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Cellular Concept Dr. Abdel Fattah Ahmed Sheta 1 EE EE424 424 Communication Communication Systems Systems Abdel Fattah Abdel Fattah Sheta Sheta Part III Part III-A Wireless Communications Wireless Communications The Cellular Concept Large Coverage Area using: Single High Power Transmitter Early Mobile Radio Single High Power Transmitter Antenna Mounted on a tall Tower Good Coverage Difficult to reuse the same frequencies throughout the system due to significant interference (No spectrum sharing a lot of bandwidth is dedicated to a single call) Limited capacity 1947 – 1977 1946 FCC allocates 33 FM channels in 33, 150 , 450 MHz bands 1960 Direct dialing from automobile in home area

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Page 1: Cellular-Concept

Cellular Concept

Dr. Abdel Fattah Ahmed Sheta 1

EEEE424 424 Communication Communication SystemsSystems

Abdel Fattah Abdel Fattah ShetaShetaPart IIIPart III--AA

Wireless CommunicationsWireless Communications

The Cellular Concept

Large Coverage Area using:Single High Power Transmitter

Early Mobile Radio

Single High Power TransmitterAntenna Mounted on a tall Tower

• Good Coverage• Difficult to reuse the same frequencies throughout the system

due to significant interference (No spectrum sharing a lot of bandwidth is dedicated to a single call)

Limited capacity• 1947 – 1977

• 1946 FCC allocates 33 FM channels in 33, 150 , 450 MHz bands• 1960 Direct dialing from automobile in home area

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Dr. Abdel Fattah Ahmed Sheta 2

The Cellular Concept Cont’d.

• developed by Bell Labs 1960’s-70’s• areas divided into cells• The cell is served by a base station with lower power

transmitter• Each cell gets portion of total number of channels• Neighboring cells assigned different groups of channels, to

minimize interference• The available channels can be reused as many times as

necessary as long as the interference between co-channelnecessary as long as the interference between co-channel stations is kept below acceptable levels

• Cells using the same channels should be spaced enough to reduce co-channel interference

The First Generation (1G)

USA Advance Mobile Phone Service (AMPS)• used FDMA with 30 KHz FM-modulated voice channels. • The FCC initially allocated 40 MHz of spectrum to this

system which was increased to 50 MHz shortly after servicesystem, which was increased to 50 MHz shortly after service introduction to support more users.

• This total bandwidth was divided into two 25 MHz bands, one for mobile-to-base station channels and the other for base station-to-mobile channels.

Europe Total Access Communication System (ETACS)

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Cellular Concept

Dr. Abdel Fattah Ahmed Sheta 3

Many of the first generation cellular systems in Europe were incompatible, and the Europeans quickly converged on a uniform standard for second generation (2G) digital systems called GSM.

The Second Generation (2G)

standard for second generation (2G) digital systems called GSM.

(GSM) Groupe Spéciale Mobile changed to GlobalSystems for Mobile Communications.

In USA two standards in the 900 MHz cellular frequency band:

IS-54, which uses a combination of TDMA and FDMA and phase-shift keyed modulation

IS-95, which uses direct-sequence CDMA with binary modulation and coding.

• In Japan The digital cellular standard is similar to IS-54 and IS-136 but in a different frequency band

• The GSM system in Europe is at a different frequency than the GSM systems in the U.S.

• Incompatible standards makes it impossible to roam betweensystems nationwide or globally without a multi-mode phone and/or multiple phones (and phone numbers).

• The second generation digital cellular standards have been enhanced to support high rate packet data services [15] (2.5 G) GSM systems provide data rates of up to 100 Kbps by aggregating all timeslots together for a single user.

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Dr. Abdel Fattah Ahmed Sheta 4

• Add broadband data to support video, internet access and other high speed data services for mobile devices.

• It is based on a wideband CDMA

The 3G

• The standard, initially called International Mobile Telecommunications 2000 (IMT-2000), provides different data rates depending on mobility and location, from 384 Kbps for pedestrian use to 144 Kbps for vehicular use to 2 Mbps for indoor office use.

• The 3G standard is incompatible with 2G systems• Service providers must invest in a new infrastructure before they can

id 3G iprovide 3G service.

• The first 3G systems were deployed in Japan.

• In fact 3G systems have not grown as anticipated in Europe, and it appears that data enhancements to 2G systems may satisfy user demands.

The Cellular Concept

Frequency Re-use

• Cells with the same letter use h f f ithe same set of frequencies

• A cell cluster is outlined in bold• A cell cluster is replicated over

the coverage area• Cluster size N = 7 cells• Frequency reuse factor = 1/7

(each cell contains one seventh(each cell contains one-seventh of the total number of channels

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Cellular Concept

Dr. Abdel Fattah Ahmed Sheta 5

Cell Shape

• The actual radio coverage of a cell is known as the footprint and is determined from field measurement or propagation prediction models

• A real footprint is amorphous in nature

• A cell must be designed to serve the weakest signal in the footprint.

