Wireless Communications:
System Design
Dr. Mustafa Shakir
Issues in cell to cell moving
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What are different levels of handoff.
(1) Intra Cell (2) Inter cell (3) Inter system
Importance of handoff.
When no priority to handoff call blocking would be equal
for call initiation and call handoff.
There are two strategies to give a priority to handoff.
(1) Guard Channel: 100% guaranty for successful
handoff but It will cause low trunking efficiency.
(2) Queuing Of Handoff Request: There can be
unsuccessful handoffs due to long delay in queue.
--“Probability of forced termination” decreases at the
cost of reduced Total Carried Traffic.
-- Queuing is possible because of the time available
between the Threshold power level and the Hand off
power level.
Umbrella Cell Approach
Solution For More Handoffs
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
Umbrella Cell Approach:
Micro
cells
inside
A
macro
cell.
---- Macro cell is defined by high power and
lengthy
tower.
---- Micro cells are defined inside the macro
cell with less power and less height towers.
---- High speed MS are handled by macro cell
and low speed subscribers are handled by
micro
cells.
---- This strategy increases the no of capacity
channels per unit area and decreases the no of
handoffs.
Umbrella Cell Approach
INTERFERENCE AND SYSTEM
CAPACITY
Interference
 It is a major limiting factor in the performance of cellular radio
systems. (In comparison with wired comm. Systems, the amount
and sources of interferences in Wireless Systems are greater.)
 Creates bottleneck in increasing capacity
 Sources of interference are:
1. Mobile Stations
2. Neighboring Cells
3. The same frequency cells 4. Non-cellular signals in the
same spectrum
 Interference in Voice Channels: Cross-Talk
 Urban areas usually have more interference, because of:
a)Greater RF Noise Floor,
b) More Number of Mobiles
Major Types Of Interference
1) Co-Channel Interference (CCI)
2) Adjacent Channel Interference (ACI)
3) Other services: like a competitor cellular service in the
same area
1) Co-Channel Interference and System Capacity

The cells that use the same set of frequencies are called co-channel cells.

The interference between signals from these cells is called Co-Channel
Interference (CCI).

Cannot be controlled by increasing RF power. Rather, this will increase CCI.

Depends on minimum distance between co-channel cells.
The yellow cells use the
same set of frequency
channels, and hence,
interfere with each other.
In case of N=7, there are
6 first-layer co-channels.

In constant cell size and RF power, CCI is a function of Distance between the
co-channel cells(D), and the size of each cell (R).

Increasing ratio D/R, CCI decreases.

Define Channel Reuse Ratio = Q = D/R

Signal-to-interference ratio
SIR =
S
6
I
k
k 1

S is the power of the signal of interest and Ik is the power
of kth interference.
 The signal strength at distance d from a source is
S d
n
 That is, received signal power is inversely related to nth
power of the distance.
 where n = path loss exponent
 For hexagonal geometry, D/R can be calculated:
R
D
Q  D / R  3N
 Smaller Q provides larger
capacity, since that would
Q  3N
mean smaller N. (Capacity
 1/N).
 Larger Q improves quality, owing to less CCI.

for
N=3,
N=7,
Q=3,
Q=4.58,
N=12, Q=6,
N=13, Q=6.24
 Then we can express the SIR in terms of distance
S/I  SIR=
Rn
6
D
k 1
n
k
 where the denominator represents the
neighboring clusters using the same channel.

users
in
Let D k=D be the distance between cell centers. Then
( D / R) n ( 3N ) n
S/I 

6
6
 Note how S/I improves with the frequency reuse N.
 Analog systems: U.S. AMPS required S/I ~= 18dB For n =
4, the reuse factor for AMPS is N  6.49, so N = 7.
 Now, let us consider the worst case for a cluster size of
N= 7. The mobile is at the edge of the cell. Express C/I as
a function of actual distances.
Worst Case Design
Worst case carrier-to-interference ratio
S
Rn

I 2( D  R) n  2D  n  2( D  R) n
D+R
D
D-R
D+R
Let n = 4 and D/R = q,
D-R
D
S
1

I 2(q  1)4  2q 4  2(q  1) 4
Let reuse N = 7, then
q  3  7  4.6
Compute C/I and get C/I = 17.3 dB
E If S/I min = 15 dB, what is the capacity for n = 4, n = 3
(a) n = 4, N = 7
D / R  3  7  4.58
S ( D / R) n ( 3N ) n (4.58) n



