ho umts vs wimax

8/7/2019 HO UMTS Vs WIMAX

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Rate Service (ERT-VR). The ERT-VR, definednewly in 802.16e, is designed to support applicationswith variable data rates, which require guaranteeddata rate and delay (e.g. VoIP with silence suppres-sion). The service terminology mentioned above is

accordance to 802.16e ([2]). In 802.16-2004 ([1]) is

similar service type as RT-VR and NRT-VR calledReal-Time Polling Service (rtPS) and Non-Real-Time Polling Service (nrtPS) respectively.

802.16e standard is an amendment of 802.16standards and provides portability and mobility.

Next table shows different types of access, their mobility requirements and support of handovers

([3]).

Tab. 1. Comparison of 802.16-2004 and 802.16e

Access Location/speed Handover 802.162004

802.16e

Fixed

access

single/

stationaryno yes yes

Nomadic

access

multiple/

stationaryno yes yes

Portabil-ity

multiple/walking speed

hardhandover

no yes

Simplemobility

multiple/low vehicular

speed

hardhandover

no yes

Fullmobility

multiple/high vehicular

speed

softhandover

no yes

3. HANDOVERS IN UMTS

The basic mean of handover is to provide thecontinuous connection when moving among cells.The handover is realized by changing the radiochannel. There is number of handover types.

Hard handover. Hard handover is appliedwhen the user’s equipment communicates with onlyjust one NodeB. Connection with the old NodeB isbroken before the new connection is established.Handover is executed after the signal strength fromneighbor’s cell exceeding the signal strength from

the current cell. This situation is shown in Fig.2.

Fig. 2. Hard handover principle

Soft handover. During soft handover user equipment is located in the coverage area of two or more different NodeBs. The user equipment simul-taneously communicates with two or more NodeBsvia two or more radio channels. A received signal in

NodeB is routed to the RNC (Radio Network Con-

troller). The RNC compares the signal on the frame-by-frame basis. The best frame is selected for thenext processing; the others frames are discarded([7]). In the downlink direction, the same signal istransmitted by both NodeBs. Combining of these

signals (in UE) provides rake receiver ([8]).The number of NodeBs simultaneously com-

municating with UE is given by number of NodeBsin Active Set list. In active set are only the NodeBswith sufficient signal quality. In the UMTS istypically used channel CPICH Ec/Io (Common Pilot

Channel) ([9]) for quality of channel measurement.The neighbor cells, which pilots are not strength

enough to be added to the active set are placed intoMonitored (or Neighbor) Set. These cells are con-tinuously measured by UE. The maximum number of cells in monitored set is 32. The typical maximum

of cells in the active set is 2 or 3. There are twoalgorithms to add/ remove cells into/out of the

active set. The first of algorithm is IS-95. Thisalgorithm uses absolutes handover threshold. toprocess handover. The second algorithm is calledUTRA. This algorithm uses relative handover

threshold. The handover proceeding level is notfixed, but is depend on the relation between signal

strength of two or more NodeBs. In UMTS is used

UTRA algorithm ([10]).Softer handover. Softer handover is similar to

soft handover. The main difference between thesetwo handovers resides in fact that a UE is located inthe coverage area of two sectors of one NodeB (see

Fig. 3 and Fig. 4). The UE communicates with oneNodeB via two radio channels. In the downlink direction the situation of combining the signals issame as in the case of soft handover. In the uplink direction, the situation is different. The signal

received in the NodeB is not routed to the RNC, butcombination of the signals is realized in thisNodeB’s rake receiver.

Fig. 3. Soft handover principle.



Fig. 4. Softer handover principle

Inter/intra-system handovers. Inter-systemhandover is realized when the UE moves amongcells belonging to two different radio access sys-tems. For example, between the UMTS and GSMsystems. Intra-system handover occurs within one

communication system. Intra-system handover canbe further divided into inter-frequency and intra

frequency handovers.Intra/inter-frequency handover. Intra-systemhandover can be classified as intra-frequency hand-over and inter-frequency handover. Intra-frequency

handover can occurs when the user is movingbetween two cells, but the operating frequency stillremains the same. Inter-frequency handover canoccurs while the UE moves among cells but theoperating frequency changes. Inter-frequencyhandover is used in order to balance loading among

the carriers and to extend the coverage area ([10]).

4. HANDOVER IN WIMAX

WiMAX handover is the process where a mo-bile station migrates from air-interface of one basestation to another air-interface provided by another base station. In order to provide the session continu-ity of IP layer during handover, the IP handover

between different IP subnets is available ([11]). Thisis a transparent way for higher-level connections.

Standard Mobile IPv6 is often unacceptable to real-time traffic such as VoIP because of latency. Tooverride this Mobile IPv6 Fast Handover protocol(FMIPv6) has been proposed to improve the hand-

over latency by predicting and preparing the

impending handover. Cooperation between link layer and IP layer is necessary.

There are two possibilities in the network archi-tecture. In Fig. 5 is shown the deployment

Fig. 5. Architecture with two subnets

with two different IP subnets. An Access Router AR1 and three Base Stations BS1, BS2 and BS3form single subnet. The second subnet is formed byAR2 with BS4, BS5 and BS6. Handover in IP layer is not required when Mobile Station (MS) moves

from BS1 to BS2 or BS3 because it moves within

same subnet. When MS moves between BS3 andBS4, the IP handover is required because MS movesfrom one IP subnet to another IP subnet.

Network deployment in Fig. 6 shows BSs inte-grated with ARs. In this case every handover means

changing subnet resulting in IP handover.

IP backbone

AR1

BS1

AR3

BS3

AR2

BS2

Fig. 6. Architecture with BS integrated in AR

In 802.16e standard there are three basic kindsof handover ([12]): Hard Handover (HHO), Fast BS

Switching (FBSS) and Macro Diversity Handover (MDHO). HHO is mandatory and FBSS and MDHOare optional modes.

