WO BT1002 E01 0 UMTS Key Technologies

UMTS Key Technologies

Contents

1 Power Control.............................................................................................................................................1

1.1 Open Loop Control............................................................................................................................2

1.2 Inner Loop Control............................................................................................................................3

1.3 Outer Loop Control...........................................................................................................................3

2 Handover.....................................................................................................................................................5

3 RAKE Receiver........................................................................................................................................13

4 Code Resource Allocation........................................................................................................................15

4.1 Channelization Code.......................................................................................................................15

4.2 Scrambling Code.............................................................................................................................18

4.3 Synchronization Code.....................................................................................................................22

4.4 Measurement Related to Admission Control...................................................................................24

5 Load Control/Congestion Control..............................................................................................................27

i

1 Power Control

Quality Of Service (QOS) that Radio cell network provides for each subscriber mainly

depends on signal-to-interference ratio (SIR) of subscriber receiving signals. For

CDMA cell system, all subscribers in same cell use same band and timeslot, and

subscribers are isolated with each other only by the (quasi-) orthogonalization of

spreading code. Correlation characteristics between each subscriber signals are not so

good and signals of other subscribers interfere signals of current subscribers, due to

multipath and delay of the radio channels.

Increasing of subscribers or power of other subscribers may enhance interference on

current subscriber. Therefore, CDMA system is a strong power-restricted system and

strength of interference influences system capacity directly.

Power control is regarded as one of the key technologies of CDMA system. Power

control adjusts transmission power of each subscriber, compensates channel

attenuation, countervails near-far effect and maintains all subscribers at lowest standard

of normal communication. It reduces interference on other subscribers at most,

increases system capacity and prolongs holding time of mobile phones.

Power control is an important part in the UMTS system. If all the MSs in a cell

transmit signals at the same power, the signals from a near MS to the BTS are stronger,

and the signals from a far MS to the BTS are weaker. As a result, the strong signals

override the weak signals. This is called Near/Far Effect in the mobile communication.

UMTS is a self-interference system and all users use the same frequency. Therefore,

the "near-far effect" is more serious. In addition, for the UMTS system, the downlink

of the BTS is power restricted. To achieve acceptable call quality when the TX power

is small, both the BTS and the MS are required to adjust power needed by the

transmitter in real time according to the communication distance and link quality. This

process is called "power control".

Power control of UMTS includes inner loop power control and outer loop power

control by effect.

Inner loop power control is used to combat channel fade and loss, so that the SIR or

1

power of the received signals can reach the specific target value. Outer loop power

control generates the SIR or power threshold for inner power control according to the

QoS in the specific environment. By link, there is uplink power control and downlink

power control. Since the CDMA system capacity is mainly restricted by that of the

uplink, uplink power control is particularly important.

By link type, there is open loop power control and closed loop power control. Open

loop power control is based on the assumption that the uplink and downlink channels

are symmetric. It can counteract path loss and shadow fade. Close loop power control

does not need this assumption, and it can counteract fast fade.

1.1 Open Loop Control

1. Uplink open loop control

In the UMTS system, every MS is calculating the path loss from the BTS to the MS all

the time. When the signal received by the MS from a BTS is very strong, it indicates

that either the MS is very close to the BTS or the transmission path is excellent. In this

case, the MS can lower the TX power, while the BTS can still receive signals normally.

On the other way around, when the signal received by the MS is very weak, its TX

power can be increased to counteract the attenuation. Open loop power control occurs

only when the MS is powered on and only once.

2. Downlink outer loop control

It is the process of estimating the initial TX power of a new requested service. The

system can estimate the initial TX power of the downlink channel according to the

signal quality of the primary common pilot channel (P-CPICH) measured by the UE.

At the same time, the following factors have to be taken into account: QoS, data rate,

quality factor Eb/No, real-time total TX power of the downlink, interferences on this

cell by other cells, and so on.

Open loop power control is simple and direct, without needing exchange of control

information between the MS and the BTS. In addition, it features a higher control

speed and needs few overheads. However, in the UMTS system, different frequencies

are used in uplink/downlink transmission. The frequency difference is far greater than

the coherent bandwidth of channels. Therefore, it cannot be assumed that the fade

characteristic of the downlink channel is equal to that of the uplink channel. This is the

limitation of the open loop power control.

