International Journal of Engineering Trends and Technology- Volume4Issue3- 2013
A significant way to optimize the Call Blocking Probability
Anuj Kumar, Shilpi Srivastava, Alok Aggrawal and Narendra Kumar
Abstract: This paper tries to impregnate the
possible of solution to counter cell blocking
issues in a typical cellular network. When a
mobile user changes his location his base station
also changes and the ongoing call requires a
new channel in the new base station.
Keywords:
Hierarchical
cell
structure,
handover, fast and slow speed moving user.
Introduction:
Wireless networks and mobile computing
systems have evolved as one of the most
promising
&
interesting
areas
in
telecommunications industries. Mobility is the
most vital aspect of a wireless cellular
communication system, which supports a wide
variety of services such as voice, data, and
multimedia contents to the users on the move.
Mobile customers can make a phone call as in
wired telephone or make an Internet connection
to retrieve information such as emails or stock
quotes, to surf the Internet, or to do business
over the Internet while listening to one’s
favorite
music
online.
Thus,
mobile
communications systems have experienced a
rapid increase in the number of subscribers,
which in turn, places extra demands on their
capacity. This increase leads to the requirement
of a new network architecture where the cells
are designed to be increasingly smaller.
To achieve this goal, wireless networks must
have to be designed with desired quality-ofservice (QoS) requirements, i.e. call blocking
probability and handover blocking probability.
In second-generation cellular systems, the call
blocking probability is lower than 5% while the
handover blocking probability is lower than 2%
for voice service. For the third-generation
networks, where data or multimedia services are
in showcase features , these two blocking
probabilities can be used to characterize the
quality of call connections, which are then used
for the general QoS characterization.
To evaluate the performance of a wireless
network, the following are considered:
 Call dropping probability
 Handover probability
 Handover rate
 Actual call holding times for a complete
call and an incomplete call.
Call dropping probability is the probability when
a call is immaturely terminated due to lack of
channels in the network, and is closely related to
handover blocking probability. The handover rate
is used to find the handover traffic arrival rate,
which is needed to find the call blocking
probability and handover blocking probability.
The handover probability is used to design
channel reservation schemes, and the actual call
holding time can be used to design service
charging rate. Furthermore, to obtain some
analytical results following assumptions are used :
the call holding time which vary with the new
applications, the inter-arrival time of cell traffic
and the channel holding times which depends on
the mobility of the customers, the geographic
situations, and the channel allocation schemes.
The most common and damaging problem that
arises is the handover issue. This problem
becomes more serious, for fast speed mobile user
(FSMU) where the handover rate increases and
the probability that an ongoing call will be
dropped due to the lack of a free traffic channel
becomes high. The handover blocking probability
is considered to be more important than the
blocking probability of new calls because the call
is already active and the QoS is more sensitive for
the handover calls. In this paper, we first provide
how calls are handover in the system under
different traffic policies in a network and then we
find expressions for the handover blocking
probabilities in two mutually inter-dependent
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International Journal of Engineering Trends and Technology- Volume4Issue3- 2013
scenarios , i.e., for slow speed mobile users
(SSMU) and for fast speed mobile users
(FSMU) , and show the fundamental differences
between these blocking probabilities. We also
introduce new two-layer architecture to achieve
handover call blocking probability performance
of FSMU in a packed urban area. For lower
layer of the architecture we propose a
microcellular solution, for absorbing the traffic
loads of both the SSMU and the new calls of
FSMU. And for higher layer, we use macro cell
umbrella solution, for absorbing the traffic load
of the existed handover calls of the FSMU.
Hierarchical Cell Structure
Hierarchical
cell
structure
in
mobile
telecommunication means splitting of cells in
cascaded manner. This type of cell structure
allows the network to effectively use the
geographical area and serve ever increasing
population.
A typical network organization is implemented,
first by coverage considerations and later on by
capacity requirements. In first stage, coverage of
the network area is achieved easily by using
umbrella cells and macro cells. Within the
evolution of the network, the next layer is
achieved by implementing small cells, i.e micro
cells and pico cells, in order to increase
capacity. Thus, fully developed network is
characterized by a layered structure of
hierarchical cells.
Umbrella and Macro cells are characterized by :
 High transmit power
 Wide area coverage (large cell radius)
 Antenna above rooftop level
 Fast moving mobile user (speed
sensitive handover)
 Redundant capacity ( for traffic overflow
from macro cells)
Micro and Pico cells are characterized by :
 Low transmit power
 Small area coverage (e.g – hot spots in
city center, airport)
 Antenna below rooftop level or indoor
 Slow moving mobile user
Handover
As such, a typical wireless network essentially
provides communication services to large number
of mobile users. The design of such network is
based on a cellular architecture which consists of
a backbone network with finite number of base
stations, and allows efficient use of the limited
available and allocated spectrum. The geographic
area within which mobile units can communicate
with a particular base station is referred as a cell.
To ensuring continuity of communications
neighboring cells marginally overlap with each
other, in order to ensure seamless connectivity,
when the users move from one cell to another.
When a mobile user wants to communicate with
another user or a base station, it must first obtain a
channel from one of the base stations that hears it.