• Regular shapes:Regular shapes:SquareEquilateral triangle and Hexagonal

• adjacent circles can not be overlaid upon a map without leaving gaps or creating overlapping regions.

Ex. hexagon geometry cell shape• Designed to serve the weakest mobiles within the footprint

(typically located at the edge)

Cell Shape

• The hexagon has the largest area of the three regular shapes.

• Simplistic model, Universally adopted

• fewest number of cells can cover a geographic region

• Approximate circular shape no gapsno overlap equal area

systematic system design

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Dr. Abdel Fattah Ahmed Sheta 6

Geometry of a Hexagon

R

R R

Surface area is 6R2 √3 / 4

• Base station location:• At the center of the cell (Omni-directional antenna)• At the vertices of three cells (directional antennas)

Base Station Location

Practical considerations usually do not allow base stations to be placed exactly as they appear in the hexagonal layout (~1/4 cell radius away from the ideal location)

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Assume the following system parameters:K Number of channels in a cell

Cluster Size and System Capacity

N Number of cells/cluster (Cluster size)M Number of times the cluster is repeated S = KN Number of channels in a clusterC Total number of channelsC = MkN = MS

A cluster has N cells with unique channels

Cluster size N (with cell size const) more clusters are required to cover a given area C Co-channel cells become closer

Cluster Size & System Capacity

Cluster size N (with cell size const) the ratio between cell size and the distance between co-channel cells is large

Design Objectives for Cluster Size1. High spectrum efficiency

• many users per cell• small cluster size gives much bandwidth per cell

2. High performance• Little interference• Large cluster sizes

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The effect of decreasing cell size• Increased user capacity• Increased number of handovers per call• Increased complexity in locating the subscriberIncreased complexity in locating the subscriber• Lower power consumption in mobile terminal:

· Longer talk time,· Safer operation

• Different propagation environment, shorter delay spreads• Different cell layout,

· lower path loss exponent, more interference· cells follow street pattern· more difficult to predict and plan· more flexible, self-organizing system needed (cf. DECT vs. GSM)

• The power transmitted by each base station d b l h i ll

Transmit Power Constraint

needs to be large enough to cover its own cell, but small enough to not cause too much interference in the co-channel cells

• As cells get smaller, transmit power is reduced

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• There are only certain cluster sizes and cell layout which are possible in order to connect without gaps b dj ll

Cluster Size and System Capacity Cont.

between adjacent cells

• N = i2 + ij + j2 where i and j are non-negative integers• Example i = 2, j = 1

– N = 22 + 2(1) + 12 = 4 + 2 + 1 = 7N 2 2(1) 1 4 2 1 7

Typical Cluster Sizes

N = 1, 3, 4, 7, 9, 12, 13, 16, 19, 21 ……………

• Frequency Reuse is the core concept of cellular mobile radio

Frequency Reuse Again

• Users in different geographical areas (in different cells) may simultaneously use the same frequency

• Frequency reuse drastically increases i ffi iuser capacity and spectrum efficiency

• Frequency reuse causes mutual interference (trade off link quality versus subscriber capacity)

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Dr. Abdel Fattah Ahmed Sheta 10

Frequency Reuse

Example N=19(i=3, j=2)

To find the nearest co-

Nearest co-channel

channel neighbors of a particular cell:

• move i cells along any chain or hexagon.

• then turn 60 degrees counterclockwise and move j cells.

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Dr. Abdel Fattah Ahmed Sheta 11

Nearest co-channel

Example 3.1

• 33 MHz bandwidth is allocated to a particular FDD cellular telephone system. Two 25 kHz simplex channels to provide full duplex voice and control channels,to provide full duplex voice and control channels,

(1) compute the number of channels available per cell if a system uses (a) 4-cell reuse, (b) 7-cell reuse (c) 12-cell reuse.

If 1 MHz of the allocated spectrum is dedicated to control channels,

(2) determine an equitable distribution of control channels and voice channels in each cell for each of the three systems.

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Dr. Abdel Fattah Ahmed Sheta 12

Total bandwidth 33 MHzChannel BW=25 kHz×2 simplex channels =50 kHz/duplex channelTotal available channels = 33 000/50 = 660 channels

Example 2.1

Total available channels = 33,000/50 = 660 channels

(a) For N= 4,total number of channels available per cell 660/4 = 165 channels.

(b) For N= 7, total number of channels available per cell = 660/7 = 95 channels.

(c) For N= 12,total number of channels available per cell = 660/12 = 55 channels.1 MHz = 20 control channels

• Fixed Channel Assignments

– Each cell is allocated a predetermined set of voice channels.

Channel Assignment Strategies

– If all the channels in that cell are occupied, the call is blocked, and the subscriber does not receive service.

– Variation includes a borrowing strategy: a cell is allowed to borrow channels from a neighboring cell if all its own g gchannels are occupied.

– This is supervised by the Mobile Switch Center: Connects cells to wide area network; Manages call setup; Handles mobility

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• Dynamic Channel Assignments

• Voice channels are not allocated to different cells permanently.

