3G Cellular

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Wireless Information Transmission System Lab.

National Sun Yat-sen University Institute of Communications Engineering

The Cellular Concept

System Design Fundamentals

Table of ContentsFrequency Reuse

Channel Assignment Strategies

Handoff StrategiesPrioritizing Handoffs

Practical Handoff ConsiderationsInterference and System Capacity

Power Control

Trunking and Grade of ServiceImproving Coverage and Capacity in Cellular Systems

Trunking Theory

Frequency Reuse

Cellular System Design ConsiderationsTo solve problems of spectral congestion and user capacity.

Replacing a single, high power transmitter with manylow power transmitters.

Neighboring base stations are assigned different groups

of channels so that the interference between basestations is minimized.

Available Channels are distributed throughput the

geographic region and may be reused as many times asnecessary.

With fixed number of channels to support an arbitrarily

large number of subscribers.

Concepts of Frequency Reuse

Cellular Networks and Frequency ReuseOne important characteristic of cellular networks isthe reuse of frequencies in different cells.

By reuse frequencies, a high capacity can be achieved.

However, the reuse distance has to be high enough,so that the interference caused by subscribers usingthe same frequency (or an adjacent frequency) inanother cells is sufficiently low.

To guarantee an appropriate speech quality, thecarrier-to-interference-power-ratio (CIR) has toexceed a certain threshold CIR min which is 9 dB for the GSM system.

Hexagons Hexagonal cell shape has been universally adopted.

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

Base stations can be placed at:

The cell center – center-excited cells – omni-directionalantennas.

The cell vertices – edge-excited cells – sectored directional

antennas.

Reuse Factor

Reuse Factor = i2+ij+ j2; i, j are non-negative integers.

Due to the fact that the hexagonal geometry has exactly six equidistant neighborsand that the lines joining the centers of any cell and each of its neighbors areseparated by multiples of 60 degrees, there are only certain cluster sizes and cell

layouts which are possible.

Co-channel Neighbor Cells

Move i cells along any

chain of hexagons.Turn 60o counter-clockwise and move j cells.

For example: N =19: i =3, j =2;

N =12: i =2, j =2;

N =7; i =2, j =1;

Channel Assignment Strategies

Channel Assignment StrategiesFixed Channel Allocation

Dynamic Channel Allocation

Hybrid Channel Allocation

Borrowed Channel Allocation

Fixed Channel AssignmentEach cell is allocated a predetermined set of voicechannels.

Any call attempt within the cell can only be served bythe unused channels in that particular cell.

Probability of blocking is high.

Dynamic Channel Assignment StrategyChannels are not allocated to different cells permanently.

Each time a call request is made, the serving base stationrequests a channel from the MSC.

The MSC allocates a channel to the requested cell followingan algorithm that takes into account the likelihood of future

blocking within the cell, the frequency of use of the candidatechannel, the reuse distance of the channel, and other costfunctions.

MSC only allocates a given frequency if that frequency is not presently in use in the cell or any other cell which falls withinthe minimum restricted distance of frequency reuse to avoid co-channel interference.

Dynamic Channel Assignment Strategy

MSC has to collect real-time data on channel

occupancy, traffic distribution, and radio signalstrength indications (RSSI) of all channels on acontinuous basis.

Reduce the likelihood of blocking at the expense of increasing the storage and computational load.

Borrowing Strategy

Modified from fixed channel assignment strategies.

A cell is allowed to borrow channels from aneighboring cell if all of its own channels are already

occupied.

The MSC supervises such borrowing procedures and

ensures that the borrowing of a channel does notdisrupt or interfere with any of the calls in progress inthe donor cell.

Handoff Strategies

Handoff / HandoverIn a cellular network, the process to transfer the ownership of a MS from a BS to another BS.

Handoff not only involves identifying a new BS, but alsorequires that the notice and control signals be allocated tochannels associated with the new base station.

Usually, priority of handoff requests is higher than callinitiation requests when allocating unused channels.

Handoffs must be performed successfully and as infrequentlyas possible and be imperceptible to the uses.

Handoff / HandoverHandover Occasions

Bad signal quality on current channel

noise or interferenceTraffic overload in current cell

load balancing

Handover Indicator: The parameters to monitor todetermine HO occasion

RSSI, in ensemble average sense.

Bit Error Rate (BER)/Packet Error Rate (PER), more accurate.

Handoff / Handover Need to specify an optimum signal level to initiate

a handoff.Minimum useable signal for acceptable voicequality at the base station receiver is normally

taken as between -90 dBm to -100 dBm.

