PATENT DOCUMENT

Publication Number: US-8838106-B2
Application Number: US-76947310-A
Country: US
Kind Code: B2

Title: Method and apparatus for cell reselection

Abstract:
A method of reselecting a mobile wireless communication device to a network subsystem before initiating a connection with a wireless communication network. A connection request is received by the mobile wireless communication device in response to a user input. Attributes of signals received by the mobile wireless communication device from multiple network subsystems in the wireless communication network are measured over a period of time. The mobile wireless communication device is associated with one of the network subsystems and not associated with other network subsystems. The mobile wireless communication device reselects to one of the network subsystems based on quality metrics generated from the measured received signal attributes. After reselection, the mobile wireless communication device initiates a connection through the reselected network subsystem.

Claims:
What is claimed is: 
     
       1. A method occurring while a wireless communication device is powered on and not connected to a serving cell, the method comprising:
 receiving, at the wireless communication device, control signaling information from one or more neighbor cells; 
 determining, at the wireless communication device, at least one signal quality metric for the one or more neighbor cells from the received control signaling information; 
 ranking, at the wireless communication device, the serving cell and the one or more neighbor cells based at least in part on the determined at least one signal quality metric for the one or more neighbor cells and at least one signal quality metric for the serving cell; 
 repeating the determining and ranking, at the wireless communication device, until a network connection request is generated by the wireless communication device; 
 identifying a preferred neighbor cell of the one or more neighbor cells having a better signal quality than the serving cell based at least in part on the ranking at the wireless communication device at the time of the network connection request; and 
 associating the wireless communication device with the preferred neighbor cell in response to the network connection request. 
 
     
     
       2. The method as recited in  claim 1 , further comprising creating a historical record of ranked cells comprising one or more previously determined signal quality metrics associated with the serving cell and one or more previously determined signal quality metrics associated with the one or more neighbor cells. 
     
     
       3. The method as recited in  claim 2 , wherein the ranking of the serving cell and the one or more neighbor cells is further based on a comparison of at least one signal quality metric associated with the serving cell and stored in the historical record of ranked cells to at least one signal quality metric associated with the one or more neighbor cells and stored in the historical record of ranked cells. 
     
     
       4. The method as recited in  claim 2 , further comprising:
 predicting, at the wireless communication device, a future signal quality associated with the serving cell based at least in part on one or more previously determined signal quality metrics associated with the serving cell that is stored in the historical record of ranked cells; and 
 identifying the preferred neighbor cell based at least in part on the predicted future signal quality associated with the serving cell. 
 
     
     
       5. The method as recited in  claim 2 , further comprising:
 predicting, at the wireless communication device, a future signal quality associated with the one or more neighbor cells based at least in part on one or more previously determined signal quality metrics associated with the one or more neighbor cells that is stored in the historical record of ranked cells; and 
 identifying the preferred neighbor cell based at least in part on the predicted future signal quality associated with the one or more neighbor cells. 
 
     
     
       6. The method as recited in  claim 1 , further comprising predicting, at the wireless communication device, a future signal quality associated with the serving cell based at least in part on the ranking at the wireless communication device at the time of the network connection request. 
     
     
       7. The method as recited in  claim 6 , further comprising predicting, at the wireless communication device, a future signal quality associated with the one or more neighbor cells based at least in part on the ranking at the wireless communication device at the time of the network connection request. 
     
     
       8. The method as recited in  claim 7 , further comprising identifying the preferred neighbor cell based at least in part on the predicted future signal quality associated with the serving cell and the future signal quality associated with the one or more neighbor cells. 
     
     
       9. A method, comprising:
 receiving, at a wireless communication device, a call origination request from a serving cell, while the wireless communication device is powered on and not connected to the serving cell; 
 receiving, at the wireless communication device, control signaling information from one or more neighbor cells; 
 measuring, at the wireless communication device, the received control signaling information to determine at least one signal quality metric associated with the one or more neighbor cells; 
 comparing, at the wireless communication device, the determined at least one signal quality metric associated with the one or more neighbor cells to at least one signal quality metric associated with the serving cell; 
 repeating the measuring and comparing, at the wireless communication device, until receiving the call origination request from the serving cell; 
 identifying a preferred neighbor cell of the one or more neighbor cells having a better signal quality than the serving cell based at least in part on the comparing; and 
 in response to receiving the call origination request from the serving cell, initiating a connection between the wireless communication device and the preferred neighbor cell. 
 
     
     
       10. The method as recited in  claim 9 , wherein the control signaling information received from the one or more neighbor cells corresponds to at least one of periodic control signaling information received via a frequency correction channel (FCCH) and periodic control signaling information received via a synchronization channel (SCH). 
     
     
       11. The method as recited in  claim 9 , wherein the determined at least one signal quality metric associated with the one or more neighbor cells corresponds to a signal-to-noise ratio (SNR). 
     
     
       12. The method as recited in  claim 9 , wherein the identifying the preferred neighbor cell further comprises comparing at least one location area identifier (LAI) associated with the one or more neighbor cells to an LAI of the serving cell. 
     