2

Chapter 4 Code Resource Allocation

1.2 Inner Loop Control

In this case, the receiver compares the signal-interference ratio of the received signal

with the target value of the control channel. Then, it returns a transmission power

control (TPC) command to the sender. The sender determines whether to increase or

reduce the TX power based on the closed loop power control algorithm specified by the

upper layer, and makes adjustments at the specified step according to the received

command.

1.3 Outer Loop Control

Outer loop power control is a supplement to closed loop power control. The working

principle of uplink outer loop power control is: Compare the actual block error (BLER)

ratio of the transmission channel with the target block error (BLER) ratio, then, slowly

adjust the target signal-interference ratio (SIRTarget) so that the service quality is not

affected by the change of the radio environment, and that a relatively constant

communication quality can be maintained.

Outer loop power control usually adjusts the target SIR based on BLER, to make the

QoS meet the requirements. Since different services have different QoS, there are

different target SIRs.

Downlink outer loop power control is similar to downlink inner loop power control.

3

2 Handover

Concept of handover: UTRAN distributes radio resources for UE in the new cell

because of UE’s moving and system load. Then UE synchronizes with the new cell and

transmits data each other. Handover is a very important technology in mobile

communication system networking.

Handover falls into:

1. Soft handover

Soft handover of cells under same Node B (softer handover)

Soft handover of cells between different Nodes B

Soft handover of cells in same band between different RNCs (involving Iur

interface)

2. Hard handover

Hard handover between different operators

Hard handover in same operator (forced hard handover)

Hard handover between systems (such as, with GSM)

Hard handover between different modes (such as, between FDD and TDD)

Handover refers to the redistribution of radio resource in mobile communication

system in cell structure, to keep discontinuous communication of mobile phones when

moving a mobile station from a district to another. Figure 1 Handover Types in UMTS

shows handover types.

5

Handoverin the mode

Handoverbetween modes

Handoverbetween systems

Soft handover (Micro diversity)

Softer handover (Among sectors)

Hard handover (Among frequencies)

Handover of FDD-TDD

UTRA FDD - GSM

R4 UTRA FDD-CDMA2000

Figure 1 Handover Types in UMTS

Hard handover refers to deleting the old link and then building a new link.

Services break off during the handover, as shown in Figure 2.

Soft handover refers to adding a new link (the old and the new links exist at the

same time) and deleting the old one after stabilization. Services continue in the

handover, as shown in Figure 3.

Node B

Node B

Cell 1 Cell 2

Figure 2 Hard Handover

6


Node B

Node B

Cell 1 Cell 2

Figure 3 Soft Handover

Softer handover refers to handover between cells with same frequency and in

same base station. Difference of soft handover and softer handover is that

combination of radio links is realized by RNC for soft handover while is realized

in Node B (decided by RNC) for softer handover.

Inter-frequency hard handover refers to handover between cells with different

carriers in UMTS.

Inter-system hard handover refers to handover between UMTS and other systems (such

as, GSM).

General flow of handover:

1) Measurement (UE)

2) Reoport on measurement results (report from UE to RNC)

3) RNC decides whether to perform handover accpridng to the report and algorithm

(RNC sends the command of handover to UE if it is necessary).

Figure 4 shows detailed flow of handover.

7


(A) RNC send measurementcontrol message to UE

(B) UE measures according toRNC's requirement and sendmeasurement report messageto RNC

(C) RNC stores its measuredresults of each cell in differentcarriers, for different UE

(D) Estimate signal quality ofeach carrier on the report(between bands and systems)

(E) Qualityjudgment

Quality of currentcarrier is good

Quality of othersystems is good

Quality of othercarriers is good

(F) Maintainavtivated set andmonitoring set

(inclduing currentand different carrier)

(G) Corresponds toallocation resourcesof virtue activated

set cell, prepared forhandover

(H) In correspondingcell, allocateresources andprepared for

handover

(I) Determine the target cell and send the handover command if handover isrequired

Figure 4 Handover Flow

In 3GPP, events correlative to handover are:

Event correlative to soft handover in the band: 1A ~ 1F

Measured value is usually Ec/N0 of pilot channel, reflecting the quality of a certain

cell. 3GPP defines a series of measurement events in the band. UE reports

corresponding events when meeting definitions.

