Cellular SystemsCellular Systems

Most Widespread Example: GSMMost Widespread Example: GSM

The total bandwidth is divided into a number of narrowband channels (200kHz in GSM)Users are given time slots in each narrowband channel (8 users)So GSM is a combination of FDM and TDMMultiple access is orthogonal: users within the cell never interfere with each other

890 MHz 915 MHz 935 MHz 960 MHz20 MHzGuardBand

Up-Link Down-Link124

124

0 025 MHz25 MHz

The GSM SpecificationThe GSM Specification

Access Method Access Method TDMA/FDMATDMA/FDMA

Freq. BandFreq. Band 900/1800900/1800

No. of Channel No. of Channel 125 radio carriers125 radio carriers

Max no. of channels Max no. of channels 125 * 8 = 1000 channels125 * 8 = 1000 channels

Channel BW 200 KHzChannel BW 200 KHz

Uplink Freq. BW Uplink Freq. BW 890 to 915MHz (MS890 to 915MHz (MS BTS)BTS)

Downlink Freq. BW Downlink Freq. BW 935 to 960MHz (BTS935 to 960MHz (BTS MS)MS)

No, of channel/carrier No, of channel/carrier 8 channels/carrier8 channels/carrier

Modulation Digital GMSK (Modulation Digital GMSK (Gaussian Minimum Gaussian Minimum Shift Keying that is a type of Shift Keying that is a type of phase phase modulation)modulation)

Speech CodingSpeech Coding RPERPE--LTPLTP (Regular pulse excited (Regular pulse excited -- long long term prediction)term prediction)

Speech coding bit rateSpeech coding bit rate 13 kbps13 kbps

Data coding bit rateData coding bit rate 12 kbps12 kbps

ServiceService Voice and DataVoice and Data

What is the cellular concept?What is the cellular concept?Assume we have S frequency channels available Assume we have S frequency channels available for our mobile system. for our mobile system. For GSM, we have 125 frequency channels and 8 For GSM, we have 125 frequency channels and 8 time slots per channel time slots per channel --> S is equal to 1000.> S is equal to 1000.How many people can speak at the same time in How many people can speak at the same time in a city with that many channels?a city with that many channels?If we use 1000 channels for the whole city with If we use 1000 channels for the whole city with one Base station then obviously 1000 people can one Base station then obviously 1000 people can talk at the same time. talk at the same time. Then, how many people are actually speaking Then, how many people are actually speaking simultaneously with their phones in Tehran?simultaneously with their phones in Tehran?

Cellular Systems:Cellular Systems:Reuse channels to maximize capacityReuse channels to maximize capacity

Divide the area into Divide the area into ““CellsCells”” with different Base Stationswith different Base StationsFrequencies/timeslots/codes reused at spatiallyFrequencies/timeslots/codes reused at spatially--separated separated locations.locations.Cells should be far enough so that signals do not interfereCells should be far enough so that signals do not interfere

BASESTATION MSCs

Cellular Systems:Cellular Systems:Reuse channels to maximize capacityReuse channels to maximize capacityBase stations/Mobile Switching Centers (MSC) coordinate handoff and control functionsShrinking cell size increases capacity, as well as networking cost and burden

BASESTATION MSCs

Cellular conceptCellular concept

1) Why hexagonal cells?1) Why hexagonal cells?•• They give the maximum distance to center for They give the maximum distance to center for

minimum number of cells to cover an areaminimum number of cells to cover an area

2) Are cells so uniform in reality?2) Are cells so uniform in reality?•• They are designed based on uniform design. They are designed based on uniform design. •• In reality base stations are close to designed In reality base stations are close to designed

locations.locations.•• Real Footprints should be simulated or Real Footprints should be simulated or

measured in real deploymentsmeasured in real deployments

SaadatSaadat--Abad Cell SitesAbad Cell Sites

Reuse conceptReuse concept

S : Total number of channelsS : Total number of channelsK : Number of channels in each cellK : Number of channels in each cellN : Number of cells that in total use N : Number of cells that in total use all S channelsall S channelsN cells form a N cells form a ““clustercluster””N is usually 1, 3, 4, 7, or 12N is usually 1, 3, 4, 7, or 12

How to choose cluster size?How to choose cluster size?Note that choice of cluster size (N) is Note that choice of cluster size (N) is independent from actual size of cells, so independent from actual size of cells, so for fair comparison, assume identical cell for fair comparison, assume identical cell sizes and decide on best value of N.sizes and decide on best value of N.

N = 7 N = 3

How to choose cluster size?How to choose cluster size?If we use M clusters in an area, the total capacity If we use M clusters in an area, the total capacity (number of users that can talk at the same time) will (number of users that can talk at the same time) will be: C=MS = MKNbe: C=MS = MKNAssume, number of cells in the area is fixed (cell size is Assume, number of cells in the area is fixed (cell size is fixed) fixed) --> MN is fixed> MN is fixedTherefore, to maximize C, should maximize K (number Therefore, to maximize C, should maximize K (number of channels per cell)of channels per cell)S = KN S = KN --> minimize N for highest capacity> minimize N for highest capacityNN↓↓•• --> Higher capacity> Higher capacity•• --> But higher interference as well> But higher interference as well

Therefore, for a given mobile system we should first Therefore, for a given mobile system we should first specify how much interference we can toleratespecify how much interference we can tolerateThen, choose the Then, choose the smallestsmallest value of N that ensures that value of N that ensures that amount of interferenceamount of interference

Cluster sizeCluster sizeNeed to find relationship between Need to find relationship between cluster size (N) and interference cluster size (N) and interference levelslevels

For hexagonal cells we have:For hexagonal cells we have:

•• N = iN = i22+ij+j+ij+j22

•• D = D = √√3 R 3 R √√(i(i22+ij+j+ij+j22) ) --> Reuse > Reuse distancedistance

Example: AMPSExample: AMPS•• Total BW = 25MHz, Each channel = Total BW = 25MHz, Each channel =

