17 PERFORMANCE EVALUATION OF HANDOFF PARAMETERS … · handoff it is necessary that the handoff parameters will be set well. There are several performance indicators to evaluate the

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –

6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME

164

PERFORMANCE EVALUATION OF HANDOFF PARAMETERS IN

MOBILE SYSTEMS

S. Sri Gowri

1, K.Venkata Satya Anvesh

2, K. Sri Pavan Kumar

2

1. Professor, ECE Dept, SRK Institute of Technology.

[email protected]

2. IV/IV Students of ECE Dept, SRK Institute of Technology.

[email protected], [email protected]

ABSTRACT

Handoff is an essential component of mobile cellular system. Mobility causes dynamic

variations in link quality and interference levels in cellular system. There are so many

advantages of using soft handoff than hard handoff. So, in order to reap the benefits of soft

handoff it is necessary that the handoff parameters will be set well. There are several

performance indicators to evaluate the system, namely Ec/Io - the link quality indicator, Tc - the

carried traffic which is a good resource allocation indicator. The tradeoffs and parameter settings

have been mostly in the form of simulation. In this work the quantitative tradeoffs that are

interference vs. capacity and call drop probability vs. offered traffic in hard and soft handoffs are

investigated. It has also been shown that by implementing the macroscopic diversity using

Maximal Ratio Combining technique, Drop Call Rate will be reduced effectively in the boundary

conditions.

1 INTRODUCTION

Cellular is one of the fastest growing and most demanding telecommunications

applications. Today, it represents a continuously increasing percentage of all new telephone

subscriptions around the world. It is forecasted that cellular systems using a digital technology

will become the universal method of telecommunications. Increases in demand and the poor

quality of existing service led mobile service providers to research ways to improve the quality

of service and to support more users in their systems. Because the amount of frequency spectrum

available for mobile cellular use was limited, efficient use of the required frequencies was

needed for mobile cellular coverage Deployment parameters, such as amount of cell-splitting and

cell sizes, are determined by engineers experienced in cellular system architecture. Provisioning

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ISSN 0976 – 6464(Print)

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for each region is planned according to an engineering plan that includes cells, clusters,

frequency reuse, and handovers. The cellular communications system consists of the following

four major components that work together to provide mobile service to subscribers. Public

Switched Telephone Network (PSTN), Mobile Telephone Switching Office (MTSO), cell site

with antenna system and Mobile subscriber unit (MSU).

2 HANDOFF

In cellular telecommunications, the term handover or handoff refers to the process of

transferring an ongoing call or data session from one channel connected to the core network to

another. In telecommunications there may be different reasons why a handover might be

conducted. When the phone is moving away from the area covered by one cell and entering the

area covered by another cell, When the capacity for connecting new calls of a given cell is used

up and an existing or new call from a phone which is located in an area overlapped by another

cell, In non-CDMA networks when the channel used by the phone becomes interfered by another

phone using the same channel in a different cell -in all the above cases a Handoff is required.

Generally, when a mobile user is moving away from the center cell, then the received

signal strengths (RSS) will degrade gradually. So when the RSS drops below the threshold level

hand-off will be initiated by handing over the mobile user to another cell which is the nearest and

having free channels.

2.1 Soft Handoff

Soft handoff is used in voice-centric cellular networks such as GSM or CDMA. It uses a

make-before-break approach whereas a connection to the next BS is established before a SS

leaves an ongoing connection to a BS. This technique is suitable to handle voice and other

latency-sensitive services such as Internet multiplayer game and video conference. When used for

delivering data traffic (such as web browsing and e-mail), soft handoff will result in lower

spectral efficiency because this type of traffic is bursty and does not require continues handover

from one BS to another.

2.2 Hard Hand-off

A Hard handoff can be practically employed with more efficiency in FDMA

(Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) network

access systems, because in these systems channel interference is minimized since different

frequency ranges are used from adjacent channels. Mostly CDMA (Code Division Multiple

Access)-based technologies employ Soft handoffs. A Hard handoff mechanism is particularly

suitable for delay-tolerant communication traffic such as in broadband technology-enabled

Internet, VoIP, mobile networking technology such as mobile WiMax. Broadband Internet access

and emailing are more efficient and reliable when a Hard handoff mechanism is used.

3 MODELING OF CALL DROPPING IN CELLULAR NETWORKS

3.1 Drop-Call Probability

Drop-call probability is given by [1]:



166

tvn

d den

tvnYP

−==!

)()( , n ≥ 0

Here, vd is the drop-call rate, t the call duration, while Y is a random variable that counts the

number of drops and n is the confirmed calls dropped. This is a Poisson Probability function with a

discrete variable which counts the number of dropped calls.

