Abstract:
A method of reducing an audio gap in a connection between communication hardware and a mobile unit during a handoff between a first cell and a second cell in a communication network, wherein a first voice processor is communicatively coupled to a mobile switching center that services the first cell, is disclosed. The method includes the steps of determining when the mobile unit is likely to move from the first cell to the second cell, establishing inbound and outbound communicative coupling between the mobile switching center and a second voice processor that services the second cell when the mobile unit is likely to move from the first cell to the second cell and determining that the mobile unit has moved from the first cell to the second cell.

Description:
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&#39;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&#39;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&#39;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&#39;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&#39;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 broken or “torn down.” The voice processor  76  is only connected to either the BTS  64  or the BTS  66  because the voice processor  76  can only process one inbound signal at a time (e.g., either the signal from the BTS  64  or the signal from the BTS  66 ). The mobile unit  86  does not switch from the frequency or code associated with the cell  68  to the frequency or code associated with the cell  70  until a handoff takes place. As with the communication network  10  of FIG. 1, ideally when a handoff takes place, the switch  80  disconnects its inbound path from the BTS  64  and connects its inbound path to the BTS  66  at exactly the same time the mobile unit  86  switches. However, in reality this is rarely possible and, therefore, an inbound audio mute having a duration equal to the difference in switching times between the switch  80  and the mobile unit  86  results. 
     Therefore, there is a need for a method and an apparatus for reducing audio gaps during a handoff in a communication network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a known communication network having both an inbound audio mute and an outbound audio mute during a handoff. 
     FIG. 2 is a block diagram illustrating a known communication network having only an inbound audio mute during a handoff. 
     FIGS. 3 is a block diagram illustrating a communication network designed in accordance with the teachings of the present invention. 
     FIG. 4 is a flow diagram illustrating the process by which the communication handoff illustrated in FIGS. 3,  5  and  6  is carried out. 
     FIGS. 5 and 6 are block diagrams illustrating a communication handoff carried out on a communication network designed in accordance with the teachings of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In general, the teachings of the present invention pertain to the reduction or elimination of inbound and outbound audio mutes that occur during a handoff in a communication network. Although the communication network disclosed herein is associated with a cellular system, those having ordinary skill in the art will readily appreciate that the teachings of the present invention may be applied to any communication networks that utilize handoffs. Such communication networks may include terrestrial networks, personal communication networks and the like. 
     Referring now to FIG. 3, a communication network  102  generally includes a plurality of BTS&#39;s, only two of which are shown in FIG. 3 as  106  and  110 . The BTS&#39;s  106 ,  110  generate coverage cells  114 ,  116 , respectively. A mobile unit  120  is shown inside the cell  114 . Each of the BTS&#39;s  106 ,  110  are connected to a BSC  124 . The BSC  124  is communicatively coupled to an MSC  125 , which is further coupled to a PSTN  126 . The MSC  124  may include voice processors  128 ,  130 , communication switches  140 ,  142  and a central processing unit (CPU)  145 . The CPU  145  may include a processor  146  and a memory  148 . 
     In general, the voice processor  128  provides communication service between the mobile unit  120  in the cell  114  and the PSTN  126 , via the MSC  125  and the switch  140 . Similarly, the voice processor  130  provides communication service between the cell  116  and the PSTN  126 . However, because there are no mobile units within the cell  116 , the voice processor  130  need not be connected to the MSC  125 , as there is no audio to be exchanged between the cell  116  to the PSTN  126 . The voice processor  128  performs the function of compressing data that is to be transmitted to the mobile unit  120  via the BTS  106 . Additionally, the voice processor  128  performs the function of decompressing data received from the mobile unit  120 , via the BTS  106 , and couples the decompressed data to the PSTN  126 , via the switch  140  and the MSC  125 . Similarly, the voice processor  130  performs the functions of compressing and decompressing data that is exchanged between the MSC  125  and the BTS  110 , which provides the coverage cell  116 . 
     The switches  140 ,  142  may be embodied in kiloport switches that are known to those having ordinary skill in the art and that are capable of establishing and tearing down communication paths. The switches  140 ,  142  are controlled by the CPU  145  to communicatively couple the voice processors  128 ,  130 , respectively, to the MSC  125 . For example, the switches  140 ,  142  may be controlled by the CPU  145  to establish or tear down communication paths between one another, between the voice processors  128 ,  130  and the BTS&#39;s  106 ,  110  or between the voice processors  128 ,  130  and the MSC  125 . As a further example, the switch  140  may couple information from the MSC  125  to the voice processor  128 , and the switch  142  may couple information from the MSC  125  to the voice processor  130 . 
     The CPU  145 , which may include the processor  146  and the memory  148 , may be communicatively coupled to the MSC  125  and the BTS&#39;s  106 ,  110 . The CPU  145  is also coupled to, and programmed to control, the switches  140 ,  142 . In particular, either the processor  146  or the memory  148  of the CPU  145  may be programmed with software that controls the switches  140 ,  142  to establish and tear down communication paths between the switches  140 ,  142  themselves, the voice processors  128 ,  130 , the MSC  125  and the BTS&#39;s  106 ,  110 . Further information regarding the software or programming of the CPU  145  to carry out various aspects of the present invention will be described below with respect to FIG.  4 . 
