1. EEL 6591 Wireless Networks Mobility Management
Wireless Information Networking Group (WING)
EEL 6591 Wireless Networks
Mobility Management

2. Outline Handoff management Location management handoff strategies
Wireless Information Networking Group (WING)
Outline
Handoff management
handoff strategies
Location management
location update
paging
call delivery

3. Mobility Management General network architecture
Wireless Information Networking Group (WING)
Mobility Management
General network architecture

4. Wireless Information Networking Group (WING)
Handoff Basics
The ultimate purpose is to support the call without any interruption
Performance metrics for design
call blocking probability
handoff blocking probability
call dropping probability
handoff rate
duration of interruption
delay
handoff probability

5. Handoff Basics Hard handoff Soft handoff Softer handoff
Wireless Information Networking Group (WING)
Handoff Basics
Hard handoff
MS connects with one BS only at any time instant
Break before connect
common in TDMA systems
Soft handoff
MS can use multiple BSs for transmit/receive
similar to diversity technique
mostly for CDMA systems
Softer handoff
handoff between sectors in one cell (channel handoff)

6. Handoff Initiation Relative signal strength
Wireless Information Networking Group (WING)
Handoff Initiation
Relative signal strength
choose the BS with the strongest signal at all times
disadvantage: too many unnecessary handoffs
Relative signal strength with threshold
switch only if the received signal drops sufficiently weak (a threshold is used) and the other BS has stronger received signal
P1<Ph and P2>Ph, switch to BS2
ping-pong effect: switch back and forth

7. Handoff Initiation Relative signal strength with hysteresis
Wireless Information Networking Group (WING)
Handoff Initiation
Relative signal strength with hysteresis
switch only if the new BS is sufficiently stronger than the old BS
P2- P1>h, switch BS2
Relative signal strength with hysteresis and threshold
switch only if the current signal level drops below a threshold and the target BS is stronger than the current BS by a hysteresis margin (h)
Prediction techniques
switch based on the future value of received signal

8. Handoff Initiation Two approaches for averaging
Wireless Information Networking Group (WING)
Handoff Initiation
Two approaches for averaging
window averaging: using window size w sk = mk+mk-1+…+mk-w+1=sk-1 +mk-mk-m mk=sk/w
leaky-bucket integration (exponential average) sk=a sk-1+mk, a<1 the forgetting factor
more general approach: a(z)sk=b(z)mk (filtering)
smoothing technique
overcome certain irregular and fast variations

9. Handoff Strategies Link transfer types (types of handoffs)
Wireless Information Networking Group (WING)
Handoff Strategies
Link transfer types (types of handoffs)

10. Handoff Strategies Mobile-controlled handoff (MCHO)
Wireless Information Networking Group (WING)
Handoff Strategies
Mobile-controlled handoff (MCHO)
popular in low-tier system, such as PCS
MS monitoring the strength and quality of signal from serving BS, when some handoff criteria are met, the MS checks the “best” candidate BS for a traffic channel and launches a handoff
ongoing measurements and processing of measured data
triggering decision
searching for new channel
execution of automatic link transfer

11. Handoff Strategies-MCHO
Wireless Information Networking Group (WING)
Handoff Strategies-MCHO

12. Handoff Strategies Network-controlled handoff (NCHO)
Wireless Information Networking Group (WING)
Handoff Strategies
Network-controlled handoff (NCHO)
BS monitors the strength and quality of signal from MS, when signal is weak, the network arranges for handoff (assigns a BS)
BS asks surrounding BSs to also measure signals from MS and report the measurements
MSC issues commands for signal strength measurements while the serving BS supervises the quality of all connections (no direct link between BSs)

13. Handoff Strategies Mobile-Assisted Handoff (MAHO) a variation of NCHO
Wireless Information Networking Group (WING)
Handoff Strategies
Mobile-Assisted Handoff (MAHO)
a variation of NCHO
network asks MS to measure the strength and quality of signals from surrounding BSs (pilot signals) and to report them back to serving BS so that handoff decision can be made
GSM is using MAHO: efficient for TDMA system
One further twist: all measurements can be used to predict the location of the mobile

14. Handoff Strategies-NCHO/MAHO
Wireless Information Networking Group (WING)
Handoff Strategies-NCHO/MAHO

15. Handoff Failures Reasons for handoff failures
Wireless Information Networking Group (WING)
Handoff Failures
Reasons for handoff failures
No channel is available at target BS
handoff is denied by the network for lack of resources: available resource is less than the requested, or rerouting fails
it takes too long to setup the handoff after handoff initiation
target link fails in some way during the execution of handoff

16. Intersystem Handoff Handoff between two MSCs (dashed: signaling)
Wireless Information Networking Group (WING)
Intersystem Handoff
Handoff between two MSCs (dashed: signaling)

17. Wireless Information Networking Group (WING)
Mobility Management
Location management or location tracking: keeping track of mobile users in the system
Two-level hierarchy
Home Location Register (HLR): a database storing mobile users’ subscription information
directory service
profile information
current information
validation period
Visitor Location Register (VLR): temporarily storing subscription for mobile users who visits the area

18. Mobility Management Two basic operations
Wireless Information Networking Group (WING)
Mobility Management
Two basic operations
Registration (location update): a process that a mobile user informs the system about its location
update the location information in HLR
location info: the VLR’s ID the MS is currently visiting
Paging (location tracking): a process that the system locates the MS
MSC associated with the VLR for the MS directs all BSs to broadcast a message for the MS
paging procedure
sequential paging
parallel paging
selective paging