30KHz (simplex)30KHz (simplex)•• For N=4, 7, 12 For N=4, 7, 12 --> How many > How many

channels per cell?channels per cell?Answer : 104, 59, 34Answer : 104, 59, 34HW: Do the same computations for HW: Do the same computations for GSMGSM

R

D

i= 1, j= 2 -> N=7

Interference and System CapacityInterference and System CapacitySide note: The terminology of Capacity Side note: The terminology of Capacity for cellular systems is not related to for cellular systems is not related to Shannon Capacity!!Shannon Capacity!!•• It generally means the number of users a It generally means the number of users a

cellular system can supportcellular system can support

CoCo--channel interference inherent in channel interference inherent in cellular systems due to frequency reusecellular systems due to frequency reuseImportant note: CoImportant note: Co--channel interference channel interference CAN NOT be reduced by increasing Tx CAN NOT be reduced by increasing Tx power!power!Only can be reduced by having enough Only can be reduced by having enough separation between coseparation between co--channel siteschannel sites

Interference and System Interference and System CapacityCapacity

Q=D/R Q=D/R -->Cochannel Reuse ratio>Cochannel Reuse ratioFor hexagonal cells For hexagonal cells --> > Q =Q =If we decrease Q, we get higher capacity If we decrease Q, we get higher capacity but higher interference as well.but higher interference as well.Mobile systems are designed to be Mobile systems are designed to be interference limited and not noise limitedinterference limited and not noise limitedTherefore, interference computation is the Therefore, interference computation is the main way of estimating capacitymain way of estimating capacity

3N

Choosing the correct N?Choosing the correct N?

But how should we find the best But how should we find the best possible N?possible N?Where should we start?Where should we start?In a mobile system design the In a mobile system design the starting point is the min acceptable starting point is the min acceptable SIR (Signal to Interference Ratio) SIR (Signal to Interference Ratio) level at receiver level at receiver --> SIR> SIRminmin

Then, we need to find out the Then, we need to find out the relation between SIRrelation between SIRminmin and Nand N

SIR ComputationSIR Computationii00 : The number of co: The number of co--channel cells channel cells -->>For hexagonal cells For hexagonal cells --> > ii00 = 6= 6

How to estimate power levels How to estimate power levels coming from different locations?coming from different locations?Use propagation models Use propagation models -->>PP0 0 : Power at distance d: Power at distance d00

S/ I= S

∑i = 1

i 0

Ii

00

nr

dP =P( )d

−

00

1

//n n

in

ii=

R (D R)S I =i

D

−

−

≅

∑

Interference and CapacityInterference and Capacity

--> For hexagonal cells:> For hexagonal cells:

Example: AMPS (Analog Mobile)Example: AMPS (Analog Mobile)FM Signal FM Signal --> SIR> SIRminmin=18db, =18db, Assume n = 4 (propagation factor)Assume n = 4 (propagation factor)

--> N > 6.5 > N > 6.5 --> N = 7> N = 7

More exact analysis shows that N=7 does not More exact analysis shows that N=7 does not satisfy SIRsatisfy SIRmin min in worst case scenariosin worst case scenariosBut still use that in practice resulting in But still use that in practice resulting in interference limited designinterference limited design

HW: Do the computations for GSM (SIRHW: Do the computations for GSM (SIRmin min = = 12db)12db)

3N/6

n( )S I =

ExampleExampleAssume SIR = 15db, find cluster size N for n = 4 Assume SIR = 15db, find cluster size N for n = 4 and n = 3:and n = 3:

For n=4, find cluster size:For n=4, find cluster size:

N = 4.52 N = 4.52 --> Choose N = 7> Choose N = 7

For n =3 For n =3 --> N=11.06 > N=11.06 --> Choose N = 12> Choose N = 12

So , So , ““betterbetter”” propagation will lead to larger values propagation will lead to larger values of N, which will decrease capacityof N, which will decrease capacity

3N/6

n( )S I =63SIRN=->

Channel AssignmentChannel Assignment

Fixed AssignmentFixed Assignment•• Can also use borrowing in advances Can also use borrowing in advances

systemssystems

Dynamic AssignmentDynamic Assignment•• Measure RSSI (Radio Signal Strength Measure RSSI (Radio Signal Strength

Indicator), traffic distribution, and Indicator), traffic distribution, and channel occupancychannel occupancy

Mostly fixed assignment used in Mostly fixed assignment used in practicepractice

Channel Planning in real systemsChannel Planning in real systemsIn reality, a complex optimization problem In reality, a complex optimization problem --> > special program used for thatspecial program used for thatPart of mobile channels are devoted to signaling Part of mobile channels are devoted to signaling --> Use better reuse ratio (for example 12 instead > Use better reuse ratio (for example 12 instead of 7)of 7)

In CDMA systems in theory, reuse factor = 1 can In CDMA systems in theory, reuse factor = 1 can be usedbe usedIn practice, will not assign coIn practice, will not assign co--channel frequencies channel frequencies to neighboring cells to control interference levelsto neighboring cells to control interference levelsAlso an Also an ““equivalentequivalent”” reuse factor can be defined reuse factor can be defined for CDMA systems for CDMA systems --> discussed later> discussed later

Adjacent Channel InterferenceAdjacent Channel Interference

If Rx filters were ideal then AdjIf Rx filters were ideal then Adj--Ch Ch Interference was not importantInterference was not importantBut in reality filters are not ideal and so But in reality filters are not ideal and so adjacent channel signals may also enter adjacent channel signals may also enter the receiverthe receiverHighest Adj Ch Int. when desired user at Highest Adj Ch Int. when desired user at cell boundaries cell boundaries In practice Adj channels are not used in In practice Adj channels are not used in the same cell or even in neighboring cellsthe same cell or even in neighboring cells

