Seamless Realtime Traffic Handover Policy forIEEE 802.16m Mobile WiMAX

Sachin Lal Shrestha*, Nah-Oak Songt, and Song Chong**School of EECS, Korea Advanced Institute of Science and Technology (KAIST),

Email: sachinls@netsys.kaist.ac.kr.song@ee.kaist.ac.krtMobile Media Platform Center, KAIST

Email: nsong@mmpc.kaist.ac.kr

Abstract-IEEE 802.16m standard is defining several improve­ments on IEEE 802.16e standard, one of them being handover(DO). This paper provides a policy to satisfy IEEE 802.16mbenchmark of 150msec or less for a hard handover (DO)between two base stations (BSs). To facilitate such fast DO,the target BSs are chosen before DO is sought. Such choiceis made based on previously acquired knowledge of humanmobility behavior. There are several initial ranging opportunitiesprovided before the actual DO. Before the connection breakswith serving BS, target BS sets up its MAC states to allowinstantaneous connection of service flows that are prevalent inthe ongoing connection. Once the connection is broken, servingBS redirects the traffic to target BS while MSS, using theranging parameters provided right before the connection break­up, initiates a connection request to the target BS. Several timersperform as check points so that critical operations are performedin timely manner.

Index Terms-Dandover, Mobile WiMAX, 802.16, MobilityManagement.

I. INTRODUCTION

Worldwide Interoperability for Microwave Access(WiMAX) IEEE 802.16d [1] describes Medium AccessControl (MAC) and Physical layer functionalities. Thisstandard considered fixed Mobile Subscriber Station (MSS)and Base Stations (BSs). Mobility support was added onwith WiMAX IEEE 802.16e [2] standard where a MSSuses hard handover (HO) scheme while moving from oneBS to another in an OFDMA system. IEEE has adoptedadditional HO schemes like Soft Handover and Fast BSSwitching operations. In hard HO, the connection is brokenwith the serving (connected) BS and then a new connection isestablished with the target (chosen) BS; break before make.In contrary, soft HO allows an MSS to connect to two BSssimultaneously; make before break. But present standard[2] though permits a seamless HO, the performance forrealtime traffic for high speed users remarkably degrades asMSS undergoes a HO procedure. New developing WiMAXstandard IEEE 802.16m [3] aims to overcome HO latency.According to the IEEE 802.16m HO requirements, the latencyinterruption in connection between MSS and network shouldnot be more than 150msec in case of inter-frequency HO andless than 50msec in case of intra-frequency HO.

This paper presents an HO implementation with referenceto IEEE 802.16e standard to fulfill the requirements of IEEE802.16m standard. Our algorithm builds intelligence in the

MSS and requires minimum changes at BS to achieve IEEE802.16m HO requirements. The algorithm utilizes mobilitypattern of users or MSS to select target BS and preempt HOprocedures. This approach is efficient for MSSs traveling athigh speed as they usually take some predefined path at highprobability and, therefore, the HO procedure can be initiatedwell before the actual HO is required.

Our algorithm uses hard HO scheme to establish a reliable,fast and seamless HO to satisfy the IEEE 802.16m require­ments. A close inspection of hard HO reveals the most criticalportion of HO procedure: disconnected state of a MSS fromthe network during actual HO. By definition of hard HO, thestate can not be eliminated but can be reduced. The objectiveof this paper is to improvize the HO procedure so that thedisconnected state is minimum and actual HO procedure isaccurate to minimize redundancy.

Remainder of the paper discusses IEEE 802.16e standardsand points out some critical procedures that dictate the hardHO latency. We discuss our algorithm in retrospect of humanmobility pattern and establish a preliminary network discoveryprotocol implemented at MSS side. We also formalize the hardHO protocol with minimum setup time during actual HO.

