On Modeling Speed-Based Vertical Handovers in Vehicular Networks Dad, slow down, I am watching the movie

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1 On Modeling Speed-Based Verical Handovers in Vehicular Neworks Dad slow down I am waching he movie Flavio Esposio flavio@cs.bu.edu Anna Maria Vegni avegni@uniroma3.i Ibrahim Maa maa@cs.bu.edu Alessandro Neri neri@uniroma3.i Compuer Science Dep. Boson Universiy Boson MA Applied Elecronics Dep. Universiy of Roma Tre Rome Ialy Compuer Science Dep. Boson Universiy Boson MA Applied Elecronics Dep. Universiy of Roma Tre Rome Ialy Absrac Alhough vehicular ad hoc neworks are emerging as a novel paradigm for safey services supporing real-ime applicaions (e.g. video-sreaming Inerne browsing online gaming ec.) while mainaining ubiquious conneciviy remains a challenge due o boh high vehicle speed and non-homogeneous naure of he nework access infrasrucure. To guaranee accepable Qualiy-of-Service and o suppor seamless conneciviy verical handovers across differen access neworks are performed. In his work we prove he counerinuiive resul ha in vehicular environmens even if a candidae nework has significanly higher bandwidh i is no always beneficial o abandon he serving nework. To his we inroduce an analyical model for a verical handover algorihm based on vehicle speed. We argue ha he proposed approach may help providers incenivize safey by forcing vehicular speed reducion o guaranee accepable Qualiy-of-Service for real-ime applicaions. I. INTRODUCTION In Vehicular Ad hoc NETworks (VANETs) vehicles owed wih sophisicaed on-board equipmen communicae wih each oher and wih he wireless and cellular nework infrasrucure by means of several nework inerface cards (i.e. IEEE UMTS HSDPA and so forh [1]). Fuure neworked vehicles represen he fuure convergence of compuers communicaions infrasrucure and auomobiles [2]. An envisioned goal is o embed human-vehicleinerfaces such as color reconfigurable head-up and headdown displays and large ouch screen acive marix liquid crysal displays for high-qualiy video-sreaming services [3]. Passengers can enjoy heir raveling ime by means of realime applicaions e.g. video sreaming and online gaming using individual erminals nex o heir seas (Fig. 1 (a)). To guaranee he delivery of accepable Qualiy-of-Service (QoS) in such environmens Vehicle-o-Infrasrucure (V2I) communicaions represen a viable soluion. However V2I proocols sill lack seamless conneciviy: when vehicles encouner an area wih overlapping wireless neworks a decision on wheher or no a conneciviy swich should be execued has o be aken. The mechanism preserving on-he-move user s conneciviy is defined as Verical Handover (VHO) [4]. In his paper we show ha in VANET scenarios wih a heerogeneous nework access infrasrucure bandwidh gains are ighly coupled wih he proocol overhead handover laency and he speed of he vehicle. Leveraging on his consideraion we propose a new speed-based QoS-oriened Verical Handover algorihm for vehicular neworks. In he res of he paper we refer o our algorihm as S-VHO. The idea of using he vehicle speed as assessmen crierion for verical handovers has been floaed before [5] [6]. However our emphasis lays on real-ime applicaions for VANETs. As a baseline of comparison for our simulaions we consider he recen work of Yan e al. [5] whose algorihm is based on boh he Received Signal Srengh (RSS) and he erminal speed and we show ha our S-VHO algorihm ouperforms heir approach in erms of hroughpu delay jier and overhead (number of verical handovers). The paper is organized as follows. In Secion II we presen an analyical model useful o compue a speed upper bound used by our S-VHO algorihm described in Secion III. Analyical and simulaion resuls are shown in Secions IV and V respecively. We discuss some relaed work in Secion VI and in Secion VII we conclude our paper. II. ANALYTICAL MODEL In his secion we presen he counerinuiive resul ha in heerogeneous vehicular neworks conneciviy swiches are jusified only when he vehicle speed saisfies a given speed upper bound. To compue his bound we need o formally define (i) a valid handover in VANETs (ii) he cell crossing ime (i.e. he ime a vehicle sps inside a wireless cell) and (iii) he handover laency. Definiion 1 (Valid Handover in VANETs): A vehicle crossing an area covered by a leas wo wireless neworks performs a valid handover if and only if he handover resuls in a hroughpu increase. 1 Our model assumpions are depiced in Fig. 1 (b). A vehicle is moving a speed v in a vehicular environmen wih a nework infrasrucure composed of several overlapping heerogeneous wireless access neworks parially covering he road. The vehicle s rajecory follows a Manhaan mobiliy model and i is consrained by he road composed of sraigh lanes. Moreover each vehicle is assumed o be equipped wih an on-board Global Posiioning Sysem (GPS) nework inerface 1 Noe ha here exis wo main ypes of handover horizonal when he wo cells involved in he process belong o he same echnology and verical when he wo cells belong o differen echnologies. Alhough Definiion 1 holds for any ype of handover in his work we focus on verical handovers beween WLAN and UMTS and vice versa.

