Cellular Principles

Cellular Principles

Cellular Principles 2

Cellular HierarchyMEGA CELLS MACRO CELLS MICRO CELLS PICO CELLS

Coverage Large Large Small Small

Radii

100 to 500 km(the cell radius is afunction 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 tohigh

Cells Remote areas(the cells move)

Outdoor cells Outdoor cells Indoor cell

Mobilespeeds

Low mobility aswell very high-

mobilityUp to 500 km/h Up to 100 km/h

Up to 10km/h

Antennas Low-orbit satellites

Directional,mounted above therooftops on towersor on the tops ofbuilding.

Mounted below therooftops onlampposts or onbuilding walls


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 preferencesAvailable NetworksMS capabilitiesNetwork capabilitiesMS mobilityService requirements


System Management

Cell reselectionUnsuitability of current cell due to interference or output

power requirementsRadio link failureNetwork requestTraffic load considerationsUser request

Channel Selection/Assignment Channel assignment algorithms usually take into account

the following:System loadTraffic patternsService typesService priorities Interference situations


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 handoverbetween cells in the same cell layer: Intercell handoverbetween cells of different layers: Interlayer handoverbetween cells of different networks: Internetwork handover

Hard handover In FDMA and TDMA wireless network

Soft-type handoverSoft handover (boundary of the cell)Softer handover (boundary of the coverage area of the sector)Soft-softer handover (both) In CDMA wireless network


System Management

The following criteria may be used to initiate a handover for radio transmission reasons: Signal strength measurementsSignal-to-interference ratioBit error ratesDistance between MS and BSMS speedMS Mobility trendsOthers


System Management

Mobility Support Logon-logoff Location Updating


System Performance

Interference Control Diversity Strategies

Diversity strategies are used to combat fadingSpaceFrequencyTime

Variable Data Rate Control Direct support of variable data rates over the air interface Variation of the number of bearer channel Packet access


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 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


Macro cellular Reuse Pattern

Circles x Regular Polygons (Equilateral triangles, squares, and hexagons)

Hexagonal cellular geometry Propagation symmetry Low-capacity systems



u

(u 2, v 2)

(u 1, v 1)

R3

R

v

D



R = Cell radius d = The distance between the center of two cells.

D = Reuse distance, that is, the distance between two co-cells.

A =Area of the hexagonal cluster.

a = Area of the hexagonal cell.

N = Reuse Factor (Number of cells per cluster)

222 jijid 12 uui 12 vvj

22 jijiN

222 jijiD

2DaA

N




Macrocellular Reuse Pattern

(1,2)

(1,2)

(1,2)

(1,2)

(1,2)

(1,2)i

n

j

k

l

m



Co-channel Reuse Ratio

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

NR

D3


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



D

u

v

R2

R

(u1, v1)

(u2, v2)

Reuse distance

Reuse Factor (Number of Cells per Cluster)

Reuse Ratio

222 jid 2DA

22 jiN

NRD

2



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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


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)


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:

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

KdP



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)



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 co-cells)

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


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:

(x) is the Riemann function: (1)=, (2)=2/6, (3)=1.2021, and (4)=2/6.

1

)(n

xnx


1

)(6n

nDn

R

I

C

NRD 3

1

)1(6

)3(

n

N

I

C

6

)3( N

I

C1n

good approximation


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:


1

)(6n

RnDn

R

I

C

NRD 3 1

1

)13(6

n

NnnI

C

6

)13(

N

I

C1n

good approximation


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:

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


)1(6

)3(

sN

I

C6

)3( sN

I

C

1n

1

1

)13(6

n

Nns

n

I

C

6

)13( sN

I

C

1n


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


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)



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.

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



Figure 2.7



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

In these Fig’s, 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 Figures 2.8

A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E A

BC

D E A

BC

D E A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E A

BC

D E

A

BC

D E A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E

A

BC

D E

A

B

E

A

D E

ABC

A

C

D

A

BC

D E



(a)



( b)( b)



(c)



(d)


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



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:

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


dKP NLOSNLOS


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:

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


12

21

B

LOSLOS d

d

d

KP


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



Uplink Interference By using PLOS for both wanted and interfering signals:

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.


1

1222

12

])(1[4

)(1

LLL Knnr

rk

I

C 1L 22

21

21

)(14

)(1

rkr

knn

I

C

good

approximation


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:

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 ...