Adjacent Channel InterferenceAdjacent Channel InterferenceExample: Example: •• In same cell scenario, if another user is much closer to In same cell scenario, if another user is much closer to

the Base than the desired user, its adjacent channel the Base than the desired user, its adjacent channel signal can cause significant interferencesignal can cause significant interference

•• For example, if D1/D2, the ratio of distances from the For example, if D1/D2, the ratio of distances from the two sources to the Base, is equal to 20 then S/I = (20)two sources to the Base, is equal to 20 then S/I = (20)--nn

•• n=4 n=4 --> SIR = > SIR = --52db52db•• Rx filter slope = 20db/oct Rx filter slope = 20db/oct --> At least 6 channel > At least 6 channel

separation between users is requiredseparation between users is required

In practice some sort of In practice some sort of ““power controlpower control”” is used is used (more important in CDMA)(more important in CDMA)

Handoff (or Handover)Handoff (or Handover)Need to change base stations as we cross Need to change base stations as we cross cell boundariescell boundariesSounds easy , but is not!!Sounds easy , but is not!!Handoff mechanism should be such that:Handoff mechanism should be such that:•• Least number of handoffsLeast number of handoffs•• More successful handoffsMore successful handoffs

Threshold Setting:Threshold Setting:•• Lower Lower ΔΔ --> More call cutoff probability> More call cutoff probability•• Higher Higher ΔΔ --> More Handoffs> More Handoffs

Δ

Min Accpetable signal level

HO Threshold level

Handoff (or Handover)Handoff (or Handover)

Signals should also be averaged over Signals should also be averaged over time, otherwise signals get disconnected time, otherwise signals get disconnected due to fadingdue to fadingShould also consider velocity in HO Should also consider velocity in HO processprocessShould monitor other Base Station Should monitor other Base Station signals as wellsignals as wellChannel Reservation for HOChannel Reservation for HOSoft Handoff in CDMA SystemsSoft Handoff in CDMA Systems

Switching BasicsSwitching BasicsLets assume we want to cover a city with Lets assume we want to cover a city with our mobile systemour mobile system

Simple question: Simple question:

For a given number of subscribers, how For a given number of subscribers, how many channels should we assign to that many channels should we assign to that

area?area?

Answer is not so trivial!!Answer is not so trivial!!

Basics of SwitchingBasics of SwitchingIn practice, for a telephony system when In practice, for a telephony system when you want to cover N subscribers, you do you want to cover N subscribers, you do not assign N channel to them not assign N channel to them --> This is > This is due to the fact that subscribers connect to due to the fact that subscribers connect to network only network only ““occasionallyoccasionally””For example, in an office with N For example, in an office with N employees, the number of lines that talk employees, the number of lines that talk at the same time is only a fraction of Nat the same time is only a fraction of NObviously we need to define an Obviously we need to define an ““acceptableacceptable”” quality level that based on quality level that based on that we can find required number of that we can find required number of channels for a given Nchannels for a given N

Trunking TheoryTrunking Theory

Trunking Concept:Trunking Concept:

Allowing large number of users to Allowing large number of users to share relatively small number of share relatively small number of

channels based on the fact that users channels based on the fact that users use the channel statistically over timeuse the channel statistically over time

Based on a Based on a ““quantitativequantitative”” measure of measure of acceptable service, N can be foundacceptable service, N can be found

Grade of ServiceGrade of ServiceAcceptable service quality in trunking Acceptable service quality in trunking theory is defined based on the measurable theory is defined based on the measurable Grade of Service (GoS) parameterGrade of Service (GoS) parameter

GoS is usually defined based on the GoS is usually defined based on the following two parameters:following two parameters:

1) Blocking Rate (probability of getting a busy 1) Blocking Rate (probability of getting a busy tone when trying to dial a number)tone when trying to dial a number)

2) Waiting Period to get the number through (for 2) Waiting Period to get the number through (for queued systems)queued systems)

Trunking Theory: Basic DefinitionsTrunking Theory: Basic Definitions

Blocked callBlocked call: A call that can not get through: A call that can not get throughHolding TimeHolding Time: Average duration of a call : H (in : Average duration of a call : H (in sec.)sec.)Request RateRequest Rate: Average number of requested : Average number of requested calls in unit of time for each user(calls in unit of time for each user(λλ in 1/sec)in 1/sec)ErlangErlang: One Erlang is the amount of traffic that : One Erlang is the amount of traffic that will completely occupy the channel for the given will completely occupy the channel for the given period of timeperiod of time•• Example: If a channel in one hour is only occupied for Example: If a channel in one hour is only occupied for

30 minutes then the traffic of that channel is 0.5 30 minutes then the traffic of that channel is 0.5 ErlangsErlangs

Basic DefinitionsBasic Definitions

Traffic IntensityTraffic Intensity: Average occupancy of one : Average occupancy of one or more channel , A (in Erlangs)or more channel , A (in Erlangs)GoSGoS: : •• Possibility of Blockage of a call (Erlang B formula)Possibility of Blockage of a call (Erlang B formula)•• Possibility of a connection with a delay more than Possibility of a connection with a delay more than

a specific value (Erlang C formula)a specific value (Erlang C formula)

Traffic parametersTraffic parametersBased on the above definitions:Based on the above definitions:

•• AAuu : Average traffic offered by each user: Average traffic offered by each user•• A : Offered traffic by U users A : Offered traffic by U users •• AAc c : Average traffic of each channel (if C channels are : Average traffic of each channel (if C channels are

available)available)

Therefore, we will have:Therefore, we will have:

AAuu = = λλHHA = UAA = UAuu

AAc c = A/C= A/C

Traffic ParametersTraffic ParametersNote that if too much traffic is offered to a Note that if too much traffic is offered to a system, than the actual traffic supported system, than the actual traffic supported will at most be C Erlangs if C channels are will at most be C Erlangs if C channels are availableavailableThe GoS defined for wireless systems is The GoS defined for wireless systems is different from wired values:different from wired values:•• For wired systems GoS is about 0.5%For wired systems GoS is about 0.5%•• For wireless systems GoS of 2For wireless systems GoS of 2--5% is usually 5% is usually

specified specified •• For example for 2% GoS, at peak hours, 2 out For example for 2% GoS, at peak hours, 2 out

of 100 calls attempted by subscribers will be of 100 calls attempted by subscribers will be blockedblocked

•• You can guess what is the current GoS we You can guess what is the current GoS we have in Tehran!!have in Tehran!!