2 erogeneous wireless neworks parially covering he road. The vehicle s rajecory is consrained by he road composed by a sraigh lane (i.e. Manhaan mobiliy model) [8]. Moreover each vehicle is equipped by an on-board Global Posiioning Sysem (GPS) nework inerface card so ha he vehicle posiion is always known. BS UMTS Cell Crossing Time B. Modeling 5 AP 1 We WLAN sar modeling he cell crossing ime he ime ha a vehicler sps inside a wireless cell. Formally we have: Pin in Definiion! 2 (Cell Crossing Time): Given a vehicle V raversing an area covered by a wireless cell C wih a!x! > he cell crossing ime of consan speed! v such ha v V in C denoed as T is he overall ime ha V can sp inside C. 15 Pou ou (a) 4 (b) (c) Denoing byforin he ime in wih which a vehicle eners an area Fig. 1. (a) Video-sreaming applicaions for on-he-move users (b) Manhaan mobiliy model a VANET scenario heerogeneous overlapping wireless neworks (c) Simulaed VANET scenario. covered by a considered wireless cell we model he exi ime from he cell as: card so ha he vehicle posiion is ubiquiously known. Le a vehicle be conneced o a x SN enering he wireless = in (1) Definiion 2 (Cell Crossing Time): Given a vehicle V range of a CandidaeouNework (CN).! v In his heerogeneous raversing an area covered by a wireless cell C a consanwhere scenario he daa delivered beween he wocell imein x iswe hemodel disance covered inside he wireless speed he cell crossing ime of V in C denoed as The insans and as a posiive range funcion γ : < ime inerval ou R in ou in. In paricular if we denoe as < \ {} he is he overall ime ha V can sp under C s coverage. # defined as: radius of he omnidirecional wireless cell and Denoing by in he ime a which a vehicle eners in anas φ [ π] he angle beween he vehicle s line of sigh wih γ = α (BCN δ)movemen ( T L) (1 α) B Tvehicle (4) he direcion ofsnhe area covered by a wireless cell we model he exi ime fromhe cell s anenna and we have x = 2R cos φ. he same cell as: where α is an funcion ha αassumed = 1 when a Noice ha xindicaor is known since such we have each x verical handover is execued and zerosooherwise. Noe haof (1)vehicle ou = in equipped wih a GPS receiver ha coordinaes α = and 1 γ exi is equivalen hecell hroughpu in he he when enrance poins ofohe are easyobained calculaed as: (xwhile ) and (xou! you ). Therefore for α =Pou γ is he hroughpu in he in yin where x is he disance covered inside he wireless cell inpin CN " SN. he angle ou yin compued as φloss = arcan. he daa due o hexyverical handover laency he ime inerval T = ou in. In paricular if we denoeφ can γbecapures ou xou as R < \ {} he radius of he omnidirecional wireless L. Since during he raveling ime of a vehicle i is desirable o 1 ha here exis wo our mainechnique ypes of handover he maximize hroughpu iniiaeshorizonal when a handover only cell and as φ [ π] he angle beween he vehicle s line ofwonoice cells involved in he process belong o he same wireless echnology i is valid ha is when so when verical when he wo wireless cellsγbelong echnologies. sigh wih he base saion and he vehicle moion direcionand when α=1 >oγdifferen α= and Alhough definiion 1 holds for any ype of handover in his work we focus he inequaliy: by simple geomeric consideraion we obain x = 2R cos φ.on verical handovers for WLAN and UMTS echnologies. Since we have assumed ha each vehicle is equipped wih a BSN δ (5) BCN > GPS receiver he coordinaes Pin (xin yin ) of he enrance 1 L/ T and Pou (xou you ) of he exi poin of he new (candidae) cell wih respec o a coordinae sysem cenered in he cell holds where δ < is a hyseresis