The number of dropped calls is calculated from the relation [1]:

�� =� . ��

� . ��

3.2 Derivation of Call Drop Probability

To evaluate Pd, let us consider, for sake of simplicity, the probability that a call is normally

terminated, Pnt, related to Pd by the following expression [2]:

ntd PP −=1

A call request is served by a generic channel, randomly selected, and the call will finish, if

correctly terminated, after a duration time, T. We can state that the call duration, T, is the sum of

the two random variables Tr and Tc which model the ringing and conversation times, respectively as

shown in Figure 1.

Fig. 1: Time Diagram to describe Call

The random variable (r.v.) Tr models the ringing duration with a pdf fTr(t). The r.v. Tc

models the conversation duration with a lognormal pdf fTc(t). Assuming that Tr and Tc are

independent, the pdf fT(t) of the call duration for the normally terminated calls can be obtained as

the following convolution between PDFs [2]:

.)().()()()(0

τττ dftftftftft

TcTTcTT rr ∫ −=∗=

Using the total probability theorem, summing over all the possible numbers of

contemporary active calls, the probability that a call is normally terminated with duration t is [2]:

),(),()(1

kPktTPtTP d

k

ntnt ⋅=== ∑∞

= Pnt, by simply considering every possible call duration [2]:



167

∫∞

==0

)()( dttftTPP Tntnt ,)(

!1

1

10

/∑ ∫∞

=

∞−

−=

k

ktv

T

k

dtetfke

dρ

ρ

where fT(t) is the PDF of call duration.

Finally, it results that the drop-call probability is [2]:

.)(!1

11

10

/∑ ∫∞

=

∞−

−−=

k

ktv

T

k

d dtetfke

P dρ

ρ

4 REVERSE LINK CAPACITY FOR HARD HANDOFF

While coverage is essentially symmetrical, applying equally to forward and reverse link

propagation, capacity is fundamentally asymmetric. We consider here only reverse link capacity,

which involves many- to- one multiple access. The affect of soft handoff here is tightly coupled

with the fast closed loop power control techniques described previously. These guarantee, that

for the controlling base station, each user’s received signal energy-to-interference is normalized

to be equal to that of all other users. This limits each other-cell user’s interference to a

normalized value less than unity, for otherwise those users would be controlled by the given cell.

Capacity is then reduced by the aggregate of all other-cell users’ relative energy. Hence it is

inversely proportional to 1+ f where [3]:

f ≜ average total interference from other cell-users

��

ku =average number of users per cell (at capacity).

The relative average interference at the given cell due to all users in all other cells,

denoted as the region 0S as shown in Figure 2 is [3] [4]:

=0S

I E ∫ ∫0 0

1

]10),(

10),([

10/

0

10/

1

S yxr

yxrζµ

ζµ

kdA(x,y)

Then we obtain for the mean other-cell interference normalized by the number of users

per cell as [3]:

f ≜ ]),(),(33

2[

0

2)(20

1∫ ∫=S

b

u

SyxdAyxRe

k

Iµβσ

Fig. 2: Hexagonal Cell Boundaries and Distances



168

5 REVERSE LINK CAPACITY FOR SOFT HANDOFF

To approach the performance of a soft handoff system, we consider other-cell

interference when the user is permitted to be in soft handoff to only its two nearest cells. Again

taking the zeroth cell as the one under consideration, the region for which this cell can be in soft

handoff, which we denote S0, is the six-pointed star which contains the cell, shown as the hatched

area in Fig.3. Within so, any user which is communicating with one of the six nearest neighbors

will introduce interference into the zeroth base station. But this happens only if the propagation

loss to that neighbor is less than to the zeroth base station, in which case it is power controlled by

the former. Thus the mean total interference to the zeroth base station from within the S0 region

as shown in Figure 3 is [3][4]:

1 0

0 01

0

( )/10

1 /10/10

1 0

10 ;( , ) ( , )

( , )10 ( , )10S

S

I R x y E kdA x yr x y r x y

ζ ζ

µ

ζζµ µ

− =

< ∫∫

Where the expectation is over the sample space for which the inequality is satisfied.

Dropping again the spatial notation, the first integral is evaluated, using and the

independence of the ξi variables, as follows [3]:

Fig. 3: Region S0 and Distances for best of Two Cells

Where the last equality follows from the circular symmetry of the joint density function and the

linear boundary of the region of integration. Now the second integral I2 is the same as I1, with M1

and M2 interchanged. Thus we obtain, for b = �

√� , the relative interference as the zeroth cell

base station from all users not controlled by its base station is [3]:

2__

00

0

( ) /2

1 01

2

3 3

SS

uS

I IM Me

f R Q dAk

βσµ βσ

σ

+− = = × +

∫∫

__ __

0 0

1 2 2 11 2

2 2S S

M M M MR Q dA R Q dA

µ µβσ βσ

σ σ

− − + + + +

∫∫ ∫∫ Again

in the spatial integrals over S0 and 0S , R1(x,y) , R2(x,y) and M1(x,y), M2(x,y) refer to the base

stations nearest and second nearest to the user at (x,y).

