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d.

Improving coverage in capacity in cellular system

• Due to growing demand number of channels assigned to a cell eventually becomes in sufficient.

• Techniques for increasing the capacity:

A) Cell Splitting

B) Sectoring

C) Coverage Zone

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1. Cell Splitting• Cell splitting is the process of sub-dividing a congested

cell in to smaller cells.

• Each new cell has its own base station with reduced antenna height and transmitted power.

• Increases the capacity as channel reuse increases.

• New cells are smaller in radius (micro cells) and installed between existing cell in a manner that channel allocation scheme required to maintain the minimum separation between co channel cells is not disturbed. (R is reduced but D/R is fixed)

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Base station A is surrounded by 6 –new micro cells. Smaller cells preserve the frequency reuse plan. E.g Microcell G is placed halfway between two larger stations using the same channel

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• For microcells the transmitted power is reduced, new cells with radius R/2 can be found by examining the received power Pr at the new and old boundary and setting them equal,

Pr [at old cell boundary] α Pt1 R-n

&

Pr [at New cell boundary] α Pt2 (R/2)-n

Pt2 = (Pt1/16) for n=4Where

Pt1, Pt2 are transmitted power of larger and smaller cells base station

n is the path loss exponent

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d.

• In Practice all cells are not split at the same time. So larger and smaller cells coexist.

• Large cells transmits high power where as smaller one low power ,when they co exist channel assignment is complex.

• If large power used by all cells then some channels used by smaller cell may not be at sufficient distance from co channel cells.

• If small transmit power is used by all the cells, in larger cell some part may remain unserved.

Solution

So channels in the old cell must be broken in to two groups, one that cater smaller cell reuse requirement and other corresponds to the larger cell reuse requirements.

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d.

• The two channel group size depends on cell splitting process.

• In beginning of splitting process there will be fewer channels in smaller power group.

• As demand grows it will channels in smaller groups will increase and splitting process may continue till all the channels allotted to smaller power group.

• Antenna down-tilting focuses radiation energy focused to ground to limit the radio coverage in a smaller cell.

• Hand-off is addressed by Umbrella approach bigger cell cater high speed traffic and smaller cell to slow user.

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d.

2. Sectoring• In cell splitting R is decreased but D/R kept fixed to

increase capacity by increasing channels per unit area..

• In Sectoring R is kept fixed but D/R is reduced to increase capacity. Sectoring involves two steps

Step1 : S/I ratio is increased by using direction antennas.

Step2 : Capacity is improved by reducing N (cluster size).

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d.

Step-1• To reduce relative interference without decreasing

transmit power, single omni directional antenna is replaced by several directional antennas, each radiating within a specified area called Sector.

•By using Directional antennas a given cell will receive and transmit interference with only a fraction of available co-channels.

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d.

• The technique for decreasing co-channel interference and thus increasing the system performance by directional antennas is called sectoring.

• Channel in the cell are broken down to in to sectored groups and are used only within a particular sector.

• For N= 7, 120º sectoring the number of interferers in first tier is reduced from six to two. (S/I increased from 17 to 24.2 dB)

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d.

Problems with sectoring:• More number of antennas at base stations are required.

• Reduced trunking efficiency ( ability to accommodate a large number of users in limited spectrum) due to channel sectoring.

• Sectoring reduces coverage area of a channel so more hand-off is required (but not a major concern as new technology allows handoffs in sectoring without intervention of MSC)

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d.

3. Microcell zone concept• Increased number of Hand-off’s are required when sectoring

employed resulting in to increased load on links and MSC.

• Lee gave concept of microcell for 7-cell reuse

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d.

• Cell is divided in to zones and zones are connected together to a single base station and share the same set of radio equipments.

• Zones are connected by coaxial cable, fiber optic cable or microwave link.

• Zones and a single BS make up a cell.• Within a cell mobile is served by the zone with strongest

signal. Within a cell user can move between zones without handoff as it retains the same channel. BS station simply switches the channel to different zone site.

• A given channel is active only in one of the zone and hence base station radiation is localized and interference is reduced.

• Channels are distributed in time and space by all three zones

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d.

Advantages of micro cell concept over sectoring

• Antennas are placed at the outer edge of cell and any base station channel may be assigned to any zone by the base station.

• No handoff is required when mobile travels within zones in a celle as same channel is used within a cell/zone.

• While Cell maintains a particular coverage radius, the co channel interference is reduced since a large central base station is replaced by a several low power transmitters.

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d.

Each hexagon representing a zone and three together makes a cell.

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d.

• For N=7 , D/R=4.6 to achieve min S/I of 18 dB

• In Zone micro cell system DZ/RZ =4.6

Where

DZ =Min. distance between co-channel zones

RZ = Zone Radius

For a DZ/RZ =4.6 geometry shoes that D/R =3

(where R=2 Rz)

D/R=3 corresponds to N=3. So a reduction in N from 7 to 3 , increases the capacity by almost 2.33 times.