     
       13. A wireless communication device, comprising:
 a transceiver device configured to receive a wireless communication from a serving cell or one or more neighbor cells; and 
 one or more processors coupled to the transceiver, wherein the one or more processors are configured to execute program instructions that cause the wireless communication device to:
 receive, at the wireless communication device, control signaling information from the one or more neighbor cells; 
 determine, at the wireless communication device, at least one signal quality metric for the one or more neighbor cells based at least in part on the received control signaling information; 
 rank, at the wireless communication device, the serving cell and the one or more neighbor cells based at least in part on the determined at least one signal quality metric for the one or more neighbor cells and at least one signal quality metric for the serving cell; 
 repeat determination of the at least one signal quality metric for the one or more neighbor cells and ranking of the serving cell and the one or more neighbor cells until receiving a call origination request from the serving cell; 
 compare, at the wireless communication device, the corresponding rank of the serving cell to that of the one or more neighbor cells; 
 identify a preferred neighbor cell of the one or more neighboring cells based at least in part on the comparison; and 
 associate the wireless communication device with the preferred neighbor cell in response to receipt of the call origination request from the serving cell. 
 
 
     
     
       14. The wireless communication device as recited in  claim 13 , wherein the control signaling information received from the one or more neighbor cells corresponds to at least one of periodic control signaling information received via a frequency correction channel (FCCH) and periodic control signaling information received via a synchronization channel (SCH). 
     
     
       15. The wireless communication device as recited in  claim 13 , wherein the one or more processors are configured to execute program instructions that cause the wireless communication device to compare at least one local area identifier (LAI) associated with the one or more neighbor cells to an LAI of the serving cell, to further identify the preferred neighbor cell. 
     
     
       16. A method of associating a wireless communication device with a network cell before initiating a network connection attempt, the method comprising:
 receiving, at the wireless communication device, a network connection request from a serving cell; 
 receiving, at the wireless communication device, control signaling information from one or more neighbor cells; 
 measuring, at the wireless communication device, the received control signaling information to determine at least one signal quality metric associated with the one or more neighbor cells; 
 identifying a preferred neighbor cell of the one or more neighbor cells based at least in part on a comparison of the at least one signal quality metric associated with the one or more neighbor cells to a signal quality metric associated with the serving cell; 
 repeating the measuring and identifying until receiving the network connection request from the serving cell; 
 associating the wireless communication device with the preferred neighbor cell in response to receiving the network connection request from the serving cell; and 
 initiating a network connection of the wireless communication device to the preferred neighbor cell. 
 
     
     
       17. The method as recited in  claim 16 , further comprising ranking the serving cell and the one or more neighbor cells based at least in part on comparing the determined at least one signal quality metric associated with the one or more neighbor cells to the signal quality metric of the serving cell. 
     
     
       18. The method as recited in  claim 16 , further comprising creating a historical record of ranked cells comprising one or more previously determined signal quality metrics associated with the one or more neighbor cells. 
     
     
       19. The method as recited in  claim 18 , further comprising comparing the signal quality metric associated with the serving cell to at least one previously determined signal quality metric associated with the one or more neighbor cells and stored in the historical record of ranked cells to determine a corresponding ranking for each of the one or more neighbor cells. 
     
     
       20. A wireless communication device, comprising:
 a transceiver configured to receive wireless communication from a serving cell or from one or more neighbor cells; and 
 one or more processors coupled to the transceiver, wherein the one or more processors are configured to execute program instructions that cause the wireless communication device to:
 receive, at the wireless communication device, control signaling information from the one or more neighbor cells; 
 measure, at the wireless communication device, the received control signaling information to determine at least one signal quality metric associated with the one or more neighbor cells; 
 identify a preferred neighbor cell of the one or more neighbor cells based at least in part on a comparison of the at least one signal quality metric associated with the one or more neighbor cells to a signal quality metric associated with the serving cells; 
 repeat measuring and identifying until receiving a network connection request from the serving cell; and 
 associate the wireless communication device with the preferred neighbor cell in response to receiving the network connection request from the serving cell. 
 
 
     
     
       21. The wireless communication device as recited in  claim 20 , wherein the one or more processors are configured to execute program instructions that cause the wireless communication device to rank the serving cell and the one or more neighbor cells based at least in part on comparing the determined at least one signal quality metric associated with the one or more neighbor cells to the signal quality metric of the serving cell. 
     
     
       22. The wireless communication device as recited in  claim 21 , wherein the one or more processors are configured to execute program instructions that cause the wireless communication device to compare the signal quality metric associated with the serving cell to at least one previously determined signal quality metric associated with the one or more neighbor cells and stored in the historical record of ranked cells to determine a corresponding ranking for each of the one or more neighbor cells. 
     
     
       23. The wireless communication device as recited in  claim 20 , wherein the one or more processors are configured to execute program instructions that cause the wireless communication device to generate a historical record of ranked cells comprising one or more previously determined signal quality metrics associated with the one or more neighbor cells.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/307,373 entitled CELL RE-SELECTION CALL ORIGINATION by Venkatasubramanian Ramasamy et al., filed Feb. 23, 2010 which is incorporated by reference in its entirety for all purposes. 
    
