نیمسال اوّل 94-93 افشین همّت یار دانشکده مهندسی کامپیوتر...

9393--9494نیمسال اّو�ل نیمسال اّو�ل یاریار افشین هم�تافشین هم�ت

دانشکده مهندسی کامپیوتر

�ار ) �ار )مخابرات سی ((4040--626626مخابرات سی

های سلولیهای سلولی سیستمسیستم

Most Widespread Example: GSMMost Widespread Example: GSM

The total Bandwidth is divided into a number of narrowband )200KHz( channels. 8 Users are given time slots in each narrowband channel. So GSM is a combination of FDMA and TDMA. Multiple access is orthogonal that means users within the cell never interfere with each other.

22

The GSM SpecificationsThe GSM Specifications

Access Method FDMA/TDMA

Frequency Band 900MHz and 1800MHz

No. of Channels 125 radio carriers

Max No. of user channels 125*8 = 1000 channels

Channel Bandwidth 200KHz

Uplink )MS BS( Freq. BW. 890 to 915MHz

Downlink )BS -> MS( Freq. BW 935 to 960MHz

Modulation Digital GMSK )Gaussian Minimum Shift Keying(

Speech Coding RPE-LTP )Regular Pulse Excited- Long Term Prediction(

Speech Bit Rate 13Kbps

Data Bit Rate 12Kbps 33

Why We Use Cellular Systems?Why We Use Cellular Systems?

Assume we have S frequency channels available for our mobile system.

For GSM, we have 125 frequency channels and 8 time slots per channel thus S is equal to 1000.

How many people can speak at the same time in a city with that many channels?

If we use 1000 channels for the whole city with one Base station then obviously 1000 people can talk at the same time.

Then, how many people are actually speaking simultaneously with their phones in Tehran?

44

Cells and Base StationsCells and Base Stations

Divide the area into “Cells” with different “Base Stations”

Frequencies/Timeslots/Codes reused at spatially-separated locations.

Cells should be far enough so that signals do not interfere.

55

Mobile Switching CenterMobile Switching Center

Base Stations/”Mobile Switching Centers” )MSC( coordinate handoff and control functions.

Shrinking cell size increases capacity, as well as networking cost and burden.

66

Cell ShapeCell Shape

Why hexagonal cells? o They give the maximum distance to center for minimum number of cells to cover an area.

Are cells so uniform in reality?o They are designed based on uniform design.o In reality base stations are closed to designed locations.o real Footprints should be simulated or measured in real deployments.

77

Reuse ConceptReuse Concept

S: Total number of channels

K: Number of channels in each cell

N: Number of cells that in total use all S channels

N cells form a “cluster”

N is usually 1, 3, 4, 7, or 12

88

Cluster Size )1(Cluster Size )1(

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

99

Cluster Size )2(Cluster Size )2(

If we use M clusters in an area, the total capacity )number of users that can talk at the same time( will be: C = M.S = M.K.N

Assume, number of cells in the area is fixed )cell size is fixed( thus M.N is fixed.

Therefore , to maximize C, should maximize K )number of channels per cell(

S = K.N thus minimize N for highest capacity

Decreasing N causes Higher capacity but higher interference as well

Therefore, for a given mobile system we should first specify how much interference we can tolerate.

Then, choose the smallest value of N that ensures that amount of interference.

1010

i.dj.d R

Adjacent Cells

d = 2Rcos30 o

= R√3

Same Cells

D = d√)i2 - 2ijcos120o + j2(

= d√)i2 + ij + j2( = R√3 √)i2 + ij + j2(

Distance between CellsDistance between Cells

1111

No. of Channels per CellNo. of Channels per Cell

For hexagonal cells we have:

N = i2 + i.j + j2 Reuse distance is D = R√)3N(

Example: AMPS

o Total BW = 25MHz

o Each channel = 30KHz simplex

o For N = 4, 7, 12 we will have

104, 59 and 34 channels per cell

1212

Interference and System Capacity )1(Interference and System Capacity )1(

The terminology of capacity for cellular system is not related to Shannon capacity! Here capacity means the number of users a cellular system can support.

