Patent Application
20080076436
The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:
An embodiment of the present invention is a method of call setup in a wireless communication system in which traffic channels at a serving base station and one or more neighbor base stations are assigned to a mobile station during system access to improve call setup, particularly in cross carrier channel assignments when the mobile station is in a cross carrier cell (or coverage area) of one of the neighbor base stations.
Carrier X is the carrier frequency used by mobile station 130 to access the wireless communication system and establish a communication link with the wireless communication system, as will be described later herein. Carrier X is also referred to herein as “access carrier X.” Carrier Y is any carrier frequency which is not access carrier X. Carrier Y is also referred to herein as “cross carrier Y.”
Traffic channels may be assigned to mobile station 130 on either access carrier X or cross carrier Y. If the assigned traffic channel is on access carrier X, the channel assignment is referred to herein as a “same carrier channel assignment.” If the assigned traffic channel is on cross carrier Y, the channel assignment is referred to herein as a “cross carrier channel assignment.”
Base stations 110 and 120 provide telecommunication services to mobile stations within their respective cell or geographical coverage areas 115 and 125, respectively. Each cell 115 and 125 comprises an access carrier X cell and a cross carrier Y cell in which telecommunication services are provided to mobile stations over carriers X and Y. Differences in propagation characteristics between carriers X and Y may affect the size and/or shape of the respective cells. In an ideal or optimized multi-carrier wireless communication system, the access carrier X and cross carrier Y cells in each cell 115 and 125 are configured to be approximately the same size and shape, thus completely (or almost completely) overlapping each other.
Cross carrier channel assignments in ideal multi-carrier wireless communication systems should not result in call setup failure due to differences in propagation characteristics of access carrier X and cross carrier Y. However, configuring the access carrier X and cross carrier Y cells to be identical in size and shape would require extra resources, such as antennas and radios, thereby increasing cost. For illustrative purposes, the cross carrier Y cells 220 and 240 are shown as slightly larger and rounder than the access carrier X cells 210 and 230, respectively, even though such cells may be identical in size and/or shape.
In a non-ideal or non-optimized multi-carrier wireless communication system, the access carrier X and cross carrier Y cells for each base station would not be the same size and/or shape, thus not completely overlapping each other.
In the non-ideal multi-carrier wireless communication system, signal quality of a pilot channel in access carrier X would not be a reliable indicator of signal quality of a traffic channel in cross carrier Y. There may be an appreciable signal quality difference between the access carrier X pilot channel and the cross carrier Y traffic channel. In cross carrier channel assignments, call setup may fail when propagation characteristics differences result in the cross carrier Y cell covering less geographical area than the access carrier X cell.
An embodiment of the present invention addresses this cross carrier channel assignment problem by assigning cross carrier Y traffic channels at a serving base station and one or more neighbor base stations during system access. If the cross carrier Y cell of the serving base station covers less geographical area than the corresponding access carrier X cell, then the mobile station may be in the cross carrier Y cell of one of the neighbor base stations. In such a case, the mobile station should be able to establish a communication link with that neighbor base station even though it may not be able to establish a communication link with the serving base station, thereby improving call setup in cross carrier channel assignments.
In step 415, mobile station 130 transmits an access request, also known as an “access probe,” to the serving base station over an access channel (or some other control channel) on one of the uplink carrier frequencies. The access request is a request to the serving base station for a traffic channel assignment. The access request may also indicate other base stations with strong pilot signal strength measurements on the access carrier X. In step 420, the serving base station and RNC allocate resources to mobile station 130 for setting up a traffic channel (i.e., assign a traffic channel) in response to the access request. The assigned traffic channel may either be on access carrier X or cross carrier Y.
If a cross carrier Y traffic channel is assigned, i.e., cross carrier channel assignment, then flowchart 400 continues to step 425. Otherwise, if an access carrier X traffic channel is assigned, i.e., same carrier channel assignment, then flowchart 400 continues to step 455. In step 425, the serving base station will send a traffic channel request to RNC 140. The traffic channel request is a request for the assignment of traffic channels in cross carrier Y from one or more selected base stations neighboring the serving base station, i.e., neighbor base stations. In another embodiment, the traffic channel request may be a request for the assignment of traffic channels on access carrier X, or both access carrier X and cross carrier Y, from one or more neighbor base stations.