If Δis too large, unnecessary handoffs may occur.

If Δis too small, there may be insufficient time tocomplete a handoff.

useableminimumhandoff Pr Pr −−=Δ

Illustration of a handoff scenario at cell

boundary

Illustration of a Handoff Scenario at Cell

BoundaryFigure (a) demonstrates the case where a handoff is notmade and the signal drops below the minimum

acceptable level to keep the channel active.The dropped call event in figure (a) can happen whenthere is an excessive delay by the MSC in assigning a

handoff or when the threshold Δ is set too small for thehandoff time the system.

Excessive delays may occur during high traffic

conditions due to computational loading at the MSC or due to the fact that no channels are available on any of the nearby base stations.

Handoff / HandoverDuring handoff, it is important to ensure that the drop inthe measured signal level is not due to momentary

fading and that the mobile is actually moving away fromthe serving base station.

The base station monitors the signal level for a certain

period of time before a hand-off is initiated.The time over which a call may be maintained within acell, without hand-off, is called the dwell time.

Handoff in 2nd Generation TDMA Systems

Handoff decisions are mobile assisted.

In mobile assisted handoff (MAHO), every mobile

measures the received power from surrounding basestations and reports the results to the serving base station.

MAHO enables the call to be handed over at a muchfaster rate.

Handoff or Handover

Mobile Assistant Handover – more efficient.

MS monitors all BSsMS reports the measurements to the BS

MSC makes decision

USDC (IS-54/136):BSs monitor all MSs.

When a MS is leaving the cell, the BS sends it a measurement order

The MS begins its measurement and reports

MSC makes the Handover decision.

Handover Algorithms (IS-95 vs. WCDMA)

Basic IS-95 handover algorithm usesabsolute threshold algorithm.

WCDMA handover algorithm usersrelative threshold algorithm.

Absolute Threshold Handover

Th_Add

Th_Drop

Neighbor Set

CandidateSet

ActiveSet

Neighbor Set

(1) (2) (3) (4) (5) (6)

Basic IS-95 HO Algorithm

1. Pilot strength exceed T_Add. MS sends a Pilot StrengthMeasurement Message and transfers pilot to the Candidate Set.

BS sends a Handover Direction Message.3. Mobile station transfers pilot to the Active Set and sends a

Hanover Completion Message.

4. Pilot strength drops below T_Drop. MS starts the handover drop

timer.5. Handover drop timer expires. MS sends a Pilot Strength

Measurement Message.

6. BS sends a Handover Direction Message. MS moves pilot fromthe Active Set to the Neighbor Set and sends a Handover Completion Message.

Problems with Absolute Threshold

Algorithm

Some locations in the cell receive only weak pilots (requiring a lower handover threshold).

Some locations in the cell receive a few strong andominant pilots (requiring a higher handover threshold).

Relative Threshold HO

Strongest Pilot in Active SetEc/Io

Window_Add

Window_Drop

T_DropT_Add Time

MS_Ec/Io

MS AS MS

AS_ThAS_Th_Hyst

AS_Th_Hyst

Active vs. Monitored Set

Active Set (AS): User information is sent from allthese cells and they are simultaneously demodulated

and coherently combined.

Monitored Set (MS): Cells, which are not included in

the active set, but are monitored according to aneighboring list assigned by the UTRAN.

Soft Handover Algorithm

(for Active Set limit = 2)

AS_Th -AS_Th_HystAS_Rep_Hyst

AS_Th + AS_Th_Hyst

Cell 1 Connected

CPICH 1

CPICH 2

CPICH 3

ΔT ΔT ΔT

Event AAdd Cell 2

Event BReplace Cell 1 with Cell 3

Event CRemove Cell 3

Intersystem Handoff

Intersystem handoff happens when a mobile moves fromone cellular system to a different cellular system.

The MSCs involved in the two cellular systems aredifferent.

Compatibility between the two MSCs must be

determined.

Prioritizing Handover

Guard Channel Concept : Use reserved guard channelfor handover.

Disadvantage: Reducing the total carrier traffic.

Queuing of Handover Requests: To prevent forced

termination by queuing the request.Queuing of handoffs is possible due to the fact that there is afinite time interval between the time the received signal leveldrops below the handoff threshold and the time the call is

terminated due to insufficient signal level.

The Umbrella Cell Approach

Practical Handover Consideration

Problem 2: Cell Dragging

Caused by pedestrian users that provide a very strong signal tothe base station.

Often occurs in an urban environment when there is a line-of-sight (LOS) radio path between the subscriber and the basestation.

