AN OPTIMIZED METHOD FOR SCHEDULING PROCESS OF SCANNING FOR HANDOVER IN MOBILE WIMAX MOHAMMAD KAWSER, NAFIZ IMTIAZ BIN HAMID, M.A.NASER, ADNAN MAHMUD Department of Electrical and Electronic Engineering (EEE), Islamic University of Technology (IUT), BoardBazar Gazipur-1704, Bangladesh

IEEE 802.16e standard for 4G Cellular Technology-Mobile WiMAX is proposed to support mobility. As a result, handover has become one of the most important QoS factors. Because of lack of clarification in the conventional algorithm for handover process, there exists wastage of channel resource along with undesired delay. Again, it causes significant degradation in the overall system performance. Existing standard for handover includes scanning as one of the most vital processes. But scanning requests without proper response and delayed initiation of scanning only deteriorates the scenario. In this paper, a modification of the scheduling process of scanning is proposed to reduce redundant scanning requests and overall handover operation delay.

1.

Introduction

Again, a scheme for the reduction of data transmission delay and packet loss probability for realtime downlink service had been proposed in [5]. A delay timer to reduce the initiation of unnecessary handover had been proposed in [7]. In that proposal the timer deals with unnecessary handover caused by inaccurate signal level assessment. In our paper, an optimized scheduling method of scanning is proposed to reduce wireless resource waste and delay in the initiation of scanning process. The remainder of this paper is organized as follows. Section 2 provides an overview of the handover process of the IEEE 802.16e. Section 3 shows the shortcomings in the scheduling process of scanning. Section 4 depicts our proposed scheme. Analysis of the performance of our proposal and conclusion of this paper are given respectively in section 5 and 6.

Mobile WiMAX is one of the most promising technologies for broadband wireless communication based on the IEEE 802.16e-2005 [1] standard. IEEE 802.16e provides enhancements to 802.16-2004 [2] to support subscriber stations moving at vehicular speeds. Key advantages of 802.16e include- tolerance to Multipath and Self-Interference, Scalable Channel Bandwidth, Orthogonal Uplink Multiple Access, Support for Spectrally-Efficient TDD, Frequency Selective Scheduling, Fractional Frequency Reuse, Fine Quality of Service (QoS), Advanced Antenna Technology [1] [2]. The Physical layer of Mobile WiMAX is based on OFDMA technology. The new technologies employed for Mobile WiMAX result in lower equipment complexity and simpler mobility management due to the all IP core network and provide many other advantages over CDMA based 3G systems [6]. As in the current standard, there is the support for mobility; handover has become a very vital issue. Handover of IEEE 802.16e broadband wireless network had been studied in several literatures. Up to now various fast handover schemes had been proposed. An enhanced fast handover algorithm was proposed to reduce the waste of the wireless channel resources and handover delay in [3] [4]. Target BS estimation using mean CINR and arrival time differences had been proposed for reducing unnecessary neighbor BS scanning and association process in [3]. Single neighbor BS scanning, fast ranging and pre-registration had been proposed in [4].

2.

Handover in IEEE 802.16e

In IEEE 802.16e standard, three kinds of handover types can be supported- Hard Handover (HHO), Macro Diversity Handover (MDHO) and Fast Base Station Switching (FBSS). HHO is the simplest one among them; whether other two types having more complicacy are regarded as optional. Again, handover can be both MS and BS initiated. In this paper, when handover is mentioned, it refers to MS initiated HHO. MS initiated hard handover as seen by MS starts from neighbor BS advertisement message and goes on through request and response related messages for scanning and HO; provided that the corresponding criteria are met. Figure 1 shows the step by step 1

2 procedure for this. The final indication of handover process is performed via transmission of MOB_HOIND message. Scanning interval and normal operations appear alternately by frame basis. This is shown in figure 2. for scanning without association case. Figure 3. shows the same process for scanning with non coordinated association. To our view, reduction of unnecessary scanning and HO delay mostly deals with the request, response and other MAC management messages regarding handover.