Co-channel interference inherent in cellular systems due to frequency reuse . Mobile systems are designed to be interference limited and not noise limited. Co-channel interference can not be reduced by increasing Tx power, only can be reduced by having enough separation between co-channel sites. Therefore, interference computation is the main way of estimating capacity.

1313


How should we find the best possible N?

The starting point in mobile system design is the minimum acceptable Signal to Interference Ratio )SIRmin ( level at receiver.

Then, we need to find out the relation between SIRmin and Nmin.

Pr=P0)d/d0(-n S=P0)R/d0(-n )R is radius of the cell(

and Ii=P0)Di/d0(-n )Di is distance between co-channel cells( Ii is ith co-channel interference thus Di is the same for all co-channel cells )Di=D(.

SIR=S/∑Ii=R-n/∑Di-n

=)D/R(n/NC )NC is the number of co-channel cells(

For hexagonal cells NC=6 thus SIR=)D/R(n/61414


Q=D/R is Co-channel Reuse Ratio

For Hexagonal cells Q=D/R=R√)3N(/R=√)3N( SIR=)√)3N((n/6 N=10)2log)6SIR(/n(/3

Example1: AMPS SIRmin=18dB=63.1

Assuming n=4 Nmin=7

Example2: GSM SIRmin=12dB=15.8

Assuming n=4 Nmin=4

Example3: SIRmin=15dB=31.6

Assuming n=4 Nmin=7

Assuming n=3 Nmin=12

So better propagation will lead to larger values of N, which will decrease capacity. 1515

Channel Assignment )1(Channel Assignment )1(

Fixed Assignment

o Can also use borrowing in advances system

Dynamic assignment

o Measure RSSI )Radio Signal Strength Indicator(, Traffic distribution, and channel occupancy

Mostly fixed assignment used in practice.

1616

Channel Assignment )2(Channel Assignment )2(

In reality, channel planning is a complex optimization problem, that need to use special program.

Part of mobile channels are devoted to signaling. Thus we should to use better reuse ratio )for example 12 instead of 7(

In CDMA systems in theory, reuse ratio “1” can be used.

In practice, we don’t assign co-channel frequencies to neighboring cells to control interference levels.

Also an “equivalent reuse ratio” can be defined for CDMA systems.

1717

Adjacent Channel Interference )1(Adjacent Channel Interference )1(

If Rx filters were ideal then “Adjacent Channel Interference” was not important.

But in reality filters are not ideal and so adjacent channel signals may also enter the receiver.

Highest Adjacent Channel Interference when desired user at cell boundaries.

In practice Adjacent channels are not used in the same cell or even in neighboring cells.

1818

Adjacent Channel Interference )2(Adjacent Channel Interference )2(

Example:o In same cell scenario, if another user is much closer to the base than the desired user, its adjacent channel signal can cause significant interference. o Assume the ratio of distance from the two sources to the base )D1/D2(, is equal to 20, then SIR =20-n which for n=4 is equal to -52dB.o If Rx filter slope is 20dB/oct, at least 6 channel separation )each 200KHz( between users is required.

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

1919

Handoff )1(Handoff )1(

Need to change base stations as we cross cell boundaries.

Sounds easy, but is not!

Handoff )or Handover( mechanism should be such that:

o Least number of handoffso More successful handoffs

Threshold Setting:o Lower threshold -> More call cutoff probabilityo Higher threshold More Handoffs.

2020

Handoff )2(Handoff )2(

Signals should also be averaged over time, otherwise signals get disconnected due to fading.

Should also consider velocity in Handoff process.

Should monitor other Base Station signals as well.

Channel Reservation for Handoff

Soft Handoff in CDMA systems.

2121

Trunking Theory )1(Trunking Theory )1(

Lets assume we have to cover a city with our mobile system. Simple question:

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

In practice, for a telephony system when you want to cover N subscribers, you do not assign N channel to them. This is due to the fact that subscribers connect to network only occasionally. For example, in an office with N employees, the number of lines that talk at the same time is only a fraction of N. Obviously we need to define an acceptable quality level that based on that we can find required number of channels for a given N.