FBSS. In FBSS mode, the MS and BS maintain

a list of BSs that are involved in FBSS. This set iscalled an Active Set. MS continuously monitors the

base stations in the active set and defines an Anchor BS. Anchor BS is one base station of the active set

that MS communicates with only for all uplink anddownlink traffic including management messages. A

FBSS handover begins with a decision by an MS.The anchor BS can be changed from frame to frame.

This means every frame can be sent via different BSin active set.

MDHO. When MDHO mode is supported byMS and BS, the active set of BSs involved in

MDHO is maintained by MS and BS. MS communi-cates with all BSs in the active set of uplink and

downlink unicast messages and traffic. For downlink MDHO, two or more BSs transmit data to MS suchthat diversity combining can be performed at the

MS. For uplink MDHO, MS transmission is receivedby multiple BSs where selection diversity of thereceived information is performed.

Hard Handover. When MS stays in link, it lis-tens to L2 (link-layer) messages. A BS periodicallybroadcasts a Neighbor Advertisement Message(MOB_NBR-ADV) for identification the network and to define the characteristics of the neighbor BS.After that MS is able to scan the neighbor BS and

measure the signal parameters. For future handover MS can perform ranging and association procedures.The handover is divided into two steps: handover preparation and handover execution.



Handover preparation. Either MS or servingBS may initiate the handover. When MS initiates thehandover, it sends MOB_MSHO-REQ message. Theserving BS replies with MOB_BSHO-RSP messagecontaining recommended BSs after negotiation with

candidate BSs. When BS initiates handover, it sends

MOB_BSHO-REQ message only.Handover execution. After handover prepara-

tion, handover execution is following. When thetarget BS is finally selected and only switching linksremains, MS sends MOB_HO-IND message. After

sending this message all communication betweenMS and serving BS discontinues. As soon as MS

switches the link, it shall execute ranging with targetBS. This means the MS can acquire the timing,power and frequency adjustment information of thetarget BS. Then MS negotiates basic capabilities,

performs authentication and finally registers with thetarget BS. Some of these procedures can be omitted

if the target BS learned these information formerlythrough the backbone. Since this time, the target BSstarts to serve the MS; it becomes serving BS.Communication with MS via new serving BS is

available now.If MS moves to different IP subnet, it should re-

establish IP connection; IP handover should beperformed as mentioned above.

5. CONCLUSIONS

This paper analyzes the current handover situa-

tion in the UMTS and WiMAX networks. TheUMTS technology was designed naturally as amobile network. However, in the first versions of WiMAX standards, the mobility was not supportedat all. By the time became a need of user mobility.

Because of this reason several types of handover inWiMAX technology was introduced.

The first type, hard handover, is analogical tohard handover in UMTS. In both cases the MS (inUMTS called UE) communicates with just one BS(denoted NodeB in UMTS). Hard handover allows

only low speed mobility (portability or simplemobility). For higher speed mobility (full mobility)

were FBSS and MDHO implemented.MDHO is similar as soft (softer) handover in

UMTS technology. MS (UE) can communicate

simultaneously with all BSs (NodeBs) in active set.In uplink (downlink), BSs (MS) are capable of diversity combining of received signals.

FBSS was created as a new type of handover inWiMAX. In contrast to MDHO, in FBSS the dataare sent to all BSs in active set but without diversity

combining. Further, the data are processed in anchor BS only. An advantage of this handover type is not

using of explicit handover signaling messages whenanchor BS is changed.

Acknowledgement

This work has been performed in the framework of the FP6 project FIREWORKS IST-27675 STP,which is funded by the European Community. The

Authors would like to acknowledge the contribu-

tions of their colleagues from FIREWORKSConsortium (http://fireworks.intranet.gr).

REFERENCES

[1] IEEE 802.16-2004: Air Interface for Fixed Broadband Wireless Access Systems (October

2004).[2] IEEE P802.16e/D11: Amendment for Physical

and Medium Access Control Layers for Com-bined Fixed and Mobile Operation in Licensed Bands (September 2005).

[3] WiMAX Forum: Fixed, nomadic, portable and mobile applications for 802.16-2004 and 802.16e WiMAX networks (November 2005).

[4] H. Y Wei, S. Ganguly, R. Izmailov, Z. J. Hass:Interference-Aware IEEE 802.16 WiMAX MeshNetwork , Stockholm (May 2005).

[5] WiMAX Forum: WiMAX’s technology for LOS

and NLOS environments. [6] C. Eklund, R. B. Marks, K. L. Stanwood, S.

Wang: IEEE Standard 802.16: Wireless MAN Air Interface for Broadband Wireless Access,IEEE Communications Magazine (June 2002).

[7] S. N. P. Van Cauwenberge: Study of soft hand-

over in UMTS, Technical University of Denmark, University of Gent (July 2003).

[8] H. Lee, Y. Lin, Y. Harel, M. Woh, S. Mahlke,T. Mudge, K. Flautner: Software Defined Radio– A High Performance Embedded Challenge(2005).

[9] H. Holma, A. Toskala: WCDMA for UMTS –

Second Edition, Wiley (2002). [10] Y. Chen: Soft Handover Issues in Radio Re-

source Management for 3G WCDMA Networks(September 2003).

[11] H. Jang, J. Jee, Y. H. Han, S. D. Park, J. Cha:Mobile IPv6 Fast Handovers over IEEE

802.16e Networks draft (December 2005). [12] WiMAX Forum: Mobile WiMAX – Part I: A

Technical Overview and Performance Evalua-tion (February 2006).

ho umts vs wimax

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