GoS Equations GoS Equations (Case 1: No queue)(Case 1: No queue)

Users try to call; if get busy signal, try at a later time.Users try to call; if get busy signal, try at a later time.Call Requests are independent and have a Poisson Call Requests are independent and have a Poisson distribution:distribution:

λλ = total mean call arrival rate for all users= total mean call arrival rate for all users= average of t= average of t11, t, t22 , , ……

Call durations (holding times) have exponential Call durations (holding times) have exponential distribution:distribution: 1/1/μμ = average of = average of ττ11, , ττ22, , ττ33, , ……

1 2 3

t1 t2 t3

4

τ1 τ2 τ3

n λt

n(λt) eP(t)=n!

−

Blocking RateBlocking RateThen, for C channels in the system, it can be shown Then, for C channels in the system, it can be shown that the probability of Blocking is given by:that the probability of Blocking is given by:

where A = where A = λλH = H = λλ//μμ is the total traffic offered to the is the total traffic offered to the systemsystem

The above equation is called the The above equation is called the Erlang BErlang B formulaformula

11 ≥≥ GoS GoS ≥≥ 00Bad GoodBad Good

0

/

/

C

Ck

k=

A C!GOS= prob(blocking)=A k!∑

Capacity of an Erlang B SystemCapacity of an Erlang B System

75.277.480.984.1100

49.25153.756.170

24.525.727.32940

12.21314.215.324

9.4110.111.11220

3.093.433.964.4610

0.7620.91.131.365

0.4390.5350.7010.8694

0.0460.0650.1050.1532

Capacity (Erlangs) for GoS= 0.01 = 0.005 =0.002 =0.001

Number of Channels C

Example (1)Example (1)How many users can be supported for 0.5% blocking How many users can be supported for 0.5% blocking probability for the following number of trunked probability for the following number of trunked channels:channels: 5, 10, 100 channels (Assume 0.1 Erlang 5, 10, 100 channels (Assume 0.1 Erlang traffic for each user)traffic for each user)

C = 5 C = 5 --> 1.13 Erlang traffic > 1.13 Erlang traffic --> 11 users> 11 usersC = 10 C = 10 --> 3.96 Erlang traffic > 3.96 Erlang traffic --> 39 users> 39 usersC = 100 C = 100 --> 80.9 Erlang traffic > 80.9 Erlang traffic --> 809 users> 809 users

Quick Observation: For smaller number of channels, Quick Observation: For smaller number of channels, the ratio of users to channels is around 2 but for higher the ratio of users to channels is around 2 but for higher C, the ratio is around 8 C, the ratio is around 8 --> having larger pool of channels creates > having larger pool of channels creates ““more more

efficientefficient”” trunkingtrunking

Example (2)Example (2)Assume 1Million residents in a cityAssume 1Million residents in a cityAssume 49 cells with 100 channels per cell Assume 49 cells with 100 channels per cell assignedassignedFor 1% GoS and average calls of two 3 minutes For 1% GoS and average calls of two 3 minutes calls per hour, find percentage of market calls per hour, find percentage of market penetration.penetration.

AAuu==λλH = 2 *(3/60) = 0.1 ErlangH = 2 *(3/60) = 0.1 ErlangC = 100 C = 100 --> A=Total traffic = 84.1 Erlang> A=Total traffic = 84.1 Erlang--> U = A / A> U = A / Au u = 84.1 / 0.1 = 841 user/cell= 84.1 / 0.1 = 841 user/cellTotal subscribers = 49 * 841 = 41209Total subscribers = 49 * 841 = 41209Penetration = 74849/1,000,000 = 4.12%Penetration = 74849/1,000,000 = 4.12%

Example (3)Example (3)City Area = 1300 sqCity Area = 1300 sq--km, hexagonal cell radius = 4kmkm, hexagonal cell radius = 4kmN = 7N = 7Total BW = 49 MHz, full duplex BW = 100KHzTotal BW = 49 MHz, full duplex BW = 100KHzGoS = 1%, Offered traffic/user = 30mErlangGoS = 1%, Offered traffic/user = 30mErlang

Find:Find:a) number of cellsa) number of cellsb) number of channels /cellb) number of channels /cellc) traffic of each cellc) traffic of each celld) maximum carried trafficd) maximum carried traffice) Total number of users that can be servede) Total number of users that can be servedf) Maximum number of served users that can talk f) Maximum number of served users that can talk

at the same timeat the same time

Example (3)Example (3)a) Cell Area = 2.5981Ra) Cell Area = 2.5981R22 = 41.57sq= 41.57sq--km km --> > Number of cells = 1300 / 41.57 = 31 cellNumber of cells = 1300 / 41.57 = 31 cellb) Total number of channels /cell = b) Total number of channels /cell = 49MHz/100KHz *1/7 = 70 channel49MHz/100KHz *1/7 = 70 channelc) C = 70, GoS = 1% c) C = 70, GoS = 1% --> A = 56.1 Erlang/cell> A = 56.1 Erlang/celld) Maximum carried traffic = 56.1 * 31 = 1739.1d) Maximum carried traffic = 56.1 * 31 = 1739.1e) Total number of users = 1739.1/0.03 = 57970 e) Total number of users = 1739.1/0.03 = 57970 useruserf) Max number of simultaneous calls = Number of f) Max number of simultaneous calls = Number of all channels = 70 * 31 = 2170 callsall channels = 70 * 31 = 2170 callsSo, about 2170/57970 = 3.7% of users in the So, about 2170/57970 = 3.7% of users in the area can talk at the same timearea can talk at the same time

