2. Cellular Principles 2
Cellular Hierarchy
MEGA CELLS MACRO CELLS MICRO CELLS PICO CELLS
Coverage Large Large Small Small
Radii
100 to 500 km
(the cell radius is a
function of satellite
altitude, power,
and antenna
aperture)
Up to 35 km Up to 1 km Up to 50 m
Traffic Low Medium Medium to high
Medium to
high
Cells
Remote areas
(the cells move)
Outdoor cells Outdoor cells Indoor cell
Mobile
speeds
Low mobility as
well very high-
mobility
Up to 500 km/h Up to 100 km/h
Up to 10
km/h
Antennas Low-orbit satellites
Directional,
mounted above the
rooftops on towers
or on the tops of
building.
Mounted below the
rooftops on
lampposts or on
building walls
3. Cellular Principles 3
System Management
īą Link Quality Measurement
īą Forward and reverse links are continually monitored
īą Parameters: received signal quality and the bit error
rates
īą Cell Selection
īą Choice of operator
īąUser preferences
īąAvailable Networks
īąMS capabilities
īąNetwork capabilities
īąMS mobility
īąService requirements
4. Cellular Principles 4
System Management
īą Cell reselection
īąUnsuitability of current cell due to interference or output
power requirements
īąRadio link failure
īąNetwork request
īąTraffic load considerations
īąUser request
īą Channel Selection/Assignment
īą Channel assignment algorithms usually take into
account the following:
īąSystem load
īąTraffic patterns
īąService types
īąService priorities
īąInterference situations
5. Cellular Principles 5
System Management
īą Handover (Handoff)
īą âThe change of Physical Channel(s) involved in a call
whilst maintaining the callâ
īą Handovers may take place in several conditions:
īąwithin the cell: Intracell handover
īąbetween cells in the same cell layer: Intercell handover
īąbetween cells of different layers: Interlayer handover
īąbetween cells of different networks: Internetwork
handover
īą Hard handover
īąIn FDMA and TDMA wireless network
īą Soft-type handover
īąSoft handover (boundary of the cell)
īąSofter handover (boundary of the coverage area of the
sector)
īąSoft-softer handover (both)
īąIn CDMA wireless network
6. Cellular Principles 6
System Management
īą The following criteria may be used to initiate a
handover for radio transmission reasons:
īąSignal strength measurements
īąSignal-to-interference ratio
īąBit error rates
īąDistance between MS and BS
īąMS speed
īąMS Mobility trends
īąOthers
8. Cellular Principles 8
System Performance
īą Interference Control
īą Diversity Strategies
īą Diversity strategies are used to combat fading
īąSpace
īąFrequency
īąTime
īą Variable Data Rate Control
īą Direct support of variable data rates over the air
interface
īą Variation of the number of bearer channel
īą Packet access
9. Cellular Principles 9
System Performance
īą Capacity Improvement Techniques
īą Slow frequency hopping
īą Dynamic power control
īą Dynamic channel allocation
īą Discontinuous transmission for voice, including voice
activity detection
īą Nonvoice services
īą Battery-Saving Techniques
īą Output power control
īą Discontinuous reception
īą Discontinuous transmission
10. Cellular Principles 10
Cellular Reuse Pattern
īą Co-cells: Cells using the same carrier frequency
īą Cluster: A group of cells among which the
whole spectrum is shared and within which no
frequency reuse exists
īą The number of cells per cluster defines the
reuse pattern and this is a function of the
cellular geometry
11. Cellular Principles 11
Macro cellular Reuse Pattern
īą Circles x Regular Polygons (Equilateral
triangles, squares, and hexagons)
īą Hexagonal cellular geometry
īą Propagation symmetry
īą Low-capacity systems
13. Cellular Principles 13
Macro cellular Reuse Pattern
īą R = Cell radius
īą d = The distance between the center of two
cells.
īą D = Reuse distance, that is, the distance
between two co-cells.