 18.66 dB>15 dB
I
i0
i0
6
N =7 can be used
(b) n = 3, N = 7
S
4.58

 12.05 dB <15 dB
I
6
Need larger N
3
D/ R 
3  12  6.0
S
6

 36  15.56 dB >15 dB
I
6
3
(2) Adjacent Channel Interference
 Interference from channels that are adjacent in frequency,
 The primary reason for that is Imperfect Receiver Filters
which cause the adjacent channel energy to leak into your
spectrum.
 Problem is severer if the user of adjacent channel is in
close proximity.  Near-Far Effect
 Near-Far Effect: The other transmitter(who may or may
not be of the same type) captures the receiver of the
subscriber.
 Also, when a Mobile Station close to the Base Station
transmits on a channel close to the one being used by a
weaker mobile: The BS faces difficulty in discriminating
the desired mobile user from the “bleed over” of the
adjacent channel mobile.
Near-Far Effect: Case 1
Uninte
nded
Tx
Strong “bleed
over”
Mobile User
Rx
BS as Tx
Weaker signal
The Mobile receiver is captured by the unintended, unknown
transmitter, instead of the desired base station
Near-Far Effect: Case 2
BS as Rx
Weaker signal
Strong “bleed
over”
Desired Mobile
Tx
Adjacent
Channel
Mobile Tx
The Base Station faces difficulty in recognizing the actual
mobile user, when the adjacent channel bleed over is too
high.
Minimization of ACI
(1) Careful Filtering ---- min. leakage or sharp transition
(2) Better Channel Assignment Strategy
 Channels in a cell need not be adjacent: For channels
within a cell, Keep frequency separation as large as
possible.
 Sequentially assigning cells the successive frequency
channels.
 Also, secondary level of interference can be reduced
by not assigning adjacent channels to neighboring
cells.
 For tolerable ACI, we either need to increase the
frequency separation or reduce the pass band BW.
Power Control in Mobile Com
What is power control ?
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Both the BS and MS transmitter powers are adjusted
dynamically over a wide range.
Typical cellular systems adjust their transmitter powers
based on received signal strength.
TYPES OF POWER CONTROL
o Open Loop Power Control
It depends solely on mobile unit, not as accurate as
closed loop, but can react quicker to fluctuation in signal
strength. In this there is no feed back from BS.
o Closed Loop Power Control
In this BS makes power adjustment decisions and
communicates to mobile on control channels
Why power control ?
Near-far effect
 Mechanism to compensate for “channel
fading”
 Interference reduction,
 prolong battery life