As the user travels away from the base station at a very lowspeed, the average signal strength does not decay rapidly and the received signal at the BS may be above the handoff threshold, thus a handoff may not be made.

Creates a potential interference and traffic management problem.

Solution: Careful arrangement of handoff threshold and radio coverage parameters.

Handoff Control Parameters

Δ≅ 6~12 dB (AMPS)→Δ≅0~6 dB (GSM)

Typical time to make a handoff – once the signal

level is blow the handoff threshold : 10 sec (AMPS)→1~2 sec (GSM)

The faster handoff process supports a much greater

range of options for handling high speed and low speed users and provides the MSC with substantial time torescue a call that is in need of handoff.

useableminimumhandoff Pr Pr −−=Δ

Handoff Miscellaneous

Intra-frequency Handoff : handoffs in the samesystem and carrier.

Inter-frequency Handoff : handoffs between samesystems and different carriers.

May be used for handoff between different cell layers of

the multi-layered cellular network, when the cell layers usedifferent carrier frequencies.

Inter-system Handoffs: handoffs between different

systems.Inter-frequency and inter-system handoffs may beused for coverage or load balancing reasons.

Co-channel Cells and Interference

In a given coverage area, there are several cells that use thesame set of frequencies. These cells are called co-channel

cells.The interference between signals from co-channel cells iscalled co-channel interference.

Unlike thermal noise which can be overcome by increasingthe signal-to-noise ratio (SNR), co-channel interferencecan＇t be overcome by simply increasing the carrier power

because an increase in carrier power increases the interference

to neighboring co-channel cells.To reduce co-channel interference, co-channel cells must be

physically separated by a minimum distance.

Hexagon

A cell centered at (u,v)

vuvu y

rjrivu

+=+⋅=

⋅=⋅=

2)30sin(

3)30cos(

)2,2(),(

Hexagonal Cluster

Each cluster is surrounded by six similar clusters with thesame orientation

Each cluster has a total area equivalent to what can becalled a“super-hexagon＂

view a cluster as a“hexagon＂

2 22 D r i j ij= ⋅ + +

7-Cell Co-Channel Signal to Interference

gRatio– An Approximation

2( ) 2( ) 2

I D R D R D

− − −=

− + + +

( ) ( )4 4 4

2 1 2 1 2

I Q Q Q− − −

=− + + +

Power Control in 3G (WCDMA)

Tight and fast power control is perhaps the mostimportant aspect in WCDMA in particular on the

uplink. Without it, a single overpowered mobilecould block a whole cell.

Near-Far problem of CDMA: A MS close to the basestation may be overpowered and block a large part of the cell.Power control in WCDMA:

Open-loop power control

Close-loop power controlInner-loop power control

Outer-loop power control

Open Loop Power Control in WCDMA

Attempt to make a rough estimation of path loss bymeasuring downlink beacon signal.

Disadvantage: Far too inaccurate, because fast fading isessentially uncorrelated between uplink and downlink,due to the large frequency separation of uplink and downlink band of the WCDMA FDD mode.

Open-loop power control is used in WCDMA to providea coarse initial power setting of the MS at the beginningof a connection.

Inner-loop Power Control in WCDMAU li k

Base station performs frequent estimates of thereceived Signal-to-Interference Ratio (SIR) and

compares it to a target SIR.If the measured SIR is higher than the target SIR, the basestation will command the MS to lower the power.

If SIR is too low, it will command the MS to increase its power.

The power control is operated at a rate of 1500 times

per second.

Uplink

Trunking System

Trunking system: A mechanism to allow many user toshare fewer number of channels.

Not every user calls at the same time.

Penalty: Blocking Effect.

If traffic is too heavy, call is blocked!!

Small blocking probability is desired.

There is a trade-off between the number of availablecircuits and blocking probability.

Grade of Service (GOS)

Erlang: The amount of traffic intensity carried by achannel that is completely occupied.

For example, a radio channel that is occupied for 30minutes during an hour carries 0.5 Erlangs of traffic.

Grade of Service (GOS) is a measure of the ability of a

user to access a trunked system during the busiest hour.GOS is typically given as the likelihood that a call is blocked, or the likelihood of a call experiencing a delay

greater than a certain queuing time.

Definitions of Terms Used in TrunkingTheory

Theory

Load : Traffic intensity across the entire trunked radiosystem, measured in Erlangs.

Grade of Service (GOS): A measure of congestionwhich is specified as the probability of a call being blocked (Erlang B), or the probability of a call beingdelayed beyond a certain amount of time (Erlang C).

