Cellular Principles
Cellular Hierarchy
Coverage
Radii
Traffic
Cells
Mobile
speeds
Antennas
MEGA CELLS
MACRO CELLS
Large
Large
100 to 500 km
(the cell radius is a
function of satellite
Up to 35 km
altitude, power,
and antenna
aperture)
Low
Remote areas
(the cells move)
Low mobility as
well very highmobility
MICRO CELLS
Small
PICO CELLS
Small
Up to 1 km
Up to 50 m
Medium
Medium to high
Medium to
high
Outdoor cells
Outdoor cells
Up to 500 km/h
Up to 100 km/h
Directional,
mounted above the
rooftops on towers
Low-orbit satellites
or on the tops of
building.
Cellular Principles
Indoor cell
Up to 10
km/h
Mounted below the
rooftops on
lampposts or on
building walls
2
System Management
 Link Quality Measurement
 Forward and reverse links are continually monitored
 Parameters: received signal quality and the bit error
rates
 Cell Selection
 Choice of operator
 User preferences
 Available Networks
 MS capabilities
 Network capabilities
 MS mobility
 Service requirements
Cellular Principles
3
System Management
 Cell reselection
 Unsuitability of current cell due to interference or output
power requirements
 Radio link failure
 Network request
 Traffic load considerations
 User request
 Channel Selection/Assignment
 Channel assignment algorithms usually take into
account the following:
 System load
 Traffic patterns
 Service types
 Service priorities
 Interference situations
Cellular Principles
4
System Management
 Handover (Handoff)
 “The change of Physical Channel(s) involved in a call
whilst maintaining the call”
 Handovers may take place in several conditions:
 within the cell: Intracell handover
 between cells in the same cell layer: Intercell handover
 between cells of different layers: Interlayer handover
 between cells of different networks: Internetwork
handover
 Hard handover
 In FDMA and TDMA wireless network
 Soft-type handover
 Soft handover (boundary of the cell)
 Softer handover (boundary of the coverage area of the
sector)
 Soft-softer handover (both)
 In CDMA wireless network
Cellular Principles
5
System Management
 The following criteria may be used to initiate a
handover for radio transmission reasons:
 Signal strength measurements
 Signal-to-interference ratio
 Bit error rates
 Distance between MS and BS
 MS speed
 MS Mobility trends
 Others
Cellular Principles
6
System Management
 Mobility Support
 Logon-logoff
 Location Updating
Cellular Principles
7
System Performance
 Interference Control
 Diversity Strategies
 Diversity strategies are used to combat fading
 Space
 Frequency
 Time
 Variable Data Rate Control
 Direct support of variable data rates over the air
interface
 Variation of the number of bearer channel
 Packet access
Cellular Principles
8
System Performance
 Capacity Improvement Techniques
 Slow frequency hopping
 Dynamic power control
 Dynamic channel allocation
 Discontinuous transmission for voice, including voice
activity detection
 Nonvoice services
 Battery-Saving Techniques
 Output power control
 Discontinuous reception
 Discontinuous transmission
Cellular Principles
9
Cellular Reuse Pattern
 Co-cells: Cells using the same carrier frequency
 Cluster: A group of cells among which the
whole spectrum is shared and within which no
frequency reuse exists
 The number of cells per cluster defines the
reuse pattern and this is a function of the
cellular geometry
Cellular Principles
10
Macro cellular Reuse Pattern
 Circles x Regular Polygons (Equilateral
triangles, squares, and hexagons)
 Hexagonal cellular geometry
 Propagation symmetry
 Low-capacity systems
Cellular Principles
11
Macro cellular Reuse Pattern
v
(u1, v1)
u
D
(u2, v2)
R
3R
Cellular Principles
12
Macro cellular Reuse Pattern
 R = Cell radius
 d = The distance between the center of two
cells.
i  u2  u1
j  v2  v1
d 2  i 2  ij  j 2
 D = Reuse distance, that is, the distance
between two co-cells.
D 2  i 2  ij  j 2
 A =Area of the hexagonal cluster.
a = Area of
the hexagonal cell.
A
N   D2
a
 N = Reuse Factor (Number of cells per cluster)
N  i 2  ij  j 2
Cellular Principles
13
Macro cellular Reuse Pattern
Cellular Principles
14
Macrocellular Reuse Pattern
j
(1,2)
k
i
(1,2)
(1,2)
(1,2)
l
(1,2)
(1,2)
n
m
Cellular Principles
15
Macro cellular Reuse Pattern
 Co-channel Reuse Ratio
D
 3N
R
 The reuse ratio gives a qualitative measure of
the signal quality (carrier-to-interference ratio)
as a function of the cluster size.
 Positioning of the Co-Cells
 There are 6n co-cells on the nth tier
Cellular Principles
16
Micro cellular Reuse Pattern
 Square cellular geometry
 High traffic demand in dense urban regions
 Low mobility
 The propagation direction of the radio waves is
greatly influenced by the environment
 Inherent asymmetry
 A much greater number of BS
 The per-subscriber cost is determinant
 The interference is dependent not only on the
distance between transmitter and receiver but
also, and mainly, on the LOS
Cellular Principles
17
Micro cellular Reuse Pattern
v
(u2, v2)
D
(u1, v1)
u
 Reuse distance
R
2R
d 2  i2  j2
A  D2
 Reuse Factor (Number of Cells per Cluster)
 Reuse Ratio
N  i2  j2
D
 2N
R
Cellular Principles
18
Micro cellular Reuse Pattern
.
. . .
. . .
. . .
.
.
. .
. .
j
. (1,2)
. . . i
.
(-2,1)
. .
. . . .
. .
(2,-1)
.
.
. . .
. . .
.
.
.
. . .
. . . . .
. . .
.
(-1,-2)
Cellular Principles
19
Micro cellular Reuse Pattern
j
j
.
.
.
.
(-2,1)
. . .
.
. (1,2)
.
.
.
.
.
.
.
.
.
.
. . .
(-1,-2)
.
.
.
.
.
.
. (2,3)
. .
. .
. .
. (-3,2)
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. .
. .
. (-2,-3)
.
.
i
.
.
. .
(2,-1)
.
.
.
.
.
.
.
. .
(3,-2)
. . .
.
.
i
.
.
.
.
Cellular Principles
20
Interference in Narrowband (NB) and Wideband
(WB) Systems
 NB and WB systems are affected
differently by interference
 NB System:
 Interference is caused by a small number of high-power
signals
 There are different interference patterns between
Macrocellular and Microcellular networks
 Macrocellular systems:
 Uplinks and downlinks present approximately the same interference
performance (Note: regardless of the system, the uplink performance is always
worse)
 The larger the reuse pattern (N), the better the interference performance
 Microcellular systems:
 Interference Performance of uplinks and downlinks are very dissimilar
 In general, the larger the reuse pattern (N), the better the interference
performance
Cellular Principles
21
Interference in Narrowband (NB) and Wideband
(WB) Systems
 WB System:
 Interference is caused by a large number of low-power
signals
 Traffic profile and channel activity have great influence
on interference performance
 Uplinks and downlinks have different performances
 The interference performance analysis of a Cellular
System is performed in terms of:
 carrier-to-interference ratio (C/I)
 efficiency of frequency reuse (f)
Cellular Principles
22
Interference in Narrowband Macrocellular
Systems
 The propagation is characterized by an
NLOS (non line-of-sight) condition
 The Mean Power (P) received at a
distance (d) from the transmitter is:
P  Kd