Channel Assignment Strategies

Voice channels are not allocated to different cells permanently.

• Each time a call request is made, the serving base station requests a channel from the MSC.

• MSC then allocates a channel to the requested call according to l i h ki i diff f falgorithm taking into account different factors: frequency re-use

of candidate channel and cost factors.

• Dynamic channel assignment is more complex (real time), but reduces likelihood of blocking

• Reasons for handoverMoving out of rangeLoad balancing

Handoff

• Handover scenariosIntra-cell handover (e.g., change frequency due to narrowband interference)Inter-cell, intra-BSC handover (e.g., movement across cells)

SC i SC h d ( SC)Inter-BSC, intra-MSC handover (e.g., movement across BSC)Inter MSC handover (e.g., movement across MSC)

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Dr. Abdel Fattah Ahmed Sheta 14

• Designers must specify an optimum signal level at which to initiate a handoff.

Handoff

• Margin (Δ) is defined Δ = handoff threshold - Minimum acceptable signal to maintain the call

• If Δ too small:Insufficient time to complete handoff

Improper handoff situation

before call is lostMore call losses

• If Δ too large:Too many handoffsBurden for MSC

Proper handoff situation

Call Dropped

Handoff is not made and call is dropped if:

• Large delay by the MSC in assigning a handoff.a ge de ay by t e SC ass g g a a do .

• Threshold margin (Δ) is set too small for the handoff time in the system.

• Excessive delays may occur during high traffic conditions due to computational loading at the MSC

• No channels are available on any of the nearby base stations (thus forcing the MSC to wait until a channel in a nearby cell becomes free

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Dr. Abdel Fattah Ahmed Sheta 15

• The base station monitors the signal level for a certain period of time before a hand- off is initiated. So that unnecessary handoffs are avoided (signal drop may be

Handoff is Necessary?

unnecessary handoffs are avoided. (signal drop may be due to momentary fading).

• The length of time needed to decide if a handoff is necessary depends on the speed at which the vehicle is moving.

• If the slope of the short-term average received signal level in a given time interval is steep, the handoff should be made quickly.

It is the time over which a call may be maintained within a cell, without handoff

Dwell Time

• Depends on: Propagation, interference, distance betweenthe subscriber and the base station, and other time varying effects. (the speed of the user and the type of radio coverage)

• Even a stationary subscriber may have a random and finite dwell Even a stationary subscriber may have a random and finite dwell time due to fading effect.

• Statistics of dwell time are important in practical design of handoff algorithms.

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Dr. Abdel Fattah Ahmed Sheta 16

Styles of Handoff• Network Controlled Handoff (NCHO)

– in first generation cellular system, each base station constantly monitors signal strength from mobiles in its cell

– based on the measures, MSC decides if handoff necessary– mobile plays passive role in process– burden on MSC

• Mobile Assisted Handoff (MAHO)– present in second generation systems– mobile measures received power from surrounding basemobile measures received power from surrounding base

stations and report to serving base station– handoff initiated when power received from a neighboring cell

exceeds current value by a certain level or for a certain period of time

– faster since measurements made by mobiles, MSC don’t need monitor signal strength

Intersystem Handoff

• If a mobile moves from one cellular system to different cellular system controlled by a differentdifferent cellular system controlled by a different compatible MSC.

• When a mobile signal becomes weak in a given cell and the MSC cannot find another cell within its system to which it can transfer the call in progress.

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Dr. Abdel Fattah Ahmed Sheta 17

Prioritizing Handoff

Dropping a call is more annoying than line busy• Guard channel concept (Decrease the probability of forced termination due to lack of

available channels)available channels)Reserve some channels for handoffsWaste of bandwidthBut can be dynamically predicted

• Queuing of handoff requests (due to lack of available channels)There is a finite time interval between time for handoff and time to drop (signal goes below the handoff threshold).Better tradeoff between dropping call probability and network traffic.

(1) Practically, several problems arise when attempting to design for a wide range of mobile velocities.

Practical Handoff Considerations

• High speed vehicles pass through the coverage region of a cell within a matter of seconds, whereas pedestrian users may never need a handoff during a call.

• Particularly with the addition of microcells to provide capacity, the MSC can quickly become burdened if high speed users are

l b i d b ll llconstantly being passed between very small cells.

(2) Another practical limitation is the ability to obtain new cell sites. Inpractice it is difficult for cellular service providers to obtain newphysical cell site locations in urban areas.

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Dr. Abdel Fattah Ahmed Sheta 18

• Is used to provide large area coverage to high speed users while providing small area coverage to users traveling at low speeds.

The umbrella Cell Solution

• By using different antenna heights (often on the same building or tower) and different power levels, it is possible to provide large and small cells which are co-located at a single location.

• # handoffs is minimized for high speed users and provides additional microcell channels for pedestrian users.

• If a high speed user in the large umbrella cell is approaching the base station, and its velocity is rapidly decreasing, the base station may decide to hand the user into the co-located microcell, without MSC intervention.

