Derivation of Q in cellular communication

8/11/2019 Derivation of Q in cellular communication

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Lecture -7 Interference and System

capaci ty & Improving Capacity in Cellular

Sys tems

Mobile and Cellular Communications

10EC345-Mobile and Cellular Communication


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Small-scale and large-scale fading


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Outline of the Lecture Review of topics covered in last

classInterference and System Capacity

Co-channel Interference Adjacent Channel Interference

Power Control for Reducing Interference

Improving Capacity in Cellular Systems

Cell SplittingSectoringMicrocell Zone Concept10EC345-Mobile and Cellular Communication2


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Interference and System Capacity

Sources of interference Another mobile in the same cell A call in progress in the neighboring cellOther base stations operating in the same frequency bandNon-cellular system leaks energy into the cellular frequency band

Two major cellular interferenceCo-channel interference

Adjacent channel interference10EC345-Mobile and Cellular Communication3


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Co-channel & Adjacent channel Interference

Co-channelcells

Adjacent-channelcells

Co-channelinterference

Adjacent-channelinterference


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Co-channel Interference and System Capacity

Frequency reuse - there are several cells that use the same set of frequencies,This

leads to existence of co-channel interferenceTo reduce co-channel interference, co-

channel cell must be separated by a minimum distance.

10EC345-Mobile and Cellular Communication4


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Co-channel Interference and System

Capacity When the size of the each cell is approximately the sameCo-channel interference is independent of the transmitted powerco-channel interference is a function of

R : Radius of the cell D: distance to the center of the nearest co-channel cell

Increasing the ratio , the interference isreduced.

is called the co-channel reuse ratio10EC345-Mobile and Cellular Communication5 11-Jan-13


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For a hexagonal geometry

A small value of Q provides large capacity A large value of Q improves the transmission quality - smaller level of co-channel

interference A tradeoff must be made between these two objectives



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Derivation of Q = 3 N


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Let i 0 be the number of co-channel interfering cells.The signal-to- interference ratio (SIR) for a mobile receiver can be expressed as

S: desired signal power : Interference power caused by the iinterfering co-channel cellbase stationThe average received power at a distance d from the transmitting antenna is approximated by

is the path loss exponent which ranges between 2 and 4.



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When the transmission power of each base station isequal, SIR for a mobile can be approximated as

Consider only the

first layer of interferers



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For hexagonal geometry with 7-cell cluster, with the mobile unit being at the cell boundary, the signal-to-interferenceratio for the worst case can be approximated as



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For the case where the mobile unit is at the cell

boundary in a 7-cell cluster ( the worst case ). The distances fromthe co-channel interfering cells are approximated to

D-R, D and D+R .Assuming n= 4

444

4

2)(2)(2 D R D R D R

I

S

444 2)1(2)1(21

QQQ I S

the worst case SIR


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For N=7 , Q= 4.58 from

From(worst case)

S/I = 49.56 (17 dB)

But by using

S/I = 17.8 dB(average)Hence for a 7-cell cluster, the S/I ratio is slightly less that 18 dB in

the worst case.

444 2)1(2)1(21

QQQ I S

0

1

)(i

i

n

i

n

D

R

I

S


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1. Compute Co-channel S/I ratio for N=3


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Adjacent Channel Interference Adjacent channel interference: Interference from adjacent infrequency to the desired signal.Imperfect receiver filters allow nearby frequencies to leak into the pass bandPerformance degrade seriously due to near-far effect.


Filter

Signal on Adjacent Channel Signal on Adjacent ChannelReceiving filter response

Interference Interference

Desired Signal

Desired Signal


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Adjacent channel interference can be minimized through careful filtering and channel assignment .Keep the frequency separation between each channel in a given cell as large as possible

A channel separation greater than six is needed to bring the adjacent channel interference to an

acceptable level.If adjacent channels are assigned to different cells, and no cell is assigned contiguous set of channels adjacent channel interference can be reduced significantly.

