Wireless Com 31st March Lec13

8/10/2019 Wireless Com 31st March Lec13

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Wireless Communications:

System Design

Dr. Mustafa Shakir


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What is power control ?

Both the BS and MS transmitter powers are adjusteddynamically over a wide range.

Typical cellular systems adjust their transmitter powersbased on received signal strength.

TYPES OF POWER CONTROLo Open Loop Power Control

It depends solely on mobile unit, not as accurate asclosed loop, but can react quicker to fluctuation in signalstrength. In this there is no feed back from BS.

o Closed Loop Power ControlIn this BS makes power adjustment decisions andcommunicates to mobile on control channels


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Why power control ?

Near-far effect

Mechanism to compensate for channel

fading

Interference reduction,

prolong battery life


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Improving Capacity in Cellular Systems

Cost of a cellular network is proportional to the numberof Base Stations. The income is proportional to thenumber of users.

Ways to increase capacity:

New spectrum expensive. PCS bands were sold for$20B.

Architectural approaches: cell splitting, cell sectoring,microcell zones.

Dynamic allocation of channels according to load inthe cell (non-uniform distribution of channels).

Improve access technologies.


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Cell Splitting

Cell Splitting is the process of subdividing thecongested cell into smaller cells (microcells), Eachwith its own base station and a correspondingreduction in antenna height and transmitter power.

Cell Splitting increases the capacity since number ofclusters over coverage region would be increased thusincreasing the number of channels.

New cells added having smaller radius than originalcells and by installing these smaller cells (calledmicrocells ) between existing cells , capacity increasesdue to additional number of channels per unit area.


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Cell splitting diagram 1


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An Example

The area covered by a circle with radius R isfour times the area covered by the circle withradius R/2 The number of cells is increased fourtimes

The number of clusters the number of channelsand the capacity in the coverage area are

increased Cell Splitting does not change the co-channel re-use ratio Q =D/R


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Transmit Power

New cells are smaller, so the transmit power ofthe new cells must be reduced

How to determine the transmit power? The transmit power of the new cells can be

found by examining the received power at thenew and old cell boundaries and setting themequal

Pr(at the old cell boundary) is proportional to

Pt1 * R-n Pr(at the new cell boundary) is proportional to

Pt2 * (R/2)-n


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Transmit Power

Take n=4, we get

Pt2 = Pt1/16

We find that the transmit power must bereduced by 16 times or 12 dB in order to usethe microcells to cover the original area.While maintaining the same S/I.


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Application of cell splitting

When there are two cell sizes one cant simply use

the same transmit power for all cells. If larger transmit power used for all cells some smaller cellswould not be sufficiently separated from co channelcells. Using smaller Pt the larger cells might be left

unserved. So old channel broken to two channel groups

corresponding to smaller and larger cell reuse.

Larger cell for less frequent hand off.

Antenna down tilting focusing radiated energy frombase station to the ground to limit radio coverage ofnewly formed cells.


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Cell Sectoring Co channel interference may be reduced by replacing

omni directional antenna by several directional antennas. Given cell will receive interference and would transmit

with fraction of available co channel cells.

Each sector uses directional antenna at the B.S and

assigned a set of channels. Partitioning into three 120 deg. sectors or six 60 deg.

sectors.

Amount of CCI reduced by number of sectors.

Reduced Tx Power

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Cell Sectoring


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Example for sectoring


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Explanation For Cell Sectoring


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Effects of Sectoring Reduction in interference offered by sectoring would

enable to reduce the cluster size N and additionaldegree of freedom in channel assignment.

Increased number of antennas with shrinking cluster sizeand decrease in trunking efficiency due to channel

sectoring at base station.

Since sectoring reduces the coverage area of aparticular group of channels the number of handoffsincreases

Available channels subdivided and dedicated to aspecific antenna thus making up of several smaller poolscontributing to decrease in trunking efficiency.


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Repeaters

To provide dedicated coverage for hard to reachareas

Radio retransmitters for range extension.

Upon receiving signals from base station

forward link the repeater amplifies andreradiates the base station signals to specificcoverage region.

In building wireless coverage by installingDistributed Antenna Systems.

Repeaters must be provisioned to match theavailable capacity from the serving base station.


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Repeaters For Range Extension


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Microcell Zone The increased number of handoff as a result of sectoring

would result in an increased load on switching andcontrol link elements of the mobile system.

Division into microcell zones and each of the three areconnected to a single base station and share the same

radio equipment. Zones connected by a coaxial cable, fiber optic cable or

microwave link to the base station.

Handoff not required while mobile travels between zones

within cell. Channel switching and a channel active only within zone

of travelling.


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Scenario

In Micro cell zone scenario each hexagonrepresents a zone while the group of threehexagons represent a cell.

Zone Radius Rz is one hexagon radius.

Capacity of Microcell is directly related todistance betw. Cochannel cells and notzones.

No handoffs is required at the MSC.

The base station radiation is localized andinterference is reduced


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Trunking & Grade Of Service


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Trunking and Grade of Service (GOS)

Trunking A means for providing access to users on

demand from available pool of channels. With trunking, a small number of

channels can accommodate largenumber of random users. Telephone companies use trunking

theory to determine number of circuitsrequired.

Trunking theory is about how a populationcan be handled by a limited number of servers.


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Terminologies Erlang:

One Erlang:When a circuit is busy for one hour it handled a traffic of one erlang.

Grade of Service (GOS):

probability that a call is blocked (or delayed).

Set-Up Time:

Traffic intensity is measured in Erlangs:

time to allocate a channel.

Blocked Call:

Call that cannot be completed at time of request

due to congestion. Also referred to as Lost Call.


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Terminologies

Contd. Holding Time: (H)Average duration of typical call.

Load:

Traffic intensity across thewhole system.

Request Rate: ()

Average number of call requests per unittime.


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Traffic Measurement (Erlangs)

Traffic per userAu = H where is the request rate

and H is the holding time. For U users the load isA= UAu If traffic is trunked in C channels, then the traffic

intensity per channel isAc= UAu /C

Erlang B:


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The Erlang B Chart


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Example Example : An urban area has 2 million

residents. Three competing cellular systemsprovide service:

System A 394 cells x 19 channels/cell. System B 98 cells x 57 channels/cell. System C 49 cells x 100 channels/cell. For each user = 2 calls/hr, H= 3min, GOS

= 2% blocking. Find the number of users thatcan be supported by each system. Note thatthese are not simultaneous users.

System A: Au= H= 2 x 3/60 = 0.1 Erlangs. From the curve for GOS = 0.02 and C = 19 => A = 12 Er. Users per cell (U) =A/Au= 12/0.1 = 120 users 120 users/cell x 394 cells = 47,280 users can be served.

Market penetration = 2.36%.


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No. of subscribers System C: Prob Blocking = 2% = 0.02 C =100 Au = H= 2 x 3/60 = 0.1 Erlangs. From table,A = 88 Erlangs. Users per cell U =A/Au = 88/0.1 =880 users 880 users/cell x 49 cells = 43,120.

Market penetration = 2.156%.

System B: Prob Blocking = 2% = 0.02 C =57 Au = H= 2 x 3/60 = 0.1 Erlangs. From table,A = 45 Erlangs Users per cell U =A/Au= 45/0.1 = 450 users

450 users/cell x 98 cells = 44,100. Market penetration = 2.21%.


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Total No. of supported users = 47,280 + 44,100

+ 43,120

= 134,500 users.

Total market penetration for 3 systems =6.725%


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Documents

Wireless Com 31st March Lec13