1. Handoff Management

2. Outline
Handoff Basics
Handoff Management
Handoff Detection
Handoff Assignment
Example: Handoff in GSM
Handoff Requirements and Goals
Advanced Issues of Handoff Management
Handoff Failure
Radio Link Transfer
Soft Handoff
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 2

3. Review:
Signal Strength within Cells
The contours may not be concentric and could be distorted by
atmospheric conditions and topographical contours
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 3

4. Handoff
A most common reason that the handoff is conducted:
When the phone is moving away from the area covered by
one cell and entering the area covered by another cell, the
call is transferred to the second cell to avoid call termination
when the phone gets outside the range of the first cell
The handoff process is of major importance within
any cellular networks
Failure for it to perform reliably can result in dropped calls,
and this is one of the key factors that can lead to customer
dissatisfaction
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 4

5. Basics of Handoff Management
Handover (Europe) = handoff (North America)
Handoff is the process that occurs when a mobile
is “handed over” from one access point to another,
i.e., the access point which the mobile is using
changes
Either handover or handoff can be abbreviated as
HO hereafter
Handoff Management
(Handoff detection) How do you detect that you
should handoff?
(Handoff assignment) Who initiates handoff?
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 5

6. How HO Happens?
As the MS moves away from the BS of the cell, the signal
strength weakens and the radio connection of MS is changed to
another adjacent cell
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 6

7. How HO Happens? (cont’d)
By starting to look for a new BS before you need it, there is
time to make a decision
X3: Start looking for a new BS
X5: Time to switch
TL: Threshold for looking around
TH: Threshold for handoff
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 7

8. Outline
Handoff Basics
Handoff Management
Handoff Detection
Handoff Assignment
Example: Handoff in GSM
Handoff Requirements and Goals
Advanced Issues of Handoff Management
Handoff Failure
Radio Link Transfer
Soft Handoff
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 8

9. “Ping-Pong” Effect
The name of the phenomenon implies that the MS
moves back and forth between the overlapped area
of two adjacent cells
Celli
Cellj
The solution is to allow MS continue maintain a radio
link with the current BSi, until the signal strength
from BSj exceeds that of BSi by some pre-specified
threshold value E (shown as Xth)
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 9

10. How HO Happens? (cont’d)
To avoid “ping-pong” effect, the MS continues to
maintain a radio link with BSi until Xth
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 10

11. Reality is More Complex
Three common measurements of the channel:
Word Error Indicator (WEI)
Based on if the receiver is able to decode the received signal
correctly
Received Signal Strength Indication (RSSI)
A measure of the received signal strength
Quality Indicator (QI)
Related to the signal to interference & noise ratio (S/I)
Handoff may depend more reliably on WEI of the
current channel rather than RSSI
If WEI is good, then handoff is not performed
However, it is necessary to accumulate WEI measurements
over a period of time, whereas RSSI is known
instantaneously
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 11

12. Reality is More Complex (cont’d)
To make the handoff decision accurately and quickly,
it is desirable to use both WEI and RSSI
However, to use a simple policy for handoff detection can be
feasible in practice
For example, GSM Specification introduces 6 basic
types of causes of HO with priority order
Uplink quality
Downlink quality
Uplink level
Downlink level
MS-BS distance
Power budget: To improve transmission quality in the lower
power level
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 12

13. Who Makes the HO Decision?
NCHO (network controlled handoff)
The network makes the decision
Used in CT-2 Plus and AMPS
MCHO (mobile controlled handoff)
The mobile decides for itself
Forward: the mobile initiates HO and sends the request to the
new AP
Backward: the mobile initiates HO and sends the request to the
old AP
Used in DECT, PACS and Mobile IP
MAHO (mobile assisted handoff)
The mobile provides data for the network to make the
decision
Used in GSM and IS-95 CDMA
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 13

14. NCHO
Network Controlled Handoff
The BS monitors the signal strength and quality from the BS
The network uses multiple (current and surrounding) BSs to
supervise the quality of all current connections by making
measurements of RSSI
The MSC makes the decision when and where to effect the
handoff
Drawback: Heavy network signaling traffic and
limited radio resources at BSs prevent frequent
measurements of neighboring links
Long handoff time: up to 10 sec. or more
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 14