 K is a proportionality constant that depends
on several parameters, such as: f, Base
Station (BS) antenna height and gain, Mobile
Station (MS) antenna height and gain,
environment, etc.
  is the propagation path loss coefficient and
usually ranges between 2 and 6
Cellular Principles
23
Interference in Narrowband Macrocellular
Systems
 Subsequent calculations assume that:
 K and  remain constant
 MS is positioned for the worst-case
condition, that is, at the border of the serving
cell (distance R from the BS)
 C/I ratio for the downlink is calculated at
the MS:
 C is the signal power received from the
serving BS
 I is the sum of the signal powers received
from the interfering BS’s (co-cells)
Cellular Principles
24
Interference in Narrowband Macrocellular
Systems
 C/I ratio for the uplink is calculated at
the BS:
 C is the signal power received from the
wanted MS
 I is the sum of the signal powers received from
the interfering MS’s (from the various cocells)
Macrocellular network:
 In this network, it is convenient to investigate
the effects of interference by using:
 omnidirectional antennas: 6n interferers for the nth
tier (all possible)
 directional antennas: reduction to  6n/s interferers,
where ´s´ is the number of sectors used in the cell
Cellular Principles
25
Interference in Narrowband Macrocellular
Systems
 Downlink Interference - Omnidirectional
Antenna
 For the worst-case condition, the MS is positioned
at a distance R from the BS. It is assumed that the
6n interfering BS’s in the nth ring are  at a distance
of nD. Therefore:
C

I
R 

 6n(nD)
n 1
D R 3N

C

I
( 3N )

 6(  1)
good
approximation
n 1
C ( 3N )

I
6
n 1
 (x) is the Riemann function: (1)=, (2)=2/6,
(3)=1.2021, and (4)=2/6.

