II Cellular4

7/31/2019 II Cellular4

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Wireless Communications

lecture # (7)

The Cellular Concept

System Design Fundamentals

Improving Capacity in Cellular Systems

Cellular design techniques are needed to provide more

channels per unit coverage area.

1. Cell splitting: allows an orderly growth of the system.

(increases the number of base stations in order to increasecapacity)

2. Sectoring: uses directional antennas to further control

.

(rely on base station antenna placements)

3. Coverage zone approaches: distributes the coverage

of a cell and extends the cell boundary to hard-to-reach

places.


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Cell SplittingCell Splitting

The process of subdividing a congested cell into smallercells. (each with its own base station and a correspondingreduction in antenna hei ht and transmitter ower

By defining and installing new cells which have a smallerradius than the original cells (microcells).

Cell s littin reserves the eometr of the architecture and

therefore simply scales the geometry of the architecture

The increased number of cells would increase the number ofclusters which in turn would increase the number ofchannels reused, and capacity

Cell Splitting (Cell Splitting (22))

if every cell were reduced in such a way that the radius ofevery cell was cut in half. In order to cover the entireservice area with smaller cells, approximately four times asmany cells would be required.

Cell splitting not upsetting the channel allocation schemerequired to maintain the minimum co-channel reuse ratio Q

- .


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In Figure 3.8, the base stations are placed at corners of the cells,and the area served by base station A is assumed to be saturatedwith traffic

i.e, the blockin of base station A exceeds acce table rates .

Cell Splitting (Cell Splitting (33))

Cell Splitting is applied, note that the original base station Ahas been surrounded by six new microcell base stations.

(the smaller cells were added in such a way as to preserve the

frequency reuse plan of the system).

Microcell G was placed half way between two larger stationsutilizing the same channel set G

(also, for other microcells in the figure).

Cells are split to add channels with no new

spectrum usage


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(How much the transmit power must be reduced for the new smaller cells?

the received power (Pr) at the new and old cell boundaries and setting them

equal to each other. ( to ensure that the frequency reuse and S/I is the same)

where Pt1 an Pt2, are the transmit powers of the larger and smaller cell base

stations, respectively, and n is the path loss exponent.

= ,

the transmit power must be reduced by 12 dB in order to fill in theoriginal coverage area with microcells, while maintaining the S/Irequirement.

in practice, not all cells are split at the same time therefore,different cell sizes will exist simultaneously.

Two different transmitted ower levels for small and lar e

Practical considerations for cell splitting

cells are used.

Channels in the old cell must be broken down into twochannel groups, one for smaller cell and other for largercell.

The larger cell is usually dedicated to high speed traffic sothat handoffs occur less frequently.

Antenna downtilting, which focuses radiated energy from the basestation towards the ground (rather than towards the horizon), is oftenused to limit the radio coverage of newly formed microcells.


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Example 3.8 Consider Figure 3.9. Assume each base station uses 60

channels, regardless of cell size. If each original cell has aradius of1 km and each microcell has a radius of0.5 km,

n e num er o c anne s con a ne n a m ykm square centered around A,

(a) without the use of microcells,

(b) when the lettered microcells as shown in Fig 3.9 are used

c a e or g na ase s a ons are rep ace y m croce s.

Assume cells on the edge of the square to be contained withinthe square.


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Solution to Example 3.8

(a) Without the use of microcellsA cell radius of 1 km implies that the sides of the largerhexagons are also 1 km in length.

To cover the 3 km by 3 km square centered around basestation A, we need to cover 1.5 km (1.5 times thehexagon radius) towards the right, left, top, and bottom ofbase station A.

This is shown in Fi ure 3.9. From Fi ure 3.9 we see that

this area contains 5 base stations. Since each basestation has 60 channels,

The total number of channels without cell splitting isequal to 5 60 = 300 channels.

(b) With the use of the microcells as shown in Figure 3.9:

In Figure 3.9, the base station A is surrounded by 6microcells.

Therefore, the total number of base stations in the squarearea under study is equal to 5 + 6 = 11.

Since each base station has 60 channels,

the total number of channels will be equal to 11 60=660 channels. This is a 2.2 times increase in capacitywhen compared to case (a).


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(c) if all the base stations are replaced by microcells:From Figure 2.9, we see that there are a total of

5 + 12 = 17 base stations in the square region understudy. Since each base station has 60 channels, the

total number of channels will be equal to 17 x 60 =1020channels. This is a 3.4 times increase in capacity whencompared to case (a).

Theoretically, if all cells were microcells having half the

radius of the original cell, the capacity increase wouldapproach 4.

