1. Adv. Wireless Comm. Systems - Cellular Networks -
Objectives
Understand the advantage of a cellular network topology
Acquire basic methods for capacity planning
Outline
The cellular network concept
Frequency planning in cellular networks – uniform traffic
Methods to increase network capacity
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

2. Adv. Wireless Comm. Systems - Cellular Networks -
Frequency Reuse
Bandwidth (spectrum) is scarce
Tradeoff between transmission-capacity and reception-
quality that best utilize a given spectrum
A single base station accommodates channels
Hz total spectrum
Hz per channel
Fig. 3.1
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

3. Frequency Reuse (cont.)
Transmission power attenuates with distance
Reuse channel frequency is sufficiently apart
Sufficiently apart distance
Fig. 3.2
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

4. Frequency Reuse (cont.)
Partition the service region into cells
A cell comprises the BTS coverage area
Mobiles at each cell are served by cell’s BTS
Fig. 3.3
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

5. Frequency Reuse (cont.)
Hexagon is a geometrical shape that is most close to a circle
that covers a region w/o overlapping
Commonly accepted abstraction for resource and capacity
planning in cellular networks
Characterized by its radius R
We address the questions:
For a given traffic (calls/unit area), how to select R ?
How to select transmitter power as a function of R ?
How to allocate frequencies (channels) among cells ? R
Fig. 3.4
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

6. Frequency Reuse (cont.)
The Reuse Principle for Uniform Traffic
Uniformly distribute all channels to a cell cluster of size N
Example: A cluster with N=7
1/N is called “frequency reuse factor”
Keep co-channel cells as far apart B G C A F D
The feasible N are determined from: E
Fig. 3.5
(3.1)
integers j
Examples: N=3,4,7,9,12 i
Fig. 3.6
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

7. Frequency Reuse (cont.)
The Reuse Principle (cont.) B
Replicate cluster keeping co-channel cells as far apart
Transmission capacity: G C A F D E B
(3.2) B G C G C A
S – Total no. of channels
N – Cluster size A F D F D E
Lowering N
Increases capacity C
Increases co-channel interference E
Fig. 3.7
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

8. Frequency Reuse (cont.)
Trading off Capacity and Interference
Measurements show that the average received power
approximately follows the “exponent decay law”
(3.3)
(Power in dB)
- Power received at reference point
Fig. 3.8
- Path loss exponent (typically between 2 – 4)
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

9. Frequency Reuse (cont.)
Trading off Capacity and Interference (cont.)
A common measure for co-channel interference is the
Signal Interference Ratio (SIR), namely
The ratio between the received signal power and the co-channel interference power
Exercise: Verify that the worst case is given by:
(3.4)
No. of cells in the 1st tier of
co-channel interfering cells
Fig. 3.9
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

10. Frequency Reuse (cont.)
Trading off Capacity and Interference (cont.)
If all cells have the same size and base stations transmit in the same power, then
is independent of the transmitter power
Exercise: Verify that the “co-channel reuse ratio” satisfies
(3.5)
Hence, the “capacity” and the worst SIR become ;
(3.6)
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

11. Frequency Reuse (cont.)
Trading off Capacity and Interference (cont.)
A sensible tradeoff rule is to maximize
Subject to:
(3.7)
Thus, we take the smallest N that satisfies Eq. (3.7) N 3 4 7 9 12
SIR (dB)
11.3
13.8
18.66
20.85
23.34
n = 4
Example: US AMPS cellular system requires SIR of at least 18 dB
Exercise: Refine the calculation ([Rappaport, Ch ] )
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

12. Frequency Reuse (cont.)
Cell Radius Selection
From Eq. (3.6), the selectable N above is independent of the cell radius R
The radius R is determined by other considerations
Base station cost drives to large R
Mobile battery power however, limits R by the constraint
(3.8) or
- Minimum received power at base station
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

13. Frequency Reuse (cont.)
The Micro-cell Concept
A smaller R generally implies
Lower transmitter power
Better area coverage
Higher capacity
These are the basic principles of micro-cells discussed below.
No. of cells per unit area however, is
Thus, halving R results in 4 times more cells
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

14. Frequency Reuse (cont.)
The Micro-cell Concept (cont.)
Traffic is not uniform in general and some spots need more allocated channels than others
One way to resolve it is to:
Add base stations at hot spots which transmit at a lower power
Without changing the frequency assignment to the macro-cells
N =3
3 micro-cells
Fig. 3.10
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

15. Frequency Reuse (cont.)
The Micro-cell Concept (cont.)
The transmit power at the micro-cells is determined by its radius and through the equation
(3.9)
, for some c
- transmission power in the macro-cell
- transmission power in the micro-cell
For k=2 and n=4 we have
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

16. Frequency Reuse (cont.)
The Micro-cell Concept (cont.)
Frequency planning with micro-cells is not obvious
In a ”3-cluster cell plan” we may embed ”6 micro-cell clusters”
N =3
Fig. 3.11: An oversimplified example
6 micro-cells with radii R/2
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

17. Frequency Reuse (cont.)
The Micro-cell Concept (cont.)
In a “6-cluster” cell plan” we may embed “6 micro-cell clusters”
N = 6
Fig. 3.12: An oversimplified example
6 micro-cells with radii R/2
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

18. Frequency Reuse (cont.)
Sectoring
Cell splitting into micro-cells increases capacity by rescaling
It decreases R while keeping D/R constant.
Hence packing more channels per unit area
“Sectoring” decreases D/R while keeping R constant
Base station uses several directional rather one omni-directional antenna, which increases the receiver’s SIR 1 1 6 2 2 3 5 3 4
Fig. 3.13: 3 sectors per cell
Fig. 3.14: 6 sectors per cell
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -

19. Frequency Reuse (cont.)
Sectoring
With 3 sectors per cell and a cluster size of 7, the number of 1st tier interfering co-channel cells reduces from 6 to 2, and with 6 sectors, to 1.
Fig. 3.14
Lecture 3: Frequency reuse for uniform traffic
Adv. Wireless Comm. Systems Cellular Networks -