re use OF THE FREQUENCY FOR BETTER UTILIZATION

1. 3.2 FREQUENCY RE USE
The key characteristic of a cellular network is the ability to re-use frequencies
to increase both coverage and capacity.
AJAL.A.J
Assistant Professor –Dept of ECE,
Federal Institute of Science And Technology (FISAT) TM
MAIL: ec2reach@gmail.com

2. Cellular Concept
 The limited capacity of the first mobile radio-telephone services was related
to the spectrum used…not much sharing and a lot of bandwidth dedicated to
a single call.
– good coverage
– interference: impossible to reuse the same frequency
 The cellular concept addressed many of the shortcomings of the first mobile
telephones
– Frequency reuse
– Wasted spectrum allocated to a single user
 In 1968, Bell Labs proposed the cellular telephony concept to the FCC
 It was approved and then the work began!
– FCC allocated spectrum (took away TV UHF channels 70-83) in the
825-845 MHz and 870-890 MHz bands

3. Cellular Concept
 developed by Bell Labs 1960’s-70’s
 areas divided into cells
 a system approach, no major technological changes
 a few hundred meters in some cities, 10s km at country side
 each served by base station with lower power transmitter
 each gets portion of total number of channels
 neighboring cells assigned different groups of channels,
interference minimized
 hexagon geometry cell shape

4. What is a Cell?
• Cell is the Basic Union in The System
– defined as the area where radio coverage is given by
one base station.
• A cell has one or several frequencies, depending on
traffic load.
– Fundamental idea: Frequencies are reused, but not in
neighboring cells due to interference.

5. Cellular Concept
 Limited number of frequencies => limited channels
 Single high power antenna => limited number of users
 Smaller cells => frequency reuse possible => more number of users
 Base stations (BS): implement space division multiplex
– Each BS covers a certain transmission area (cell)
– Each BS is allocated a portion of the total number of channels available
– Cluster: group of nearby BSs that together use all available channels
 Mobile stations communicate only via the base station
– FDMA, TDMA, CDMA may be used within a cell
 As demand increases (more channels are needed)
– Number of base stations is increased
– Transmitter power is decreased correspondingly to avoid interference

6. Cellular Concept
 Cell size:
– 100 m in cities to 35 km on the country side (GSM)
– even less for higher frequencies
– Umbrella cell: large cell that includes several smaller cells
• Avoid frequent handoffs for fast moving traffic
 Cell shape:
– Hexagonal is useful for theoretical analysis
– Practical footprint (radio coverage area) is amorphous
 BS placement:
– Center-excited cell: BS near center of cell
• omni-directional antenna
– Edge-excited cell: BSs on three of the six cell vertices
• sectored directional antennas

7. Cellular Concept
 Advantages:
– higher capacity, higher number of users
– less transmission power needed
– more robust, decentralized
– base station deals with interference, transmission area etc. locally
 Problems:
– fixed network needed for the base stations
– handover (changing from one cell to another) necessary
– interference with other cells: co-channel, adjacent-channel
 Important Issues:
– Cell sizing
– Frequency reuse planning
– Channel allocation strategies
Bottom line: Attempt to maximize availability of channels in an area

8. • Cells labeled with the same letter use the same
group of channels.
• Cell Cluster: group of N cells using complete
set of available channels
• Many base stations, lower power, and shorter
towers
• Small coverage areas called “cells”
• Each cell allocated a % of the total number of
available channels
• Nearby (adjacent) cells assigned different
channel groups
– to prevent interference between neighboring
base stations and mobile users

9. Cell characteristics
• Implements space division multiplex: base
station covers a certain transmission area
(cell)
• Mobile stations communicate only via the
base station
• Advantages of cell structures:
– higher capacity, higher number of users
– less transmission power needed
– more robust, decentralized
– base station deals with interference, transmission area etc. locally
• Problems:
– fixed network needed for the base stations
– handover (changing from one cell to another) necessary
– interference with other cells
• Cell sizes from some 100 m in cities to, e.g., 35 km
on the country side (GSM) - even less for higher
frequencies

