Wireless cellular networks divide geographic areas into smaller sections called cells to improve capacity and coverage. Each cell uses a subset of available frequencies and is served by a base station. As users move between cells, their active connections are handed off between base stations through a process managed by the mobile switching center. Cell sizes and the frequency reuse plan must be optimized to balance capacity, coverage, and interference between cells using the same frequencies.

1. Wireless Communications
The Cellular Concept
Transferring knowledge to future leaders
Reference:
Professor Johnson I Agbinya
(University of the Western Cape)

2. Single Cell ‘Network’
2

3. History of Cellular Networks
Why cellular networks?
To address requirement for greater capacity
For efficient use of frequency
To address the poor quality of non cellular mobile
networks and increases coverage
– replaces a large transmitter with smaller ones in cells
– smaller transmitting power
– each cell serves a small geographical service area
– each cell is assigned a portion of the total frequency
3

4. Description of a Cell
Approximated to be a hexagonal coverage
– best approximation of a circular area
Served by a base station
– low powered transceiver
– antenna system and a
– a mast
– may be divided into 6 equilateral triangles
– length of base of each triangle = 0.5R (radius)
– different groups of channels assigned to base stations
4
R
R 0.87R
2
3 »

5. Mathematical Description of a Cell
Area 6x R x R R R cell = = =
5
Area of a cell is:
2
Perimeter of a cell = 6R
2
2
2.598
3 3
2
2
3
Capacity of network = N x number of users per cell
N = number of cells in network
– we assumed cells of equal capacity - can differ in practice

6. Typical Cellular Network
(For a Sparsely Populated Country)
6

7. Multi-Cellular Network
Densely Populated City
7

8. Definitions
Definition of ‘channel’
depends on the system
– frequency (radio station)
– frequency band (TV transmissions)
– time slot of a frequency (GSM, GPRS)
– orthogonal code (CDMA networks)
8

9. Structure of a Mobile Communication
System
9

10. Types of Mobile Communication Cells
The size of a cell is dictated by capacity demand
10
Macrocell
– large, covering a wide area
– range of several hundred kilometres (km) to ten km
– mostly deployed in rural and sparsely populated areas
Microcell
– medium cell, coverage area smaller than in macro cells
– range of several hundred metres to a couple of metres
– deployed mostly in crowded areas, stadiums, shopping malls

11. Types of Mobile Communication Cells (1)
The size of a cell is dictated by capacity demand
11
Picocell
– small, covering a very small area
– range of several tens of metres
– low power antennas
– can be mounted on walls or ceilings
– used in densely populated areas, offices, lifts, tunnels etc

12. Means of Increasing Cells Capacity
There are several approaches for increasing
cellular system capacity including:
12
Cell clustering
Sectoring of cells
Cell splitting
Frequency reuse
Reduction of adjacent cell interference and co-channel
interference

13. Cell Structure
13
F1 F2 F3
F1 F2
F3
F3
F2
F4
F1
F1
F2
F3
F5 F4
F6
F7
(a) Line Structure (b) Plan Structure
Note: Fx is set of frequency, i.e., frequency group.

14. Cell Clusters
Service areas are normally divided into clusters
of cells to facilitate system design and increased
capacity
14
Definition
a group of cells in which each cell is assigned a
different frequency
– cell clusters may contain any number of cells, but clusters
of 3, 4, 5, 7 and 9 cells are very popular in practice

15. Cell Clusters
15
A cluster of 7 cells
2
1
5
7
6
4
3
the pattern of cluster is repeated throughout the
network
channels are reused within clusters
cell clusters are used in frequency planning for
the network
Coverage area of cluster called a ‘footprint’

16. Cell Clusters (1)
A network of cell clusters in Cape Town
7
2
16
2
1
5
6
4
3
1
5
7
6
4
3
2
1
5
7
6
4
3
2
1
5
7
6
4
3
2
1
5
7
6
4
7
2
7
1 3
5
6
4
2
1
3
5
6
4
3