Umbrella Cell Approach

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Dr. Abdel Fattah Ahmed Sheta 19

• As the user travels away from the base station at a very slow speed, the average signal strength does not decay rapidly.

Cell Dragging (Pedestrian users emit very strong signal to the base station)

• Even when the user has traveled well beyond the designed range of the cell, the received signal at the base station may be above the handoff threshold, thus a handoff may not be made.

Interference and traffic management problem, since the user has meanwhile traveled deep within a neighboring cell.To solve this problem, handoff thresholds and radio coverage parameters must be adjusted carefully.

Interference is the major limiting factor in performance of cellular radio systems

• Sources of interference:Mobiles in same cell

Interference and System Capacity

– Mobiles in same cell– A call in progress in a neighboring cell– Other base stations operating in the same frequency band– Non-cellular system leaking energy into the cellular

frequency band• Effect of interference:

Cross talk in voice channels– Cross talk in voice channels– For control channels missed or blocked calls

• The two main types are:– co-channel interference– adjacent channel interference

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Dr. Abdel Fattah Ahmed Sheta 20

Co-channel cells: Cells that use the same set of frequencies

Unlike thermal noise which can be overcome by increasing the

Co-channel Interference

Unlike thermal noise which can be overcome by increasing thesignal-to- noise ration (SNR), co-channel interference cannot becombated by simply increasing the carrier power of a transmitter.

To reduce co-channel interference, co-channel cells must bephysically separated by a minimum distance to provide p y y p y psufficient isolation due to propagation.

Co-channel Interference

When the size of each cell is the same, and the BSs transmit the same power, the co-channel interference ratio depends on:• The radius of the cell (R)

• Co-channel reuse ratio: Q = D/R = (3N) (Hexagonal Geometry)

• Q increases Interference decreases

• The distance between centers of the nearest co-channel cells (D)

• Q decreases Interference increases (cluster size N decreases and system capacity increases)

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Dr. Abdel Fattah Ahmed Sheta 21

Co-channel Reuse Ratio

Signal-to-Interference Ratio

The signal-to-interference ratio (S/I or SIR) for a mobile receiver which monitors a forward channel (Down Link Channel) =

∑=

=0

1

i

iiI

SIS

C a e )

S : The desired signal power from the desired base stationS : The desired signal power from the desired base stationIi : The interference power caused by the ith interfering

co-channel cell base station.i0 : The number of co-channel interfering cells.

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Dr. Abdel Fattah Ahmed Sheta 22

Average Received Power

(Propagation measurements)The average received power Pr at a distance d from the transmitting antenna is approximated bytransmitting antenna is approximated by

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

⎟⎟⎠

⎞⎜⎜⎝

⎛=

0

0

log10)()(ddndBmPdBmP

ddPP

or

n

or

Where P0 is the power received at a close-in reference point in the far field region of the antenna at a small distance d0 from the transmitting antenna, and n is the path loss exponent. n ~ 2 to 4 in urban cellular systems.

Assumptions• The interference is due to co-channel base stations.• The transmit power of each base station is equal• The path loss exponent is the same throughout the coverage area,S/I f bil b i t d

Co-channel Interference

S/I for a mobile can be approximated as

AMPS (FM 30 KHz channel bandwidth) S/I=18dB (sufficient quality) If n=4 N needs to be larger than 6 49 ~ 7

00

1

)3()/(

)(0 i

Ni

RD

D

RIS nn

i

i

ni

n

===

∑=

If n=4, N needs to be larger than 6.49 ~ 7

Thus a minimum cluster size of seven is required to meet S/I = 18 dB.

All the interfering cells are assumed to be equidistant from the base station receiver. (Good for large N)

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Dr. Abdel Fattah Ahmed Sheta 23

Co-Channel cell for 7 cells reuse

Assume n=4, the signal-to- interference ratio for the worst case can be closely approximated as

In terms of co-channel reuse ratio Q = 4.6 for N = 7

17 dB (f N 7)

Co-channel Interference

17 dB (for N = 7)

Exact solution using the equation

17.8 dB.∑ −

=0

)(i

ni

n

D

RIS

=1i

Slightly less than 18 dB

it would be necessary to increase N to the next largest size, which is found to be 12 (corresponding to i = j = 2).

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Dr. Abdel Fattah Ahmed Sheta 24

Example 3.2

S/I = 15 dB , Frequency reuse factor 1/N? n=4

(a) n=4 ( )Let N = 7 , then Q = D/R = 3N

dBiN

IS 66.18)3(

0

4

== Which is greater than 15 dB, N=7 is good value

(b) n=3 Let N = 7 , then Q = D/R = 3N

dBiN

IS 05.12)3(

0

3

== It is less than required 15 dB, N=7 More N should be used 9,

12, 19, ……. check

Adjacent Channel Interference

Origin: Arising from signals which are adjacent in frequency to the desired signal

Become serious by• Imperfect receiver filters which allow nearby

frequencies to leak into the passband(near-far-effect)

• A mobile close to a base station transmits on a channel closeA mobile close to a base station transmits on a channel close to one being used by a weak mobile. The base station may have difficulty in discriminating the desired mobile user.

