lecture 2_cellular concept

7/29/2019 Lecture 2_Cellular Concept

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Lecture 2:

Cellular ConceptDr. Osama M. H. AminElectrical Engineering Department

Assiut University

Wireless Communications

November 11, 2012 1


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What is this Lecture about?

Introduction Frequency Reuse and Mobility Management Cell Cluster Concept

Co-channel and Adjacent Channel Interference Call Blocking and Delay at the Cell Site Mechanisms for Capacity Increase Channel Assignment Strategies First Suggested Project Reference: Chapter 5: Fundamentals of Cellular

Communications, Wireless Communications and Networking, John W.

Mark and Weihua Zhuang, Prentice-Hall Inc.

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1- Introduction

Wireless channel exhibits impairments more severe than guidedwire channels, yielding

u Bad Signal-to-Interference and Noise ratio and (SINR)u Low coverageu High Bit Error Rate (BER)u Degraded throughputu Reduction of usable spectral width

Mitigation of interference can enlarge the spectral width, thenmore users can be supported (larger throughput).

How?u Modulation and coding techniquesu Detection techniquesu

Proper design of cellular communication system.

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Serving larger population large geographical area coverage. How? Use single transmitter with very high power and very high

antenna.

Is this sufficient?u No, all users will share the same frequency set and maximum number of

users will be limited, i.e. limited system capacity.

u The reason of this limitation is that the radio resources is not efficientlyused.

How to increase system capacity?u Frequency reuse

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2-Frequency Reuse and Cellular Communication

Assign the same set of radio resources to serve a smallergeographical area and then reused to serve another small

geographical area, and so on

The small geographical area called cell, or afootprint. Whichtopology is best fit as cell model ? Circle or one of the regularshapes?

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Each cell has is served by:1. Single base station (access point (AP)), or a cluster of cells. Channel from

base station to user is called forward channel or downlink. And Channelfrom a user the base station is called reverse channel or uplink.

2. Mobile switching center (MSC) connected to a the base station bywireline. MSC has more computing power than the base stations,therefore most of the communication operations are handled by the MSC.

Advantages:1. Lower power transmitters with lower antennas height is used.2. Total system capacity is increased.

Disadvantages:1. Interference2. Because of the differences in terrain and population densities, real

footprints are irregular in nature.

3. Cellular design must permit serving mobile users between cells, i.e. theconnection must handed off from the serving base station to the new

one.

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3- Mobility Management

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Handoff Managementu Definition: When a mobile moves into a different cell while a session is in a

progress, the session has to be transferred to a new channel belonging tothe new cell, Handoff.

uOperation: Handoff involves the identification of a new base station andthe allocation of channels to support both data and control signals in thenew base stations.

u Handled by: MSC Location Managementu A mobile host (MH) is assigned a home network and identified by a home

address by its home agent.

u When MH moves away from the home network and enters a foreignnetwork, it registers with its home agent through the foreign agent.


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4- Cell Cluster Concept

The channels used in the forward and reverse directions areseparated in time or in frequency to permit duplexing.

The capacity of a cellular system is defined by the total numberof channels available. And this depend on how the available

channels are deployed.

Cells which use the same set of frequencies are referred tocochannel cells, and the resulting interference is called cochannel

interference.

Cell cluster : a group of cells that use a different set offrequencies in each cell. Cluster size =Ncells

Each cell is allocatedJchannelsAvailable channels in a cluster is K=JN

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5- Capacity Expansion by Frequency Reuse

TheNcells use the the complete set of available frequencies orchannels (K) K = JN

For a given available channels, the decrease in the cluster size isaccompanied by an increase in the number of channelsJ

allocated per cell, which can increase the cell capacity.

The entire cellular communication system consists of replicatedMclusters. Thus the system capacityCis given by

C=MJN

For the same available channels per cluster (K) and the samecoverage geographical area:

u Decreasing number of cells per cluster (N),Jmust be increased to maintainKconstant.

u Mhas to be increased to maintain the same coverage area.

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u Then the system capacity is increased.WhenNis minimized, Cis maximized for the same geographicalcoverage area and total number of channels without reuse K.What is the cost?

Increasing cochannel Interference

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6- Cellular Layout for frequency Reuse


Rule for determining the nearest cochannel neighbors:1. Move icells along any chain of hexagons2. Turn 60 degrees counterclockwise and movejcells.u N=i2+ij+j2


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Advantages of Cellular Systems:a) Use low power transmitterb) An allowance for frequency reuse

Frequency reuse needs to be structured so that cochannelinterference is kept at an acceptable level.

As the distance between cochannel cells increases, cochannelinterference will decrease.

If the cell size is fixed, the average signal to cochannelinterference ratio will be independent of the transmitted power

of each cell.



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7- Geometry of Hexagonal Cells

Dis the radius between two adjacent cochannel


Dnorm2

= j2

cos2

30( )+ i + jsin 30( )( )2

= i2+ j

2+ ij

Dnorm

= N

D = Dnorm

3R = 3N R


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8- Frequency Reuse Ratio


The frequency reuse ratio, q, is defined as:q =

D

R= 3N


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9- Cochannel and Adjacent Channel Interference



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Worst Case Cochannel InterferenceN=7


S

I=

Rk

2 D R( )k+ 2D

k+ 2 D + R( )

k

=1

2 q1( )

k

+ 2qk

+ 2 q + q( )k

N b 11 2012 20


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Adjacent Channel Interference


N b 11 2012 21


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N b 11 2012 22


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Other Mechanisms for Capacity Increase

A) Cell Splitting One way to perform cell splitting is to divide congested cell into

smaller cells, each with its own base station and a corresponding

reduction in antenna height and transmit power.