10. Shape of Cells
 Square
 Width d cell has four neighbors at distance d and four at
distance 2d
 Better if all adjacent antennas equidistant
 Simplifies choosing and switching to new antenna
 Hexagon
 Provides equidistant antennas
 Radius defined as radius of circum-circle
 Distance from center to vertex equals length of side
 Distance between centers of cells radius R is 3 R
 Not always precise hexagons
 Topographical limitations
 Local signal propagation conditions
 Location of antennas

11. Cellular Geometries

12. Cell Footprint

13. Cellular Network Architecture
Mobile Public
Switching
Center
Telephone
network
and Internet
Mobile
Switching
Center
Wired network

14. Frequency Reuse
 Adjacent cells assigned different frequencies to avoid
interference or crosstalk
 Objective is to reuse frequency in nearby cells
– 10 to 50 frequencies assigned to each cell
– transmission power controlled to limit power at that frequency
escaping to adjacent cells
– the issue is to determine how many cells must intervene between
two cells using the same frequency

15. Frequency Reuse
 each cell allocated a group k channels
– a cluster has N cells with unique and disjoint channel
 groups, N typically 4, 7, 12
 total number of duplex channels S = kN
 Cluster repeated M times in a system
 Total number of channels that can be used (capacity)
– C = MkN = MS
 Smaller cells  higher M  higher C
+ Channel reuse  higher capacity
+ Lower power requirements for mobiles
– Additional base stations required
– More frequent handoffs
– Greater chance of ‘hot spots’

16. Frequency planning
f3 f3 f3
f2 f2
f1 f1 f1
f3 f3 3 cell cluster
f2 f2 f2
f1 f1
f3 f3 f3
f2 f3 f7
f5 f2
f4 f6 f5
7 cell cluster f1 f4
f3 f7 f1
f2 f3
f6 f5 f2
f2 f2 f2
f1 f f1 f f1 f
h h
3 cell cluster
3 3 3
h 2 h 2
g2 1 h3 g2 1 h3 g2
g1 g1 g1
g3 g3 g3 with 3 sector antennas

17. Figure Frequency reuse patterns

18. Frequency planning
• Frequency reuse only with a certain
distance between the base stations
• Standard model using 7 frequencies:
f3
f5 f2
f4 f6 f5
f1 f4
f3 f7 f1
f2

19. FREQUENCY REUSE
• The allotted frequency spectrum for mobile
communication
• during the 1 G is in the range of  (800- 900 )
Mhz
• but ,Using this small bandwidth, thousands of
subscribers need to be served over a large
area.
• To manage this situation, a technique called
‘’frequency reuse ‘’ is adopted.

20. FREQUENCY REUSE

22. Cellular Networks
• Propagation models represent cell as a circular area
• Approximate cell coverage with a hexagon - allows easier
• analysis
• Frequency assignment of F MHz for the system
• The multiple access techniques translates F to T traffic channels
• Cluster of cells K = group of adjacent cells which use all of the systems
frequency assignment

24. Frequency reuse concept
ALPHABETICAL
REPRESENTATION

25. Wrong implementation

26. END Result of Wrong implementation

27. Correct implementation

31. Freq reuse Advantage
Cluster3
Freq reuse: Several cells in
coverage area that use same set
of frequencies
Cochannel cells: Those cells
Cluster1 using same freqs
Cochannel interference: The
mobile in cell A also gets
signals from other Co channel
cells
Cluster2
31

32. Cellular Concepts
In this case
N=19 (i=3, j=2)

33. Frequency Reuse
 Power of base transceiver controlled
 Allow communications within cell on given frequency
 Limit escaping power to adjacent cells
 Allow re-use of frequencies in nearby cells
 Use same frequency for multiple conversations
 10 – 50 frequencies per cell
 E.g.
 The pattern consists of N cells
 K total number of frequencies used in systems
 Each cell has K/N frequencies
 Advanced Mobile Phone Service (AMPS) K=395, N=7 giving 57
frequencies per cell on average