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Dr. Abdel Fattah Ahmed Sheta 25

Adjacent Channel Interference Example

• Adjacent channel interference can be minimized through careful filtering (High Q filters) and channel assignments

Adjacent Channel Interference

careful filtering (High Q filters) and channel assignments.

• Since each cell is given only a fraction of the available channels, a cell need not be assigned channels which are all adjacent in frequency. (By keeping the frequency separation between each channel in a given cell as large as possible).

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If a mobile is 20 times as close to the base station as another mobile and has energy spill out of its passband, the signal-to- interference ratio at the base station for the

Example

weak mobile (before receiver filtering) is approximately

For a path loss exponent n = 4, this is equal to -52 dB. If the intermediate frequency (IF) filter of the base station receiver has a slope of 20 dB/octavehas a slope of 20 dB/octave

Then an adjacent channel interferer must be displaced by at least six times the passband bandwidth from the center of the receiver frequency passband to achieve 52 dB attenuation

How channels of AMPS are divided into subsets to minimize adjacent channel interference?

Case Study (Example 3.3)

666 duplex channels

In 1989, the FCC allocated an additional 10 MHz (166 new channels) There are now 832 channels

Forward channels 1 666 (870.030 889.98 MHz) Reverse channel s 1 666 (825 030 844 98 MHz )Reverse channel s 1 666 (825.030 844.98 MHz )

Extended channels 667 799 and 990 1023.

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Case Study cont.

Two operators (416 channels for each)

Channels distinguished as block A and block B

Case Study cont.

416 channels 395 voice & 21 control channels

1 312 voice channels313 333 control channels

334 354 control channels

block A channels

block B 334 354 control channels355 666 voice channels

667 716 & 991 1023 extended Block A voice channels

717 799 extended Block A voice channels

channels

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Dr. Abdel Fattah Ahmed Sheta 28

395 voice channels = 21 subsets x ~ 19 channels

In each subset the closest adjacent channel is 21 channels

Case Study cont.

In each subset, the closest adjacent channel is 21 channels away.

In a 7-cell reuse system, each cell uses 3 subsets of channels

The 3 subsets are assigned such that every channel in the cell is assured of being separated from every other channel by at least g p y y7 channel spacing.

In the following Table:

E h ll h l i th b t i + i + i

Case Study cont.

Each cell uses channels in the subsets, iA + iB + iC, where i is an integer from 1 to 7.

The total number of voice channels in a cell is about 57

The channels listed in the upper half of the chart belong to block A and those in the lower half belong to block Bblock A and those in the lower half belong to block B.

The shaded set of numbers correspond to the control channels which are standard to all cellular systems in North America

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Dr. Abdel Fattah Ahmed Sheta 29

Power Control for reducing Interference

The power levels transmitted by every subscriber unit are under constant control by the serving base stations.

• Each mobile transmits the smallest power necessary to maintain good quality link

• Power control prolong battery life for the subscriber unit

• Power control reduces the reverse channel S/I in the system.

• Power control is especially important for emerging CDMA spread spectrum systems that allow every user in every cell to share the same radio channel.

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Trunking and Grade of Service

• Trunking is the aggregation of multiple user circuits into a single channel.

• The aggregation is achieved using some form of multiplexing.C ll l di t l t ki t d t• Cellular radio systems rely on trunking to accommodate a large number of users in a limited radio spectrum.

(SLC) Subscriber line concentrator

The Concept of Trunking

• Large number of users share small number of channels.

• Assigning users channels on demand

• Each cell has pool of channels

• When user requires service, channel allocated to user

• When user no longer requires service, channel returned to pool to be allocated to next user

• The user is blocked (denied access) when all radio channels are already in use.

• A queue may be used to hold the requesting users until a channel becomes available.

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Important to design trunked radio systems that can handle a specific capacity at a specific grade of service, GOS

T ki th d l d b E l

Trunking Theory

Trunking theory was developed by Erlang

• Erlang based his studies of the statistical nature of the arrival and the length of calls. The measure of traffic intensity bears his name

• One Erlang represents the amount of traffic intensity carried by a channel that is completely occupied (i e 1 call-hour perby a channel that is completely occupied (i.e. 1 call-hour per hour or 1 call-minute per minute).

• For example, a radio channel that is occupied for thirty minutes

during an hour carries 0.5 Erlangs of traffic

The Grade of Service (GOS)

• The grade of service (GOS) is a measure of the ability of a user to access a trunked system during the busiest hour

• It is used to define the desired performance of a particular trunked system.

• GOS is typically given as the probability that a call is blocked, or the probability of a call experiencing a delay, greater than a certain queuing time.

