Mobile Technology

Mobile Technologies- Cell Cluster& Frequency Reuse(SRP)
Cell
A cell is a certain, geographic location that is served by a
particular tower. If you are moving, your cellphone signal is
transferred from cell to cell, depending on your location. In
order to prevent you from losing the call. All of these cells are
connected to a mobile switching center. Signals are passed
from the phone, through the antenna, and sent through the
switching center where they will either be passed to another
cell or converted to a signal that can be sent along traditional
telephone lines. The entire, complicated process only takes a
second to complete. Maintaining the system is very important to
keeping the lines of communication open.
Every city is divided into small areas each area is called as a
cell thus a cell is defined as a basic geographical unit of a
cellular communication system seriously pay attention this is
important as the city is geographically divided into small cells. It
is mandatory that all cells must be symmetrical in shape based
on this.

We have four geometrical shapes such as Circle Square
equilateral triangle and hexagon. If we select the shape of a cell
as a circle then the area between the two circles will not get
covered by the base station and any attempt of communication
from that area will fail. Thus we eliminate the circular shape.
Hexagon has the highest area as compared to the other two
shapes. Thus we divide the geographical area into hexagonal
cells.
Cluster group:
The next concept is a cluster a group of cells is called as a
cluster. The cluster size is not fixed. It depends on the
requirements of the area.

Basic structure of mobile phone:
Let’s study the mobile phone system now the basic structure of
the mobile phone system is as shown.
Where MS means mobile station BTS is base transceiver
station. BSC is base station controller and MSC is mobile
switching center. In this diagram MS is nothing but the mobile
phone of a user.
Call from one mobile to another mobile:
Every cell has its own base transceiver station at its center.
Whenever a call is set up the first signal is sent to the base
transceiver station of the cell from this base transceiver station
it goes to the central base station controller which controls the
working of all the base stations from BSC it then goes to MSC
or mobile switching center which is the master controller of the
entire system. These MSC s are different for different areas
from MSC of area one the signal is transmitted to MSC of area
2 where it follows the reverse sequence as MSC to BSC, BSC
to BTS and from BTS – MS. Whenever a user makes an
attempt to call someone a separate channel is assigned to the
user by the MSC if all the channels are already occupied by
other users then this user has to wait for a channel to become

free in such a case the user gets the notification such as call
cannot be completed or network error etc
Features of cellular concept:
Frequency reuse:
Now let’s see the features of the cellular concept its first feature
is frequency reuse in frequency reuse same set of frequencies
are used for radio channels located in different areas as shown
in the diagram below.
Every cell named as a uses the same set of frequencies the
advantages of frequency reuse are many transmitters of small
output power operating at the same frequency can be used. It
reduces the minimum height of the transmitting antenna as
each antenna has to cover a small distance but the
disadvantage is that if the system is not properly designed then
it may lead to co-channel interference. Co-channel interference
is a phenomenon in which two frequency signals of adjacent
channels interfere with each other.

Cell splitting:
The second feature of the cellular concept is cell splitting. In
cell splitting technique each cell is divided into smaller cells
known as micro cells. The radius of these small cells is half of
the original radius. Cell splitting technique proves advantages
when the traffic of cell phone users increases beyond the limit
of a regular cell.
Handoff:
Let’s see what do we mean by a handoff procedure consider a
situation where we are traveling from place A to place B via a
car. Sometimes it happens that for a short period of 2 or 3
seconds the signal strength on mobiles becomes very low and
again it increases and reaches its normal level. Have you ever
thought why this happens? The answer to this question is the
handoff procedure when someone user X travels in area A he
receives the signal from base station that is antenna a has a
stronghold on out devices signal but as he moves away from
antenna a the signal strength gradually decreases at a
boundary of cell a the whole of antenna a on the device is
minimum at the same time when the device is about to enter
into cell B base station B starts to take hold on the device thus
at the border of cell a and cell B. Both the antennas have equal
amount of hold on the device of user X as user X crosses the
border and enters into cell be area B strength of base station. A
very rapidly decreases and strength of base station B rapidly
increases in short the device receives the signal network from
base station B. But during this procedure the call is still on.
Hence base station a handovers this call to base station B
without any effect on a call this procedure is known as the
handoff procedure. This process happens so rapidly that the
user never notices it.

