# Mobile Cellular Networks

In mobile cellular network the geographical area is divided into different number of hexagonal
cells. Each cell is serviced by the station known as base station, which is located at the center of
the cell. The cell is defined by the coverage area of the base station .The mobile Host in each cell
is operated by this base station and all base stations of each cell are connected with each other.
Each cell uses different set of frequencies from its neighboring cells, to avoid interference.

1.1 Cellular Network

A Cellular network is a radio network, distributed over land areas called cells and each cell

is operated by one station known as a base station[1]. Mobile Hosts (MHs) in each cell is

operated by the base station or mobile service station (MSS).

1.1.1 Cell size

The size of cell is defined by the coverage area or the signal strength of the Base Station or

the technology and frequency band used with in a cell. The shape of cell is hexagonal. The

signal strength of the base station gets weaker as its goes far from the Mobile station.

1.1.2 Frequency Bandwidth

The bandwidth is defined as the range of frequencies contained in a composite signal. The

amount of data that can be carried from one point to another in a given time period. Suppose

a composite signal contains frequency between 1000 and 5000, then its bandwidth is

difference between highest and lowest frequency i.e 5000-1000 = 4000.

1.1.3 Hot Spot Cells

A Cell becomes hotspot when the bandwidth available in that cell is not enough to sustain

the users demand and call will be blocked or dropped.

1.2 Frequency Reuse

Since there is a limited amount of frequency spectrum which is suitable for communication

purpose, and very large geographical area to cover, so same set of frequency is used again

and again or frequency reuse is necessary. Using same set of frequencies at different location

may cause channel interference[4]. So to avoid channel interference, there is a minimum

reuse distance between two cells containing same set of frequencies. The minimu reuseable

distance is represented by Dmin.

Dmin =

1.3 Handoff Calls

When the currently used channel by any mobile host is replaced by any other channel, then

that process is called handoff process[2]. Handoff call is of two types intracell handoff and

intercell handoff. During handoff process when the currently used channel is replaced by

other channel with the same base station,then the process is called the intracell handoff. And

when the currently used channel is replaced by some other new channel from the new base

station the it is called the intercell handoff.

1.3.1 Handoff Schemes

Handoff schemes are classified into two type i.e. hard handoff and soft handoff. In hard
handoff on going call get disconnected before connecting with the new channel[11]. In hard
handoff there is a short time interruption before get connected with the new channel. Where
as in soft handoff there no interruption while disconnecting with the old channel and
connecting with new channel.

1.4 Resource Planning Model

Resource Planning Model is the model through which, we can plan or arrange our

resources so that we can use them to get as an optimal output. We can use resource

planning model in two ways as SEARCH MODE and UPDATE MODE.

1.4.1 Search Mode

In search mode the auxiliary station sends requests to their neighboring cells for the availability
of the free channels. All the neighboring cells sends their free channels to the auxiliary station
and auxiliary station choose one channel which is common among all the cells and connects the
holding calls to that free channel to avoid channel interference. Search mode requests channels
on demand, means when it needs channel then only it sends request to their neighbor cells for
channel availability.

1.4.2 Update Mode

In update mode the auxiliary station always updated with all free channels available to

their neighboring cells[2]. In update mode there is no need to send request again and

again to their neighboring cells for the availability of the free channels. In this mode

message complexity is more as compared to the search mode because in this auxiliary

station always keep them updated with the free channels available to their neighboring

cells by message passing and receiving.

1.5 Channels

The bandwidth spectrum is divided into number of channels. Each cell is allocated with

the number of fixed channels. The channels of its own cell is known as Primary channels

and the channels of the other cell which is borrowed when needed is known as secondary

channel.

1.6 Blocking

When the base station has no any free channels to connect the mobile user, then the

incoming calls and handoff calls get blocked. The blocking occurs in two ways, new call

blocking and handoff call blocking. The new call blocking is blocking of the new calls

which do not initiated or which do not get any channel to become active. On the other

hand the ongoing calls when get out of the coverage area of the home cell requires the

another channel to continue the call from the other cell or other base station and due to the

unavailability of the channel that call get dropped is called handoff call blocking.

