owj103104 wcdma load control issue1.1.doc

Course OWJ103104

WCDMA Load Control

ISSUE 1.1

OWJ103104 WCDMA Load Control ISSUE1.1 Table of Contents

Table of Contents

Chapter 1 Load Control.................................................................................................................. 21.1 Abbreviations...................................................................................................................... 21.2 Introduction......................................................................................................................... 21.3 Load Measurement.............................................................................................................3

1.3.1 WCDMA Load Concept............................................................................................31.3.2 UL Load and Measurement......................................................................................51.3.3 DL Load Definition....................................................................................................61.3.4 The Load Situations..................................................................................................7

1.4 Call Admission Control........................................................................................................81.4.2 UL CAC Algorithm 1.................................................................................................91.4.3 DL CAC Algorithm 1...............................................................................................101.4.4 DL CAC Algorithm 2, UL CAC algorithm 2..............................................................10

1.5 Load Balance.................................................................................................................... 111.5.1 Inter-freq LDB.........................................................................................................121.5.2 Intra-freq LDB.........................................................................................................12

1.6 Congestion Control...........................................................................................................13

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OWJ103104 WCDMA Load Control ISSUE1.1 Chapter 1 Load Control

Chapter 1 Load Control

1.1 Abbreviations

CAC： Call Admission Control

RTWP: Received Total Wideband Power

UL: Uplink

DL: Downlink

MAI: Multiple Access Interference.

TCP: Transmitted Carrier Power

SAC: Same Area Cell

LCC: Load Congestion Control

BE: Best Effort

TF: Transport Format

1.2 Introduction

Load control is needed for WCDMA system as well as other systems to keep the system working in stable state. However for a CDMA based system, cell capacity is varied as the user moving and the propagation conditions varied in cells, further more in 3G system, comparing with other wireless systems, more service types are included, the load of one connection is different for different type of service. For data service, the data rate is varying dramatically as time being. It means even for a data connection, the load caused is varying. So we can see that the load control for WCDMA system is much more important and should pay more attentions to.

The 1st topic we’ll face is how to measure the system load, since the WCDMA system capacity is “soft”, i.e. varying.

Based on the load measurement, kinds of load control methods can be applied. They are admission control, load balance and congestion control.

Admission control is used to evaluate if a resource request can be accepted or not.

Load balance is used to balance the load among neighboring cells.

And the congestion control is used to “draw” the system back from very high load status.

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In the following sections, the load measurement, admission control, load balance and congestion control are described in detail.

1.3 Load Measurement

Generally, “load” means system capability consumption. Since the load of any system is limited, it’s important for any system to control the load under the safe range. However, for a whole system, the capacity or the load is usually decided by the bottle-neck part. For example, in a computer, you can see the CPU load, the hard-disk W/R rate, the I/O rate, the bus speed, etc. It’s the one with the lowest capacity deciding the capacity of the computer.

For wireless communication systems, we should focus more on the air interface, since the radio resource is much more precious.

1.3.1 WCDMA Load Concept

Note:

1) In this section, we discussed why the capacity on the air interface is limited. In discussion, some simplifications are used. These simplifications are only applied in this section.

2) In discussion, the UL is taken as example, but the principle deduced is also applicable to the DL, although, in downlink there’re some differences.

We know that for a connection which can work well, the only requirement is that the received signal is good enough:

…… (1)

Here the Eb/No is the required minimum bit energy and noise density ratio, and Ri is the data rate, is the activity factor, W is the chip rate, pi is the received signal power and I is the total interference.

To find out the system capacity theory, we can just look at the minimum requirement, i.e.:

…… (2)

We define and obtain the load factor of one connection.

…… (3)

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Let’s consider a single cell with number of users in it, and take the uplink into consideration. Then for each user/connection, the interference consists of three parts: other user’s power, the interference from the user itself, and the background noise.

…… (4)

All the other users’ power will directly cause the interference to the user in concern.

…… (5)

To simplify the deduction, we can assume that the other users’ power is much larger than ith user power, then:

…… (6)

Together with equation (2), we can get:

…… (7)

Where is defined as the load factor.

Assuming that in the cell, only one kind of service is in use. Then:

Here, n is the number of users in the cell.

Since the interference power must be positive and finite, the number of users “n” must be limited.

Example:

Provided that all services in a cell are same, then we can simplify the equation as below:

Here n is the number of users in the cell. NR is the noise rise .



We can see from the equation that the NR is rise as the number of users increasing, at the same time, the higher the data rate R, the NR is higher, also the higher the

required signal quality , the NR will be higher.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

2

4

6

8

10

12

14

LoadFactor

Noi

seR

ise(

dB)

Figure 1-1 Noise vs. Load factor.

Generally, why the air interface capacity is limited is because of the multiple access interference (MAI), every user’s power will disturb other users’ communication.