īą A =Area of the hexagonal cluster.
īąa = Area of the hexagonal cell.
īą N = Reuse Factor (Number of cells per cluster)
2
2
2
j
ij
i
d īĢ
īĢ
īŊ 1
2 u
u
i ī
īŊ 1
2 v
v
j ī
īŊ
2
2
j
ij
i
N īĢ
īĢ
īŊ
2
2
2
j
ij
i
D īĢ
īĢ
īŊ
2
D
a
A
N īŊ
īŊ
16. Cellular Principles 16
Macro cellular Reuse Pattern
īą Co-channel Reuse Ratio
īą The reuse ratio gives a qualitative measure of
the signal quality (carrier-to-interference ratio)
as a function of the cluster size.
īą Positioning of the Co-Cells
īą There are 6n co-cells on the nth tier
N
R
D
3
īŊ
17. Cellular Principles 17
Micro cellular Reuse Pattern
īą Square cellular geometry
īą High traffic demand in dense urban regions
īą Low mobility
īą The propagation direction of the radio waves is
greatly influenced by the environment
īą Inherent asymmetry
īą A much greater number of BS
īą The per-subscriber cost is determinant
īą The interference is dependent not only on the
distance between transmitter and receiver but
also, and mainly, on the LOS
18. Cellular Principles 18
Micro cellular Reuse Pattern
D
u
v
R
2
R
(u1, v1)
(u2, v2)
īą Reuse distance
īą Reuse Factor (Number of Cells per Cluster)
īą Reuse Ratio
2
2
2
j
i
d īĢ
īŊ 2
D
A īŊ
2
2
j
i
N īĢ
īŊ
N
R
D
2
īŊ
21. Cellular Principles 21
Interference in Narrowband (NB) and Wideband
(WB) Systems
īą NB and WB systems are affected
differently by interference
īą NB System:
īą Interference is caused by a small number of high-power
signals
īą There are different interference patterns between
Macrocellular and Microcellular networks
īą Macrocellular systems:
īą Uplinks and downlinks present approximately the same interference
performance (Note: regardless of the system, the uplink performance is always
worse)
īą The larger the reuse pattern (N), the better the interference performance
īą Microcellular systems:
īą Interference Performance of uplinks and downlinks are very dissimilar
īą In general, the larger the reuse pattern (N), the better the interference
performance
22. Cellular Principles 22
Interference in Narrowband (NB) and Wideband
(WB) Systems
īą WB System:
īą Interference is caused by a large number of low-power
signals
īą Traffic profile and channel activity have great influence
on interference performance
īą Uplinks and downlinks have different performances
īą The interference performance analysis of a Cellular
System is performed in terms of:
īą carrier-to-interference ratio (C/I)
īą efficiency of frequency reuse (f)
23. Cellular Principles 23
Interference in Narrowband Macrocellular
Systems
īą The propagation is characterized by an
NLOS (non line-of-sight) condition
īą The Mean Power (P) received at a
distance (d) from the transmitter is:
īą 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.
īą īĄ is the propagation path loss coefficient and
usually ranges between 2 and 6
īĄ
ī
īŊ Kd
P
24. Cellular Principles 24
Interference in Narrowband Macrocellular
Systems
īą Subsequent calculations assume that:
īą K and īĄ remain constant
īą MS is positioned for the worst-case
condition, that is, at the border of the serving
cell (distance R from the BS)
īą C/I ratio for the downlink is calculated at
the MS:
īą C is the signal power received from the
serving BS
īą I is the sum of the signal powers received
from the interfering BSâs (co-cells)
25. Cellular Principles 25
Interference in Narrowband Macrocellular
Systems
īą C/I ratio for the uplink is calculated at
the BS:
īą C is the signal power received from the
wanted MS
īą I is the sum of the signal powers received from
the interfering MSâs (from the various co-
cells)
īąMacrocellular network:
īą In this network, it is convenient to investigate
the effects of interference by using:
īą omnidirectional antennas: 6n interferers for the nth
tier (all possible)
īą directional antennas: reduction to īģ 6n/s interferers,
where ´s´ is the number of sectors used in the cell
26. Cellular Principles 26
īą Downlink Interference - Omnidirectional
Antenna
īą For the worst-case condition, the MS is positioned
at a distance R from the BS. It is assumed that the
6n interfering BSâs in the nth ring are īģ at a distance
of nD. Therefore:
īą ī(x) is the Riemann function: ī(1)=īĨ, ī(2)=ī°2/6,
ī(3)=1.2021, and ī(4)=ī°2/6.
