Slide UMTS.10std

8/8/2019 Slide UMTS.10std

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Trng i hc Bch Khoa H Ni

Khoa in t Vin thng

Thng tin di ngMobile Communications

TS. Trng TunB mn K thut thng tin

H Ni, 10-2010


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2

Mng thng tin di ng 3GUMTS / W-CDMA

(Universal Mobile Telecommunications System)

4. Mt s c ch c bn ti UTRAN4. Mt s c ch c bn ti UTRAN--FDDFDD


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Cell Interference

3

Need to control: Power Range (coverage) Processing Gain/Spreading Factor per user

Overall loading


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Cell Breathing

A UE on the cell edge is transmitting with maxpower

Another UE becomes active

Increased interference

the received signal from the UE on the cell edge istoo weak!

Effective cell size decreases with increasingnumber of users

There is a trade-off between capacity and coverage

Cell size depends on both maximum Tx power andnumber of active users (in the same and othercells) which results in cell breathing


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Cell breathing phenomena

Cas 1 : 10 utilisateurs Cas 2 : 20 utilisateurs

-10 < C/I < -5 dB -15 < C/I < -10 dB-15 < C/I < -50 dB cellules

Case 1: 20 usersCase 2: 10 users

cells


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Cell Breathing


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Cell Breathing

Effective range of cell is reduced on higher loading due to interferencecaused by additional channels Adjacent cells also breathed

Soft handover region reduces


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Cell Breathing


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Voice Activity Detection

Reducing multiple access interference

Human speech: 42%

results in a capacity gain

FDMA and TDMA cellular systems

Frequencies are permanently assigned

Capacity in FDMA and TDMA systems is fixed and

primarily bandwidth limited.


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The relationship between the received power and the number of users


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Received power and the number of users

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Figure 2.20 shows a plot of the required received power, C, against thenumber of users N based on Equation (2.2) and assuming a processinggain of 256, a value for Eb/I0 +N0 of 7 dB, a value for of 50%, a value fori of 55% and a receiver noise figure of 5 dB. This shows the manner inwhich the required received power increases as the number of usersincreases. The increase in received power is gradual at first, but then it

starts to increase more rapidly as more users are added to the network.At some point we reach a value for N that causes the denominator inEquation (2.2) to become zero and, hence, C goes to infinity. Since nopractical transmitter can generate an infinite amount of power, this valueof N can never be reached in a practical system and it is termed the polecapacity of the network. If a practical network starts to approach its pole

capacity then it can become unstable, with the transmit powerrequirements of the UEs varying dramatically for very small changes in thenetwork load. Therefore, practical networks are usually designed tooperate at a certain fraction of their pole capacity and new calls arerejected once this limit is reached.


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Noise Rise

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The system load in the uplink direction can be measured in terms of equivalent

noise rise at the Node B, which is defined as the additional power that must

be delivered by a UE at the Node B to overcome the interference generated by

other UEs. Returning to Figure 2.20, we can see that with a single user on the

network, this UE must be received with a power of1205 dBm. However, if thenetwork load increases to 10 users, then each UE must deliver a power of

1199 dBm at the Node B receiver, i.e. an increase or noise rise of 0.6 dB. In a

practical network, an operator may choose to limit the network load to 75% of

the pole capacity and this equates to a noise rise of 6 dB. Once the Node B

detects that the total received noise and interference power at its receiver is 6

dB greater than the thermal noise alone, it will reject any new calls.


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Cell radius and noise riseCell radius and noise rise

R

Charge de la cellule = 20 % de

la capacit maximum

Niveau dinterfrence =y dB

RR

Charge de la cellule = 50 % de

la capacit maximum

Noise Rise = 2 dB

Niveau dinterfrence =y + 2 dB

R

R etRsont les rayons des

cellules dans les deux

situations de charge

Cell load = 20% of the

maximum capacity

Interference level=y dB

R andRare the cell

radius in the 2 load

situations

Cell load = 50% of the

maximum capacity

Interference level=y + 2 dB


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CELL BREATHING


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In CDMA coverage and capacity are tight together: When the number of users increases, the interference levels increases and

therefore the needed powers in order to keep constant quality. Due to infinite

power resources this means that the coverage decreases.

