Mobility Management in Wireless Communication – Chapter 12

ByDonavon M. Norwood

CS286Dr. Moh

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Introduction - 12.1

Mobility is very important in mobile communication, and can be classified as radio mobility and network mobility. Radio mobility is mainly concerned with the handoff process, whereas network mobility deals with mobile location management (i.e., location and updating). The public land mobile network (PLMN) is a cellular network that provides wireless services to mobile subscribers to other networks and network services, which include other PLMN's. The PLMN is divided into regions called LA's, (Location Areas), which are made up of one or more cell areas. A mobile station (MS) registers with the visitor location register (VLR) each time it enters a new LA, and is free to move inside the LA without registration. Each cell has a unique identification number called a cell global identification (CGI), which contains location area identification (LAI) and the cell identification (CI). The LAI consists of mobile country code (MCC), mobile network code (MNC), and location area code (LAC). Mobility management in wireless networks is the primary set of functions important to a network in order to enable subscriber mobility, which includes enabling the network to keep track of a subscribers status and location in order to deliver calls to the subscriber. The key component to mobility management is the subscribers service profile, which is a database record in the network that contains information about each subscriber. The data in the record is dynamic such as current location and status of subscriber and permanent data such as service profile, international mobile subscriber identity (IMSI), etc., of the subscriber.

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Figure 12.1 Wireless PLMN

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Figure 12.2 Cell global identification

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Mobility Management Functions - 12.2

Mobility management generally deals with automatic roaming, authentication, and intersystem handoff. Automatic roaming includes a set of network functions that allow the subscriber obtain service outside the home service provider area. These functions are automatic and do not require special subscriber actions. Automatic roaming functions are divided into:

I. Mobile station (MS) service qualificationII. MS location managementIII. MS state managementIV. Home location register (HLR) and VLR fault recovery

The authentication process requires that the end users of the system are authenticated. Handoff is one of the essential features that guarantee the subscriber mobility in a mobile network where the subscriber can move around. The handoff function allows the moving subscriber to maintain a connection. In simple terms the handoff functions works when a subscriber moves into a new cell a new connection has to be established and the cell in which the subscriber left from has to be disconnected.

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Mobile Location Management - 12.3

Location management schemes are based on subscribers mobility and incoming call rate characteristics. The location update (LU) is used by the network to keep track of the mobile subscribers location to direct a incoming call. A paging process transmits paging messages to all the cells were mobile terminals are located. If a LU cost is high, the the paging cost is low and paging is performed over a small area. When LU cost is low, then paging cost is high and paging is performed over a wider area. Location management uses either the periodic LU or LU-on-LA-crossing. The VLR stores the LAI, and the HLR keeps the VLR identifier. Using the periodic LU method, the MS periodically sends its identity to the network, but the drawback to using this method is resource consumption, and an example would be if a MS does not move in a new LA for several hours. In the LU-on-LA-crossing method, every base station will periodically broadcast the identity of the LA. The MS is required to listen to network broadcast information and store the current LA identity if the one stored in the MS is different. The LU procedure is is automatically done by the MS. The advantage of this method is that LU's are only generated when the MS moves.

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Mobility Model - 12.3.1

A mobility model describes the occurrence of procedures such as LU and handoff, and several models have been proposed.

Fluid model: This model assumes that the traffic flow to be like the flow of fluid. The model suggests that the amount of traffic flowing out of the area is proportional to the population density of the area, average velocity of movement and the length of the area boundary. For an area the average number of crossings per unit time is given as:

= L

=number of crossings per unit of time ,=average populationdensity ,

=averagemovement velocity of the area , L= perimeter of the area

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Mobility Model – 12.3.1 (Continued)

Markovian model: Known as the random walk model, this model describes individual movement. Also in this model a mobile subscriber remains within a region or moves to an adjacent region according to a transition probability distribution. One of the limitations of this model is that there is no concept of trips.

Gravity model: Different types of this model have been employed in transportation research to model human movement behavior. This model also has been applied to regions of varying sizes, from city models to national and international models. This model is difficult because many parameters are required in the calculations, therefore it is difficult to model geography with many regions.

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Mobile Registration - 12.4

The detection of a subscriber in a new serving system is an example of a registration event, which is a subset of automatic roaming functions, which consists of MS service qualification and MS location management. These two functions allow the subscriber to register with the network, that is to indicate to the network location register functional entities of the location and status of the MS. The registration functions need to be performed before the network can provide service to the subscriber, with the exception of emergency services like 911. Registration may be initiated by the MS or the network, or may be implied during the MS access. Upon receiving the registration request from a MS, the base station (BS) constructs the Registration Update Request message and sends it to the MSC (Mobile Switching Center), which contains the MS's identification and location information and may contain authorization parameters. The MSC may respond with a request for authorization (optional) and finally with a Registration Update Response message. The MSC sends the Registration Update Response to the BS when the registration procedure has been successfully completed, and this message also indicates whether the MS's registration has been accepted or rejected. The three possible results in the registration request are: successful registration, unsuccessful registration, and cancellation of registration.