Conclusion

Cell splitting achieves capacity

improvement by essentially rescaling

the system. By decreasing the cellradius R and keeping the co-channel

reuse ratio D/R unchan ed cell

splitting increases the number of

channels per unit area.


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EEG473 Mobile Communications

lecture # (12)

The Cellular Concept

System Design Fundamentals

3.7.2 Sectoring

Another way to increase capacity is to keep the cellradius unchanged and seek methods to decrease theD /R ratio.

Is the technique for decreasing co-channelinterference and thus increasing capacity by

replacing a single omni-directional antenna at thebase station by several directional antennas,

with only a fraction of the available co-channel cells.

The factor of co-channel interference reductiondepends on the amount of sectoring used.


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A cellcell is normally partitioned into three 1200 sectors

or six 60 sectors as shown in Figure 3.10(a) and (b).

the channels used in a particular cell are broken

own n o sec ore groups an are use on y w n

a particular sector

Assuming 7-cell reuse, for the case of 120 sectors,

the number of interferers in the first tier is reduced

rom o . s s ecause on y o e co-channel cells receive interference with a particular

sectored channel group.

Sectoring improves S/I


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Figure 3.11, consider the interference experienced by amobile located in the right-most sector in the center cell

labeled 5.

There are 3 co-channel cell sectors labeled 5 to theri ht of the center cell and 3 to the left of the center cell.

Out of these 6 co-channel cells, only 2 cells have sectorswith antenna patterns which radiate into the center cell,and hence a mobile in the center cell will experienceinterference on the forward link from only these twosectors.

The resulting S/I for this case can be found using equation(3.8) to be 24.2 dB, which is a significant improvement over theomni-directional case in Section 3.5, where the worst case S/I wasshown to be 17 dB.

Sectoring improves S/I

(3.8)


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The minimum required S/I of18 dB can be easily

achieved that with 120o sectoring, with 7-cellreuse or 12-cell reuse in the unsectored case ( forthe worst possible situation see Section 3.5.1).

Thus, sectoring reduces interference which

amounts to an increase in capacity by a factor of

12/7, or 1.714.

In practice, the reduction in interference offered by

sectoring enable planners to reduce the cluster

size N, and provides an additional degree of

freedom in assigning channels.

The penalty of sectoring

1. the number of handoffs increases.

or una e y, many mo ern ase s a ons suppor sec or za on an a ow

mobiles to be handed off from sector to sector within the same cell

without intervention from the MSC, so the handoff problem is often not

a major concern.)

2. decrease in trunking efficiency,

(see next example)


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Example 3.9Consider a cellular system in which:

An average call lasts 2 minutes, the probability of blocking is to beno more than 1%. Assume that every subscriber makes 1 call perhour, on average.

If there are a total of 395 traffic channels for a 7-cell reuse system,there will be about 57 traffic channels er cell.

Assume that blocked calls are cleared so the blocking is describedby the Erlang B distribution. From the Erlang B distribution, it canbe found that the unsectored system may handle

44.2 Erlangs or 1326 calls per hour.

Now employing 120 sectoring, there are only 19 channels perantenna sector (57/3 antennas).

For the same probability of blocking and average call length, it canbe found from the Erlang B distribution that each sector can handle

11.2 Erlangs or 336 calls per hour.

Since each cell consists of 3 sectors, this provides a cell capacity:

3 X 336 = 1008 calls per hour,

which amounts to a 24% decrease compared to the unsectored case.

Thus, sectoring decreases the trunkingefficiency while improving the S/I for

each user in the system.


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It can be found that using 60 sectors improves the S/I evenmore. In this case the number of first tier interferers is reducedfrom 6 to only 1. This results in S/I 29 dB for a 7-cell systemand enables 4-cell reuse.

Of course, using 6 sectors per cell reducesthe trunking efficiency and increases thenumber of necessary handoffs even more.

If the unsectored system is compared to the 6 sector

case, the degradation in trun ing efficiency can beshown to be 44 %(The proof of this is left as an exercise).

2.7.3 A Novel Microcell Zone Concept Proposed by [Lee] as a solution to the problem of increased

number of handoffs required when sectoring is employed asillustrated in Figure 3.12.

In this scheme, each of the 3 (or more) zone sites (represented as Tx/Rxin Figure 3.13) are connected to a single base station and share thesame radio equipment.

The zones are connected by coaxial cable, fiber optic cable (Radioover fiber), or microwave link to the base station. Multiple zonesand a single base station make up a cell.

As a mobile travels within the cell, it is served by the zonewith the strongest signal.