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• Set-up Time: The time required to allocate a trunked radio channel to requesting user

• Blocked Call: Call which cannot be completed at the time of request

Some Definitions used in Trunking Theory

• Holding Time (H) : Average duration of a typical call (H in seconds)

• Traffic Intensity (A): Measure of channel time utilization (average channel occupancy measured in Erlangs)

• Load: Traffic intensity across the entire trunked radio system (Erlangs)

• Grade of service (GOS): A measure of congestion which is specified as the probability of a call being blocked (Erlang B), or the probability of a call being delayed beyond a certain amount of time (Erlang C)

• Request Rate (λ): The average number of call request per unit time (S-1 )

Traffic Intensity (A)

The traffic intensity offered by each user (Au) is Au = call request rate × Holding time Au = λ H

Total offered traffic intensity (A) is A = U Auwhere U is the number of users in a given system

In a C channel trunked system, and if the traffic is equally distributedThe traffic intensity per channel AC = U Au/C

At a given time, if the offered traffic exceeds the capacity of the system (e.g., UAu > C), calls are blocked

The AMPS is designed for 2% GOS. I.e, 2 out of 100 calls will be blocked due to channel occupancy during the busiest hour

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Types of trunked systems

There are two systems commonly used.

1. System with no queue for call requests. If h l i il bl t ti d th i- If a channel is available, no setup time and the user is

given immediate access to the available channel

- If no channels are available, the requesting user is blocked without access and is free to try again later

-This type of trunking is called-This type of trunking is called (blocked calls cleared) - Erlangs B

2. System which a queue is provided to hold calls which are blocked. (Blocked Calls Delayed) Erlangs C

Erlang B formula

• Blocked calls cleared• The probability of blocking during the busy hour

GOS

kA

CA

blockingC

k

k

C

==

∑=0 !

!)Pr(

– Can use plot of Erlang B formula to determine one of the parameters: Pr(blocking), C, A

Au = λ H & A = U Au & AC = U Au/C

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Erlang B PlotNumber of trunked channels C

ing

Prob

abili

ty o

f Blo

cki

Traffic Intensity in Erlangs

Erlang B Example

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Blocked Calls Delayed• Blocking calls are delayed until channels are available, queuing• Erlang CThe probability of a call not

∑−

=> 1)0Pr( C k

C

AAdelay

having immediate access to a channel• The probability that a delayed call will have to wait longer than t

= e-(C-A)t/H

• Probability of delay larger than tPr[delay>t]= Pr[delay>0] e-(C-A)t/H

∑=

−+0 !

)1(!k

CAC

kACA

• The average delay D D = Pr[delay>0] H/(C-A)

GOS is defined in this case as the probability that a call is blocked after waiting a specific length of time in the queue.

Erlang C PlotNumber of trunked channels C

Prob

abili

ty o

f D

elay

Traffic Intensity in Erlangs

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Cellular Concept

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3.4 How many users can be supported for .5% blocking probability for the following number of trunked channels in a blocked calls cleared system? 1 5 10 20 100 Assume each user generates

Some Examples

cleared system? 1, 5,10, 20, 100. Assume each user generates 0.1 Erlangs of traffic (Au = 0.1).

For GOS = 0.005, then use Fig. 3.9 U = A/Au = A/0.1

C = 1, U = 1 User C = 5, U = 11Users

C= 10 , U = 39 Users C= 20, U = 110 Users

C = 100,U = 809 Users

• Pr(blocking) = 2%• Each user averages 2 calls per hour at an average s duration of 3 min./call

• System A: 394 cells, 19 channels/cell• System B: 98 cells, 57 channels/cell• System C: 49 cells, 100 channels/cell

Ex. 3.5 (2 million residents in an urban area)

• Find number of subscribers U that can be supported in each cell

• Traffic intensity by user Au = λ H = (2/60)(3) = 0.1 & Pr(blocked) = 0.02

• System A: C = 19, From Erlang B plot, A ~ 12 ErlangsU = A/Au = 12/0.1 = 120 users/cell, N = 120 users/cell * 394 cells = 47,280

• System B: C = 57 From Erlang B plot A ~ 45 Erlangs• System B: C = 57, From Erlang B plot, A ~ 45 ErlangsU = A/Au = 45/0.1 = 450 users/cell N = 450 users/cell * 98 cells = 44,100

• System C: C= 100, From Erlang B plot, A ~ 88 ErlangsU = A/Au = 88/0.1 = 880 users/cell N = 880 users/cell * 49 cells = 43,120

Percentage market penetration for systems A, B, and C and the market penetration of the three systems

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Ex. 3.6 A certain city has an area of 1,300 square miles and is covered by a cellularsystem using a seven-cell reuse pattern. Each cell has a radius of four milesand the city is allocated 40 MHz of spectrum with a full duplex channel bandwidth of 60 kHz. Assume a GOS of 2% for an Erlang B system is specified. If the offered traffic per user is 0.03 Erlangs, compute: (a) the number of cells in the service area(a) the number of cells in the service area,