We all know that there are two types of systems available as
GSM and CDMA. We will only study the basics of GSM system
GSM stands for global system for mobile GSM is nothing but a
big system made up of few small systems. Such as mobile
stations MS base station subsystem, BSS network and
switching subsystem NSS operating subsystems OSS. The
architecture of this GSM network is as shown mobile station
MS nothing but the device used for communication. Such as
cell phone fax machine etc base station subsystem BSS. BSS
gets connected to ms wire radio interface. it has two different
blocks as BTS base transceiver system and BSC base station
controller practically. Every MS gets connected to BTS of that
area this beauty s sends the signal to BSC many BTS are
connected to one BSC and at the end of this BSC is connected
to MSC. Hence BSS system consists of BTS and BSC network
and switching subsystem NSS. This system mainly contains
MSC. MSC is the backbone of the entire network it controls all
the operations from setting up a call till the handoff procedure
other blocks of NSS are h l our home location register which
keeps the database of all the users who reside in the same
geographical area VL our visitor location register keeps the
track of all the users who are visitors for that particular
geographical area mainly roaming customers authentication
center mainly controls. The authentication of the users by
checking their sim numbers etc and sends the required
information to the MSC
SIM:
what does a SIM card actually do we all know a mobile would
not work without one but why let’s start by looking at what SIM
actually stands for it stands for subscriber identity module. So
fundamentally it identifies your phone so when you send a
message. Your phone knows which network to use and your
operator knows who to bail for it though SIM contains more

information. It contains of course your number the information
needed to bill you for a lot SIM. The SIM obviously knows the
unlock pin though you can’t get it out even if you use scissors
and it typically has contacts and messages saves on its
memory because of this you can easily move the SIM between
phones and keep your number. A simple way to think about it is
that once you insert a SIM card the phone becomes your
phone. The carrier extracts the information from a number this
number is written on the SIM card.
It is called the integrated circuit card identifier the shiny bit is an
integrated circuit basically just a bunch of transistors and this is
mounted on a card. It is a pointless name really but Telkom
people seem to love the raku names anyway the number is
made up of an issuer identification number and IIN more
acronyms starting with an industry identifier which is almost
always 89 have a look at your SIM card it probably starts with
89 then the country code, 44 for United Kingdom then we have
the individual account identification number that is unique to
you and finally we have a check digit that ensures that the
previous numbers are correct. So it checks for any miss prints
or errors and this is done using a clever algorithm developed
IBM which is Lunz checksum. Finally let’s look at something the
very few people know about there are two main ways of
managing a network GSM and CDMA. GSM is a global system
for mobile that requires a SIM and is used by almost everyone

but you also have the entirely valid code division multiple
access system which does not require a SIM how is this
possible it’s quite complicated but basically what is going on is
the phone has a serial number and the carriers have a list of
the phone’s if your phone is on the list you can use the network.

Cells
A cell can be defined as an area of radio coverage from one BTS
antenna system. It is the smallest building block in a mobile network
and a cell can be represented by a hexagon.
There are two types of cells;
1. Omni directional cell:
An omni-directional cell (or omnicell) is served by a BTS with an
antenna which transmits equally in all directions (360 degrees).
2. Sector cell:
A sector cell is the area of coverage from an antenna, which transmits, in
a given direction only. For example, this may be equal to 120 degrees or
180 degrees of an equivalent omni- directional cell. One BTS can serve
one of these sector cells with a collection of BTS’s at a site serving more
than one, leading to terms such as two-sectored sites and more
commonly, three-sectored sites.