Event Description

1AQuality of target cell improves, entering a report range of relatively

activating set quality

Event 1BQuality of target cell decreases, depart from a report range of

relatively activating set quality

Event 1CThe quality of a non-activated set cell is better than that of a certain

activated set cell

Event 1D Best cell generates change

Event 1E Quality of target cell improves, better than an absolute threshold

Event 1F Quality of target cell decreases, worse than an absolute threshold

Event correlative to hard handover between bands: 2A ~ 2F

8


Ec/N0 is measured value, reflecting quality of operators by measuring cells at different

bands. UE reports corresponding events when meeting definitions.

Event Description

Event 2A Best band generates change

Event 2BQuality of currently-used band is worse than an absolute threshold

and that of non-used band is better than an absolute threshold

Event 2C Quality of non-used band is better than an absolute threshold

Event 2D Quality of currently-used band is worse than an absolute threshold

Event 2E Quality of non-used band is worse than an absolute threshold

Event 2F Quality of currently-used band is better than an absolute threshold

Event correlative to handover between systems: 3A ~ 3D

RSSI is measured value for GSM. UE reports corresponding events when meeting

definitions.

Event Description

Event 3A

Quality of currently-used UTRAN operator is worse than an

absolute threshold and quality of other radio systems is better than

an absolute threshold

Event 3B Quality of other radio systems is worse than an absolute threshold

Event 3C Quality of other radio systems is better than an absolute threshold

Event 3D Best cell in other systems generates change

For example: Adding a radio link in soft handover and Figure 5 shows signaling

process of handover.

9


UE Target Node B Source Node B RNC

RRC: Measurement Report(Event 1a) (From Source Node B to RNC)

NBAP: Radio Link Setup Request

NBAP: Radio Link Setup Response

Executing handoverjudgement andadding a radio linkin Target Node B

Start to receive

Distributing transmission resources on Iub interface

Start to send

RRC: Active Set Update(E1a) (From Source Node B to UE)

RRC: Active Set Update Complete (From Source & Target Node B to RNC

simutaneously)

UE connects to Source Node B and Target Node B simutaneously

Figure 5 Soft Handover (Adding a Link) Signaling Flow

2．Compression mode

UE has only one RF reception unit and can only decode signals of one frequency at one

time. Therefore, compression mode is necessary if measuring two cells of different

bands or I different cells. Figure 6 shows compression mode priciple.

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Data Compressi on1 Frame/ 10 ms

1 f rameCompressed

I dl e t i me forhetero- f requency

measurement

1 f rameuncompressed

Figure 6 Compression Mode Principle

In compression mode, Node B compresses data when sending some downlink channel

frames. UE takes advantage of remaining time for measurement of different bands.

Compression mode can be realized by drilling and halving spreading factors, and so on.

When measuring between frequencies, the system negotiates with UE whther to

support and select compression mode.

11

3 RAKE Receiver

Since multipath signals contain useful information, CDMA receivers improve the S/N

of the received signals by combining multipath signals. What a RAKE receiver does is

to receive the various channels of signals from multipath signals through multiple

related detectors, and then combine them. The RAKE receiver is a classical diversity

receiver specially designed for the CDMA system. Its theoretical basis is that when the

propagation delay exceeds one chip code cycle, multipath signals are actually seen to

be mutually irrelevant.

RAKE reception separates and combines multipath signals. Different from the IS-95 A,

UMTS has three times of multipath resolving power. In addition, in a UMTS system,

the pilot information sent by the user can be used for coherent combination on the

reverse link. The theoretical analysis of UMTS shows that if the reverse link uses 8-

path RAKE reception, over 75% signal energies are used. The suppression of the

RAKE reception for multiple access interference depends on the correlation between

the different user characteristic codes.

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4 Code Resource Allocation

In UMTS, code resources fall into channelization codes, scrambling codes and

synchronization codes.

In ＷＣＤＭＡ mobile communication system, primary scrambling codes are to

differentiate cells, channelization codes are to differ physical channels on the downlink,

and scrambling codes are to differentiate users on the uplink. Orthogonal Variable

Spreading Factor (OVSF) is precious ans scarce resource, so one cell corresponds to

one code table. To access more users and to increase system capacity, make use of code

resources reasonably. It is very important to plan and manage downlink channelization

code resources.