īĨ
īĨ
īŊ
ī
īŊ
ī
1
)
(
n
x
n
x
Interference in Narrowband Macrocellular
Systems
īĨ
īĨ
īŊ
ī
ī
īŊ
1
)
(
6
n
nD
n
R
I
C
īĄ
īĄ N
R
D 3
īŊ
īĨ
īĨ
īŊ
ī
ī
īŊ
1
)
1
(
6
)
3
(
n
N
I
C
īĄ
īĄ
6
)
3
( īĄ
N
I
C
īŊ
1
īŊ
n
good
approximation
27. Cellular Principles 27
īą Consider īĄ = 4 and N = 7:
īą Exact C/I = 61.14 = 19.9 dB
īą Approximate C/I = 73.5 = 18.7 dB
īąUplink Interference - Omnidirectional
Antenna
īą For the worst-case condition, the MS is positioned
at a distance R from the BS. It is assumed that the
6n interfering MSâs in the nth ring are īģ at a
distance of (nD - R), which is the closest distance
that the MS can be with respect to the interfered
BS. Therefore:
Interference in Narrowband Macrocellular
Systems
īĨ
īĨ
īŊ
ī
ī
ī
īŊ
1
)
(
6
n
R
nD
n
R
I
C
īĄ
īĄ N
R
D 3
īŊ
1
1
)
1
3
(
6
ī
īĨ
īŊ
ī
īē
īģ
īš
īĒ
īĢ
īŠ
ī
īŊ īĨ
n
N
n
n
I
C īĄ
6
)
1
3
( īĄ
ī
īŊ
N
I
C
1
īŊ
n
good
approximation
28. Cellular Principles 28
īąConsider īĄ = 4 and N = 7:
īą Exact C/I = 25.27 = 14.0 dB
īą Approximate C/I = 27.45 = 14.38 dB
īąDownlink Interference - Directional Antenna
īą Following the same procedure above:
īą Consider īĄ = 4, N = 7 and s = 3 (Three-sector cell):
īą Exact C/I = 183.42 = 22.6 dB
īą Approximate C/I = 220.5 = 23.4 dB
īą Uplink Interference - Directional Antenna
Interference in Narrowband Macrocellular
Systems
)
1
(
6
)
3
(
ī
ī
īŊ
īĄ
īĄ
s
N
I
C
6
)
3
( s
N
I
C īĄ
īŊ
1
īŊ
n
1
1
)
1
3
(
6
ī
īĨ
īŊ
ī
īē
īģ
īš
īĒ
īĢ
īŠ
ī
īŊ īĨ
n
N
n
s
n
I
C īĄ
6
)
1
3
( s
N
I
C īĄ
ī
īŊ
1
īŊ
n
29. Cellular Principles 29
īą Consider īĄ = 4, N = 7 and s = 3 (Three-sector cell):
īą Exact C/I = 75.81 = 18.8 dB
īą Approximate C/I = 82.35 = 19.16 dB
īą Examples:
īą The table below gives some examples of C/I figures for īĄ = 4 and
for several reuse patterns, with omnidirectional and directional
(1200 antennas, or three-sectored cells) antennas
Interference in Narrowband Macrocellular
Systems
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
30. Cellular Principles 30
īą NOTE that the use of directional antennas
substantially improves the C/I ratio
īą The choice of which antenna to use depends on
how tolerant the technology is with respect to
interference
īą N = 7 and N = 4 are reuse patterns widely deployed
with 1200 antennas (they are referred as 7x21 and
4x12, respectively)
Interference in Narrowband Macrocellular
Systems
31. Cellular Principles 31
īą 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.
