1. Fundamentals of Cellular Networks (Part IV)
Wireless Networks
Lecture 14
Fundamentals of Cellular Networks (Part IV)
Dr. Ghalib A. Shah

2. Outlines Trunking and Grade of Service Improving Coverage and Capacity
Measuring Traffic Intensity
Trunked Systems
Blocked Calls Cleared
Blocked Calls Delayed
Erlang Charts
Improving Coverage and Capacity
Cell Splitting
Sectoring
Repeaters for Range Extension
Microcell Zone Concept

3. Last lecture review Interference and system capacity
Co-channel interference and capacity
Adjacent channel interference and capacity
Channel Planning for Wireless System

4. Trunking
Allows a large number of users to share a small number of channels
Channel allocated per call basis from a pool of available channels
Relies on statistical behavior of users so that a fixed number of channels (circuits) may accommodate a large random user community
Trunking theory is used to determine number of channels for particular area (users)
Tradeoff between the number of available channels and likelihood of call blocking during peak calling hours

5. Trunking Theory
Developed by Erlang, Danish Mathematician, how a large population can be accommodated by a limited number of servers, in late 19th century
Today, used to measure traffic intensity
1 Erlang represents the amount of traffic intensity carried by a completely occupied channel
i.e. one call-hour per hour or one call-minute per minute
0.5 Erlang: Radio channel occupied 30 minutes during 1 hour

6. Grade of Service
GOS is a benchmark used to define performance of a particular trunked system
Measure of the ability of a user to access trunked system during the busiest hour.
Busy hour is based on the demands in an hour during a week, month or year.
Typically occur during rush hours between 4 pm to 6 pm.
GOS is typically given as likelihood of call blocking or delay experienced greater than certain queue time

7. Traffic intensity
Traffic intensity is measured as call request rate multiplied by call holding time
User traffic intensity of Au Erlang is
(1) Au= λH
Where H is average call duration or holding time and λ is average number of call requests.
For system of U users and unspecified channels, the total offered traffic intensity A is
(2) A = UAu
In a C channel trunked system, traffic equally distributed, traffic intensity per channel Ac is
(3) Ac= UAu/C

8. In Erlang, max possible carried traffic is the number of channels C
Note that traffic is not necessarily the carried traffic but offered to the trunked system
If offered load increases the system capacity, the carried traffic becomes limited
In Erlang, max possible carried traffic is the number of channels C
AMPS is designed for a GOS of 2% blocking
i.e. 2 out of 100 calls will be blocked due to channel occupancy
There are two types of commonly used trunked systems
Blocked Calls Cleared
Blocked Calls Delayed

9. Block Calls Cleared
User is given immediate request if a channel is available.
If no channel available, the requesting user is blocked and free to try later
Assume call arrivals as Poisson Distribution
the Erlang B formula determines the probability that call is blocked with no queuing, is a measure of GOS for trunked system

10. Capacity (Erlangs) For GOS
Erlang B Trunking GOS
Capacity of an Erlang B System
Number of Channels C
Capacity (Erlangs) For GOS
= 0.01
= 0.005
= 0.002
= 0.001 2
0.153
0.105
0.065
0.046 4
0.869
0.701
0.535
0.439 5
1.36
1.13
0.900
0.762 10
4.46
3.96
3.43
3.09 20
12.0
11.1
10.1
9.41 24
15.3
14.2
13.0
12.2 40
29.0
27.3
25.7
24.5 70
56.1
53.7
51.0
49.2
100
84.1
80.9
77.4
75.2
B Erlang is more conservative approach

11. Erlang B
Fig. 2.8

12. Block Calls Delayed Queue is provided to hold blocked calls.
Call request may be delayed until a channel becomes available
Its measure of GOS is defined as the probability that a call is blocked after waiting specific length of time in the queue
The likelihood of a call not having immediate access is determined by Erlang C formula

13. Erlang C
Fig. 2.9

14. if no channels are available immediately, the call is delayed, probability that call is forced to wait more than t seconds is
Average delay D in all calls in queued system is

15. Trunking Efficiency
A measure of the number of users which can be offered a particular GOS with particular configuration of channels
The way channels are grouped can alter the number of users handled
For example, From table
10 trunked channels at GOS of 0.01 can support 4.46 Erlang of traffic
Whereas 2 groups of 5 channels can support 2x1.36=2.72 Erlangs of traffic, 60% lesser

16. Improving Coverage and Capacity
As demand increases, number of channels per cell become insufficient
Cellular design techniques needed to provide more channels per unit coverage area
Various techniques developed to expand the capacity of system
Cell splitting
Sectoring
Micro cell zone concept

17. Cell Splitting
Achieve capacity improvement by decreasing R and keeping D/R (cell reuse ratio) unchanged
Divide the congested cells into smaller cells
Smaller cells are called micro cells
If radius of cell is cut to half, approximately four cells would be required
Increased number of cells would increase the number of clusters, which in turn increase the capacity
Allows a system to grow by replacing larger cells with smaller cells without upsetting the allocation scheme

19. For new cells to be smaller in size, tx power must be reduced
For new cells to be smaller in size, tx power must be reduced. By which factor?
If n = 4 then the received powers equal to each other becomes
Power must be reduced by 12 dB in order to maintain S/I requirements

20. Thus low speed and high speed users can simultaneously handled
Channels in old cell must be broken down into two groups corresponding to smaller and larger cells
At beginning of cell splitting, fewer channels to smaller power groups.
As demand grows, more channels will be required and thus more micro cells
In the end, the whole system will be replaced with micro cells

21. Sectoring Keep cell radius unchanged and decrease D/R
Increases SIR so that cluster size may be reduced
SIR is improved using directional antennas
Hence increasing frequency reuse without changing transmission power
Cell is partitioned into 3 120o sectors or 6 60o sectors as shown in Fig

24. Instead of interference from 6 cells, only 2 sectors interfere
thus S/I can be found to be 24.2 dB, where it is 17 dB in worst case presented before
This S/I improvements allow designers to decrease cluster size N and hence enhances capacity
Drawbacks
Increased number of handoffs

25. Microcell Zone Concept
A cell is divided into zones with a single BS sharing the same radio equipment
Zones are connected through coaxial cable, fiber optics or microwave links to the BS
Superior to sectoring since antennas are placed at outer edges of the cells and any channel may be assigned to any zone by BS
As mobile travels from one zone to other, it retains same channel, BS simply switches the channel to a different zone.
Co-channel interference is minimized becuase
Large BS is replaced by several low powered tx
Improves S/I