facor inroduced o cenre are easily calculaed so ha x is known. I follows avoid verical handover occurrence when he wo compeing neworks have negligible bandwidh difference. From (5) we ha he cell crossing ime is expressed as 2 : noe how swiching decisions may no be necessary even x 2R you yin T = = cos arcan. (2) hough he bandwidh BCN is higher han BSN. Swiching xou xin becomes necessary only if he ime ha he vehicle will sp in he cell wih higher bandwidh is long enough o compensae Given he above assumpions and definiions we can model for he daa loss due o he swiching overhead namely only he hroughpu Θ ha a vehicle would experience by remaining if L < T holds. This observaion leads o he conclusion conneced wih he Serving Nework (SN) during he cell ha he hroughpu is influenced no only by he bandwidh of crossing ime T = ou in as a funcion of he bandwidh he considered echnologies bu by a larger se of parameers: he crossing ime he vehicle speed and he overhead of BSN assumed o be consan during T ; namely we have: he conrol-plane proocols adoped (handover laency). We Θ = T BSN. (3) formalize his inuiion wih he following: Noe ha so far we have only modeled he hroughpu in Theorem (Speed Upper Bound): Given a vehicle V rava VANET nework where vehicles are covered by a single eling wih an average speed ~v in a heerogeneous vehicular service nework; we now ex he model o heerogeneous environmen for a disance x a valid handover for V occurs if is bounded as follows: environmens by capuring nework swiches as well. Definiion 3 (Handover Laency): Given a vehicle V raversing an area covered by a leas wo wireless cells he handover laency L is he ime inerval during which V does no receive any daa due o conrol plane (socke swiching) signaling messages exchange. 2 This compuaion is direcly performed by he vehicle by assuming consan vehicle speed and knowledge of he new cell radio coverage. < x (BCN BSN δ). (BCN δ)l (6) Proof: he claim follows from (5) and from he definiion of average speed of a vehicle: ~v = ~x/ T. 2 In he res of he paper we show how he above resul is useful in designing vehicular-o-infrasrucure proocols as From ha he he Serv ou consan Noic in a VA service C. New In h in our vehicle handove parame Definii raversi he han does no message Le a enering In his beween ou as: where γ γ and α i handove α = 1 while fo Throu due o

3 well as o promoe vehicle safey applicaions. Providers may in fac offer lower daa rae in hose areas where he speed limi is lower o induce vehicles o mainain lower speeds in order o experience accepable QoS levels low jier and high hroughpu hroughou valid handovers. III. SPEED-BASED VERTICAL HANDOVER ALGORITHM In his secion we presen our S-VHO algorihm and we discuss he relaed work Speed Probabiliy-Based VHO [5] () which we compare agains in our simulaions. In boh algorihms he speed of he vehicle is used as handover assessmen crierion. Consider Algorihm 1. Our S-VHO acceps hree inpus: he vehicle speed v he ingress ime in of he vehicle ino a wireless cell and he GPS locaion informaion P in and reurns he handover decision variable α { 1}. Le a vehicle conneced o a SN enering ino an area wih also a CN coverage. Afer each handover execuion he algorihm eners in idle mode for an iner-swich waiing ime period T w. For example if a vehicle ravels a 15 m/s a 1 seconds inerswich waiing ime resuls in 15 meers covered by he vehicle before he algorihm is re-acivaed. This shrewdness is necessary o avoid a high handover frequency ha may occur when vehicles ravel on a border line beween wo wireless cells (effec known as