Clusters
Groups of frequencies can be placed together into patterns of cells
called clusters. A cluster is a group of cells in which all available
frequencies have been used once and only once.
Since the same frequencies can be used in neighboring clusters,
interference may become a problem. Therefore, the frequency re-use
distance must be kept as large as possible. However, to maximize
capacity the frequency re-use distance should be kept as low as
possible.
The re-use patterns recommended for GSM are the 4/12 and the
3/9 pattern. 4/12 means that there are four three-sector sites
supporting twelve cells using twelve frequency groups.
In the 3/9 cell pattern there are always 9 channels separating each
frequency in a cell. However, when compared with the 4/12 pattern,
cells A1 and C3 are neighbors and use adjacent frequencies.
Therefore, the C/A interference will increase. In this case, an operator
may use frequency hopping which, if planned correctly, could reduce
the possibility of such adjacent channel interference

Frequency Reuse
Frequency Reuse is the scheme in which allocation and reuse of
channels throughout a coverage region is done. Each cellular base
station is allocated a group of radio channels or Frequency sub-bands
to be used within a small geographic area known as a cell. The shape
of the cell is Hexagonal. The process of selecting and allocating the
frequency sub-bands for all of the cellular base station within a system
is called Frequency reuse or Frequency Planning.
Silent Features of using Frequency Reuse:
 Frequency reuse improve the spectral efficiency and signal Quality
(QoS).
 Frequency reuse classical scheme proposed for GSM systems
offers a protection against interference.
 The number of times a frequency can be reused is depend on the
tolerance capacity of the radio channel from the nearby transmitter
that is using the same frequencies.
 In Frequency Reuse scheme, total bandwidth is divided into different
sub-bands that are used by cells.
 Frequency reuse scheme allow WiMax system operators to reuse
the same frequencies at different cell sites.

Cell with the same letter uses the same set of channels group or frequencies
sub-band.

TOTAL NUMBER OF CHANNELS (S)
To find the total number of channel allocated to a cell:
S = Total number of duplex channels available to use
k = Channels allocated to each cell (k<S)
N = Total number of cells or Cluster Size
Then Total number of channels (S) will be,
S = kN
FREQUENCY REUSE FACTOR
Frequency Reuse Factor = 1/N
In the above diagram cluster size is 7 (A,B,C,D,E,F,G) thus frequency reuse
factor is 1/7.
THE NUMBER OF CELLS
N is the number of cells which collectively use the complete set of available
frequencies is called a Cluster. The value of N is calculated by the following
formula:
N = I2
+ I*J + J2
Where I,J = 0,1,2,3…

CAPACITY(C)-
Hence, possible values of N are 1,3,4,7,9,12,13,16,19 and so
on.
If a Cluster is replicated or repeated M times within the cellular
system, then Capacity, C, will be,
C = MkN = MS
In Frequency reuse there are several cells that use the same
set of frequencies.
These cells are called Co-Channel Cells. These Co-Channel
cells results in interference.
So to avoid the Interference cells that use the same set of
channels or frequencies are separated from one another by a
larger distance.
The distance between any two Co-Channels can be
calculated by the following formula:
D = R * (3 * N)1/2
Where,
R = Radius of a cell
N = Number of cells in a given cluster

BELOW IS THE PYTHON CODE FOR VISUALIZING THE
FREQUENCY REUSE CONCEPT
#!/usr/bin/python
from math import *
# import everything from Tkinter module
from tkinter import *
# Base class for Hexagon shape
class Hexagon(object):
def __init__(self, parent, x, y, length, color, tags):
self.parent = parent
self.x = x
self.y = y
self.length = length
self.color = color
self.size = None
self.tags = tags
self.draw_hex()
# draw one hexagon
def draw_hex(self):
start_x = self.x
start_y = self.y
angle = 60
coords = []
for i in range(6):

end_x = start_x + self.length * cos(radians(angle * i))
end_y = start_y + self.length * sin(radians(angle * i))
coords.append([start_x, start_y])
start_x = end_x
start_y = end_y
self.parent.create_polygon(coords[0][0],
coords[0][1],
coords[1][0],
coords[1][1],
coords[2][0],
coords[2][1],
coords[3][0],
coords[3][1],
coords[4][0],
coords[4][1],
coords[5][0],
coords[5][1],
fill=self.color,
outline="black",
tags=self.tags)
# class holds frequency reuse logic and related methods
class FrequencyReuse(Tk):
CANVAS_WIDTH = 800
CANVAS_HEIGHT = 650
TOP_LEFT = (20, 20)
BOTTOM_LEFT = (790, 560)

