[0001] METHOD AND SYSTEM FOR

STATION LOCATION BASED NEIGHBOR DETERMINATION AND HANDOVER PROBABILITY ESTIMATION

[0002] FIELD OF INVENTION

[0003] The present invention is related to wireless communication system.

More particularly, the present invention is a method and system for station location based neighbor determination and handover probability estimation.

[0004] BACKGROUND

[0005] Integration of wireless local area networks (WLANs) with other wireless access technologies has become increasingly important among the various standards that develop wireless access technologies. Handover mechanisms and procedures that might allow seamless delivery of services across heterogeneous networks are of prime importance towards multi-technology integration.

[0006] In particular within IEEE standards, 802.21 has been tasked with providing mechanisms and notification that enable other technologies to react quickly to changing conditions within the WLAN network itself. These notifications provide details with regard to the current state of the WLAN or access technology supplying a service.

[0007] Link quality, (e.g., received signal strength indicator (RSSI), bit error rate (BER), packet error rate (PER)), link capabilities to support a particular service, (e.g., voice or high speed data), and details on the service provider that delivers the service are examples of the information that 802.21 can provide. This information allows existing mobility management mechanisms to set up alternate access resources in order to continue the delivery of services with the same or similar level of quality as originally requested by the user.

[0008] It is important to note that it is up to an access technology to determine the course of action to follow, at the upper layer, upon receipt of the notification and triggers. However, in order to avoid the so-called "ping-pong"

effect, notifications hinting the need for a handover need to provide a validity period and action time that will allow the upper layer function to determine when the handover needs to occur.

[0009] In general handovers are triggered using two main criteria. Either the current link is no longer suitable or a better candidate has been found.

Handover across heterogeneous networks entails determining the best candidate across all available access technologies within a region.

[0010] There are current proposals addressing selection and discovery of handover candidates that base their criteria on measurements and information centrally stored in the access point. Information is stored in the form of neighboring maps that provide a snap shot of the network and associate this information to different zones. This poses a problem as measurements are taken from the access point's perspective and not from the client station's perspective.

This is particularly problematic when the base station or access points are moved frequently, as maps are not updated that frequently.

[0011] Although measurements and settings are kept centrally at the access point, the handover decision might be made at the client station (STA).

However, said current proposals do not take into consideration measurements taken at the STA, even thought the STA itself is in the best position to measure on prospective candidates for handover.

[0012] Therefore, it would be desirable to provide a method and system for station location based neighbor determination and handover probability without the limitations of the prior art.

[0013] SUMMARY

[0014] The present invention provides a method and system to estimate the best possible neighbor to handover to when the current link is no longer suitable to provide a service. The method and system also estimates the expected time at which a handover is to be executed within a particular cell and determines the degree of certainty that handover will occur towards a particular cell.

[0015] BRIEF DESCRIPTION OF THE DRAWINGS

[0016] A more detailed understanding of the invention may be had from the following description of a preferred example, given by way of example and to be understood in conjunction with the accompanying drawing wherein:

[0017] Figure 1 shows a scheme of station location-based neighbor determination in accordance with the present invention; and

[0018] Figure 2 shows a scheme of handover based on station location information in accordance with the present invention.

[0019] Figure 3 shows a signaling diagram for measurement message exchanges between a serving access point and a station for handover decision calculations.

[0020] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0021] Hereafter, a station (STA) includes but is not limited to a user equipment, a wireless transmit/receive unit, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, an access point (AP) includes but is not limited to a Node-B, a site controller, a base station or any other type of interfacing device in a wireless environment.

[0022] Figure 1 shows a wireless local area network (WLAN) 100 of multiple access points API, AP2, AP3, AP4 and AP5, an access controller AC, a base station BS and a target station STAl. The handover method described herein is also applicable in heterogeneous networks such that surrounding access points and/or base stations in the vicinity of station STAl belong to different networks rather than a common network as with WLAN 100. In accordance with the present invention, the handover decision is preferably based on knowledge of the target station STAl with respect to the direction of its movement, the position of the target station STAl relative to its currently associated access point API and neighboring candidate access points AP2-AP5, and the measurements reported from both the target station STAl and by historical

measurements of other STAs that once were located at or near the same coordinates.

[0023] A preferred method of STA location-based neighbor determination in accordance with the present invention will be described in reference the network 100. Initially, the mobile station STAl is associated with access point API. The direction of motion 101 is away from the access point API and at the edge of API's cell. At this location, a handover of the station STAl is necessary so that its communication link to the network remains unbroken. To facilitate handover, an access controller (AC) or any other central entity controlling the APs in the WLAN assesses the speed and direction of the station STAl's movement, determines the locations of handover candidate APs or base stations, and determines the probability that STAl is approaching the boundary of the cell, and therefore the need for handover.

