Improved Handover interruption time in WiMAX, using GPS

Hossein Pirkomaji

Department of E-learning center Iran University of science and technology

Tehran, Iran E-mail: pirkomaji@gmail.com

Dr. Vahid Taba Taba Vakily

Department of Electrical Engineering Iran University of science and technology

Tehran, Iran E-mail: vakily@iust.ac.ir

Abstract WiMAX (Worldwide Interoperability Microwave Access) has been one of the most important technologies in telecommunication networks during last few years. Mobility is the most challenging research in WiMAX networks that are highlighted in broadband wireless communications.

There is more demand for new data services which need high data rate. But the long interruption of handovers is not acceptable for this kind of services. The real time services like: IPTV and VoIP are sensitive to packet loss. This paper is focused on procedures and the interruption time of full handover in mobile WiMAX. All events that affect on handover during movement of a mobile station will be considered. We have presented a new algorithm to improve the handover interruption time and to decrease the amount of signaling transaction during the handover procedure. In this algorithm we have used GPS (Global Position system) to perform handover faster. Our focus in this algorithm is on the distance between MS (Mobile Station) and BS (Base Station). The MS finds its position using GPS and calculates distance to the SBS (Source Base Station) and neighbor BSs. In the next step, MS selects the target BS according to the distance.

Keywords- WiMAX, Handover, GPS

I. INTRODUCTION Mobile WiMAX is a wireless system based on the

IEEE 802.16e standard. Both hard and soft handover are supported in WiMAX but the implementation of hard handover is mandatory and soft handovers like FBSS (fast base station switching) and MDHO (macro diversity handover) are optional in this standard [1].

Two frequency ranges are used in this communication technology. Frequency ranges 2-11 GHz for NLOS (Non-Line of Sight) and 10-66 GHz for LOS (Line Of Sight). In the case of NOS transmission, the coverage area is about 8 km with bit rate up to 70 Mbps and in the case of LOS transmission, the coverage distance is about 50 km.

WiMAX can provide broadband wireless access up to 8,000 square km of coverage. This is what allows WiMAX to achieve its maximum range.

A WiMAX tower can connect straight to another tower using of microwave link that referred as a backhaul.

In the case of NLOS, a small antenna on your computer is connected to the tower. In this mode, WiMAX uses a lower frequency range 2 GHz to 11 GHz (similar to WiFi). Lower wave length transmissions are not disrupted easily by physical obstructions - they are better able to diffract, or bend, around obstacles. In the other kind of service which is introduced as LOS, a fixed dish antenna points straight at the WiMAX tower from a rooftop or pole. The line-of-sight connection is stronger and more

stable, so it's able to send a lot of data with fewer errors. Line-of-sight transmissions use higher frequencies, with ranges reaching a possible 66 GHz. At higher frequencies, there is less interference and lots more bandwidth [2].

How could MS make faster decision? How does MS choice the target faster? How could we decrease the amount of signaling transactions during the handover procedure? Answering to these questions will help us to find the new solutions to decrease the interruption time. We present a new algorithm and try to answer these questions. The name of this algorithm is HWG (Handover-WiMAX-GPS). The HWG algorithm considers the distance as a most important factor in hard handover in mobile WiMAX.

In HWG algorithm just the hard handover will be considered.

There has been some previous work to reduce the interruption time [3], [4].

In [3] the distance and the direction have been considered as important factors. The distance is determined by the MS with sending and receiving signal to the SBS. But in our algorithm distance is determined by MS with using GPS. So signaling load in each BS will be decreased. Another point is the accuracy of the GPS. With new GPS receivers the accuracy is acceptable [5]. The aberration is less than 2 meters, so we can trust them and use them in the future networks.

In [6] an active list for those BSs that have best situations for handover is considered. But in this algorithm, MS consider one BS for handover.

The rest of the paper is organized as follows: in the next section HWG algorithm will be introduce. The simulation result will be present in section 3. The last section contains the conclusion.

II. EXECUTE HANDOVER WITH USED OF GPS The number of handover execution depends on area

covered by the antenna. As already explained the WiMAX tower, covers up to 8,000 square km that means the big area will be covered by just one tower. The distance between two towers can be 30 to 50 km. We use this fact as a positive point that helps users to do less handovers. The HWG algorithm uses the distance parameter as a major factor for performing the handover. All MSs and BSs have a position. The MS position will be determined by GPS and the BS position will be considered by the network in the implementation stage. In this way the distance between MS and BSs are actually determined by MSs. The position of the MS can be determine by GPS in a period of time less than 200 ms. this time can be decreased to 60 ms in the situation that satellites are in the best

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situation from MS point of view. Every BS has a fixed position that could be configured when one BS is connected to the network for the first time. All MSs that are connected to one BS should be informed about the position of SBS and its neighbor BSs. This can be happen by receiving a message like MOB_NBR-ADV in the NTAP phase or a new message like MOV_NBP-REQ (NBP: Neighbor BS Position) that is introduced in this paper. When the position of MS and all BSs are determined, MS can calculate its distance to SBS and all neighbor BSs using this formula:

22 )()( MSBSMSBSBSMS YYXXD += ( :BSMSD Distance between MS and BS,

X=Longitude, Y =Latitude). In the other researches like [3] the distance parameter

may be considered but the distance between MSs and BSs are calculated by sending and receiving the signal to all BSs. In that case the signaling load related to every BS is too much and the signaling transaction to all neighbor BSs waste the time.