Request Rate: The average number of call requests

per unit time. Denoted byμ

second -1

Computation of GOS

There are two types of trunked systems:

No queueing for call requests:for every user who requests service, it is assumed there is no setup timeand the user is given immediate access to a channel if one is available.

If no channels are available, the requesting user is blocked withoutaccess and is free to try again later.

Called blocked calls cleared .

GOS: Erlang B formula determines the probability that a call is blocked.

A queue is provided to hold calls which are blocked.If a channel is not available immediately, the call request may be

delayed until a channel becomes available.Called blocked calls delayed .

GOS: Erlang C formula gives the likelihood that a call is initiallydenied access to the system.

Trunked Systems with no Queuing forCall Requests– blocked calls cleared

Blocking Probability -- Erlang B Formula:

There are infinite number of users.Call requests are memoryless; both new and blocked usersmay request a channel at any time.

Service time of a user is exponentially distributed

Traffic requests are described by Poisson model.Inter-arrival times of call requests are independent and exponentially distributed.

blocking C

∑=0 !

!]Pr[ AMPS cellular system is designed for a GOS of 2% blocking.

Trunked Systems with Queuing for CallRequests– Blocked Calls Delayed

The average delay D for all calls in a queued system isgiven by:

The average delay for those calls which are queued isgiven by:

AC H delay D−

>= ]0Pr[

Improving Capacity in Cellular Systems

System Expansion Techniques

Adding New Channels

Frequency borrowing

Cell SplittingSectoring / Sectorization

Change of Cell Pattern

Coverage zone

Cell Splitting

Cell splitting⇒ small cells (microcells)

Sameservice

More cells in the service area, more capacity.

Transmit Power for Split cell

The transmit power of the split cell must be reduced.

For example, if new cell radius is half of that of old celland the path loss exponent n = 4:

t r RPP−

∝ 1 boundary]cellold at[n

t r RPP−∝ )2( boundary]cellnewat[ 2

boundary]cellold at[ boundary]cellnewat[ r r PP=

Sectoring

A Novel Microcell Zone Concept

Summaries of Improving Capacity inCellular Systems

While cell splitting increases the number of base stationsin order to increase capacity, sectoring and zonemicrocells rely on base station antenna placements to

improve capacity by reducing co-channel interference.Cell splitting and zone microcell techniques do notsuffer the trunking inefficiencies experienced by

sectored cells, and enable the base station to oversee allhandoff chores related to the microcells, thus reducingthe computational load at the MSC.

T ki Th

Trunking Theory

Introduction

There are two major classes of trunked radio systems: Lost Call Cleared ( LCC )

Lost Call Delayed ( LCD)

Lost Call Cleared systemQueueing is not provided for call requests.

When a user requests service, the user is given immediate

access to a channel if one is available.If no new channels are available, the call is blocked withoutaccess to the system. The user is free to try the call again later.

Calls are assumed to arrive with aPoisson distribution

.The time between successive calls is exponentially distributed.

The Erlang B formula describes the grade of service (GOS) asthe probability that an user will experience a blocked call.

Introduction

Assumed that there are infinite number of users.

Assume that all blocked calls are instantly returned to aninfinite user pool, and may be retried at any time in the future.

Lost Call Delayed system:Queues are used to hold call requests that are initially blocked.

When a channel is not immediately available, the call request

may be delayed until a channel becomes available.The Erlang C formula gives the likelihood that all channels arein use. It is also necessary to know the probability of howlong the cal must be delayed before a channel is available.

The GOS is measured by the probability that calls will havedelayed greater than t seconds.

Assume that all calls in the queue are eventually serviced.

Erlang B Formula

Derivation of Erlang B

a(t) is the number of call requests (arrivals) that have occurred

a(t ) is the number of call requests (arrivals) that have occurred since t = 0.

The Poisson process implies that the time of the nth call arrival

and the interarrival times between successive call requests aremutually independent.

The interarrival times between call requests are exponentially

distributed and mutually independent, and the probability that theinterarrival time will be less than some time s is given byPr(τn≤s)=1-e-λs, s≥0 whereτn is the interarrival time of the ntharrival and τn=t n+1-t n, where t n is the time at which the nth call

request arrived.The probability density function for τn is:

( ) , 0.n

n n p e

λτ τ λ τ −= ≥

The user service time is the duration of a particular call that has

The user service time is the duration of a particular call that hassuccessfully accessed the trunked system.