( x)   n  x
n 1
Cellular Principles
26
Interference in Narrowband Macrocellular
Systems
 Consider  = 4 and N = 7:
 Exact C/I = 61.14 = 19.9 dB
 Approximate C/I = 73.5 = 18.7 dB
Uplink Interference - Omnidirectional
Antenna
 For the worst-case condition, the MS is positioned
at a distance R from the BS. It is assumed that the
6n interfering MS’s in the nth ring are  at a
distance of (nD - R), which is the closest distance
that the MS can be with respect to the interfered
BS. Therefore:
C

I
R 

D R 3N
 6n(nD  R) 

C 

  6n(n 3N  1)  
I  n 1


1
n 1
good
approximation
n 1
Cellular Principles
C ( 3N  1)

I
6
27
Interference in Narrowband Macrocellular
Systems
Consider  = 4 and N = 7:
 Exact C/I = 25.27 = 14.0 dB
 Approximate C/I = 27.45 = 14.38 dB
Downlink Interference - Directional Antenna
 Following the same procedure above:
n 1
C ( 3N ) s

I 6(  1)
C ( 3N ) s

I
6
 Consider  = 4, N = 7 and s = 3 (Three-sector cell):
 Exact C/I = 183.42 = 22.6 dB
 Approximate C/I = 220.5 = 23.4 dB
 Uplink Interference - Directional Antenna
C   6n

  (n 3N  1)  
I  n 1 s

1
n 1
Cellular Principles
C ( 3N  1) s

I
6
28
Interference in Narrowband Macrocellular
Systems
 Consider  = 4, N = 7 and s = 3 (Three-sector cell):
 Exact C/I = 75.81 = 18.8 dB
 Approximate C/I = 82.35 = 19.16 dB
 Examples:
 The table below gives some examples of C/I figures for  = 4 and
for several reuse patterns, with omnidirectional and directional
(1200 antennas, or three-sectored cells) antennas
Uplink (dB)
Downlink (dB)
N
Omni
Directional
Omni
Directional
3
4.0
8.7
10.5
15.3
4
7.5
12.3
13.0
17.7
7
14.0
18.7
17.9
22.7
9
16.7
21.5
20.0
24.7
12
19.8
24.5
22.5
27.3
Cellular Principles
29
Interference in Narrowband Macrocellular
Systems
 NOTE that the use of directional antennas
substantially improves the C/I ratio
 The choice of which antenna to use depends on
how tolerant the technology is with respect to
interference
 N = 7 and N = 4 are reuse patterns widely deployed
with 1200 antennas (they are referred as 7x21 and
4x12, respectively)
Cellular Principles
30
Interference in Narrowband Microcellular
Systems
 nL is the distance between the interferers
at the co-cell of the L-th layer and at the
target cell (reference) normalized with
respect to the cell radius. It is then given
in number of cell radii.
 nL is used to investigate the performance
of different microcellular reuse patterns
 nL is greatly dependent on the reuse
pattern (N).
 nL can be obtained by simple visual
inspection, but Appendix D shows a
general formulation for calculating it.
Cellular Principles
31
Interference in Narrowband Microcellular
Systems
 The subsequent performance analysis
considers a square cellular pattern with
BS’s positioned at every other
intersection of streets. Then, BS’s are
collinear and each micro cell covers a
square area comprising four 900 sectors,
each sector corresponding to half a
block, with the streets running on the
diagonals of this square.
 In Fig 2.7, the horizontal and vertical
lines correspond to the streets, and
diagonal lines represent the borders of
microcells
Cellular Principles
32
Interference in Narrowband Microcellular
Systems
 Figure 2.7
Cellular Principles
33
Interference in Narrowband Microcellular
Systems
 Figures 2.8 and 2.9 show the complete
tessellation for clusters with 5 (Fig 2.8),
8, 9, 10, and 13 (Fig 2.9) microcells, in
which the highlighted cluster
accommodates the target cell, and the
other dark cells correspond to the comicrocells that at certain time may
interfere with the BS or MS of interest
 In these Fig’s, stars indicate the sites
contributing to the C/I of the downlink,
whereas the circles indicate the worstcase location of the MS affecting the
performance of the uplink
Cellular Principles
34
Interference in Narrowband Microcellular
Systems
 Figures 2.8
D
C
E
A
C
B
C
B
D
B
D
D
C
C
B
D
C
B
D
D
C
B
C
C
B
B
D
B
D
E
A
E
C
B
B
D
A
A
C
A
B
E
D
E
C
D
A
E
D
A
B
B
E
C
D
A
E
A
C
C
B
B
E
E
C
D
A
E
C
B
B
D
A
E
C
A
B
D
A
E
C
B
A
E
C
A
B
D
A
E
C
A
B
E
D
E
C
D
A
E
D
A
B
B
E
C
D
A
E
A
D
B
B
E
C
C
D
A
E
C
B
B
E
A
E
C
D
A
E
C
A
B
D
A
E
C
D
A
E
A
E
C
A
D
A
E
B
D
D
D
A
E
A
E
C
B
A
Cellular Principles
35
Interference in Narrowband Microcellular
Systems
 Figures 2.9
(a)
Cellular Principles
36
Interference in Narrowband Microcellular
Systems
 Figures 2.9
(b )
(b )
Cellular Principles
37
Interference in Narrowband Microcellular
Systems
 Figures 2.9
(c)
Cellular Principles
38
Interference in Narrowband Microcellular
Systems
 Figures 2.9
(d)
Cellular Principles
39
Interference in Narrowband Microcellular
Systems
 Note that distinct situations can affect in
different ways the performance of the
downlink and the uplink
 In general, the set of micro cells
affecting the downlink is a subset of
those influencing the uplink
 Note that the staggered nature of some
patterns implies that the closest interferers
are either completely obstructed or
obstructed for most of the time with a
LOS interferer appearing many blocks
away
Cellular Principles
40
Interference in Narrowband Microcellular
Systems
 For clusters constituted by a prime
number of cells (Fig 2.8), the interfering
BS in the downlink changes as the target
MS moves along the street
 Propagation
 it is characterized by both LOS and NLOS modes
 For NLOS mode, the mean power received at
distance d from the transmitter is:
PNLOS  K NLOS d

