15.4 Intersystem Paging

In a multitier wireless service area consisting of dissimilar systems, it is desirable to consider some factors that will influence the radio connections of the mobile terminals (MTs) roaming between different systems. ^[16] Consider there are two systems, Y and W, in the microcell tier, that may use different protocols such as DCS1800 and PCS1900. Each hexagon represents a location area (LA) within a stand-alone system and each LA is composed of a cluster of microcells. The terminals are required to update their location information with the system whenever they enter a new LA; therefore, the system knows the residing LA of a terminal all the time. In the macrocell tier there are also two systems, X and Z, in which different protocols (e.g., GSM and IS-41) are applied. For macrocell systems, one LA can be one macrocell. It is possible that systems X and W, although in different tiers, may employ similar protocols such as IS-95, GSM, or any other protocol. There are two types of roaming: intra- or intersystem. Intrasystem roaming refers to an MT's movement between the LAs within a system such as Y and Z. Intersystem roaming refers to the MTs that move between different systems. For example, mobile users may travel from a macrocell system within an IS-41 network to a region that uses GSM standard.

For intersystem location update, a boundary region called boundary location area (BLA) exists at the boundary between two systems in different tiers. ^[17] In addition to the concept of BLA, a boundary location register (BLR) is embedded in the BIU. A BLR is a database cache to maintain the roaming information of MTs moving between different systems. The roaming information is captured when the MT requests a location registration in the BLA. The BLRs enable the intersystem paging to be implemented within the appropriate system that an MT is currently residing in, thus reducing the paging costs. Therefore, the BLR and the BIU are accessible to the two adjacent systems and are colocated to handle the intersystem roaming of MTs. On the contrary, the VLR and the MSC provide roaming information within a system and deal with the intrasystem roaming of MTs. Besides, there is only one BLR and one BIU between a pair of neighboring systems, but there may be many VLRs and MSCs within a stand-alone system.

When a call connection request arrives at X, the call will be routed to the last registered LA of the called MT. Given that the last registered LA within X is adjacent to Y, the system needs to perform the following steps to locate the MT:

Send a query signal to the BLR between X and Y to obtain the MT's location information. This step is used to ascertain whether the MT has crossed the boundary.
If the MT has already moved to Y, only the LA in Y needs to be searched. Otherwise, the last registered LA within X will be searched. Within network X or Y, one or multiple polling messages are sent to the cells in the LA according to a specific paging scheme.

As a result, only one system (X or Y) is searched in the paging process for intersystem roaming terminals. This approach will significantly reduce the signaling cost caused by intersystem paging. In particular, it is very suitable for the high-traffic environment because it omits searching in two adjacent systems. Moreover, because the BLR is an additional level of cache database, it will not affect the original database architecture. Another advantage of the BLR is that it reduces the zigzag effect caused by intersystem roaming. For example, when an MT is moving back and forth on the boundary, it only needs to update the information in the BLR instead of contacting the HLRs. If the new BLR concept is not used, the intersystem paging can still take place. The system will search X first, if the called MT cannot be found, then Y will be searched. This method increases the paging cost as well as the paging delay, thus degrading the system performance.

^[16]Akyildiz, I.F. and Wang, W., A dynamic location management scheme for next generation multi-tier PCS systems, IEEE Trans. Wireless Commun., Jan. 2002.

^[17]Akyildiz, I.F. and Wang, W., A dynamic location management scheme for next generation multi-tier PCS systems, IEEE Trans. Wireless Commun., Jan. 2002.