MOB_NBR-ADV message, for each Configuration Change Count for the compression of neighbor BSIDs in MOB_SCN-REP and MOB_MSHO-REQ message. Parameters included in the MOB_NBR-ADV are mainly Operator ID, Configuration Change Count, N_NEIGHBORS. For each advertised neighbor, important parameters are: Length, Neighbor BSID, Preamble Index/Sub channel Index, PHY Profile ID, FA Index, BS EIRP. Scheduling Service Support, HO Process Optimization. 2.2. Scanning Interval Allocation Request (MOB_SCN-REQ) Message A MOB_SCN-REQ message may be transmitted by an MS to request a scanning interval for the purpose of seeking available BSs and determining their suitability as targets for HO. An MS may request the scanning allocation to perform scanning or non-contention Association ranging. BS#1 (serving)

MS

BS#2 (target)

BS#2 (target)

MOB_NBR-ADV (N_NEIGHBORS = 2) Iteration #1

MOB_SCN-REQ (Scan duration = N frames, Interleaving interval = P frames, Iteration=T times) MOB_SCN-RSP

M frames

Scanning Interval Duration =N frames

Synchronize with BS#2, measure metrics Synchronize With BS#3, measure metrics

Non-scanning interleaving interval (P frames)

Figure 1. MS initiated hard handover as seen by MS.

So, for the level of significance of those messages in our proposal along with the clarification purpose we here state a brief explanation of some of them as follows:

Scanning Interval Duration =N frames

Data Traffic (if any)

Synchronize with BS#2, measure metrics Synchronize With BS#3, measure metrics

Iteration #2 Alternation of scanning interval and interleaving interval

Scanning Interval Duration =N frames

2.1. Neighbor Advertisement (MOB_NBR-ADV) Message BS supporting mobile functionality is capable of transmitting a MOB_NBR-ADV management message at a periodic interval to identify the network and defines the characteristics of neighbor BSs to potential MS seeking initial network entry or handover. Serving BS keeps mapping-tables of neighbor BS MAC addresses and neighbor BS indexes transmitted through

(Start frame = M frames) (Duration = N frames)

Synchronize with BS#2, measure metrics Synchronize With BS#3, measure metrics

Iteration #3

Figure 2. Neighbor BS advertisement and scanning without association by MS request.

&

Vital parameters of this message are: Scan duration iteration, Interleaving Interval,

3 N_Recommended_BS_Index, Configuration Change Count for MOB_NBR-ADV, Neighbor_BS_Index, Scanning type- whether Scanning without or with association; N_Recommended_BS_Full, Recommended BS ID. 2.3. Scanning Interval Allocation Response (MOB_SCN-RSP) Message A MOB_SCN-RSP message is transmitted by the BS either unsolicited or in response to an MOB_SCN-REQ message sent by an MS. A BS may transmit MOB_SCN-RSP to start MS scan reporting with or without scanning allocation. If the Report mode is event-triggered, the MS responds once according to the trigger action specified in the MOB_NBR-ADV or the serving BS DCD. A subsequent MOB_SCN-RSP message may be sent to reset the trigger procedure. Parameters included in MOB_SCN-RSP message are: Scan duration, Report mode, period and metric containing BS CINR, RSSI; Start Frame, Interleaving interval, Scan iteration, Rendezvous time, CDMA code, Transmission opportunity offset. BS#1 (serving)

MS

Receive parameters for BS#2 and BS#3

BS#2 (target)

BS#2 (target)

MOB_NBR-ADV (N_NEIGHBORS = 2) MOB_SCN-REQ (Duration = N frames) MOB_SCN-RSP

M frames

(Start frame = M frames) (Duration = N frames) Synchronize with BS#2, measure metrics

Scanning Interval Duration=N frames Synchronize with BS#3, measure metrics

Association: Initial Ranging RNG-RSP (with service level)

Association: Initial Ranging RNG-RSP (with service level)

Figure 3. Neighbor BS advertisement and scanning with noncoordinated association by MS request.

3.

Shortcoming in Scheduling Process for Scanning in IEEE 802.16e

The IEEE 802.16e standard has the provision of T44 timer for retransmitting scanning request [1]. It has been found as one of the main causes that deteriorates the scheduling process. According to this standard, when the Trigger action in the DCD message is encoded as 0x3 the trigger condition is met and MS sends the MOB_SCN-REQ

message to the BS to begin the neighbor BS scanning process. MS sends the scanning request message repeatedly until serving BS suggests any neighbor BS for scanning. Through the network analysis, it has been observed that MS keeps trying with MOB_SCN-REQ message repeatedly, but the serving BS keeps sending MOB_SCN-RSP suggesting no neighbor BSs for scanning which is not desired at all. Such repeated exchange of MOB_SCN-REQ and MOB_SCN-RSP messages is obviously a waste of wireless resource. Again, throughput also gets reduced as scanning and data traffic can’t take place simultaneously [1]. So, it makes certain degradation in system performance. The current standard also does not explicitly suggest the beginning of scanning when the signal quality is getting too poor but still serving BS denies the request for scanning. 4.