19. Registration Process VLR-HLR-VLR signaling
Wireless Information Networking Group (WING)
Registration Process
VLR-HLR-VLR signaling

20. Call Delivery Procedure
Wireless Information Networking Group (WING)
Call Delivery Procedure
Call delivery or call termination

21. SS7 Signaling for Roaming Management
Wireless Information Networking Group (WING)
SS7 Signaling for Roaming Management
Common Channel Signaling (CCS), e.g., SS7
The signaling network (SS7) is overlaying over the wireless networks to handle the control signaling exchanges

22. Location Update Schemes
Wireless Information Networking Group (WING)
Location Update Schemes
Registration area (RA) or location area (LA): a group of cells for location management purpose
Registration is needed when
the MS is power on/off
the MS moves out of the registration area
active location update is used
De-registration is sometimes performed
if MS moves out, the memory at the old VLR will be reclaimed back through de-registration

23. Location Update Schemes
Wireless Information Networking Group (WING)
Location Update Schemes
Three dynamic location update schemes
time-based: periodic location update
movement-based: an MS performs a location update whenever it makes a predefined number of movements across cell boundaries (can be detected by MS using BS switches)
distance-based: an MS performs a location update when its distance from the cell where it performed the last location exceeds a predefined value
Performance: distance-based>movement-based >time-based

24. Wireless Information Networking Group (WING)
Terminal Paging
Paging under delay constraints: choose a paging scheme within the delay constraints, selective paging may be used
paging all cells at the same time: delay is the minimum, but too much paging traffic
paging one cell at a time: minimum in signaling traffic, but delay is too long
selective paging: divide the cells in the paging area into groups, then page each group at a time, this can control the delay and the signaling traffic

25. Tradeoff of Signaling Traffic
Wireless Information Networking Group (WING)
Tradeoff of Signaling Traffic
Observations
more frequent location updates shrink the uncertainty region of MS location, which leads fewer paging cost
less frequent location updates result in larger uncertainty region, which leads to more paging cost
Tradeoff analysis minimize (location cost+paging cost)
some papers by Ho, Akyildiz & Lin (‘97), Li et al (‘00), Fang (‘01)

26. Reduction of Signaling Traffic
Wireless Information Networking Group (WING)
Reduction of Signaling Traffic
Observation: signaling traffic mostly occurs between MS and HLR, HLR is the bottleneck!
Idea: reduce the signaling traffic to HLR
Dynamic hierarchical database architecture
add a new level between the two-level mobility data base architecture in IS-41 or GSM MAP: Directory Register (DR) storing location pointer
Three types of DR
local pointer: at local DR pointing to the current MSC of MS
direct remote pointer: at remote DR pointing to current MSC
indirect remote pointer: at remote DR pointing to current DR

27. Reduction of Signaling Traffic
Wireless Information Networking Group (WING)
Reduction of Signaling Traffic
Per-user location caching
caching the MS and location binding at STP (Signal Transfer Point): (M,VLR), VLR is the recently known location of M
cache hit means signal traffic saving (no need to query HLR)
User profile replication
user profiles are replicated at selected local mobility databases
when call arrives, such databases will be searched first

28. Reduction of Signaling Traffic
Wireless Information Networking Group (WING)
Reduction of Signaling Traffic
Pointer forwarding
Instead of reporting a location change to HLR every time an MS moves out a LA, a pointer is setup at the old VLR, pointing to the new VLR
A threshold is set in order to avoid long chain of pointers
Local anchoring
A VLR close to the MT is selected as its local anchor, location changes are reported to local anchor instead of HLR

29. Reduction of Signaling Traffic
Wireless Information Networking Group (WING)
Reduction of Signaling Traffic
Two location algorithm (TLA)
in mobility databases (HLR/VLR), use two entries in location information field: current VLR and previous VLR
location update: done only when the new VLR is NOT the two VLRs in the location information entry
Similar to the one-step pointer forwarding

30. Location Prediction (Tracking)
Wireless Information Networking Group (WING)
Location Prediction (Tracking)
Location profiling
it is per-user-based, home-way-work
e.g., 7:30-8:30am, on the way, 8:30am-12:00pm, at work, 12:00-1:00pm, lunch, 1:00-5:30pm, at work, …
probability distribution model: a user is at place i with probability p(i)
Use the past history or measurements
Measurements (power) from at least three BSs can determine the MS’s location
Kalman’s filtering technqiues can be used to track a user (Liu, Bahl and Chlamtac)

31. Location Prediction (Tracking)
Wireless Information Networking Group (WING)
Location Prediction (Tracking)
Two-level pointer forwarding scheme
add another level of database between HLR and VLR
mobility anchor (MA): a group of RAs form a regional area of interest handled by an MA, which acts as a “local HLR”
At MA level, we form a chain of pointers while the original pointer is used
3G systems are a natural three-level hierarchy: HLR-GLR-VLR
GLR: gateway location register

32. Geolocation FCC mandate
Wireless Information Networking Group (WING)
Geolocation
FCC mandate
E-911 service: wireless systems should make 911 service possible--quickly locate an MS at reasonable time with reasonable accuracy
Geolocation study: locate a user in a system
GPS
power levels
angle of arrival (AOA)
time difference of arrival (TDOA)
a special issue on IEEE Communications Magazine

33. Further Reading Pollini’s paper (course webpage)
Wireless Information Networking Group (WING)
Further Reading
Pollini’s paper (course webpage)
Lin & Chlamtac, Chap. 2, 3, 4
Lin’s paper in Wiley International Journal on Wireless Communications and Mobile Computing
A paper by Ma and Fang in IEEE Transactions on Mobile Computing (to appear)