[0024] The metrics regarding the speed and direction of the station STAl's movement are preferably measured as a function of explicit geographical location and signal strength measurements from different APs and BSs. The metric regarding the location of the candidate APs and BSs is measured as a function of the signal strength received at both the target station STAl and the current access point API. The AC gathers information about the current location of the APs within the WLAN. Each access point AP1-AP5 performs measurements of its current location for reporting to the access controller AC or other central entity. Each access point AP1-AP5 also performs measurements of transmitted signals and received signals to determine its cell boundary. These measurements may be performed with assistance from GPS. The access points AP1-AP5 store the information for reference by other network entities, such as the access controller AC and the target station STAl. The location and cell boundary measurements by each access point AP1-AP5 are preferably made on a periodic basis to account for any repositioning of the access point that may occur either deliberately or inadvertently. Based on the STAl metrics and the AP1-AP5/BS metrics, the access controller AC assists the station STAl in selection of the best candidate for handover from the access point API.

[0025] Alternatively, the cell boundaries are determined with the assistance of neighboring network stations. For simplicity, Figure 1 shows only the target station STAl. However, the network may include a plurality of additional stations located within one or more of the cells of access points. [0026] According to the present invention, any of these additional stations within the network report their measurements on their current connection, including received signal strength and link quality (e.g., bit error rate (BER)). From these reported measurements, the access controller AC determines an estimate of the boundary locations of all the cells in the WLAN and the distance from these cell boundaries and the current location of the target station STAl at any point within the network. In a mobile environment where the stations and/or the access points are not stationary, measurements may be taken as frequently as the movement itself demands. This includes fixed periodic measurements, and measurements triggered by the movement itself. With this assistance of the neighboring stations, the target station STAl now has accurate location information available to it provided by the access controller AC, so that the handover from access point API to the best candidate access point is improved. Furthermore, power consumption is reduced thereby conserving battery resources. The above measurements of access point AP1-AP5 location and their respective cell boundaries can also be made for surrounding base station BS and any other network entity.

[0027] The access controller AC communicates the network topology information for WLAN 100, such as the location and the cell boundary information of the access points AP1-AP5 and the base station BS, to the target station STAl and any other mobile or stationary stations within the WLAN 100. To receive this information, the target station STAl requests the network topology information from the access controller AC at system access or as required by the station STAl. Along with the request, the station STAl may provide its current location coordinates. The station STAl knows its own location based on both GPS and the network map provided by the network. The network

map provides detail on the network topology that can help calculate its location by triangulation, for example.

[0028] Alternatively, the serving access point API may use an estimate of the station STAl's current position using radio propagation information. One example of a technique utilized by the API for this step involves retrieving GPS coordinates from the station STAl and tracking the change of these coordinates over time to determine directional information and the position of the station STAl. This is then used to estimate the expected time until the station STAl will reach the cell boundary and at which point handover will be required to commence.

[0029] After the initial report of the WLAN topology information provided by the access controller AC is delivered to the station STAl, subsequent updates of the information are provided to accommodate the mobility of the station STAl. These updates may be periodic or upon request by the station STAl. The station STAl may request updates from the access controller AC as movement is detected and signal quality thresholds are crossed. The frequency of requests preferably depends upon the time the last topology update was retrieved and the speed and direction of the movement. The station might detect its own movement based on GPS and measurements from other access points AP2-AP5 and current access point API. The AC or other network entity might calculate the station STAl's movement based on measurements coming from the station STAl. The WLAN topology information received by the station STAl is based on the current position of the station STAl. As the station STAl moves toward the outside boundary of the current cell, the station STAl uses the topology information and knowledge of current movement, including its speed and direction of its movement to determine the probability to reach the boundary of a particular candidate cell. The probability refers to how quickly the station STAl will reach the cell boundary for API within a predefined time. For example, it may be estimated that the station STAl will reach the cell boundary within 5 ms with 95% probability. This is calculated by sampling speed and direction at specific intervals along with knowledge of the distance to the cell boundary.

[0030] Figure 2 shows a WLAN 200 comprising several access points API,

AP2, AP3, AP4 and AP5 with a mobile station STAl. In this second embodiment, handover of the station STAl is achieved without the assistance of an access controller. The following example describes a location based handover according to this second embodiment. Initially, the station STAl is associated with the access point API. The geographical position of the station STAl along with its direction of movement is determined by the access point API similar techniques as described above in reference to Figure 1.