    
     TECHNICAL FIELD 
     The described embodiments relate generally to wireless mobile communications. More particularly, a method is described for performing cell reselection prior to call origination by a mobile wireless communication device. 
     BACKGROUND OF THE INVENTION 
     Mobile wireless communication devices, such as a cellular telephone or a wireless personal digital assistant, can provide a wide variety of communication services including, for example, voice communication, text messaging, internet browsing, and electronic mail. Mobile wireless communication devices can operate in a wireless communication network of overlapping “cells”, each cell providing a geographic area of wireless signal coverage that extends from a base transceiver station (BTS). Whether idle or actively connected, a mobile wireless communication device can be associated with a “serving” cell in a wireless communication network and be aware of neighbor cells to which the mobile wireless communication device can also associate. The quality of a communication link between the mobile wireless communication device and the BTS can vary based on the distance between them and on interference included in received signals at either end of the communication link. As the mobile wireless communication device moves further away from the BTS, eventually a neighbor cell can provide an equal or better performing communication link than the current serving cell. The mobile wireless communication device can include a process for determining if and when to switch cells with which it associates. If the mobile wireless communication device is actively connected to the serving cell, then the process of switching to a neighbor cell is known as “handoff.” For a mobile wireless communication device that is associated with a serving cell in an “idle” state, the process of associating with a neighbor cell is known as “cell reselection.” 
     When a mobile wireless communication device seeks to associate with a wireless communication network, such as after a power on initialization, the mobile wireless communication device can search for cells located in its vicinity. If a cell is located that has sufficient performance quality to provide a wireless communication link, then the mobile wireless communication device can associate with that cell. The mobile wireless communication device can then be “camped” on a particular “serving” cell in the wireless communication network of cells. While camped on the serving cell, the mobile wireless communication device can listen to messages broadcast from the serving cell BTS as well as from other BTS located in neighbor cells. If the mobile wireless communication device determines that a neighbor cell can provide a higher quality communication link than a current serving cell, then the mobile wireless communication device can disassociate from the current serving cell and associate with the neighbor cell, in a process known as “cell reselection,” typically after waiting for a certain time period. By waiting before performing cell reselection, the mobile wireless communication device can avoid bouncing between different cells as the quality of the communication links between the mobile wireless communication device and several nearby cells changes. 
     Waiting a fixed period of time before cell reselection, however, can result in a mobile wireless communication device continuing to associate with a serving cell to which the performance of a connection is degrading when the mobile wireless communication device seeks to originate an active connection. An active connection between the “idle” mobile wireless communication device and the BTS in the current serving cell can be initiated by the mobile wireless communication device or by the BTS through an exchange of a series of messages. While camped on a serving cell, a wireless mobile communication device can initiate a connection with the serving cell with which it is associated, but not with a neighbor cell, even if the performance of a connection between the mobile wireless communication device and the neighbor cell can result in a higher quality connection. The performance of a connection between the mobile wireless communication device and the serving cell can degrade sufficiently rapidly that the mobile wireless communication device cannot handoff to a neighbor cell before the connection drops. 
     Thus there exists a need to evaluate a quality of a link between a serving cell and a mobile wireless communication device and to reselect to a better quality cell if available prior to call origination by the mobile wireless communication device. Monitoring the quality of multiple neighbor cells in addition to the serving cell can improve a success rate of call origination. 
     SUMMARY OF THE DESCRIBED EMBODIMENTS 
     This paper describes various embodiments that relate to methods to associate a mobile wireless communication device with network subsystems before initiating a connection of the mobile wireless communication device with a wireless communication network. 
     In one embodiment, a method is described. The method can be performed at a mobile wireless communication device. In the described embodiment, when the mobile wireless communication device is powered and not connected to a wireless network the mobile wireless communication device can evaluate wireless signal attributes of wireless signals received from a plurality of wireless network subsystems and identify one of the plurality of wireless network subsystems based on the evaluating until a network connection request is generated by the mobile wireless communication device. In response to the connection request, the mobile communication device can associate with the identified wireless network subsystem and initiate the connection to the wireless network using the identified wireless network subsystem. 
     In another embodiment, a method can be performed by a mobile wireless communication device. The mobile wireless communication device can generate a request to connect to a wireless network. In response to the request, the mobile wireless communication device can receive a wireless signal having one or more wireless signal attributes from a plurality of network subsystems, measure the at least one or more wireless signal attributes, compare the measured one or more signal attributes with each other, identify one of the plurality of network subsystems based on the comparing, and use the identified one of the plurality of network subsystems to connect to the wireless network. 