 Note that this power strength is similar to that one
of macrocellular systems
 KNLOS is a proportionality constant that depends on
frequency, antenna heights, environment, etc
Cellular Principles
41
Interference in Narrowband Microcellular
Systems
 For LOS condition, and for a transmitting antenna
height ht, a receiving antenna height hr, and a
wavelength , the received mean power at distance
d is approximately:
PLOS
K LOS
 2
d
  d 
1   
  d B 
2



1
 KLOS is a proportionality constant and depends on
frequency, antenna heights, environment, etc
 dB is the breakpoint distance (4hthr/ )
 Note that LOS and NLOS propagation modes a
rather different
 For NLOS condition, the mean signal strength
decreases monotonically with the distance
Cellular Principles
42
Interference in Narrowband Microcellular
Systems
 For LOS condition and d < dB, the mean signal
strength decreases monotonically with a power law
close to the free space condition (  2). However,
for d > dB, the power law follows closely that of the
plane earth propagation (  4)
 For calculation purposes, it is defined r = d/R as the
distance of the serving BS to the MS normalized
with respect to the cell radius (0  r  1), and
k = R/dB as the ratio between the cell radius and the
breakpoint distance (K  0)
 It is interesting to investigate the C/I performance
as the mobile moves away from the serving BS
along the radial street. Note: this pattern is
different from the macrocellular one, whose
interference pattern is approximately maintained
throughout the cell
Cellular Principles
43
Interference in Narrowband Microcellular
Systems
 Uplink Interference
 By using PLOS for both wanted and interfering
signals:
C

I
1  (rk ) 
2 1

4r
2
L 1
 n [1  (nL K ) ]
L 1
2
L
2 1
good
approximation

C n12 1  (n1k ) 2
 2
I 4r 1  (rk ) 2



Downlink Interference
 Following the same procedure as the uplink
interference, C/I can be found. However, since this
ratio greatly depends on the position of the target
MS within the cell, three different interfering
conditions may be identified as MS moves along the
street: (1) at the vicinity of the serving BS, (2) away
from both the vicinity of the serving BS and the cell
border, and (3) near the cell border.
Cellular Principles
44
Interference in Narrowband Microcellular
Systems
 at the vicinity of the serving base station, more
specifically at the intersection of the streets (r 
normalized distance from the cell site to the
beginning of the block), the MS has a good radio
path to its serving BS, but it also has radio paths to
the interfering BS on both crossing streets. Then:
C

I
2


2 1
r 1  (rk )
 (n  r )  2 [1  ( n  r ) 2 k 2 ]1  (n  r )  2 [1  (n  r ) 2 k 2 ]1 
 L

