chapter 2 cellular 1

Chapter 2: The Cellular System

Graduate ProgramGraduate Program Department of Electrical and Computer Engineering

Goal of the Chapter

In cellular system, the available radio spectrum is limited E.g., because of regulatory issues Hence the number of simultaneous call supported is limited Hence, the number of simultaneous call supported is limited

How to achieve high capacity (or support simultaneous calls) at the same time covering very large areas?calls) at the same time covering very large areas? Frequency reuse by using cells?

Overview system design fundamentals on cellular y gcommunication Cell formation and the associated frequency reuse, handoff, and

power controlpower control

Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 2

Overview

Cellular system Cell shape

F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity


Cellular System - Architecture

R di tRadio tower

Mobile Switching

PSTNTelephoneNetwork


Mobile SwitchingCenter

Cellular System ….

High capacity is achieved by limiting the coverage of each base stations to a small geographic region called a cell Single high power transmitter (large cell) are replaced with many Single, high power transmitter (large cell) are replaced with many

low power transmitters (small cells)

A portion of the total number of channels is allocated to peach cell or base station Available group of channels are assigned to a small number of

neighboring base stations called clusterneighboring base stations called cluster Near by base stations are assigned d/t groups of channels to

minimize interference

Same channels (frequencies/timeslots/codes) are reusedby spatially separated base stations Reuse distance and frequency reuse planning?

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Reuse distance and frequency reuse planning?

Wireless Communications - Ch. 2 – Cellular System 5

Cellular System ….

A switching technique called handoff enables a call to proceed from one cell to another

As demand (or # of users) increases the number of base As demand (or # of users) increases, the number of base stations may be increased to provide additional capacity Smaller cells, e.g., Microcells, Picocell, Femtocell Also cell sites in trucks to replace downed cell towers after natural

disasters, or to create additional capacity for large gatherings (football games, rock concerts)

Transmission power reduction => interference decreases

Typical power transmitted by the radios in a cell systemBase station: Maximum Effective Radiated Power (ERP) is Base station: Maximum Effective Radiated Power (ERP) is 100W, or up to 500 W in rural areas

Mobile station: Typically 0.5 W. For CDMA, transmit power is lowered when close to a BS

Sem. II, 2010/11

lowered when close to a BS


Forward and Reverse Channels

Forward Voice Channel (FVC): Used for voice transmission from BS to MS

Reverse Voice Channel (RVC): Used for voice transmission from MS to BS

C C ( CC) f Forward Control Channel (FCC): Used for initiating a call from BS to MS

R C t l Ch l (RCC) U d f i iti ti ll Reverse Control Channel (RCC): Used for initiating a call from MS to BS


Anatomy of a Cellular Call

A cell phone, when turned on, (though not yet engaged in a call) scans the group of FCC to determine the one with the strongest signalstrongest signal

It monitors the channel until it drops below the usable thresholdthreshold

It then scans for another channel with the strongest signal

Control channels are defined and standardized throughout Control channels are defined and standardized throughout the service area

Typically the control channels use up to 5% of the total number of channels


A Call TO a Mobile User

The MSC dispatches the request to all the base stations The Mobile Identification Number (MIN) is broadcast as a paging

message over all FCC throughout the service area.g g

The MS receives the paging message from the BS it is monitoringg

It responds by identifying itself over the RCC

The BS conveys the handshake to the MSCy The MSC instructs the BS to move to an unused voice

channel


A Call TO a Mobile User. . .

The BS signals the MS to change over to unused FVC and RVC

A data message (called ‘alert’) is transmitted over the FVC to instruct the mobile to ring

f f f All of these sequences of events occur in just few seconds, and are not noticeable to the user

Whil th ll i i th MSC dj t th While the call is in progress, the MSC adjusts the transmitted power in order to maintain the call quality


A Call FROM a Mobile User

A call initiation request is sent to the RCC

Along with this, the MS transmits its MIN, Electronic Serial Number (ESN) and the phone number of the called party

The MS also transmits the Station Class Mark (SCM) which findicates the maximum transmitter power level for the

particular user

Th BS f d th d t t th MSC hi h lid t th The BS forwards the data to the MSC, which validates the data and makes connection to the called party through the PSTN


Overview




Cell Shape – Why hexagon?

• The hexagonal shape is a simplistic assumption

(a) is theoretical coverage area and (b) measured coverage area h d bl d ll i di t i l t th iwhere red, blue, green, and yellow indicate signal strength, in

decreasing order Footprint: Actual radio coverage and obtained experimentally

Actual shape is a random that depends on the environment Circular (theoretical): If path loss was a decreasing function

of distance say 1/dn where d is the distance b/n BS & MS

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of distance, say, 1/d , where d is the distance b/n BS & MS


Cell Shape – Required

Geometric shape that approximates the theoretical shape?theoretical shape?

Shape whose non-overlapping and repetitive placement covers an entire region?eg o

Possible shapes Triangles, squares, g , q ,

hexagons Which one to choose?

Has dead zonesHas dead zones


Cell Shape . . .

RR

RR

aT = 33/2 R2/4aR = 2R2 aH= 33/2 R2/2

Hexagonal cell is conceptual, however, it is universally g p , , yadopted for most theoretical treatment because:

Hexagons are a geometric shape that approximates a circle (for O i di ti l di ti )Omni-directional radiation)

Using a hexagon geometry, fewest number of cells can cover the entire geographic region


Cell Shape . . .

When using hexagon to model coverage areas Center-excited cell: Base station (BS) depicted as being in the

center of the cell Omni-directional antenna is used

Edge-excited cell: on three of the six cell verticesS t d di ti t i d Sectored direction antenna is used


Overview




Frequency Reuse – Example

Assume a city of 10 Million mobile users Let every user is allocated a radio spectrum for analog speech of 4

kHz bandwidth Thus the required bandwidth is 4 kHz * 10 Million users = 40 GHz!