     A mobile wireless communication device is described that includes at least a wireless transceiver and a processor. In the described embodiment, the wireless transceiver is arranged to receive a wireless signal from a plurality of wireless network subsystems, the wireless signals having one or more signal attributes that includes at least a wireless signal strength and a wireless signal quality. The processor is arranged to execute instructions for measuring the one or more signal attributes of the wireless signal received by the mobile wireless communication device, compare the measured one or more signal attributes with each other, associating the mobile wireless communication device with one of the plurality of network subsystems based on the comparing, and initiating a connection between the mobile wireless communication device and the associated one of the plurality of network subsystems. 
     A method of reselecting a mobile wireless communication device between network subsystems before initiating a connection with a wireless communication network is described. The method can be performed by receiving a request to connect to the wireless communication network by the mobile wireless communication device, measuring one or more attributes of signals received by the mobile wireless communication device from a first network subsystem associated with the mobile wireless communication device and from a second network subsystem in the wireless communication network not associated with the mobile wireless communication device, reselecting the mobile wireless communication device to the first or second network subsystem based on the measured attributes of signals, and initiating the connection of the mobile wireless communication device with the wireless communication network through the reselected network subsystem. 
     Computer program product encoded in a tangible computer readable medium for associating a mobile wireless communication device with a wireless network substation of a wireless network based upon measured signal attributes of a wireless signal received at the mobile wireless communication device prior to the mobile wireless communication device joining the wireless network is described. The computer program product includes at least computer code for receiving the wireless signal at the mobile wireless communication device from a plurality of network subsystems of the wireless network, the wireless signal having one or more wireless signal attributes, computer code for measuring the at least one or more wireless signal attributes, computer code for comparing the measured one or more signal attributes with each other, computer code for identifying one of the plurality of network subsystems. 
     A mobile wireless communication device is described that includes at least a processor. When the mobile wireless communication device is not connected to a wireless network, the processor evaluates wireless signal attributes of wireless signals received from a plurality of wireless network subsystems, identifies the mobile wireless communication device with one of the plurality of wireless network subsystems based on the evaluating, repeats the evaluating and associating until a network connection request is generated by the mobile wireless communication device, and in response to the connection request, initiates the connection to the wireless network using the identified one of the plurality of wireless network subsystems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and the advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings. 
         FIG. 1  illustrates a mobile wireless communication device located within a wireless cellular communication network. 
         FIG. 2  illustrates varying received signal strengths for a mobile wireless communication device traversing several cells in a wireless cellular communication network. 
         FIG. 3  illustrates a control signaling multi-frame structure for a wireless communication network. 
         FIG. 4  illustrates a method for changing association of a mobile wireless communication device between network subsystems. 
         FIG. 5  illustrates a representative method for cell reselection by a mobile wireless communication device after receiving a call origination request. 
         FIG. 6  illustrates a representative method for monitoring and ranking a set of cells by a mobile wireless communication device. 
         FIG. 7  illustrates a representative method for cell reselection by a mobile wireless communication device after receiving a paging request. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the concepts underlying the described embodiments. It will be apparent, however, to one skilled in the art that the described embodiments may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the underlying concepts. 
     A mobile wireless communication device  101  can include the capability to connect seamlessly with a wireless communication network  100  of overlapping wireless communication cells, each wireless communication cell covering a geographic area extending from a base transceiver station (BTS) as illustrated in  FIG. 1 . The mobile wireless communication device  101  can receive communication signals from a number of different cells in the wireless communication network  100 , each cell located at a different distance from the mobile wireless communication device. As signal strength of a wireless communication signal decays proportionally to the square of the distance between a transmitting end and a receiving end, the mobile wireless communication device  101  can monitor the received signal strength from several different cells in the wireless communication network  100  to determine a cell having the strongest received signal at any given time. As the mobile wireless communication device  101  changes locations within a wireless communication network  100 , the received signal strength can vary substantially, and thus the cell from which the mobile wireless communication device  101  receives the strongest signal can change. The mobile wireless communication device  101  can maintain a dynamic table of cells from which it can receive signals and a corresponding measure of signal strength received from each. 