L
L
L



2
2 1
2
2 2 1

 2(nL  r ) [1  (nL  r )k ]
L 1 

 Away from the vicinity of the serving BS and away
from the cell border, which correspond to most of
the paths, the MS enters the block and loses LOS to
those BS located on the perpendicular street ...
Cellular Principles
45
Interference in Narrowband Microcellular
Systems
 Then:
C

I
2

r 1  (rk )

2 1
 (n

L 1
2
2 2 1
2
2 2 1

r
)
[
1

(
n

r
)
k
]

(
n

r
)
[
1

(
n

r
)
k ]
L
L
L
L

 At the border of the cell, new interferers appear in
the LOS condition. However, this is not the case for
all reuse patterns. This phenomenon only happens
for clusters with a prime number of cells. For this
clusters, considering that the MS is away from its
serving BS (1- r  normalized distance from the site
to the beginning of the block) and r  1  r :
C

I
2


2 1
r 1  (rk )
 (n  r )  2 [1  ( n  r ) 2 k 2 ]1  (n  r )  2 [1  (n  r ) 2 k 2 ]1 
 L

L
L
L



2
2 1
2
2 2 1

 (nL  r ) [1  (nL  r )k ]
L 1 

Cellular Principles
46
Interference in Narrowband Microcellular
Systems
 A good approximation for the downlink C/I can be
obtained by simply considering L=1
 Examples
 C/I performance for clusters with 5, 8, 9, 10, 13
micro cells are illustrated. The performance has
been evaluated with the central micro cell as the
target cell and with the MS departing from the cell
center towards its edge (see arrow in Fig 2.8, which
also shows, in gray, the micro-cells that at certain
time may interfere with the wanted MS in a LOS
condition).
 For numerical results, the calculations considered:
R=100 m, street width of 15 m, ht=4 m, hr=1.5 m,
f=890 MHz ( = 3/8.9 m), and then, K=1.405 (note
that R is 40.5% greater than dB). The network was
considered to have an infinite number of cells (in
practice, 600 layers of interfering cells)
Cellular Principles
47
Interference in Narrowband Microcellular
Systems
 Figs 2.10 and 2.11 show, respectively, the uplink
and downlink performances for N = 5, 8, 9, 10, and
13 as a function of the normalized distance.
 In general, the larger the cluster, the better the C/I.
However, the five-micro-cell cluster exhibits a
remarkable behavior. Its uplink C/I curve coincides
with that for N=8 (lower curve in Fig 2.10), and its
downlink C/I curve coincides with that for N=10 for
most of the path extension (curve below the upper
curve in Fig 2.11). In the latter, the separation of
the curves occurs at the edge of the micro cell,
where 2 interferers appears in a LOS condition.
 Note also that in Fig 2.10, the C/I curves for N=9
and N=13 are also coincident
 Fig 2.12 compares the performance between 5- and
10- micro cell clusters.
Cellular Principles
48
Interference in Narrowband Microcellular
Systems
 Fig 2.12 shows how different the performances
between uplink and downlink are for an specific N,
and how they get progressively smaller and smaller
as N increases
 Fig 2.13 and 2.14 examine how the number of
interfering layers influences on both downlink and
uplink performance analyses for N=5- and N=10clusters, respectively. Both figures provide the
performances as functions of the normalized
distance to the BS using L=1 and L=
 Note that the difference between the C/I ratio for
an infinite-cell network and for a one-layer network
is NEGLIGIBLE! This conclusion also applies to the
other patterns, with the largest difference found in
similar analyses for all reuse patterns being less
than 0.35 dB
Cellular Principles
49
Interference in Narrowband Microcellular
Systems
 Therefore, very accurate estimates can be
achieved by only considering the closest layer to
the target cell
Cellular Principles
50
Interference in Narrowband Microcellular
Systems
 Figure 2.10
Uplink 5
Uplink 8
Uplink 9
70
Uplink 10
Uplink 13
60
Carrier/Interference [dB]
50
40
30
20
10
0,2
0,4
0,6
0,8
1,0
Normalized Distance from Site
Cellular Principles
51
Interference in Narrowband Microcellular
Systems
 Figure 2.11
Downlink
Downlink
Downlink
Downlink
Downlink
90
Carrier/Interference [dB]
80
70
5
8
9
10
13
60
50
40
30
20
10
0.2
0.4
0.6
0.8
1.0
Normalized Distance from Site
Cellular Principles
52
Interference in Narrowband Microcellular
Systems
 Figure 2.12
Uplink 5
Downlink 5
Uplink 10
Downlink 10
Carrier/Interference [dB]
70
60
50
40
30
20
10
0.2
0.4
0.6
0.8
1.0
Normalized Distance from Site
Cellular Principles
53
Interference in Narrowband Microcellular
Systems
 Figure 2.13
5 Cell Clusters
Uplink oo layers
Uplink 1 layer
Downlink oo layers
Downlink 1 layer
Carrier/Interference [dB]
70
60
50
40
30
20
10
0.2
0.4
0.6
0.8
1.0
Normalized Distance from Site
Cellular Principles
54
Interference in Narrowband Microcellular
Systems
 Figure 2.14
8 Cell Cluster
Carrier/Interference [dB]
60
Uplink oo layers
Uplink 1 layer
Downlink oo layers
Downlink 1 layer
50
40
30
20
10
0.2
0.4
0.6
0.8
1.0
Normalized Distance from Site
Cellular Principles
55
Interference in Wideband Systems
 Wideband systems operate with a unity
frequency reuse factor.
 The channelization is carried out by means of
codes sequences.
 In an ideal situation, with the use of orthogonal
code sequences and the orthogonality kept in
all circumstances, no interference occurs (the
efficiency of frequency reuse is 100%)
 But in real situations, the systems are led to
operate in an interference environment (the
efficiency of the reuse factor is less than 100%)
Cellular Principles
56
Interference in Wideband Systems
 The frequency reuse efficiency ƒ is defined as:
IS
f 
I S  IO
where IS is the total power of the signals within
the target cell and IO is the interference power
due to the signals of all the other cells.
 Let I= IO/ IS be the interference ratio. Thus,
1
f 
1 I
Cellular Principles
57
Interference in Wideband Systems
 Because within a system the traffic may vary
from cell to cell, the frequency reuse efficiency
can be defined per cell.
 For an N-cell system, let j be the target cell and
i the interfering cell. Therefore, for cell j, the
frequency reuse efficiency, ƒj , can be written
as:
Ij
f 
Ij 
N