īą nL is used to investigate the performance
of different microcellular reuse patterns
īą nL is greatly dependent on the reuse
pattern (N).
īą nL can be obtained by simple visual
inspection, but Appendix D shows a
general formulation for calculating it.
Interference in Narrowband Microcellular
Systems
32. Cellular Principles 32
īą The subsequent performance analysis
considers a square cellular pattern with
BSâs positioned at every other
intersection of streets. Then, BSâs 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.
īą In Fig 2.7, the horizontal and vertical
lines correspond to the streets, and
diagonal lines represent the borders of
microcells
Interference in Narrowband Microcellular
Systems
34. Cellular Principles 34
īą 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 co-
microcells that at certain time may
interfere with the BS or MS of interest
īą In these Figâs, stars indicate the sites
contributing to the C/I of the downlink,
whereas the circles indicate the worst-
case location of the MS affecting the
performance of the uplink
Interference in Narrowband Microcellular
Systems
35. Cellular Principles 35
Interference in Narrowband Microcellular
Systems
īą Figures 2.8
A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E A
B
C
D E A
B
C
D E A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E A
B
C
D E
A
B
C
D E A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
C
D E
A
B
E
A
D E
A
B
C
A
C
D
A
B
C
D E
40. Cellular Principles 40
īą Note that distinct situations can affect in
different ways the performance of the
downlink and the uplink
īą In general, the set of micro cells
affecting the downlink is a subset of
those influencing the uplink
īą 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
Interference in Narrowband Microcellular
Systems
41. Cellular Principles 41
īą 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
īą Propagation
īą it is characterized by both LOS and NLOS modes
īą For NLOS mode, the mean power received at
distance d from the transmitter is:
īą Note that this power strength is similar to that one
of macrocellular systems
īą KNLOS is a proportionality constant that depends on
frequency, antenna heights, environment, etc
Interference in Narrowband Microcellular
Systems
īĄ
ī
īŊ d
K
P NLOS
NLOS
42. Cellular Principles 42
īą For LOS condition, and for a transmitting antenna
height ht, a receiving antenna height hr, and a
wavelength īŦ, the received mean power at distance
d is approximately:
īą KLOS is a proportionality constant and depends on
frequency, antenna heights, environment, etc
īą dB is the breakpoint distance (4hthr/ īŦ)
īą Note that LOS and NLOS propagation modes a
rather different
īą For NLOS condition, the mean signal strength
decreases monotonically with the distance
Interference in Narrowband Microcellular
Systems
1
2
2
1
ī
īē
īē
īģ
īš
īĒ
īĒ
īĢ
īŠ
īˇ
īˇ
ī¸
īļ
ī§
ī§
ī¨
īĻ
īĢ
īŊ
B
LOS
LOS
d
d
d
K
P
43. Cellular Principles 43
īą For LOS condition and d < dB, the mean signal
strength decreases monotonically with a power law
close to the free space condition (īĄ īģ 2). However,
for d > dB, the power law follows closely that of the
plane earth propagation (īĄ īģ 4)
īą 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 īŧ r īŖ 1), and
k = R/dB as the ratio between the cell radius and the
breakpoint distance (K īŗ 0)
īą 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
Interference in Narrowband Microcellular
Systems
44. Cellular Principles 44
īą Uplink Interference
īą By using PLOS for both wanted and interfering
signals:
īąDownlink Interference
īą 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.