     After a “power on” initialization, or similarly when entering a “new” wireless communication network, the mobile wireless communication device  101  can seek to associate with a cell in the wireless communication network  100 . A BTS in each cell of the wireless communication network  100  can broadcast a set of messages at regular intervals that identify salient characteristics of the BTS. Based on measurements of received signal strength, or other quality metrics such as carrier to interference ratios, the mobile wireless communication device  101  can associate with a base transceiver station  104 , thereby choosing a particular cell within the wireless communication network  100  as a “serving” cell  102 . The process of associating with a cell can be also known as “camping” on that cell. After associating with the serving cell  102 , the mobile wireless communication device can initiate connections (such as voice or data calls) with the serving cell  102 . The base transceiver station  104  in the serving cell  102  can also initiate connections to the mobile wireless communication device  101 , as the wireless network  100  can be aware of the association between the serving cell  102  and the mobile wireless communication device  101 . While “camped” on the serving cell  102 , the mobile wireless communication device  101  can continue to monitor broadcast signals from neighbor cells, each signal possibly traversing a different distance between its associated BTS and the mobile wireless communication device  101 . As shown in  FIG. 1 , the mobile wireless communication device  101  can receive detectable signals from neighbor cells  103  A, B and C, while neighbor cell D  103  can be located at too great a distance for any broadcast signals to be received reliably. In addition to distance, which attenuates signal strength, interference received at a mobile wireless communication device  101  can also affect the quality of reception and detection of signals from different cells within the wireless network  100 . 
       FIG. 2  illustrates a graph  200  of receive signal strength  201  for a mobile wireless communication device  101  traversing through several overlapping cells of a wireless network. Initially at time  0 , the mobile wireless communication device  101  can receive stronger signals from a BTS within a serving cell  203  than from a BTS in a neighbor cell  204 . The distance between the mobile wireless communication device  101  and the BTS in the serving cell  203  can increase as time advances resulting in declining received signal strength, while simultaneously the distance to the BTS in the neighbor cell  204  can decrease resulting in rising received signal strength. The mobile wireless communication device  101  can regularly monitor received signal strength  201  from BTS located in several different cells to determine from which cell the strongest signal is received. If signals from a neighbor cell exceed those from the serving cell for a period of time, then the mobile wireless communication device  101  can choose to “reselect” from the current serving cell to a neighbor cell. In GSM (Global System for Mobile Communication) systems, a delay time for reselection can be set to wait at least five seconds before switching from a current serving cell to a new neighbor cell. 
     As shown in  FIG. 2 , the signal strength from neighbor cell  204  can exceed the signal strength from the serving cell  203  at a time  202  between 0 and 1. Accounting for some processing time for detection of the change in signal strength, the mobile wireless communication device  101  can detect that the neighbor cell is better than the serving cell  203  by a time value of 1. At the same time value of 1, the mobile wireless communication device  101  can also choose to initiate a connection. As the mobile wireless communication device  101  is associated with serving cell  203 , a connection with the network can first be realized by communicating through the BTS of the serving cell  203 . Although the receive signal strength  201  from the serving cell  203  can be lower than the signal strength received from the neighbor cell  204 , the mobile wireless communication device  101  cannot choose to connect with the neighbor cell until after reselection completes. Once a connection between the mobile wireless communication device  101  and the serving cell  203  has begun, the mobile wireless communication device  101  can transfer the connection to the neighbor cell  204  through a process known as “handoff” under control of the wireless network. The “handoff” process can take a substantial length of time to complete because the mobile wireless communication device  101  can be required to measure received signal performance and communicate these measurements to the wireless network before the wireless network can decide when handoff can occur. 
     In some situations, the signal strength of the connection between the mobile wireless communication device  101  and the BTS in the serving cell  203  can be sufficiently weak that the handoff from the serving cell  203  to the neighbor cell  204  cannot be realized. In this case, as shown in  FIG. 2 , the connection between the mobile wireless communication device  101  and the wireless network can terminate (call drop) when the receive signal strength  201  falls below a certain threshold (for example −110 dBm). Thus, while the mobile wireless communication device  101  can know that a better neighbor cell exists before starting a call, a connection with the neighbor cell cannot be realized when a call origination with the network precedes a neighbor cell reselection. Connections between the mobile wireless communication device  101  and the wireless network can drop because the connection never completes or because a completed connection with a serving cell is too weak to communicate messages to switch to a better neighbor cell. 
     Current methods can use received signal strength as a measure of quality for determining reselection; however, better and faster methods can also use other quality metrics. The quality of a connection between a mobile wireless communication device and a BTS can combine multiple measurements at the mobile wireless communication device based on signals broadcast from the BTS. One such measurement can include the receive signal strength  201  as indicated in  FIG. 2  for the current serving cell  203  and for one or more neighbor cells, such as neighbor cell  204 . (Although not shown in  FIG. 2 , multiple neighbor cells can be available as shown in  FIG. 1 , and the receive signal strength  201  from each neighbor cell can be monitored.) In addition to an instantaneous or average receive signal strength, the mobile wireless communication device  101  can also monitor changes in receive signal strength for each cell, such as the slopes of the receive signal strength lines shown in  FIG. 2 . Thus a mobile wireless communication device can estimate not only the current quality of cells but can also predict the future quality of connections to each cell. 
