admission control and load control in umts network

8/14/2019 Admission Control and Load Control in Umts Network

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ADMISSION CONTROL AND LOAD CONTROL IN UMTSNETWORK

Zdenk RIKA, Stanislav HANUSInstitute of Radio Electronics, Brno University of Technology

Purkyova 118, 612 00 Brno, Czech Republic

e-mail: [email protected]:+420-54114 9126

Abstract

This article deals with ways of guarding UMTS radio access network against congestion.There are described basic principles and several methods of Admission Control and Load

Control. These algorithms are fundamental for mobile network based on CDMA.

Introduction

Admission control and load control are processes, which prevent UMTS air interfaceoverloading. In this network, overloading may cause dropping calls, decreasing of QoS, etc.Admission control (AC) algorithm is placed in Radio Network Controller (RNC). Its task

is to admit or deny new subscriber access into the network. Permit is based on current state ofair interface and type of service requested by user.

Load control (LC) guards the overloading in the network. When the congestion occurred,LC execute action, which decreases the interference level.

Admission Control

In UMTS network isnt exact limit of number of subscribers. This case is called soft limit. Number of submitted users depends on interference level. Each admitted user increasesinterference in his cell and probably in neighbour cells. It also increases cell load. It is need todefine threshold for interference level or cell load. If value crosses this boundary, sometrouble may occur. Cell load have direct effect on cell range and coverage. A new admittedcall can decrease the QoS of other calls or even drop some calls from cell border.

Admission control must be performed for both of uplink and downlink. When bothalgorithms are passed, new connection can be established.

The admission control algorithm has to estimate the increasing of the resources usage thenew user would cause. It is quite difficult, because a new user can also affect the resourceconsumption of existing users. The more of cell load means the more of load increase by newuser. It is clear from Figure 1.

There are several types of admission control algorithms shown below. Interference BasedAdmission Control and Throughput Based Admission Control are described in [1]. Otherthree algorithms are mentioned in [2].

Interference Based Admission Control

This admission control strategy may be used for both uplink and downlink. However, thedirections must be considered separately.

In the uplink a new user is admitted if a new total interference level is under the thresholdvalue:

thresholdoldtotal III


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Important task is estimation of interference increase I, which is caused by new user. Twodifferent methods of this estimation are shown below. Both take into account the load curve(depicted in Figure 1) and load factor of the new connection presented in Equation (2):

RNEW

L

b +

=

0/1

1

(2)

where Wis the chip rate,R is the bit rate of new user,Eb/N0 is the assumedEb/N0 of the newconnection and is the assumed voice activity of the new connection.The first one is the derivative method. It is based on the derivative of uplink interference withrespect to the uplink load factor which can be calculated as follows:

( )21111

_

=

=

== NtotalNtotalN

total P

d

dIPI

P

IriseNoise

(3)

Figure 1: Uplink load curve and the estimation of the interference

increase due to a new user

The change in uplink interference can be obtained from assumption that the power increase isthe derivative of the old uplink interference power with the respect to the uplink load factor,multiplied by the load factor of the new userL:

( )L

IIL

PI

d

dI

L

I totalNtotal

=

==

11 2

(4)

The second method is the integration method, in which the derivative of

interference with respect to the load factor is integrated from the old value ofthe load factor to the new value of the load factor+L:


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( )L

L

IId

PdII total

L L

Ntotal

=

==

+ +

11 2

(5)

The downlink admission control strategy is the same as in the uplink. The user is admitted if

the new total downlink transmission power does not exceed the predefined value:

thresholdoldtotal PPP


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includes influence to neighbour base stations. New user is admitted only if transmitted powerfrom the base station does not exceed threshold of all neighbour base stations (in set B).

Load Control

This mechanism tries to settle the problems with network overloading. Its function seems tobe the same as the function of admission control. The difference is that load control takesplace after overload occurs. Admission control tries to prevent overload situation.

If the system is good planned and admission control and packet scheduling algorithmsworks sufficiently, overload situations should be exceptional. But absolutely effective AC is avery complicated task. In addition interference level in the cell may change over time and fastmoving user cause more interference than a stationary or slow moving user.

If congestion occurs, the load control must decrease the load to the limits defined bynetwork planning. There are several methods for load reduction.

First two possibilities are uplink fast power control and downlink fast load control. Indownlink it means prevention of power-up commands from mobile. These actions are done

within a base station. It is fast prioritisation of the different services. These actions onlydecrease the rate of services which are not sensitive to delay and maintain the quality of theconversational services.

The other load control actions are slower. It is possibly reduce the throughput of packetdata traffic. This action is provided by packet scheduler, see [1]. Next chance is to perform thehandover. Target of this handover may be other WCDMA carrier (for example handover frommicrocell to macrocell in hierarchical structure of radio access network) or GSM network.This method of load decreasing must look out for sense of this action. Handover of fastmoving user from macrocell to microcell may cause problems. Some services likevideoconference cannot be redirected to GSM network.

Load control algorithm also can decrease bit rates of real-time users. One example of this

is AMR (Adaptive Multi Rate) speech codec.Last possibility is dropping of existing calls. This action is the last chance for reduce

overload of network and must be performed in a controlled fashion.

Conclusion

Interference level and overloading belongs to fundamental problems of radio access networkin UMTS. Admission and load control are basic tools for prevention these problems. Properlydesigned algorithms of admission and load control may rapidly improve quality of mobilenetwork and services provided by network.

Acknowledgement

This research has been supported by the Grant Agency of the Czech Republic under thecontract no. 102/03/H109 and no. 102/04/2080.

References

[1] Holma, H., Toskala, A. WCDMA for UMTS: Radio Access for Third Generation MobileCommunications. New York: John Willey & Sons, 2000

[2] Korhonen, J.Intoduction to 3G Mobile Communications.Nordwood: Artech House, 2001[3] Mishra, R. A. Cellular Network Planning and Optimisation, Chichester: John Willey &

Sons, 2004

admission control and load control in umts network

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