[0031] Concurrently, the neighboring access points AP2, AP3, AP4 and AP5 report their estimates of the location of their cell boundaries. Under this embodiment, the station STAl is able to detect and receive signals from the neighboring access points AP2-AP5 as the station STAl approaches their cell boundaries. The station STAl is able to read this information during so called silent periods. Thus, the station STAl obtains the cell boundary information directly from the network without the central access controller intervention. In addition, the access point API can read signals of the other access points AP2- AP5. While the access points AP1-AP5 may not necessarily report to each other, the access point API can report to the station STAl the cell boundary information read by API from the access points AP2-AP5. Preferably, four geographical coordinates of each cell are provided (e.g., S, N, E, W coordinates). Alternatively, additional coordinates may be provided for better resolution of the cell boundary. If the access points AP1-AP5 are mobile, the cell fringe estimates are updated on a periodic basis. The access points AP1-AP5 may use the assistance of GPS technology for deriving the coordinates. [0032] The station STAl requests WLAN 200 topology information from the serving access point API either at system access or as needed. The request provides the station STAl's current location coordinates and measurements of signal strength received from the serving access point API. Optionally, the request provides measurements made with respect to received signals from the access points AP2-AP5, and/or measurements made by the access points APl- AP5 of each other.

[0033] The initially serving access point API determines probability for the station STAl to reach the cell boundaries of the neighboring access points AP2- AP5 for handover, using metrics as described above in reference to Figure 1. The serving access point API associates a weighting factor for the accessible neighboring access points AP2-AP5 so that the station STAl can select the best handover candidate. Based on the weighting factor, the handover candidates AP2-AP5 are ranked in order of probability of a successful handover with unbroken communication link. Alternatively, the access point API provides WLAN 200 network topology information, such as access point location and quality of signal, and the STAl performs the calculation necessary to estimate the distances to the cell boundaries of the neighboring access points AP2-AP5 and thereby select the best candidate for handover.

[0034] In addition to determining the best handover candidate, the time when a handover is likely to happen is also calculated by either the access point API or the station STAl. This handover timing is estimated according to the proximity of the cell boundary of each handover candidate access point as well as the direction and speed of the mobile station STAl as it approaches each respective handover candidate cell boundary.

[0035] As the serving access point API determines the weighting factors, each factor may be adjusted and tuned based on the received signal quality of neighboring access points AP2-AP5 as measured by the station STAl and as reported by the station STAl to the access point API. The station STAl may take these measurements of the access points AP2-AP5 during periods when the station STAl is not transmitting to the access point API. [0036] Figure 3 shows a signaling diagram for measurement message exchanges between station STAl and serving access point API. Station STAl comprises a measurement processor 301 and access point API comprises a measurement processor 302 for making the following measurements used for the handover probability and estimated time until the expected handover. Upon determining its geographical location coordinates using GPS, station STAl reports the coordinates to the access point API at successive messages 303i to

303n. Based on the multiple location, coordinates, the serving access point API tracks the motion of the station STAl to calculate the speed and direction of the station STAl and reports the results back to the station STAl at message 304. Station STAl also reports received signal strength measurements with respect to neighboring access points AP2-AP5 at message 305. The station STAl requests the network topology information at message 306, which includes cell boundary definitions for the neighboring access points AP2-AP5. Based on the received signal strength measurements, the serving access point API updates the network topology information, and reports it back to the station STAl at message 307. Using the network topology information and the movement measurements received by the station STAl, calculation 308 is performed to determine the probability for handover to any particular neighboring access point AP2-AP5 and the estimated time that such a handover will occur.

[0037] The present invention is applicable to any wireless communication system including, but not limited to, IEEE 802 technologies, cellular standards such as 3GPP or 3GPP2, and other standardized or proprietary wireless technologies similar to IEEE 802 WLANs, including 802.15 Bluethooth, HIPERLAN/2, etc. More particularly, the applicable IEEE 802 technologies include:

• WLAN baseline air interface standards: o 802.11 baseline o 802.11a OFDM 5GHz WLAN o 802.11b HR-DSSS 2.4GHz WLAN o 802.1Ig OFDM 2.4GHz WLAN o 802.11J OFDM 10 MHz option WLAN o 802.1 In High-Throughput WLAN

• WLAN standards supplements to extend operation for particular scenarios: o 802.1 Ie QoS extensions (including WMM and WMM/2 brands) o 802.11s ESS Mesh

o 802.11k Radio Resource Measurement o 802. Hv Wireless Network Management o 802.21 Media Independent Handover

[0038] The present invention may be implemented in any type of wireless communication system, as desired. By way of example, the present invention may be implemented in any type of 802 type system, including but not limited to 802.11, 802.16 and 802.21 or any other type of wireless communication system. The present invention may also be implemented as software, middleware or application-based. The invention is applicable to the data link layer, the network layer and transport layer of a wireless communication system or device. [0039] Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.