In this scenario, blocked users will In this scenario, blocked users will enter a queue.enter a queue.The new model for such scenario is The new model for such scenario is called Erlang Ccalled Erlang C

GoS Equations (Case 2: Blocked users queued)

Channel DistributionChannel DistributionFor example, if we have 10 channels, what For example, if we have 10 channels, what happens if we divide the set to two 5 happens if we divide the set to two 5 channel sets?channel sets?Using bigger channel pools is better in Using bigger channel pools is better in statistical access scenariosstatistical access scenariosFor the above example: For the above example: •• For 10 channels For 10 channels --> 4.46Erlang> 4.46Erlang•• For 5 channels For 5 channels --> 1.36Erlang > 1.36Erlang

1.36*2 = 2.72 < 4.46Erlang1.36*2 = 2.72 < 4.46Erlang•• A set of 10 channels about 60% more traffic A set of 10 channels about 60% more traffic

than two 5 channel setsthan two 5 channel sets

Trunking EfficiencyTrunking Efficiency

A measure of efficiency of trunking systemsA measure of efficiency of trunking systems

ηη==Traffic (Erlang)/Number of Channels*100Traffic (Erlang)/Number of Channels*100

For smaller number of channles, the efficiency is For smaller number of channles, the efficiency is smallersmaller

0102030405060708090

10 20 30 40 50 60 70

N (Number of Channels)

Traffic (Erlang)

Trunking Efficiency

How to increase capacity?How to increase capacity?If we want to have more subscribers If we want to have more subscribers in an area, what can we do?in an area, what can we do?Some common approaches:Some common approaches:•• Cell SplittingCell Splitting•• Interference ReductionInterference Reduction

SectoringSectoringAntenna adjustmentAntenna adjustmentVoice activity monitoringVoice activity monitoringFrequency HoppingFrequency HoppingSmart AntennasSmart AntennasInterference CancellationInterference Cancellation

Cell SplittingCell Splitting

Dividing cells into smaller cellsDividing cells into smaller cellsMore BTSs required with smaller More BTSs required with smaller height and smaller powerheight and smaller powerUse smaller cells (microcells and Use smaller cells (microcells and picocells) in more crowded areaspicocells) in more crowded areasFrequency assignment more complex Frequency assignment more complex in variousin various--size cells, but no other size cells, but no other choicechoice

SectoringSectoringUsing directional antenna vs. omnidirectional Using directional antenna vs. omnidirectional antennasantennasWhy using directional antenna reduces Why using directional antenna reduces interference?interference?

Use of 120Use of 120oo antenna:antenna:For N =3 For N =3 --> > Number of coNumber of co--channels channels down from 6 to 3down from 6 to 3

For N = 7 For N = 7 --> > Number of coNumber of co--channelschannelsdown from 6 to 2down from 6 to 2--> SIR : 17db > SIR : 17db --> 21.7db> 21.7db

SectoringSectoring

So, by sectoring can get higher SIR So, by sectoring can get higher SIR and so can use smaller Nand so can use smaller NBut, what are the disadvantages of But, what are the disadvantages of

using sectoring?using sectoring?

0

3N n( )SIR=I

I0 = 6 Omni Antenna

-> I0 = 2 1200 Antenna

I0 = 1 600 Antenna

Sectoring: Negative SidesSectoring: Negative Sides

More equipment at BTS sitesMore equipment at BTS sitesMore HandoffsMore HandoffsSmaller trunking efficiency due to Smaller trunking efficiency due to dividing channels in each cell into dividing channels in each cell into three groups (Erlang B formulas)three groups (Erlang B formulas)

But , in general, sectoring is widely But , in general, sectoring is widely used in practical mobile deploymentsused in practical mobile deployments

Sectoring (Example)Sectoring (Example)Assume GoS = 1%, Avg Call duration = 2 minutes, Avg 1 call per Assume GoS = 1%, Avg Call duration = 2 minutes, Avg 1 call per hour, Total number of channels = 395, N=7hour, Total number of channels = 395, N=7

For Omni case:For Omni case:395/7 Channel 395/7 Channel --> 44.2 Erlang traffic > 44.2 Erlang traffic --> 1326 call/hour> 1326 call/hour

Now use 120Now use 12000 antenna:antenna:SIR increases by a factor of 3 for 7SIR increases by a factor of 3 for 7--reusereuse395/7/3 channels/sector 395/7/3 channels/sector --> 11.2 Erlang/sector > 11.2 Erlang/sector --> 33.6 Erlang > 33.6 Erlang --> > 1008 call/hour1008 call/hour

So, if we do not change N, we improve SIR, but lose capacitySo, if we do not change N, we improve SIR, but lose capacity

Now for 60Now for 6000 antenna:antenna:SIR increases by a factor of 6 SIR increases by a factor of 6 --> can reduce N from 7 to 4> can reduce N from 7 to 4395/4/6 channels/sector 395/4/6 channels/sector --> 9 Erlang/sector > 9 Erlang/sector --> 54 Erlang > 54 Erlang --> 1620 > 1620 calls/hourcalls/hour

So, if we can reduce N, by using sectored antenna, we may regainSo, if we can reduce N, by using sectored antenna, we may regainthe lost trunking efficiencythe lost trunking efficiency

Other means for interference reductionOther means for interference reductionAntenna adjustment:Antenna adjustment:•• Height and TiltHeight and Tilt