Although there are many scrambling codes on the uplink, it is necessary to plan

scrambling of RNC, to avoid different users in different RNC use same scrambling

codes.

4.1 Channelization Code

1. Brief introduction

Channelization codes are based on OVSF technology, which can change the spreading

factor and ensure orthogonality between different spreading codes of different length.

On the downlink, channelization codes are to differentiate transmission from the same

source, that is, downlinks within a sector, while on the uplink, to differentiate dedicated

physical channels of all uplinks from a UE (including dedicated physical data channel

and dedicated physical control channel).

2. Generation principle

Figure 7 shows principle of channelization code generation.

Cch，n，m indicates channelization codes with spreading factor (SF) of n and code of m.

15

C C

C C

C C

C C

C C

C C

C

C

C

C

C

C

1 1

1 1

C C

C C

C

C

1C

1-ch,2n,2n1-ch,2n,2n

1-ch,2n,2n1-ch,2n,2n

ch,2n,1 ch,2n,1

ch,2n,1ch,2n,1

ch,2n,0 ch,2n,0

ch,2n,0ch,2n,0

1-1)1),2(nnch,2

2-1)1),2(nch,2(n

1),3ch,2(n

1),2ch,2(n

,11nch,2

01)ch,2(n

ch,1,0ch,1,0

ch,1,001ch

12ch

2,0ch

ch,1,0

－

－

－

＝

－＝

－＝

＝

＋＋（

＋

）＋（

，

，，

，，

，

Figure 7 Channelization Code Generation Principle

3. Selection rule

Codes are selected from the code tree shown in Figure 8.

SF = 1 SF = 2 SF = 4

C ch,1,0 = (1)

C ch,2,0 = (1,1)

C ch,2,1 =

(1,-1)

C ch,4,0 =(1,1,1,1)

C ch,4,1 =

(1,1,-1,-1)

C ch,4,2 =

(1,-1,1,-1)

C ch,4,3 =

(1,-1,-1,1)

Figure 8 Channelization Code Tree

There are some restrictions to channelization codes used by same information source.

16


For physical channelization, all codes in a lower branch of the code tree are not in use,

that is, all high-order spreading factors after the code cannot be used. Likewise, the

low-order spreading factors from the branch to the root cannot be used. As shown in

Figure 9, red codes are those that have been allocated and blue codes are those that

cannot be allocated.

SF=8

SF=32

SF=16

Figure 9 Example for Code Allocation

4. Specific allocation

1) Allocation of downlink common channelization code

Channelization codes used on primary common pilot channel (P_CPICH) and primary

common control physical channel (P_CCPCH) are fixed, which are Cch,256,0 and

Cch,256,1 respectively.

For S_CPICH, PICH, AICH, AP-AICH, CSICH and CD/CA_ICH common channels,

channelization codes with the spreading factor of 256 are used. Generally, several such

channels have been planned when a cell is set up and the channelization codes for them

have been allocated in advance (therefore, dynamic application is unnecessary).

For S_CCPCH, since the data rate borne on it is not fixed, its spreading factor can be

selected between 4 and 256. Generally, a corresponding spreading factor is determined

for it according to the permitted transmittable data rate during common channel setup

and then a channelization code is allocated for it.

2) Allocation of downlink dedicated channelization code

As the parameters vary, the principle of optimized and dynamic code allocation is

followed for dedicated physical channels.

3) Allocation of uplink common channelization code

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PRACH: Its preamble signature s (0< = s< = 15) refers to the 16 nodes on the code

tree, which corresponds to length 16 of the channelization code. The sub-tree of the

specified node is used to spread the message part. Spreading code of the control part:

Cc = Cch,256,m, m = 16 × s + 15 spreading factor of the data part: Ranging between 32

and 256. More exactly, spreading code Cd = Cch,SF,m, m = SF × s/16.

4) Allocation of uplink dedicated channelization code

PCPCH: Control channel Cc = Cch,256,0, data channel Cd = Cch,SF,k, SF = {256, 128, …, 4},

k = SF/4, and power control forward channelization code Cch,256,0.

4.2 Scrambling Code

1. Brief introduction

Scrambling codes are used after spreading, which will not change signal bandwidth.

They are only used to differentiate UEs or Nodes B.