17. Frequency Plan
Intelligent allocation of frequencies used
– Each base station is allocated a group of channels to be
used within its geographical area of coverage called a
‘cell’
Adjacent cell base stations are assigned completely
different channel groups to their neighbors
base stations antennas designed to provide just the
cell coverage, so frequency reuse is possible
17

18. Frequency Reuse Concept
Assign to each cluster a group of radio channels to
be used within its geographical footprint
ensure this group of frequencies is completely different
from that assigned to neighbors of the cells
Therefore this group of frequencies can be reused in
a cell cluster ‘far away’ from this one
Cells with the same number have the same sets of
frequencies
18

19. Frequency Reuse Factor
19
Definition
When each cell in a cluster of N cells uses one of N
frequencies, the frequency reuse factor is 1/N
Frequency reuse is an example of space diversity
multiplex access
frequency reuse limits adjacent cell interference
because cells using same frequencies are separated
far from each other

20. Factors Affecting Frequency Reuse
Factors affecting frequency reuse include:
Types of antenna used
– omni-directional or sectored
placement of base stations
coverage for distance (highways) vs area (city)
capacity through macro and micro overlays
20

21. Excitation of Cells
Once a frequency reuse plan is agreed upon overlay the
frequency reuse plan on the coverage map and assign
frequencies
The location of the base station within the cell is referred
to as cell excitation
In hexagonal cells, base stations transmitters are either:
– centre-excited, base station is at the centre of the cell or
– edge-excited, base station at 3 of the 6 cell vertices
21

22. Frequency Assignment Plan
22
Example
S duplex channels
S channels divided into N cells
Each cell allocated a group of k channels
Total # available channels S = k N
Replicate a cluster M times in the network
Total number of duplex channels, C=MkN=MS

23. Characterization of Frequency Reuse
23
F1
F2
F3
F5 F4
F6
F7
F1
F2
F3
F5 F4
F6
F7
F1
Reuse distance D
• For hexagonal cells, the reuse
distance is given by
D = 3NR
R
where R is cell radius and N is the
reuse pattern (the cluster size or the
number of cells per cluster).
N
• Reuse factor is
q = D = 3
R
Cluster

24. Characterization of Frequency Reuse
 The cluster size or the number of cells per cluster is given by
24
N = i2 + ij + j2
where i and j are integers.
i
j
60o

25. Finding the Nearest Co-Channel
(1) Move i cells along any chain of hexagons
(2) Turn 600 counter-clockwise and move j cells, to
reach the next cell using same frequency sets
this distance D is required for a given frequency reuse
to provide enough reduced same channel interference
i.e. after every distance D we could reuse a set of
frequencies in a new cell
25

26. Channel Assignment Strategies
Three assignment approaches
Fixed and static (most common)
26
Dynamic
Hybrid
Fixed channel assignment
all channels in a cell could be in use all the time
– new calls are then blocked (no channels left)
– may be solved by borrowing spare channels from
nearby cells

27. Channel Assignment Strategies (1)
Dynamic channel assignment
MSC allocates frequencies when a call is made
Provides high channel utilization
To do this it needs real-time information on
channel occupancy
traffic distribution and
radio signal strength indication (RSSI)
high computational load and increased storage
27

28. Cell Splitting
Often large cells need to be split into smaller ones
because the population of users in the big cell has
increased beyond what it can support
Cell splitting increases system capacity
Is used in high density subscriber areas (e.g. Water Front)
Results to increased costs (e.g.. new base stations)
28

29. A Split Cell
29

30. Handover (Handoff)
Provides continuity of communication across cells
30
Difficulty
dropping a call before reconnecting is unacceptable
different cells use different frequencies
mobile phone users usually move from place to place
and very quickly too
therefore the current location of the mobile phone must
somehow be known and kept