15. MCHO
Mobile Controlled Handoff
The mobile decides for itself by monitoring signal strength
and quality from the current and candidate BSs
When it finds a better candidate, it initiates a handoff
In MCHO, most of the work is done by the mobile (as
it knows who it can hear, how well it can hear them,
and can even consider its battery level, etc)
Handoff time
DECT: 100~500 ms.
PACS: 20~50 ms.
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 15

16. MAHO
Mobile Assisted Handoff
The mobile provides data which the network uses to make
the decision
Essentially, it is a variant of NCHO but uses the mobile to
help reduce the handoff time
Example: In GSM, the MS transmits measurements
twice a second ⇒ GSM handoff time ~ 1 sec.
Note in both NCHO and MAHO, if the network cannot
tell the mobile about the new channel / time slot / ...
to use before the link quality has decayed too far,
then the call may be terminated
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 16

17. Handoff in GSM
A handoff is decided by the network, not the MS
BSS criteria
Received signal level
Channel quality
Distance between MS and BTS
Network operation criteria
Current traffic load of the cell
Ongoing maintenance work
Link control for handoff preparation
Periodically, the MS checks the downlink signal strength of its
current BS and the neighboring BSs
Uplink signal quality is monitored by the network side
MS sends measurement report to its current BS and this report is to
be evaluated for handoff decisions by the network side
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 17

18. Handoff in GSM (cont’d)
Handoffs include several procedures: measurements,
decision and destination selection
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 18

19. Handoff in GSM (cont’d)
Use measurement results from the MS and BS
To identify possible other BSs as targets for handovers
To determine the optimal moment to execute the HO
Objective: to keep (#HO per cell change) small
Ideally, (#HO per cell change)=1
In reality, radio conditions are often not very stable when an
MS leaves the BS => (#HO per cell change)=1.5~5
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 19

20. Outline
Handoff Basics
Handoff Management
Handoff Detection
Handoff Assignment
Example: Handoff in GSM
Handoff Requirements and Goals
Advanced Issues of Handoff Management
Handoff Failure
Radio Link Transfer
Soft Handoff
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 20

21. Handoff Requirements
Requirements from a handoff scheme are:
Latency: the time required to effect the handoff should be
appropriate for the rate of mobility of the mobile terminal,
as well as the nature of data transferred
Scalability: the handoff procedure should support handoffs
within the same cell, between different base stations in the
same or in different networks
Minimal drop-off and fast recovery
Quality of service should be maintained or re-negotiatied
after the handoff is completed
Minimal additional signaling
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 21

22. Handoff Goals
Minimal impact on traffic
Making a handoff at the “right” time
Tolerance / adaptation for congestion and capacity
The new and old cells may have different levels of utilization,
available bandwidth, ...
Efficiency
The handoff should result in improved efficiency (in terms of traffic,
energy consumption, reduced interference, …)
The handoff process should try to minimize the resources it
consumes
Improve availability
Handoff should result in using a BS which provides better
bandwidth, lower cost, lower delay, low delay variance, ...
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 22

23. Handoff Goals (cont’d)
The mobile should be able to use the maximum set
of BSs (which may involve changing spreading code,
modulation, coding, or changing to a different radio
module) to achieve a better system optima
Fast handoff
Optimal BTS selection
Reduced signaling overhead
Increased speech quality
Avoidance of far-away-cell effect
Consideration of adjacent-cell capacity
Avoidance of ping-pong effect
Reduced handover failure
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 23

24. Outline
Handoff Basics
Handoff Management
Handoff Detection
Handoff Assignment
Example: Handoff in GSM
Handoff Requirements and Goals
Advanced Issues of Handoff Management
Handoff Failure
Radio Link Transfer
Soft Handoff
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 24

25. Handoff Failure
What happens if there are insufficient resources in
the new BS?
Existing calls cannot be handed over ⇒ forced terminations
New calls are blocked
Goals of channel assignment
To achieve high spectrum utilization
To maintain a given service quality
To use a simple algorithm
To require a minimum number of database lookups
Unfortunately, it is hard to do all of these at once!
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 25