2. Generation

1) Long scrambling code sequence

Long scrambling code sequences Clong,1,n and Clong,2,n are generated from 38400

chip packet mode 2 of two 25-order polynomial m sequences.

Suppose that x and y stands for two M sequences. x sequence is generated by using

multinomial X25+X3+1 and y sequence is generated by using multinomial

X25+X3+X2+X+1. They form a Gold sequence. Clong,2,n is the 16777232 code chip

shifted from sequence Clong,1,n.

The construction of xn and y of M sequence is:

Initial conditions:

xn(0) = n0, xn(1) = n1, …,xn(22) = n22, xn(23) = n23,xn(24) = 1.

y(0) = y(1) = … = y(23) = y(24) = 1.

Recursive expression:

xn(i+25) = xn(i+3)+ xn(i)modulo 2, i = 0, …, 225-27.

y(i+25) = y(i+3)+y(i+2)+y(i+1)+y(i) modulo 2, i = 0, …, 225-27.

Definition:

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zn(i) = xn(i)+ y(i)modulo 2, i = 0, 1, 2, …, 225-2.

。22，，1，01)(1

0)(1)( 25

iforizif

izifiZ

n

nn

clong, 1, n(i) = Zn(i), i = 0, 1, 2, …, 225 – 2

clong, 2, n(i) = Zn((i + 16777232) modulo (225 – 1)), i = 0, 1, 2, …, 225 – 2.

Finally, the definition of long scrambling code sequence is as follows:

2/211)()( ,2,,1,, icjiciC nlongi

nlongnlong

i = 0, 1, ..., 225 - 2; indicates round-off.

Figure 10 shows generation of a long scrambling code sequence.

c long,1,n

c long,2,n

MSB LSB

Figure 10 Long Scrambling Code Sequence Generator

2) Short scrambling code sequence

zn(i) = a(i)+2b(i)+2d(i) modulo 4, i = 0, 1, …, 254;

Here, "a (i)" is generated from polynomial expression.

a(0) = 2n0 + 1 modulo 4;

a(i) = 2ni modulo 4, i = 1, 2, …, 7;

a(i) = 3a(i-3)+ a(i-5)+3a(i-6)+2a(i-7)+3a(i-8)modulo 4,

i = 8, 9, …, 254;

Here, "b (i)" is recursive from polynomial expression.

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b(i) = n8+i modulo 2, i = 0, 1, …, 7,

b(i) = b(i-1)+ b(i-3)+ b(i-7)+ b(i-8) modulo 2, i = 8, 9, …, 254,

where, g2(x) = x8+x7+x5+x4+1 as;

where, d(i) = n16+i modulo 2, i = 0, 1, …, 7;

d(i) = d(i-1)+ d(i-3)+ d(i-4)+ d(i-8)modulo 2, i = 8, 9, …, 254,

where, zn(255) = zn(0).

Finally, the definition of a short scrambling code sequence is as follows:

2/256mod211)256mod()( ,2,,1,, icjiciC nshorti

nshortnshort

Figure 11 shows generation of a short scrambling code sequence.

0 7 4

+ mod n addition

d(i) 1 2 3 5 6

2

mod 2

0 7 4 b(i)

1 2 3 5 6

2

mod 2

+ mod 4 multiplication

Zn(i)

0 7 4 1 2 3 5 6

+ mod 4

Mapper

C short,1,n(i)

a(i)

+ + +

+ + +

+ + +

3 3

3 2

C short,2,n(i)

Figure 11 Short Scrambling Code Sequence Generator

3. Usage

1) Downlink

There are altogether 24,576 downlink scrambling codes numbered as n = 0,..., 24575.

They fall into following three parts:

k = 0, 1, 2,...8191, correspond to 8,192 ordinary scrambling codes used in normal

mode.

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k+8192, k = 0, 1, 2, ...8191, are alternative scrambling codes used in compression

mode when "n" is less than SF/2. They are called left scrambling codes and there are

altogether 8,192.

k+16384, k = 0, 1, 2, ...8191, are alternative scrambling codes used in compression

mode when "n" is more than or equal to SF/2. They are called right scrambling codes

and there are altogether 8,192.

Here: n is allocated channelization code No.