Clearly impractical!N th i ibl i di t i i No other services possible using a radio transmission

Most of the spectrum will be unused most of the time

Cellular radio systems rely on intelligent allocation and reuse of channels through out the coverage area Available group of channels are assigned to a cluster Available group of channels are assigned to a cluster Same group of frequencies are reused to cover another cell

separated by a large enough distance, i.e., there is a tradeoff


Frequency Reuse – Example

Example: Consider a cluster of 7 cells

Same color labeled cells use the same frequencyfrequency

Frequency reuse factor is 1/7 since each cell

t i thcontains one-seventh of the total available channels


Geometry of HexagonsU

Vy

U

x

D


Geometry of Hexagons …

Axes U and V intersect at 60o

Unit distance is the distance between cell centersIf ll di t i t f h i R th If cell radius to point of hexagon is R, then 2Rcos 30o = 1 or R = 1/√3 (Normalized radius of a cell)

To find the distance of a point P(u,v) from the origin, use X-To find the distance of a point P(u,v) from the origin, use XY to U-V coordinate transformation

0

0

222

30i30cosuxyxr

21

22

0

)(

30sin

uuvvr

uvy


Geometry of Hexagons …

Using these equations, to locate the co-channel cells, start from a reference cell and move i-hexagons along the U-axis and i hexagons along the U axis and j-hexagons along the V-axis

The distance, D, between co-channel cells in adjacent clusters is given by

22 jijiD

The number of cells in a cluster, N, is given by22 jijiN

where i and j are non-negative integers There are only certain cluster sizes and layouts possible

T i l l f N 1 3 4 7 12

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Typical values of N are 1, 3, 4, 7, 12, ……


Example

Re-use Coordinates Number of Cells in the cluster

Normalized repeat distance

i j N Di j N D1 0 1 1

1 1 3 1 7321 1 3 1.732

2 1 7 2.646

2 2 12 3 4642 2 12 3.464

1 3 13 3.606

3 2 19 4 3593 2 19 4.359

1 4 21 4.583


Locating Co-Channel Cells: Example N=7, i=2 & j=1

V To find out the

nearest co-channel neighbors of a

BS1

neighbors of a particular cell, do the following

M i ll i th U

U Move i cells in the U

direction Then turn 60 degree

t l k i d

BS1

BS1

counter clockwise and move j cells in the V direction

BS1

1/3


Locating Co-Channel Cells: Example N=19, i=3, j=2


Re-use Factor

For Hexagonal cells, the re-use distance is i b

Dgiven by:

RNRD 3

Where R = cell size and N = cluster size

Re use factor is

R

Re-use factor is defined as:BS1

BS1 DBS1N

RDq 3


Overview




Cell Capacity and Reuse

Consider a cellular system Which has S duplex channels available for re-use Each cell allocated a group of k channels Each cell allocated a group of k channels Let the S channels be divided among N cells (unique and disjoint)

then, kNS

Cluster: N cells, which collectively use the complete set of available frequencies

If a cluster is replicated M times in the system, the total number of duplex channels, C, as a measure of capacity is

SMNkMC


Cell Capacity and Reuse . . .

If cluster size N is reduced while cell size is kept constant More clusters are required to cover area of interest, i.e., So capacity is directly prop to replication factor for fixed area

CM So capacity is directly prop. to replication factor for fixed area

However, small cluster size means co-channel cells are located much closer togetherlocated much closer together Results in larger co-channel interference May result in lower Quality of Service (QoS)

Conversely, large cluster size indicates that co-channel cells are far from each other Less co channel interference and frequency utilization Less co-channel interference and frequency utilization

The value of N is a function of how much interference a mobile or BS can tolerate

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mobile or BS can tolerate


Cell Capacity and Reuse: Example 1

Assume that: 50 MHz is available for

forward channels GSM is deployed Each channel is 200 kHz In GSM TDMA is used so In GSM, TDMA is used so

that 8 simultaneous calls can be made on each channel

How large is k? How many forward calls

can be madecan be made simultaneously for the deployment containing 28

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cells as in the figure?


Cell Capacity and Reuse: Solution

Solution: There are 50 MHz / 0.2 MHz = 250 channels per cluster With N = 4 then k = 250/4 = 62 5 With N 4, then k 250/4 62.5

With 62.5 channels, 8(62.5) = 500 simultaneous calls can be made in each cell

There are 28 cells on the cell map in Figure, so the total forward calls is 28(500) = 14×103 calls can be made simultaneously


Cell Capacity and Reuse: Example 2

Suppose 33 MHz BW allocated to particular FDD cellular system, where two 25 KHz simplex channel to provide full-duplex for voice/dataduplex for voice/data

Compute the number of channels per cell if a system uses Four-cell reuse

S ll Seven-cell reuse Twelve-cell reuse

Solution: Given that Total BW = 33 MHz, channel BW = 25 KHz x 2 = 50 KHz/duplex

channel S = 33,000/50 = 660 channels

For N = 4, k = 660 / 4 ≈ 165 channels For N = 7, k = 660 / 7 ≈ 95 channels

Sem. II, 2010/11

For N = 12, k = 660 / 12 ≈ 55 channels


Overview




Channel Assignment Strategies

Which channels should be assigned to a cell? Channel assignment strategies can be classified as either

fixed or dynamicfixed or dynamic

Within a cluster, separate channels in as much as possibleWithin a cluster, separate channels in as much as possible This reduces adjacent channel interference

A scheme for increasing capacity and minimizing g p y ginterference is required


Fixed Channel Assignment

Each cell is assigned a fixed number of voice channels Any call attempt within the cell can only be served by the unused

channels in that particular cell

If all the channels in the cell are in use, the call is blocked I.e., the user will not get service

