Patent Application
20080070576
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:
For purposes of discussion, the present invention will be described herein with respect to
Wireless communication system 200 comprises base stations 210, 220 and 230, mobile station 240 and radio network controller (RNC) 250. Base stations 210, 220 and 230 provide telecommunication services to mobile stations within their geographical coverage areas or cells 215, 225 and 235, wherein each cell 215, 225 and 235 includes a plurality of co-located cells. Each cell 215, 225 and 235 (or co-located cells) can be divided into a plurality of sectors A, B and C.
Telecommunication services are provided to mobile stations within each of the plurality of co-located cells using a radio interface and a frequency band. In one embodiment, each cell 215, 225 and 235 comprises of a plurality of co-located cells which use different radio interfaces, thus, each cell 215, 225 and 235 is associated with a set of radio interfaces. Specifically, in this embodiment, cells 215 and 225 each comprises a co-located cell which uses the well-known CDMA2000 1x radio interface (also referred to herein as a “3G1x cell”) and a co-located cell which uses the well-known CDMA2000 EV-DO Rev 0 radio interface (also referred to herein as a “DO Rev 0 cell”), and cell 235 comprises a 3G1x cell and a co-located cell which uses the well-known CDMA2000 EV-DO Rev A radio interface (also referred to herein as a “DO Rev A cell”). Circuit calls are supported by 3G1x cells, wherein Voice over Internet Protocol (VoIP) calls are supported by DO Rev A cells. DO Rev 0 cells do not support VoIP calls nor circuit calls.
In 3G1x cells, telecommunication services are provided over a first frequency band, referred to herein as a “3G1x band.” In DO Rev 0 and DO Rev A cells, telecommunication services are provided over a second frequency band, referred to herein as a “DO band.” A set of communication channels are used for communication over each of the frequency bands. Each set of communications channels include a pilot channel, a control channel and a plurality of traffic channels.
When a mobile station travels across the coverage area of wireless communication system 200, the call is handed off from one cell to another in order to keep the call active. The present invention is a method of handoff which utilizes a combined signal quality measurements from a plurality of cells to trigger the handoff. The present invention will be described herein with respect to an illustrative embodiment involving an inter-frequency handoff in which a VoIP call is converted to a circuit call. This should not be construed to limit the present invention in any manner.
In the illustrative embodiment, mobile station 240 is in a VoIP call and traveling along path 245 in cell 235, which as a 3G1x/DO Rev A cell. Base station 230 is its serving base station, and base stations 210 and 220 are its neighbor base stations. As mobile station 240 approaches cells 215 and 225, the VoIP call will be handed off to neighbor base stations 210 and 220 and converted to a circuit call because cells 215 and 225 are 3G1x/DO Rev 0 cells, which do not support VoIP calls. VoIP calls are carried over the DO band, whereas circuit calls are carried over the 3G1x band, thus the handoff will be an inter-frequency handoff.
In step 315, mobile station 240 transmits to serving base station 230 a measurement message indicating the signal strength measurements and offsets of the DO pilots, such as a Route Update Message (RUM). In step 320, serving base station 230 determines whether to initiate an inter-frequency handoff to one or more neighboring 3G1x sectors (or cells) based on the RUM. Such a handoff is also referred to herein as a “DO-3G1x handoff.” The manner of determining whether to initiate an inter-frequency handoff will now be described herein with reference to two embodiments illustrated in
If mobile station 240 is not in a border coverage area, then flowchart 400 determines in step 410 not to initiate the inter-frequency handoff. Otherwise, flowchart 400 continues to step 415 where base station 230 combines the DO pilot signal strength measurements to determine whether to trigger or initiate the DO-3G1x handoff. Specifically, base station 230 determines whether a sum (or other combination) of the DO pilot signal strength measurements associated with two or more DO Rev 0 sectors (or cells), hereinafter referred to as “Rev 0 sum,” is greater than a first Rev 0 handoff threshold “T_Rev—0,” i.e., Rev 0 sum>T_Rev—0. In one embodiment, the Rev 0 sum can correspond to a summation of DO Rev 0 pilot signal strength measurements associated with a pair of adjacent DO Rev 0 sectors (or cells) which, for example, may include sectors of the serving base station and a neighbor base station. In another embodiment, the Rev 0 sum can correspond to a summation of DO Rev 0 pilot signal strength measurements associated with any number of DO Rev 0 sectors (or cells) indicated in, for example, the Route Update Message (RUM). Note that a combined DO pilot signal strength measurement is used to trigger the DO-3G1x handoff instead of individual DO pilot signal strength measurements. Such combination allows the DO-3G1x handoff to be triggered earlier when, for example, the mobile station is traveling along a boundary shared by neighbor cells, thereby reducing interference the mobile station would have caused if individual DO pilot signal strength measurements were used to trigger the DO-3G1x handoff.