Rules for ordinary scrambling code allocation

Former 8,192 scrambling codes fall into 512 sets and each comprises of a primary

scrambling code and 15 secondary scrambling codes following the primary one. Every

cell corresponds to a set of downlink scrambling codes.

Primary scrambling code sequence number: n = 16*I, I = 0, …, 511,

Scrambling code No. corresponding to the secondary scrambling code sets: n = 16 * I +

K K = 1, …15.

Rule for scrambling code selection during scrambling for downlink physical

channel

For PCIPCH, PCCPCH, S_CPICH, PICH, AICH, AP-AICH, CSICH and CD/CA_ICH

channels, primary scrambling codes are adopted for scrambling.

For their downlink physical channels, either primary scrambling codes or secondary

scrambling codes can be adopted for scrambling.

2) Uplink

PRACH scrambling code

Construction of preamble code:

4095...,30,1,2k ekCkSkCk

24j

ssig,nr_pre,sn,pre, ，，＝，＝＋

Here, message part scrambling code:

Sr_msg，n（i）=Clong，n（I+4096），i=0，1，…，38399

Scrambling code of the message part begins with 4096th code chip of scrambling code

sequence. The first 4096 code chips serve as preamble scrambling codes of PRACH.

That is to say, same scrambling code sequence No. is used for preamble scrambling

21


code and message scrambling code of PRACH. In addition, there are also altogether

8,192 scrambling codes in the message part.

Here, preamble scrambling code is:

Sr_pre，n（i）=Clong，n（i），i=0，1，…，4095; n=0，1，2，…，8191

There are 8,192 PRACH preamble scrambling codes in total. They fall into 512 sets

and each comprises of 16 scrambling codes. There are 16 preamble scrambling codes

in each cell in total. The relation between scrambling code sequence number and

corresponding primary scrambling code sequence of the corresponding cell is: n =

16*m+k m = 0, 1, …, 511; k = 0, 1, …, 15.

DPCH scrambling code

There are numerous scrambling codes available for DPCH. In addition, scrambling

codes generated from a long scrambling code generator or from a short scrambling

code generator can be used.

Use of long scrambling code:

38399...0,1,i iCiS nlong,nDPCH, ＝，＝

Use of short scrambling code:

38399,...,1,0i ,,, ＝iCiS nshortnDPCH

Available scrambling codes on UMTS uplink is 224 and codes of 0 to 4095 are

allocated to PRACH, 4095 to 40959 are allocated to PCPCH, and the remaining 224 to

40960 are all for DPCH. During the allocation, ensure that different uplink scrambling

codes are allocated to different UE.

Since scrambling codes are allocated in RNC, uplink scrambling code resources are

same for different RNC, which are 224 to 40960. During the network planning, for

those cells at the edge between different RNCs, keep their downlink primary

scrambling codes different to ensure their scrambling code sequence different.

4.3 Synchronization Code

Synchronization codes fall into primary synchronization codes and secondary

synchronization codes, which are the detection objects of cell search by a UE. All cells

have same primary synchronization code.

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1. Composition of a primary synchronization code

a a, a,- a, a,- a, a, a, a,- a,- a, a,- a,- a, a, a,j1C psc＝

11,-1,-1,1,-1,1,-1,1,-1,-1,1,1,1,1,1, x,, x, x,xa 16321 ＝＝

A primary synchronization code is composed of 256 bits in total, which correspond one

by one to 256 chips transmitted from each time slot of a synchronization channel.

2. Composition of a secondary synchronization code

255z255...2z2),1()1(0z0j1C kscc mmmm hhzhh ，，，＝，

Here, 1k16m －＝

z sequence is a fixed code sequence:

16 15,1413121110987654321 x-x- ,x- ,x- ,x- ,x- ,x- ,x- , x, x, x, x, x, x, x,xb ＝

b- b,- b,- b,- b,- b, b,- b, b,- b,- b, b, b,- b, b, b,z ＝

Here, the values of x1, x2, ... x15, x16 are the same as X values that constitute a

primary synchronization code. h sequence is constituted according to the following rule

when k is 8:

1k H H

H HH

1H

1k1k

1k1kk

0

，

－＝

＝

－－

－－

Call admission control decides to accept or refuse a new subscriber, new Radio Access

Bearer (RAB) and new Radio Link (RL)according to current resource (such as,

handover). Call admission control is applicable to original UE access, RAB

designation, reconfiguration and handover. It may lead to different results because of

PRI and actual situations.