i 1,i  j
Ii
Cellular Principles
58
Interference in Wideband Systems
 The interference conditions for the uplink and
for the downlink are rather dissimilar.
 The multipoint-to-point communication (reverse
link) operates asynchronously. In such a case,
the orthogonality of codes used to separate the
users is lost and all the users are potentially
interferers.
 The point-to-multipoint communication (forward
link) operates synchronously but because of the
multipath propagation, and if there is sufficient
delay spread in the radio channel, orthogonality
is partially lost and the target mobile receives
interference from other users within the same
cell.
Cellular Principles
59
Interference in Wideband Systems
 Uplink Interference
 Because of power control, the signals of all active mobile
users within a given cell arrive at the serving base
station with a constant and identical power (κ).
 The total power from the active users within a cell j is:
 
I J      AJ  d AJ
 where  A
is the traffic density (users per area) of cell
J
j, whose area is Aj.
 The interference condition in the reverse link:
desired
mobile station
ri , j
target cell
interfering
mobile station
ri ,i
interfering cell
Cellular Principles
60
Interference in Wideband Systems
 For any active user i, κ is the power at its serving base
station i.

 The power transmitted from the mobile station is r .
ii
 The power received at the base station j (interfering
 
power) is r r .
ii ij
 For all users in cell i the total interfering power at base
station j is


I i      Ai rii rij dAi
 Hence,
fj
  A  dA


 