II. RELATED WORK

IEEE 802.16e standard has added mobility support to itspredecessor IEEE 802.16d. MSS HO between WiMAX BSstakes place in a predefined sequence of events. The cellreselection process is a periodic process in which MOB_NRB­ADV mobile neighborhood advertisement messages from serv­ing BS are received at MS; MOB_NRB-ADV message con­tains synchronizing parameters of neighboring BSs in caseof possible HO. Once a network topology around an MSSis constructed using MOB_NRB-ADV message, MSS startsscanning the neighboring BSs using MOB_SCN-REQ mo­bile scan request and MOB_SCN-RSP mobile scan responsemessages. MSS provides a set of target BSs in MOB_SCN­REQ message and serving BS negotiates with all or selectedBSs. Through backbone network the serving BS collectsthe allocated rendezvous time set by each target BS. Thisreport is sent to MSS using MOB_SCN-RSP message. Atthe scanning time or rendezvous time, the MSS can scaneither one or more target BSs. During scanning, after a targetBS ID is confirmed, the MSS tries to synchronize with its

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downlink transmission and estimates the quality of the physicalchannel. The synchronization happens if association is doneduring scanning, else it is done only after HO decision. Theassociation enables the MSS to acquire and record rangingparameters and service availability information for the purposeof proper HO selection and/or expediting a potential futureHO. Recorded ranging parameters of an associated BS may befurther used for setting initial ranging values in future rangingevents during actual HO. The association parameters (PHYoffsets, etc.) stored in the MSS are obtained from rangingresponse RNG-RSP ranging response message of the targetBS.

Actual HO begins with the decision that may originate eitherat the MSS, the serving BS or on the network. HO proceedswith the notification of either MOB_MSHO-REQ mobileHO request or MOB_BSHO-REQ base station HO request.The response messages MOB_BSHO-RSP base station HOresponse holds list of target BS. The serving BS negotiateswith the possible target BSs in the list given in MOB_MSHO­REQ. The MOB_BSHO-RSP contains the rendezvous timeand opportunity for contention free coordinated ranging pro­cessing with the possible target BSs. During the rendezvoustime either obtained from the association table in MSS or inMOB_BSHO-RSP message, the MSS synchronizes with thedownlink transmission of possible target BSs.

Once the MSS synchronizes with the downlink transmissionof target BS, the MSS scans target BS for UL-MAP thatincludes either a contention- or non-contention-based MSS ini­tial ranging opportunity. In either case, MSS sends RNG-REQranging request message that may include MSS MAC Addressor HO_ID assigned in MOB_BSHO-REQ or MOB_BSHO­RSP. RNG-REQ message contains serving BSID and RangingPurpose Indication. On successful Ranging, target BS assignsto the MSS Basic CID and Primary CID in the RNG-RSPmanagement message. RNG-RSP reception indicates success­ful network re-entry whereby MSS sends a Bandwidth Requestheader.

At some stage during the HO process, the MSS terminatesservice with the serving BS. This is accomplished by sendinga MOB_HO-IND handover indication message with the HO­IND_type=ObOO value indicating serving BS release. Theserving BS starts the resource retain timer from value Re­source_Retain_Time. The serving BS retains the connections,MAC state machine and PDUs associated with the MSS forservice continuation until the expiration of the timer.

HO cancelation can happen in two different ways. First,when MSS transmits MOB_HO-IND message with HO canceloption (HO_IND_type = ObOl), and second, a drop wherean MSS stops communicating with its serving BS beforethe normal HO sequence (outlined in Cell Selection andTermination) with the serving BS has been completed. In eithercase, the MSS can send HO_IND_type = ObOl and if theserving BS receives the message before the resource retaintimer has expired, the MSS and serving BS immediately startsnormal operation. Our HO algorithm doesn't entirely omit thecurrent Mobile WiMAX HO standard. The algorithm's focal

point is to reduce the disconnected time of MSS with thenetwork during HO and thus introduces number of changesin the HO algorithm with different neighbor advertisementalgorithm, scanning, HO initiation and actual HO processesthat includes HO cancelation as well.

III. HUMAN MOBILITY PATTERN AND ITS ApPLICATIONON HANDOVER

Human mobility pattern is an upcoming field in mobilenetwork technology. This article focuses in a specific aspectof mobility pattern that includes a long distance flight ortravel. Our recent effort [4] accurately described the leviwalk nature of human where long distance travel is alsodocumented. But as most of the mobility pattern papers, itonly focuses primarily on how humans move rather than whenand which route they take. The answer to how humans travelhas enormous implication for mobile wireless network suchas DTN networks where Internet as we know is in motion,but for more traditional wireless network infrastructure suchas WCDMA and Mobile WiMAX the answer to the questionwhen and which route the human takes has greater impacton handover decision, load balancing and network deploymentitself. Since the objective of this article is to utilize the mobilitypattern for HO, we keep the human mobility pattern discussionrestricted to our survey only.

