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Evolution of Mobile Radio Comm
Figure 1.1 The growth of mobile telephony as compared with other
popular inventions of the 20th
century.
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Evolution of Mobile Radio Comm
The first generation digital cellular wireless
network was the Advanced Mobile Phone
System (AMPS) The second generation wireless systems
are the popular Global System for Mobile
Communications (GSM) and Personal
Communication Service (PCS)
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Mobile Standards
AMPS - Advanced Mobile Phone System
USDC - US Digital Cellular
CDPD - Cellular Digital Packet Data
IS-95 - Interim Standard- 95
JTACKS -Japanese Total Access Cellular Systems
PDC - Pacific Digital Cellular
NTT - Nippon Telephone and Telegraph Company
PHS - Personal Handy Phone Service
ETACS - European Total Access Cellular System GSM - Global System for Mobile
CT2 - Cordless Telephone
DECT - Digital European Cordless Telephone
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Examples of Wireless
Communication System Garage Door Openers
Remote Controllers
Paging system Cordless System
Hand held walkie - talkie
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Paging System
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Disadvantages:
Wide area paging system are more
complex in design for they require a
network of telephone lines, many base
station transmitters, and large radio
towers.
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Cordless Telephone System
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Cellular or Mobile System
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Timing diagram illustrating
how a call to a mobile user
initiated by a landline
subscriber is established.
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Timing diagram illustrating how
a call initiated by a mobile
is established.
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First Generation Wireless networks
Second generation wireless networks Third Generation wireless networks
Modern Wireless CommunicationModern Wireless Communication
SystemsSystems
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Basic Concepts in a Cellular
System Frequency Reuse
Handoff
Interference System capacity
Improving Coverage and Capacity in
cellular system
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Frequency Reuse
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The design process of selecting and
allocating channel groups for all of thecellular base stations within a system is
called frequency reuse or frequency
planning
Each cellular base station is allocated a
group of radio channels that is to be used
within a small geographic area called cell
The actual radio coverage of a cell isknown as the footprint
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Concept:
Consider a cellular system which has atotal ofS channels for use and the system
is divided into N cells.
If each cell is allocated a group ofk
(k
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The N cells which collectively use the
complete set of available frequencies is
called a cluster.
A measure of capacity of the system
is
C = MkN or C = MS
here N is called the cluster size and
can have values which satisfy the equation
given below
N= i2 + ij + j2
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Method of locating co-channel cells in a cellular system. In
this example, N= 19 (i.e., I= 3, j= 2).
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Definition:
The process of transferring a
mobile from one channel or base station to
another is called as handoff.
Dwell time:
The time over which a call
maybe maintained in a cell without handoff
is called the dwell time.
Handoff
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Intersystem Handoff:
During the course of a call, ifa mobile moves from one cellular system
to a different cellular system controlled by
a different MSC then it is known asIntersystem handoff.
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Prioritizing Handoff:
Two methods discussed
1. Guard Channel concept
2. Queuing of handoff requests
Some handoff issues and their solutions
1. High speed moving vehicles
Sol: Umbrella approach
2. Pedestrian users Cell Dragging
Sol: Handoff thresholds and radio
coverage parameters should be
adjusted carefully
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Umbrella approach
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Improving Coverage and Capacity
in cellular system
Two ways
1. Reducing cell size
2. Decreasing the channel reuseratio (Q=D/R)
Techniques used to expand the cellcapacity are
1. Cell Splitting
2. Sectoring
3.Coverage zone approaches
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1. Cell Splitting
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Cell splitting is the process of subdividing the
cell into smaller cells (micro cells) each with its
own base station.
It increases the cell capacity by increasing the
number of times the channels are
used. The transmitted power and antenna height is
also reduced
The small cells are divided into more number of
cells, so more number of channels are allocated
for the cell to increase the channel capacity
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Transmit Power:
Pr[ at old cell boundary] Pt1R-nPr[ at new cell boundary] Pt2(R/2)
-n
Pt1,Pt2-Transmit powers of larger and smallerbase stations
n- Path loss component
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2. Sectoring
The technique for decreasing co-channel
interference by using directional antennas is
called sectoring.
It also used to increase the capacity of thesystem
A cell is normally partitioned into three 120
sectors or six 60 sectors.
When sectoring is employed ,the channels in aparticular cell are broken down into sectored
groups and are used within a particular sector.
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Sectoring improves S/I
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Disadvantage:
1. Increased number of handoff results
in an increased load on switching andcontrol link elements.
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The Zone Cell Concept
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In this scheme, each of three zone sites
are connected to single base station.
They share the same radio equipment.
The zones are connected by coaxial cable,
fiber optic cable or microwave link.
As a mobile travels within the cell it isserved by the zone with the strongest
signal.
As a mobile travels from one zone toanother within the cell it retains the same
channel. Thus handoff is avoided.
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Advantages:
1. Handoff is not required
2. Co-channel interference is
decreased.
3. Improved signal quality.
4. The channels are distributed in
space and time by all three zones
and are also reused in co-channel
cells in normal fashion. Thus
capacity is increased
I t f
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Interference
System capacity
Some sources of interference:
Another mobile in the same cell
A call in progress in a neighbouring cellOther base stations operating in the
same frequency band
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Two major interference
1. Co-channel interference2. Adjacent channel interference
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Indoor propagation model
It has been observed that propagation within buildings isstrongly influenced by specific features such as thelayout of the
building, the construction materials, and the building type.
Signal level vary depending on whether interior doors areopen or closed inside a building.
Some key models are given, they are Partition losses(same floor), partition losses between floors, Longdistance path loss model,
Ericsson multiple breakpoint model and Attenuation factormodel.
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Partition losses( same floor)
Hard partitions partitions that are formed
as part of the building structure
Soft partitions partitions that may movedand do not span to the building
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Partition loss (between floor)
The losses between floors of a building
are determined by the external dimensions
and materials of the building as well as thetype of construction used to create the
floors and the external surroundings.
Floor attenuation factor
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Long distance path loss model
PL (dB) = PL(d0) + 10 log (d/d0) +X
Where the value of n depends on thesurroundings and building type and X
represents normal random variable in dB
having a standard deviation of dB.
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Ericsson multiple breakpoint model
Ericsson radio system model was obtained
by measurements in a multiple floor office
building .
This model also assumes that there is 30
dB attenuation at d0 = 1m,which can be
shown to be accurate for f = 900Mhz
The path loss based on the function of
distance
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Attenuation factor model
This model provides flexibility and to reduce the standarddeviation between measured and predicted path loss toaround 4 dB as compared to 13dB
The attenuation factor model is given by
nSF represents the exponent value for the same floormeasurement.
FAF represents a floor attenuation factor for a specificobstruction encountered by a a ray drawn between thetransmitter and receiver in 3-D
The technique of drawing a single ray between thetransmitter and receiver is called primary ray tracing