Frequency reuse

Frequency spectrum, Frequency Reuse Pattern

1. Frequency Reuse

2. 2
Why are Cells Required?
•Original mobile voice networks used transmitter with large power
to cover very large area
•Capacity severely limited by available bandwidth
•Spectrum limited, so could not increase capacity by adding new
channels
•Cellular concept was born
–Ushered in modern communication systems

3. 3
Frequency Reuse…
•Due to Co-channel Interference (CCI), cannot use same frequency
in adjacent cell
•Cells that use same frequencies must be separated by distances large
enough to keep interference levels low
•Frequencies assigned to different cells using frequency reuse plan
•Adjacent cells assigned different frequencies to avoid interference
or crosstalk

4. 4
Frequency Reuse…
•Objective is to reuse frequency in nearby cells
•10 to 50 frequencies assigned to each cell
•Transmission power controlled to limit power at that frequency
escaping to adjacent cells
•The issue is to determine how many cells must intervene between
two cells using the same frequency

5. Frequency spectrum

6. 6
Frequency Reuse
•Design cells to be non-overlapping and cover entire
region
•Cells depicted as hexagons
–Conceptual design allowing easy analysis of system
–Close to circular shape achieved by omnidirectional
antennas
•“Footprint”: actual radio coverage of a cell
–Determined from field measurements or propagation
prediction models
–Amorphous in nature
–Use hexagon to approximate shape

7. 7
Frequency Reuse…
•Cells with same letter use the same set of frequency channels
•Using hexagonal cells, BS located at center of cell
•MS at edge of cell receives weak signal from BS, i.e., low Carrier
to Interference ratio (C/I)
A
F
E
D
B
G C
A
F
E
D
B
G C
A
F
E
D
B
G C

8. 8
Frequency Reuse…
•Suppose system has S total channels & k channels per cell (k < S)
•Channels divided among N cells into disjoint groups, S = kN, N
cells which use all S channels called “cluster” (N = cluster size,
typically 4, 7, 12)
•Clusters replicated in system
•Typically cluster size N = i2 + ij + j2
–N=7  i=2, j=1
–N=3  i=1, j=1
•Move i cells in any direction
•Turn 60o CCW
•Move j cells in this direction

9. 9
Frequency Reuse…
A
F
E
D
B
G C
A
F
E
D
B
G C
N = 7
i=2, j=1
A
B
C
B
B
C C
C
B
B
A
C
A
A A
N = 3
i=1, j=1

10. 10
Frequency Reuse Pattern
2
3
1
N=3
1
2
4
3
4
1
3
6
5
2 7
N=4
N=7
1
1
1
1
N=1

11. 11
Cell Geometry
D
R
R
R Nq
R
D
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