Simple, but a busy cell will run out of channels before a neighboring cell Service variations of fixed assignment strategy exit System performance will be limited by the most crowded cell


Fixed Channel Assignment …

In a variant of the fixed channel assignment, a cell can borrow channels from its neighboring cells if its own channels are fullchannels are full MSC supervises such procedures and ensures that the borrowing

of a channel does not disturb any call in the donor cell


Dynamic Channel Assignment

In dynamic channel assignment (DCA), channels are not assigned to cells permanently Each basestation can change the channels it uses Each basestation can change the channels it uses

When a call request is made, the BS requests a channel from the MSCfrom the MSC MSC only allocates the channel after verifying that the channel is

not presently in use

To ensure a required QoS, the MSC allocates a given frequency if that frequency is not currently used in The cell or The cell, or In any other cell which falls within the limiting reuse distance, i.e.,

channels in neighboring cells must still be different


Dynamic Channel Assignment . . .

The MSC allocates a channel taking into account The likelihood of future call blocking The frequency usage of the candidate channel The frequency usage of the candidate channel The reuse distance of the channel Other cost functions

DCA reduces the likelihood of blocking, thus increasing the capacity of the system

DCA strategies require the MSC to collect real-time data on channel occupancy and traffic distribution on a continuous basiscontinuous basis DAC requires more careful control as it gives extra load to

the MSC


Overview




Handoff

The process of transferring a call, which is in progress from one channel or BS to another is called handoff or handover

Handoff is required when a MS moves into a different cell MSC facilitates the transfer

In general handoff involves In general, handoff involves Identifying the new BS Allocation of voice and control channels in the new BS

Prioritize handoff requests over call initiation requests when allocating unused channels in a cell site


Handoff Region – Power Strength

P1(x)BS-1 BS-2P2(x)

By looking at the variation of signal strength from either base station, it is possible to decide on the optimum area where handoff can take place

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handoff can take place


Handoff

Handoffs must be performed Successfully As infrequently as possible and As infrequently as possible, and Must be imperceptible to the user

To meet these requirements, a minimum usable signalTo meet these requirements, a minimum usable signal level must be specified for acceptable voice quality at the base station

If the received power drops too low prior to handoff the call will be If the received power drops too low prior to handoff, the call will be dropped so that users complain about dropped calls


Handoff Region . . .


Handoff Margin

Consider the following two power levels Pr,min. usable be the minimum received power in dB, below which a

call cannot be received A handoff has to be initiated much prior to this point

Pr,handoff be a higher threshold in dB at which the MSC initiates the h d ff dhandoff procedure Handoff is made when the received signal at the BS falls below the

threshold

Define handoff margin in dB as ∆ = Pr,handoff − Pr,min. usable


Handoff Margin …

How much margin is needed to handle a mobile at driving speeds?

The margin ∆ should not be too large or too small The margin ∆ should not be too large or too small The handoff threshold power is selected such that it is slightly

greater than the minimum usable signal power for an acceptable voice qualityvoice quality

If ∆ is too large, it may lead to unnecessary handoffs which may burden the MSCmay burden the MSC The call may be headed over to the neighboring BS when the MS

is well inside the home cell

If ∆ is too small, there may be insufficient time to complete a handoff before a call is lost due to weak signal conditions

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g


Handoff Margin … A ll d l h h th i i A call drop can also happen when there is an excessive delay by the MSC in assigning a channel E.g., during high traffic conditionsg g g

To effect handoff, it is important to ensure that the mobile is actually moving away from the serving base station The measured signal level drop may be due to momentary fading In order to ensure this, BS monitors signal level for a certain

period of time before a handoff is initiated p The length of monitoring depends on the speed of mobile units Where to get information about the mobile speed?

At high mobile speeds, handoff needs to happen quickly In GSM, handoff is typically within 1-2 seconds In AMPS this was 10 seconds (higher potential for dropped calls!)

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In AMPS, this was 10 seconds (higher potential for dropped calls!)


Handoff Margin – Example

Assume that A mobile moving at a speed of v = 35 mps (~125 Kph) Path-loss exponent n = 4 Path loss exponent n 4 Cell radius of 500 meters (the distance at which the call is

dropped) 2 second handoff 2 second handoff

What is the required handoff margin?


Handoff Margin - Solution

Assume the mobile is driving directly away from the BS So distance d changes by 70 meters in two seconds

Consider the received power at the two times Consider the received power at the two timesPr,min. usable = 0 − 10nlog10dPr,handoff = 0 − 10nlog10(d−70)

Taking the difference of the 2nd and the 1st equations,∆ = 10nlog10d − 10nlog10(d − 70) = 10n log10(d/(d − 70))

Taking that the call is dropped at d = 500 meters, we have∆ = 40 log10(500/430) = 2.6 dB

Note: In this example, the propagation equation used is for “large scale path loss” only, which changes slowly


Handoff Strategies

1. MSC controlled Used in the 1st generation analog cellular systems Signal strength measurements are made by the BS and Signal strength measurements are made by the BS and

supervised by the MSC A spare receiver in each BS, called the location receiver, is used to

determine signal strengths of mobile users which are indetermine signal strengths of mobile users which are in neighboring cells (and appear to be in need of handoff)


Handoff Strategies ….

2. Mobile-assisted hand-off (MAHO) Used in the 2nd generation systems MSs measures the received power from surrounding BSs and MSs measures the received power from surrounding BSs and

report the results to home BS Handoff is initiated when the received power at the MS from the

neighboring BS begins to exceed the home BS by a certain levelneighboring BS begins to exceed the home BS by a certain level for a certain period of time

The MAHO performs at a much faster rate, and is particularly suited for micro cellular environmentssuited for micro cellular environments


Handoff Strategies ….