Voice Activity Voice Activity --> Every user is only talking at about > Every user is only talking at about 40% of time40% of time•• VA is efficiently used in CDMAVA is efficiently used in CDMA•• Also incorporated in GSM as DTX (discontinuous transmission) Also incorporated in GSM as DTX (discontinuous transmission)

(not enabled in country)(not enabled in country)Frequency Hopping:Frequency Hopping:•• Use a set a frequencies that can be swapped randomly over Use a set a frequencies that can be swapped randomly over

time , instead of one fixed channeltime , instead of one fixed channel•• Hop rate around 200Hz (slow freq hopping)Hop rate around 200Hz (slow freq hopping)•• Since all channels are not used all the time, statistically Since all channels are not used all the time, statistically

reduces the interference (about 2db lower SIR requirement)reduces the interference (about 2db lower SIR requirement)•• In theory, can marginally achieve N=1 cluster size (full reuse)In theory, can marginally achieve N=1 cluster size (full reuse)•• In practice, N=3 and N=4 are commonIn practice, N=3 and N=4 are common•• Widely used in GSM Widely used in GSM --> Also improves resistance to fading > Also improves resistance to fading

(discussed later)(discussed later)•• Two modes in GSM: Baseband and Synthesized (recently Two modes in GSM: Baseband and Synthesized (recently

enabled in the country in BB mode)enabled in the country in BB mode)

Capacity of different fixed MA Capacity of different fixed MA schemesschemes

How can we fairly compare different MA schemes? How can we fairly compare different MA schemes? Main parameters that determine capacity are: Main parameters that determine capacity are: •• Total available BW: BTotal available BW: Btt

•• BW of each channel: BBW of each channel: Bcc

•• Frequency distribution over cells: dependent on Freq Reuse Frequency distribution over cells: dependent on Freq Reuse factorfactor

To obtain a fair comparison, should keep main To obtain a fair comparison, should keep main parameters the same: We will consider same Bparameters the same: We will consider same Btt and and same cell sizesame cell sizeThe question is then which MA schemes provides The question is then which MA schemes provides more number of channels (in time, freq, or code)more number of channels (in time, freq, or code)

Interference and CapacityInterference and CapacityAs we saw before, smaller cluster size (N) As we saw before, smaller cluster size (N) and frequency reuse factor will lead to higher and frequency reuse factor will lead to higher capacitycapacity

# of channels /cell = m = B# of channels /cell = m = Btt /(B/(BccN) N) We saw before the relation between N and We saw before the relation between N and C/I:C/I:

N = D/(RN = D/(R√√3)3)For 6 coFor 6 co--channels: (C/I)channels: (C/I)min min < 1/6 (R/D)< 1/6 (R/D)--nn

2 /min/ 2

6 /3

t

nc n

Bm=B ( (C I) )

min

22 /3

t

c

Bn = m=B (C I)

⇒

Interference and CapacityInterference and CapacityAs this equation shows, C/I effect is the same as As this equation shows, C/I effect is the same as square of BW:square of BW:

Also note that, (C/I)Also note that, (C/I)minmin is different for different is different for different modulation schemes:modulation schemes:•• For Analog FM: (C/I)For Analog FM: (C/I)minmin = 18db= 18db•• For GSM: (C/I)For GSM: (C/I)minmin = 10 to 12 db= 10 to 12 db

Note: For TDMA systems that transmit multiple slots Note: For TDMA systems that transmit multiple slots per each frequency, the actual Bper each frequency, the actual Bcc is obtained by is obtained by dividing system Bdividing system Bcc by number of slots. For example by number of slots. For example for GSM: Bfor GSM: Bcc = 200Khz = 200Khz --> Effective B> Effective Bc c = 200KHz/8 = = 200KHz/8 = 25KHz25KHz

min2 /3

t

c

Bm=B (C I)

Capacity ComparisonCapacity Comparison

27.7250/333

4/312db1000200KHz/8

25MHzGSM

11.9119718db83330KHz25MHzAMPS

Traffic (Er/km

2)

Channel/cell

Freq Resue (C/I)min

Voice channel

sBcBt

• So GSM leads to a capacity improvement of 2.3 to 3.4 over AMPS

Capacity of CDMA systemsCapacity of CDMA systemsIn CDMA capacity is highly interference limited. In CDMA capacity is highly interference limited. In TDMA/FDMA system, interference indirectly In TDMA/FDMA system, interference indirectly affects capacity through cluster size change, but affects capacity through cluster size change, but in CDMA it directly affects capacityin CDMA it directly affects capacityInterference in CDMA mainly comes from other Interference in CDMA mainly comes from other users in the system that are using the same users in the system that are using the same frequency channel. frequency channel. Since the source of interference is quite different Since the source of interference is quite different in CDMA, previous results relating reuse factor in CDMA, previous results relating reuse factor and interference is not valid any moreand interference is not valid any moreAlso, interference reduction schemes (such as Also, interference reduction schemes (such as directional antennas or silence suppression) now directional antennas or silence suppression) now directly affect capacitydirectly affect capacity

Capacity of singleCapacity of single--cell CDMA systemscell CDMA systemsAssume users receive power is equal to S (assuming Assume users receive power is equal to S (assuming perfect power control)perfect power control)Since interference is mainly caused by imperfect Since interference is mainly caused by imperfect orthogonality of codes, for desired user, the output will orthogonality of codes, for desired user, the output will be proportional to PG, but for other users, the output be proportional to PG, but for other users, the output will be proportional to will be proportional to √√PGPGSo, if we consider N users:So, if we consider N users:

SIR = PG/(NSIR = PG/(N--1)1)Considering the effect of white noise we will have:Considering the effect of white noise we will have:

SINR = PG / ((NSINR = PG / ((N--1) + 1) + ηηBW/PG)BW/PG)--> N = 1 + PG/SINR > N = 1 + PG/SINR -- ηηBW/PGBW/PG

CDMA CapacityCDMA CapacityNow if we use 120Now if we use 120oo sectored antennas, number of sectored antennas, number of interferers reduces from (Ninterferers reduces from (N--1) to about N/31) to about N/3

N N ≅≅ 1 + 3PG/SINR1 + 3PG/SINRAlso if we consider voice activity factor of Also if we consider voice activity factor of αα::

N N ≅≅ 1 + 3PG/(1 + 3PG/(ααSINR)SINR)For example, for 120For example, for 120oo sectored antenna and sectored antenna and αα = 3/8, we will get an improvement in capacity = 3/8, we will get an improvement in capacity of almost 8 times.of almost 8 times.Example: For ISExample: For IS--95, R95, Rcc=1.2288Mchip/sec, R=1.2288Mchip/sec, Rbb = = 9.6kbps (PG = 128), (E/N9.6kbps (PG = 128), (E/N00))minmin = 6.8db (reverse = 6.8db (reverse link), so for link), so for αα = 3/8:= 3/8:•• For omni antenna (and no VA reduction): For omni antenna (and no VA reduction):

N N ≅≅ 1 + 1.2288e6 /9600 /4.78 = 32 user/cell1 + 1.2288e6 /9600 /4.78 = 32 user/cell

•• For 120For 120oo antenna and VA: antenna and VA: N N ≅≅ 1+ 3/.375 (1.2288e6/9600/4.78) = 225 user/cell1+ 3/.375 (1.2288e6/9600/4.78) = 225 user/cell

CDMA multiCDMA multi--cell capacitycell capacityNaturally, signals coming from adjacent cells will Naturally, signals coming from adjacent cells will appear as interference as wellappear as interference as wellAlso, these signals are only powerAlso, these signals are only power--controlled for controlled for their own cell and not for other neighboring cellstheir own cell and not for other neighboring cellsTherefore, although reuse factor is in theory Therefore, although reuse factor is in theory equal to one for CDMA systems, in practice we equal to one for CDMA systems, in practice we will have smaller reuse factor numbers (note that will have smaller reuse factor numbers (note that in analog FDMA, resue factor = 1/7)in analog FDMA, resue factor = 1/7)In practice, an equivalent reuse factor f = 0.3 to In practice, an equivalent reuse factor f = 0.3 to 0.7 will be experienced for CDMA systems, which 0.7 will be experienced for CDMA systems, which will be equivalent to cluster sizes of about 1.5 to will be equivalent to cluster sizes of about 1.5 to 3 (note in GSM, cluster size = 3 or 4)3 (note in GSM, cluster size = 3 or 4)Taking all these into account, the actual capacity Taking all these into account, the actual capacity of ISof IS--95 with 12095 with 120oo antenna will be about 30antenna will be about 30--50 50 users/cell (for a single 1.25MHz channel) or users/cell (for a single 1.25MHz channel) or about 2about 2--3 times of GSM3 times of GSM

Other means for interference reductionOther means for interference reduction

Smart Antennas Smart Antennas --> More intelligent use of > More intelligent use of antennas antennas •• Long time thought as dream Long time thought as dream •• Now proposed for 3G and recently highly utilized in Now proposed for 3G and recently highly utilized in

IEEE 802.11n standard for next generation WLANIEEE 802.11n standard for next generation WLANMultiuser Detection Schemes:Multiuser Detection Schemes:•• Do not consider other users signals as interference Do not consider other users signals as interference

but try to detect them simultaneously with the but try to detect them simultaneously with the desired signaldesired signal

•• First detect the strongest signal, then subtract that First detect the strongest signal, then subtract that and go for the next strongest,and go for the next strongest,……

•• Lots of theoretical work done for CDMALots of theoretical work done for CDMA•• Recently, targeted for implementation in 3G, Recently, targeted for implementation in 3G,

although not so widespreadalthough not so widespread

Real Examples of Mobile System Real Examples of Mobile System Design and OptimizationDesign and Optimization

Real ToolsReal Tools

Planning: Where to put the base Planning: Where to put the base stations and with what capacity?stations and with what capacity?•• Tools:Tools:

ASSET (main tool used by MCI)ASSET (main tool used by MCI)Locally developed tools (for example Locally developed tools (for example HiplanHiplanfrom from KavoshComKavoshCom Asia)Asia)

How the actual system is working?How the actual system is working?•• Drive Test Tools (TEMS by Ericsson)Drive Test Tools (TEMS by Ericsson)•• Analysis Tools (custom designed)Analysis Tools (custom designed)

Asset Coverage ModelAsset Coverage Model

HiplanHiplan Coverage OutputCoverage Output

Best Server Plots (Best Server Plots (HiplanHiplan))

Carrier to Interference Plots (Carrier to Interference Plots (HiplanHiplan))((YasujYasuj Area)Area)

Before Optimum Freq Planning After Optimum Freq Planning

Sample Drive Tests in TehranSample Drive Tests in Tehran(using TEMS tools)(using TEMS tools)

What information is extracted from Drive Tests?

What information is extracted from What information is extracted from Drive Tests?Drive Tests?

