Capacity Design, Traffic Engineering and Sectoring





Read pages 178-182 Course Pack supplementary Notes on System Design
Call Blocking: Call Blocking occurs when all channels are in use and the call
cannot be completed. If this happens too often, the customer will be very unhappy
and switch to another Service provider.
Call Dropping: Call Dropping occurs when a mobile phone moves from one cell
to another that has all its channels occupied. Which means that the hand-off
cannot be completed and the call is dropped. This will make the customer
extremely angry. It could be also very embarrassing because the customer may
think the other party has hung up the phone.
Trunking Theory : Attempts to study the traffic pattern and using statistical
analysis, suggests ways to optimize channel capacity and predict Blocking
Pattern.
Erlang Theory: A.K Erlang, a Swedish Engineer studied this problem and using
Statistical analysis found that
(a) More channels, less Blocking for a given amount of traffic. This is obvious,
however, what is not obvious is that, he also found that
(b) With more channels, the amount of possible traffic increases for a given blocking
probability. This is called Trunking Gain (discuss the example of Bank Teller)
 The Phone traffic is defined in Erlang (E), which is a dimensionless number. One
Erlang = one continuous phone call of a fixed duration say 60 minutes (one hour)
 Thus if 1000 customer use the phone 10% of the time, they generate 100E of
traffic.
 The traffic generated by each user in Erlang = E = H, where  = average
number of calls generated by the customer and H is the average duration of the
call.
 If there are a total of N users, the total traffic generated will be ET = NE = NH.
 Furthermore, if there are C channels, and assuming that the traffic is equally
distributed among the channels, then the traffic intensity per channel is ETC =
ET/C
 Note that the traffic generated is not necessarily the traffic carried by the
Network.
 From this equation it is clear that this traffic analysis does not consider the
random nature of real situations, such as
(a) A customer may use the phone continuously for an hour and then nothing for the
rest of the day
(b) Lots of people using the phone at the same time (rush hours or peak hours traffic
usually occurs between 4PM and 6PM.).
(c) Some business people use phone continuously during business hours and then in
the evening hour very little phone use
(d) Some use it mainly for emergencies.
(e) Evening and weekend usage are much lower (that’s why rates are lower in these
hours
Hyder Khoja
Page 1
2/15/2016










What is the most obvious way to avoid blocking? _________________?
Ans:Increase the number of channels
However, the number of channels are fixed, for example, for 12 cells system, the
# of available channels /Cell = 395/12 = 33
How many channels are available /cell for a 7 Cell system? _________
Cellular system operators consider 2% blocking as acceptable.
GOS is defined as the measure of the ability of a user to access a trunked system
during the busiest hour. It is specified as the probability that a call will be
blocked.
Table 2.4 and Fig 2.6 Rappaport as well as Table 10.4 (P392 B) shows traffic
levels in E for different # of channels with various blocking probabilities.
Notice from the table and graphs that for a given blocking probability, as the # of
channel increases the Trunking Efficiency(Total Traffic in E/# of channels
Required) _________ .
Also, as the Blocking Probability (GOS) decreases, the traffic intensity
__________ for the same # of channels.
The Trunking Efficiency is a measure of the number of users that can be
supported with a particular GOS and a particular configuration of a fixed number
of channels. From table 2.4 it is clear that a group of 10 trunked channels at a
GOS of 0.01 can support higher level of traffic (4.46E) as compared to two
groups of 5 channels (21.36=2.72E). Thus 10-channel system can support 60%
more traffic. Thus, the way channels are grouped has a profound effect on the
overall system capacity.
Example 1: A cellular system uses 12-cell pattern with a total of 120 Cell. And
20,000 customer. Each customer uses phone on an average 30 minutes /day, but
on evenings 10% of those minutes are used during peak hour. Calculate
(a) Average and peak traffic in E for the whole system
(b) The average and peak traffic/ell, assuming callers are evenly distributed over the
system
(c) The approximate average call blocking probability
(d) The approximate call blocking probability during peak hour.
Hyder Khoja
Page 2
2/15/2016

Example 2: How many users can be supported for 0.5% blocking probability for
the following number of trunked channels. Assume each user generates 0.1 E of
traffic. (a) 1 (b) 5 (c) 10 (d) 20 (e) 50 (f) 100.. Use Traffic intensity graph
or Table from your Course pack

Example 3: An Urban area has a population of 2 million. Three competing
trunked mobile Networks ( Such as Bell Mobily, Telus and Rogers provide
service in Metro Toronto) provide service in this area. Assume bell mobility has
394 cells with 19 channels each, Telus has 98 cells with 57 channels each, Rogers
has 49 cells each with 100 channels (these numbers are fictitious). Find the
numbers of users that can be supported with 2% blocking if each user averages 2
calls per hour at an average duration of 3 minutes. Assuming all 3 are operated at
maximum capacity, calculate the percentage market share of each.
Hyder Khoja
Page 3
2/15/2016

Assignment: A certain city has an area of 1300 KM and is covered by a
Cellular system using a 7-cell reuse pattern. Each cell has a radius of 4 Km
and the city has allocated 40 MHz of spectrum with a FDX channel BW of
60KHz. Assuming a GOS of 2% for an Erlang B system and the offered
traffic per user of 0.03 E, calculate (a) The Number of cells in the service
area, (b) The number of channels per cell, (c) traffic Intensity of each cell, (d)
the maximum carried traffic, (e) the total number of users that can be
supported for 2% GOS, (f) the number of mobiles per channel, (g) the
theoretical maximum number of users that can be served at one time by this
system
Hyder Khoja
Page 4
2/15/2016
Increasing Capacity

Two additional techniques used for increasing capacity are
(a) Cell Splitting
(b) Sectoring
Cell Splitting:


Cell splitting is the process of subdividing a cell into smaller cells, each with its
own Base Station and a corresponding reduction in antenna height and transmitted
power.
P169 Course pack (take a slide), also p55, 58,59 R
Sectoring:




Sectoring is the process of dividing a cell into smaller sectors (usually 3 or 6) and
using directional antennas to concentrate the transmitted power to only those
sectors.
This reduces co-channel interference and thus increases capacity.
Normally 120 degree or 60 degree sectoring is used as shown in the fig.
P171 Course Pack and P60 R
Modulation Schemes:
Analog Modulation

Define and explain the processes of AM, FM and PM. Show waveforms
Digital Modulations:



P 50, 323, 324,333,337,338 Hioki
ASK, FSK, PSK, DPSK, QAM
Channel BW and Capacity calculations, P113-124 Blake
Hyder Khoja
Page 5
2/15/2016