Interference in Narrowband Microcellular
Systems
ī ī
īĨ
īĨ
īŊ
ī
ī
ī
īĢ
īĢ
īŊ
1
1
2
2
2
1
2
]
)
(
1
[
4
)
(
1
L
L
L K
n
n
r
rk
I
C 1
īŊ
L ī ī
ī ī
2
2
2
1
2
1
)
(
1
4
)
(
1
rk
r
k
n
n
I
C
īĢ
īĢ
īŊ
good
approximation
45. Cellular Principles 45
īą at the vicinity of the serving base station, more
specifically at the intersection of the streets (r īŖ
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:
īą 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 ...
Interference in Narrowband Microcellular
Systems
ī ī
īĨ
īĨ
īŊ
ī
ī
ī
ī
ī
ī
ī
ī
ī¯
īž
ī¯
īŊ
īŧ
ī¯
īŽ
ī¯
ī
īŦ
īĢ
īĢ
īĢ
īĢ
ī
īĢ
ī
īĢ
īĢ
īĢ
īĢ
īĢ
īŊ
1
1
2
2
2
1
2
2
1
2
2
2
1
2
2
2
1
2
2
]
)
(
1
[
)
(
2
]
)
(
1
[
)
(
]
)
(
1
[
)
(
)
(
1
L L
L
L
L
L
L
k
r
n
r
n
k
r
n
r
n
k
r
n
r
n
rk
r
I
C
46. Cellular Principles 46
īą Then:
īą 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 īŖ normalized distance from the site
to the beginning of the block) and :
Interference in Narrowband Microcellular
Systems
ī ī
īģ īŊ
īĨ
īĨ
īŊ
ī
ī
ī
ī
ī
ī
ī
īĢ
ī
īĢ
īĢ
īĢ
īĢ
īĢ
īŊ
1
1
2
2
2
1
2
2
2
1
2
2
]
)
(
1
[
)
(
]
)
(
1
[
)
(
)
(
1
L
L
L
L
L k
r
n
r
n
k
r
n
r
n
rk
r
I
C
ī ī
īĨ
īĨ
īŊ
ī
ī
ī
ī
ī
ī
ī
ī
ī¯
īž
ī¯
īŊ
īŧ
ī¯
īŽ
ī¯
ī
īŦ
īĢ
īĢ
īĢ
īĢ
ī
īĢ
ī
īĢ
īĢ
īĢ
īĢ
īĢ
īŊ
1
1
2
2
2
1
2
2
1
2
2
2
1
2
2
2
1
2
2
]
)
(
1
[
)
(
]
)
(
1
[
)
(
]
)
(
1
[
)
(
)
(
1
L L
L
L
L
L
L
k
r
n
r
n
k
r
n
r
n
k
r
n
r
n
rk
r
I
C
r
r ī
īŊ1
47. Cellular Principles 47
īą A good approximation for the downlink C/I can be
obtained by simply considering L=1
īą Examples
īą 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).
īą For numerical results, the calculations considered:
R=100 m, street width of 15 m, ht=4 m, hr=1.5 m,
f=890 MHz (īŦ = 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)
Interference in Narrowband Microcellular
Systems
48. Cellular Principles 48
īą 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.
īą 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.
īą Note also that in Fig 2.10, the C/I curves for N=9
and N=13 are also coincident
īą Fig 2.12 compares the performance between 5- and
10- micro cell clusters.
Interference in Narrowband Microcellular
Systems
49. Cellular Principles 49
īą 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
īą 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=10-
clusters, respectively. Both figures provide the
performances as functions of the normalized
distance to the BS using L=1 and L=īĨ
īą 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
Interference in Narrowband Microcellular
Systems
50. Cellular Principles 50
īą Therefore, very accurate estimates can be
achieved by only considering the closest layer to
the target cell
Interference in Narrowband Microcellular
Systems
51. Cellular Principles 51
Interference in Narrowband Microcellular
Systems
īą Figure 2.10
0,2 0,4 0,6 0,8 1,0
10
20
30
40
50
60
70
Uplink 5
Uplink 8
Uplink 9
Uplink 10
Uplink 13
Carrier/Interference
[dB]
Normalized Distance from Site
56. Cellular Principles 56
Interference in Wideband Systems
īą Wideband systems operate with a unity
frequency reuse factor.