A
r
r dA



j
N
i 1
i
j

ii ij
Cellular Principles
i
61
Interference in Wideband Systems
 The frequency reuse efficiency depends on both the
traffic distribution as well as on the propagation
conditions (path loss and fading).
 For uniform traffic distribution and for an infinite number
of cells, all cells present the same frequency reuse
efficiency.
 A common practice in cellular design is to use ƒ=0.6.
Cellular Principles
62
Interference in Wideband Systems
 Downlink Interference
 The constant-power situation, as experienced in the
reverse link, no longer applies.
 The interference is a function of the distance of the
mobile station to the interferers.
 The frequency reuse efficiency ƒj(x,y) is a function of the
mobile position variables (x,y).
 The interference condition in the forward link is
illustrated bellow:
desired
base station
ri ,i
ri , j
target cell
interfering
base station
interfering cell
Cellular Principles
63
Interference in Wideband Systems
 The mean frequency reuse efficiency is defined as:
1
f j  x, y  
Aj
 f  x, y  dxdy
j
 The own-cell interference at the mobile station depends
on the degree of orthogonality of the codes.
 For an ideal condition, no own-cell interference occurs
and the frequency reuse efficiency is 1.
 For a complete loss of orthogonality, the own-cell
interference reaches its maximum and the reuse
efficiency its minimum.
 A common practice in cellular design is to use ƒ=0.6.
Cellular Principles
64
Network Capacity
 A measure of network capacity can be provided
by the spectrum efficiency.
 The spectrum efficiency (η) is defined as the
number of simultaneous conversations per cell
(M) per assigned bandwidth (W).
 In cellular networks, efficiency is directly
affected by two type of technologies:
compression technology (CT) and access
technology (AT).
 CTs increase the spectrum efficiency by
packing signals into narrower-frequency bands,
e.g. low-bit-rate source coding and bandwidthefficient modulations.
Cellular Principles
65
Network Capacity
 ATs may be used to increase the spectrum
efficiency by providing the signals with a better
tolerance for interference, e.g., reuse factor and
digital signal processing techniques.
 Narrowband systems are less immune to
interference as compared to wideband systems,
so a reuse factor greater than 1 is necessarily
used, while wideband systems are
characterized by a reuse factor equal to 1.
 A loss in capacity occurs in wideband systems
because the frequency reuse efficiency is
usually substantially smaller than 1.
Cellular Principles
66
Network Capacity
 Narrowband systems are usually based on
FDMA or TDMA access technologies. Wideband
systems, in general, make use of CDMA access
technology.
 Narrowband systems
 The assigned bandwidth is split into a number of
subbands. The total time of each subband channel may
be further split into a number of slots.
 If C is the number of slots per subband times number of
subbands, the spectrum efficiency is given by:
M
C


W NW
Cellular Principles
67
Network Capacity
 The ratio C/W is a direct result of the CTs used.
 The reuse factor N is chosen such that it achieves the
signal-to-interference ratio required to meet transmission
quality specifications.
 Wideband Systems
 They are typically interference limited, with the
interference given by the number of active users within
the system.
 The total interference power It is defined as: It=IS+IO+IN,
where IN is the thermal noise power, IS is the power of the
signals within the target cell and IO the interference
power due to the signals of all the other cells.
 The number of active users, their geographic distribution,
and their channel activity affect the interference
conditions of the systems.
Cellular Principles
68
Network Capacity
 Define PN as the signal power required for an adequate
operation of the receiver in the absence of interference.
Let Pt be the signal power required for an adequate
operation of the receiver in the presence of interference.
The ratio NR between these two powers is known as
noise rise and is given as
NR 
Pt
I
 t
PN I N
 In the absence of interference, NR=1, i.e., the power
required for an adequate operation of the receiver is the
power required in the presence of the thermal noise.
 If we define the load factor ρ as

 we obtain
I S  IO
I S  IO  I N
NR 
1
1 
Cellular Principles
69
Network Capacity
10
Noise Rise (dB)
8
6
4
2
0
0.0
0.2
0.4
0.6
0.8
1.0
Traffic Load ( )
Cellular Principles
70
Network Capacity
 The condition ρ=0 signifies no active users within the
system. As ρ approaches unity the noise rise tends to
infinity, and the system reaches its pole capacity.
 A system is usually designed to operate with a loading
factor smaller than 1 (typically ρ 0.5,or equivalently 3dB
of noise rise).
 The load factor is calculated differently for the uplink and
for the downlink.
 Uplink Load Factor
 Let i = Ei / Ni be the ratio between the energy per bit and
the noise spectral density for user i. Define Gi = W / Ri as
the processing gain for user i. The energy per bit is
obtained as Ei = Pi Ti = Pi / Ri , where Pi , Ti and Ri = 1/ Ti
are, respectively, the signal power received from user i,
the bit period of user i, and the bit rate of user i. The
noise spectral density is calculated as
Ni = IN /W = (It – Pi ) / W.
Cellular Principles
71
Network Capacity
 For a channel activity equal to ai , 0  ai  1
i 
Ei
WPi
Gi Pi


