EPL 476
Fundamental Concepts in
Wireless Networks
Cellular Networks

Prior Cellular Networks
 Telephone service provided by high power
transmitter

Typical system


25 channels with an effective radius 80km
Cellular Networks

Lower power transmitter of 100w

Due to low coverage of the transmitter


the area can divided into multiple cells
Each cell is served by Base Station

Transmitter, receiver, and control unit
What is a Cell?

Cell is the Basic Union in The System


defined as the area where radio coverage is given by one
base station.
A cell has one or several frequencies, depending on
traffic load.

Fundamental idea: Frequencies are reused, but not in
neighboring cells due to interference.
Shape of Cell?

Design Decision

Shape of Cell to cover the area


Square
Hexagonal,
Frequency Reuse

Transmission Power




Carefully controlled to allow communication
within the cell using a given frequency band
while limiting the power at that frequency that
escapes the cell into adjacent cells.
The same frequency is not used in the
adjacent cells
The objective is to use the same frequency
band in multiple cells at some distance from
one another.
At the same cell multiple frequency bands are
assigned, the number of bands depending on
the traffic expected.
Frequency Reuse

A key design

Determine the minimum separation between
two cells using the same frequency band so
that the two cells do not interfere with each
other.
Frequency Reuse

A key design



D=minimum distance between centers of cells
that use the same frequency band
R=radius of cell
d= distance between centers of adjacent cells

N=number of cells in repetitious pattern
K= total number of frequency allotted for the
system.

Frequencies for each cell = K / N

Frequency Reuse

In hexagonal cell pattern the following
values of N are possible:

N=1,3,4,7,9,12,13,16,19,21…..
Problem

Assume a system of 32 cells with a cell
radius of 1.6km, a total of 32 cells, a total
frequency bandwidth that supports 336
traffic channels, and a reuse factor of N=7.




What is the geographic area covered?
How many channels are per cell?
What is the total number of concurrent calls
that can be handled?
Repeat with 0.8km and 128 cells.
Solution


What is the geographic area covered?
Find the area for each cell
area of hexagonal=6.65k
total geographic area= 6.65*32=213k

How many channels are per cell?

For N=7 the number of channels per cell is
336/7=48. 48 channels per cell.
Solution

What is the total number of concurrent calls
that can be handled?

48 channels per cell* 32 cells=1536 channels
Mobile Radio Propagation Effects

Signal Strength


The strength of the signal between the base station
and the mobile unit must be strong enough to
maintain signal quality at the receiver but not so
strong to create too much cochannel interference
with channels in another cell using the same
frequency band.
Fading

Even if the signal is within an effective range, signal
propagation effects may disrupt the signal and
cause errors
Mobile Radio Propagation Effects

Path loss information HATA
Mobile Radio Propagation Effects

Path loss information HATA
Path Loss Problem

Let fc = 900MhHz, ht =40m, hr= 5m and d =
10km. Estimate the path loss for a mediumsize city.
Traffic Engineering

Consider a cell that has L potential of
customers (L mobile units) and that is
able to handle N simultaneous users
If L <= N
 No-blocking system
 If L >= N
 Blocking system

Traffic Engineering

Blocking System
What is the degree of blocking?
 What is the probability the call is
blocked?
 What is the capacity (N) is needed to
achieve a certain upper bound on the
probability of blocking

A=λh
A=traffic intensity
λ=the mean rate of calls attempt per unit time
h=the mean holding time per successful call
Traffic Engineering
A=λh
λh=ρN
A= ρN
A= is the average number of channels required
λ=the mean rate of calls attempt per unit time
h=the mean holding time per successful call
ρ=is server utilization