Location Management Methods

forThird-GenerationMobile Systems

Sami Tabbane,

IEEE Communications Magazine,August, 1997.

2

• Increasing need for mobility in the access links within the telecommunication network:– Residential line access with cordless phones– Business line with wireless PBX and wireless LAN

(IEEE 802.11 and HIPERLAN)– Paging systems (delivering voice, message, e-mail)– Cellular systems

• The growth of mobility is seen at three levels:– At a spatial level

• roam locally regionally nationally internationally globally

– Penetration rate (Sweden 30+ %)– Traffic

• voice + data

3

• Mobility can be categorized into two areas:– Radio Mobility: Handover (handoff)

– based on radio aspects– Problem: unpredictable and highly fluctuating radio channel behav

ior

– Network Mobility: Location management– based on user‘s mobility and incoming call rate characteristics– Locating: to keep the user‘s location knowledge in order to be

able to find him, e.g. in case of an incoming call.Paging: to send paging messages in all the cells where the user

could be located.– If the cost of locating is high, the paging cost will be low.

(paging over a small area) And vice versa.– First generation:

» large cell locating has little impact» unbalanced traffic (1/3- incoming calls) paging has little impact

– Current:» small cell locating is more important» balanced traffic paging is more important

4

• Level 0: No Location Management– Early wide-area wireless systems (not yet cellular)– Pagin system

• Level 1: Manual Registration– Telephone cordless system, such as CT2

• Level 2: Use of Location Areas forAutomatic Location Management

– First- and second-generation cellular systems, such NMT, GSM, IS-95.

Present Location Management Methods

5

• Level 0: No Location Management– In early wide-area wireless systems

• Human operator• Very large cell, few BSs,

low user population and call rates• Calling: from every BS

Paging: through all BSs No location management is realized

i.e. flooding algorithm• + Simplicity

No need for special database- Does not fit for large networks

with high number of users and high incoming call rate

– Same in the Paging Systems

6

• Level 1: Manual Registration– Telephone cordless system, such as CT2

• Paging:– The user register himself each time he/she moves to a new island o

f CT2 beacons.– The user keeps scanning the channels to detect paging message.– The network first transmits messages to the beacon that the user re

gistered and , if the mobile does not answer, entends the paging to neighboring beacons.

• + Simplicity- Need to register each time the user moves

7

• Level 2: Use of Location Areas for AutomaticLocation Management

– First- and second-generation cellular systems, such NMT, GSM, IS-95.

– Location magament is done automatically.

–Location Area (LA)• Paging only occurs

in the cells of the LA in which the user is located.

• + Limited resource consumption- Home & visitor databases are needed.

8• Location Updating (LU) Methods– (1) Periodic Location Updating

• The mobile periodically transmits its ID to the network.• + Simplicity

- The resource consumption is user-dependent.(unnecessary updating if the user does not mov

e)

– (2) Location Updating on LA Crossing• Each BS broadcasts it

s LA ID periodically• Mobile listens and sto

res the current LA ID.• If the received LA ID

differs from the stored one, the mobile triggers a LU.

– (3) Hybrid : LU on LA crossing + LU by mobile if no comm. for some time

9

Separate User and Paging Networks• User Network

– a user‘s data transport network

• Paging Network– a signaling network for locating users

10

GSM Example• Databases

– HLR (Home Location Register)& AuC (Authentication Center)

VLR (Visitor Location Register)

• Hybrid Location Updating (HLR VLR LA)

• Three main types of LU procedures:– intra-VLR LU– inter-VLR LU using TMSI

(Temporary Mobile Subscriber Identity)– inter-VLR LU using IMSI

(International Mobile Subscriber Identity)– IMSI Attach: triggered when the mobile is powered on

where it was powered off

11• Inter-VLR LU using IMSL

Channel request & LU request LU Update Location Ciphering New TMSI & Clear LU Channel

12

LA Partitioning• To minimize location management cost

(LU + paging traffic and processing)

• Two types of methods– Analytical

• Based on assumptions of homogeneous cell shape, LA structure, and users‘ movements

– Heuristic• Mobility traces (record actual movement behavior)

are more important than mobility models.

• More complex, as an NP-complete problem

• Only empirical methods can be used to approach the optimal solution

13

– Analytical• Subscriber mobility model

– Markovian model (random-walk model): individual behavior

– Fluid model: macroscopic movement behavior

» Optimal number of cells in LA:Nopt = ( V * PAGcost / * R * LUcost)1/2

– Heuristic• Start by LAs using analytical methods,

then iteratively reduce a mobility cost function

• Generic algorithm ?– efficiently group the cells under a mobility cost constraint

• Simulated annealing ?

• Single-move heuristc ?

• Steepest descent optimization ?

14

Location Management Methods for Third-Generation Systems

• Two major groups: memory-less & memory-based

15

Memory-less Methods• Database Architecture

– A unique centralized database• small and medium networks

with star topology

– Distributed databases• large networks

• Ex.: GSM worldwidenetwork

– Hybrid• Centralized: HLR

Distributed: VLR

• Ex.: a single GSM network

16

• Optimizing Fixed Network Architecture– In 2nd and 3rd generation cellular networks, signaling

is managed by the Intelligent Network (IN).– Appropriately organizing mobility functions and entiti

es can• reduce the signaling burden at the network side,

• reduce the network mobility costindependent of the radio interface and LA organization.

– EX: To use different degrees of decentralization of control function.

– EX: To use higher-layer VLR (VLR gateway) with a MAN such that most significant traffic for LUs in a city can be handled in MAN

17

• Combining Location Areas and Paging Areas– LA size optimization in

• locating (LA)

• paging (PA)

– Splitting an LA into several PAs.• An MS registers only once when it enters the LA.