1 0

__

0

( )

1 1 1 1 2 2;

S

I R E e M M kdAβ ζ ζµ ζ ζ− = + < + ∫∫



169

6 RESULTS

6.1 Call Drop Probability

Using statistical tools on measured data from a real network, we have characterized dropped

calls and call durations (distinguishing between ringing and conversation phases). Results of this

data analysis have driven the development of a new analytical model which relates drop-call

probability to the drop-call rate, the pdf of the call duration, and the traffic load. The developed

model can be easily extended to different cellular networks simply characterizing the

distribution of the call duration.

For the validation, in each considered cell, the drop-call probability and its confidence

interval (with confidence level 1 − α = 0.95) have been estimated. This is to establish the

acceptance region for results from our model. Then, the drop call probability has been

analytically estimated as shown in Table 1. Results coming out from the analytical model can be

considered acceptable if they fall in the confidence interval of the measured drop-call probability.

Table 1: Call Drop Probability

Call arrival

rate(λ)

%Pd

140 27.9959

150 22.8529

160 17.7098

170 12.5667

180 7.4236

6.2 Comparison of Reverse link Capacity for Hard and Soft Handoff

It has been established by a simple analytical approach, using the generally accepted

propagation model with lognormal shadowing, that soft handoff in CDMA improves coverage

by a factor of 2 to 2.5 in cell area. The value of relative interference, f, is evaluated numer-

ically and shown in Table 2 for Hard handoff. The value of relative interference, f, is evaluated

numerically and shown in Table 3 for Soft handoff. And here also the interference, f, was

decreasing when σ increasing from top to bottom and also µ from left to right. For all it and a,

the value is greatly reduced from the single cell (hard handoff) case of Table 3. Hence Capacity

increases greatly than in Hard handoff.

Table 2: Interference levels in Hard Handoff

σ\µ 2 3 4 5

0.00 1.379 0.923 0.617 0.413

0.50 1.468 0.982 0.657 0.440

1.00 1.770 1.185 0.793 0.530

1.50 2.420 1.619 1.084 0.725

2.00 3.748 2.508 1.678 1.123

2.50 6.577 4.401 2.946 1.971



170

3.00 13.071 8.752 5.857 3.920

3.50 29.465 19.718 13.197 8.832

4.00 75.213 50.338 33.690 22.548

4.50 217.568 145.612 97.454 65.223

5.00 713.124 477.274 319.427 213.783

Table 3: Interference levels in Soft Handoff

σ\µ 2 3 4 5

0.00 1.379 0.923 0.617 0.413

0.50 1.262 0.844 0.565 0.378

1.00 1.281 0.857 0.574 0.384

1.50 1.442 0.965 0.646 0.432

2.00 1.797 1.203 0.805 0.538

2.50 2.478 1.659 1.110 0.743

3.00 3.779 2.529 1.692 1.132

3.50 6.365 4.260 2.851 1.908

4.00 11.840 7.924 5.303 3.549

4.50 24.306 16.267 10.887 7.286

5.00 55.045 36.840 24.656 16.501

7 CONCLUSIONS

� We observed that, as the Call Arrival Rate increases, the Call Drop Probability was

decreased as seen from Table 1. Hence Call Drop Probability was reduced even if the

Traffic was increased.

� It was also proved that the interference levels in Hard Handoff are higher than in Soft

Handoff observed from the Table 2 and Table 3. Hence we can conclude that by using the

Soft Handoff, the Capacity of the cell increases as the interference was low compared to

Hard Handoff.

REFERENCES

[1] Nathaniel S. Tarkaa, Joseph M. Mom, Cosmas I. Ani,” Drop Call Probability Factors

in Cellular”, International Journal of Scientific & Engineering Research Volume 2,

Issue 10, October-2011.

[2] Gennaro Boggia, Pietro Camarda, and Alessandro D’Alconzo, “Modeling of Call

Dropping in Well-Established Cellular Networks”, EURASIP Journal on Wireless

Communications and Networking, ISSN:1687-1472 EISSN:1687-1499, Jan,2007.

[3] A. J. Viterbi et al., “Soft Handoff Extends CDMA Cell Coverage and Increases

Reverse Link Capacity,” IEEE JSAC, vol. 12, no. 8, Oct. 1994, pp. 1281–87.

[4] William C.Y. Lee, “Mobile Cellular Telecommunications”, Tata McGraw-Hill Edition

2006.

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17 PERFORMANCE EVALUATION OF HANDOFF PARAMETERS … · handoff it is necessary that the handoff parameters will be set well. There are several performance indicators to evaluate the