Distance-based Location Update & RouteUpdateRadius in CDMA 1X EV-DO

• Introduction

Idle-mode mobility management in cellular systems involve location updates and paging. Idle-mode mobility is not tracked at the granularity of individual cells. Instead, it is tracked at a coarser granularity of a group of contiguous cells termed as a "location area". A location update mechanism involves the reporting of this location area information by an idle mode mobile to the network, whenever it moves from one location area to another. Because the network knows the location of the mobile only at the location area-level, when there is an incoming call for the mobile, the network needs to page the mobile in all the cells in the location area. Both the location update mechanism and paging will generate signaling load on the network, and reducing the load due to one of these would involve an increased load due to the other.

In CDMA 1X EV-DO, specified in 3GPP2 C.S0024-A v1.0 (2004), a dynamic distance-based location update mechanism is used. In this scheme, a mobile makes a location update if the distance between the BTS in which it is currently camped, and the BTS where it made its last location update is greater than a parameter called RouteUpdateRadius. This scheme provides a significant performance benefit over the static location update mechanism used in GSM/GPRS/UMTS networks. However, the distance-based mechanism does not utilize the knowledge of the direction of mobiles’ movement.

• 1X EV-DO Route Update Protocol

In CDMA 1X EV-DO, the idle-mode mobility management procedures are handled by the route update protocol, and it uses the distance-based location update approach. In 1X EV-DO, each cell broadcasts its latitude, longitude, and a parameter called RouteUpdateRadius. An idle mode mobile as it moves from cell to cell, monitors these three parameters. after each cell change, the mobile computes the distance between the site locations of the current cell and the cell in which it last sent a location update message. If this distance is greater than the RouteUpdateRadius parameter broadcast in the cell in which it last sent a location update message, the mobile sends another update to the network.

Otherwise mobile does not send a location update message. To perform this operation, the mobile would have to store the latitude, longitude, and RouteUpdateRadius parameters of the last cell in which it did a location update operation.

The distance computed is the distance between the site locations, and it does not depend on the location of mobile within the serving site. The latitude and longitude information is broadcasted by each cell is used for computing the distance.

Because the mobile sends a location update message only after it moves to a cell that is sufficiently far apart from the cell from which the mobile last sent a location update, the problem of ping-ponging is eliminated. Essentiall, as soon as a mobile sends a location update, it draws a circle around the serving cell of radius RouteUpdateRadius and sends the next location update only if it goes outside that circle. Clearly, this approach eliminates the ping-ponging problem of the static location area approach.