ECS455 Chapter 2
Cellular Systems
2.3 Sectoring
Dr.Prapun Suksompong
1 prapun.com/ecs455
Improving Coverage and Capacity
As the demand for wireless service increases, the number of
channels assigned to a cell eventually becomes insufficient to
support the required number of users.
At this point, cellular design techniques are needed to
provide more channels per unit coverage area.
Easy!?
If cells can be
Atotal S reduced in size, more
C of them can be added
Acell N in a given area,
increasing the overall
capacity.
2
Sectorization (sectoring)
Use directional antennas instead of omnidirectional antennas.
When 120° sectorization is used, one cell that usually covers
360° is divided into three 120° regions.
When 60° sectorization is used, one cell that usually covers
360° is divided into six 60° regions.
These regions are called sectors.
Analogy: Flashlight
3
Sectoring (N = 7)
Ex.
With no sectoring, suppose we have m = 18 channels/cell
With 120o sectoring, we have 6 channels/sector
With 60o sectoring, we have 3 channels/sector
120° Sectoring 60° Sectoring
[Rappaport, 2002,
Figure 3.10]
Can support “the same” number of users per cell
In the next section, we will consider better definition of capacity.
From such view on capacity, sectoring will give smaller capacity.
Why is this better?
4
From previous section
1
SIR 3N
60 Degree Sectoring K
Out of the 6 co-
channel cells in the
first tier, only one of
them interfere with
the center cell.
If omnidirectional
antennas were used at
each base station, all
6 co-channel cells
would interfere the
the center cell.
5
The value of K changes from 6 to 1!
1
SIR 3N
Sectoring (N = 3 , 60) K
K=2 K=1
6
1
SIR 3N
Sectoring (N = 3, 120) K
K=3 K=2
7
Sectoring (N = 7, 120)
Assuming seven-cell reuse,
for the case of 120 sectors,
the number of interferers in
the first tier is reduced from
six to two.
K=2
[Rappaport, 2002, Fig 3.11]
8
Summary:
S = total # available duplex radio channels for the system
Frequency reuse with cluster size N Path loss exponent
1 D 1
Atotal S S kR
“Capacity” C 3N
Acell N Tradeoff I K kD K R
K
m = # channels allocated to
each cell.
Omni-directional: K = 6
120 Sectoring: K = 2
60 Sectoring: K = 1
9
Sectoring
Advantages
Reduce interference by reducing K
Increase SIR (better call quality).
The increase in SIR can be traded with reducing the cluster size (N)
which increase the capacity.
Disadvantages
Cost: Increase number of antennas at each base station.
Next section: Decrease trunking efficiency due to channel
sectoring at the base station.
The available channels in the cell must be subdivided and dedicated to a
specific antenna.
10
Location of the BS
Center vs. Corner
11
Visualizing the Cellular Signals
Artist: Nickolay Lamm
Use data from antennasearch.com to approximate the number of stations in each city
and imposed a theoretical hexagonal grid over Chicago and New York.
Color representation:
The area within each sector antenna radiation pattern has different users being assigned
different frequencies and their signals combine to form a single perceived color in that
instant.
Different channel combinations from sector to sector are indicated by different colors.
The channel combinations are not static, but rather change rapidly in time as different
users are assigned different channels. But, if you were to take a photo of these rapid
changes, you’d likely see a wide array of colors as seen in the illustration.
With some technical check by
Danilo Erricolo, professor of electrical and computer engineering at the University of
Illinois, and
Fran Harackiewicz, a professor at Southern Illinois University Carbondale who teaches
antenna theory and design.
12 [http://motherboard.vice.com/blog/here-are-what-cell-phone-signals-would-look-like-if-you-could-see-them]