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With more cells, there will be more clusters in the same coveragearea, which is equivalent to replicating a cell cluster more times,

i.e. replication factorMis increased.Advantages of Cell splitting

1. Capacity Increase2. Reducing the call blocking probability3.

Reduce transmit power

4. Reduce antenna height


Pr= P

0

d

d0

k

Pt1R

k= P

t2

R

2

k

Pt1

Pt2

= 2k

k= 2

4

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B) Directional Antennas (Sectoring)

In the basic form, antennas are omnidirectional. Directional antenna can increase the system capacity relative to

that of omnidirectional antennas


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The worst case SIR is given by

WhereNI depends on the form of antenna used. For 7-cell reuseand omnidirectional case NI=6, while for 3-sector directional

case,NI=2. Assume Di=D


S

I=

Rk

Di( )

k

i=1

NI

SI

omni

=1

6q

k

S

I

120

o

=

1

2q

k

q =D

R

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Worst Case Scenario in 120o

When the mobile is located at the corner of the cell, Ris the cellradius and Dis the distance between the adjacent cochannel

cells.


S

I

120o

=R

4

D4+ D + 0.7R( )

4

=1

q4+ q + 0.7( )

4

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C) Channel Assignment Strategies

Fixed Channel Assignment (FCA)u In FCA, each cell is allocated a predetermined set of voice channels. Any

attempt within can only be served by the unused channels in that particularcell.

u To improve utilization, a borrowing option may be considered, where a cellis allowed to borrow channels from neighboring cell if all of its ownchannels are already occupied and the neighboring cell has spare channels.

u Borrowing is normally supervised by the MSC Dynamic Channel Assignment (DCA)u In DCA, voice channels are not assigned to cells in a permanent basis.u MSC determines dynamically the availability of a channel and executes its

allocation procedure accordingly.

u MSC needs to collect real-time data on channel occupancy.


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Coordinated Multi-Point Transmission (CoMP)


CoMP was introduced in LTE-Advanced technology to to improvethe spectral efficiency and data throughput of LTE systems.

Coordinated multipoint or cooperative MIMO is one of thepromising concepts to improve cell edge user data rate and spectralefficiency beyond what is possible with MIMO OFDM in the first

versions of LTE or WiMAX.

Interference can be exploited or mitigated by cooperation betweensectors or different sites. Significant gains can be shown for both theuplink and downlink.

Multiple sectors of one base station(eNB in 3GPP LTE terminology)

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CoMP Challenges

Clustering: Suitable clusters of cooperating base stations haveto be found, which can be done in a static way or dynamically.

Synchronization: Cooperating base stations have to besynchronized in frequency such that intercarrier interference is

avoided, and in time in order to avoid both intersymbol andintercarrier interference. The maximum distance of cooperatingbase stations is limited since different propagation delays of

different terminals may conflict with the guard interval. Thisaspect may be compensated through a more complexequalization.

Channel estimation: A large number of eNBs in the COMPcluster in the UL will require a larger number of orthogonal ULpilot sequences.




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At some cluster sizes, the COMP gains are outweighed bycapacity losses due to additional pilot effort.

Complexity: The above mentioned field trials have beenperformed using orthogonal frequency-division multiple access

(OFDMA) in the UL, as this enables a subcarrier and symbol-wise MIMO equalization and detection in the frequency domain.

If single-carrier (SC)-FDMA was used as in LTE Release 8,equalization would be more complex.

Backhaul: It can be a severe issue if centralized decoding isapplied. Hence, adaptive decentralized/centralized cooperationappears to be an interesting option. Furthermore, source codingschemes appear interesting for backhaul compression.




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Femtocell

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Femtocells are small, low-cost base stations located in the smallspaces such as houses and offices. The maximum allowed

transmit power is low compared to transmit power of a

macrocell base station. Thus, the subscribers demands of higher

data rates with low delays are granted by femtocell deployment.



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Why Femtoceolls

why current cellular systems need to implement femtocells aresummarized as follows:

1. Coverage: Macrocells are inadequate when providing indoor coveragedue to the signal attenuation while penetrating the outer walls of thebuildings. However, the signal strength is good when there is small

distance between the transmitter and the receiver.2. Capacity: Since the coverage area of the femtocells is smaller than

that of macrocells, there is less number of users in the cells and eachuser has a larger share of radio resources compared to themacrocellular networks.

3. Power:The macrocells handle a large number of users. When someusers are passed on to femtocell base stations which decreases theload of the macrocells. Hence the air interface is maintainedeffectively for both outdoor and indoor users and the powerconsumed at the macrocell is reduced.

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Femtocell Benefits to End Usersu Reduced in home call chargesu Improved indoor coverageu

Continued use of current handsetu Reduced battery drainu One consolidated billu Multiple users/linesu Landline support

Femtocell Benefits to Mobile Operatorsu Improves coverageu Reduces backhaul trafficu Provides capacity enhancementsu Enables triple playu Addresses the VoIP threatu

Stimulates 3G usageu Captures termination feesu Allows for multiple users/linesu Addresses the fixed mobile convergence market with a highly attractive and efficient

solution

,



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Thank you!

Next Lecture will be on:Characterization of Wireless Channel

Dont forgetu First Exam next week on cellular concept

Suggested First Projectu Coordinated Multipoint: Concepts, Performance, Deployment Scenarios

and Operational Challenges and Field Trial Results

u Massive MIMO rule in Cellular Conceptu Effect of cell size on previous items.

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lecture 2_cellular concept

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