This leads to Cell Breathing: the coverage area changes as the load of the cell

changes Therefore, the coverage and the capacity has to plan simultaneously

Coverage and capacity planningCoverage and capacity planning


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Cell Interference

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Near-far problem

The uplink issue

D1 D2

D1> D2

UEs closer to Node B may create too much interference.

Requirements: fast power control in UE

Target: all UEs are received at the Node B with

the same power


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despreading

despreading

Power control

Transmit Power Control

MS

MS

MS

MS

Near far problem

Node B

Node B

TPCis essential

Minimizethe Tx power

Increasethe system capacity


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Power Control

Aims to reduce interference Near-far problem Reduces power consumption in

the MS Methodologies

Open-loop Sum of transmit power

and the received poweris kept constant

Closed-loop Signifies the other party

to increase or decreasetransmit power by apre-defined power step

d1

d2

Base Station

c1

c2

Distance

Pr2

Pr1

Pt1: Power transmitted from c1

Pt2: Power transmitted from c2

Pr1: Power received at base station from c1

Pr2: Power received at base station from c2

Pr1 = Pr2

Pt1

Pt2


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Power Control


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Power Control


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Closed-Loop Power Control Feedback loop with 1.5kHz cycle to

adjust uplink / downlink power to its

minimum

Even faster than the speed ofRayleigh fading for moderate mobile

speeds

Outer Loop Power Control

Adjust the target SIR (Signal toInterference Ratio) setpoint in base

station according to the target BER,

commanded by RNC

Power Control


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Inner Loop Power Control in the Downlink : This procedure enables abase station to adjust its transmit power in response to TPC commands from

the UE. Power is adjusted using a step size of 0.5 or 1 dB. The objective

here is to maintain a satisfactory signal-to-interference ratio at a UE using as

little base station transmitter signal power as possible.

Inner Loop Power Control in the Uplink This procedure is used by the UE

to adjust its transmit power in response to a TPC command from a base

station.With each TPC command, the UE transmit power is adjusted in steps

of 1, 2, or 3 dB in the slot immediately following the decoding of TPC

commands.

A TPC command may be either 0 or 1. If it is 0, it means that the transmitter

power has to be decreased. If it is 1, the transmitter power is to be increased.

Power Control


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Power Control


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Closed loop transmit power control in the Uplink

Transmit Power Control

Power Control: Manages radio link quality - Uplink is handled permobile (UE), downlink per physical channel

Ensures that transmission powers are kept at a minimum level and

that there is adequate signal quality and level at the receiving end

SIR measurement

TPC bit

Target SIRSIR

Up Link

Transmit power control

Tslot

Down Link

1dB step


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TPC and TPC and NearNear--farfar problem problem


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Fast closed loop PC (TPC)Fast closed loop PC (TPC)


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Fast closed loop PC (TPC)Fast closed loop PC (TPC)


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Handoff :

Cellular system tracks mobile stations in order to maintain their communication links.

When mobile station goes to neighbor cell, communication link switches from current cell

to the neighbor cell.

Hard Handoff :

In FDMA or TDMA cellular system, new communication establishes after breaking current

communication at the moment doing handoff. Communication between MS and BSbreaks at the moment switching frequency or time slot.

Hard handoff : connect (new cell B) after break (old cell A)

switching

Cell BCell A

Handoff (1/2)


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Cell

BCell A

Soft handoff : break (old cell A) after connect (new cell B)

transmitting same signal from both BS A andBS B simultaneously to the MS

Soft Handoff :

In CDMA cellular system, communication does not break even at the moment doing

handoff, because switching frequency or time slot is not required.

Soft Handoff (2/2)


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Mobility/Handoff in Umbrella Cells

Avoids multiple handoffs.


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Hand Over


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Soft Hand Over


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Soft Hand Over

UE combines symbols received from each Node B.

RNC selects the best radio frame from each Node B

As the mobile moves away from Node B #1, the link between the mobile and

Node B #1 becomes weaker. Before the link becomes marginal or breaks,

another link is established between the mobile and the second Node B. This

is known as a soft handover. If one link experiences a deep fade (e.g., due to

shadowing of the radio signal or interference in congested areas), the call will

stay up as long as the other link is maintained. This makes soft handovers

more reliable than hard handovers, where only a single link is maintained at

any given time.