ping-pong [7]). The echnique insead focuses on an adapive handover mechanism beween WLAN and UMTS [5] based on he evaluaion of a handover probabiliy P ho obained from power measuremens. The handover decision is hen aken by comparing he handover probabiliy wih a fixed probabiliy hreshold P T deping on he vehicle speed and on handover laency among he wo neworks. In paricular he conrol decision equaion o assess wheher P ho = λ RX h RX W λ RX h RX h > 2R L v 2R B = P U T (7) L B W iniiaing an handover (o he WLAN hospo) when he inequaliy holds. In (7) λ is a coefficien whose larger value denoed more difficuly o perform handover RX h is he hreshold value of RSS ha denoes he successful receiving of packes RX W he currenly measured RSS of he WLAN R is he radius of he WLAN cell L denoes he average handover laency beween WLAN and UMTS while B U and B W are he daa rae of UMTS and WLAN respecively. IV. ANALYTICAL RESULTS In his secion we dissec he impac of boh he handover laency and he hyseresis effecs on he speed upper bound compued in Secion II wih some analyical resuls. In Fig. 2 (a) we show he impac of he handover laency on he speed upper bound for a given bandwidh raio of available echnologies. The bandwidh ranges were chosen according o WLAN [8] and UMTS [9] requiremens. The hyseresis facor δ was se o zero o isolae he impac of he single parameer Inpu: v in P in Oupu: α (handover decision) while inside area wih a leas wo overlapped cells do if (B CN > B SN / ( ) 1 L T δ) hen α 1 (VHO execued) se a decreasing couner o T w [s]. while T w > do idle mode else α Algorihm 1: Speed-based Verical Handover Algorihm. L (handover laency) and he range of speed was bounded by 35 m/s being a common highway speed limi. The firs ake-home message confirms he validiy of our model revealing ha for higher values of handover laency he speed bound (i.e. he maximum speed a which vehicles experience valid handovers) decreases. This makes sense since vehicles raveling a higher speed may no sp enough ime under higher daa-rae cells o jusify he degraded performance inroduced by he handover overhead of he signaling messages. The second resul comes by observing he epigraph he se of poins above he drawn curves. Any poin belonging o he epigraph represens no performance gain in iniiaing handovers even if he CN has higher bandwidh han he SN. In conras for any poin in he hypograph he se of poins below he curve valid handovers occur. As a limi case sudy noe how he curve wih zero bandwidh gap has empy hypograph; his follows direcly from he definiion of valid handover: a handover canno be valid when he daa raes are equal. In Fig. 2 (b) we show he impac of he hyseresis δ [Mbps] on he speed bound given he handover laency L [s]. We considered he hyseresis range o be δ [ B CN B SN ] and we have simulaed he case B CN B SN = 16 [Mbps] a ypical gap in daa raes beween UMTS and WLAN [8] [9]. I is useful o noe ha B CN B SN is he maximum δ afer which no valid handover would occur. The message for his simulaion seing concerns he difference in he hypographic area for differen values of L: when he handover laency increases he hypographic area significanly reduces. From his observaion i follows ha handover laency should be aken ino accoun when designing proocols for seamless conneciviy in VANET insead of considering only physical parameers or speed of he vehicles. V. SIMULATION RESULTS In his secion we repor on nework performance i.e. hroughpu delay and jier as well as he number of verical handovers obained wih our even-driven simulaor. Deails of he simulaor can be found in [1].

