCs”), such as that referenced MSC
1
, each of the MSCs co-operating with external networks, such as, in particular, Public Switched Telephone Networks (PSTNs) or Integrated Services Digital Networks (ISDNs), and controlling a subset of Base Station Controllers from the point of view of managing calls; in the example shown, the Mobile Switching Center MSC
1
controls the Base Station Controllers BSC
1
and BSC
2
.
In such a system, a decision to hand over a call is made by the network on the basis of a certain number of parameters, such as, in particular, the results of measurements taken on radio signals for the current server cell and for cells neighboring the current server cell.
The cell to which the call is to be handed over may be referred to as the “new server cell”, and it is more precisely chosen from a set of cells referred to as “candidate cells” (determined by the network from said neighboring cells on the basis of said measurement results) in the following manner.
Firstly a request is made to hand over the call to the best of the candidate cells. When such handover is possible, the best candidate cell constitutes the new server cell. When such handover is not possible, a request is made to hand over the call to the following candidate cell, in the order of preference of candidate cells, and so on.
A candidate cell to which handover is requested may also be referred to as the “target cell”.
A target cell to which a requested handover is not possible is also referred to as a “rejected cell”. A target cell can be rejected in particular because the mobile station does not succeed in setting up a link with the network in the target cell, or else because no radio resource is available in said target cell, because of traffic overload, or because a testing or maintenance operation is in progress in the target cell, this list naturally not being exhaustive.
When the target cell is controlled by the same BSC as the current server cell (in
FIG. 1
, this situation corresponds, for example, to handover being requested from cell C
1
to cell C
2
, or from cell C
3
to cell C
4
, such handover also being referred to as “internal handover” or “intra-BSC handover”), the BSC in question is capable, by itself and internally, of determining whether or not handover to the target cell is possible, i.e. whether or not the target cell is a rejected cell, since said BSC manages the radio resources for both of the cells.
When the target cell is controlled by a BSC (referred to as the “target BSC”) other than the BSC (referred to as the “current server BSC” or more simply below as the “server BSC”) that controls the current server cell (also referred to more simply below as the “server cell”), this situation corresponding for example, in
FIG. 1
, to a request to hand over a call from one or other of the cells C
1
and C
2
to one or other of the cells C
3
and C
4
(such handover also being referred to as “external handover”, or “inter-BSC handover”), the server BSC is not capable itself and internally of determining whether or not handover to the target cell is possible, i.e. whether or not the target cell is a rejected cell, and the procedure is then as shown in one or other of
FIGS. 2 and 3
, namely:
the server BSC, referenced BSC
s
, firstly sends a handover request message (referenced HO REQ in this example for “handover required”) to the MSC;
the MSC in turn sends a message of the same type (referenced HO REQ′ in this example for “handover request”) to the target BSC, referenced BSC
c
;
after having determined internally whether or not such a handover to the target cell is possible, i.e. whether or not said target cell is a rejected cell, the target BSC sends to the MSC either, when the target cell is not a rejected cell (FIG.
2
), a handover request acknowledgment message (referenced HO REQ ACK in this example) itself containing a handover command message, or else, when the target cell is a rejected cell (FIG.
3
), a message indicating that it is impossible to perform handover to said target cell (this message being referenced HO FAIL in this example for “handover failure”); and
the MSC in turn sends a message of the same type to the server BSC, namely either a handover command message (referenced HO COMM in this example), or else a message indicating that it is impossible to perform handover to said target cell (this message being referenced HO REJ for “handover request rejected”).
As shown in
FIGS. 4 and 5
, once a handover (or “transfer”) T
1
from one cell C
i
to another cell C
j
has actually been performed, it is possible that a request might be made for a handover T
2
to a cell C
k
that was rejected previously during the handover T
1
. If the cell C
k
was rejected previously, then a handover to it was requested previously, i.e. it was probably one of the best candidate cells, and it is thus normal, in particular if the request for handover T
2
is made soon after handover T
1
, that it reappears as one of the best candidate cells for handover T
2
, and thus that handover to it is requested once again.
As shown in
FIG. 4
, when both of the cells C
i
and C
j
are controlled by the same BSC, referenced BSC
ij
, the BSC is aware of that fact that the cell C
k
was rejected during handover T
1
, and is thus able to decide that, temporarily, a request for such a handover T
2
to cell C
k
must not be made.
As shown in
FIG. 5
, when the two cells C
i
and C
j
are not controlled by the same BSC, the current server BSC, referenced BSC
j
, is not aware of the fact that cell C
k
was rejected during handover T
1
(because only the initial server BSC, referenced BSC
i
, is aware of that fact), and it is thus not able to decide that, temporarily, such a request for handover T
2
to cell C
k
must not be made.
Thus:
if the cells C
j
and C
k
are not controlled by the same BSC, then the current server BSC (BSC
j
) uses the procedure recalled above with reference to
FIGS. 2 and 3
to send a handover request to the target BSC (not shown) controlling the cell C
k
, and it then might receive from the target BSC a message indicating that it is impossible to perform handover to the cell C
k
. An unnecessary handover request will then have been made, thereby lengthening handover time unnecessarily, and giving rise to unnecessary consumption of transmission resources for the purpose of interchanging the corresponding signalling messages, and thus degrading the quality of service; and
if the cells C
j
and C
k
are controlled by the same BSC, the current server BSC (BSC
j
) might unnecessarily and internally have to determine whether a handover t
Roberts Michael
Saada Stéphane
Alcatel
Chin Vivian
Ramos-Feliciano Eliseo
LandOfFree
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