Our goals in this section are:1. To understand peoples mobility patterns so we can

anticipate the mobility patterns of MSS.2. To demonstrate that significant opportunity exists to

optimize HO performance in an emerging data and voicenetwork by exploiting the users' mobility characteristics.

Human mobility information such as route and time infor­mation can be maintained at an MSS with the encounter ofeach BS that the MSS utilizes to connect to the network duringa flight or a travel. Our focus is on an MSS that travel withhigh speed rather than pedestrian speed; at the later speedMSS with ongoing call need one or no HO. Calls indulged inrealtime traffic are critical when HO is considered for a highspeed MSS. This scenario only occurs for a long distance flightor travel where several HOs are possible.

Researches in environmental psychology show that peo­ple naturally structure their experience around personally orsocially meaningful places, such as homes, offices, schools,churches, coffee-shops, pubs, etc. [5] and [6]. The humantravel is motivated by socio-economic aspects of culture thatmake up the social network. The long distance travel occurs indaily basis while going to office, or to schools or to malls. Inan urban setup, a travel from one stationary place to anothercan take place through several alternative routes that we callroad-ways. Stationary points refer to a place where an MSSis connected to a single BS for a substantially long period oftime, such as from 9am till 5pm for office workers or 8amtill 3pm for school students. Hence, our analysis is centeredon answering two questions related to the temporal and socialaspects of human mobility: (1) do people behave consistently

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o No. of Flights/Travel I?J Same Route (%) 51 Other Route (%)

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Fig. 1. Human Mobility Behavior Pattern.

over time? and (2) do people have same rendezvous places(i.e., people gather at some specific place time and again)?

We investigated to answer the above two questions onhuman mobility behavior. 20 volunteers from different walksof life embedded GPS devices with them for 2 months thattrack their movement and also register the sequence of BSsencountered based on the highest signal strength.

In Fig. 1, we have chosen popular destination pairs that ourvolunteers took during the observation period of 2 months.Most popular destination pair is home to work (office orschool) and vice versa followed by home to mall and work tofood or pubs. Interestingly, most of the people choose samepath to go to their most popular destination. 90% of the timepeople choose same route to go to their offices or schoolsor malls while 10% of the time they take detors. A differentpicture arises when we consider human mobility pattern ofafter office activities. Most of the time, different individualsgather infrequently to form a temporary social network thatleads to an array of different destinations. A prime exampleof such event is after office gathering where different peoplecome together and decision of destination is influence by thesocial hierarchy, culture and age. Therefore, fewer number oftimes people tend to visit same place thus registering lowfrequency of same route. Such observation is depicted inWork-Drink/Food and Work-Recreation subtitles in Fig. 1.

How can such a mobility pattern be exploited for a faster,resource efficient HO? In our algorithm, we device the solutionby partly offloading the HO procedure to handheld devicesor MSS. Due to its ubiquitous and mobile nature, MSS notonly provide information exchange service but are also capableto holding vital statistics that can facilitate a better service.Our MSS prototype maintains a sequence of BS ID that itconnects to while in a flight or travel. A chain of BS ID orBS sequence is formed that can have branches; each MSStraces a tree of BS sequences that are unique to each MSS.Each BS ID is accompanied by time duration that the MSSusually spends in the BS cell. These two states are enoughto expedite possible future HOs. An MSS in motion, whenenters a network, can look up for the next possible targetBS and inform the serving BS to beginning negotiations for

possible HO via backhaul network. Typical cellular networkdoes not allow such preemptive HO process since the targetBS is not confirmed. Utilizing human mobility statistics, ourprototype MSS can determine the target BS with near centpercent guarantee. The authentication, authorization and evenMAC CS layer context can be transferred to target BS beforethe actual HO initiates. In next section we discuss our protocolwith elaboration on cell selection based on human mobilityprinciple.

IV. HANDOVER PROTOCOL DRIVEN BY LEARNED MSSMOVEMENTS

In this section, we discuss several aspects of our inter­frequency HO algorithm including HO cancelation or abort.Intra-frequency HO has similar algorithmic construction buthas simpler procedure. Due to the space constraint, we decideto exclude our intra-frequency HO procedure from this article.

