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Mobile communications
Chapter # 3 (Rappaport)
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Cellular Systems
Cellular concept is major break - through in
solving the problem of :
(i) Congestion
(ii) Capacity enhancement of telephone
system
It can offers very high capacity with some
technological changes.
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Cellular System (Contd)
The basic idea behind cellular system design is
frequency reuse which exploits the fact that
The power of a transmitted signal falls off with
distance. Thus, two users can operate on the same
frequency at spatially-separate locations with
minimal interference between them.
Separation varies with pico-cell, micro-cell
and macro-cell.
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Frequency Reuse
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Cellular Systems Contd.
The coverage area of a cellular system isdivided into non overlapping cells where a
subset of channels is assigned to each cell.
This same channel set is used in another cell
some distance away, as shown in Figure below,
3 polygons tesselate completely Triangle,
Square, and a Hexagon
We need minimum number of cells when
hexagon is used. Min. cost.
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Frequency Reuse Contd.
CLUSTER OF SIZE 7Clusters
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Reuse Distance
The separation distance of cells that reuse the
same channel set is known as reuse distance.
In orderto increase the spectral efficiency of the
Cellular communication system reuse distanceshould be as small as possible so that
frequencies are reused as often as possible
However, as reuse distance decreases inter-cell
interference increases due to smaller
propagation distance between interfering cells.
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Reuse Distance Contd.
Since intercell interference must remain
below a given threshold for acceptable
system performance(AMPS = 18dB),
reuse distance cannot be reduced belowsome minimum value thus limiting the
capacity of the cellular system.
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Mobile Stations & Base Stations
A Cellular system consists of several mobile
stations & a base stations.
A mobile station is either hand-held personal
units (portables) or installed on vehicles(mobiles), in the cellular radio service, intended
for use while in motion at unspecified locations.
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Mobile Stations & Base Stations Contd.
Base station is a fixed station in a mobile radio
system used for radio communication with the
mobile stations.
Base stations are located at the centerof acoverage region.
They consists of radio channels and transmitter
and receiver antennas mounted on top of a
tower.
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Mobile Stations & Base Stations Contd.
Radio tower
Base Station
Mobile Station
Hand offProcess of a redirecting an ongoing phone call fromone cell to another cell using different frequency pair
without interrupting the call.
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Frequency Reuse Contd.
To find size of a
cluster
ij
Size of a cluster, N = i2+ij+j2
i= Move i cell in any chain of
Hexagon
j= Turn 60 degrees counterclock wise and move j cell
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Frequency Reuse Contd.
Find cluster size for the following cellular system.
Size of a cluster, N = i2+ij+j2
Here i =3 and j=2
N = 19
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Frequency Reuse Capacity
expansion in one cluster
S = Total duplex channels in a system
k = Available channels per cell
N = Total number of cells in a cluster
Therefore, S = kN
N cells collectively use complete set ofavailable frequencies. Each cell usedlesser freqs. This leads to SystemCapacity expansion.
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Frequency Reuse Multi-clusters
M = No of times cluster is replicated
C= Total number of duplex channels in a
system
Therefore, C= M k N
=> C=MS (Total channel
capacity of system) => C is directly proportional to M
N iscalled cluster Size
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Example from Text (Self read)
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Frequency Reuse (Contd.)
If N is reduced while cell size is kept constant thenmore clusters are required to cover a given area.Hence, more capacity (large value of C) isachieved. But this may increase interference.
From design view point smallest possible value ofN is desirable in order to maximize the capacity.
Frequency reuse factorof cellular system is givenby 1/N.
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Problem with Smaller Cluster
size
Interfering cells are closer by when cluster size is smaller.
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Problem with Smaller Cluster
size If interfering cells are closer. Thus total interference
power will be larger.
With higher interference power, the quality of thesignal (speech or data) will deteriorate.
To reduce the interference power, we can make thecells larger.
With larger cell, the number of users covered perunit area reduces. So, the capacity (total number ofusers supported) of a smaller cluster size is not ashigh as we think.
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Inter-Cell Interference (Co channel
Interference)
Because same set of channels is used in
another cell some distance away therefore
interference is possible.
The interference caused by users in different
cells operating on the same channel set is called
inter-cell interference( Co-Chl Interference).
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Causes of CCI
Undesired Transmitter Signals located in
some other cells that are using the same
frequencies (co-channel signals) arrive at
the receiver.
Adverse weather and propagation
conditions
Poor frequency planning
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Example of CCI
D1
D6
D2
D3
D4
D5
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Minimizing CCI
By increasing D/R ratio, the spatial separation between the
co-channel cells relative to the coverage distance of cell is
increased. Thus co-channel interference is reduced.
D = Distance between the centers of nearest co-channelcell
R= Radius of the cell
This ratio (D/R) is called co-channel reuse ratio
represented by Q.Where N is cluster size.
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Co- Channel Interference
Hence, Q determines the spatial separation relative to
the coverage distance of a cell.
Smaller value of Q provides larger capacity, but
higher CCI
Hence there is a tradeoffbetween the Capacity and
Interference
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Co-channel reuse Ratio Table
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Reuse Distance
The separation distance of cells that reuse the
same channel set is known as reuse distance.
In orderto increase the spectral efficiency of the
Cellular communication system reuse distanceshould be as small as possible so that
frequencies are reused as often as possible
However, as reuse distance decreases inter-cell
interference increases due to smaller
propagation distance between interfering cells.
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Books
1. T. S. Rappaport, Wireless Communications, PHI, 2002.
2. William C.Y. Lee, Mobile Cellular Telecommunications-
Analog & Digital Systems, Mc.Graw Hill, 1995.
3. V.K.Garg, Principles and Applications of GSM, Pearson
Education, 1999.
4. V.K.Garg, IS-95 CDMA and cdma 2000, Pearson
Education, 2000.