31. Handover Process
Parties in communication share the same channels
Received signal weakens as mobile moves out of cell
Cell site at some point requests handover to cell with
stronger signal strength
MSC switches call to new cell after allocating channels, and
informing the two mobiles of the new channels (voice and
control channels)
31

32. Handover Process (1)
32
BS1
BS2
BS3
PSTN MSC Trunks

33. Handover Process (2)
Handover must not be too frequent or
system is kept busy servicing handover requests
handover threshold is set with the above in mind
Minimum usable signal level is normally set to be between
-90 dBm and -100 dBm
33

34. Setting Handover Thresholds
34

35. Choosing Handover Margins
35
Handover margin D
D = P r handover - Pr minimum usable
If D is too large unnecessary handover will occur, burdening
the MSC
If D is too small, there maybe insufficient time to complete
the handover before a call is lost due to weak signals
Therefore D is chosen carefully

36. Interference Sources in Mobile Networks
Interference comes from many sources:
36
Multipath
Mobile phones in the same cell
Other users in neighboring cells
Environmental effects
base stations operating at the same frequencies
Radiation leakage from other sources into the
mobile communication band

37. Types of Channel Interference
Interference is either system or environment related.
System capacity as affected by interference related
to the frequency channel used by a cellular system
is treated in this lecture. The critical ones are:
Co-channel interference
Adjacent channel interference (ACI)
Near end Far end interference
MAI
37
Inter cell interference
Intra cell interference
Adjacent channel interference
Co-channel interference

38. Capacity and Co-Channel Interference
38
Definition
Interference from co-channel cells is called co-channel
interference. Such cells use identical frequencies or
channels
– Independent of transmitted power for cells of same size
– Is a function of the radius (R) of cell and distance (D) to co-channel
cell
– Cannot be overcome by increased transmission power
– Overcome by separating such cells by a required minimum
distance - by increasing the ratio Q = D/R, where Q is the
co-channel reuse ratio

39. Capacity and Co-Channel Interference (1)
Desired Signal
39
For hexagonal cells:
N
Q = D = 3
R
i
0
å=
=
1
i
I i
SIR
where SIR=signal to interference ratio and
Ii is the interference from ith co-channel cell

40. Capacity and Co-Channel Interference (2)
For 6 interfering cells, carrier to interference ratio C/I is
C I = {R} -a {6D-a }
If they are equidistant from the cell of interest then
C I = 1/{6(D R) -a }
RF power decreases with inverse power of distance
Decay of received power with distance is given as:
where, d0 is reference distance, P0 is Prec at d0 and a is the path
loss exponent and lies between 2 and 5 for urban areas
40

41. Effects of Q on Capacity
A small value of Q is desirable for larger capacity. This
means a small value of N (small clusters)
A large value of Q improves transmission quality due to
small values of co-channel interference
In practice the value of Q is a trade off between capacity
and transmission quality
41

42. Relationship Between Q and SIR
n
å= -
S
n is the path loss exponent
Assume all interfering base stations are equidistant (D)
from desired base station, then
S n n
N
D R
( / ) ( 3 )
i
r = =
For a hexagonal-shaped cell, i0 = 6, D = Di, and Q = D/R
S = Q =æ
S
n
42
-
=
0
1
i
i
n
i
r
D
R
I
0 0
i
I
n
r Q
I
6
r
ö I
çè
1
6 ÷ø

43. Distance between co-channel cells
43
D
D-R/2
D-R
D-R
D+R
D+R/2

44. Distance between co-channel cells (1)
Assume a reuse ratio of 7 and cells are equidistant
from the cell in centre. The separations between the
6 outer cells and the inner one are approx. D+R, D- R,
D, D - R/2 and D+ R/2, and CI is approximately:
R
-
= =
å0 2 2 2
( ) n ( ) n n
n
R
S
Let n = 4 and Q=D/R, then
1
=
I Q Q Q
44
n
i
i
n
i
r
D R D R D
D
I
- - -
=
-
-
- + + +
1
2( - 1) - 4 + 2( + 1) - 4 + 2 -
4
Sr