This approach is superior to sectoring since antennas areplaced at the outer edges of the cell, and any base stationchannel may be assigned to any zone by the base station


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In-building deployment is the next great growth phase

The Zone Cell Concept


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As a mobile travels from one zone to another within thecell, it retains the same channel. The base station simply

switches the channel to a different zone site. (Thus, unlike insectoring, a handoff is not required at the MSC when the mobile travels betweenzones within the cell.)

reduced.The channels are distributed in time and space byall three zones and are also reused in co-channel cells inthe normal fashion.

This technique is particularly useful for urban traffic

Decreased co-channel interference improves the signalquality and also leads to an increase in capacity, withoutthe degradation in trunking efficiency caused bysectoring.

As mentioned earlier, an S/I of18 dB istypically required for satisfactory systemerformance in narrowband FM. For a s stem

with N = 7, a D/R of 4.6 was shown to achievethis.

With respect to the zone microcell system,since transmission at any instant is confined toa part cu ar zone, t s mp es t at a z z o4.6 (where Dz is the minimum distancebetween active co-channel zones and Rz is thezone radius) can achieve the required linkperformance.


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Zone Cell Concept

In Figure 3.12, let each individual hexagon represents a zone, whileeach group of three hexagons represents a cell.

The zone radius R is approximately equal to one hexagon radius.Now, the capacity of the zone microcell system is directly related tothe distance between co-channel cells, and not zones. This distance isre resented as D in Fi ure 3.14.

For a Dz/Rz value of 4.6, it can be seen from the geometry of Figure3.14 that the value of co-channel reuse ratio, D/R, is equal to 3,where R is the radius of the cell and is equal to twice the length of thehexagon radius.

= = . , .This reduction in the cluster size from N = 7 to N = 3 amounts to a2.33 times increase in capacity for a system completely based on thezone microcell concept.

Hence for the same S/I requirement of 18 dB, this systemprovides a significant increase in capacity over conventionalcellular planning.


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By examining Figure 3.13 and using equation (3.8)[Lee] the exact worst case S/I of the zone microcellsystem can be estimated to be 20 dB.

Thus, in the worst case, the system provides a marginof 2 dB over the required signal-to- interference ratiowhile increasing the capacity by 2.33 times over aconventional 7-cell system using omni-directionalantennas.

No loss in trunking efficiency is experienced.Zone cell architectures are being adopted in manycellular and personal communication systems.

Summary of Chapter (3)

the fundamental concepts of handoff,

frequency reuse,

,

frequency planning.

The capacity of a cellular system.

The S/I limits.

cell splitting,

sectoring,

zone microcell technique

The radio propagation characteristics influence theeffectiveness of all of these methods in an actual system.Radio propagation is the subject of the following twochapters.


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Slides from Vodafone seminar

(taken from source with permission)

Radio Coverage

A visible pattern of sound waves


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GSM Overv iew

Radio Coverage

Cell Geometr y

Dead Spots

Problem of omni directional antennas

To solve the dead spot problem

GSM Overv iew

Radio Coverage

Cell Geomet ri cal Shape

R R R

The number of cells required to cover a given area.

The cell transceiver power.

ra eo s


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GSM Overv iew

Radio Coverage

Transceiver Antenna

Omni-Directional AntennaSectorial Antenna

GSM Overv iew

Radio Coverage

Sectorial Antenna

Sectorial Antenna

The cells will take the form of overlapping circles.

Due to the obstacles in the coverage area the actual shape of the

cells would be Random.


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GSM Overv iew

Radio Coverage

Cell ClassificationMacrocell

Overlaid &

Underlaid CellsNormal Cell Normal Cell

Fast moving subscribers

Microcell

Slow moving subscribers

Picocell

In buildingcoverage

A3

A2

A1

B3

B2

B1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

GSM Overv iew

Radio Coverage

3/ 9 Cluster

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

B3

B2

A3

A2

B3

B2

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A1 B1

C3

C2

C1

A3

A2

A1

B3

B2

B1

C3

C2

C1

A1 B1

C3

C2

C1

3/9 cluster in which theavailable frequenciesare divided into 9groups and distributedbetween 3 sites

C1


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Carrier to interference ratio

GSM Overv iew

Radio Coverage

Which Cluster Size to use?

Its the difference in power level between the carrier in a givencell and the same carrier received from the nearest cell that reusesthe same frequency.

Number of frequenciesper site

Traffic ChannelsC/I Ratio

4/12 Medium Medium Medium

7/21 Low Low High

Chapter 3 (problems) 3nd Edition

3.1

3.3

3.5

3.8

3.9 (XXX)

3.10

3.15

II Cellular4

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