     When the mobile unit  120  is well within the cell  114 , the various components of the communication network  102  are connected as shown in FIG.  3 . However, when the mobile unit  120  nears the edge of the cell  114  that abuts the cell  116  (as shown in FIG.  5 ), the MSC  125  of the communication network  102  detects that a handoff is likely to occur between the cell  114  (which may be referred to as the source cell) and the cell  116  (which may be referred to as the target cell). As will be appreciated by those having ordinary skill in the art, a cell and associated hardware from which a mobile unit is switching (or “handing off ) is commonly referred to using the adjective “source.” Likewise, it will be appreciated that a cell and associated hardware to which a mobile unit is switching is commonly referred to by the adjective “target.” For example, a mobile unit switches from a source cell to a target cell. When the MSC  125  determines that a handoff is likely to occur, the MSC  125  informs the CPU  145 , which performs the various steps shown in FIG.  4  and described in connection with FIGS. 5 and 6. 
     FIG. 4 illustrates a handoff process  150  that includes a number of steps that the CPU  145  performs to complete a handoff. The handoff process  150  may be programmed as software or instructions adapted to be executed by the processor  146 . The software or instructions may be stored in the memory  148 , which may be a read only memory (ROM), random access memory (RAM) or any suitable combination thereof. In particular, at step  152  the CPU  145  receives an indication from the MSC  125  that a handoff between two cells (e.g., cells  114  and  116 ) is likely to occur. The MSC  125  may indicate that a handoff is likely to occur when, for example, the mobile unit  120  is close to the boundary between two cells, such a situation is shown in FIG.  5 . 
     After the CPU  145  receives from the MSC  125  an indication that a handoff is likely to occur, the CPU  145 , at step  154 , controls the switches  140 ,  142  to establish inbound and outbound connections between the switch currently handling communications (e.g., switch  140 ) and a switch likely to handle communications after the handoff occurs (e.g., switch  142 ). Such connections are represented by the dashed lines between switch  140  and switch  142  in FIG.  5 . In particular, the CPU  145  controls the switch  140  to couple information from the MSC  125  to both of the voice processor  128  and the switch  142 . The switch  142 , in turn, couples information to the voice processor  130 , which communicates with the BTS  110 . Thus, outbound audio is coupled from the MSC  125  to each of the cells  114 ,  116  before the mobile unit  120  switches over to communicate with the cell  116 , thereby eliminating any outbound audio mute. 
     Regarding the inbound audio path, when a handoff between cells  114  and  116  is anticipated by the MSC  125 , the CPU  145  controls the switches so that the BTS  110  couples information to the switch  142 , which further couples the information to the voice processor  130  for processing. After the inbound audio information from the cell  116  has been processed (e.g., decompressed) by the voice processor  130 , the information is coupled to the switch  140 , via the switch  142 . Thus, inbound audio from cells  114 ,  116  is coupled to the MSC  125 , thereby eliminating any inbound audio mute. 
     In the inbound path, the audio from the cell  116  is coupled from the BTS  110  to the MSC  125  via the switch  142 , the voice processor  130  and the switch  140 . The switch  140  adds the information from the voice processor  128  to the information from the voice processor  130  (provided by the switch  142 ) and provides the combined information to the MSC  125 . This addition is possible because the BTS (e.g.,  106  or  110 ) associated with the cell with which the mobile unit  120  is communicating is the only BTS (e.g.,  106  or  110 ) providing audio to the switch  140 . Furthermore, this addition is typically performed by the switch  140  because the voice processor  128  may not be able to handle more than one audio signal source (i.e., audio signals from more than one BTS). The BTS that is not handling communication with the mobile unit  120  provides no audio to the switch  140 . Therefore, the sum of the audio from the BTS handling communication with mobile unit  120  and the BTS not handling communication with the mobile unit  120  is merely the audio from the BTS handling the communication with the mobile unit  120 . 
     Returning to FIG. 4, when the MSC  125  determines that a handoff between the two cells (e.g.,  114 ,  116 ) has occurred and in complete, the MSC  125  communicates such a determination to the CPU  145 , which receives the indication from the MSC  125  at step  156  of the handoff process  150 . Upon receiving an indication that a handoff has occurred, the CPU  145 , at step  158 , controls the switches  140 ,  142  to establish inbound and outbound connections between the MSC  125  and the switch  142  associated with the voice processor  130  handling communications with the mobile unit  120  (shown as the dashed lines in FIG.  6 ). After step  158  has completed the CPU  145 , at step  160 , controls the switches  140 ,  142  to tear down the connection between the switch currently handling communications (e.g., switch  142 ) and the switch that was previously handling communications (e.g., switch  140 ). Thus, inbound and outbound communications are provided between the cell  116  and the MSC  125  without the use of the switch  140  or the voice processor  128 . 
     FIG. 6 represents the state of the communication network  102  after a handoff to cell  116  has taken place (e.g., after step  160  of FIG. 4 has been executed) and when a handoff from cell  116  to any other cell is not anticipated. As shown in FIG. 6, after the handoff between cell  114  and cell  116  is carried out, all connections between the mobile switches  140 ,  142  are torn down by the CPU  145 . The operation of the communication network  102  as shown in FIG. 6 is substantially identical to the operation of the communication network  102  as shown in FIG. 3, except that communications between the mobile unit  120  and the MSC  125  are exclusively handled by the BTS  110 , the voice processor  130  and the switch  142 . 
     The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.