Adjacent Channel Interference



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Originally 666 channel allocated. Later 166channels are added (Totally 832 channels).Forward channel at 870.030 MHz along with its reverse channel at 825.030 MHz is numbered as

channel 1.Extended AMPS has channels numbered as 667 to799 and 990 to 1023.Segregated as block A(Non-wire) and block B(Wireproviders). Each block is having 416 channels.1-312 are allotted for voice and 313 to 333(21 nos) control channels in Block A. 355-366 for voice and

334 to 354 control channels in block B.

AMPS Channel Allocation



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Out of 21, one Control channel is allotted for a bank

of (or) trunked voice channels

In AMPS 1 Control channel per cell is allocated with

in 7 cluster sizeNeighbouring two clusters uses the remaining 14

control channels. Hence uses 21 cell reuse

scheme.

Extended AMPS has channels 667 through 716

and 717 through 799(82) are extended Block B




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Out of 416 channels each 395 voice channels are

divided in to 21 subsets(each containing 19

channels)

In 7 cell reuse system each cell is assigned 3

subsets.

The closest adjacent cell is separated by 21

channels

The channels are assigned such a way that they

are separated at-least by 7 channels




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ProblemAssume a system of 32 cells with a cell radius

of 1.6km, a total of 32 cells, a total frequency

bandwidth that supports 336 traffic channels,

and a reuse factor of N=7.What is the geographic area covered?How many channels are per cell?

What is the total number of concurrent calls that can be handled?Repeat with 0.8km and 128 cells .


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SolutionWhat is the total number of concurrent calls that

can be handled?

48 channels per cell* 32 cells=1536 channels


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1. Compute Co-channel S/I ratio for N=3


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Power Control For Reducing InterferenceIn practical cellular radio and personal communication systems the

power levels transmitted by every subscriber unit are controlled bythe serving base stations

Need for Power Control:

Received power must be sufficiently above the background noisefor effective communication

Desirable to minimize power in the transmitted signal from the

mobile. Reduce co-channel interference, alleviate health concerns,save battery power

In Spread Spectrum systems using CDMA, its desirable to equalize

the received power level from all mobile units at the Base station.


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Power control enable the transmitter side to adapt

its output power according to pilot signals strength.

Aims to solve the near-far problem with the goal

to achieve uniform signal to interference ratio (SIR)for all the active users.

Two methods:

a) open-loop Power Control

b) closed-loop Power Control.

Power Control


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Open-loop Power ControlIn open-loop PC , the mobile measures the pilot strength

(continuously transmitted by the BS) which is related to path loss.If the pilot gets weak it powers-up, when the pilot gets strong it

powers down.

It assumes that the forward and reverse link are similar.--- notaccurately (incase of FDD).

Quickly react. e.g. mobile emerges from a behind a large

building.It gives quite results with the TDD mode.

It is used in WCDMA-FDD mode but only to provide initial power

setting of the mobile station at the beginning of the connection.


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Closed-loop Power ControlIn closed-loop PC , feedback is used whereby the base-

station measures the signal-quality of mobile; e.g. frameerror rate (FER) and commands each mobile to power up and

down accordingly in order to keep the overall FER at desired

level. reverse link.

The mobile provides information about received signal

quality to the BS, which then adjusts transmitted power.

forward link.

The UTRA-FDD mode uses fast closed-loop power control

technique both in uplink and downlink.


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TWO SPECIAL CASES FOR FAST CLOSEDLOOP POWER CONTROL

Soft Handover: How to react to multiple powercontrol commands from several sources. At the

mobile, a power down command has higher

priority over power up command.

Compressed Mode: Large step size is used after a

compressed frame to allow the power level toconverge more quickly to the correct value after the

break.

GSM 900MH t itt l


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GSM 900MHz transmitter classes

The transmission power in the handset is limited to a maximum of 2watts in GSM850/900 and 1 watt in GSM1800/1900.


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GSM Power class Not all mobiles have the same maximum power output

level.

In order that the base station knows the maximum power

level number that it can send to the mobile, GSM power

class number is allocated to a mobile.