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Mobile Registration – 12.4 (Continued)Registration Types

Distance-based registration: when the distance between the current cell and the cell where the mobile last registered exceeds a threshold.

Geographic-based registration: whenever a mobile station enters a new area of the same system.

Parameter change registration: when specific operating parameters in the mobile are changed.

Periodic registration: when the system sets parameters on the forward control channel to indicate that all or some of the mobile stations must register.

Power-down registration: when the mobile is switched off. This allows the network to deregister a mobile immediately upon its power-down.

Power-up registration: when power is applied to the mobile, and used to notify the network that the mobile is now active and ready to place or receive calls.

Timer-based registration: when a timer expires in the mobile.

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Figure 12.3 Call flows for MS registration of all mobiles listening to a control channel (ANSI-41 Standard)

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GSM Token-Based Registration - 12.4.1

When an MS registers with the new network, it sends its TMSI and LAI. The LAI informs the system where to find the old VLR. The network then queries the old VLR for data and uses the data to authenticate the MS. The new VLR then communicates with the HLR to update the location of the MS. The HLR sends a registration cancellation message to the old VLR. The operation of the token-based system is slightly different from the ANSI-41 system in that the segmentation of call processing between the BS, MSC and VLR is defined differently. Here are the steps for Token-Based registration:

1. MS sends registration message to the visiting system with its old TMSI and old LAI.2. Visiting system queries the old VLR for data.3. Old VLR returns security-related information.4. Visiting system issues a challenge to the MS.5. MS responds to challenge.6. Visiting system assigns new TMSI.7. Visiting system sends a message to the HLR with location update information8. HLR updates its its location database with new location of the MS.9. HLR acknowledges messages and may send other security related information.10. HLR sends a registration cancellation to old system.11. Visiting system sends an encrypted message to the MS with new TMSI.12. MS acknowledges the message.

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Figure 12.4 Token-based registration

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IMSI Attach and ISMSI Detach (Registration and Deregistration) in GSM - 12.4.2

In GSM a mobile power-up implies a IMSI attach which causes a mobile registration. See figure 12.5.

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Figure 12.5 IMSI attach process in GSM

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IMSI Attach and ISMSI Detach (Registration and Deregistration) in GSM – 12.4.2

(Continued)

In GSM a mobile power-down implies a IMSI detach which causes a mobile deregistration. See figure 12.6.

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Figure 12.6 IMSI detach process in GSM

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Paging in GSM - 12.4.3

In this scenario we assume that the mobile is already registered with the system and has acquired a TMSI. It is also assumed that the mobile is located in its home network. A land subscriber dials the number of the mobile subscriber. See Figure 12.7.

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Figure 12.7Mobile-terminated call in GSM

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Handoff – 12.5

As a mobile moves from one cell to another, an active call must undergo a switch from one channel to another. This process is called handover and handoff. In TDMA systems handoff usually involves both a change of channel carrier frequency and time slot. It also may require a reconfiguration of the wireline facilities by dropping the connection to the serving (old) BSS and switching to a connection to the new (target) BSS as shown in figure 12.8. In this example the mobile tunes from carrier frequency 6 on time slot 3 served by the old BSS to carrier frequency 9 on time slot 7 by the new BSS (Interfrequency handoff).

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Figure 12.8Inter-MSC, Inter-BSS handoff

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Handoff – 12.5 (Continued)

Handoff is initiated because of a variety of reasons. Signal strength deterioration is the most common reason for handoff at the edge of a cell. Other reasons could be due to load balancing where the handoff is network initiated to relieve traffic congestion by shifting calls in a highly congested cell to a lightly loaded cell. The handoff could be synchronous or asynchronous. Handoffs are initiated by the BSC based upon radio subsystem criteria such as RF level (RXLEV), signal quality (RXQUAL) or distance, or they are a result of network traffic loading. Appropriate decisions are made by a handoff algorithm. The measurements performed by the MS and the BTS which are collated by the BSC include:

• MS – RXLEV and RXQUAL (for serving downlink and adjacent BS's)• BTS – RXQUAL and RXLEV (uplink for serving BS)

There are two types of handoff: internal and external. If the serving and target base station are located in the same BSS, the BSC for the BSS performs the handoff process without the help of a MSC. This type of handoff is known as a intra-BSS handoff. However if the serving and target base stations are not in the same BSS, and external handoff is performed in which a MSC coordinates the handoff performs the switching task between the serving and target base stations. External handoffs can be either in the same MSC (intra-MSC) or between different MSC's (inter-MSC).