īą The channelization is carried out by means of
codes sequences.
īą 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%)
īą But in real situations, the systems are led to
operate in an interference environment (the
efficiency of the reuse factor is less than 100%)
57. Cellular Principles 57
Interference in Wideband Systems
īą The frequency reuse efficiency Æ is defined as:
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.
īą Let I= IO/ IS be the interference ratio. Thus,
S
S O
I
f
I I
īŊ
īĢ
1
1
f
I
īŊ
īĢ
58. Cellular Principles 58
Interference in Wideband Systems
īą Because within a system the traffic may vary
from cell to cell, the frequency reuse efficiency
can be defined per cell.
īą For an N-cell system, let j be the target cell and
i the interfering cell. Therefore, for cell j, the
frequency reuse efficiency, Æj , can be written
as:
1,
j
N
j i
i i j
I
f
I I
īŊ īš
īŊ
īĢ īĨ
59. Cellular Principles 59
Interference in Wideband Systems
īą The interference conditions for the uplink and
for the downlink are rather dissimilar.
īą 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.
īą 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.
60. Cellular Principles 60
Interference in Wideband Systems
īą Uplink Interference
īąBecause 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 (Îē).
īąThe total power from the active users within a cell j is:
īąwhere is the traffic density (users per area) of cell
j, whose area is Aj.
īąThe interference condition in the reverse link:
ī¨ īŠ
J J J
I A dA
īĢ
īŊ īĄ
ī˛
ī¨ īŠ
J
A
īĄ
interfering
mobile station
desired
mobile station
target cell interfering cell
j
i
r,
i
i
r,
61. Cellular Principles 61
Interference in Wideband Systems
īąFor any active user i, Îē is the power at its serving base
station i.
īąThe power transmitted from the mobile station is .
īąThe power received at the base station j (interfering
power) is .
īąFor all users in cell i the total interfering power at base
station j is
īąHence,
ii
rīĄ
īĢ
ii ij
r r
īĄ īĄ
īĢ ī
ī¨ īŠ
i i ii ij i
I A r r dA
īĄ īĄ
īĢ ī
īŊ īĄ
ī˛
ī¨ īŠ
ī¨ īŠ
1
j j
j N
i ii ij i
i
A dA
f
A r r dA
īĄ īĄ
ī
īŊ
īĄ
īŊ
īĄ
ī˛
īĨī˛
62. Cellular Principles 62
Interference in Wideband Systems
īąThe frequency reuse efficiency depends on both the
traffic distribution as well as on the propagation
conditions (path loss and fading).
īąFor uniform traffic distribution and for an infinite number
of cells, all cells present the same frequency reuse
efficiency.
īąA common practice in cellular design is to use Æ=0.6.
63. Cellular Principles 63
Interference in Wideband Systems
īą Downlink Interference
īąThe constant-power situation, as experienced in the
reverse link, no longer applies.
īąThe interference is a function of the distance of the
mobile station to the interferers.
īąThe frequency reuse efficiency Æj(x,y) is a function of the
mobile position variables (x,y).
īąThe interference condition in the forward link is
illustrated bellow:
interfering
base station
desired
base station
target cell interfering cell
j
i
r,
i
i
r,
64. Cellular Principles 64
Interference in Wideband Systems
īąThe mean frequency reuse efficiency is defined as:
īąThe own-cell interference at the mobile station depends
on the degree of orthogonality of the codes.
īąFor an ideal condition, no own-cell interference occurs
and the frequency reuse efficiency is 1.
īąFor a complete loss of orthogonality, the own-cell
interference reaches its maximum and the reuse
efficiency its minimum.
īąA common practice in cellular design is to use Æ=0.6.