When a mobile user travels from one area of
coverage or cell to another cell within a call’s
duration the call should be transferred to the new
cell’s base station. Otherwise, the call will be
dropped because the link with the current base
station becomes too weak as the mobile recedes.
This process of changing the channel (frequency,
time slot, spreading code, or combination of them)
during an ongoing call is called handoff or
handover. Some of the major reasons of handover
are –
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International Journal of Engineering Trends and Technology- Volume4Issue3- 2013






When user is crossing a cell boundary or
by a deterioration in quality of the signal
in the current channel.
Due to a sudden mal-functioning &
reduction in current cell’s signal
strength.
Due to BSS system overload in existing
serving cell.
Co-channel
&
adjacent
channel
interference are major reason of
handover in a typical GSM network. If
there is interference on the channel being
used by a subscriber, due to another
subscriber using the same channel in a
different neighboring cell, the call is
handover by BSC to a different channel
in the same cell or a different channel in
another cell to avoid the interference.
This comes under Radio Resource
Management in BSSAP ( Base
substation system application protocol )
Another major reason of handover is
driven by the mobility of subscriber.
There are two types of users. SSMU
(Slow Speed Mobile User) , and FSMU (
Fast Speed Mobile User ) When a fast
speed mobile user, connected to a large,
umbrella-type
of
cell,
becomes
stationary , his call may be handed over
to a smaller macro cell or even to a
micro cell . This handover is again
controlled by BSC using BSSAP (Base
substation system application protocol).
By this way, the capacity in umbrella
cell is regained for other fast speed
moving users and this also helps to
reduce the possibility of potential
interference to other cells or users.
Handover in CDMA network is
sometimes triggered to reduce the
interference to a smaller neighboring cell
because of the "near-far" effect. This
handover takes place despite the user
having good connection strength toward
its serving cell.
Handover Process –
Handover Traffic Cases
Handover can be used for load balancing between
cells. During a call setup in a congested cell, the
MS can be transferred to a cell with less traffic if
an acceptable connection quality is likely to be
obtained. There are several types of handover,
including:
 Intra-cell – Handover between channels in
the same cell
 Intra BSC - Handover between cells
controlled by the same BSC
 Inter BSC - Handover between cells
controlled by different BSC’s, but the
same MSC
 Inter MSC - Handover between cells
controlled by different MSC
 Intra-Cell Handover

 It is performed when the BSC considers
the quality of the connection too low, but
receives no indication from the
measurements that another cell would be
better. In that case the BSC identifies
 another channel in the same cell which
may offer a better quality, and the MS is
ordered to retune to it.
Intra BSC Handover

When performing a handover between two
cells controlled by the same BSC, the
mobile switching center (MSC) is not
involved. However, the MSC/VLR will be
informed when a handover has taken
place. If the handover involves different
LA’s, location updating is performed once
the call is finished.
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International Journal of Engineering Trends and Technology- Volume4Issue3- 2013
Consider traffic in existing mobile networks with
prioritized handover procedure with following
assumptions –
1. In every microcell total C channels are
available and the priority technique for
handover calls assigns a guard channel
(Ch) exclusively for handover calls. The
remaining channels (C - Ch) are shared
equally by new and handover calls.
2. The users are categorized either as slow
speed mobile user (SSMU) or fast speed
mobile user (FSMU) according to their
moving speed .
3. In all microcells traffic is homogenous
with same capacity and same mean
holding time (T).
1. The BSC commands the new BTS to
activate a traffic channel.
2. The BSC sends a message to the MS, via
the old BTS, containing information
about the frequency and time slot to
change and also the output power to use.
This information is sent to the MS using
fast
associated
control
channel
(FACCH).
3. The MS tunes to the new frequency, and
transmits handover access bursts in the
correct time slot. Since the MS has no
information yet on TA, the handover
bursts are very short.
4. When the new BTS detects the handover
bursts, it sends information about TA.
This is also sent via FACCH.
5. The MS sends a handover complete
message to the BSC via the new BTS.
6. The BSC tells the old BTS to release the
old traffic channel.
Present Model with Prioritized Handover
In microcell, the mean rate (R) of new and
handover calls of SSMU are generated by Poisson
point process, are
and
respectively, while
new and handover calls of FSMU have mean rates
of
and
per cell. Thus, the total relative
mobility (M) for both SSMU and FSMU is
defined as
…………………. (1)
Where
for SSMU
for FSMU
The total offered load (L) per cell is
…………(2)
Where
Let n be the number of microcells in the
microcellular region. Then The total offered load
in the system is:
…………….(3)
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International Journal of Engineering Trends and Technology- Volume4Issue3- 2013
and total number of channels in the system are –
If all channels are free in every microcell, then
the probability (P) of occupying a channel by a
new user is :
………………..(4)
Let j channels are active in each microcell, then
steady state probability can be derived as
for j = 1, 2, ….., C - Ch
Also
for j = C-Ch+1,…..,C
………(5)
…….(6)
In every microcell, the blocking probability (Pb)
for a new call placed either by FSMU or SSMU
is :
sum of probabilities that the state
number j of the microcell  (C-Ch)
i.e
………(7)
Also, the probability of handover failure (Phf) is
equal to the probability of the microcell with the
state number C i.e
…………(8)
Thus, the handover failure probability for
FSMU is :
SSMU and FSMU are considered, the mean call
blocking probability is :
…………..( 9)
Multilayer Cellular Structure
Now assume that total Channel Capacity of
system is Csys and n be the number of microcells
in the microcellular layer. Since the priority is
always given to Handover attempts in typical
microcellular layer, by assigning guard channels
(Ch) exclusively for handover calls of SSMU
among the C channels in a cell, so the leftover
channels (C-Ch) are mutually shared by both new
call attempts of FSMU& SSMU and handed over
calls of SSMU.