A. Handover Procedure

Fig. 2 is the sequence diagram of overall HO procedure.A prominent phase of the HO procedure is learning andknowledge acquisition phase where MOB_NRB-ADV is usedby MSS to discover neighboring BSs. Once a sufficient pool ofsamples have been acquired to construct a sequence of BSs, theusability and thus the derived application of MOB_NRB-ADVis replaced by the acquired knowledge. As MSS re-enters thenetwork, MSS provides likely target BSs list to new servingBS based on the previously acquired knowledge. The next HOprocedure is scanning followed by actual HO.

Fig. 2 shows the exchange of control messages betweenMSS and service BS and target BSs. Generic flowcharts inFig. 3 show our HO algorithm and message transfer betweenMSS and service BS. As described above, MSS chooses targetBSs based on acquired knowledge and compiles a list to sendto serving BS Next_BS_List(BSs) shown in Fig. 3(a). ServingBS sends request signal through backhaul network to the targetBSs in the list obtained in Next_BS_List message (shown inFig. 2). This message contains MSS MAC address. The targetBS uses this MAC address to identify any future communi­cation with the particular MSS. Along with MSS MAC ad­dress, serving BS sends SBS_HO_CDMA_Code that uniquelyidentify MSS. Target BS reply with BS_Add_MS_Rsp(UCD,DCD) message to serving BS. Serving BS collects re­sponse messages from the target BSs and sends a messageto MSS Target_BS_Association_Report with scan_intervaland SBS_HO_CDMA_Code (Fig. 3(a)). The scan_intervalis the rendezvous time provided by the target BSs wherecontention free scanning and optional association are pos­sible; a feature well documented in the related art. TheSBS_HO_CDMA_Code is issued by serving BS for the pur­pose of HO related message transfers. MSS uses this code tostart initial ranging and actual HO with the target BS. The useof the code dramatically reduces the airtime overhead. MSSstores the scanning time periods along with the code (Fig.3(a)).

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Save Target BSs scanningintervals and HO_COMA_Code

Normal Operation,wait for another

scanning opportunityfor the BS

=:----Continu

Success

Ranging_status

Receive Initial HO ranging response reportfrom Serving BS: HO_lniCRng_Report

Save the Ranging Parametersand set validity timer Tv

(a) Flowchart showing pre-HO decision processes (b) Flowchart showing post-HO decision processes

Fig. 3. Inter-frequency Handover Flowchart.

At scanning time, the MSS sendsHO_Init_Rng_CDMA_Code message that in fact is aCDMA code SBS_HO_CDMA_Code that was assignedby the serving BS. During HO Scan Period the MSS cansend the HO code to either one or several target BSs.HO_Init_Rng_Rsp(RangingParam, Ranging_status) messagesare sent to serving BS by target BSs via backhaul (Fig.2). Serving BS compiles the report and sends to MSSin HO_Init_Rng_Rsp message with ranging parametersfrom the target BSs. One of the ranging parameters is theRanging_status. When the target BS receives the HO codeand successfully allocates resources for the MSS for possibleHO, the target BS sends Ranging_status as 'successful'else the state is 'unsuccessful'. Scan periods and thus theinitial ranging processes repeat periodically and for thoseunsuccessful scan responses, the MSS sends the CDMAcode again. Serving BS communicates with the target BS viabackhaul and collects the ranging parameters and sends toMSS. The collected ranging parameters are stored in MSS.

The MSS scans for signal strength of the serving BS andthat of neighboring BSs (or target BSs) (Fig. 3(b)). As wededuced from the human mobility behavior model in SectionIII, with a probability of over 90% MSS tends to take sametarget BS for HO repetitively. Therefore, our prototype MSSneeds to scan signal strength of two target BSs at most. MSS

sends MS_HO_REQ signal to serving BS with selected targetBS ID. The serving BS migrates MSS related service flowinformation to target BS (Fig. 2). Serving BS is responsibleto inform other target BSs to release resources reserved inanticipation of a possible HO (Release_resource with MSSMAC as parameter). The MSS sends a HO_Rng_Req (Fig.3(b)) request for obtaining an update on the ranging param­eter of the chosen target BS. The normal scan period isused for the purpose of 'Final HO Ranging Process.' TheHO_Rng_Req is again the HO CDMA code assigned by theserving BS at the beginning of the HO procedure. Uponsuccessfully creating MAC CS layer service flow parametersand reception of SBS_HO_CDMA_Code, chosen target BSsends Ready_to_Receive message to serving BS via backhaulwith MAC layer service flow parameters such as ConnectionID (CID), ranging parameters, and higher layer parametersuch as IP address (for managed MSS). This message alsohas Ranging_status parameter that is used for same purposeas used in HO_init_Rng_Rsp message. Serving BS sendsTarget_BS_ready message to MSS with all the parametersobtained from the target BS. Once the MSS receives Tar­get_BS_Ready message, it replies the BS with HO_Indicationmessage. Till this point MSS has been connected to servingBS in normal operation phase. However, when serving BSreceives HO_Indication message, serving BS redirects all the

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Fig. 2. Inter-frequency Handover Sequence.