2222


Trunking concept: Allowing large number of users to share

relatively small number of channels based on the fact users use the channel statistically over time

Based on a quantitative measure of acceptable service, N can be found. Acceptable service quality in trunking theory is defined based on the measureable “Grade of Service” )GoS(_ parameter. GoS is usually defined based on the following two parameters:

o “Blocking Rate” which is the probability of getting a busy tone when trying to dial a number.o “Waiting Period” to get the number through )for queued systems(.

2323


Basic definition: Blocked Call: A call that can not get through Holding Time: Average duration of a call )H in sec( Request Rate: Average number of requested calls in unit of time for each user )λ in 1/sec( Erlang: One Erlang is the amount of traffic that will completely occupy the channel for the given period of time. Example: If a channel in one hour is only occupied for 30 minutes then the traffic of that channel is 0.5 Erlangs. Traffic Intensity: Average occupancy of one or more channel )A in Erlangs( GoS:

o Possibility of blockage of a call )Erlang B formula(o Possibility of a connection with a delay more than a specific value )Erlang C formula( 2424


Traffic parameters: Au: Average traffic offered by each user A: Offered traffic by U users AC: Average traffic of each channel C: Number of available channels

Au=λ.H A=U.Au AC=A/C Note that if too much traffic is offered to a system, than the actual traffic supported will at most be C Erlangs if C channels are available. The Gos defined for wireless system is different from wired values: For wired system GoS is about 0.5% For wireless systems GoS of 2-5% is usually specified. For example for 2% GoS at peak hours, 2 out of 100 calls attempted by subscribers will be blocked. You can guess what is the current GoS we have in Tehran!

2525


GoS Equations (Case 1: No queue) Users try to call; if get busy signal, try at a later time. Call requests are independent and have a Poisson distribution:

λ=mean call arrival rate for all users=average of t1, t2, . . .

Call duration )holding times( have exponential distribution: 1/μ = average of τ1, τ2, . . .

2626


GoS Equations (Case 1: No queue) Then, for C channels in the system, it can be shown that the probability of blocking is given by:

A=λ.H=λ./μ is the total traffic offered to the system

The above equation is called the Erlang B formula.

)Bad( 1 ≥ GoS ≥ 0 )Good(

2727


Number of channels

Capacity (Erlangs) for GoS

=0.01 =0.005 =0.002 =0.001

2 0.153 0.105 0.065 0.046

4 0.869 0.701 0.535 0.439

5 1.360 1.130 0.900 0.762

10 4.46 3.96 3.43 3.09

20 12.0 11.1 10.1 9.41

24 15.3 14.2 13.0 12.2

40 29 27.3 25.7 24.5

70 56.1 53.7 51.0 49.2

100 84.1 80.9 77.4 75.2

Capacity of an Erlang B System

2828


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

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

Quick observation: for smaller number of channels, the ratio of users to channels is around 2 but for higher C, the ratio is around 8

having larger pool of channels creates more efficient trunking.

2929


Example 2 Assume one million residents in a city. Assume 49 cells with 100 channels per cell assigned For 1% GoS and average calls of two 3 minute calls per hour, find percentage of market penetration.

Au=λ.H=2)3/60(=0.1 Erlang

C=100 A=84.1 Erlang )total traffic(

U=A/Au=84.1/0.1=841 user/cell

Total subscribers = 49x841= 41209

Penetration =41209/1000000 x 100% = 4.12%

3030

Truncing Theory )10(Truncing Theory )10(

Example 3 City Area = 1300Km2

Hexagonal cells radius = 4Km , and N = 7 Total BW = 49MHz, and full duplex BW = 100KHz GoS = 1%, and Offered traffic/user = 30 mErlang

Find:a( Number of cellsb( Number of channels/cellc( Traffic of each celld( Maximum carried traffice( Total number of users that can be servedf( Maximum number of serves users that can talk

at the same time

3131

Truncing Theory )11(Truncing Theory )11(

Example 3

a(Cell area = 3√3/2 R2 ≈ 2.6R2 = 41.6Km2

number of cells = 1300/41.6 ≈ 31 cells

b(Total number of channels/cell = 49MHz/100KHz/7 = 70 channels

c(C = 70, GoS = 1% A = 56.1 Erlang/cell

d(Maximum carried traffic = 56.1x31 = 1739.1

e(Total number of users = 1739.1/0.03 = 57970 user

f(Maximum number of simultaneous calls = number of all channels = 70x31 = 2170 calls So, about 2170/57970 x100% = 3.7% of users in the same area can talk at the same time.