Now, based on this information, let’s assume that
Cu be the channels assigned to umbrella cell to
serve only handed over calls of FSMU. So, the
Channel Capacity of system will be defined as:
C sys = n.C+Cu
………….(10)
We know that the mean rate of handover calls of
FSMU is
per cell
So, the mean rate generated in the umbrella cell
by FSMU is n.
, where n is the total number of
microcells.
In order to improve the blocking probability of
handover calls of FSMU in this structure , we try
to find out the ratio between Cu and Csys, by
taking help of MS, MF, MFS and
.
If all channels are free in every microcell, then the
probability (P) of occupying a channel by a new
user is:
To conclude, in microcellular region for all the
microcells, when the new and handover calls of
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International Journal of Engineering Trends and Technology- Volume4Issue3- 2013
Significantly, it comes out that probability of
handover failure in umbrella cell is equal to the
probability with the static number Cu number of
cells , i.e
….(14)
Now, the steady state probabilities when q
number of channels are busy in a microcell is :
for j = 1, 2, ….., C - Ch
Also
Thus, the mean call blocking probability for the
microcellular layer having n microcells, in view
of the new calls of SSMU and FSMU and
handover calls of SSMU is :
………(11)
…….(15)
for j = C-Ch+1,…..,C
…….(12)
For any FSMU or SSMU per microcell, the
blocking probability of a new call is either
achieved by the sum of probabilities so that the
static number of the microcell is greater than or
equal to (C – Ch). Thus,
…………..(13)
The probability of handover failure (Phf) is equal
to the probability of the microcell with the static
number C i.e
….(14)
Now, for the umbrella cell, the steady-state
probabilities when j channels are busy is :
for j = 1, 2, ….., Cu
Where
………….(15)
Therefore, we achieve the objective of the
projected structure i.e to guarantee the required
QoS for handover calls and handover blocking
probability for FSMU while allowing high
utilization of channels.
Outcome
In order to see the effect & behavior of proposed
structure on handover blocking probability of
FSMU and mean call blocking probability of
microcellular layer against total offered traffic
load in the system, we consider following four
scenarios under a typical wireless system, which
may consist of
(i) Csys = 120 channels
(ii) All calls are served by microcells and there is
no umbrella layer.
(iii) C sys = 120 and C = 42.
Following three consider the above , wireless
system where handover calls of FSMU are served
by the umbrella layer and the new calls of FSMU
and SSMU, and the handover calls of SSMU by
the microcellular layer.
(ii) There is umbrella layer and Csys = 120, Cu =
26 and C = 34.
(iii) There is umbrella layer and Csys = 120, Cu =
50 and C = 26.
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International Journal of Engineering Trends and Technology- Volume4Issue3- 2013
(iv) There is umbrella layer and Csys = 120, Cu =
74 and C = 18.
To calculate
,
,
and
we also
consider the following parameters :
Ch = 0.1 C, Tch = 80 sec , MS = 0.4, MF = 0.6, MFS
= 0.46 and
0
In the proposed structure, when we adjust the ratio
of Cu against Csys according to the values of MS ,
MF , MFS and
, it is clearly visible by the
curves of figure 1 and 2 , that handover call
blocking probability of FSMU is enhanced . This
enhancement is directly proportional to the
number of channels which have been assigned to
the umbrella cell. The graphs above substantiate
the fact that ratio of Cu against Csys optimizes the
umbrella layer with the least possible effect on the
lower layer. This optimization subsequently refers
to a decrease in blocking probability of the
umbrella layer with the minimum increase in
blocking probability of microcellular layer.
Fig1. Handover blocking probability of FSMU
against total offered traffic load in the system in
above four scenario.
Conclusion
In order to achieve low handover call blocking
probability, the Two Layered network structure
must be implemented. The handover call blocking
probability of FSMU has been reorganized in
accordance with call blocking probability of
microcellular layer, so as to achieve minimum
impact in terms of handover failures. As the same
time the Umbrella Cell concept has been
implemented in parallel to serve handover calls of
FSMU.
This setup of network in form of layered
architecture has significant positive traits to
achieve maximum handover success rate, wide
across difficult terrain, network capacity situation
& subscriber behavior.
References
Fig2. Mean call blocking probability of
microcellular layer total offered traffic load in
the system in above four scenario.
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International Journal of Engineering Trends and Technology- Volume4Issue3- 2013
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