45. Distance between co-channel cells (3)
Assume in this case the base station is centre-excited
The all 6 other co-channel interfering cells are D metres
away from it, and
D2 = 3R2 (i2 + ij + j 2 )
Area of first-tier circle is
Area of circle surrounding inner cell at centre is
Ratio of areas is equal to the number of cells that can be fitted into the
first-tier of cells and is:
i ij j D
A
l e 3 3 2
– Omnidirectional cells interfere with all their co-channel
neighbors
– interference is reduced by sectoring
45
( 2 ) 2 ( 2 2 )
arg A k D 3R i ij j l e = = + +
A k(R2 ) small =
( ) N
R
A
small
2
arg = 2 + + 2 = =

46. Distance between co-channel cells (4)
Consider base stations use sectored antennas, i.e. they are edge-excited
Assume N=7 and antenna sectors are 120 degrees wide, the worst
case carrier to interference ration is;
i.e.
= -4 -4 21 0.7 21
46
D
and
or C/I = 24.5 dB
{( ) 4 4 }
4
0.7 - -
-
+ +
=
D R D
C I R
= Q = 3x7 = 21
R
( ) ( ) þ ý ü
î í ì
+ +
C I 1

47. Adjacent Channel Interference (ACI)
Interference resulting from signals that are adjacent in
frequency to the desired signal is called adjacent
channel interference
ACI is caused by imperfect receiver filters that allow
radiation to leak out into the passband of adjacent cells
é
For omni-directional cells: æ
ACI Log d
Filter isolation
47
d
n
ù
ö
i +
c
ú ú
û
ê ê
ë
÷ ÷ø
ç çè
= -10

48. How to Reduce Adjacent Channel
Interference
ACI is a function of the distance between the
two cells under consideration
ACI can be minimized through:
careful channel assignment
48
filtering
using isolation filters is not the best solution
Worst case ACI occurs when one of the mobiles is
close to the base station and the other is at the edge
of the cell

49. Near End Far End Interference
– In the uplink, signals are attenuated differently
because they take different paths
Signals to and from mobiles nearest to the base
station are stronger than signals from mobiles
located much farther away
– In the downlink however, mobiles at the cell edge
experience larger degradation and interference
compared to mobiles close to the base station
49
A
d1
d0
B

50. Solution to Near Far Effect
The Mobiles experience greater interference from own
base station compared to from far base stations
Mobiles close to base stations therefore cause more
interference particularly in terms of ACI
Power control is used to mitigate “near far effect”, by
equalizing the power received by all mobiles
50
Absence of Power Control
Received
Power at
BS
MS1
MS2
MS3
With Power Control
Received
Power at
BS
MS1
MS2
MS3

51. Traffic Engineering
Problems with Connecting Phones with Switches
Many switches required - to connect n phones
together, s = (n-1)*n/2 switches are required
slow connection speeds
too many regular faults
high maintenance costs and cost of switches
51

52. Traffic Engineering – Design Objectives
52
Traffic
System Capacity
Quality of Service

53. Traffic Engineering - Considerations
Design for flexibility and account for low
and high traffic periods
peak traffic period occur sometimes in the
mornings and afternoons. Low traffic weekends
high traffic usually 10 to 20% of total capacity,
all users need not be directly connected
cellular systems depend on trunking to connect
a large number of users
53

54. Trunking
In a trunked radio system, each user is allocated a
channel on a per call basis, and on termination of
call, previously occupied channel is immediately
returned to the pool of available channels
Therefore a large number of users share a small
pool of channels in a cell on a per call basis
Access is provided to each user on demand
When all channels are in use, a new user or
demand is (denied) blocked
54

55. Unit of Traffic - Erlang
The unit of telephone traffic intensity is called the
Erlang, in honor of a Danish mathematician
Definition: One Erlang is one channel occupied
continuously for one hour. In data communications,
an 1 E = 64 kbps
55

