WiMAX Location Tracking Mechanism

IEEE 802.16e mobile Worldwide Interoperability for Microwave Access (WiMAX) pro-vides broadband wireless services with wide service coverage, high data throughput, and high mobility [75, 76, 77]. The WiMAX network architecture can be simplified as shown in Figure 3.1, which consists of the Connectivity Service Networks (CSNs; see Figure 3.1

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Figure 3.1: A Simplified WiMAX Network Architecture

(a)) and the Access Service Networks (ASNs; see Figure 3.1 (b)). An ASN that comprises ASN Gateways (ASN-GWs; see Figure 3.1 (c)) and WiMAX Base Stations (BSs; see Figure 3.1 (d)) provides radio access (such as radio resource management, paging and lo-cation management) to the WiMAX Mobile Station (MS; Figure 3.1 (i)). Every ASN-GW connects to several BSs. The ASN-GWs are also connected to each other to coordinate MS mobility. A CSN consists of network nodes such as the Mobile IP (MIP) [39] Home Agent (HA; see Figure 3.1 (f)), the Authentication, Authorization, and Accounting (AAA) server (see Figure 3.1 (g)) and the Dynamic Host Configuration Protocol (DHCP) server

(see Figure 3.1 (h)). The CSN provides IP connectivity (such as Internet access and IP address allocation) to a WiMAX MS and interworks with the ASNs to support capabili-ties such as AAA and mobility management. Before an MS is allowed to access WiMAX servcies, it must be authenticated by the ASN-GW (which serves as the authenticator) and the AAA server in the CSN.

In Taiwan, a linear layout with 27 WiMAX BSs has been deployed from Taipei to Taoyuan International Airport to cover highway and local road traffics (the distance is 30km). The network is anticipated to extend to 679 BSs in north Taiwan for more general broadband wireless applications [44]. One of the applications aims for Intelligent Transportation Systems (ITS) where the WiMAX BSs can be viewed as roadside units and the MSs are onboard units [36, 37, 51, 82]. In ITS applications, high mobility of vehicles may significantly affect network signalling overhead for location tracking, which is investigated in this chapter.

We first introduce the WiMAX location tracking mechanism. In WiMAX, two sub-scriber modes characterize the activities of an MS attached to the network. In the normal mode, the MS sends or receives packets to/from a BS. When there is no data transmis-sion for a period, the MS switches from the normal mode to the idle mode to conserve resources.

Several procedures defined in WiMAX are exercised when the MS is in the idle mode.

For example, the Location Update (LU) procedure is exercised for location tracking of an MS [27]. When there are incoming packets for the idle MS, the paging procedure is exercised to alert the MS. Then the MS performs the idle mode exit procedure to return to the normal mode and starts data transmission. In the control plane, the Paging Controller (PC) in an ASN-GW (see Figure 3.1 (c)) handles the location tracking and the paging operations for the idle MSs. All BSs connected to a PC are partitioned into several Paging Groups (PGs; see Figure 3.1 (e)). The PGs are used for MS tracking. For each MS in the idle mode, the network assigns a PC called the Anchor PC (or APC) to the MS.

Every APC is associated with a database called Location Register (LR) that contains the MS tracking and paging information of the idle mode MSs. The information includes the current PG where the MS resides, the paging parameters, the QoS profiles, etc. Suppose that an MS in the normal mode enters the idle mode through PG 0 in Figure 3.1, and PC 0 serves as the APC of the MS. When this idle MS moves from PG 1 to PG 2, it performs LU procedure to inform its APC (i.e., PC 0) of the new location through PC 1.

After location update, PC 1 serves as the relay PC for the MS and all PC-related control messages are delivered between PC 0 (the APC) and the MS indirectly through PC 1.

When the MS enters the normal mode, data transmission with handover is supported by the MIP. In Figure 3.1, assume that an MS first enters the WiMAX network (in the normal mode) and obtains an IP address through ASN-GW 0. The MS then registers to the HA (see Figure 3.1 (f)) in the CSN to indicate that the Foreign Agent (FA) is FA 0 associated with ASN-GW 0 (see Figure 3.1 (c)). At this point, the MS can start data transmission with a Corresponding Node (CN; see Figure 3.1 (j)). When the MS moves from BS 4 to BS 5, the local ASN-GW is changed from ASN-GW 0 to ASN-GW 1. The data between FA 0 and the MS is then delivered through ASN-GW 0, ASN-GW 1 and BS 5.

In WiMAX, the APC is only defined in the idle mode and does not play any role in the normal mode. When an MS switches from the idle mode to the normal mode, the APC is no longer associated with the MS; i.e., the MS information is removed from the LR of the APC (through the idle mode exit procedure). When the MS enters the idle mode again, a new APC is assigned to the MS by exercising the idle mode entry procedure.

In the idle mode, WiMAX provides two alternatives to reassign the APC during the MS movement: static or dynamic. When the MS moves from an old PG to a new PG, the APC can be dynamically reassigned during the LU procedure. If the APC reassignment occurs frequently, these APC/LR relocations result in extra network signalling overhead.

On the other hand, if the APC reassignment occurs infrequently, the APC may be far

away from the MS after several movements. In this case, long delays for message exchange will be expected in the LU procedures. This chapter analyzes the cost for the APC/LR reassignment under Taiwan’s linear WiMAX BS layout for ITS.

This chapter is organized as follows. In Section 3.2, we describe the LU procedure.

Section 3.3 illustrates two scenarios to study the cost for the APC/LR relocation due to MS (vehicle) mobility. Section 3.4 numerically compares these two scenarios.