Intersystem handoff When a mobile user moves from one cellular system to a different

cellular system controlled by a different MSCy y It may become a long-distance call and a roamer Compatibility between the two MSCs need to be determined


Handoff Strategies - Prioritizing Handoffs

Having a call abruptly terminated while in the middle of a conversation is more annoying than being blocked occasionally on a new call attemptoccasionally on a new call attempt

Concept of guard channels A fraction of the total available channel is reserved for handoffA fraction of the total available channel is reserved for handoff

requests, which then are not offered to mobiles making new calls It may reduce the total carried traffic However it offers efficient spectrum utilization when dynamic However, it offers efficient spectrum utilization when dynamic

channel assignment strategies are used

Queuing of handoff requestsg q Does not guarantee a zero probability of forced termination


Handoff Strategies - Practical Handoff Considerations

How to handle the simultaneous traffic of high speed and low speed users while minimizing the handoff intervention from the MSC?from the MSC? Using microcells to increase capacity also increases burden on

MSC

Another practical limitation is the abilit to obtain ne cell Another practical limitation is the ability to obtain new cell sites, particularly in an urban environment


Handoff Strategies - Umbrella Cell

By using different antenna heights (often at the same building or tower) and different power levels, “large” and “small” cells are co-located at a single locationsmall cells are co located at a single location

Minimizes the number of handoffs for high speed users and provides additional microcell channels for pedestrian users


Handoff Strategies – Hard Handoff

Hard handoff: The channel in the source cell is released only when the channel in the target cell is engaged I e assign different radio channels during a handoff I.e., assign different radio channels during a handoff

For 1st generation analog systems, if takes about 10 seconds and the value for ∆ is on the order of 6dB to 12dB

For 2nd generation digital systems, typically requires only 1 or 2seconds, and ∆ usually is between 0 dB and 6 dB

In 2nd generation systems, handoff decision is also based on a co-channel and adjacent channel interference levels


Handoff Strategies – Soft Handoff in CDMA

The channel in the source cell is retained and used for a while in parallel with the channel in the target cell

Used in CDMA system In CDMA, users share the same channel in every cell Consequently, handoff does not mean a physical change in theConsequently, handoff does not mean a physical change in the

assigned channel, rather that a different base station handles the radio communication task

B i lt l l ti th i i l f i l By simultaneously evaluating the receiver signals from a single subscriber at several neighboring base stations, the MSC may actually decide which version of the user’s signal is best at any moment in timemoment in time


Overview


F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y

Co-channel interference Adjacent channel interference Power control for reducing interferences Power control for reducing interferences

Trunking and grade of service Improving capacity

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p g p y


Interference

Interference is a major limiting factor in the performance of cellular radio It limits capacity thereby increasing the number of dropped calls It limits capacity thereby increasing the number of dropped calls

Interference are difficult to control in practice largely due to random propagation effectsrandom propagation effects

Sources of interference include Another mobile in the same cell or in a neighboring cellg g Other BSs operating in the same frequency band Any cellular (e.g., from competing cellular carriers) or non-cellular

system which inadvertently leaks energy into the cellular frequencysystem which inadvertently leaks energy into the cellular frequency band

…


Interference – Effects

Interference in the voice channels causes crosstalk A subscriber hears interference in the background due to an

undesired transmission

Interference in the control channels causes error in digital signaling which causessignaling which causes Missed calls Blocked calls Dropped calls

Interference is more severe in urban areas, due to the t RF i fl d th l b f bgreater RF noise floor and the large number of base

stations and mobiles


Interference – Types

There are two major types of Interferences: Co-channel Interference (CCI) Adjacent channel Interference (ACI) Adjacent channel Interference (ACI)

CCI is caused due to the cells that reuse the same frequency setfrequency set These cells using the same frequency set are referred to as co-

channel cells

ACI is caused due to signals that are adjacent in frequency


Co-Channel Interference – First-tier Interference


Co-Channel Interference – First-tier Interference

First-tier co-channel BSs

D1D 1

D2D5

D6

S i B

D3D4

Serving Base Station


Co-Channel Interference …

Unlike thermal noise, CCI cannot be overcome by increasing the carrier power of a transmitter This is because any increase in the transmitter power also increases This is because, any increase in the transmitter power also increases

the interference to other co-channel cells

Instead, co-channel cells must be physically separated by a p y y p yminimum distance to provide sufficient isolation due to propagation To reduce CCI the co-channel cells must be sufficiently separated To reduce CCI the co-channel cells must be sufficiently separated

Co-channel interference is a function of The radius of the cell R and The radius of the cell, R, and The distance to the center of the nearest co-channel cell, Di



For a hexagonal geometry, the co-channel reuse ratio, Q is related to the cluster size

D

It determines the spatial separation relative to the coverage

NRDQ 3

It determines the spatial separation relative to the coverage distance of the cell

N small gives Q smallg Provides a larger capacity (i.e., can re-use more), but higher CCI

N large means Q largeB tt t i i lit d t ll l l f h l Better transmission quality due to a small level of co-channel interference but small capacity

Hence there is capacity vs interference tradeoff

Sem. II, 2010/11

Hence there is capacity vs. interference tradeoff


Signal-to-Interference Ratio

Signal-to-interference ratio (S/I) for a mobile which monitors a forward channel is

SS

Where S: desired signal power, Ij: interference caused by the jth co-

m

jjII

1

jchannel cell, and m: first-tier co-channels cells

The average received power at a distance d from the transmitting antenna is approx bytransmitting antenna is approx. by

or n

or ddPP

)log(10)()(

00 d

dndBPdBPr

Where Po is the received power at a close-in reference distance in the far-field and n is the path-loss exponent

od 0d

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The path loss exponent, n, ranges between 2 and 6


Signal-to-Interference Ratio …If D i th di t f th ith i t f th i d If Di is the distance of the ith interferer, the received power is proportional to