1.3.2 UL Load and Measurement

Based on the discussion in the above section, taken into account the service activity factor, other cell’s interference and the interference of the user itself that is already included in the Eb/No requirements, we can rewrite the formula (6) as :

…… (8)

Here the is the service activity factor; is the ratio of other cell to own cell.



The is UL load factor, directly corresponding to the UL load as illustrated in figure 1.

From the equation above, we can have two difference definition of the UL load measurement: (1) interference based definition and (2) Traffic based definition.

1. Interference based definition: RTWP

Look at the left side of the equation (7), since the background noise PN is a constant, the interference I can be used as the measurement of UL load. The interference I in fact can be measured on by NodeB received total wideband power (RTWP).

Then we can have the load factor measurement as:

…… (9)

Based on the definition above, we can get the system load by RTWP measurement.

2. Traffic based definition: NTF

Look at the right side of the equation (7) and the definition of ,

……. (10)

We can also directly get load information from the services themselves.

Further more, we can define an ideal standard service as a 12.2 kbps speech service with activity factor equal to 1. Then any one kind of service can be normalized by this standard service as number of standard services. The normalized number is “normalization traffic factor (NTF)”.

…… (11)

By this way, we can just count the number of standard services in the system to get the system Load, without measurement on any power information.

1.3.3 DL Load Definition

For DL, if we left out common channel power, i.e. the total cell transmission power is the sum of the channel power of all users in the cell. In the similar way as deduction the UL load, we can get the equation as below:

…… (12)

Here, is path loss between ith base station and the jth mobile, Ptotal is the base station transmitted power, is the non-orthogonal factor because WCDMA employs



orthogonal codes in the downlink to separate users, and without any multipath propagation the orthogonaliry remains when the base station signal is received by the mobile.

Comparing with equation (7), we can see the downlink has the similar limitation on the air interface due to MAI interference.

So we can use the DL total transmission power as the DL load measurement.

Also we can just use the NTF as the load measurement.

1.3.4 The Load Situations

Once we have the load measurement, we can evaluate the load situations.

As the following figure shown, the load situation can be divided into 3 phase: stable state, just stable state and the unstable state.

Figure 1-2 Load phases ( take the UL as example)

Just as the figure 1-2 shown, taking UL as example, in the stable load phase, the noise will rise slowly as the load increased, in the just stable phase the noise will rise faster as the load increased, while in the unstable phase, i.e. the congestion phase, the noise will increase dramatically as the load increased.



According to the different load situations, different load control mechanism should be applied.

The CAC is always running, and only admit resource request that will not cause the unstable situation of the load.

The congestion control is started once the system goes into the unstable situation to reduce the load and make the system load back to the just stable or stable situation.

1.4 Call Admission Control

The call admission work flow is as below figure.

c a l l a r r iv e

G e t th e s e r v ic e c h a r a c t e r i s t i c a n d t h e c u r r e n t l o a d

S u p p o s e t h e u s e r i s a c c e p t e d , a n d c a l c u l a t e t h e p r e d i c t e dU L lo a d f a c t o r . A n d t h e n c o m p a r e d w i t h t h e t h r e s h o l d .

a d m i t t e d ?

S u p p o s e t h e u s e r i s a c c e p t e d , a n dc a l c u l a t e t h e p r e d ic t e d D L l o a d f a c to r .A n d th e n c o m p a r e d w i th th e t h r e s h o ld .

a d m i t t e d ?

c a l l a d m i t t e d c a l l r e j e c t e d

e n d

n

y

y

n

Figure 1-3 CAC work flow

We can see from the figure that when new resource request arrived, both the UL and DL load should be checked.

We should also notice that the CAC is triggered by the resource requests which could include both new call request and channel reconfiguration request.

Based on the load definition in section 1.3 , there’re 2 CAC algorithms.



1.4.1 UL CAC Algorithm 1

Figure 1-4 UL CAC algorithm 1

We can see from figure 1-4 that the UL CAC algorithm 1 employed the load definition based on RTWP as the load measurement.

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Admission request

Smaller than thethreshold?

admitted

y

end of UL CAC

rejected

n

Get current RTWP, and calculate the current load factor.

RTWPPN

UL 1

Get the traffic characteristic, and estimate the increment of load factor. RN

EWb

0

1

1

Calculate the predicted load factor.

0_ ULpredictedUL

9


1.4.2 DL CAC Algorithm 1

Figure 1-5 DL CAC algorithm 1

In figure 1-5, the DL CAC algorithm 1, the DL transmitted carrier power (TCP, i.e. the DL total transmission power) is used as the downlink load indicator.

1.4.3 DL CAC Algorithm 2, UL CAC algorithm 2

In fact, the same algorithm work flow is used for both DL algorithm #2 and UL algorithm #2.


Admission request


admitted

y

end of DL CAC

rejected

n

Get current TCP. )(NP

Get the traffic characteristic, andestimate the increment of tcp.