= Total area/area of cell = 31 cells(b) the number of channels per cell,

=Total channels/7 = 40MHz/(7x60KHz) = 95 Channels/cell(c) traffic intensity of each cell,

From Erlangs B graph GOS=0.02, Au = 0.03, C=95 channels A … (d) the maximum carried traffic,

= number of cells × traffic intensity per cell = 31 × 84 = 2604 Erlangs.y p g(e) the total number of users that can be served for 2% GOS,

= Total traffic (Atot)/trafic intensity per user = 2604/0.03 = 86800(f) the number of mobiles per unique channel (where it is understood that channels are reused),= number of users/number of channels = 86800/666(g) the theoretical maximum number of users that could be served at one

time by the system = Number of the available channels in the system

A hexagonal cell within a four-cell system has a radius of 1.387 km. A totalof 60 channels are used within the entire system. If the load per user is0.029 Erlangs, and λ = 1 call/hour, compute the following for an Erlang Csystem that has a 5% probability of a delayed call:

(a) How many users per square kilometer will this system support?(b) What is the probability that a delayed call will have to wait for more

Ex. 3.7

(b) What is the probability that a delayed call will have to wait for more than 10 s?

(c) What is the probability that a call will be delayed for more than 10 seconds?

C = 60 channels, Au = 0.029, λ = 1/60 × 60 H = Au/ λ C/cell 60/4 = 15

(a) U = A/Au ( From Erlangs C graph find A for C = 15 and probability of delay = 0.05)

(b) The probability that a delayed call will have to wait longer than 10 s isPr [delay >t |delay]= e(–(C – A)t /H) = e(–(15 – 9.0)10/104.4) = 56.29%

(c) Probability that a call is delayed more than 10 seconds,Pr [delay >10] = Pr [delay >0]Pr [delay >t |delay] = 0.05 x 0.5629 = 2.81%

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Improving Coverage & Capacity Increasing

Demand of Services Increases?More channels per unit coverage area are needed

T h i t d th it f ll l tTechniques to expand the capacity of cellular systems:

1. Cell splitting – cells in areas of high usage can be split into smaller cells.Increase capacity by increasing the number of base stations.

2. Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels (Directional antennas tosectors, each with their own set of channels (Directional antennas to control interference) . Increase the load of the MSC and reducing trunking efficiency. Improve capacity by reducing co-channel interference.

3. Microcell zone (distributes the coverage of a cell and extends the cell boundary to hard -to-reach places.) Improve capacity by reducing co-channel interference.

Cell splitting

Cell splitting is achieved through:• Subdividing a congested cell into smaller cells (reducing cell radius

and keeping the D/R ratio unchanged)p g g )• Reduction in antenna height and transmitter power (different cells

will have different transmit power requirements to support cells of different sizes)

Some properties– Cell splitting enables more spatial reuse (greater system capacity)– Cell splitting preserves original frequency reuse plan– In practice, cells might have different coverage areas due to practical p , g g p

BS placement issues– Cell splitting causes increased handoff

• Can use “umbrella” cells where fast-moving mobiles covered by original cell and slower mobiles covered by microcells

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If we assume that the radius of every cell is reduced to half of its original value

Cell splitting Cont.

Four times as many cells would be required to cover the same area

Number of clusters over the

Number of channels increases

fcoverage region increases

Example

• The base stations are placed at corners of the cells • The original base station A is surrounded by six new

microcells• In this example the smaller cells added in such a way as to p y

preserve the frequency reuse plan of the system • Each microcell base station is placed half way between two

larger stations utilizing the same channel

Cell splitting simply scales the geometry of the cluster

The radius of each new microcell is half that of the original cell

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A

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• The transmit power of the new 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 Pr

Example Cont.

g y g p rat the new and old cell boundaries and setting them equal to each other

• Pr [at old cell boundary] ∝ Pt1R-n

• Pr [at new cell boundary] ∝ Pt2 (R/2)-n

• For n = 4 and set the received powers equal to each other

Pt2 = Pt1/16 the transmit power must be reduced by 12 dB in order to fill in the original coverage area with microcells, while maintaining the S/I requirement.

Channel Assignment• Not all cells are split at the same time • It is often difficult to find real estate that is perfectly situated

Practical problems in Cell splitting Cont.

for cell splitting • Different cell sizes will exist simultaneously• Special care needs to be taken to keep the distance between co-

channel cells at the required minimum, and hence channel assignments become more complicated

ffHandoff:High speed and low speed traffic should be simultaneously accommodated (the umbrella cell approach is commonly used).

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Practical problems in Cell splitting Cont.

In practice different cell sizes will exist simultaneously

• If the larger transmit power is used for ll ll h l d b h

Dall cells, some channels used by the smaller cells would not be sufficiently separated from co-channel cells

• If the smaller transmit power is used for all the cells, there would be parts of the larger cells left unserved

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Channels in the old broken into two channel groups: 1. The first one corresponds to the smaller cell reuse requirements2. The second corresponds to the larger cell

Practical problems in Cell splitting Cont.