If transmit power of each BS is equal & n is the same

niD )(

If transmit power of each BS is equal & n is the same throughout the coverage area, S/I for a mobile is approx. as

nRS

To simplify, assume all first-tier interferers are equidistance

m

i

niDI

1)(

To simplify, assume all first tier interferers are equidistance

mN

mR

D

IS

nn3

This relates S/I to the cluster size, and in turn determines the overall capacity of the system

mmI

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Puts a limits on how low we may set N


Signal-to-Interference Ratio …

For a hexagonal cluster of cells with the MS situated at the edge of the cell Rthe edge of the cell

nn

NRD

IS 3

61

61

DD

As long as all cells are of the i S/I i

RI 66 D

DD

same size, S/I is independent of the cell radius, R

D D


Signal-to-Interference Ratio - Example 1

Design parameters: Desired S/I = 15dB Path-loss exponent n = 4 Path loss exponent n 4 Assume that there are six co-channel cells in the first tier and all of

them are at the same distance from the mobile

What is the required re-use factor and cluster size that should be used for maximum capacity?


Signal-to-Interference Ratio – Example 1 …

Six co-channel cells in the first tiertier


Signal-to-Interference Ratio - Example 1 …

Let’s try for N= 4. The co-channel re-use ratio is

D

• Let’s try: N= 7D 58.4

And the signal-to-interference ratio is

46.3RD

dBISR

66.185.73

58.461

58.4

4

ratio is

• Which is greater than the desired

dBIS 8.132446.3

61 4

Smaller than the desired 15 dB

We m st mo e to the ne t re se

• Hence, N=7 can be used

• The frequency reuse We must move to the next reuse distance

The frequency reuse factor = 1/7


Example 2 - Repeat Example 1 for n = 3

Solution Let’s try for a seven-cell reuse pattern, i.e. N= 7. Like the previous

examplep

Which is smaller than the desired 15 dB, hence we need to use larger N

dBISand

RD 05.1204.1658.4

6158.4 3

larger N Let us try N=12

dBSandD 5615360061006 3

Since this is greater than 15 dB, N=12 can be used

N t 3 i t i l l f b b

dBI

andR

56.153600.66

00.6

Note: n=3 is typical value for sub-urban area

Exercise: Try for n=2, which represents rural area!


Summary - Re-Use Factor for n=2, n=3, and n=4

30

20

25 Path loss n= 2Path loss n = 3Path loss n=4

10

15

IR in

dB

N=12N=7

0

5

SI

0 2 4 6 8 10 12 14 16 18 20-5

0

Cluster Size N


Cluster Size, N

Worst Case Calculation of S/I

The MS is at the cell boundary

The approximate S/I is The approximate S/I is given by:

nnn

n

RDDRDR

IS

222

S

1 nnn QQQI

12212


Overview





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p g p y


Adjacent Channel Interference (ACI)

Results from signals that are adjacent in frequency to the desired signal Due to imperfect receiver filters that allow nearby frequencies to Due to imperfect receiver filters, that allow nearby frequencies to

leak

Near-far effect: The adjacent channel interference is jparticularly serious. This occurs when:

When an interferer close to the BS radiates in the adjacent h l hil th b ib i f f th BSchannel, while the subscriber is far away from the BS The BS may not discriminate the desired mobile user from the “bleed

over” caused by the close adjacent channel mobile

Or, an interferer which is in close range to the subscriber’s receiver, is transmitting while the receiver receives from the BS


Near-Far Effect - Interferer Close to BS

• One solution is power control, i.e., reducing the power level transmitted by mobiles close to the BS

SubscriberInterferer


Near-Far Effect - Interferer Close to MS

SubscriberInterferer


Adjacent Channel Interference …

ACI can be reduced by Careful filtering Careful channel assignment Careful channel assignment

The frequency separation between each channel in a cell should be made as large as possibleshould be made as large as possible Assign non-adjacent channels within each cell’s channel group

Example: Assign S = 50 channels into groups for N = 7.p g g p Solution

There are about k = 50/7 ≈ 7 channels per cellF 1 f d h l {1 8 15 22 29 36 43 50} For group 1, use forward channels {1, 8, 15, 22, 29, 36, 43, 50}

For group i, i = 2, . . . 7, let the channels for group i consist of {i, i+7, i + 14, i + 21, i + 28, i + 35, i + 42}



Example: The frequency separation between each channel in a cell should be made as large as possible while assigning themassigning them



If a subscriber is at a distance d1 and the interferer is d2from the base station, then SIR (prior to filtering) is:

n

n

dd

IS

2

1

Example Suppose a subscriber is at d1 = 1000m from the BS and an

dj t h l i t f i t d 100 f th BSadjacent channel interferer is at d2 = 100m from the BS Assume: Path-loss exponent is n = 3 The signal-to-Interference ratio prior to filtering is then

dBdd

IS

n

3010100

1000 33

2

1


Overview





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p g p y


Power Control to Reduce Interference

In practice, power levels transmitted by every subscriber are under constant control by the serving BS Each MS transmits with the smallest power necessary Each MS transmits with the smallest power necessary

In power control1. Reduces the transmit power level of MSs close to the BS since a1. Reduces the transmit power level of MSs close to the BS since a

high TX power is not necessary in this case2. MSs located far away must transmit with larger power than those

nearbynearby

Advantages of power control Reduces out-of-band interference Prolongs battery life and Even reduces even co-channel interference on the reverse

channel

Sem. II, 2010/11

channel


Power Control to Reduce Interference …

However, power control requires well control Controlling a mobile means communication from the BS to the

mobile to inform it whether to increase or decrease its power, p ,which incurs overhead

In CDMA systems, every user in every cell share the same radio channel means a tight power control is required The “near-far problem” is even more of a problem in CDMA Need to reduce the co-channel interferenceNeed to reduce the co channel interference Reduced interference leads to higher capacity


Overview


F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service

Basic definitions Bl k d ll l d Blocked calls cleared

Blocked calls delayed

Improving capacity

Sem. II, 2010/11

p g p y


Trunking

Trunking: How to accommodate a large number of users in a limited radio spectrum?