4 35 }= {182} [Mbps] 6 B SN = 18 Mbps B CN = 2 Mbps Speed Upper Bound [m/s] }= {185} [Mbps] }= {189} [Mbps] }= {1814} [Mbps] }= {1818} [Mbps] Speed Upper Bound [m/s] L =.1 [s] L =.2 [s] L =.4 [s] L =.8 [s] 1!2 1!1 Handover Laency [s] (a) 1! Hyseresis (!) [Mbps] (b) Fig. 2. Simulaion Scenario: A vehicle eners from a random locaion and is resriced o ravel along a grid of srees and inersecions following a pah inside a grid. Fig. 1 (c) depics one of he simulaion scenarios in erms of daa rae disribuion from hree UMTS base saions and weny WLAN access poins in a region of 2 km 2. Typical values have been considered for UMTS and WLAN respecively [8] [9]. The locaion of each wireless cell has been generaed uniformly a random and a vehicle moves in his area wih speed in he range [5 35] m/s. A vehicle downloads a series of video frames. Nework Performance: In Fig. 3 (a) we show he hroughpu as cumulaive received bis in a downlink connecion for boh S-VHO and echniques versus he iner-swich waiing ime. The effeciveness of S-VHO is clear when vehicle speed is below a given limi (e.g. 2 m/s). On he oher hand does no appear sensiive o eiher speed or iner-swich waiing ime and is hroughpu is limied. Noe however he S-VHO hroughpu drops when he vehicle speed exceeds he desired limi. Fig. 3 (b) shows wih 95% confidence inervals how he average frame delay for boh S- VHO and increases for higher speeds. This is because here is no enough ime o download he nex frame before he signal from he SN ges oo weak. Moreover S-VHO experiences lower delays compared o since on average i performs less handovers. Jier performance from S-VHO and have been compared in Fig. 4 (a) (b) (c) for differen values of speed and a fixed iner-swich waiing ime value of T w = 1 [s]. Each poin represens he cumulaive jier defined as he difference beween maximum and minimum frame delay averaged over 1 simulaions. As we can see jier increases wih speed since wo frames may be more ofen coming from differen wireless neworks and also because he cell crossing ime decreases when he speed increases. Overhead (Handover Frequency): Fig. 3 (c) depics wih 95% confidence inervals he average number of handovers for differen values of iner-swich waiing ime T w [ 5] s. As expeced he number of verical handovers decreases when he sysem is idle for longer periods (T w increases). Since our simulaions coun all he handovers (valid or invalid) he gap beween he S-VHO and curves represens he number Performance of Speed Upper Bound. Impac of (a) handover laency and (b) hyseresis. of invalid handovers ha are execued no aking ino accoun he handover laency L. VI. RELATED WORK A VHO decision is usually aken on he basis of (i) physical parameers e.g. received signal srengh level [7] signal-onoise and inerference raio [11] and (ii) QoS merics [12]. QoS-based verical handover algorihms mosly sugges ha he user conneciviy should be swiched o a candidae nework whenever he bandwidh is higher han he currenly experienced in he serving nework in order o improve perceived received qualiy [13]. Alhough his sraegy seems reasonable in vehicular environmens i may fail due o he speed and he ime ha he vehicle is going o sp in he new cell. In VANETs vehicles move a high speed herefore handovers should be performed on he basis of specific facors as vehicle mobiliy paern and localiy informaion raher han sandalone QoS requiremens. Pas soluions have parially bu no fully considered hese aspecs: in [14] for example he auhors deal wih a novel nework mobiliy proocol for VANETs o reduce boh handover delay and packe loss rae while Olivera e al. [15] proposed he Always Bes Conneced paradigm o achieve seamless conneciviy beween WLAN and UMTS neworks. Our mehod insead focuses on a vehicle-conrolled VHO due o smar on-board compuer equipped wih GPS conneciviy [1]. The idea of handover decisions based on boh vehicle speed and handover laency was previously inroduced in [16]. We augmen our conribuions by exing he algorihm s usefulness o realime applicaions compleing he performance evaluaion and significanly exing he analyical resuls. We compare our echnique wih respec o anoher speed based VHO algorihm [5] ha considered a probabiliy-driven handover scheme beween WLAN and UMTS based on a vehicle raveling disance predicion wihin a wireless cell. Alhough heir main resuls also address he minimizaion of verical handovers we ouperform he performance of heir approach and we focus more on joinly improving hree QoS merics: delay jier and hroughpu. VII. CONCLUSIONS AND FUTURE WORK We have presened via analyical modeling and a simulaion sudy a counerinuiive resul for verical handovers in