In one embodiment, a list of neighbor base stations, i.e., neighbor list, may be used to identify which DO pilot signal strength measurement are associated with DO Rev 0 sectors (or cells) and DO Rev A sectors (or cells). The neighbor list can indicate whether a base station or its associated sector (or cell) utilizes a 3G1x, DO Rev 0 and/or DO Rev A radio interface. The neighbor list may be maintained by each base station 210, 220 and 230, or by some other entity, such as RNC 250.
If, in step 415, it is determined that the Rev 0 sum is not greater than the first Rev 0 handoff threshold T_Rev—0, then base station 230 does not initiate the inter-frequency handoff in step 420. If it is determined that the Rev 0 sum is greater than the first Rev 0 handoff threshold T_Rev—0, then base station 230 determines in step 425 whether a sum (or other combination) of the DO pilot signal strength measurements for the DO Rev A sectors (or cells), hereinafter referred to as “Rev A sum,” minus the Rev 0 sum is less than a differential handoff threshold “T_Diff,” i.e., Rev A sum−Rev 0 sum<T_Diff. In one embodiment, the Rev A sum can correspond to a summation of DO Rev A pilot signal strength measurements for base stations in the active set. In another embodiment, the Rev A sum can correspond to a summation of DO Rev A pilot signal strength measurement for only the serving base station or DO Rev A pilot signal strength measurements for any number of base stations indicated in the RUM.
If the difference between Rev A sum and Rev 0 sum is not less than the differential handoff threshold T_Diff, then base station 230 determines not to initiate the inter-frequency handoff in step 430. Otherwise, base station 230 determines to initiate the inter-frequency handoff in step 435 to the base stations associated with the pilots combined in the Rev 0 sum.
Note that, in step 415, the present invention is checking to make sure that the difference between the Rev A sum and the Rev 0 sum is not less than the differential handoff threshold T_Diff. This check provides assurances against triggering the handoff too early, especially when the DO band signal quality between mobile station 240 and the base stations in its active set, i.e., DO Rev A base stations, is stronger than the DO band signal quality between mobile station 240 and the neighbor DO Rev 0 base stations.
Otherwise, in step 515, base station determines whether at least one DO pilot signal strength measurement associated with a DO Rev 0 sector (or cell) of a neighbor base station, hereinafter referred to as “Rev 0 pilot,” is greater than a second Rev 0 handoff threshold “T_Rev—0—2,” i.e., Rev 0 pilot>T_Rev—0—2. If a DO pilot signal strength measurement from at least one neighbor base station is greater than the second Rev 0 handoff threshold T_Rev—0—2, then the serving base station determines in step 520 to initiate the inter-frequency handoff to the neighbor base station associated with that DO pilot signal strength measurement.
Otherwise, flowchart 500 continues to step 525 where base station 230 determines whether the Rev 0 sum, e.g., summation of DO pilot signal strength measurements of two or more DO Rev 0 sectors or cells, is greater than the first Rev 0 handoff threshold T_Rev—0, i.e., Rev 0 sum>T_Rev—0. In one embodiment, the first Rev 0 handoff threshold T_Rev—0 is set greater than the second Rev 0 handoff threshold T_Rev—0—2.