T ki f t h i fi d d ll b f Trunking refers to sharing a fixed and small number of channels among a large and random user community

Each user demands access from a pool of channel Each user demands access from a pool of channel infrequently & at random times A channel is allocated on a per call basis and a channel is returned

to the pool up on termination of a call So a dedicated channel for each user is not required If U be number of users and C be number of channels, for any C < y

U, possibility of more requests than channels

Trunking exploits statistical behavior of users so that a fixed b f h l d t l d

Sem. II, 2010/11

number of channels accommodate a large, random user


Trunking …

Trunking accommodates large & random users: By providing access to each user on demand from a pool of

available channels

When a user requests service and if all channels are in use

1. The user is blocked, or denied access to the system, y2. In some systems, a queue may be used to hold the requesting

users until a channel becomes available

Upon termination of the call the previously occupied channel is Upon termination of the call, the previously occupied channel is immediately returned to the pool

Designing a trunked system, that can handle a givenDesigning a trunked system, that can handle a given capacity at a specific “grade of service”, requires trunking and queuing theories


Trunking – Definition of Terms . . .

Setup time: The time required to allocate a radio channel to a requesting user Users request may be blocked or have to wait Users request may be blocked or have to wait

Blocked Call: A call that cannot be completed at the time of request due to congestion Also called lost call => lost revenue, e.g., pick hours, holidays, …

Holding time: Average call duration in seconds, denoted H Depends on users and operator's tariff

Request (or call) rate: Average number of calls per unit time, denoted λ seconds-1denoted λ seconds Typically taken to be at the busiest time of day Depends on type of users community: Office, residential, call center,

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…


Trunking – Definition of Terms . . .

Traffic Intensity: A measure of channel time utilization Is the average channel occupancy measured in Erlang, denoted by A

Load: Traffic intensity across the entire trunked radio system Load: Traffic intensity across the entire trunked radio system Measured in Erlang

Erlang: A “unit” of measure of usage or traffic intensity Erlang: A unit of measure of usage or traffic intensity A channel kept busy for one hour is defined as having a load of one

Erlang

Grade of Service (GoS): Measure of congestion (or ability of a user to access a trunked system) during the busiest hour Typically given as likelihood that a call is blocked called Erlang B or Typically given as likelihood that a call is blocked, called Erlang B or The likelihood of a call experiencing a delay greater than a certain

amount of time, called Erlang C


Trunking …

Average arrival rate, λ: Average number of MSs requesting service (call request/time)

Average hold time H Average hold-time, H Average duration of a call (or time for which MS requires service)

An average traffic intensity offered (generated) by each user An average traffic intensity offered (generated) by each user

Example 1: If a user makes on average two calls per hour

)(ErlangsHAu

Example 1: If a user makes on average two calls per hour, and that a call lasts an average of 3 minutes

ErlangA 10min32 ErlangAu 1.0min3

min60


Trunking …

Example 2: In a cell with 100 MSs average of 30 requests are generated in an hour with average holding time of 6 minutes6 minutes

The arrival rate: sec/360030 requests

Offered load is: ErlangsCallSeconds

SecondsCallsAu 3360*

360030


Trunking …

The total offered traffic intensity for U users Note: A is not necessarily the traffic carried by the trunked system

uUAA

In a C channel trunked system, if traffic is distributed equally among channels, then traffic intensity per channel

AUA

I E l 1 th t th 100 d 20

CA

CUAA u

C

In Example 1, assume that there are 100 users and 20 channels Then A = 100(0.1)= 10 and Ac = 10/20 = 0.5( ) c

Note: Ac is a measure of the efficiency of channels utilization


Trunking and GoS

Offered traffic is not necessarily the traffic carried by the trunked system, only that is offered to the system Maximum possible carried traffic is the total number of channels C Maximum possible carried traffic is the total number of channels, C,

in Erlangs

AMPS system is designed for a GOS of 2% blockingy g g Channel allocations for cells are designed so that 2 out of 100 calls

will be blocked due to channel occupancy during the busiest hour

What do we do when a call is offered (requested) but all channels are full?

Blocked calls cleared? Offers no queuing for call requests Erlang B Blocked calls cleared? Offers no queuing for call requests, Erlang B Blocked calls delayed? Erlang C


Overview





Improving capacity

Sem. II, 2010/11

p g p y


Trunking – Blocked Calls Cleared1 C ll i l t f ll P i di t ib ti1. Calls arrival request follows a Poisson distribution2. Memoryless arrivals of requests

I e all users including blocked users may request a channel at I.e., all users, including blocked users, may request a channel at any time

3. The probability of a call durations (or a user occupying a channel) is exponentially distributedchannel) is exponentially distributed I.e., longer calls are less likely to occur

4. There are “infinite number of users” and “finite channels” Rather than a finite number U of users each requesting Au traffic,

set the total offered traffic as a constant A, and then let U and Au 0 in a way that preserves A = UAuu y p u

These assumptions leads to the Erlang B formula Also known as the “blocked calls cleared formula”


Trunking – Erlang B Formula

The probability of an arriving call being blocked is:

GOS!][ C k

c

rCA

blockingP

Where C: number of trunked channels and A: total offered traffic

!