56. How To Estimate Telephone Traffic
A h u = l
56
Definitions, let
Au Erlangs be traffic intensity generated by each user
h be average duration of a call (hour)
l is the average number of call requests per hour. Then
For a system containing U users, the total offered traffic intensity
A is
A =UA
u In a trunked system of C channels, the traffic intensity per
A = UA /
C channel is
e u

57. Busy Hour
The traffic level is an average, taken over several days,
and over the busiest period. The period is usually 1
Hour, and average over that hour is called “Busy
Hour” traffic.
Example: if the circuit is said to carry 0.6 Erlangs, it will
be busy, an average, for 0.6 hours (36 min) during the
busy hour.
57

58. Grade of Service (GoS)
A measure of the performance of a telephone system
GOS is a measure of the ability of a user to access a
trunked system during the busiest hour
Also an indication of the user not being able to secure a
channel during the busiest hour
Telephone networks are designed with specified GOS,
usually for the busiest hour. If a subscriber is able to
make a call during the busiest hour, he will be able to
make a call at any other time
58

59. Grade of Service (1)
59
Definition
GOS is the probability of having a call blocked during the
busiest hour. For example, if GOS = 0.05, one call in 20
will be blocked during the busiest hour because of
insufficient capacity
GOS is used to determine the number of channels required;
GOS could be determined by
– competition between operators (measure of good service)
– regulation - a national communication authority might
decide to impose a grade of service on its operators

60. Types of Trunked Systems
Two types of trunked systems are used
(a) blocked calls cleared (Erlang B, M/M/m queue)
(b) blocked calls delayed (Erlang C formula)
Characteristics of Blocked calls Cleared Model
Call arrival rate = Poisson (exponential) distribution
Infinite number of users
Memoryless, channel requests at any time
infinite number of channels in pool
60

61. Traffic Intensity Models
Three traffic intensity model tables are used in
practice
Erlang B tables (blocked calls cleared); can over estimate
Engset formula (probability of blocking in low density
areas); used where Erlang B model fails
Erlang C tables (blocked calls delayed or held in queue
indefinitely)
Poisson tables (blocked calls held in queue for a limited
time only)
61

62. Erlang B Formula
Determines the probability that a call is blocked
Is a measure of the GOS for trunked systems with blocked
calls cleared
[ ] !
Erlang B formula: GOS
62
A
k
A
C
P blocking C
k
k
C
r = =
å=
0 !

63. Using the Erlang B Table
The objective is to determine the number of trunks
required for a given Erlang value and a blockage
level. Three steps are required:
Locate the column with the desired blockage level;
While staying in the same column, find the row with
the desired Erlang value (round off the Erlang value
as necessary);
Find the number of trunks in the selected row (at the
intersection);
63

64. Planning for Cell Capacity
Assume that in a telephone network the call arrival rate is l calls
per hour and the mean holding time for a call is tn (hours per call).
Example: There are 100 subscribers with the following telephone traffic
profile: 20 make 1 call/hour for 6 minutes; 20 make 3 calls/hour for
half a minute; 60 make 1 call/hour for 1 minute. The traffic they
generate is:
64
20x1x (6/60) = 2 E
20x3x(0.5/60) = 0.5 E
60x1x(1/60) = 1 E
i.e. a total of 3.5 E. On average, each subscriber generates 35 mE.
In practice on average telephone subscribers generate between
25 to 35 mE during the busiest hour

65. Planning for Cell Capacity
Example: Use the Erlang B table to compute the number of channels
required for a cell when the expected number of calls per hour is 3000,
blocking probability of 2% and the average length of a call is 1.8
minutes.
Solution: The offered traffic for this case is A = qxT/60 = 3000x1.8/60
= 90 Erlangs. Erlang B table indicates that 103 channels
are required.
65

66. Questions and Answers
66