TOP_RIGHT = (780, 20)
BOTTOM_RIGHT = (780, 560)
def __init__(self, cluster_size, columns=16, rows=10, edge_len=30):
Tk.__init__(self)
self.textbox = None
self.curr_angle = 330
self.first_click = True
self.reset = False
self.edge_len = edge_len
self.cluster_size = cluster_size
self.reuse_list = []
self.all_selected = False
self.curr_count = 0
self.hexagons = []
self.co_cell_endp = []
self.reuse_xy = []
self.canvas = Canvas(self,
width=self.CANVAS_WIDTH,
height=self.CANVAS_HEIGHT,
bg="#4dd0e1")
self.canvas.bind("<Button-1>", self.call_back)
self.canvas.focus_set()
self.canvas.bind('<Shift-R>', self.resets)
self.canvas.pack()
self.title("Frequency reuse and co-channel selection")
self.create_grid(16, 10)

self.create_textbox()
self.cluster_reuse_calc()
# show lines joining all co-channel cells
def show_lines(self):
# center(x,y) of first hexagon
approx_center = self.co_cell_endp[0]
self.line_ids = []
for k in range(1, len(self.co_cell_endp)):
end_xx = (self.co_cell_endp[k])[0]
end_yy = (self.co_cell_endp[k])[1]
# move i^th steps
l_id = self.canvas.create_line(approx_center[0], approx_center[1],
end_xx, end_yy)
if j == 0:
self.line_ids.append(l_id)
dist = 0
elif i >= j and j != 0:
dist = j
# rotate counter-clockwise and move j^th step
l_id = self.canvas.create_line(
end_xx, end_yy, end_xx + self.center_dist * dist *
cos(radians(self.curr_angle - 60)),
end_yy + self.center_dist * dist *

sin(radians(self.curr_angle - 60)))
self.curr_angle -= 60
def create_textbox(self):
txt = Text(self.canvas,
width=80,
height=1,
font=("Helvatica", 12),
padx=10,
pady=10)
txt.tag_configure("center", justify="center")
txt.insert("1.0", "Select a Hexagon")
txt.tag_add("center", "1.0", "end")
self.canvas.create_window((0, 600), anchor='w', window=txt)
txt.config(state=DISABLED)
self.textbox = txt
def resets(self, event):
if event.char == 'R':
self.reset_grid()
# clear hexagonal grid for new i/p
def reset_grid(self, button_reset=False):
self.first_click = True
self.curr_angle = 330
self.curr_count = 0

self.co_cell_endp = []
self.reuse_list = []
for i in self.hexagons:
self.canvas.itemconfigure(i.tags, fill=i.color)
try:
self.line_ids
except AttributeError:
pass
else:
for i in self.line_ids:
self.canvas.after(0, self.canvas.delete, i)
self.line_ids = []
if button_reset:
self.write_text("Select a Hexagon")
# create a grid of Hexagons
def create_grid(self, cols, rows):
size = self.edge_len
for c in range(cols):
if c % 2 == 0:
offset = 0
else:
offset = size * sqrt(3) / 2
for r in range(rows):
x = c * (self.edge_len * 1.5) + 50