P

Calculate the predicted tcp.commPPNP )(

10


Figure 1-6 the algorithm 2 for both UL and DL.

For both DL and UL algorithm 2, the load definition based on NTF (normalized traffic factor) is used.

We shall note that, the UL and DL algorithms can be flexibly selected by algorithm switch independently.

1.5 Load Balance

In real system, the users are probably distributed unequally. That means in some cells the load could be very high while the load of its neighboring cells are low. In this situation, if we can “move” some users from high load cell to low load cell, i.e. to balance the load among cells, we can get the system capacity gain.

Here we’ll discuss two kinds of load balance mechanisms. One is between inter-frequency cells and the other is between intra-freq cells.


Admission request


admitted

y

end of UL/DL CAC

rejected

n

Get current total NTF.

N

iikNK

1

)(

Get the traffic characteristic, andestimate the increment of NTF.

dardsc

NcN NE

NEk

tan0

101

Calculate the predicted NTF. 1)()1( NkNKNK

11


1.5.1 Inter-freq LDB

The inter-freq LDB is applied only on the same area (inter-freq neighboring) cells. Once a cell’s load is higher than its SAC cell, some users will be selected to be hard-handover to the SAC cell. The inter-freq LDB work flow is as the following figure.

Measure transmi ttedcarri er power

Ptx(m), m=1, 2, . . . M

Get Ptx, max and Ptx, mi n f romPtx(m)

Ptx, max > Padj ust&

Ptx, max - Ptx, mi n >Pdi ff erence

y

I ni t i al i ze the hardhandover at tempt counter

i =0

i < N?

Sel ect one sui tabl e useri n the hi ghest l oaded

cel l , and att empt to behandover to the l owest

l oaded cel l .

y

Successf ul ?

i + 1

n

Wai t for next peri od

y

ncarri er power

Ptx(m), m=1, 2, . . . M

Get Ptx, max and Ptx, mi n f romPtx(m)

Ptx, max > Padj ust&

Ptx, max - Ptx, mi n >Pdi ff erence

y

I ni t i al i ze the hardhandover at tempt counter

i =0

i < N?

Sel ect one sui tabl e useri n the hi ghest l oaded

cel l , and att empt to behandover to the l owest

l oaded cel l .

y

Successf ul ?

i + 1

n

Wai t for next peri od

y

nn

Figure 1-7 inter-freq LDB algorithm work flow

1.5.2 Intra-freq LDB

When un-balanced load situation happens among intra-freq neighboring cells, the loads also could be balanced. The inter-freq LDB is also called cell breath.

In intra-freq LDB, once load un-balance happens between intra-freq cells, the common pilot channel power in the high load cell will be decreased and the pilot channel power of its neighboring cell will be increased. i.e. the high-load cell coverage shrinks while its neighboring cell coverage spreads. By changing the coverage, some user on the border will be handover to the low-load neighboring cells automatically.

The intra-freq LDB algorithm work flow is as the below figure:



Measure transmi ttedcarri er power Ptx

Ptx < Pexpand&

PCPI CH < PCPI CH,max

I ncrease the PCPI CHone step

y

Wai t f or next peri od

Pt x > Pshri nk&

PCPI CH > PCPICH, mi n

n

Decrease t he PCPI CHone step

y

n

Measure transmi ttedcarri er power Ptx

Ptx < Pexpand&

PCPI CH < PCPI CH,max


y



y


y


Pt x > Pshri nk&

PCPI CH > PCPICH, mi n

n

Decrease t he PCPI CHone step

y

n

Figure 1-8 intra-freq LDB

In LDB, to simplify the balance procedure and ensure the balance effectiveness, the user will high load factor will be selected.

1.6 Congestion Control

Refer to Error: Reference source not found, in the unstable or congestion phase, any minor load fluctuation may drive the system outage.

Due to the radio propagation environment varying and the mobile movement, though we have CAC and Load Balance, the system load could vary significantly and make the system goes from the stable state to the unstable state.

In the congestion (unstable) state, some kinds of fast control methods must be employed.

In the current version, the LCC (Load Congestion Control) algorithm is included. This algorithm is used to reduce the DL BE service data rate and reduce the system load quickly.

Once congestion happens, the network side MAC will quickly limit the downlink available transport formats (TF) and thus limit the DL data rate. The LCC work flow is as the following:



Monitor Cell Carrier PowerPeriodically or by event

If Carrier Power >Congestion threshold, or

receive a event

Congestion Detectedand inform MAC

Congestion Relievedand infrom MAC

MAC decrease themax TF number

Start a Adjust Timer

expi res

Kill the Adjust Timer

MAC increase themax TF number

Start a Recover Timer

expi res

Figure 1-9 LCC work flow


owj103104 wcdma load control issue1.1.doc

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