1. Adv. Wireless Comm. Systems - Cellular Networks -
Objectives
Understand mobility related issues
Be able to evaluate the “mobility cost”
Outline
Mobility and handover definitions
Tradeoff between call blocking and dropping
Network dimensioning with mobility
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

2. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Phenomena and Definitions
In cellular networks users travel across cells during a call
As a user moves from cell 1 toward cell 2 , its signal from
BTS 1 weakens and from BTS 2 strengths
At some point, a process called Handover
must take place:
The up & down channels from cell 1 are released
Available up & down channels from cell 2 are allocated 1 2
If no channels are available from cell 2, the call is dropped
Typically, some channels are reserved to handle handover
Can also use the other channels as well
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

3. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Mobile Assisted Handover
In current cellular networks, mobiles control the handover:
each mobile measures the signal strength for multiple BTS
when (watts) for at least (sec), and
channels are available in cell 2, an handover occurs
Signal strength
- from BTS 2
- from BTS 1
Time
start handover
timer occurs
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

4. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Design Issues for Mobile Assisted Handover
Design Problem 5.1:
How to select and ?
Too small values result in too frequent handovers:
too many reserved channels
too much signaling overhead
droppings due unavailable channels
Too large values result in too
infrequent handovers:
too many call droppings due to call degradation
signal strength
dropping threshold
time
call drop
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

5. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Design Issues for Mobile Assisted Handover (cont.)
Ignoring signaling overhead and given a channel allocation for
new calls and handover calls
Then the values of and should minimize the sum of
call dropping due to unavailable channels
call dropping due to signal degradation
The signaling overhead simply imposes a constraint on the
minimization problem
To evaluate phenomena 1), we need a method to allocate
channels with mobile users
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

6. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Trading Between Call Blocking & Dropping
Design Problem 5.2:
To accelerate the handover process and reduce dropping :
with FCA - each cell reserves its own handover channels
with DCA - adjacent cells can share handover channels
Reserved channels reduce the no. of channels for new calls
but - increases the call blocking probability
We need to tradeoff between call blocking and dropping ?
Typically, call dropping should be smaller than blocking
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

7. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Call Blocking & Dropping Tradeoff (cont.)
Beside offered load, other factors play a role in the tradeoff:
Cell radius
Smaller radius Higher handover rate
Smaller radius Higher system capacity
The proportion between mobile and “stationary” users
Mobile speeds
Again, it boils down into the problem of
how to allocate channels for new calls and handover calls
in a mobile environment
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

8. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Call Blocking & Dropping Tradeoff (cont.)
The following analysis proposed by Foschini, Gopinath &
Miljanic in 1993, lays the ground for such tradeoffs
We restrict our attention however, to a simplified version
Fixed Channel Allocation (FCA)
Mobiles traveling in constant speed
Uniform traffic and mobility characteristics
Blocking and dropping are treated alike
commonly referred to as “call rejection”
We want a method to determine C (no. of channels/cell)
such that the overall call rejection probability would not exceed p
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

9. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels with Mobile Users
The difference between the C’s with mobile users and w/o,
is called the mobility cost
Users are characterized by
Exogenous arrival rate per cell -
Expected call duration -
Call termination probability upon leaving a cell -
can be regarded as the mobility parameter
Higher Higher mobility
is a function of the cell size and mobile speed
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

10. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels w/o Mobile Users
The No-mobility Case ( )
The cells are independent, thus given
C is resolved from:
No closed form solution - however
increases in C
Thus, C can easily be determined numerically
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

11. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels with Mobile Users
The Mobility Case ( )
Due to uniformity we may focus on a generic cell
Call arrivals and departures are classified as follows:
exogenous arrivals -
completing calls
rejected calls
internal arrivals -
calls in transition -
Effective call arrival
rate at this cell
Expected call dwell time
in this cell
- Call rejection probability from an arbitrary cell
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

12. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels with Mobile Users
Given , we may approximate C by solving
Approximation – since we assume that blocking events in
each cell are mutually independent, hence internal arrivals
are Poisson (which is clearly not the case)
Thus, we need to express , with known params
for which we also need
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

13. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels with Mobile Users (cont.)
Computing the Overall Call Rejection Prob. -
A call can be rejected at any cell along its route if there are
no available channels at that cell. Thus,
The hop-number of the cell where the call is rejected
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

14. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels with Mobile Users (cont.)
To express as a function of known parameters we solve:
Which yields:
(5.1)
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

15. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels with Mobile Users (cont.)
Computing the Effective Arrival Rate -
Definition
Call conservation
Equations and
Equation and
(5.2)
(5.3)
(5.2)
(5.3)
(5.4)
(5.4)
(5.1)
(5.5)
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

16. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels with Mobile Users (cont.)
Computing the Effective Service Rate -
- The no. of cells visited by a call until normal completion
- Dwelling time of a call in cell
- Call duration
For identically distributed , that are also
independent of , we have:
(5.6)
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

17. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels with Mobile Users (cont.)
Computing the Effective Service Rate (cont.)
From (5.6):
(5.7)
By definition:
(5.8)
Number of visited cells until termination
Equations (5.7) and (5.8) imply
(5.9)
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

18. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels with Mobile Users (cont.)
Effective Offered Load to an Arbitrary Cell
Effective arrival rate to a cell:
Effective service rate in a cell:
Thus, the effective offered is:
(5.10)
Offered load w/o mobility
Mobility & dropping effect
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

19. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Required Channels with Mobile Users (cont.)
Erlang-B Formula
Recall that the required no. of
channels is determined by:
and from (5.1) :
Thus, C is numerically resolved from:
(5.11)
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

20. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Differentiating Between Blocking & Dropping
Analysis Outline
We need to determine and , where
- no. of channels per cell reserved for handover
- no. of channels that can be used by new calls
Subject to the constraints that
Call dropping prob. of handovers
Call blocking prob. of new arrivals
A reserved handover channel is used only if all
non-reserved are taken
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

21. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Differentiating Between Blocking & Dropping
Analysis Outline (cont.)
To differentiate between blocking and dropping probs.
- Call dropping prob. from an arbitrary cell
- Prob. that a new arriving call is blocked
Note that calls subject to dropping are only the non-blocked
is computed as before - but overall dropping prob.
should satisfy rather , hence
(5.12)
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

22. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Differentiating Between Blocking & Dropping
Analysis Outline (cont.)
Since N is the same, the effective service rate in a cell is
computed as before, yielding:
(5.13)
The effective arrival rate is slightly more complex
Solving: ;
and using (5.12) provides: ;
(5.14)
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

23. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Differentiating Between Blocking & Dropping
Analysis Outline (cont.)
Thus, (5.13) and (5.14) imply that
- Total offered load to a cell
- Internal offered load to a cell
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

24. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Differentiating Between Blocking & Dropping
Analysis Outline (cont.)
To resolve ,
observe:
ext. blocking calls
ext. + int.
int. overflow calls
int. only
int. dropped calls
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

25. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Differentiating Between Blocking & Dropping
Analysis Outline (cont.)
Thus, assuming all blocking events are independent,
and are resolved from: ;
Call blocking requirement
Call dropping requirement
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -

26. Adv. Wireless Comm. Systems - Cellular Networks -
Mobility and Handover
Differentiating Between Blocking & Dropping
Exercise: Complete and verify the derivation of
and
Lecture 6: Mobility & Handover
Adv. Wireless Comm. Systems Cellular Networks -