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Handoff Techniques - 12.5.1

Handoff techniques can be classified as mobile-controlled handoff (MCHO), network-controlled handoff (NCHO), and mobile-assisted handoff (MAHO). In MCHO themobile continuously monitors the signal strength and quality from the serving base stationand several handoff candidate base stations. When the handoff criteria are met, themobile checks the candidate base station for an available traffic channel and launches ahandoff request. NCHO is employed by a low-tier CT-2+ and a high-tier AMPS system. In this case the base station monitors the strength and signal quality from the mobile. When these fall below a certain threshold, the network arranges for a handoff to anotherstation. The network asks all surrounding base stations to monitor the signal strengthfrom the mobile and report the measurements back to the network. The networkselects a new base station for handoff and informs both the mobile and new base station.MAHO strategy has been employed by high-tier 2G (GSM, IS-95 CDMA, IS-136 TDMA)and 3G (WCDMA, cdma200) systems. In this approach the network provides the mobilewith a list of base station frequencies (those of nearby base stations). The network asksthe mobile to measure the signal strengths and signal quality from the surrounding basestations and report the measurements back to the serving base stations so the networkcan decide whether a handoff is required and to which base station.

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Handoff Types - 12.5.2

Handoff can be categorized as hard handoff, soft handoff, and softer handoff. Hardhandoff can be further divided into intrafrequency and interfrequency hard handoffs. Ahard handoff occurs when the old connection terminated before making a newconnection. In the case of a interfrequency hard handoff, the carrier frequency is differentfrom the old carrier frequency which the MS was connected to. If the new frequency ofthe carrier is the same as the old frequency in which the MS was connected to, this iscalled a intrafrequency handoff. In 2G TDMA systems the majority of the handoffs areintrafrequency handoffs. Interfrequency handoffs may occur between two different radioaccess networks. In this case it can also be called a intersystem handoff. A intersystemhandoff is always a type of interfrequency since different frequencies are used in differentsystems. A soft handoff occurs when a new connection is made before the oldconnections is released. In 3G systems the majority of handoffs are intrafrequencyhandoffs. A 2-way soft handoff happens when two sectors with different BS's that donot belong to the same BSC. A softer handoff occurs when a BS transmit through onesector but receives responses from more than one sector. When a soft and softerhandoff occurs at the same time, this is called a soft-softer handoff.

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Handoff Process and Algorithms - 12.5.3

A basic handoff process consists of three main phases which include measurements,decision and execution phase.

• Measurement phase - the MS continuously measures the signal strength of of the serving and neighboring cells, and reports the results to the network.

• Decision phase - consists of an assessment of the overall quality of service (QoS) of the connection and comparing it with the requested QoS attributes and estimates from neighboring cells. A handoff my occur depending on the outcome of the comparisons.

• Execution phase - involves handoff signaling and radio resource allocation. Radio signal strength (RSS) measurements from the serving cell and neighboring cells are primarily used in most networks.

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Handoff Process and Algorithms – 12.5.3(Continued)

The following parameters are generally used in the handoff algorithm:

• Upper threshold: the level at which the signal strength of the connection is at the maximum acceptable level with respect to the required QoS.

• Lower threshold: the minimum level of the signal strength to satisfy the required QoS, thus the signal strength of the connection can not fall below this level.

• Handoff margin: a predefined parameter set in which the signal strength of neighboring cell exceeds that of the serving cell by a certain amount or a certain time.

Some of the traditional handoff algorithms are as follows:

i. RSS type: the BS with the largest signal strength is selectedii. RSS plus threshold type: a handoff is performed if the RSS of the new BS exceeds that of

the serving BS and the signal strength of the serving BS is below the lower threshold value.iii. RSS plus handoff margin type: a handoff is performed if the RSS of the new BS is more

than that of the serving BS by a handoff margin/

* RSS (Radio Signal Strength)

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Figure 12.9 Handoff process

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Handoff Call Flows (Intra-MSC Handoff) - 12.5.4

In Intra-MSC Handoff the MS constantly monitors the signal

quality of the BSS link, or optionally the BSS will forward its

measurements to the MS. When the signal quality os poor, the MS

will attempt to maintain the desired signal quality of the radio link

By requesting a handoff. The following steps on the left occur in a

Intra-MSC Handoff.

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Figure 12.10 Call flow for intra-MSC handoff

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Handoff Call Flows (Inter-MSC Handoff) - 12.5.4

In this scenario we assume that a call has already been established. The serving BSS is connected to the servingMSC, and the target BSS to the target MSC (see figure 12.11).

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Figure 12.11Call flow for inter-MSC handoff

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Figure 12.11Call flow for inter-MSC handoff

(Continued)

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Thank You

References: Wireless Communications and Networking, Vijay K. Garg.