Chapter 2

2.1 LITERATURE SURVEY

In cellular system the whole geographical area is divided into small number of regions called

cells and each cell is attached with one station called base station. This Base station

provides channels to the Mobile Host[MH]. Since base station has fixed number of channels

so it can provide channels to the fixed number of mobile host. When all the channels of the

base stations are busy and it has no free channels, then the new calls originated and handoff

calls coming to the base station get blocked and dropped. In this way the quality of service

decreases.

2.2 Problem Statement

In the Cellular network each base station has a fixed number of channels and so they can

provide services to only fixed number of mobile hosts. In [3] auxiliary station is used to

reduce the call dropping and blocking rate. An auxiliary station is placed in the cell, when all

the channels of the base station are busy then new incoming calls are received by the

auxiliary station and its hold it for some fixed amount of time and during this time it

regularly checks for base station to availability of free channels. In this way call blocking

and dropping rates are reduced to some extend. But when all the channels of both base

station as well as auxiliary stations are busy then again call blocking problem arises.

2.3. Channel Allocation

Cellular network cover the whole geographical area and whole geographical area is divided

into number of cell in hexagonal shape[3]. A base station is used to serve each cell and

located at the center of the cell. Every base station of each cell is connected with each other

through wired network. Mobile switching center is used to connection between the base

stations and landline networks such as PSTN, ISDN. Each cell has fixed number of

channels. Two cells of the same network cannot use the same channel at the same time,

when they are within the minimum reuse distance, otherwise channel interference occurs.

2.4 Handoff Prioritization Schemes

Call dropping is due to the result of handoff. Handoff occurs when any Mobile Host [MH or
MSS] crosses its home cell and enters into any foreign cell during on going call. When its enters
into foreign cell it requires new channel from foreign cell to continue its call, if any channel is
available in the foreign cell then old channel from the home cell is disconnected and call is
connected to the new channel from the foreign cell and if no free channel is available then the
call is dropped. Handoff call failure is reduced by giving priority to handoff calls over new calls.
This will make optimal utilization of the frequency bandwidth and also improves the quality of
Service (Qos) . Some schemes are used for handoff prioritization such as guard channel(GC) call

admission control(CAC), and handover queuing schemes. Better result of service is obtained by
using all these three schemes simultaneously.

2.4.1 Hard Handover
In hard handover the old radio connection on going with the mobile host disconnected before
connecting with the new radio channel. In hard handover there is a short duration disconnection
before connecting with new channel. Mobile host can feel this short duration disconnection.
Hard handover cannot hold more than one channel at all time. And it also have some
disadvantages such as there are many chances of call dropping. Fig 4 showing below describes
the whole process how hard handover takes place.

2.4.2 Soft Handover
Soft handover is introduced during the CDMA technology. In soft handover the call is connected
with the new channel before disconnecting the old channel. In soft hand over call dropping
probability is reduced. Fig 5 below is the example of soft handover, which describes the whole
process how soft handover takes place.

2.5 Guard Channel
Guard channel (GC) is used to improve the performance of the system by reserving some
channels for the handover purpose[7]. Guard channel is decreases the call dropping rate of the
handoff calls, because some fixed number of channels are reserved for the handover purpose.
The left channels are shared between handover calls and new calls.

Chapter 3

PROPOSED WORK

In [3][5] an auxiliary station is placed in the cell for handling the call for fixed amount of

time when all the channels of the base station are busy. Auxiliary station holds the call for

some time and in between this it checks the base station whether free channel is available or

not if there is free channel the it transfer the call to the base station and channel on auxiliary

station becomes free to handle new calls. In this way call blocking and dropping probability

get reduced. But when all channels of both, base station as well as auxiliary stations are busy

then again this blocking and dropping happens.

