Lect-2 Mobcom ch#3.pptx

7/27/2019 Lect-2 Mobcom ch#3.pptx

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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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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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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.

Documents

Lect-2 Mobcom ch#3.pptx