In HWG method, each MS can use one table to maintain the position of BSs. So for finding the distance, it needs to MS determines its position with using GPS.

For implementing this algorithm, two thresholds are considered: one threshold clarifies bad quality indicating the handover is necessary and the second threshold uses for determining if it is necessary to start finding out the position. So we defined THO1 (threshold1) as a standard threshold value for performing handover, this value can be set as usual and THO2 (threshold2) as a new parameter that clarify the starting position measurement time.

According to the IEEE 802.16e standard [7], BS requests the report within 10 seconds by sending REP-REQ message to all MSs and receives their responses by REP-RSP message. So SINR can be determined by these transactions.

With comparing of thresholds (THO1 and THO2) and SINR, MS decides to start taking it's position or making a handover to TBS.

MS or BS can start the handover procedure. In the first scenario, MS starts handover. MS compares the THO2 with SINR when the SINR is less than THO2, MS takes its position with GPS and calculates the distance with all neighbor BSs. MS choices two BSs that have less distances and then with comparing their SINR, selects the best one as a TBS. When the target BS is selected, MS informs SBS with sending a MOB_MSSHO-REQ message (or maybe use a new message) and then asks SBS to prepare the condition for doing easy handover to TBS. during that time MS measures the SBS SINR as soon as SINR becomes less than THO1, MS makes a handover to TBS. The deference between THO1 and THO2 gives MS enough time to do necessary actions.

In Another scenario, SBS starts handover procedure. In this case, SBS sends a MOB NBR-ADV message to MS and informs MS about the neighbor BSs. This message may include the information about weak quality of service (QoS). In this scenario the comparison between SINR and THO1 or THO2 can be done by SBS. So SBS informs MS about performing a handover to another BS.

The scanning procedure steps can be decreased as it is shown in figure1 and figure 2. Another point that we can

find out in this figures is that the signaling transaction has been reduced. So the signaling transaction and signaling load in each BS has been reduced and this is the highlighted point in HWG algorithm and could affect on interruption time and the quality of service. As already mentioned MS uses GPS instead of BS for finding out its position and scanning procedure will be performed for one BS, thus it is not necessary to connect to the all BSs for finding out their distance. This is a point that signaling load is decreased in the HWG algorithm.

As it shown in figure2 some messages are removed in comparison with figure1.

Our algorithm has been present for hard handover. In hard handover, MS communicates just with one BS in each time. Two other handovers FBSS and MDHO are not discussed in this proposal.

III. SIMULATION RESULT For simulating the scenario we used the QualNet4.5.

We choose the environment that contains 8 BS and every BS has one subnet. In this scenario, handover is performed in BS1, BS2, BS3 and BS4. All BS are connected to one switching center. Handover is performed just in the first access level. We didnt consider handovers between ASNs. One MS moves and connects to four BSs that are in the path. Some MSs are connected to the BSs but just MS5 has movement during the simulating handover.

In [6] some explanations about timer during handover are presented. In this paper we have followed the standard 802.16e.

To consider all statements between pedestrian and vehicle, the movement speed is assumed to be between 0 to 120 km/h.

We consider 7 MSs in our proposal and MS5 is moving and connecting to BS 1,2,3,4 consequently. The OFDMA model is considered for this simulating. We use FTP and Telnet protocol in this simulation. The Antenna efficiency is 0.8 and antenna model is omni directional.

The radio parameters are summarized in table 1. In table 2 you can find the values of the other parameters that we used in our simulation.

In this simulation we consider two cases: for the first one we run the scenario according to standard 802.16e and all procedure will be considered as a standard in this method. In the second method we run the HWG that has selected TBS before NTAP phase.

MS5 performs the entry procedure and connect to the BS1 and then handover to BS2, BS3 and BS4 consequently. The movement path of the MS is passed near from those BSs. We consider the longitude and latitude related to each position in the path. So when MS decide to make a handover uses the longitude and latitude of it's position, it is the same as MS will be used GPS for finding it's position. Then MS calculates the distance to BSs, and select two nearest BSs. After that, MS will measure their SINR and then select one BS as a target and make pre coordinating to SBS and TBS, such as the method presented in [3].

This part happens before Handover procedure. So in this algorithm TBS will be selected before the Handover procedure has been started. This means we save at least 200 ms of scanning time during Handover procedure.

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Figure 1. handover with scanning procedure

In figure3 the handover delay is presented. As you can

find in this figure the HWG algorithm improves the delay time. Depending on the condition; the delay time is decreased about 10-15 ms in comparison to the standard 802.16e.

If we consider all time related to a handover, for two NTAP and AHOP phases, HWG algorithm, decreases the delay time. Thus for the whole handover procedure the interruption time is lessened.