So to improve it further, we implementing the resource planning model with the auxiliary

Station. Resource planning model collects all the free available channels of its neighbor cells

and select a common channels among the and transfer the holding call by auxiliary station to

that channel. In this way channels at the auxiliary station becomes free to handle more calls.

A resource planning model is used with auxiliary stations to borrow free channels from their

neighboring cells. In the fig below suppose cells Ao, Bo, Co are allocated with the following

set of channels.

Ao= 1-10
Bo= 11-20
Co= 21-30
When cell Ao sends a request for free channel allocation to its neighbor cell, all neighbour cells send their free channels to Ao. Suppose received channels by Ao are as

Bo= 11, 12, 13 Co= 22, 24, 27
B1= 12, 14 C1= 23, 24, 28
B2= 12, 15 C2= 21, 24, 29

So in cells Bo, B1, B2 channel no.12 is common and in cells Co, C1, C2 channel no. 24 is
common. These two channel are assigned to the calls which are handled by the auxiliary Station,
and now two channels are free at the auxiliary station to handle new the incoming calls. This will
reduces the call blocking rate and dropping rate probability.

1. Mobile station request for channel to the base station.

2. If (channel !=0)
3. Connect to the base station
Else
Connect to the Auxiliary Station
4. AS check for free channel at base station at fixed time interval
If(channel !=0)
Disconnect call from AS and connect to base station
Else
AS gets all free neighbour channels
5. Select common channel among all and disconnect ongoing call from AS and connect it to that free channel
6. New incoming calls handled by AS

7. End.

CHAPTER 4

SIMULATION RESULT

4.1 Tool Introduction
Implementation is done by using the network simulator OMNeT ++.
Framework used is Mixim.
Omnet ++ tool is based on C++.
Omnet ++ tool consists of simple modules and compound modules. A module consists of more
than one simple module is known as compound module.

The above graph shown in Fig 2 represents the comparision between previous call blocking rate
and new call blocking rate i.e. after implementing resource planning model with the auxiliary
station. Green line showing that call blocking rate is decreasing as the interval time increasing.

The above graph shows the reducing rate of call blocking and dropping. Red line graph

showing the existing blocking rate and green line shows the new improved rate of call

blocking, which shows as the number of auxiliary station increases the blocking rate is

decreasing continuously.

CHAPTER 5

CONCLUSION

In cellular network call blocking and call dropping is the basic problem due to the non
availability of channels at the base stations. It is because base station have limited channels as
well as limited bandwidth that is used for communication purpose. This problem can be reduced
by placing auxiliary stations in the cell. An auxiliary station can hold calls for a limited time
period. But when channels of both auxiliary station and base station are busy then again call
blocking starts happening. This can be further reduced by implementing resource planning
model, which borrows free channels from its neighboring cells and transfer holding calls to that
channels and its again ready to handle new calls. By implementing Resource Planning
Mode l(RPC) , the quality of service increases and provide maximum optimal utilization of radio
frequency.

REFRENCES

[1] M.P. Mishra, P.C Saxena 'Survey of Channel Allocation Algorithms Research for
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[3] Mrs. Mahalungkar Seema Pankaj, Prof. Santosh S.Sambare 'Improved Call Blocking
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Engineering Research Volume 4, Issue3, March 2013.

[4] Lalit Gehlod, Vaibhav Jain, Gourav Sharma ' Handover Management using Adaptive and
Prioritization Scheme in Cellular Mobile Systems' International Journal of Advanced Research
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[11] V.S. Kolate, G.I. Patil, A.S. Bhide 'Call Admission Control Schemes and Handoff
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[12] Debabrata Sardar, Soumya Das, Dipsikha Ganguly, Suman Kumar Sikder 'Reducing The
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[13] Moshe Sidiabd David Starobinski 'New Call blocking versus handoff blocking in cellular
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Essay UK, Mobile Cellular Networks. Available from: <https://www.essay.uk.com/free-essays/information-technology/mobile-cellular-network.php> [06-06-20].