     Besides receive signal strength, the mobile wireless communication device  101  can determine and track other quality metrics that can account for received interference as well as received signal. These quality metrics can include correlations of received signals with known data patterns, estimates of signal to noise ratios based on equalizer outputs, and indications of errors from decoding units within the receiver of the mobile wireless communication device  101 . As shown in  FIG. 3 , in some embodiments, a BTS can broadcast control signaling information repeatedly at regular intervals. A control signaling multi-frame  301  can include several different channels transmitted on a fixed time slot  0 , and each frame in the multi-frame  301  can contain information for a different control channel. For a GSM wireless communication network, a BTS can broadcast a frequency correction channel (FCCH)  302  and a synchronization channel (SCH)  303  once every 10 frames in a 51 frame multi-frame  301 . The FCCH  302  can consist of a burst of 148 consecutive zeroes that can generate a signal at a fixed frequency. While the mobile wireless communication device  101  can use the received fixed frequency signal from the FCCH  302  to align its local oscillator to the BTS transmitter, it can also use the FCCH  302  to measure a quality of the communication path between the BTS and itself. In an embodiment, the mobile wireless communication device  101  can correlate bursts received on the FCCH  302  in frames  0 ,  10 ,  20 , etc. with a fixed pattern of zeroes to determine effectively signal strength in the presence of interference. The correlation  305  can be compared against a fixed or adaptive threshold  306  to assess the quality of the connection between the BTS and the mobile wireless communication device  101 . Similarly the SCH  303  can include a fixed, known pattern of 64 bits that can be correlated with received signals received in frames  1 ,  11 ,  21 , etc. to also determine a quality of the connection between the mobile wireless communication device  101  and the BTS. Thresholds used for the correlation of the FCCH  302  and the SCH  303  can be independent. 
     The mobile wireless communication device  101  can also monitor a decode success rate for bursts received on the broadcast control channel (BCCH)  304 , on the FCCH  302  and on the SCH  303 . In a representative embodiment, the mobile wireless communication device  101  can maintain a database of quality for the serving cell and for one or more neighbor cells ranking each cell&#39;s performance using quality metrics as described above. The mobile wireless communication device can monitor the BCCH  304  in the serving cell and the FCCH  302  and the SCH  303  in known neighbor cells. (For GSM networks, the serving cell can broadcast on the BCCH a list of neighbor cells.) A neighbor cell can be considered a candidate for reselection if a minimum threshold is met for one or more of the monitored quality metrics. For example, a threshold can be set for the decode success of the FCCH and SCH that indicates that minimal or no decoding errors are expected for communication with a reselected neighbor cell. The mobile wireless communication device  101  can include a battery, and to minimize power consumption by the battery, the mobile wireless communication device can reduce computational cycles by limiting the monitoring and ranking of connection quality to periods when the serving cell&#39;s performance is degrading rather than all of the time. 
       FIG. 4  illustrates an embodiment of a method for reselecting a mobile wireless communication device  101  with a wireless network subsystem, such as a BTS in a cell of a wireless communication network. In step  401 , the mobile wireless communication device  101  can be associated with a first network subsystem. For example, the first network system can be a BTS in a serving cell of a wireless communication network. In step  402 , the mobile wireless communication device  101  can receive a connection request. The connection request can originate from a processing block within the mobile wireless communication device  101  that controls establishing links with an external wireless communication network. In step  403 , the mobile wireless communication device  101  can determine if a second network subsystem can offer a higher quality connection that the current network subsystem with which the mobile wireless communication device  101  is associated. For example, a BTS in a neighbor cell can have a higher quality currently or can be estimated to have a higher quality in the near future when a connection is completed with the mobile wireless communication device  101  compared to the BTS in the presently associated serving cell. If the current first network subsystem is best, then in step  405  the mobile wireless communication device  101  can initiate a connection with the current first network subsystem. If the current first network subsystem is not the best, then in step  404 , the mobile wireless communication device  101  can first reselect to the higher quality second network subsystem to which a connection can then be initiated in step  405 . 
       FIG. 5  illustrates a more detailed, specific embodiment of a method to reselect a mobile wireless communication device  101  to one or more network subsystems in a wireless communication network. In step  501 , the mobile wireless communication device  101  can be in an “idle” mode, i.e. associated with a “serving” cell&#39;s BTS in the wireless communication network but not actively connected. In step  502 , the mobile wireless communication device  101  can receive a call origination request to form an active connection with the wireless communication network. The call origination request can be generated by a user of the mobile wireless communication device  101 . 
     In step  503 , the mobile wireless communication device  101  can determine if the current serving cell&#39;s signal strength falls below a first threshold, thereby detecting if the performance of a connection between the mobile wireless communication device  101  and the wireless communication network through the BTS in the serving cell can potentially prove inadequate. In an embodiment, the mobile wireless communication device  101  can determine if the receive signal strength of the serving cell falls below a first threshold. If the current serving cell&#39;s signal strength exceeds the first threshold, then in step  506 , the mobile wireless communication device  101  can originate a call with the BTS in the serving cell. Otherwise, in step  504 , the mobile wireless communication device  101  can determine if the current serving cell&#39;s signal quality level falls below a second threshold. While the test for signal strength in step  503  can determine if signals received from the BTS in the serving cell can be low, the interference level can also be low and the quality of a connection with the serving cell, e.g. based on a signal to interference ratio, can be adequately high for effective communication. Quality metrics such as measuring the decode success rate of FCCH and SCH frames and examining receive equalizer outputs can be analyzed in the mobile wireless communication device  101  to assess a serving cell&#39;s quality of connection. While the method outlined in  FIG. 5  proceeds to testing for a neighbor cell in step  505  when both the serving cell&#39;s signal strength and quality fall below thresholds, an alternative implementation could proceed to step  505  when either the serving cell&#39;s signal strength or the serving cell quality falls below a respective threshold. 