3232


GoS Equations (Case 2: Blocked users queued) In this scenario, blocked users will enter a queue.

The new model for such scenario is called Elrang C formula.

3333


Channel Distribution For example, if we have 10 channels, what happens if we divide the set to two 5 channel sets?

Using bigger channel pools is better in statistical access scenarios.

For the above example: for 10 channels 4.46 Erlang for 5 channels 1.36 Erlang 1.36x2 = 2.72 < 4.46 Erlang

A set of 10 channels supports about 60% more traffic than two 5 channel sets.

3434


Trunking Efficiency A measure of efficiency of trunking systems is:

η = Traffic (Erlang)/Number of channels x 100%

For smaller number of channels, the efficiency is smaller.

3535

Capacity Increase )1(Capacity Increase )1(

If we want to have more subscribers in an area, what can we do?

Some common approaches: Cell Splitting Interference Reduction

o Sectoringo Antenna Adjustmento Voice Activity Monitoringo Frequency Hoppingo Smart Antennaso Interference Cancellation

3636


Cell Splitting Diving cells into smaller cells.

More BTSs required with smaller height and smaller power.

Use smaller cells )micro-cells and pico-cells( in more crowded areas.

Frequency assignment more complex in various-size cells, but no other choice.

3737


Sectoring Using directional antenna vs. omni-directional antennas

Why using directional antenna reduces interference?

Use of 120° antenna: for N=3 number of co-channels down from 6 to 3

SIR: 17dB 20dB

for N=7 number of co-channels down from 6 to 2 SIR: 17dB 21.7dB

3838


Sectoring

SIR=)√)3N((n/NC

Omni Antenna NC= 6 120° Antenna NC= 2 60° Antenna NC= 1

So, by sectoring can get higher SIR and so can use smaller N.

Disadvantages:o More equipment at BTS siteso More handoffso Smaller trunking efficiency due to dividing channels in each cell into three groups )Erlang B formula(

In general, sectoring is widely used in practical mobile deployments.

3939


Sectoring Example Assume GoS=1%, Avg. call duration=2minutes, Avg. 1 call per hour, total number of channels =395, and N=7 For omni antenna: 395/7 channel 44.2 Erlang traffic 1326 call/hour For 120° antenna: SIR increases by a factor of 3 for 7-reuse 395/7/3 channels/sector 11.2 Erlang/sector 33.6 Erlang 1008 call/hour So, if we don’t change N, we improve SIR, but lose capacity. For 60° antenna: SIR increases by a factor of 6 can reduce N from 7 to 4 395/4/6 channels/sector 9 Erlang/sector 54 Erlang 1620 call/hour So, if we can reduce N, by using sectored antenna, we may regain the lost trunking efficiency. 4040


Other Means Antenna height and tilt adjustment

Voice Activity Every user is only talking at about 40% of time V.A. is efficiency used in CDMA Also incorporated in GSM as DTX )discontinuous transmission(

Frequency Hopping Use a set of frequencies that can be swapped randomly over time, instead of one fixed channel. Hop rate around 200Hz )slow rate hopping( Since all channels are not used all the time statistically reduces the interference )~2dB lower SIR requirement( In theory, can marginally achieve N=1 )full reuse( In practice, N=3 and N=4 are common Widely used in GSM also improves resistance to fading Two modes in GSM: Baseband and Synthesized 4141

SummarySummary

In a cellular design always pay attention to the roles of two main factors:

Interference Cluster size, Trunking Efficiency and Capacity

A change in system may improve one but degrade another one.

In general, mobile planning and optimization is a quite complicated and time consuming task relying mainly on lots of experience.

4242

نیمسال اوّل 94-93 افشین همّت یار دانشکده مهندسی کامپیوتر...

Documents