y = (r * (self.edge_len * sqrt(3))) + offset + 15
hx = Hexagon(self.canvas, x, y, self.edge_len, "#fafafa",
"{},{}".format(r, c))
self.hexagons.append(hx)
# calculate reuse distance, center distance and radius of the hexagon
def cluster_reuse_calc(self):
self.hex_radius = sqrt(3) / 2 * self.edge_len
self.center_dist = sqrt(3) * self.hex_radius
self.reuse_dist = self.hex_radius * sqrt(3 * self.cluster_size)
def write_text(self, text):
self.textbox.config(state=NORMAL)
self.textbox.delete('1.0', END)
self.textbox.insert('1.0', text, "center")
self.textbox.config(state=DISABLED)
#check if the co-channels are within visible canvas
def is_within_bound(self, coords):
if self.TOP_LEFT[0] < coords[0] < self.BOTTOM_RIGHT[0]
and self.TOP_RIGHT[1] < coords[1] < self.BOTTOM_RIGHT[1]:
return True
return False
#gets called when user selects a hexagon
#This function applies frequency reuse logic in order to
#figure out the positions of the co-channels

def call_back(self, evt):
selected_hex_id = self.canvas.find_closest(evt.x, evt.y)[0]
hexagon = self.hexagons[int(selected_hex_id - 1)]
s_x, s_y = hexagon.x, hexagon.y
approx_center = (s_x + 15, s_y + 25)
if self.first_click:
self.first_click = False
self.write_text(
"""Now, select another hexagon such
that it should be a co-cell of
the original hexagon."""
)
self.co_cell_endp.append(approx_center)
self.canvas.itemconfigure(hexagon.tags, fill="green")
for _ in range(6):
end_xx = approx_center[0] + self.center_dist * i * cos(
radians(self.curr_angle))
end_yy = approx_center[1] + self.center_dist * i * sin(
radians(self.curr_angle))
reuse_x = end_xx + (self.center_dist * j) * cos(
radians(self.curr_angle - 60))
reuse_y = end_yy + (self.center_dist * j) * sin(

radians(self.curr_angle - 60))
if not self.is_within_bound((reuse_x, reuse_y)):
self.write_text(
"""co-cells are exceeding canvas boundary.
Select cell in the center"""
)
self.reset_grid()
break
if j == 0:
self.reuse_list.append(
self.canvas.find_closest(end_xx, end_yy)[0])
elif i >= j and j != 0:
self.reuse_list.append(
self.canvas.find_closest(reuse_x, reuse_y)[0])
self.co_cell_endp.append((end_xx, end_yy))
self.curr_angle -= 60
else:
curr = self.canvas.find_closest(s_x, s_y)[0]
if curr in self.reuse_list:
self.canvas.itemconfigure(hexagon.tags, fill="green")
self.write_text("Correct! Cell {} is a co-cell.".format(
hexagon.tags))
if self.curr_count == len(self.reuse_list) - 1:

self.write_text("Great! Press Shift-R to restart")
self.show_lines()
self.curr_count += 1
else:
self.write_text("Incorrect! Cell {} is not a co-cell.".format(
hexagon.tags))
self.canvas.itemconfigure(hexagon.tags, fill="red")
if __name__ == '__main__':
print(
"""Enter i & j values. common (i,j) values are:
(1,0), (1,1), (2,0), (2,1), (3,0), (2,2)"""
)
i = int(input("Enter i: "))
j = int(input("Enter j: "))
if i == 0 and j == 0:
raise ValueError("i & j both cannot be zero")
elif j > i:
raise ValueError("value of j cannot be greater than i")
else:
N = (i**2 + i * j + j**2)
print("N is {}".format(N))
freqreuse = FrequencyReuse(cluster_size=N)
freqreuse.mainloop()
OUTPUT:

Useful Study links-
1. Mobile Communication –
https://www.youtube.com/watch?v=tt1-
Ohe9QQU&list=RDCMUCpuCKBmQA70bNy39-
VJU_4g&index=2
2. Frequency Reuse - Cell Splitting - Handoff Procedure
https://www.youtube.com/watch?v=ioj6F1v59yw
3. How Mobile Phone Works
https://www.youtube.com/watch?v=1JZG9x_VOwA
4. Cellular Concepts
https://www.tutorialspoint.com/umts/umts_cellular_c
oncepts_introduction.htm

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