][

0

C

k

kr

kA

g

Erlang B is a measure of the GOS for a trunked system which provides no queuing for blocked calls

Setting the desired GOS, one can derive Number of channels needed The maximum number of users we can support as A = UA or The maximum number of users we can support as A = UAU or The maximum AU we can support (and set the number of minutes

on our calling plans accordingly)

S C

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Since C is very high, it is easier to use table or graph


Erlang B Formula - Table Form


Erlang B Formula - Graphical Form


Overview





Improving capacity

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p g p y


Trunking – Blocked Calls Delayed

Instead of clearing a call, put it in a queue and have it wait until a channel is available First-in first-out line: Calls will be processed in the order received First in, first out line: Calls will be processed in the order received

There are two things to determine here 1. The probability a call will be delayed (enter the queue), and1. The probability a call will be delayed (enter the queue), and 2. The probability that the delay will be longer than t seconds

The first is no longer the same as Erlang Bg g It goes up, because blocked calls aren’t cleared, they “stick

around” and wait for the first open channel

Meaning of GOS The probability that a call will be forced into the queue AND it will

wait longer than t seconds before being served (for some given t)

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g g ( g )


Trunking - Blocked Calls Delayed …

Additional assumptions:1. The queue is infinitely long: Translates to infinite memory2 No one who is queued gives up/hangs up (rather than wait)2. No one who is queued gives up/hangs up (rather than wait)

The probability of an arriving call not having an immediate access to a channel (or being delayed) is given by Erlangaccess to a channel (or being delayed) is given by Erlang C Formula

]0[cAdelayP

1

0 !)1(!

]0[ C

k

kc

r

kA

CACA

delayP

It is typically easiest to find a result from a chart


Trunking - Calls Delayed …

Once it enters the queue, the probability that the delay is greater than t (for t > 0) is given as

AC

GOS: The marginal (overall) probability that a call will be

t

HACdelaytdelayPr exp]0[

GOS: The marginal (overall) probability that a call will be delayed AND experience a delay greater than t is then

delaytdelayPdelayPtdelayP rrr ]0|[]0[][

t

HACdelayP

yyyy

r

rrr

exp]0[

]|[][][

The average delay for all calls in a queued system

ACHdelayPD r ]0[


AC

Erlang C Formula - Graphical Form


Trunking - Example 1

Consider a system with 100 cells Each cell has C = 20 channels Each cell has C 20 channels Generates on average λ = 2 calls/hour The average duration of each call (H) = 3 Minutes

f f How many number of users can be supported if the allowed probability of blocking is 2%?

S l ti Solution: From Erlang B Chart, total carried traffic = 13 Erlangs Traffic intensity per user AU = λH = 0.1 Erlangsy p U g The total number of users that can be supported by a cell = 13/0.1

= 130 Users/cell Therefore, the total number of users in the system is 13,000

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Therefore, the total number of users in the system is 13,000



Consider a system with 100 cells, each cell has C = 20 channels Generates on average λ = 2 calls/hour Generates on average λ 2 calls/hour The average duration of each call (H) = 3 Minutes

How many number of users can be supported if theHow many number of users can be supported if the allowed probability of blocking is 0.2%?

Solution Again from Erlang B Chart, total carried traffic = 10 Erlangs Traffic intensity per user AU = λH = 0.1 Erlangs The total number of users that can be supported by a cell = 10/0 1 The total number of users that can be supported by a cell = 10/0.1

= 100 Users/cell Therefore, the total number of users in the system is 10,000

W t l b f

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We support less number of users


Trunking - Example 3C id t ith Consider a system with Total number of channels = 20 Probability of blocking = 1%

How shall we use this set of channels? Approach 1: Divide 20 channels into 4 trunks of 5 channels each

Traffic capacity of one trunk (5 channels) = 1 36 Erlangs Traffic capacity of one trunk (5 channels) = 1.36 Erlangs Traffic capacity of four trunks (20 channels) = 5.44 Erlangs

Approach 2: Divide 20 channels into 2 trunks of 10 channels eachT ffi it f t k (10 h l ) 4 46 E l Traffic capacity of one trunk (10 channels) = 4.46 Erlangs

Traffic capacity of two trunks (20 channels) = 8.92 Erlangs Approach 3: Use the 20 channels as they are

Traffic capacity of one trunk (20 channels) =12.0 Erlangs

Better to make a large pool instead of dividing Allocation of channels in a trunked radio system has a major impact

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y j pon overall system capacity



Given An urban area has a population of 2 million residents Three competing trunked mobile networks (system A B and C) Three competing trunked mobile networks (system A, B, and C)

provide cellular service in this area System A has 394 cells with 19 channels each System B has 98 cells with 57 channels each System B has 98 cells with 57 channels each System C has 49 cells each with 100 channels

Each user averages 2 calls per hour at an average call duration of 3 minutes3 minutes

Required The number of users that can be supported at 2% blocking? Assuming that all three trunked systems are operated at maximum

capacity, compute the percentage market penetration of each cellular provider


Overview


F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity and coverage

Cell splitting Sectoring Microcell zoning and use of repeaters

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Microcell zoning and use of repeaters


Improving Capacity

A network may need to expand because of Increase in traffic or demand for service Or because of a change in the environment (e g a new building) Or because of a change in the environment (e.g., a new building)

As traffic increases, the channels originally assigned to each cell will be congested

System designers have to provide more channels per unit coverage area

Common techniques Common techniques Cell splitting, sectoring, microcell zoning, and use of repeaters


Cell Splitting

Cell splitting: Process of subdividing a congested cell into smaller cells (called microcells), where each cell has Its own BS (increase in BSs deployed) and Its own BS (increase in BSs deployed) and Reduction in the transmitter power and antenna height

Splitting the cells reduces the cell size and thus moreSplitting the cells reduces the cell size and thus more number of cells have to be used More number of cells = > more number of clusters => more

channels => higher capacitychannels => higher capacity

Cell splitting allows a system to grow by replacing large cells by small cells without new spectrum usage Additional channels per unit area are created


Cell Splitting . . .Large cell (low density)

S ll ll

• Depending on traffic patterns, the smaller cells may be Small cell

(high density)cells may be activated/deactivated in order to efficiently

lluse cell resources

• The co-channel re-

Smaller

use factor D/R is unchanged

O l i h cell (higher density)

• Only increases the number of channels per unit area


Cell Splitting - Example 1

Suppose the radius of cell is reduced by half To cover the entire area, four times microcells are required

What is the required transmit power for these new cells? What is the required transmit power for these new cells?