34. Characterizing Frequency
Reuse distance between centers of cells that use the same
D = minimum
band of frequencies (called cochannels)
 R = radius of a cell
 d = distance between centers of adjacent cells (d = R)
 N = number of cells in repetitious pattern
 Reuse factor
 Each cell in pattern uses unique band of frequencies
 Hexagonal cell pattern, following values of N possible
 N = I2 + J2 + (I x J), I, J = 0, 1, 2, 3, …
 Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, …
 D/R= 3N
 D/d = N

35. Distance
R
D
R
35

36. R
3 D
3 6 1
6 1 4
4 7 2
7 2 5 3
5 3 6 1
3 6 1 4
6 1 4 7 2
4 7 2 5
7 2 5
5
36

37. Example of frequency reuse factor
or pattern 1/4
The frequency reuse factor is the rate at which the same frequency can
be used in the network. It is 1/K (or K according to some books) where
K is the number of cells which cannot use the same frequencies for
transmission. Common values for the frequency reuse factor are
1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation).

38. Reuse Ratio : q
Assuming hexagonal shape cells of equal size
D
= q = 3N
R
where:
D: Distance between the centres of cells
R: Radius of the cell
q: Reuse ratio
N: Cluster size
38

39. Example
For N = 7 and R 5 km
D = 3N R
D = 3× 7 × 5
D = 4.583 × 5 = 22.91
The minimum distance at which the same frequency can
be reused is approximately 4.6 times R, which is in this
case 22.91 km
39

40. Frequency
Reuse
Patterns

41. N=7, 32 cells, R=1.6km, in total 336
channels

42. So what is FREQUENCY REUSE ?
• After allotting the available total bandwidth
among a set of cells , the same frequency
band will be used in another set of cells .
• This kind of reuse can be adopted until the
entire area to be covered is exhausted .
• Note
• Seven cells in a set is the most frequently
used configuration.this configuration
usually operates with a cell diameter of 1-
3 km range

43. Cellular architecture
One low power transmitter per cell
B Frequency reuse–limited spectrum
A
Cell splitting to increase capacity
Reuse distance: minimum distance between
two cells using same channel for satisfactory
signal to noise ratio
Measured in # of cells in between

44. Reuse distance 2 – reuse pattern
One frequency can be (re)used in all cells of the same color

45. Reuse distance 3 – reuse pattern

46. Signal-to-interference ratio S/I or SIR

47. Capacity calculation—FDMA
n: capacity (number of total users)
m: number of cells to cover the area
N: frequency reuse factor (# cells/cluster)
B: bandwidth per user
W: total available bandwidth (spectrum)
mW
n=
N B 50

48. Problem
In an FDMA system calculate
frequency re use factor ?
• m= 70
• W= 100
• n=10
• B=100
• N = ? cells/cluster

49. Capacity calculation—FDMA
In the previous example,
• m=20,
• W=25 MHz,
• N=4, and
• B=30 KHz.
m W 20 25000
n= = = 4,166
N B 4 30
52

50. Capacity calculation—TDMA
• n: capacity (number of total users)
• m: number of cells to cover the area
• N: frequency reuse factor (# cells/cluster)
• B: bandwidth per user
• W: total available bandwidth (spectrum)
• Nu: number of time slots per carrier
mW
n= Nu
N B 53

51. Capacity calculation—TDMA
(contd.)
Assuming again,
• m=20,
• W=25 MHz,
• N=4,
• B=200 KHz,
• Nu=4.
mW 20 25000
n= Nu = 4 = 2,500
N B 4 200
54

52. Capacity of CDMA
n: number of users
W: total bandwidth
R: data rate
Sr: signal to noise ratio
W
n=
R × Sr
55

53. Capacity per cell (CDMA)
Assume:
W=1.25MHz=1,250,000 Hz
R=9600 bps
Sr should be larger than 3dB => 2 times
W 1250000
n= = = 65 users
R × S r 9600 × 2
56

54. System architecture
• A set of seven cells will be controlled by a
BSC  base station controller
• The terminal station located in every cell
will have access to the BSC.
• 7 base terminal stations will have one
BSC
• For every frequency reuse , 1 BSC is
required.
The elements that determine frequency reuse are
the reuse distance and the reuse factor

