Slide thông tin di động đại học bách khoa hà nội chương 10 một số cơ chế cơ bản tại UTRAN FDD tiếp

7Cell BreathingCell Breathing • Effective range of cell is reduced on higher loading due to interference caused by additional channels • Adjacent cells also breathed • Soft handover regi

Trường Đại học Bách Khoa Hà Nội

Khoa Điện tử Viễn thông

(Universal Mobile Telecommunications System)

ξ 4 Một số cơ chế cơ bản tại UTRAN-FDD

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 There is a trade-off between capacity and coverage

 Cell size depends on both maximum Tx power and

number of active users (in the same and other

cells) which results in cell breathing

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 cellu les

cells

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7Cell Breathing

Cell Breathing

• Effective range of cell is reduced on higher loading due to interference

caused by additional channels

• Adjacent cells also breathed

• Soft handover region reduces

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CELL BREATHING AND SOFT CAPACITY

9

Cell breathing is used to describe the way in which the

coverage of a Node B changes (or breathes) in response to

changes in the network load To show this effect let us start

with a very simple example of a UE with a fixed transmit

power communicating with a single nearby Node B.

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.

The relationship between the received power and the number of users

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Received power and the number of users

13

Figure 2.20 shows a plot of the required received power, C, against the

number of users N based on Equation (2.2) and assuming a processing

gain of 256, a value for Eb/I0 +N0 of 7 dB, a value for of 50%, a value for

i of 55% and a receiver noise figure of 5 dB This shows the manner in

which the required received power increases as the number of users

increases 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 in

Equation (2.2) to become zero and, hence, C goes to infinity Since no

practical transmitter can generate an infinite amount of power, this value

of N can never be reached in a practical system and it is termed the ‘pole

capacity’ of the network If a practical network starts to approach its pole

capacity then it can become unstable, with the transmit power

requirements of the UEs varying dramatically for very small changes in the

network load Therefore, practical networks are usually designed to

operate at a certain fraction of their pole capacity and new calls are

rejected once this limit is reached

Noise Rise

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 of −1205 dBm However, if the

network 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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R et R’ sont les rayons des

cellu les dans les deux

situations de charge

Cell load = 20% of the

maximum capacity

Interference level= y dB

R and R’ are the cell

radius in the 2 load

• 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 planning

Cell Interference

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• 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

is essential Minimize the Tx power Increase the system capacity

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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

Power Control

 Closed-Loop Power Control

 Feedback loop with 1.5kHz cycle to

adjust uplink / downlink power to its

minimum

 Even faster than the speed of

Rayleigh fading for moderate mobile

speeds

 Outer Loop Power Control

 Adjust the target SIR (Signal to

Interference 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 a

base 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 UplinkThis 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

ATPC commandmay 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

Power Control

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Power Control

Closed loop transmit power control in the Uplink

Transmit Power Control

Power Control: Manages radio link quality - Uplink is handled per

mobile (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

TPC and “Near-far” problem

Fast closed loop PC (TPC)

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Fast closed loop PC (TPC)

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 BS

breaks at the moment switching frequency or time slot.

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

Σ

Cell

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

transmitting same signal from both BS A and

BS 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)

Mobility/Handoff in Umbrella Cells

Avoids multiple handoffs

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Hand Over

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

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

Cell Reselection versus Handover

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 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 lower threshold  Handover Triggering

 Signal A equals lower threshold.

 Based on UE measurements, RNC recognises an available neighbouring signal (signal B), with adequate strength to improve quality of connection.

RNC adds signal B to Active Set.

 UE has two simultaneous connections to UTRAN and benefits from summed signal (signal A + B)

 When quality of signal B becomes better than signal A

 RNC keeps this as starting point for HO margin calculation.

 Signal B greater than defined lower threshold.

 strength adequate to satisfy required QoS.

 strength of summed signal exceeds defined upper threshold, causing additional interference RNC deletes signal A from Active Set.

 Handover essential to guarantee user mobility in a mobile communications

network.

(1)(2)(3) time

Upper threshold

Lower threshold

Handover Margin

Signal B Signal A

Summed Signal

Cell A Cell B

-Active set : bao gồm tất cả các cells liên quan đồng thời đến quá trình kết

nối chuyển giao mềm UE giải điều chế tín hiệu nhận được từ các cell này

và kết hợp thành tín hiệu cuối cùng tương ứng với việc phân tập với hệ số

khuếch đại khoảng 2 dB Danh sách cell tích cực (active set) bao gồm hai

hay nhiều cells trong một hệ thống FDD

- Monitored set : bao gồm các cell không nằm trong active set nhưng được

theo dõi bởi UE do thuộc danh sách các cell lân cận

- Detected set :bao gồm các cell được phát hiện bởi UE nhưng không thuộc

hai tập trên

Hand over

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- Giả thiết UE đang thu tín hiệu mạnh nhất từ cell 1, khi đó danh sách tích

cực chỉ có duy nhất cell 1

- Nếu tại thời điểm t1 xác định, kênh pilot (hoa tiêu) của cell 2 có tín hiệu đủ

lớn, nghĩa là khoảng chênh lệch công suất giữa cell 1 và cell2 nhỏ hơn một

giá trị ngưỡng Δ1 Khi đó pilot 2 có thể được sử dụng và vì vậy cell 2 sẽ

được bổ sung vào danh sách tích cực Từ thời điểm đó, UE sẽ đồng thời

liên lạc với cell 1 và cell 2 , tương đương với việc phân tập do kết hợp tín

hiệu thu từ hai cell nói trên Δ1 = L - H1, trong đó L = reporting range, và H1

= độ chênh lệch công suất bổ sung - addition hysteresis.

- Nếu tại thời điểm t2, pilot 1 có mức tín hiệu giảm và khoảng chênh lệch

giữa pilot 2 và pilot 1 lớn hơn một giá trị ngưỡng Δ2, khi đó pilot 1 sẽ không

tiếp tục được sử dụng và bị loại bỏ khỏi Active Set Do vậy, từ thời điểm t2,

UE chỉ kết nối với cell2 Ngưỡng Δ2 = L + H2, trong đó H2 = độ chênh lệch

công suất loại bỏ - removal hysteresis.

Chuyển giao mềm - SHO

Initial acquisition at power on

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Mobile Originated Voice Call Flow

Mobile Originated Voice Call Flow

RB - Radio Bearer

The service provided by the Layer 2 for the transfer of user data between UE (User

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