We have: nWe have: Power at the boundary of un-split cell:

Power at the boundary of a new microcell:

ntuu RPP

nRPP )2/( Power at the boundary of a new microcell:

Where Ptu : transmitted power for un-split cell, Pmc : transmitted

tmcmc RPP )2/(

power from for microcell

For same CCI performance Pu = Pmc impliesn

tutmc PP 2/


tutmc

Cell Splitting - Example 1 . . .

For n = 4; (a typical suburban area)

16tu

tmcPP

Thus, the transmit power must be reduced by 12dB in order to fill in the original coverage area with microcells,

16tmc

order to fill in the original coverage area with microcells, while maintaining the S/I requirement


Cell Splitting - Example 2

7 Cell 4 Cell Cl ster ClusterCluster

Smaller Cells

7 Cell Cluster 12 Cell

ClusterCluster

Typical city cellular radio cell plan – different cell sizes and clusters Combination of cell size and cluster size to increase capacity

Sem. II, 2010/11

Combination of cell size and cluster size to increase capacity


Cell Splitting - Example 3 Suppose a congested service area is Suppose a congested service area is

originally covered by 5 Cells Each with 80 Channels

Capacity = 5*80 = 400After Splitting: After Splitting: Let We now have 20 cells to cover the region

2/RRnew

New Capacity = 20*80 = 1600

In general, the relationship in capacity between cell litti d b ib dditi b dsplitting and subscriber addition can be expressed as

Where C : network capacity after “n” times cell splitting and C:CC n

n 4


Where Cn : network capacity after n times cell splitting and C: Network capacity before cell splitting

Overview




Sem. II, 2010/11



Cell SectoringS t i di ti l t t f th t l Sectoring uses directional antennas to further control interference and frequency reuse

As opposed to cell splitting, where D/R is kept constant while decreasing R, in sectoring keeps R untouched and reduces the D/R ratio

Capacity improvement is achieved by reducing the number of cells per cluster thus increasing frequency reuse

Sem. II, 2010/11

cells per cluster, thus increasing frequency reuse


Cell Sectoring . . .

In order to do this, it is necessary to reduce the relative interference without decreasing the transmitter power

CCI is reduced by replacing single omni-directional antenna by several directional antennas, each radiating within a specified sectorwithin a specified sector

A directional antenna transmits to and receives from only a fraction of the total number of co-channel cells Thus CCI is reduced

CCI reduction factor depends on the amount of sectoringp g A cell is normally partitioned into three 120⁰ sectors or six 60⁰

sectors



Assume 7 cell reuse and 1200 sector

Number of interference Number of interference in the first tier reduces from 6 to 2 Significant compared to

omni-directional case

Sectored groups g p



For a 7-cell cluster, the MS will receive signals from only 2 other clusters (instead of 6 in an omni-directional antenna)

For worst case, when mobile is at the edge of the cell

nn

n

RDDRSIR

)70( nn RDD )7.0(

Interfering co-channel cells @ D distanceDesired channel


Interfering co-channel cells @ D distanceDesired channel

Cell Sectoring – Problems

Increased number of antennas at each BS

Decrease in trunking efficiency due to sectoring Dividing the bigger pool of channels into smaller groups

Increased number of handoffs (sector-to-sector)

Good news: Many modern BSs support sectoring and related handoffs without the help of the MSC


Cell Sectoring – Modern BSs

13

21-11 3

2120o

1-21-3

CCISector in use


Overview




Sem. II, 2010/11



Microcell Zone Concept

The problems of sectoring, i.e., increased handoff, can be addressed by the Microcell Zone concept

A cell is divided into microcells or zones Each microcell (zone) is connected to the same base

station via fiber microwave link or coaxialstation via fiber, microwave link, or coaxial Each zone uses a directional antenna

As a MS travels from one zone to another it retains the As a MS travels from one zone to another, it retains the same channel, i.e., no handoff

The BS simply switches the channel to the next zone sitep y


Microcell Zone Concept …

Let each cell be divided into three zones

Zone Selector



While the cell maintains a particular coverage area, the CCI is reduces because: The large central BS is replaced by several low power transmitters The large central BS is replaced by several low power transmitters Directional antennas are used

Decreases CCI improvesDecreases CCI improves Signal Quality Capacity



Example: Suppose the desired S/I = 18 dB, Path loss exponent n = 4 Path loss exponent n 4,

How much capacity increase can occur if we use Microcell zoning with 3 zones per cell?

Solution To achieve S/I 18 dB we need N 7 To achieve S/I = 18 dB, we need N=7

Now we create 3 zones within a cell The cluster size has been reduced to N = 3 A capacity increase factor of 7/3 = 2.33


Repeaters for Range Extension

Useful for hard-to-reach areas Within buildings or basements Tunnels Tunnels Valleys

Radio transmitters, called repeaters, can be used to provide coverage in these areas

Repeaters are bi-directional Receive signals from BSs Amplify the signals Re-radiate the signalsg

Problem: received noise and interference is also reradiated!


Repeaters for Range Extension …


Summary

Concepts such as handoff, frequency reuse, trunking efficiency, and frequency planning are covered

Capacity of cellular system is a function of many things Capacity of cellular system is a function of many things, E.g., S/I that limits frequency reuse, which intern limits the number

of channels within the coverage area

Trunking efficiency limits the number of users that can access a trunked radio system

Capacity can be improved by cell splitting sectoring and Capacity can be improved by cell splitting, sectoring, and the zone microcell techniques


chapter 2 cellular 1

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