55. Scenario
• For 100 BTS we require more than 14
BSC’s
• All BSC are connected to a Mobile
Switching centre.
• Here mobile stations are connected to
BTS by wireless means
• All Base stations are connected to their
respective BSC’sby cables.
• In modern communication , fibre optic
cables are used to connect BTS

56. @ 3G
Fig: Advanced cellular mobile
communication system Architecture

57. Cell Planning (1/3)
• The K factor and Frequency Re-Use Distance
7
6 2
K = i2 + ij + j2 1
K = 22 + 2*1 + 12 5 3
j
K=4+2+1 7 R
2
K=7 6
1 i
D
5 3
4
D= √3K * R
D = 4.58R
Frequency re-use distance is based on the cluster size K
The cluster size is specified in terms of the offset of the center of a cluster from the
center of the adjacent cluster

58. Cell Planning (2/3)
7-cell reuse
pattern
A1
A3
G1 A2 B1
A1 G3 B3
A3
A2 B1 G2 C1 B2
G1 B3 C3
G3 C1 B2 F1 C2
G2 F3 D1
C3 D3
C2 D1 F2 E1 D2
F1 D3 E3
F3 D2 E2
F2 E1
E3 Frequency
E2
reuse

59. Cell Planning (3/3)
• Cell sectoring
– Directional antennas
subdivide cell into 3
or 6 sectors
– Might also increase
cell capacity by factor
of 3 or 6
• Cell splitting
– Decrease
transmission power in
base and mobile
– Results in more and
smaller cells
– Reuse frequencies in
non-contiguous cell
groups
– Example: ½ cell
radius leads 4 fold
capacity increase

60. BTS
• BTS acts as an interface between the
Mobile unit & BSC
• BTS connects mobile units by wireless
means to the BSC
BTS Antenna
• BTS has got an antenna usually at an elevation location.
• Invariantly , Roof tops,
• Small steel towerare used for erecting the BTS antenna.

61. Omni directional coverage= coverage in all
directions
• Compact power supply systems (some even
chargeable batteries) are used at the base
terminal stations.
• BTS is usually located at the centre of the cell
area for omni directional coverage.
INDIAN ANALOGY

62. Bandwidth vs mobility
• The wireless interface at BTS and wired
topologytopology between BTS- BSC & BSC
– MSC necessitates certain translation in
protocols.
• Upto the stage of BTS , air interface protocols
are used. After BTS , the transmission media
happen to be wired one , mostly fibre optic
communication.
• Hence at the BTS or BSC stage , there is a
need for the translation of these protocols

63. Illustration – of translation
requirement
• In mobile communication at the mobile
user’s unit the speech signal is connected
into 13 kbps digitized voice. This
conversion is is required to have an air
interface with bandwidth efficiency.
• But the backbone network through which
the voice has to travel in certain
applications can provide 64 kbps.
Contd…

64. Contd…
• PCM digitization is adopted in fixed
telephone network which make use of
these backbone network.
• When mobile unit converts the analog
information into 13kbps digital
information , the BSC converts the same
into 64 kbps information.

65. Frequency reuse revisited: cluster size and reuse distance
Cluster with cluster
size N
D
frequency group
A
Co-channel cells frequency group
B
[ 68 ]

66. The geometry of a hexagonal cell
Unit scale is distance between
neighboring cell centers.
For cell radius
To find the distance from
the origin, , of point ,
convert axis:
[ 69 ]

67. The geometry of a hexagonal cell [continue]
– So,
– Using this equation to locate co-channel cells, we start from a
reference cell and move i hexagons along the u-axis and then j
hexagons along the v-axis.
– The distance between co-channel cells in adjacent clusters is
– The number of cells in a cluster, N, is hence
since i and j can only take integer values.
– The frequency reuse factor , Q, is defined by
[ 70 ]

68. Frequency Reuse Factor
 Effective reuse of resources can highly enhance
the system capacity
 Frequency reuse factor (FRF) K defines frequency
reuse pattern
 With a smaller frequency reuse factor (FRF), more
available bandwidth can be obtained by each cell