Node B #1 Node B #2


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Softer Hand Over

UE combines symbols received from each cell. Node B combines symbols received from each cell.


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Inter-RAT Hard Handover


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Cell Reselection versus Handover


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Handover


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Handover Process

A basic HO process consists of three main

phases

measurement phase Intra-frequency

Inter-frequency

Traffic volume

Quality

Internal

decision phase Change of best cell.

Changes in the SIR level.

Changes in the ISCP level.

Periodical reporting.

Time-to-trigger.

execution phase. Network Evaluated Handover (NEHO)

Mobile Evaluated Handover (MEHO)

MEASUREMENT

DECISION

EXECUTION

Measuremetnt criteria

Measurement reports

Algorith parameters

Handover criteria

Handover signalling

Radio Resource Allocation


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Handover in UMTS

Handover Algorithm

Assumption: a UE, currently connected to signal A,is located in cell A and moving towards cell B.

Pilot signal A, deteriorates, approaching lowerthreshold Handover Triggering

Signal A equals lower threshold. Based on UE measurements, RNC recognises an

available neighbouring signal (signal B), withadequate strength to improve quality of connection.RNC adds signal B to Active Set.

UE has two simultaneous connections to UTRAN andbenefits from summed signal (signal A + B)

When quality of signal B becomes better than signalA

RNC keeps this as starting point for HO margincalculation.

Signal B greater than defined lower threshold. strength adequate to satisfy required QoS.

strength of summed signal exceeds defined upperthreshold, causing additional interference. RNCdeletes signal A from Active Set.

Handover essential to guarantee user mobility in a mobile communicationsnetwork.

(1)(2)(3) time

SignalStrength

Upper

threshold

Lowerthreshold

Handover

Margin

Signal BSignal A

Summed Signal

Cell A Cell B


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-Active set : bao gm tt c cc cells lin quan ng thi n qu trnh kt

ni chuyn giao mm. UE gii iu ch tn hiu nhn c t cc cell ny

v kt hp thnh tn hiu cui cng tng ng vi vic phn tp vi h s

khuch i khong 2 dB. Danh sch cell tch cc (active set) bao gm hai

hay nhiu cells trong mt h thng FDD.

- Monitored set : bao gm cc cell khng nm trong active set nhng c

theo di bi UE do thuc danh sch cc cell ln cn.

- Detected set : bao gm cc cell c pht hin bi UE nhng khng thuc

hai tp trn.

Hand over


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- Gi thit UE ang thu tn hiu mnh nht t cell 1, khi danh sch tch

cc chc duy nht cell 1.

- Nu ti thi im t1 xc nh, knh pilot (hoa tiu) ca cell 2 c tn hiu

ln, ngha l khong chnh lch cng sut gia cell 1 v cell2 nh hn mt

gi tr ngng 1. Khi pilot 2 c th c s dng v v vy cell 2 s

c b sung vo danh sch tch cc. T thi im , UE s ng thilin lc vi cell 1 v cell 2 , tng ng vi vic phn tp do kt hp tn

hiu thu t hai cell ni trn. 1 = L - H1, trong L = reporting range, v H1

= chnh lch cng sut b sung - addition hysteresis.

- Nu ti thi im t2, pilot 1 c mc tn hiu gim v khong chnh lch

gia pilot 2 v pilot 1 ln hn mt gi tr ngng 2, khi pilot 1 s khng

tip tc c s dng v b loi b khi Active Set. Do vy, t thi im t2,

UE ch kt ni vi cell2 Ngng 2 = L + H2, trong H2 = chnh lch

cng sut loi b - removal hysteresis.

Chuyn giao mm - SHO


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Initial acquisition at power on


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Mobile Originated Voice Call Flow

RB - Radio BearerThe service provided by the Layer 2 for the transfer of user data between UE (User

Equipment) and UTRAN (UMTS Terrestrial Radio Access Network).


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Slide UMTS.10std