11222122

12221222

122

])(1[)(2

])(1[)(])(1[)(

)(1

L LL

LLLL

krnrn

krnrnkrnrn

rkr

I

C


Then:

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 :


1

12221222

122

])(1[)(])(1[)(

)(1

LLLLL krnrnkrnrn

rkr

I

C

11222122

12221222

122

])(1[)(

])(1[)(])(1[)(

)(1

L LL

LLLL

krnrn

krnrnkrnrn

rkr

I

C

rr 1


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)



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.



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=10- clusters, 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



Therefore, very accurate estimates can be achieved by only considering the closest layer to the target cell



Interference in Narrowband Microcellular Systems 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

Car

rie

r/In

terf

ere

nce

[dB

]

Normalized Distance from Site


0.2 0.4 0.6 0.8 1.010

20

30

40

50

60

70

80

90 Downlink 5 Downlink 8 Downlink 9 Downlink 10 Downlink 13

Car

rier/

Inte

rfer

ence

[dB

]





0.2 0.4 0.6 0.8 1.0

10

20

30

40

50

60

70

Uplink 5 Downlink 5 Uplink 10 Downlink 10

Car

rier/

Inte

rfer

ence

[dB

]




0.2 0.4 0.6 0.8 1.0

10

20

30

40

50

60

70

5 Cell Clusters Uplink oo layers Uplink 1 layer Downlink oo layers Downlink 1 layer

Car

rier/

Inte

rfer

ence

[dB

]




0.2 0.4 0.6 0.8 1.0

10

20

30

40

50

60

8 Cell Cluster Uplink oo layers Uplink 1 layer Downlink oo layers Downlink 1 layer

Car

rier/

Inte

rfer

ence

[dB

]



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%)



The frequency reuse efficiency ƒ 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. Let I= IO/ IS be the interference ratio. Thus,

S

S O

If

I I

1

1f

I



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:

1,

j

N

j ii i j

If

I I



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.



Uplink InterferenceBecause 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:

where is the traffic density (users per area) of cell j, whose area is Aj.

The interference condition in the reverse link:

J J JI A dA JA

in terferingm obile station

desiredm obile station

target cell in terfering cell

jir ,

iir ,



For any active user i, κ is the power at its serving base station i.

The power transmitted from the mobile station is .The power received at the base station j (interfering power) is

.For all users in cell i the total interfering power at base station

j is

Hence,

iir

ii ijr r

i i ii ij iI A r r dA

1

j j

j N

i ii ij ii

A dAf

A r r dA



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.



Downlink InterferenceThe 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:

in terferingbase stationdesired

base station

target cell in terfering cell

jir ,iir ,



The mean frequency reuse efficiency is defined as:

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.

1, ,j j

j

f x y f x y dxdyA


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 bandwidth-efficient modulations.


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.


Network Capacity

Narrowband systems are usually based on FDMA or TDMA access technologies. Wideband systems, in general, make use of CDMA access technology.

Narrowband systemsThe 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:

NW

C

W

M


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 SystemsThey 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.


Network CapacityDefine 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

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

N

t

N

tR I

I

P

PN

NOS

OS

III

II

1

1RN


Network Capacity

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

10

Traffic Load ()

No

ise

Ris

e (

dB

)


Network CapacityThe 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 FactorLet 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.


Network Capacity

For a channel activity equal to ai , 0 ai 1

Solving for Pi ,

, where

Manipulating Equation 2.42, we obtain

The power IS can be calculated as

iti

ii

itii

i

i

ii PIa

PG

PIRa

WP

N

E

tii IP 1

1

ii

ii a

G

t

S

I

II 1

M

iiS PI

1


Network CapacityThe uplink load factor for a multirate wideband system is

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.

M

i ii

iM

ii a

GI

1

1

1

11


Network Capacity

For 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

We have assumed the conditionThe spectrum efficiency is

Downlink Load FactorBecause 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.

aI

GspM

1

1a

psG

WaI

Gsp

W

M

1


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

where Ii is the interference ratio and (x,y) is the mobile user

coordinates.

1

,M

i i i i

i i

a t Ix y

G


Network Capacity

For an average position within the cell, the average downlink load factor is given as

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-bit-rate service and

1

Mi i

i i

at I

G

p s G

Mt I a


Network Capacity

The spectrum efficiency is

M p s G

W t I a W


Summary

Cellular systems are built upon the frequency-reuse 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 high-power sites with antennas mounted high above the rooftops.


Summary

The macrocellular structure serves low-capacity 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.


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.


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.


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.


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.

Documents

Cellular Principles