ī¨ īŠ ī¨ īŠ
1
, ,
j j
j
f x y f x y dxdy
A
īŊ ī˛ī˛
65. Cellular Principles 65
Network Capacity
īą A measure of network capacity can be provided
by the spectrum efficiency.
īą The spectrum efficiency (Ρ) is defined as the
number of simultaneous conversations per cell
(M) per assigned bandwidth (W).
īą In cellular networks, efficiency is directly
affected by two type of technologies:
compression technology (CT) and access
technology (AT).
īą CTs increase the spectrum efficiency by
packing signals into narrower-frequency bands,
e.g. low-bit-rate source coding and bandwidth-
efficient modulations.
66. Cellular Principles 66
Network Capacity
īą 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.
īą 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.
īą A loss in capacity occurs in wideband systems
because the frequency reuse efficiency is
usually substantially smaller than 1.
67. Cellular Principles 67
Network Capacity
īą Narrowband systems are usually based on
FDMA or TDMA access technologies. Wideband
systems, in general, make use of CDMA access
technology.
īą Narrowband systems
īąThe 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.
īąIf C is the number of slots per subband times number of
subbands, the spectrum efficiency is given by:
NW
C
W
M
īŊ
īŊ
ī¨
68. Cellular Principles 68
Network Capacity
īąThe ratio C/W is a direct result of the CTs used.
īąThe reuse factor N is chosen such that it achieves the
signal-to-interference ratio required to meet transmission
quality specifications.
īą Wideband Systems
īąThey are typically interference limited, with the
interference given by the number of active users within
the system.
īąThe 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.
īąThe number of active users, their geographic distribution,
and their channel activity affect the interference
conditions of the systems.
69. Cellular Principles 69
Network Capacity
īąDefine 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
īąIn 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.
īąIf we define the load factor Ī as
īąwe obtain
N
t
N
t
R
I
I
P
P
N īŊ
īŊ
N
O
S
O
S
I
I
I
I
I
īĢ
īĢ
īĢ
īŊ
ī˛
ī˛
ī
īŊ
1
1
R
N
71. Cellular Principles 71
Network Capacity
īąThe condition Ī=0 signifies no active users within the
system. As Ī approaches unity the noise rise tends to
infinity, and the system reaches its pole capacity.
īąA system is usually designed to operate with a loading
factor smaller than 1 (typically Ī 0.5,or equivalently 3dB
of noise rise).
īąThe load factor is calculated differently for the uplink and
for the downlink.
īą Uplink Load Factor
īąLet ī§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 â Pi ) / W.
ī
72. Cellular Principles 72
Network Capacity
īąFor a channel activity equal to ai , 0 īŖ ai īŖ 1
īąSolving for Pi ,
, where
īąManipulating Equation 2.42, we obtain
īąThe power IS can be calculated as
ī¨ īŠ ī¨ īŠ
i
t
i
i
i
i
t
i
i
i
i
i
i
P
I
a
P
G
P
I
R
a
WP
N
E
ī
īŊ
ī
īŊ
īŊ
ī§
t
i
i I
P ī˛
īŊ
1
1
ī
īˇ
īˇ
ī¸
īļ
ī§
ī§
ī¨
īĻ
īĢ
īŊ
i
i
i
i
a
G
ī§
ī˛
ī¨ īŠ
t
S
I
I
I
īĢ
īŊ 1
ī˛
īĨ
īŊ
īŊ
M
i
i
S P
I
1
73. Cellular Principles 73
Network Capacity
īąThe uplink load factor for a multirate wideband system is
īąA load factor Ī =1 gives the pole capacity of the system.
īąTypically, ai assumes the value 0.67 for speech and 1.0
for data; the value of ī§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.
īąOf 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.
īąThe 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.