5 6.5 x Throughpu [Bi/s] x m/s 2 m/s 35 m/s S-VHO 5 m/s 2 m/s 35 m/s T w [s] Delay [ms] Speed [m/s] (a) (b) (c) Number of VHOs m/s 2 m/s 35 m/s 5 m/s 2 m/s 35 m/s T w [s] Fig. 3. (a) Throughpu averaged over 1 simulaions for S-VHO (whie markers) and (black markers). (b) Packe delay increases less rapidly wih vehicle speed when using S-VHO. (c) Number of Verical Handovers for S-VHO (whie markers) and (black markers) algorihms. Cumulaive Cumulaive Jier Jier [ms] [ms] Cumulaive Jier Jier [ms] [ms] Cumulaive Jier [ms] [ms] (a) v = 15 m/s T w = 1 s (b) v = 25 m/s T w = 1 s (c) v = 35 m/s T w = 1 s Fig. 4. Cumulaive jier experienced by a vehicle averaged over 1 simulaions for differen speeds noe he scale difference among differen graphs. Higher speed implies higher jier unless he number of unnecessary handovers is reduced (S-VHO). heerogeneous vehicular ad hoc neworks (VANETs) ha is when a vehicle encouners a candidae nework wih higher daa rae a connecion swich will no necessarily resul in a hroughpu improvemen. Our proposed echnique uses boh handover laency and cell crossing ime esimaion o simulaneously improve hroughpu and delay and i is driven by he vehicle speed: vehicles are required o mainain a given speed limi o mainain accepable levels of hroughpu delay and jier. The resuls presened in his paper are helpful for boh he research communiy when designing novel VANET proocols for real-ime applicaions and he business communiy as hey sugges how providers wih he help of vehicular neworks could enforce speed limis and herefore safey while delivering real-ime services as video-sreaming or online gaming. We plan o ex our analyical model ino more realisic scenarios removing he simplifying assumpions of consan hroughpu across he coverage area consan velociy and predicable vehicle moion. Moreover experimens comparing our handover algorihm wih oher QoS-based approaches (using real daa ses) are lef for fuure work. REFERENCES [1] H. Mousafa and Y. Zhang (Eds.) Vehicular Neworks: Techniques Sandards and Applicaions Auerbach publishers 29. [2] R. Lind e al. The Nework Vehicle-a glimpse ino he fuure of mobile muli-media IEEE Aerospace and Elecronic Sysems Magazine vol. 14 no. 9 pp Sep [3] Backsea child navigaion concep for kids available online a hp:// [4] G.P. Pollini Trs in handover design IEEE Communicaion Magazine vol. 34 no. 3 pp [5] Z. Yan H. Zhou H. Zhang and S. Zhang Speed-Based Probabiliy- Driven Seamless Handover Scheme beween WLAN and UMTS in MSN Wuhan China Dec [6] D. Kwak J. Mo and M. Kang Invesigaion of handoffs for IEEE neworks in vehicular environmen in ICUFN Hong Kong June pp [7] S. Balasubramaniam and J. Indulska Verical Handover Supporing Pervasive Compuing in Fuure Wireless Neworks Compuer Communicaion Journal vol. 27 no. 8 pp [8] IEEE Sandard for Informaion Technology Telecomm. and Informaion Exchange beween Sysems. Local and meropolian area neworks. Par 11: Wireless LAN MAC and PHY Layer Specificaions. [9] J. Laiho e al. Radio Nework Planning and Opimisaion for UMTS 2nd ediion Dec. 25 Chaper 6. [1] Anna Maria Vegni Mulimedia Mobile Communicaions in Heerogeneous Wireless Neworks PhD hesis Universia di Roma Tre 21. [11] K. Yang I. Gondal B. Qiu and L.S. Dooley Combined SINR based verical handoff algorihm for nex generaion heerogeneous wireless neworks in GLOBECOM Washinon DC USA Nov [12] A.M. Vegni M. Carli A. Neri and G. Ragosa QoS-based Verical Handover in Heerogeneous Neworks in Proc. of 1h In. Symp. on Wireless Personal Mulimedia Comm. Jaipur (India) Dec [13] A.M. Vegni e al. A Combined Verical Handover Decision Meric for QoS Enhancemen in Nex Generaion Neworks in WiCOM Marrakech (Morocco) Oc pp [14] Y.S. Chen C.H. Cheng C.S. Hsu and G.M. Chiu Nework Mobiliy Proocol for Vehicular Ad Hoc Nework in Proc. of IEEE Wireless Comm. and Neworking Conf. Budapes April 29. [15] J.A. Olivera e al. VANBA: a simple handover mechanism for ransparen always-on V2V communicaions in Proc. on IEEE 69h Vehicular Technology Conference (VTC29-Spring) April [16] A.M. Vegni and F. Esposio A Speed-based Verical Handover Algorihm for VANET in Proc. of 7h In. Workshop on Inelligen Transporaion Hamburg March