If it is determined that the Rev 0 sum is not greater than the Rev 0 handoff threshold T_Rev—0, then base station 230 does not initiate the inter-frequency handoff in step 530. Otherwise, base station 230 determines in step 535 whether the Rev A sum minus the Rev 0 sum is less than the differential handoff threshold T_Diff, i.e., Rev A sum−Rev 0 sum<T_Diff.
If the difference between Rev A sum and Rev 0 sum is not less than the differential handoff threshold T_Diff, then base station 230 determines not initiate the inter-frequency handoff in step 540. Otherwise, base station 230 determines to initiate the handoff in step 545 to the base stations associated with the pilots used in the Rev 0 sum.
Note that, in an alternate embodiment, if it is determined in steps 415 or 525 that the Rev 0 sum is greater than the first handoff threshold T_Rev—0, then the inter-frequency handoff may be initiated without continuing to steps 425 or 535, respectively. In yet another alternate embodiment, steps 415 and 525 may be bypassed or eliminated and inter-frequency handoff may be initiated based solely on steps 425 and 535.
Returning to
In step 340, RNC 250 sends a handoff response to serving base station 230 indicating the assigned traffic channels and associated base stations. In step 345, serving base station 230 transmits a handoff message to mobile station 240 indicating the assigned traffic channels and associated base stations. Upon receiving the handoff message, in step 350, mobile station 240 sets its active set to include the base stations indicated in the handoff message.
In step 355, mobile station 240 sends a handoff complete message to the base stations in order to establish communication links with the base stations using the assigned traffic channels. The VoIP call is dropped and a circuit call is established in the 3G1x band.
Note that the call is handed off to a plurality of base stations allowing the mobile station to establish simultaneous communication links with two or more base stations. This will increase the chance that at least one of the communication links or the combination thereof is a strong communication link, thereby improving the reliability and success rate of the handoff. In one embodiment, the plurality of base stations to which the call is handed off may or may not include the serving base station.
If it is determined in step 320 that an inter-frequency (or DO-3G1x) handoff is not to be initiated, then flowchart 300 continues to step 360 where base station 230 determines whether to initiate an intra-frequency handoff to one or more neighbor DO Rev A sectors (or cells), i.e., DO-DO handoff. Specifically, base station 230 determines whether any DO Rev A sectors (or cells) are associated with a DO pilot signal strength measurement greater than a Rev A handoff threshold “T_Rev_A.” In one embodiment, the Rev A handoff threshold T_Rev_A is less than the first Rev 0 handoff threshold T_Rev 0.
If no DO pilot signal strength measurement associated with a DO Rev A sector (or cell) is greater than T_Rev_A, flowchart 300 returns to step 310. Otherwise, flowchart 300 continues to step 365 where base station 230 sends a handoff request to RNC 250 indicating the DO Rev A sectors (or cells) to which the VoIP call is to be transferred or handed off. In step 370, RNC 250 will attempt to allocate resources for setting up traffic channels on the DO band at the base stations indicated in the handoff request. In step 375, RNC 250 transmits a traffic channel setup message to the base stations at which the resources have been allocated. The traffic channels setup message indicates the resources allocated to mobile station 240 at the relevant base stations.
In step 380, RNC 250 sends a handoff response to serving base station 230 indicating the assigned traffic channels and associated base stations. In step 385, serving base station 230 transmits a handoff message to mobile station 240 indicating the assigned traffic channels and associated base stations. Upon receiving the handoff message, in step 390, mobile station 240 adds the neighbor base station associated with the assigned traffic channels to its active set. In step 395, mobile station 240 sends a handoff complete message to the base stations in order to establish communication links with the base stations using the assigned traffic channels and complete the handoff.
The present invention have been described herein with reference to certain embodiments. This should not be construed to limit the present invention to these embodiments. Other embodiments are possible. For example, flowcharts 400 and 500 used for determining whether to initiate an inter-frequency handoff may also be used for determining whether to initiate intra-frequency handoff. Another example may involve handing off the call from a serving base station in one wireless communication system to neighbor base stations in another wireless communication system. Therefore, the spirit and scope of the present invention should not be limited to the description of the embodiments contained herein
