6 Months Industrial Training In Telecom in Chandigarh

ACCESS Method and
Cell Structure of GSM
www.arcadianlearning.com

Various Access Method www.arcadianlearning.com

Code Multiplex
 Each channel has a unique code
 All channels use the same spectrum at the
same time
 Advantages:
 Bandwidth efficient
 No coordination and synchronization
necessary
 Good protection against interference
and tapping
 Disadvantages:
 Lower user data rates
 More complex signal regeneration
 Implemented using spread spectrum
technology
k2 k3 k4 k5 k6k1
f
t
c

Cells www.arcadianlearning.com

Capacity & Spectrum Utilization
Solution
The need:
 Optimum spectrum
usage
 More capacity
 High quality of service
 Low cost
I wish I could increase capacity
without adding NEW BTS!
What can I do?
Network capacity at required QoS
with conventional frequency plan
Subscriber
growth
Time
Out of
Capacity!!!

Representation of Cells
Ideal cells Fictitious cells

Cell size and capacity
 Cell size determines number of cells
available to cover geographic area and
(with frequency reuse) the total capacity
available to all users
 Capacity within cell limited by available
bandwidth and operational requirements
 Each network operator has to size cells
to handle expected traffic demand

Cell structure
 Implements space division multiplex: base station covers a
certain transmission area (cell)
 Mobile stations communicate only via the base station
 Advantages of cell structures:
 higher capacity, higher number of users
 less transmission power needed
 more robust, decentralized
 base station deals with interference, transmission area
etc. locally
 Problems:
 fixed network needed for the base stations
 handover (changing from one cell to another) necessary
 interference with other cells
 Cell sizes from some 100 m in cities to, e.g., 35 km on the
country side (GSM) - even less for higher frequencies

Capacity of a Cellular
System
 Frequency Re-Use Distance
 The K factor or the cluster size
 Cellular coverage or Signal to
interference ratio
 Sectoring

i
j
1
2
3
4
5
6
7
Frequency re-use distance is based on the cluster size K
The cluster size is specified in terms of the offset of the center of a cluster from
the center of the adjacent cluster
K = i2 + ij + j2
K = 22 + 2*1 + 12
K = 4 + 2 + 1
K = 7
D = 3K * R
D = 4.58R
1
2
35
6
7
D
R
The K factor and Frequency Re-Use Distance

K = i2 + ij + j2
K = 22 + 2*0 + 02
K = 4 + 0 + 0
K = 4
D = 3K * R
D = 3.46R i
D
R
The Frequency Re-Use for K = 4

1
2
3
4
5
6
7
1
2
3
4
5
6
7
2
1
1
2
3
4
5
6
7
1
2
3
4
5
6
7
1
2
3
4
5
6
7
The Cell Structure for K = 7

1
2
3
4
1
1
1
1
1
12
2
2
2
2
3
3
3
3
3
4
4
4
4
4
4
3
2
Cell Structure for K = 4 www.arcadianlearning.com

1
11
1
2 2
22
3
3
3
3
4
4 4
4
5
5 5
5
6
6 6
6
7
7
7
7
8 8
88
9
99
9
10
1010
10
1111
1111
1212
12 12
Cell Structure for K = 12 www.arcadianlearning.com

Increasing cellular system
capacity
 Cell sectoring
 Directional antennas subdivide cell into 3 or
6 sectors
 Might also increase cell capacity by factor of
3 or 6

Increasing cellular system
capacity
 Cell splitting
 Decrease transmission power in base and
mobile
 Results in more and smaller cells
 Reuse frequencies in non-contiguous cell
groups
 Example: ½ cell radius leads 4 fold capacity
increase

Tri-Sector antenna for a cell

Highway
TownSuburb
Rural
Cell Distribution in a Network

Optimum use of frequency
spectrum
 Operator bandwidth of 7.2MHz (36 freq of
200 kHz)
 TDMA 8 traffic channels per carrier
 K factor = 12
 What are the number of traffic channels
available within its area for these three
cases
 Without cell splitting
 With 72 cells
 With 246 cells

One Cell = 288 traffic channels
72 Cell = 1728 traffic channels
246 Cell = 5904 traffic channels
Re-use of the frequency
8 X 36 = 288
8 X (72/12 X 36) = 1728

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6 Months Industrial Training In Telecom in Chandigarh

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6 Months Industrial Training In Telecom in Chandigarh