     If the current serving cell&#39;s quality level exceeds the second threshold, then in step  506  the mobile wireless communication device  101  can originate a call with the BTS in the serving cell. Otherwise, in step  505 , the mobile wireless communication device  101  can determine if a “better” neighbor cell, e.g. one with higher signal strength or higher quality, is available. Even if the serving cell&#39;s signal strength and quality fall below designated thresholds, in some cases. all neighbor cells can have weaker signal strengths. In this case, the mobile wireless communication device  101  can originate the call with the BTS of the serving cell in step  506 . If, however, a neighbor cell has higher signal strength than the current serving cell, then the mobile wireless communication device  101  can determine if the neighbor cell can be more suitable for completing the requested call. In a representative embodiment for a GSM wireless communication network, the mobile wireless communication device  101 , in step  505 , can determine if a C2 timer is running for a neighbor cell. The C2 timer associated with the neighbor cell can provide an indication to the mobile wireless communication device  101  that the signal strength of the neighbor cell exceeds the signal strength of the current serving cell. 
     If a neighbor cell with higher signal strength than the current serving cell exists, then in step  507 , the mobile wireless communication device  101  can determine if recent system information has been read for the neighbor cell identified in step  505 . Both the serving cell and neighbor cells can be regularly monitored for one or more metrics such as a measure of signal strength, interference levels, decoding errors, equalizer outputs or other receive parameters to assess and rank the quality of connections with the different cells to which the mobile wireless communication device  101  can connect. A history of system information for the serving cell and neighbor cell can be analyzed by and stored in the mobile wireless communication device to assist with determining a cell with which to associate and connect to the wireless network. The mobile wireless communication device  101  can choose to associate with and connect to the network through a cell with more stable signal quality.  FIG. 6 , as discussed later, illustrates a representative embodiment of a method to monitor quality of serving and neighbor cells. 
     If current system information is not available for the identified “better” neighbor cell, then in step  508  the mobile wireless communication device  101  can read and update available system information for the neighbor cell. With system information available, the mobile wireless communication device  101 , in step  509 , can determine if the quality of a connection using the neighbor cell can exceed the quality using the current serving cell. If the quality of the neighbor cell is lower than the quality of the serving cell, then the mobile wireless communication device  101  can originate the call through the current serving cell in step  506 . 
     If the neighbor cell&#39;s quality exceeds the current serving cell&#39;s quality, then in step  510  the mobile wireless communication device  101  can determine if the serving cell and neighbor cell have the same location area identifier (LAI). Sets of cells within a wireless communication network can be grouped together by a common LAI such as for a group of cells serving a particular geographic area. If the neighbor cell has a different location area identifier (LAI) than the serving cell, then the mobile wireless communication device  101  can originate a call in the serving cell (step  506 ). If the neighbor cell has the same LAI as the serving cell, then in step  511 , the mobile wireless communication device can reselect to the neighbor cell. Finally, in step  512 , the mobile wireless communication device  101  can originate a call in the neighbor cell (which can actually be a new serving cell after reselection in step  510 ). 
       FIG. 5  illustrates a representative embodiment; however, other embodiments can rearrange the order of some steps or skip some steps altogether. For example, the mobile wireless communication device  101  can search for a better neighbor cell if either the serving cell&#39;s signal strength or its quality falls below a relevant threshold rather than both as shown. Similarly, the mobile wireless communication device  101  can reselect to a neighbor cell if a quality of the serving cell is better than the serving cell independent of the neighbor cells signal strength level. A measure of the quality of a received signal can be more relevant than an absolute level of signal strength alone. 
       FIG. 6  illustrates a series of steps that the mobile wireless communication device  101  can use while in an “idle” mode to assess the quality of neighbor cells as candidates for reselection. In step  601 , the mobile wireless communication device  101  can be associated with a serving cell in an “idle” mode. In step  602 , the mobile wireless communication device  101  can schedule decoding of the FCCH and SCH frames for one or more neighbor cells. In step  603 , evaluation of the quality of the neighbor cells can include decoding and correlation of the FCCH and SCH training sequences as well as signal to noise ratio (SNR) estimates for each neighbor cell based on outputs from a receive equalizer or from soft decoders in the mobile wireless communication device  101 . In step  603 , results from the evaluation of received FCCH and SCH frames for each neighbor cell can be combined with stored estimates from previously received FCCH and SCH frames from that neighbor cell to produce peak values, mean values or other “filtered” values of each of the quality metrics monitored. Step  603  can be repeated until N successive FCCH and SCH frames have been decoded as tested in step  604 . In step  605 , after decoding and evaluating N successive FCCH and SCH frames for each of the neighbor cells, the mobile wireless communication device  101  can rank the monitored neighbor cells based on the N most recent measurements. If the mobile wireless communication device  101  continues in idle mode, then the scheduling, measurement, combining and storing of quality metrics, as well as the ranking of the neighbor cells in steps  602  to  605  can be repeated at regular intervals as indicated by step  606 . 
     The following table lists a hypothetical set of monitored quality values for a serving cell and several neighbor cells in a wireless communication network. This table can illustrate representative embodiments of the methods shown in  FIGS. 5 and 6 . 
     