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EC2. The second corresponds to the larger cell reuse requirements.

The larger cell is usually dedicated to high speed traffic so that handoffs occur less frequently.At the beginning: fewer channels in the small power groupsdemand grows: smaller groups will require more

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channels. splitting process continues until all the channels in an area are used in the lower power group cell splitting is complete within the region, and theentire system is rescaled to have a smaller radius per cell.

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Antenna downtilting, Focuses radiated energy from the base station toward the ground (rather than toward the horizon), to limit

Antenna downtilting

g ( ),the radio coverage of newly formed microcells.

Example 2.8

Rorig cell = 1 km & Rmic cell = 0.5 Km

Find the number of channels (N) contained in a

Each base station uses 60 channels, regardless of cell size

3 km by 3 km square centered around A under the following conditions:(a) without the use of microcells(b) when the lettered microcells as shown in

the figure are used(c) If all the original base stations are replaced

by microcells

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

(a) 5 cells are included N = 5×60 = 300 channels

(b) Number of cells = 5 + 6 = 11 N = 11×60 = 660 channels

(c) Number of cells = 5 + 6 + 5 = 17 N = 17×60 = 1020 channels

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Cell Sectoring

The uses of directional antennas improve S/I, then capacity improvement is achieved by reducing the number of cells in a cluster, thus increasing the frequency reuse. It is necessary to reduce

Keeping the cell radius unchanged and decreasing the D/R ratio

Number of clusters over the

the relative interference without decreasing the transmit power.

Number of channels increases

fcoverage region increases

• The factor by which the co-channel interference is reduced depends on the amount of sectoring used

• A cell is normally partitioned into three 120° sectors or

Reduction of Co-channel interference using sector antennas

A cell is normally partitioned into three 120 sectors or six 60° sectors as shown below

120° sectoring 60° sectoring

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• Out of the 6 co-channel cells in the first tier, only two of them interfere with the center cell

How 120° sectoring reduces interference from co-channel cells

A mobile in the center cell willA mobile in the center cell will experience interference on the forward link from only these two sectors. The resulting S/I for this case can be found fromto be 24.2 dB which is a significant improvementg pover the omnidirectional wherethe worst case S/I was shown to be 17 dBThis S/I improvement allows the decreasing the cluster size N in order to improve the frequency reuse, and thus the system capacity.

In practical systems, further improvement in S/I is achieved by downtilting the sector antennas such that the radiation pattern in the vertical (elevation) plane has a

Antenna downtilting

ad at o patte t e ve t ca (e evat o ) p a e as anotch at the nearest co-channel cell distance.

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• The S/I improvement is achieved at the cost of the number of antennas at each base station

• Sectoring decreases trunking efficiency due to channel sectoring at the base station

• Since sectoring reduces the coverage area of a particular group of channels, the number of handoffs increases

• Handed off from sector to sector within the same cell without intervention from the MSC

Example 2.9Consider a cellular system:H = two minutes GOS = less than 1%. λ = one call per hour Total traffic channels = 395 N = 7 blocked calls are cleared (Erlang B distribution)

channels/cell C = 395/7 = 57 traffic channels.

Unsectored (C=57) the system may handle 44.2 Erlangs or 1326 calls per hour.

120° t i C 57/3 9 h l t120° sectoring, C = 57/3 = 9 channels per antennaEach sector can handle 11.2 Erlangs or 336 calls per hourCell capacity of 3 × 336 = 1008 calls per hour (24% decrease)Thus, sectoring decreases the trunking efficiency while improving the S/I

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A Microcell Zone Concept

To solve the handoff and trunking efficiency problems raised due to sectoring option

• Large central base station is l d b l l

• Each of the three zone sites are connected to a

replaced by several lower powered transmitters (zone transmitters) on the edges of the cell.

sites are connected to a single base station and share the same radio equipment.

• Travel mobile is served by the zone with the strongest signal

• Any base station channel may be assigned to any zone by

A Microcell Zone Concept Cont.

Any base station channel may be assigned to any zone by the base station

• As a mobile travels from one zone to another within the cell, it retains the same channel and the base station simply switches the channel to a different zone site

• Decreased co-channel interference improves the signal quality and also leads to an increase in capacity without the degradation in trunking efficiency caused by sectoring.

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Dz /Rz of 4.6 can achieve the required link performance

A Microcell Zone Concept Cont.

The capacity of the system depends on the ratio D/R (not zones dependent)

Dz/Rz = 4.6 ~ equivalent to D/R = 3 which correspend to N = 3 ( bl 2 1)3 system (table 2.1)

Capacity increases by about 7/3

A Microcell Zone Concept Cont.

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Repeaters

Repeaters

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Outdoor Antennas

LNABPF

Repeaters

Indoor Antenna

LNABPF

Outdoor Antennas

LNABPF

Repeaters

Indoor Antenna

LNABPF

Fig. 3.3 Repeater bidirectional amplifier using duplexer and automatic gain control (AGC)