MSS Servi 9 BS

Neighborhood_Adv(TargeCBS_List)

NexCBS_List(BSs

Target BS2

the service flows. The security sublayer of Mobile WiMAXMAC layer functionality is assumed to take place based onthe related art and is out of the scope of this article. Securityrelated processes like authentication and authorization can takeplace via backhaul network between the serving and the targetBSs before or during the actual HO process. MSS sets up atimer Tl when HO_Rng_Reg message is sent to the targetBS. After MSS sends the SBS_HO_CDMA_Code, it waitsfor Target_BS_Ready message till Tl timer expires. Oncethe timer expires, provided that the retransmission counter isnot zero, MSS re-sends HO_Rng_Reg message and decreasesthe retransmission counter and resets the timer Tl again.The retransmission counter's maximum value and Tl timervalue is setup during MSS initialization. The timer Tl andretransmission counter values are set up such that at theretransmission counter exhaustion the MSS can choose anothertarget BS before loosing connection with the serving BS.

B. Handover Latency

We used NS-2 simulator with 20 cell sites in an area layoutwhere users can travel at up to 360km/hr speed that is ratedas high speed in WiMAX standardization. The users travelin a predefined pathway depicting user mobility behavior asdescribed above. We compared mobile WiMAX IEEE 802.16eHO with the proposed HO scheme for IEEE 802.16m (Fig.4). For low and medium speed users, both the schemes havesimilar handover performance. The users with IEEE 802.16eHO at speed beyond 90km/hr experience more handoverlatency than the users with the proposed HO scheme. At360km/hr the proposed scheme out performs IEEE 802.16eHO scheme by 35%.

Fig. 4. Handover latency at various user speed.

c. Handover Failure

HO abort process is controlled and moderated by severaltimers. Timers governing HO processes are set in servingBS and MSS. After receiving HO_Indication message fromMSS the serving BS starts Resource_Retain_Timer Tr (Fig. 5),which has same functionality as in the related art. When MSSsends HO_Indication to serving BS, MSS starts a Timer TH.When Timer TH expires as MSS still unable to synchronizedue to ranging parameters invalidity; in such a case, MSS

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service flow traffic to target BS and breaks the connection withMSS (as shown in Fig. 2), at which point actual HO starts.

MS immediately starts connection procedure with target BSafter sending HO_Indication message. Based on the Rang­ing_status received in Target_BS_Ready message, MSS can ei­ther send Initialize_Connection message or send Optional_Rng(Fig. 3(b)) based on Ranging_status value being 'Success' or'Continue' respectively. The second message is just a codeSBS_HO_CDMA_Code that tells target BS that the MSSattempting to connect to it is not successful in synchronizingto the downlink transmission and subsequently the uplinktransmission as well. Such condition, though rare, can beprevalent in a high speed user case where previously obtainedranging parameters may no longer be valid due to time laps orextensive distance traveled in a short time. In either case targetBS identifies such MSS that has already been disconnectedwith its serving BS and facilitates a fast ranging response Con­nection_Rng_Rsp message so that the actual HO time periodis under 150msec, a requirement of IEEE 802.16m standard.In a normal case, MSS initiates connection with a target BSvia Initialize_Connection message and as a response the targetBS, which is now the new serving BS, sends Connection_Ackmessage with which MSS starts normal operation and resumes

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Fig. 5. Sequence of events during handover failure.

[1] IEEE Computer Society, IEEE Microwave Theory and TechniquesSociety, "IEEE Std 802.16-2004", IEEE Standard, Oct. 2004.

[2] IEEE Computer Society, IEEE Microwave Theory and TechniquesSociety, "IEEE Std 802.16e-2005 and IEEE Std 802.16-2004/Corl­2005", IEEE Standard, Feb. 2006.

