Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-05-22
2002-05-21
Vo, Nguyen T. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S525000
Reexamination Certificate
active
06393287
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a system for switching a speech channel between loosely coupled radio base stations, and more particularly to a system for switching a speech channel that, even when a soft hand off instruction signal from a radio base station, which currently supplies speech communication service, does not reach the mobile telephone terminal, enables soft hand off control without causing the cut off of signal transmitted between the mobile telephone terminal and the radio base station.
BACKGROUND OF THE INVENTION
Soft hand off conforming to IS (Interim Standard)-95 specifications of TIA (TELECOMMUNICATIONS INDUSTRY ASSOCIATION)/EIA (ELECTRONIC INDUSTRY ASSOCIATION) is known in the art.
FIG. 1
is an illustration showing a conventional mobile telephone communication system to which soft hand off conforming to TIA/EIA IS-95 specifications is applied. The mobile telephone communication system conforming to TIA/EIA IS-95 specifications is a mobile telephone communication system of CDMA (code division multiple access) system which has a plurality of service areas
30
a
to
90
a
and a radio base station control unit
8
. Radio base stations
3
a
,
4
a
,
5
a
,
6
a
,
7
a
,
8
a
, and
9
a
are provided on the service areas
30
a
,
40
a
,
50
a
,
60
a
,
70
a
,
80
a
,
90
a
, respectively. In fact, a plurality of radio base station control units
8
are installed and are connected to a plurality of radio base stations
3
a
and
9
a
to provide a wider range of service areas.
Different pilot channels are allocated to respective neighbor radio base stations. Specifically, pilot channels
3
ap
,
4
ap
,
5
ap
,
6
ap
,
7
ap
,
8
ap
, and
9
ap
are allocated respectively to radio base stations
3
a
,
4
a
,
5
a
,
6
a
,
7
a
,
8
a
, and
9
a
to perform transmission, and a mobile telephone terminal
7
exists which moves between the service areas.
FIG. 2
shows an example of conventional hand off table. The radio base station control unit
8
previously catalogs in the hand off table, as candidate stations for soft hand off, neighbor radio base stations, to which hand off is possible, among radio base stations to be controlled by the radio base station control unit
8
.
The radio base station control unit
8
has a threshold for the addition of a speech channel and a threshold for the deletion of a speech channel used at the time of the switching of the speech channel. In practicing the soft hand off, the radio base station control unit
8
sends, as a hand off instruction signal, the threshold for the addition of a speech channel and the threshold for the deletion of a speech channel through a plurality of radio base stations to the mobile telephone terminal
7
. The mobile telephone terminal
7
measures the receive field strength of each of a group of pilot channels designated by the radio base station control unit
8
. When the measurement results are such that there is a pilot channel having a field strength above the threshold for the addition of a speech channel or a pilot channel having a field strength below the threshold for the deletion of a speech channel, the mobile telephone terminal
7
informs the radio base station control unit
8
of the field strength level of the pilot channel, requesting the switching of the speech channel.
Each radio base station sends information on pilot channel, system information, and field strength measurement list necessary for specifying a radio base station used at the time of initiation of call processing over paging channel (P-ch) to the mobile telephone terminal which exists in the service area of the radio base station. The mobile telephone terminal
7
present within the service area
80
a
recognizes from a signal on P-ch that the radio base station used at the time of the initiation of call processing is a radio base station
8
a
and that the field strength should be measured for pilot channels
3
ap
,
7
ap
,
8
ap
, and
9
ap.
As soon as the request for call connection from the mobile telephone terminal
7
is sent to the radio base station control unit
8
through the radio base station
8
a
, the radio base station control unit
8
, upon the completion of the establishment of the speech channel in the radio base station
8
a
, sends a neighbor list signal to the mobile telephone terminal
7
. The mobile telephone terminal
7
again obtains the field strength measurement list, and measures the receive field strength of each of a group of pilot channels designated by the radio base station control unit
8
. When the mobile telephone terminal
7
is moved from the radio base station
8
a
towards the radio base station
3
a
, due to the approach of the mobile telephone terminal
7
to the radio base station
3
a
, the pilot receive field strength of the pilot channel
3
ap
received by the mobile telephone terminal
7
exceeds the threshold for the addition of a speech channel. The mobile telephone terminal
7
sends a pilot field strength level report to the radio base station
8
a
. The radio base station
8
a
sends, to the radio base station control unit
8
, the pilot field strength level report received through an assembled line
9
containing control signal lines and voice signal lines. The radio base station control unit
8
specifies from the received pilot field strength level report and the hand off table that, for the radio base station
8
a
, candidate radio base stations for hand off are 3
a
,
7
a
, and
9
a
and that, among the specified candidate radio base stations for hand off, the pilot channel having a pilot receive field strength above the threshold for the addition of a speech channel is the pilot channel
3
ap
. The radio base station control unit
8
adds the speech channel of the radio base station
3
a
, and advises the mobile telephone terminal
7
through a hand off instruction signal that the speech channel of the radio base station
3
a
and the speech channel of the radio base station
8
a
would be used in soft hand off after that. After the advice, the radio base station control unit
8
also sends a neighbor list signal. Thus, the mobile telephone terminal
7
obtains a field strength measurement list after the switching of the speech channel.
The mobile telephone terminal
7
measures the receive field strength of each of a group of pilots designated by the radio base station control unit
8
. As the mobile telephone terminal
7
is further moved towards the radio base station
3
a
, the pilot receive field strength of the pilot channel
8
ap
becomes a value below the threshold for the deletion of the speech channel. In this case, the mobile telephone terminal
7
sends a pilot field strength level report to the radio base station
3
a
, and the radio base station
3
a
sends the pilot field strength level report through the assembled line
9
to the radio base station control unit
8
. The radio base station control unit
8
judges that, from the hand off table, for the radio base station
3
a
, the radio base stations
4
a
,
5
a
,
6
a
,
7
a
,
8
a
, and
9
a
are candidate base stations for hand off, that the addition of a speech channel is not carried out because the pilot receive field strength of the specified candidate radio base stations for hand off does not exceed the threshold for the addition of the speech channel, and that, from the received pilot field strength level report, the speech channel of the radio base station
8
a
having a field strength below the threshold for the deletion of the speech channel should be deleted. The radio base station control unit
8
advises the mobile telephone terminal
7
through a hand off instruction signal that only the speech channel of the radio base station
3
a
is usable by the mobile telephone terminal
7
. Thereafter, the radio base station control unit
8
deletes the speech channel of the radio base station
8
a
, and then sends a neighbor list signal to the mobile telephone terminal
7
. Thus, the mobile telephone terminal
7
obtains a field strength measurement list to be applied after
Munekata Hiroyuki
Shiotsuki Hirofumi
Nguyen Duc
Whitham Curtis & Christofferson, P.C.
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