Improving Capacity in Cellular Systems
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Cost of a cellular network is proportional to the number
of Base Stations. The income is proportional to the
number of users.
 Ways to increase capacity:
 New spectrum – expensive. PCS bands were sold for
$20B.
 Architectural approaches: cell splitting, cell sectoring,
microcell zones.
 Dynamic allocation of channels according to load in
the cell (non-uniform distribution of channels).
Improve access technologies.
Cell Splitting
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Cell Splitting is the process of subdividing the
congested cell into smaller cells (microcells), Each
with its own base station and a corresponding
reduction in antenna height and transmitter power.
Cell Splitting increases the capacity since number of
clusters over coverage region would be increased thus
increasing the number of channels.
New cells added having smaller radius than original
cells and by installing these smaller cells (called
microcells ) between existing cells , capacity increases
due to additional number of channels per unit area.
Cell splitting diagram 1
An Example
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The area covered by a circle with radius R is
four times the area covered by the circle with
radius R/2 The number of cells is increased four
times
The number of clusters the number of channels
and the capacity in the coverage area are
increased Cell Splitting does not change the cochannel re-use ratio Q =D/R
Transmit Power
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New cells are smaller, so the transmit power of
the new cells must be reduced
How to determine the transmit power?
The transmit power of the new cells can be
found by examining the received power at the
new and old cell boundaries and setting them
equal
Pr(at the old cell boundary) is proportional to
Pt1 * R-n
Pr(at the new cell boundary) is proportional to
Pt2 * (R/2)-n
Transmit Power
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Take n=4, we get
Pt2 = Pt1/16
We find that the transmit power must be
reduced by 16 times or 12 dB in order to use
the microcells to cover the original area.
While maintaining the same S/I.
Application of cell splitting
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When there are two cell sizes one cant simply use
the same transmit power for all cells. If larger
transmit power used for all cells some smaller cells
would not be sufficiently separated from co channel
cells. Using smaller Pt the larger cells might be left
unserved.
So old channel broken to two channel groups
corresponding to smaller and larger cell reuse.
Larger cell for less frequent hand off.
Antenna down tilting focusing radiated energy from
base station to the ground to limit radio coverage of
newly formed cells.
Cell Sectoring
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Co channel interference may be reduced by replacing
omni directional antenna by several directional antennas.
Given cell will receive interference and would transmit
with fraction of available co channel cells.
Each sector uses directional antenna at the B.S and
assigned a set of channels.
Partitioning into three 120 deg. sectors or six 60 deg.
sectors.
Amount of CCI reduced by number of sectors.
Reduced Tx Power…
32
Cell Sectoring
Example for sectoring
Explanation For Cell Sectoring
Effects of Sectoring
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Reduction in interference offered by sectoring would
enable to reduce the cluster size N and additional
degree of freedom in channel assignment.
Increased number of antennas with shrinking cluster size
and decrease in trunking efficiency due to channel
sectoring at base station.
Since sectoring reduces the coverage area of a
particular group of channels the number of handoffs
increases
Available channels subdivided and dedicated to a
specific antenna thus making up of several smaller pools
contributing to decrease in trunking efficiency.
Repeaters
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To provide dedicated coverage for hard to reach
areas
Radio retransmitters for range extension.
Upon receiving signals from base station
forward link the repeater amplifies and
reradiates the base station signals to specific
coverage region.
In building wireless coverage by installing
Distributed Antenna Systems.
Repeaters must be provisioned to match the
available capacity from the serving base station.
Repeaters For Range Extension
Microcell Zone
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The increased number of handoff as a result of sectoring
would result in an increased load on switching and
control link elements of the mobile system.
Division into microcell zones and each of the three are
connected to a single base station and share the same
radio equipment.
Zones connected by a coaxial cable, fiber optic cable or
microwave link to the base station.
Handoff not required while mobile travels between zones
within cell.
Channel switching and a channel active only within zone
of travelling.
Scenario
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In Micro cell zone scenario each hexagon
represents a zone while the group of three
hexagons represent a cell.
Zone Radius Rz is one hexagon radius.
Capacity of Microcell is directly related to
distance betw. Cochannel cells and not
zones.
No handoffs is required at the MSC.
The base station radiation is localized and
interference is reduced
Trunking & Grade Of Service
Trunking and Grade of Service (GOS)
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Trunking
A means for providing access to users on
demand from available pool of channels.
 With
trunking, a small number of
channels can accommodate large
number of random users.
 Telephone
companies use trunking
theory to determine number of circuits
required.
Trunking theory is about how a population
can be handled by a limited number of
servers.
Terminologies
Erlang:

One Erlang:
When a circuit is busy for one hour it handled a traffic of one erlang.

Grade of Service (GOS):
probability that a call is blocked (or delayed).

Set-Up Time:
Traffic intensity is measured in Erlangs:

time to allocate a channel.

Blocked Call:
Call that cannot be completed at time of request
due to congestion. Also referred to as Lost Call.

Terminologies
Contd.
Holding Time: (H)
Average duration of typical call.
 Load:
Traffic intensity across the
whole system.
 Request Rate: ()
Average number of call requests per unit
time.

Traffic Measurement (Erlangs)
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Traffic per user Au = H where  is the request rate
and H is the holding time.
For U users the load is A= U Au
If traffic is trunked in C channels, then the traffic
intensity per channel is Ac= UAu /C
Erlang B:
The Erlang B Chart
Example
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Example : An urban area has 2 million
residents. Three competing cellular systems
provide service:
System A 394 cells x 19 channels/cell.
System B 98 cells x 57 channels/cell.
System C 49 cells x 100 channels/cell.
For each user  = 2 calls/hr, H = 3min, GOS
= 2% blocking. Find the number of users that
can be supported by each system. Note that
these are not simultaneous users.
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System A:
Au=  H = 2 x 3/60 = 0.1 Erlangs.
From the curve for GOS = 0.02 and C = 19 => A = 12 Er.
Users per cell (U) = A/Au = 12/0.1 = 120 users
120 users/cell x 394 cells = 47,280 users can be served.
Market penetration = 2.36%.
No. of subscribers

System C:
Prob Blocking = 2% = 0.02
 C =100
 Au =  H = 2 x 3/60 = 0.1 Erlangs.
 From table, A = 88 Erlangs.
 Users per cell U = A/Au = 88/0.1 =880 users
 880 users/cell x 49 cells = 43,120.
Market penetration = 2.156%.
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System B:
Prob Blocking = 2% = 0.02
C =57
Au =  H = 2 x 3/60 = 0.1 Erlangs.
From table, A = 45 Erlangs
Users per cell U = A/Au = 45/0.1 = 450 users
450 users/cell x 98 cells = 44,100.
Market penetration = 2.21%.
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Total No. of supported users = 47,280 + 44,100
+ 43,120
= 134,500 users.
Total market penetration for 3 systems =
6.725%