[3] IEEE 802.16 Broadband Wireless Access Working Group, "Draft IEEE802.16m Requirements", IEEE Standard, Oct. 2007.

[4] Injong Rhee, Minsu Shin, Seongik Hong, Kyunghan Lee and SongChong On the Levy-walk Nature of Human Mobility: Do Humans Walklike Monkeys?, IEEE INFOCOM, May 2007.

[5] R. Genereux, L. Ward, and J. Russell, The behavioral component in themeaning ofplaces, Journal ofEnvironmental Psychology, pp. 43-55, v.3,1983.

[6] B. Kramer, Classification of generic places: Explorations with impli­cations for evaluation, Journal of Environmental Psychology, pp. 3-22,v.15, 1995.

V. CONCLUSIONS

Our HO algorithm provides smooth and seamless transitionof a MSS from one BS to another with HO latency lessthan l30msec that in turn guaranteed the minimum QoSrequirement for VoIP service level satisfaction. The usersduring HO attempt can be dropped for various reasons. Ourscheme not only assures reliable re-entry to the network butalso guarantees timely re-entry to the network « l50msec at360km/hr speed). To maintain a strict HO requirement, neigh­boring BSs including the serving BS are required to maintainseveral MAC layer states that can bottleneck resources atthe BSs. In the proposed algorithm, the target BS is chosenbased on each MSS prior movements which in fact is dictatedby several human behaviors. Human mobility behavior isrepetitive and follows same route for same source destinationpair. This paper successfully tied the human mobility behaviorand its application in determining the next possible target BS.The target BS is readily known as the MSS re-enters a newserving BS; a quality that expedites a fast HO. This aspectof the algorithm is particularly useful for HO for high speedusers. MSS with assistance of serving BS collects target BS'sranging parameters that are used in actual HO. Number oftimers moderate the HO procedure. The relationship betweenthese timers are established in such a way that MSS can regainits connectivity with any neighboring BS in a situation whereit fails to connect with both the chosen target and the servingBSs.

At our research center (Mobile Media Platform Center), weare actively developing Mobile WiMAX MSS in collaborationwith Texas Instruments. Our preliminary emulation of theproposed algorithm delivers claimed HO operation within theIEEE 802.l6m HO timing constrains.

REFERENCES

Target BS1 This ensures a timely re-entry and normal operation resump­tion of MSS when an MSS fails to HO to target BS as wellas loses connection with the serving BS.

Serving BS

HO_Indication( Init)

MSS

Initialize_Connection(NexCBS_List, MS_MAC, CMAC, HMAC)I

14-----Connection_Ack(CDMA_Allocation)I-------1

UrgenCnetwork_entry_code(SBS_HO_CDMA_Code)

1----UrgenCnetwork_entry_code(SBS_HO_COMA_Code)1-----...I

Connection_Rng_Rsp(RangingParam, Ranging_status)

connection-Rng- RSP1(RangingParam,Ranging_status)

Initialize_Connection(NexCBS_List, MS_MAC, CMAC, HMAC)I

......-----Connection_Ack(CDMA_Allocation)-----......I

14i------Data Connection setuo------...~I

sents HO_Indication to serving BS with status = 'Abort' andstarts another Timer TA. Serving BS, provided that Tr is notyet expired, responds to the abort signal and replies withHO_Indication(Conn_Resume) after which MSS and servingBS resume normal operation till the MSS attempts yet anotherHO. IfTr expires before an abort signal is received or the MSSis unable to receive HO_Indication(Conn_Resume) message,the re-connection with the serving BS is no longer possible.In such a case, MSS sends an Urgent_network_entry_code(Fig. 5) with same SBS_HO_CDMA_Code to all target BSs.The message is sent to serving and target BSs that main­tains the SBS_HO_CDMA_Code for lOOms (an optimumvalue chosen through iterative simulations) from the timeHO_Indication(Init) message is sent. MSS can receive re­sponse from either both or only one of the BSs. In eithercase, the MSS acknowledges the first received reply Connec­tion_Rng_Rsp and discards the second. Using the ranging pa­rameters obtained in Connection_Rng_Rsp message, the MSSre-enters the network with Initialize_Connection as discussedin normal HO operation before. The relation between the HOgoverning timers can be summarized with following equations:

(1)

Tr < lOOms (2)

For a seamless HO during real time traffic exchange be­tween MSS and the network, Tr is chosen less than lOOms.

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