Call admission control meets QOS of new calls as much as possible premising the

stability of the system, based on interference measurement, to avoid overloading.

Call admission control falls into:

Uplink call admission control

Downlink call admission control

UMTS is a self-interference system and there exists power climbing. The core is

power. Uplink capacity depends on whether total receiving interference power is

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beyond linear dynamic range of LNA or not. Downlink capacity depends on whether

the distribution of transmission power completes or not. Process of call admission

control: Measure current load of system cell when making calls (new access calls and

handover calls), forecast and estimate calls and judge whether to access calls. Take

QOS requirements of calls. That is, communication rate, communication quality

(signal-to-noise ratio and error code ratio) and delay into consideration when

forecasting calls. Refuse the call when it approaches some threshold.

Service calls fall into new calls and handover calls. Reserve some resources for

handover calls to ensure high ratio of successful handovers. PRI of handover calls is

distinguished by admission control threshold. Admission control threshold of new calls

is lower than that of handover calls. New calls are refused when current load of the cell

is higher than admission threshold of new calls. However, handover calls are accepted.

Handover ratio is better to be at about 35% in soft handover. Handover calls are also

refused when the load of the cell is higher than handover admission control threshold.

Load control threshold is commonly higher than admission control threshold of

handover, to prevent overloading when radio circumstances change and to ensure the

stable running of the system.

4.4 Measurement Related to Admission Control

1. Node B Common Measurement

1) DCH measurement

Major factors to influence UMTS system capacity (DCH) are uplink interference and

downlink operator emission power. DCH admission control performs admission

decision on these two parameters. Node reports RTWP and TCP of Node B common

measurement to RNC periodically, so that RNC can decide whether to access the new

call according to latest load.

2) HS-DSCH measurement

HS-DSCH admission control needs Node B common measurement information related

to HSDPA, including HS-DSCH required Power, Transmitted operator power of all

codes not used for HS-PDSCH or HS-SCCH transmission. Therefore, open these

common measurements simultaneously in cells supporting HSDPA.

3) RACH measurement

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According to policies of ZTE, to estimate the load, RACH starts up Acknowledged

PRACH preambles common measurement during the admission control, to get actual

utilization ratio of PRACH channel.

2. UE measurement

When predicting downlink power, RNC needs real-time route loss of UE. RNC can get

route loss of UE by different means according to admission requests type, such as,

reporting through some events of UE.

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5 Load Control/Congestion Control

1. Process of load control

The system measures the load of the system cell at real time continually. The system

load is high and the load enters unstable running district of the system when load

average value exceeds some threshold value in a set time. Load control is necessary at

the moment.

2． Load control threshold

The core of load control is to access as many services as possible premising that the

system is running stably, to realize high efficient running. Leave some redundancy,

excluding base line for system breakdown, as threshold value of load control.

Threshold value of load control is larger than that of admission control.

3． Load control mode

Load control works when system load approaches or exceeds load threshold value.

Main modes:

1） Reduce the load in a rapid mode, which is mainly realizes by base station (Node

B).

Downlink rapid load control: Refusing the command to increase the power from

the mobile station.

Uplink rapid load control: Reducing SIR destination value for uplink rapid power

control.

2） Reduce the load in medium or slow mode, which is mainly realized by base

station controller (RNC).

Commonly, RNC makes judgment and changes max allowed transmission power,

destination SIR value and TFCS by reconfiguring the RL. In this mode can the system

load be reduced for a long time.

Make negotiations if hoping to reduce the system load for a long time, that is, RNC

negotiates with CN to reduce resource occupation of services during the

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communications. Or, share loads with adjacent cells in RNS to reduce the load of those

overloading cells. The taking-in and sending-out of adjacent cells (covering radius of

one cell increases and that of another adjacent cell decreases) is called cell breathing.

Conclusions:

Reducing the throughput of grouping (reducing the transmission rate)

Handing over to other UMTS carrier frequency

Handing over to GSM system

Reducing the rate of real-time services

Executing the call drops

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WO BT1002 E01 0 UMTS Key Technologies

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Transcript of WO BT1002 E01 0 UMTS Key Technologies