69. Previous Frequency Reuse
Schemes
 With the usage of FRF-1, the most user terminals (UTs) are
afflicted with heavy Inter-cell interference (ICI)
 Especially near the cell edge
 The conventional method to figure out this problem is by
increasing the FRF
 mitigate the ICI efficiently
 but decrease on available bandwidth
 The most representative approaches improving cell-edge
performance while retaining spectrum efficiency
 Soft Frequency Reuse (SFR) scheme
 Incremental Frequency Reuse (IFR) scheme

70. FREQUENCY REUSE SCHEME
Soft Frequency Reuse (SFR) Scheme
Incremental Frequency Reuse (IFR) Scheme
Enhanced Fractional Frequency Reuse (EFFR)

71. Soft frequency Reuse
CCU: cell-centre users
CEU: cell-edge users

72. Soft Frequency Reuse (SFR)
Scheme CCU: cell-centre users
CEU: cell-edge users

73. Limitations of SFR
 How to define the borderline to divide cell area for
CCUs and CEUs is a key issue
 Generally, there are more CEUs than CCUs in a cell
 since the outer surface area is much larger than the inner part
 CEUs have maximum one third of the entire bandwidth to utilize,
which results in lower spectrum efficiency
 More ICI could happen even in a low traffic-load
situation, while there are still subchannels in idle and
underutilized in the system
 The resource allocation via the SFR scheme starts always from
the first subchannel up

74. Incremental Frequency Reuse
(IFR) Scheme (1)
 The only difference between the IFR design and
the classical reuse-1
 Classical reuse-1: allocate resources always from
the first subchannel
 IFR: start dispensing resources from different points

75. Incremental Frequency Reuse
(IFR) Scheme (2)
 IFR scheme can overcome the low spectrum reuse
efficiency problem and the more ICI at low
loading traffic problem
 IFRscheme only performs better when just
fewer traffic exists in a system
 When the loading factor is greater than 0.3, it is
inferior to the SFR scheme

76. Enhanced Fractional Frequency
Reuse (EFFR) (1) HYBRIDS
 Enhanced Fractional Frequency Reuse (EFFR) scheme
intends to retain the advantages of the both approaches
 Concept
 Define 3 cell types for directly contiguous cells in a cellular system
 Reserves for each cell-type a part of the whole frequency band named
Primary Segment
 The Primary Segments among different type cells should be
orthogonal
 The Primary Segment of each cell will be further divided into
 reuse-3 part: cannot be reused by directly neighboring cells
 reuse-1 part: is at the same time a part of the Secondary Segments
belonging to the other two cell-types

77. Enhanced Fractional Frequency
Reuse (EFFR) (2)

78. Power Allocation and SINR
Estimation
 Transmission Power Allocation
 Any cell-type is not allowed to use the reuse-3 subchannels
dedicated to the other two cell types
 The power allotted to the reuse-3 subchannels can be tripled
 Signal-to-Interference-Ratio (SINR) Estimation
 A cell acts on the Secondary Segment as a guest, and occupying
secondary subchannels is actually reuse the primary subchannels
belonging to the directly adjacent cells
 Reuse on the Secondary Segment should conform to two rules:
 monitor before use
 resource reuse based on SINR estimation
 Each cell listens on every secondary subchannel all the time
 Before occupation, a cell makes SINR evaluation and chooses resources
with best estimation values for reuse
 If all available secondary resources are either occupied or not good
enough to a link, this cell will give up scheduling resources

79. Worst Case Interference
 S/I ~ R-4 /[2(D-R)-4 + 2(D+R)-4 + 2D-4]

82. Frequency Reuse @ CDMA
In CDMA reuse patterns are not required.
Subscriber in every cell can use the same frequency
at the same time. Subscriber is discriminated from
another by the assignment of a unique code to every
conversation.
In GSM freq. Reuse pattern of 7 is used.

83. Problems
– Propagation path loss for signal power: quadratic or higher in distance
– fixed network needed for the base stations
– handover (changing from one cell to another) necessary
– interference with other cells:
• Co-channel interference:
Transmission on same frequency
• Adjacent channel interference:
Transmission on close frequencies

84. Solution: Topology of Different Areas
20
20 20
40
20
100 60 60 60
20
100
100
20
60 100
100
Town 20
20
Suburb
Highway 20
Rural

85. Q UERRIES ?