شناسايي عدم پوشش شناسايي عدم پوشش تداخل تداخل

مغايرتهاي همسايگي مغايرتهاي همسايگي OvershootingOvershooting

مناطق داراي انسداد كانال سيگنالينگي و ترافيكي مناطق داراي انسداد كانال سيگنالينگي و ترافيكي

Example of Changing the Example of Changing the Antenna TiltAntenna Tilt

Example of Changing BTS Antenna HeightExample of Changing BTS Antenna Height

بالف

سيستم موبايل سيستم موبايل Optimization Optimizationافزار براي افزار براي توسعه نرم توسعه نرم سازي شبكه موبايل با حجم انبوهي از اطالعات سروكار داريم كه بايد سازي شبكه موبايل با حجم انبوهي از اطالعات سروكار داريم كه بايد اصوالً در بهينهاصوالً در بهينه

..ذخيره بازيابي و به نحو مطلوب پردازش شوند ذخيره بازيابي و به نحو مطلوب پردازش شوند ::بندي كرد بندي كرد توان اين اطالعات را بصورت زيردسته توان اين اطالعات را بصورت زيردسته بطور خالصه مي بطور خالصه مي

و غيره و غيرهTiltTiltافزاري سايتها مانند ارتفاع و جهت سكتورها، افزاري سايتها مانند ارتفاع و جهت سكتورها، اطالعات سخت اطالعات سخت ••افزاري سايتها مانند فركانس مورد استفاده، پارامترهاي افزاري سايتها مانند فركانس مورد استفاده، پارامترهاي اطالعات پيكربندي نرماطالعات پيكربندي نرم••

HandoverHandover و غيره و غيره مانند تعداد مانند تعداد Key Performance Index (KPI)Key Performance Index (KPI)اطالعات عملكرد شبكه اطالعات عملكرد شبكه ••

تماسهاي قطع شده، ترافيك سكتورها و غيره تماسهاي قطع شده، ترافيك سكتورها و غيره DriveDriveاطالعات اطالعات •• TestTest ،كه وضعيت واقعي پوشش، كه وضعيت واقعي پوشش C/IC/I و غيره را مشخص و غيره را مشخص

..كنند كنند ميمي

Some Major KPISome Major KPI

Call Setup Success Rate (CSSR)Call Setup Success Rate (CSSR)Drop Call Rate (DCR)Drop Call Rate (DCR)Call Success Rate (CSR)Call Success Rate (CSR)HO Success Rate (HSR)HO Success Rate (HSR)HO Failure Rate (HFR)HO Failure Rate (HFR)Traffic Blocking Channel (Traffic Blocking Channel (TCH_CongTCH_Cong))SDCCH Blocking Channel SDCCH Blocking Channel ((Sdcch_CongSdcch_Cong))SdcchSdcch Access Success Rate Access Success Rate

كيفيت سرويس دهي مطلوب بر اساس معيارهاي تعيين شده كيفيت سرويس دهي مطلوب بر اساس معيارهاي تعيين شده كيفيت سرويس دهي مطلوب بر اساس معيارهاي تعيين شده (CSSR) > (CSSR) > 9955%% ))درصد موفقيت برقراري مكالمه درصد موفقيت برقراري مكالمه ((

DCR <DCR < 2.52.5 %% ) ) درصد قطعي مكالمهدرصد قطعي مكالمه((

HHFFR <R < 22 %% ) )HandoverHandoverدرصد خطا درصد خطا ((

(HSR) > (HSR) > 9955%% HandoverHandoverدرصد موفقيت درصد موفقيت ((

TCH TCH _cong_cong < < 33%% ))انسداد كانال ترافيكي انسداد كانال ترافيكي درصد درصد ((

SDCCH SDCCH _cong_cong < < 1.51.5%% ) ) انسداد كانال سيگنالينگ انسداد كانال سيگنالينگ درصد درصد ((

TCHTCH__Ass_FailAss_Fail < < 22%% ))تخصيص كانال ترافيكيتخصيص كانال ترافيكيدرصد خطادرصد خطا((

CommandCommand افزاري افزاري هاي نرم هاي نرمهاي غير الزم هاي غير الزم هاي جديد و حذف همسايگي هاي جديد و حذف همسايگي تعريف همسايگي تعريف همسايگي ••ها ها در همسايگي در همسايگي HandoverHandoverتغيير پارامترهاي تغيير پارامترهاي ••ها ها فركانسهاي همسايگي فركانسهاي همسايگي تصحيحتصحيح••PowerPowerتغيير تغيير ••TRXTRX نرخ كدينگ نرخ كدينگ تغييرتغيير••

AI Cong.>2% (Non-STD Cells)

0.00%5.00%

10.00%15.00%

20.00%25.00%

25.02.0627.02.0604.03.0606 .03.0611.03.0613.03.0625.03.0627.03.0603.04.0608.04.0611.04.0617.04.0619.04.0623.04.0625.04.06

Air Interface CongestionAir Interface Congestion

16%8%

AI Congestion @ Peak Hour (19:00)

CDR>3% (Non-STD Cells)

0.00%5.00%

10.00%15.00%

20.00%25.00%30.00%35.00%

25.02 .06

27.02 .06

04.03 .06

06.03 .06

11.03 .06

13.03 .06

25.03 .06

27.03 .06

03.04 .06

08.04 .06

11.04 .06

17.04 .06

19.04 .06

23.04 .06

25.04 .06

Call Drop RateCall Drop Rate

CDR @ Peak Hour (20:00)

22.5%12%

Outgoing Handover Successful Outgoing Handover Successful RateRate

OHSR @ Peak Hour (19:00)

30.7%

57.1%

Target : OHSR>=98%

OHSR>=90%

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

25.02.0

6 27.

02.06

05.03.

0607.

03.06

12.03.

0614.

03.06

26.03.

0628.

03.06

05.04.

0610.

04.06

12.04.

0618.

04.06

22.04.

0624.

04.06

29.63%

57.14%

SummarySummaryIn a cellular design In a cellular design alwaysalways pay attention pay attention to the roles of two main factors:to the roles of two main factors:•• InterferenceInterference•• Cluster size, trunking efficiency and capacityCluster size, trunking efficiency and capacity

A change in system may improve one but A change in system may improve one but degrade another onedegrade another oneIn general, mobile planning and In general, mobile planning and optimization is a quite complicated and optimization is a quite complicated and time consuming task relying mainly on time consuming task relying mainly on lots of experiencelots of experience