Service times are assumed to be exponentially distributed with

mean call duration H , whereμ=1/ H is the mean service rate.The probability density function of the service time is

where sn is the service time of the nth call.This trunking system is called an M/M/C/C queueing system. Thefirst M denotes a memoryless Poisson process for call arrivals,the 2nd M denotes an exponentially distributed service time, thefirst C denotes the number of channels available, and the last C

indicates a hard limit on the number of simultaneous users thatare served.

( ) ns

n p s eμ −=

Using Equation (A) and lettingδ→0 we obtain:

Using Equation (A) and lettingδ→0, we obtain:

, 1, 1

P O j i j i j i

λδ δ

λδ μδ δ

λδ δ

μδ δ

= − +

= − − + ≥

= + ≥

= ≠ ≠ + ≠ −

At steady state we have the Global Balance Equation:

At steady state, we have the Global Balance Equation:

As a result, we have:1 ,

n nP n P n C λδ μδ − = ≤

01 2 1 0

0 0 01 1

2 21 1

! 1 1! !

n in C C

PP P P P

P P P P n P Pn i

λ λ λ

μ μ μ λ μ λ

μ λ μ

⎛ ⎞= = = ⎜ ⎟

⎝ ⎠⎛ ⎞ ⎛ ⎞⎛ ⎞

= ⇒ = = − = −⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠ ⎝ ⎠

=⎛ ⎞⎜ ⎟⎝ ⎠

∑ ∑

E l n C F m l

Erlang C Formula

Derivation of Erlang C

Assumption: if an offered call can＇t be assigned a channel, it

Assumption: if an offered call can t be assigned a channel, itis placed in a queue which has an infinite length.

The Erlang C formula is derived by assuming that the trunked

system is a M/M/C/D queue, where C denotes the maximumnumber of simultaneous users and D is the maximum number of calls that may be held in the queue for service.

From the state diagram:

o t e state d ag a :

1for for

k k k k

P k P k C P P k C k

P C P k C P P k C C

P k C k

λ λδ μδ

− −

⎛ ⎞= ≤ ⇒ = ≤⎜ ⎟

⎝ ⎠

⎛ ⎞= ≥ ⇒ = ≥⎜ ⎟

⎝ ⎠

⎧⎛ ⎞≤⎪⎜ ⎟

⎪⎝ ⎠⇒ = ⎨

⎛ ⎞⎪≥⎜ ⎟⎪

⎝ ⎠⎩

Since =1 we have:P∞

Since =1, we have:

1 11 ...... .... 1!

1 1 11 1

k C k k c

Pk C C

μ μ λ

− ∞

−= =

⎡ ⎤⎛ ⎞ ⎛ ⎞+ + + + =⎢ ⎥⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠⎢ ⎥⎣ ⎦

⎡ ⎤⎛ ⎞ ⎛ ⎞⇒ + + =⎢ ⎥

⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠⎢ ⎥⎣ ⎦

⇒ =⎛ ⎞ ⎛ ⎞

+⎜ ⎟ ⎜ ⎟ ⎛ ⎞⎝ ⎠ ⎝ ⎠ −⎜ ⎟⎝ ⎠

∑ ∑

channels are busy

k C k C

μ μ λ

−∞

⎛ ⎞= ⎜ ⎟⎝ ⎠

⎛ ⎞ ⎛ ⎞= ⎜ ⎟ ⎜ ⎟

⎝ ⎠ ⎝ ⎠

⎛ ⎞= <⎜ ⎟

⎛ ⎞⎝ ⎠ −⎜ ⎟⎝ ⎠

Substituting for P0 C

⎛ ⎞

! 1channels are busy

11 1 1

λ μ λ λ

μ μ λ

λ λ λ

μ μ μ

⎛ ⎞⎜ ⎟⎝ ⎠=

⎛ ⎞ ⎡ ⎤−⎜ ⎟ ⎢ ⎥⎛ ⎞ ⎛ ⎞⎝ ⎠ ⎢ ⎥+⎜ ⎟ ⎜ ⎟⎢ ⎥⎛ ⎞⎝ ⎠ ⎝ ⎠ −⎜ ⎟⎢ ⎥

⎝ ⎠⎣ ⎦

⎛ ⎞⎜ ⎟⎝ ⎠

= ⎡ ⎤⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞+ −⎢ ⎥⎜ ⎟⎜ ⎟ ⎜ ⎟ ⎜ ⎟

⎝ ⎠⎝ ⎠ ⎝ ⎠ ⎝ ⎠⎢ ⎥⎣ ⎦

With A=λ/μ=λ H , the Erlang C formula is given by:

Pr channels are busy

A A A C C k

=⎛ ⎞+ −⎜ ⎟⎝ ⎠

3G Cellular

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