Proposed Scheme

The previous discussion shows that a major waste of resource may be caused by redundancy in attempts to begin scanning and association process of neighbor BSs when the serving BS rejects scanning requests. This can also cause delay in essential scanning and thus, in actual HO. Currently, neighbor BS estimation via CINR measurement prior to scanning request already exists in the proposition [3] [4]. An optional implementation of a Trigger Value and Trigger Averaging Duration in DCD from serving BS is also already in use which suggests MS to request for scanning when the CINR of the serving BS stays below this trigger value for Trigger Averaging Duration. We propose few additional steps in order to avoid redundant attempts to begin scanning as well as expedite beginning of necessary scanning under certain conditions. Thus, the main purpose of the proposal is to reduce wireless channel resource waste and latency in HO decision. Figure 4 gives a brief overview of our proposed scheme for scanning with MS initiated Hard HO. We propose introduction of a second Trigger Value (Lower Trigger Value) and associated Trigger Averaging Duration in DCD and name the existing Trigger Value as Higher Trigger Value. Regardless of the CINR of the serving BS, MS measures CINR of all BSs in MOB_NBR-ADV message which satisfies QoS requirements. When MS finds that the CINR of the serving BS is in between the Higher Trigger Value and

4 the Lower Trigger Value for Trigger Averaging Duration associated with Higher Trigger Value, MS selects up to three top neighbor BSs on the basis of their CINR values and sends MOB_SCN-REQ message for scanning of the selected neighbor BSs. If the serving BS sends MOB_SCN-RSP message suggesting certain neighbor BSs for scanning with or without association, MS will scan Neighbor BSs as per suggestion. Optionally, MS may also scan few neighbor BSs which were not suggested by serving BS. In this case, MS will select neighbor BSs only from what were requested in MOB_SCN-REQ and scan them without association. Measurement of CINR of BSs in MOB_NBR-ADV

Selection of top 3 neighbor BSs according to CINR

CINR of serving BS is below Higher Trigger Level Pause sending request for a certain time

Sending of MOB_SCN-REQ

Waiting for MOB_SCN-RSP

Response with no neighbor BS for scanning

3 scan requests sent but no scanning suggested by serving BS

Response with few suggested neighbor BSs

CINR still below higher trigger level Resend scan request Scanning suggested neighbor BSs with or without association CINR below lower trigger level Sending MOB_SCN-REQ

Optional Scanning of Neighbor BSs which were requested but not got recommended

Waiting for MOB_SCN-RSP

serving BS is still found to be below Higher Trigger Value. After waiting for certain period, MS may resume sending MOB_SCN-REQ if the CINR of serving BS is still below Higher Trigger Value. As long as the CINR of serving BS is found to be in between Higher Trigger Value and Lower Trigger Value, MS will repeat sending MOB_SCN-REQ three times and then pausing if the serving BS keeps rejecting any scanning. The manufacturer of the MS may gradually increase the period for pause with time. Now, if CINR of the serving BS falls below Lower Trigger Level and stays as such for its associated Trigger Averaging Duration, MS will immediately send MOB_SCN-REQ and check MOB_SCN-RSP from the serving BS. If the serving BS suggests certain neighbor BSs for scanning with or without association in MOB_SCN-RSP, MS will select neighbors BSs and scan them like before. However, if the serving BS suggests no neighbor BSs for scanning over MOB_SCN-RSP, MS will send scanning requests two more times like before. If the serving BS rejects any scanning in these three consecutive attempts, MS will spontaneously trigger ‘scanning without association’ without waiting for any further events. Here, MS will select only the top three neighbor BSs based on their CINR values for scanning. 5.