This GSM power class number indicates to the base

station the maximum power it can transmit and hence

the maximum power level number the base station can

instruct mobile to use that power level.44


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GSM power classes vary according to the band in use

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Equivalent isotropically radiated

power (dBW)Network Frequency Base

Stations Minimum Average Maximu

m

Vodafone 900 MHz 23 7.1 7.1 7.4 O2 900 MHz 21 0.3 7.3 11.7

Orange 1800 MHz 22 12.9 15.2 19.4

T-Mobile 1800 MHz 25 4.0 9.3 11.0

T-Mobile 2100 MHz 3 9.0 10.0 11.0

Low-height high-power mobile phone base station (under 10m)

7 dBW represents 5 watts radiated power per channel. 15 dBW (theOrange average) represents 32 watts radiated power per channel


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Improve Capacity

Cell Splitting : It allows an orderly growth of the

cellular system

Sectoring : It uses Directional antennas to control

the interference and frequency reuse of channels.Zone Microcell: It distributes the coverage of the

Cell.

More bandwidth

Borrow channel from nearby cells

CELL SPLITTING


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CELL SPLITTING

Cell Splitting : It is the process of subdividing the

congested cell into smaller cells.

Each of the smaller cells will have their own base station

with a reduction in antenna height and transmitted power.The smaller cells are known as Microcells .

Cell Splitting increases the capacity of the cellular system

as it increases the number of times the channels are

reused.


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CELL SPLITTING

The increased number of cells would increase the

number of clusters over the coverage region, which

in turn increase the number of channels, and thus

capacity in the coverage area

Cell Splitting allows the system to grow by replacing

large cells with smaller cells without changing the

co-channel re-use ratio Q.

Cells are split to add channels with no


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Cells are split to add channels with nonew spectrum usage


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For the new cells to be smaller in size, the transmit power ofthese cells must be reduced.

The transmit power of the new cells with radius half that of the

original cells can be found by examining the received power , P r ,

at the new and old cell boundaries and setting them equal to each

other.

This is necessary to ensure that the frequency reuse plan for the

new microcells behaves exactly as for the original cells.

Transmitter power levels in cell splitting


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Problem: Assume each base station has 60 channels, regardless

of size. If radius of each cell is 1 Km and each microcell has a

radius of 0.5km find the no channels for 3*3 square Km Area (a)

without the use of microcells (b) when the lettered cells as in figure

are used if all original base stations are replaced by micro cells.


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Sectoring:

The co-channel interference in a cellular system can

be decreased by replacing the omni directional

antenna at the base station by several directional

antennas, each radiating within a specified sector.

The process of reducing the co-channel interference

and thus increasing the capacity of the system by

using directional antennas is known as Sectoring.

S i


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Sectoring:

In general a cell is partitioned into three 120 degree

sectors or six 60 degree sectors.

When sectoring is employed, the channels used in a

particular cell are broken down into sectored groups

and are used only in a particular sector.


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Sector

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Sectoring improves S/I


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Sectoring improves S/I


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Typical GSM sector antenna outdoor unit

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S t A t M t


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Sector Antennas on Mast

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S t t f


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Sector antennas on roof

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T itti g t


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Transmitting tower

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Sector antennas with down tilts


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Sector antennas with down-tilts

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An antenna at bottom has bigger mechanical down-tilt

120 0 Sectoring


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120 Sectoring


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Verizon 4G LTE eNodeB the LTE antennas are the bigger ones on the outside"


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The Improvement in (S/I) suggests that the minimum

required (S/I) of 18 dB can be easily achieved with 7-cellreuse by employing 120 degree sector technique when

compared to 12-cell reuse

Therefore Sectoring reduces interference and increases the

capacity by an amount of (12/7) i.e. 1.714

In general, the reduction in interference by sectoring

enables planners to reduce the cluster size N, and provides

an additional freedom in assigning channels

Disadvantages:


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Disadvantages:

Increased number of antennas at each base stationDecrease in trunking eff iciency due to channel

sectoring at each base station

As sectoring reduces the coverage area of a

particular group of channels, the number of

handoffs increase.

The Microcell Zone Concept


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Repeaters


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Repeaters

Repeat interference also

Planning

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Summary


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Summary

In this chapter, concepts like handoff, frequency

reuse, trunking efficiency have been studied

The capacity of a cellular system, it's dependence on

several factors and the methods to increase the

capacity have also been studied

The main objective of these points is to increase the

number of users in the system

Derivation of Q in cellular communication

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