       
         
           
               
               
               
               
               
            
               
                   
               
               
                   
                 SCELL 
                 NCELL 1 
                 NCELL 2 
                 NCELL 3 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 RSSI 
                 SNR 
                 RSSI 
                 SNR 
                 RSSI 
                 SNR  
                 RSSI 
                 SNR 
               
               
                   
               
               
                 T0 
                  −90 
                 6 
                 −96 
                  8 
                 −100 
                 8 
                 −105 
                 4 
               
               
                 T1 
                  −95 
                 5 
                 −93 
                  9 
                 −100 
                 8 
                 −105 
                 4 
               
               
                 T2 
                  −97 
                 4 
                 −90 
                  9 
                  −94 
                 8 
                 −105 
                 4 
               
               
                 T3 
                 −100 
                 2 
                 −87 
                 10 
                  −94 
                 8 
                 −105 
                 4 
               
               
                   
               
            
           
         
       
     
     At time T 0 , a mobile wireless communication device  101  can be associated with a serving cell (SCELL) having a receive signal strength indicator (RSSI) of −90 dBm and a receive signal quality measure (SNR) of 6 dB. Each of three neighbor cells (NCELL 1 , NCELL 2  and NCELL  3 ) can have lower values of receive signal strength than the serving cell as indicated. The mobile wireless communication device  101  can require a minimum receive signal strength of at least −95 dBm for the first threshold in step  503  of  FIG. 5 . If a call origination request occurs at time T 0 , then the mobile wireless communication device  101  can originate the call in the serving cell as the current serving cell&#39;s signal strength suffices. At time T 1 , the receive signal strength in the serving cell can have degraded to −95 dBm (just at the minimum required) while the neighbor NCELL  1  at −93 dBm can exceed the serving cell in receive signal strength measured at the mobile wireless communication device  101 . If a call origination request occurs at time T 1 , then the mobile wireless communication device  101  can still originate the call in the serving cell, even though the neighbor cell&#39;s signal strength is higher, as the serving cell&#39;s signal strength still suffices based on the first threshold level setting. Thus setting a level for the first threshold in step  503  can determine when a neighbor cell can be considered for reselection. At time T 2 , the receive signal strength of the serving cell can have degraded to −97 dBm, below the first threshold of −95 dBm, while signals received from neighbor cells NCELL  1  and NCELL  2  at −90 dBm and −94 dBm respectively can both exceed the serving cell&#39;s receive signal strength. A call origination request at time T 2  can still result in the mobile wireless communication device  101  originating a call in the serving cell, however, if the serving cell&#39;s quality in step  504  suffices. For example, if the second threshold for serving cell&#39;s quality is set to 3 dB, then an SNR of 4 dB at time T 2  can still indicate that the serving cell can be used. At time T 3 , the receive signal strength of the serving cell can continue to degrade to −100 dBm and the receive SNR can fall to 2 dB, below the second threshold of 3 dB for serving cell quality. If a call origination request occurs at time T 3 , then the mobile wireless communication device  101  can evaluate stored and filtered quality metrics from the neighbor cells NCELL  1  and NCELL  2  to determine which neighbor cell can provide a higher quality connection as outlined in  FIGS. 5 and 6 . 
       FIG. 7  illustrates a method for cell reselection by the mobile wireless communication device  101  when the wireless communication network initiates a connection. In step  701 , the mobile wireless communication device  101  can be in an “idle” mode, i.e. associated with but not actively connected to a BTS in a serving cell. In step  702 , the mobile wireless communication device  101  can receive a page indication from the wireless communication network. Any of several BTS, each located in a different cell and sharing a common LAI, can transmit the page indication. In step  703 , the mobile wireless communication device  101  can determine if the received signal strength of the serving cell with which it is associated falls below a first threshold. If the received signal strength of the serving cell does not fall below the first threshold, then the mobile wireless communication device  101  can send a paging response to the BTS in the serving cell to originate a call, as shown in step  708 . If the serving cell&#39;s signal strength falls below the first threshold, then in step  704 , the mobile wireless communication device  101  can determine if the serving cell&#39;s quality falls below a second threshold. If the quality threshold is met, then the serving cell can be used for call origination. If the quality threshold is not met, then the mobile wireless communication device can determine if a neighbor cell has signal strength greater than the current serving cell, as indicated in step  705 . If all neighbor cells have signal strengths less than the current serving cell, then a paging response can be sent in the current serving cell. If, however, one or more of the neighbor cells have higher signal strengths higher than the current serving cell, then the mobile wireless communication device can determine if the neighbor cell&#39;s LAI is the same as for the serving cell. If the neighbor cell has the same LAI as indicated in step  706 , then the mobile wireless communication device  101  can reselect in step  707  to the neighbor cell. In step  708 , a paging response can be sent to the reselected neighbor cell (which can now be the serving cell after reselection in step  707 ). 
     Various aspects of the described embodiments can be implemented by software, hardware or a combination of hardware and software. The described embodiments can also be embodied as computer readable code on a computer readable medium for controlling manufacturing operations or as computer readable code on a computer readable medium for controlling a manufacturing line used to fabricate thermoplastic molded parts. The computer readable medium is any data storage device that can store data which can thereafter be read by a computer system. Examples of the computer readable medium include read-only memory, random-access memory, CD-ROMs, DVDs, magnetic tape, optical data storage devices, and carrier waves. The computer readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     The various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. 
     The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.

Metadata:
Filing Date: 20100428
Publication Date: 20140916
Grant Date: 20140916
Priority Date: 20100223
Inventors: RAMASAMY VENKATASUBRAMANIAN
DEIVASIGAMANI GIRI PRASSAD
VASUDEVAN SRINIVASAN
Assignee: APPLE INC
CPC Classifications: [{"code": "H04W48/20", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04W36/0061", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04W48/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W36/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W36/30", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04W36/008375", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W48/20", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 43849618