The delay time is important in AHOP phase but to have a stable service, it is necessary to consider both phases. During the NTAP phase, MS performs scanning procedure and doing downlink synchronization activities with the neighboring BSs to choose a new target BS for doing handover procedure. So this 250 ms that have been reduced during NTAP phase is valuable.

Figure 2. Handover without scanning procedure

During the AHOP phase, the MS releases its

connection with the current SBS and performs synchronization and registration procedures with the newly selected target BS to complete the handover process successfully. More scanning and synchronization activities increase unwanted handover delays.

TABLE I. THE VALUE OF THE RADIO PARAMETERS

During the scanning procedure, all uplink and

downlink data traffic are stalled or buffered [2]. Delay in sensitive traffics like VoIP and video bit stream, such a phenomenon is disruptive. On careful analysis, it takes a few hundred milliseconds of time to choice the best candidate BS for handover [3].

Unit Value Parameters

GHz 2.4 Channel frequency

Statistical Propagation Model

dBm -111 Propagation Limit

Two Ray Path loss model

Constant Shadowing Model

dB 4 Shadowing Mean

802.16 Radio Type

dBm 20 Transmission Power

Hz 20 Channel Bandwidth

SBS: Receive NBR_ADV

Is handover necessary?

Yes

No

SBS: Send MSSHO_REQ

Performing the handover

SBS: Receive NBR_ADV

Need to scan neighbor?

SBS: Send SCN_REQ

SBS: Receive SCN_RSP

Is handover necessary?

Scanning Neighbor

SBS: Send MSSHO_REQ

Yes

Yes

No

No

Performing the handover

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Limitation of scanning activity can be affect on signaling load and handover delay, so it can be an important task in WiMAX. Limiting the extent of scanning activities remains a challenging task in the IEEE 802.16e systems.

In our algorithm the focus is on the NTAP phase to reduce the handover time. As it shown in figure4 the scanning time is about 40 ms and this time for all BSs is the same, because the MS scan just one BS during NTAP phase.

TABLE II. THE VALUE OF THE PARAMETERS

IV. CONCLUSION The purpose of this research is to present the handover

issue of WiMAX with focus on time. The practical analysis has been done with using the

Qualnet4.5 simulator. This analysis determines the total time needed to complete the handover procedure. Both initiation and decision stages are considered. In standard ways the handover procedure takes, about 700 milliseconds for whole procedure. It is a need to decrease this interruption time for the real time applications.

Optimization can be considered for both NTAP and AHOP phases. The HWG algorithm focuses on NTAP to reduce the scanning time. With optimizing this phase we reduce the whole interruption time and the amount of signaling transactions really are decreased. So BSs signaling load is reduced.

In the same conditions, and in comparing with standard 802.16e, the HWG algorithm will be decreased the scanning time about 250-260ms as it has presented in figure4 and figure5.

It is clear that the location and direction of movement can help to select the best candidates for handover that causes reducing the number of unnecessary scanning activities.

It is realized that the unnecessary scanning in the standard procedure could be reduced if the TBS would be chosen before the scanning activity.

Figure 3. average handover delay for BS

Following the algorithm proposed in this paper, a BS

will be selected for handover before the scanning process that causes reducing of some handover processes. These lead to a reduction of the total time in handover process.

We can optimize the handover by reducing the messages or combining multiple messages.

Performing HWG algorithm for FBSS and MDHO and reducing the scanning time for these two methods could be consider in the future.

Implementation of handover in HWG algorithm depends on MS position, so it can be used for finding people in the emergency cases. The MS position that determine by GPS can be used for this purpose.

Unit Value Parameters

MSS 1 Link propagation delay

IPV4 Routing protocol

s 200 Simulation Time

m 1500*1500 Terrain

m 30000*30000 Dimension

Simple Linear Battery Model

2048 FFT size

8 Cyclic Prefix Factor

PHY 802.16 Packet reception model

IPV4 Network Protocol

No IPSec

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REFERENCES

[1] Lee, D.H, K.Kyamakya, et al., Fast Handover algorithm for IEEE 802.16e broadband wireless access system, 2006 IEEE.

[2] A.Mandal, pre-handover optimization using hybrid base station selection procedure, university of Canterbury, 2008.

[3] Jenhui Chen, Chih-Chieh Wang and Jiann-Der Lee, Pre-Coordination Mechanism for Fast Handover in WiMAX Networks The 2nd International Conference on Wireless Broad band and Ultra Wideband Communications, 2007 IEEE.

[4] Wenhua Jiao, Pin Jiang and Yuanyuan Ma, Fast Handover Scheme for Real-Time Applications in Mobile WiMAX, ICC 2007 proceedings.

[5] E.Calias, The Global Positioning System, Purdue University -EAS Department.

[6] Zdenek Becvar, Jan Zelenka,Implementation of Handover Delay Timer into WiMAX , http://fireworks.intranet.gr.

[7] IEEE 802.16 Working Group, IEEE standard for local and metropolitan area networks part 16: Air interface for fixed and mobile broadband wireless access systems amenment 2, IEEE Std. 802.16e-2005, Feburary 2006.

Figure 4. average scanning time in standard 802.16e

Figure 5. average scanning time in HWG.

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