Performance Analysis

Our proposed scheme for scheduling of scanning can be expected to provide the following improvement. 5.1. Stopping wastage of wireless resources

Response 3 times with no recommended neighbor BS for scanning

Start scanning the 3 pre-selected neighbor BSs without association

Figure 4. Proposed method for scheduling process of scanning.

However, if the serving BS sends MOB_SCN-RSP suggesting no neighbor BSs for scanning, MS will send MOB_SCN-REQ again. If the serving BS does not again suggest any neighbor BSs for scanning over MOB_SCN-RSP, MS will send MOB_SCN-REQ for the third time. Now, if the serving BS rejects any scanning for the third time, MS will pause sending MOB_SCN-REQ for certain period in order to reduce wireless channel resource waste even if the CINR of

Reportedly, in the current WiMAX network serving BS, sometimes, keeps rejecting any scanning request using MOB_SCN-RSP message. In this case, MS and the serving BS keep exchange MOB_SCN-REQ and MOB_SCN-RSP messages repeatedly with no benefit. Such unnecessary exchange of messages takes place numerous times. This is an evident wastage of wireless resources. In our proposal, MS pauses sending MOB_SCN-REQ message when it finds that the serving BS will most likely reject the scanning request. MS may also gradually increase the pause period in subsequent attempts with additional saving of wireless resources.

5 5.2. Justified wait time as scanning requests rejected repeatedly According to our proposal, when the serving BS keeps rejecting scanning requests, MS does not keep sending them indefinitely. Rather, after certain attempts, MS initiates either a pause or scanning depending on the situation. Our proposal includes a justified number of attempts for MS to wait here. In both the cases, when CINR of serving BS is in between Higher Trigger Value and Lower Trigger Value and when it is lower than Lower Trigger Value; MS sends MOB_SCN-REQ message exactly three times if serving BS keeps suggesting no neighbor BSs for scanning. These three attempts allow the provision that serving BS may like to check on the continuity of the changed CINR level of serving BS before suggesting neighbor BSs for scanning. On the other hand, consecutive three MOB_SCN-RSP messages suggesting no neighbor BSs can be enough to indicate that serving BS is disinclined to suggest any neighbor BSs for scanning. 5.3. Scanning confirmed before losing service When the CINR of the serving BS becomes too poor, MS may soon go out of service and so an attempt for handover is very much expected then. However, if the serving BS does not realize this dire need of MS, it might still suggest no neighbor BSs for scanning. Our proposal introduces Lower Trigger Level which allows MS to begin scanning in this situation. MS, after getting denied three times by the serving BS, will spontaneously begin scanning. This can actually expedite the decision for handover and help avoid radio link failure as well. 5.4. Scanning scheduled only when necessary MS performs scanning only under certain conditions which are deemed to substantiate its necessity. Scanning can take place when either CINR of the serving BS is in between Higher Trigger Value and Lower Trigger Value and serving BS suggests neighbor BSs for scanning. Also, it can take place when CINR of serving BS goes below Lower Trigger Value. However, scanning will not take place if these conditions are not met. An appropriate setting of values for Higher Trigger Value and Lower Trigger Value can ensure that scanning is justified only under these conditions. This

can save unnecessary time allocation and MS power consumption 5.5. Scanning scheduled only for potential BSs MS scans neighbor BSs which the serving BS suggests using MOB_SCN-RSP message. MS may optionally scan neighbor BSs which have been requested using MOB_SCN-REQ message as potential candidates for handover. Also, when CINR of the serving BS falls below Lower Trigger Level but it suggests no neighbor BS for consecutive three times, MS selects only the top three neighbor BSs based on their CINR values for scanning. Thus, it has always been ensured that MS scans only the potential neighbor BSs. 6.

Conclusion

Handover is obviously a vital factor in IEEE 802.16e mobile WiMAX network. In this paper an optimized HHO scheme is proposed with a modified scan scheduling. Method proposed here consisted of two trigger levels in DCD in terms of CINR value to reduce unnecessary scanning and quicken the initiation of justified scanning. Starting with MAC layer handover process in mobile WiMAX along with the related major MAC management messages; this paper gives the vivid idea of a probable problematic scenario of the scheduling process of scanning. It also provides the proposed solution scheme with performance analysis verifying the efficacy. In fine, the proposed scheme if properly implemented should enhance the overall performance radically solving the obstacles mentioned. References 1.

2.

3.

4.

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