It dose not register when moving around the PAs of the same LA.

• Paging message is broadcasted according to a sequence determined by certain strategies.

• Ex: the one where the MS was last located - possible delay increase due to large LAs

+ reducting the number of messages+ decreasing the paging delay at high load

18

• Multi-layer LAs– LU traffic occurs mainly

in the cells of LA border.– Okasaka‘s Multi-layer LAs:

• Each MS is assigned to a different group.

• Each group is assigned one or more layers of LAs.

• MSs in group 1 & 2 will not generate LU at the same cells

• Distributing the LU traffic load over all the cells

LU (1-3) LU (2-4) LU (3-5)

LU (2-4) LU (3-5) LU (4-6)

19

• A Procedure for Reducing the Exchanges of Signaling Messages

– Reverse Virtual Call (RVC) setup?• For the LA that is not smaller than the VLR area

• Reducing the number of signaling messages between the called and calling databases and switches

• 50% decrease in call setup delay

LA LA……

VLR

LA

VLRVLR…...

VLRVLR…...

LA

20

Memory-Based Methods– Memoryless Systems do a lot of repetitive actions w

hich can be avoided if predicted. By short-term or long-term user and system behavior observation and statistics, memory-based systems can avoid the repetitive actions.

• Short-Term Observation for Dynamic LA and PA Size Assignement/Adjustment– Multilevel LAs

• LA size is different for each level

• Dynamically assign MS to different level of LA (High incoming rate and low mobility MSs are assigned to small LAs.)

21

– A variation• The MS periodically registers in the cells where it is camped on.

• The paging occurs only in the cells last visited.

• The periodic timer has to be optimized.

– Dynamically adjust the LA size (k cells/LA) for individual user

• Minimize a mobility cost function C(k, a, uk)where a = incoming call rate of the MS uk = LU rate (mobility rate) of the MS

• Assign the MS to a particular LA with size k

• Difficult to manage

– Dynamically adjust the LA size for the whole population

22• Individual User Patterns– Since the users‘ mibility patterns are repetitive and hig

hly predictable, the alternative strategies (AS) are defined to reduce the management traffic and LUs.

– A profile• records the most probable mobility patterns of each user

• is provided and updated manually or automatically by monitoring the MS for a period of time.

• A set of LA stats (af,f) in a time period [ti,tj], f = 1…kwhere af = location area

f = the probability that the MS is located in af

• When incoming call, the system pages the MS sequentially over the ais until getting an acknowledgement.

• When the delay is important, parallel paging can be proccessed.

23

– A variant: Two Location Algorithm• The MS stores the two most rescently visited LA addresses.

• The HLR does the same.

• When the MS goes back and forth between two LAs, no LU is triggered.

• Predicting Short-Term Movement of the Subscriber

• Predict the movements of the MS according to its direction of movement, velocity, and so on.

• When actual movements do not fit with the prediction, a registration is triggered by the MS.

24• Mobility Statistics–Statistical Paging Area Selection (SPAS)

• Each MS collects the location statistics:a list of average duration the MS has been located in each LA

• The MS periodically reports the stats to the network.

• A priority rule is determined to settle the sequence of LAs visited by the MS.

• When the MS moves to an area that is not on the reported list, the MS processes a temporary location registration to the network.

–Mobile Motion Prediction• The system predict the future location of the user.

• Pre-connection and pre-assignment at the location before the user moves into it, so the user can immediately receive the service.

• Movement Circle (MC): to predict the long-term regular movementMovement track: to predict routine movement

–Use of a cache memory• To store the location of the frequently called in a local database.

• Reducing the signaling traffic between the local database and the HLR.

25Impact of LUs on Radio Resource Occupancy and

Number of MSC/VLR Transactions• Examine the impact of LUs on

– RF resource occupancy in a DCS 1800 (or PCS 1900) network (urban environmnet, small cells, and high user densities)

– At the network level, compute the number of transactions * processed by the MSC/VLR and due to LUs.

• * the messages received or transmitted by the MSC/VLR

26

• Location updating exchanged mainly uses an SDCCH (slowly dedicated control channel)– Eight MSs share alternatively the same time slot on a

TDMA 51-multiframe (a channel) SDCCH block: each MS is allocated with 4 time slots

in every 51 TDMA multiframes A TCH/F (traffic channel/full rate) channel

27•Radio Resource Occupancy–An optimistic case

•The cell is located at the border of two LAs related to the same VLRs

•Only intra-VLR LUs will be processed in the cell (IMSI Attach ignored)

•TrLU = 7.30 Erl. for 1 % blocking probability, requiring 1.75 chs.

–An pessimistic case•The cell is located at the border of two LAs related to the two different VLRs

•Only inter-VLR LUs will be processed in the cell.Assume 80 % of TMSI and 20 % of IMSI

•TrLU = 42.46 Erl. for 1 % blocking probability, requiring 7 chs.

•MSC/VLR Transaction Load–TTNLU 12 106 transactions at peak time

28

Conclusion Remarks• Under heavy load, the impact of LUs can be im

portant.– Radio channels used: 1/4 ~ 1 GSM RF carrier is use

d for location area boundary crossing– Processing at the MSC/VLR side:

trade-off between LUs and services

• Methods presented are intended to reduce the LU signaling and processing on either the radio interface or the network equipment.But, it is quite difficult to compare.Hence, it needs realistic performance evaluation models.

29

Conclusion Remarks• The resource usage on the air interface can be

important. However, the main impact of LUs is noticed at the MSC/VLR side, where the transaction rates can be very high and may be a cause of either saturation or reduction of the processing power dedicated to providing sophisticated services to subscribers.