N i ai Ri I t  Pi  ai I t  Pi 
 Solving for Pi ,
Pi  i I t , where

G 
 i  1  i 
 ai  i 
1
 Manipulating Equation 2.42, we obtain
  1  I 
IS
It
 The power IS can be calculated as
M
I S   Pi
i 1
Cellular Principles
72
Network Capacity
 The uplink load factor for a multirate wideband system is
M
M

i 1
i 1

   i  1  I  1 
Gi 

ai i 
1
 A load factor ρ =1 gives the pole capacity of the system.
 Typically, ai assumes the value 0.67 for speech and 1.0
for data; the value of I depends on the service, bit rate,
channel fading conditions, receive antenna diversity,
mobile speed, etc.; W depends on the channel bandwidth;
Ri depends on the service; and I can be taken as 0.55.
 Of course, other factors, such as power control efficiency
pi , and gain s (due to the use of s-sector directional
antennas) can be included in the capacity equation
above.
 The power control efficiency pi diminishes the capacity
by a factor of pi , whereas the use of sectored antennas
increases the capacity by a factor approximately equal to
the number s of sectors per cell.
Cellular Principles
73
Network Capacity
 For a classical all-voice network, such as the 2G CDMA
system, all M users share the same type of constant-bitrate service, In this case
  p sG
M
1  I  a  
psG
 1
 We have assumed the condition
a
 The spectrum efficiency is
M
  p sG

W 1  I  a  W
 Downlink Load Factor

 Because of the multipath propagation, and if there is
sufficient delay spread in the radio channel, orthogonality
(of the codes) is partially lost and the target mobile
receives interference from other users within the same
cell.
Cellular Principles
74
Network Capacity
 An orthogonality factor ti , 0  ti  1, can be added to
account for the loss of orthogonality: ti=0 signifies that
full orthogonality is kept; ti=1 signifies that orthogonality
is completely lost.
 The interference ratio depends on the user location
because the power received from the base stations is
sensed differently at the mobile station according to its
location.
 Following the same procedure as for the uplink case the
downlink location-dependent load factor ρ(x,y) is found to
be
ai i  ti  Ii 
  x, y   
Gi
i 1
M
where Ii is the interference ratio and (x,y) is the mobile
user coordinates.
Cellular Principles
75
Network Capacity
 For an average position within the cell, the average
downlink load factor is given as
ai i
  t  I  
i 1 Gi
M
 As for the orthogonality factor, this is typically 0.4 for
vehicular communication and 0.1 for pedestrian
communication.
 For a classical all-voice network, such as the 2G CDMA
system, all M users share the same type of constant-bitrate service and
M
  p s G
t  I   a  
Cellular Principles
76
Network Capacity
 The spectrum efficiency is
M
  p sG


W t  I   a   W
Cellular Principles
77
Summary
 Cellular systems are built upon the frequencyreuse principles.
 The service area is divided into cells and
portions of the available spectrum are
conveniently allocated to each cell.
 The number of cells per cluster defines the
reuse pattern, and this a function of the cellular
geometry.
 The macrocellular network makes use of highpower sites with antennas mounted high above
the rooftops.
Cellular Principles
78
Summary
 The macrocellular structure serves lowcapacity systems and is composed of the
hexagonal cell grid.
 In microcellular systems, with low power sites
and antennas mounted at street level, the
assumed propagation symmetry of the
macrocellular network no longer applies and
the hexagonal cell pattern does not make
sense.
 In the microcellular structure, the buildings
lining each side of the street work as
waveguides, in the radial direction, and as
obstructors, in the perpendicular direction.
Cellular Principles
79
Summary
 In this case, a cell is more likely to comply with
a diamond shape.
 A cellular hierarchy is structured that contains
several layers, each layer encompassing the
same type of cell in the hierarchy.
 The design of different cells depends on several
parameters such as mobility characteristics,
output power, and types of services utilized.
 Several aspects affect the performance of the
system: interference control, diversity
strategies, variable data rate control, capacity
improvement techniques, and battery-saving
techniques.
Cellular Principles
80
Summary
 Narrowband and wideband systems are
affected differently by interference.
 In narrowband systems, interference is caused
by a small number of high-power signals.
Macrocellular and microcellular networks
undergo different interference patterns.
 In macrocellular systems, uplink and downlink
present approximately the same interference
performance.
 In microcellular systems, the interference
performance of uplink and downlink is
dissimilar.
Cellular Principles
81
Summary
 For macrocellular systems, the larger the reuse
pattern, the better the interference
performance. For microcellular systems, it can
be said that, in general, the larger the reuse
pattern, the better the performance.
 In wideband systems, interference is caused by
a large number of low-power signals. The traffic
profile as well as the channel activity has a
great influence on the interference. Here again,
uplink and downlink perform differently.
 In narrowband systems, capacity is established
given the total amount of resources and the
reuse pattern.
Cellular Principles
82
Summary
 In wideband systems, the system capacity may
be influenced by a number of additional
parameters, such as the traffic profile, channel
activity, and others.
Cellular Principles
83