The neighbor base stations may be selected based on a list of neighbor base stations, i.e., neighbor list. Each base station 110 and 120 maintains a neighbor list. The neighbor list, in one embodiment, is a list of neighbor base stations which can be used for cross carrier Y traffic channel assignment. The neighbor list may include priority levels or rankings associated with the neighbor base stations. The neighbor base stations can be selected based on the priority levels or rankings (e.g., base stations with highest priority levels are selected first). In another embodiment, the neighbor base stations are selected based on an access request that indicates one or more neighbor base stations with strong signal strength measurements. The selected base stations may include the neighbor base stations indicated in the access request and any base stations adjacent to both the serving base station and the indicated neighbor base stations. The number of selected base stations may be limited, for example, to five neighbor base stations. In yet another embodiment, the neighbor base stations may be selected based on the location of mobile station 130. For example, if the serving base station has knowledge that mobile station 130 is in a particular sector of its cell, then serving base station may select the neighbor base stations adjacent to that sector.
In step 430, in response to the traffic channel request, RNC 140 determines whether traffic channels in cross carrier Y can be assigned to mobile station 130 at the neighbor base stations indicated in the traffic channel request. RNC 140 maintains information regarding availability of resources, such as carrier width and radio units, at each base station 110 and 120. Based on such availability, RNC 140 determines whether cross carrier Y traffic channels at the selected neighbor base stations may be assigned to mobile station 130.
If a cross carrier Y traffic channel at a neighbor base station is not available for assignment, RNC 140 sends a traffic channel non-assignment response to mobile station 130, in step 435. The traffic channel non-assignment response indicates to the serving base station that no cross carrier Y traffic channels at any of the selected neighbor base stations are available to be assigned to mobile station 130. In step 440, upon receipt of the traffic channel non-assignment response, the serving base station checks a retry count corresponding to a number of traffic channel requests sent after the initial traffic channel request. The maximum number of retries can be zero or greater. If the retry count is less than a maximum number of allowable retries, then flowchart 400 returns to step 425 where the serving base station sends another traffic channel request to RNC 140. Such traffic channel request may be a request for cross carrier Y traffic channel assignment at the same or different neighbor base stations as indicated in a previous traffic channel request. If the retry count is greater than or equal to the maximum number of allowable retries, then flowchart 400 continues to step 445 where the serving base station changes the traffic channel assignment to an access carrier X traffic channel, i.e., same carrier channel assignment.
If a cross carrier Y traffic channel at one or more neighbor base station is available for assignment, then RNC 140 sends a traffic channel assignment response to the serving base station and one or more channel assignment notifications to the neighbor base stations associated with the assigned cross carrier Y traffic channels, in step 450. The traffic channel assignment response indicates to the serving base station the assigned cross carrier Y traffic channels at its neighbor base station. The channel assignment notification indicates to a neighbor base station its cross carrier Y traffic channel being assigned.
From steps 420, 445 and 450, flowchart 400 goes to step 455 where the serving base station transmits a channel assignment message to mobile station 130 indicating the assigned traffic channels at the serving base station and, if applicable, the assigned traffic channels at the neighbor base stations. The channel assignment message is transmitted over a paging channel on access carrier X. In step 460, mobile station 130 will attempt to establish communication links with each of the base stations using the assigned traffic channels indicated in the channel assignment message. If mobile station is not able to establish at least one communication link, call setup is deemed a failure. Otherwise, call setup is completed and deemed a success.
Upon completion of call setup, mobile station 130 will manage its communication link, for example, by monitoring the pilot channel in the same carrier as its traffic channels in order to initiate handoffs, if necessary. For cross carrier Y assignments, mobile station 130 will manage its communication link in cross carrier Y. For same carrier assignments, mobile station 130 will manage its communication link in access carrier X.
Note that call setup is completed and successful if the mobile station can establish a communication link with at least one base station. In cross carrier channel assignments, a mobile station may not be able to successfully establish a communication link with the serving base station because the cross carrier Y cell may cover less geographical area than the access carrier X cell of the serving base station. In such a situation, the mobile station may be within the cross carrier Y cell of a neighbor base station instead of the cross carrier Y cell of its serving base station. By assigning a cross carrier Y traffic channel to the mobile station at one or more neighbor base stations (in addition to the serving base station), there is an increased likelihood that the mobile station will be able to establish at least one communication link, whether such communication link is with the serving base station or one of the neighbor base stations. For example, suppose mobile station 130 in
Further note that if a mobile station is able to establish a communication link with two or more base stations during call setup, the mobile station will enter into soft handoff mode. Soft handoff is a technique for maintaining or managing a communication link while the mobile station traverses from the cell of one base station to another. Soft handoff is well-known in the art.
Although the present invention has been described in considerable detail with reference to certain embodiments, other versions are possible. For example, traffic channels at a plurality of base stations may be assigned to a mobile station in same carrier channel assignments or in single carrier wireless communication systems. Therefore, the spirit and scope of the present invention should not be limited to the description of the embodiments contained herein.
