Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-06-09
2002-03-19
Maung, Nay (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S439000
Reexamination Certificate
active
06360099
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method and apparatus for use in a communication network and, in particular, to a method and apparatus for reducing audio gaps during a handoff in a communication network.
BACKGROUND OF THE INVENTION
Digital or analog communication networks such as cellular or personal communication services (PCS) networks include infrastructure hardware that produces cells of coverage in which communication services are provided. A number of cells may overlap or abut one another to provide coverage over a significant geographical area. A user located within a cell may have access to the communications network via a mobile unit, such as a hand held portable telephone or a car telephone or the like. The mobile unit may communicate with the communication network using predetermined frequencies or digital codes associated with a particular cell with which the mobile unit is communicating. The communication network may be coupled to a conventional public switched telephone network (PSTN) to enable land line users (i.e., conventional terrestrial telephone users) to exchange information with mobile unit users.
As a user operating a mobile unit moves from one cell (e.g., a source cell) to another cell (e.g., a target cell), communication handoffs occur within both the infrastructure hardware and the mobile unit. During such a handoff, the mobile unit disconnects from the infrastructure hardware at the source cell and connects to the infrastructure hardware at the target cell. In the process of disconnecting and connecting the mobile unit may perform frequency tuning or may change digital codes to enable communication with the infrastructure at the target cell. While the mobile unit disconnects from the source cell and connects to the target cell, the infrastructure hardware at the source cell prepares to end communications with the mobile unit and the infrastructure hardware at the target cell prepares to begin communicating with the mobile unit. The timing at which the handoffs in the mobile unit and the infrastructure occur may lead to audio gaps in inbound audio (i.e., audio from the mobile unit to the infrastructure) and outbound audio (i.e., audio from the infrastructure to the mobile unit).
A portion of a prior art communication network
10
in the process of a handoff is shown in FIG.
1
. The communication network
10
, which may be a cellular network, includes a number of base transceiver stations (BTS's) (only two of which are shown)
14
,
16
, each of which provides a cell of coverage
18
,
20
, respectively. Each BTS
14
,
16
is selectively interfaced to a base site controller (BSC)
22
, which is further interfaced to a mobile switching center (MSC)
23
. The MSC
23
communicatively couples the BSC
22
to a PSTN
24
.
Each base site controller (e.g., the BSC
22
) may provide communication service to one or more BTS's
14
,
16
. The BSC
22
may include one or more transcoders or voice processors
26
,
28
that process communication information that is exchanged between the MSC
23
and a mobile unit
36
, which may be disposed within one of the cells (e.g., the cell
18
). Each of the voice processors
26
,
28
may have an associated switch
30
,
32
, which may be controlled by a central processing unit (CPU)
34
. The voice processors
26
,
28
are selectively interfaced to the MSC
23
by the switches
30
,
32
. The BSC
22
may provide message transfer and call switching functionality and may be controlled by the MSC
23
, via the CPU
34
.
As shown in
FIG. 1
, the mobile unit
36
is near the interface of the cell
18
and the cell
20
. While the mobile unit
36
is within the cell
18
, communications are handled exclusively by the voice processor
26
as represented by the solid lines connecting the voice processor
30
and the BTS
14
, via the switch
30
. At such time as the mobile unit
36
traverses from the cell
18
to the cell
20
(a determination that is typically made by both the MSC
23
and the mobile unit
36
), a handoff takes place. During a handoff, the switch
30
associated with the voice processor
26
is controlled by the CPU
34
to disconnect the voice processor
26
from the BTS
14
and to connect the voice processor
26
to the BTS
16
in both the inbound and outbound directions, such a connection is represented by the dashed lines in FIG.
1
. In addition to the switching in the BSC
22
, the mobile unit
36
switches from a frequency or code corresponding to the cell
18
to a frequency or code corresponding to the cell
20
.
Ideally, the switch
30
disconnects from the BTS
14
and connects to the BTS
16
at the exact time the mobile unit
36
switches from the cell
18
to the cell
20
because the voice processor
26
can only process one audio source (e.g., one BTS
14
or
16
) at a time. However, in reality this switching is not synchronous. Accordingly, for the communication network
10
shown in
FIG. 1
, audio will be interrupted by an audio mute. The mute occurs in both the outbound path (i.e., the path from the MSC
23
to the mobile unit
36
) and the inbound path (i.e., the path from the mobile unit
36
to the MSC
23
). The duration of the mute is the length of time between when the switch
30
switches and when the mobile unit
36
switches.
A known communication network
60
that eliminates an outbound audio mute during a handoff is shown in FIG.
2
. Like the communication network
10
shown in
FIG. 1
, the communication network
60
includes a number of BTS's (only two of which are shown)
64
,
66
, each of which provides a cell of coverage
68
,
70
, respectively. Each BTS
64
,
66
is selectively interfaced to a BSC
74
, and each BSC
74
typically provides communication service to one or more BTS's
64
,
66
. The BSC
74
typically includes one or more voice processors
76
,
78
, switches
80
,
82
and a central processing unit (CPU)
84
. The voice processors
76
,
78
process communication information that is sent to and received from a mobile unit
86
. The voice processors
76
,
78
are selectively interfaced to an MSC
90
, via the switches
80
,
82
. The MSC
90
provides an interface between the BSC
74
and a PSTN
94
. The CPU
84
is provided to control the switches
80
,
82
.
While the mobile unit
86
is within the cell
68
, communications are handled by the voice processor
76
, as represented by the solid lines connecting the voice processor
76
, the switch
80
and the BTS
64
. The outbound connection between the voice processor
76
and the BTS
64
is coupled through the switch
80
, which is adapted to selectively connect to either or both of the BTS's
64
,
66
. As the mobile unit
86
traverses near the cell
70
(as shown in
FIG. 2
) the MSC
90
determines that a handoff is likely to occur and the CPU
84
controls the switch
80
to establish a link from the voice processor
76
to the BTS
66
. Such a situation is shown by the dashed line from the switch
80
to the BTS
66
in FIG.
2
. Such a link provides outbound audio to the cell
70
before the mobile unit
86
reaches that cell. Thus, when the mobile unit
86
reaches the cell
70
, outbound audio will already be present at the cell
70
. Such a configuration eliminates any outbound audio mute.
The configuration shown in
FIG. 2
does not, however, eliminate an inbound audio mute caused by the fact that the switch
80
does not switch its inbound connection from the BTS
64
to the BTS
66
until the handoff actually occurs. When the mobile unit
86
is in the cell
68
, the connection from the BTS
64
to the switch
80
is in use, as represented by the solid line from the BTS
64
to the switch
80
.
When the mobile unit
86
moves into the cell
70
, communication is established from the BTS
66
to the switch
80
and the voice processor
76
, as represented by the dashed line from the BTS
66
to the switch
80
and the voice processor
76
. Additionally, the connection from the BTS
64
to the voice processor
76
may be
Chu Andrew
Kondilis Christos
Jacobs Jeffrey K.
Maung Nay
Motorola Inc.
Trinh Sonny
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