26. Channel Assignment
Several schemes are introduced to reduce the number of forced
terminations, at the cost of increased blocking or decreased
efficiency, including:
Nonprioritized scheme (NPS)
A handoff call is treated the same as a new call
Reserved channel scheme (RCS)
To reserve some resources for handoffs
Queuing priority scheme (QPS)
To exploit the handoff area (i.e., cell overlap) to queue up mobiles
waiting for handoff
Subrating scheme (SRS)
To downgrade an existing call in the new cell and to share
resources with the call being handed over (e.g., by changing a full-
rate to a half-rate CODEC)
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 26

27. Radio Link Transfer
When performing handoff, it is sometimes necessary
to transfer radio link among system components
In the GSM network, several handoff types can be
enumerated:
Intra-cell HO
Inter-cell or inter-BS HO
Inter-BSC HO
Inter-MSC HO
Intersystem HO between two PCS networks (?!)
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 27

28. Preview:
Part of the GSM Architecture
BTS: base transceiver station
BSC: base station controller
MSC: mobile switching center
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 28

29. Different HO Types in the GSM
Network
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 29

30. Intra-cell & Inter-cell HOs
Intra-cell HO
For administrative reasons or because of channel quality (channel-
selective interferences)
Inter-cell HO
Because of weak signal field strength and bad channel quality
Also for administrative reasons (e.g., for traffic load balancing)
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 30

31. Flows of HO within One MSC
When both cells are connected to the same MSC, the
mobile can signal that it is going to change cells (by
providing measurements), then the MSC can set up
the correct resources deliver traffic in the new cell
This often involves setting up a bridge to copy traffic to both
the old and new channels
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 31

32. Inter-MSC HO
When the two cells are connected to different MSCs,
the situation is more complex
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 32

33. Inter-MSC HO (cont’d)
Anchor-based approach
Anchor
MSC
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 33

34. What Happens If the Mobile
Moves Again?
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 34

35. What Happens If the Mobile
Moves Again? (cont’d)
Note that the call always goes via the so-called
anchor MSC (in this case MSC1) to relay MSCs
This is because the phone attached to the PSTN knows
nothing about mobility and the originating exchange thinks
the call is still in existence
Without path minimization, the chain of chunks
between MSCs could continue to grow as long as the
call lasts and the mobile keeps moving to new MSCs
With voice calls, the call duration is generally rather limited,
but with data communication it could continue for a very
long time
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 35

36. Hard Handoff vs. Soft Handoff
Hard handoff
Break-before-make
The mobile connects only to a
single BS at a time
Soft handoff
Make-before-break
The mobile receives from &
transmits to multiple BSs
simultaneously
The signal of the best of all
connected channels is utilized
Generally used in CDMA systems
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 36

37. Hard Handoff
X
X
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 37

38. Soft Handoff
X
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 38

39. Softer Handoff
Softer handoff: In soft handoff, the network and the mobile
combine all the signals to produce a clearer copy of the signal in
both the up and down links
Received Power
Total at the MS
Cell A
Cell B
Distance
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 39

40. WCDMA Handoffs
Softer handoff is more advantageous to soft handoff
while soft handoff is more advantageous to hard
handoff. However, not all handoffs in CDMA systems
can be performed as softer handoffs
Example: In WCDMA systems, we have the following
scenarios:
Softer handoff
The handoff is within one cell between different sectors
Soft handoff
The handoff is between different BSs
Hard handoff
The handoff is between different frequencies or between
WCDMA and GSM
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 40

41. Soft Handoff in Multiple Forms
Some CDMA systems use very precise link level
timing to enable the signals from multiple BSs to
arrive additively at the mobile, thus leading to a
physically stronger signal
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 41

42. Advantages of Soft Handoff
Advantages of utilizing soft handoff include:
Reduces number of call drops
Increases the overall capacity
Mobile transmit power is reduced
Voice quality near the cell boundaries are
improved
Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 42