ī¨ īŠīĨ
īĨ īŊ
ī
īŊ
īˇ
īˇ
ī¸
īļ
ī§
ī§
ī¨
īĻ
īĢ
īĢ
īŊ
īŊ
M
i i
i
i
M
i
i
a
G
I
1
1
1
1
1
ī§
ī˛
ī˛
74. Cellular Principles 74
Network Capacity
īąFor a classical all-voice network, such as the 2G CDMA
system, all M users share the same type of constant-bit-
rate service, In this case
īąWe have assumed the condition
īąThe spectrum efficiency is
īą Downlink Load Factor
īąBecause 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.
ī¨ īŠ ī§
ī˛
ī´
ī´
īĢ
ī´
ī´
ī´
īŊ
a
I
G
s
p
M
1
1
īžīž
ī§
a
psG
ī¨ īŠ W
a
I
G
s
p
W
M
ī§
ī˛
ī¨
ī´
ī´
īĢ
ī´
ī´
ī´
īŊ
īŊ
1
75. Cellular Principles 75
Network Capacity
īąAn orthogonality factor ti , 0 īŖ ti īŖ 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.
īąThe 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.
īąFollowing the same procedure as for the uplink case the
downlink location-dependent load factor Ī(x,y) is found to
be
where Ii is the interference ratio and (x,y) is the mobile
user coordinates.
ī¨ īŠ
ī¨ īŠ
1
,
M
i i i i
i i
a t I
x y
G
ī§
ī˛
īŊ
īĢ
īŊ īĨ
76. Cellular Principles 76
Network Capacity
īąFor an average position within the cell, the average
downlink load factor is given as
īąAs for the orthogonality factor, this is typically 0.4 for
vehicular communication and 0.1 for pedestrian
communication.
īąFor a classical all-voice network, such as the 2G CDMA
system, all M users share the same type of constant-bit-
rate service and
ī¨ īŠ
1
M
i i
i i
a
t I
G
ī§
ī˛
īŊ
īŊ īĢ īĨ
ī¨ īŠ
p s G
M
t I a
ī˛
ī§
ī´ ī´ ī´
īŊ
īĢ ī´ ī´
77. Cellular Principles 77
Network Capacity
īąThe spectrum efficiency is
ī¨ īŠ
M p s G
W t I a W
ī˛
ī¨
ī§
ī´ ī´ ī´
īŊ īŊ
īĢ ī´ ī´
78. Cellular Principles 78
Summary
īą Cellular systems are built upon the frequency-
reuse principles.
īą The service area is divided into cells and
portions of the available spectrum are
conveniently allocated to each cell.
īą The number of cells per cluster defines the
reuse pattern, and this a function of the cellular
geometry.
īą The macrocellular network makes use of high-
power sites with antennas mounted high above
the rooftops.
79. Cellular Principles 79
Summary
īą The macrocellular structure serves low-
capacity systems and is composed of the
hexagonal cell grid.
īą 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.
īą 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.
80. Cellular Principles 80
Summary
īą In this case, a cell is more likely to comply with
a diamond shape.
īą A cellular hierarchy is structured that contains
several layers, each layer encompassing the
same type of cell in the hierarchy.
īą The design of different cells depends on several
parameters such as mobility characteristics,
output power, and types of services utilized.
īą Several aspects affect the performance of the
system: interference control, diversity
strategies, variable data rate control, capacity
improvement techniques, and battery-saving
techniques.
81. Cellular Principles 81
Summary
īą Narrowband and wideband systems are
affected differently by interference.
īą In narrowband systems, interference is caused
by a small number of high-power signals.
Macrocellular and microcellular networks
undergo different interference patterns.
īą In macrocellular systems, uplink and downlink
present approximately the same interference
performance.
īą In microcellular systems, the interference
performance of uplink and downlink is
dissimilar.
82. Cellular Principles 82
Summary
īą 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.
īą 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.
īą In narrowband systems, capacity is established
given the total amount of resources and the
reuse pattern.
83. Cellular Principles 83
Summary
īą In wideband systems, the system capacity may
be influenced by a number of additional
parameters, such as the traffic profile, channel
activity, and others.