On Random Access Channel Performance and M2M Support in Standalone LTE Unlicensed

Transcription

1 c 2017 IEEE. Persoal use of this material is permitted. Permissio from IEEE must be obtaied for all other uses, i ay curret or future media, icludig repritig/republishig this material for advertisig or promotioal purposes, creatig ew collective works, for resale or redistributio to servers or lists, or reuse of ay copyrighted compoet of this work i other works. Radom Access Chael Performace ad M2M Support i Stadaloe LTE Ulicesed Valeti Schrader, Mikhail Vilgelm, Wolfgag Kellerer Chair of Commuicatio Networks Techical Uiversity of Muich {v.schrader, mikhail.vilgelm, wolfgag.kellerer}@tum.de Abstract Next geeratio telecommuicatio systems are required to efficietly support orders of magitude larger amout of devices per cell tha the curret LTE etworks. This requiremet is causig major desig challeges for the Radom Access Chael (RACH), especially for Machie-to-Machie (M2M) applicatios. the other had, due to the icreasig spectrum demads, LTE vedors are explorig ulicesed spectrum. For example, MulteFire has bee recetly stadardized as a LTEbased techology for stadaloe deploymet i ulicesed 5GHz frequecy bads. It is reasoable to expect that the coexistece with Wi-Fi ad stadaloe LTE i the ulicesed spectrum, will amplify Radom Access problem ad worse RACH performace. Heceforth, i this paper, we quatify the Wi-Fi LTE coexistece ad its impact o the RACH performace. We cosider a sychroized activatio of a large amout of UEs i a MulteFire/LTE ulicesed cell, ad aalyze the time it takes to coect all of them to the base statio. ur results cofirm that the presece of Wi-Fi substatially degrades RACH performace, with a icrease of almost 50% per additioal Wi-Fi statio. Furthermore, we illustrate applicatios of our evaluatio for RACH resource dimesioig ad etwork plaig. Idex Terms LTE; MulteFire; Radom Access; M2M I. INTRDUCTIN Evolutio of cellular etwork stadards towards 5G brigs a great umber of ovel challeges to be addressed i the future desigs [1]. First challege o the way to 5G is the growig spectrum demad. To address it, there are ogoig developmets of LTE-Licesed Assisted Access (LAA), eablig a LTE licesed etwork to offload data traffic to ulicesed spectrum as a further expasio of Carrier Aggregatio (CA). Complemetary to LAA, there exist recet stadardizatio efforts to develop stadaloe LTE i the ulicesed spectrum. For istace, MulteFire Alliace has just released a techical report ad first draft of the stadard [2] for such techology. I cotrast to LAA, MulteFire assumes that ot oly data, but also cotrol chaels are shifted to the ulicesed spectrum. This makes MulteFire idepedet o the licesed carrier, ad implemetable as a local, stadaloe solutio. Apart from the spectrum challege, 5G systems are evisioed to support ovel applicatios, such as Machie-to- Machie (M2M). While there is o clear cosesus if M2M devices ca be deployed i LAA or MulteFire etworks, licese-free bad usage is possible for certai idoor M2M istallatio scearios, e.g., productio sites or itra-aircraft This work has bee fuded i part by the Germa Research Foudatio (DFG) grat KE1863/5-1 as part of the SPP 1914 Cyber-Physical Networkig. Wi-Fi statios Stadaloe en: slte-u/multifire Wi-Fi statios M2M Devices / Sesor & Actuators e.g. at productio sites Fig. 1: Sceario: N M2M UEs, Wi-Fi statios ad oe stadaloe LTE en i close proximity, operatig i ulicesed 5 GHz bad. commuicatio [1], [3]. However, M2M applicatios feature a massive amout of devices i a sigle cell, which are ot supported by the curretly stadardized LTE systems. I particular, massive M2M devices put a strai o LTE Radom Access procedure, creatig a otorious Radom Access Chael (RACH) bottleeck, especially i the case of a highly sychroized traffic, typical for M2M devices [4]. For stadaloe LTE/MulteFire deploymets i ulicesed bads, the problem of RACH overload might be eve amplified due to the uderlyig coexistece with other wireless techologies primarily Wi-Fi. I its covetioal form, LTE ad Wi-Fi are icompatible i terms of medium access [5]. Wi- Fi has bee developed specifically for ulicesed bads, ad uses a Carrier Sese Multiple Access (CSMA/CA) scheme ad back-offs to share the medium with other statios. the cotrary, LTE has bee developed for licesed bads, with o eed of medium sharig. Therefore, LTE-LAA ad MulteFire itroduce chael sesig Liste efore Talk (LT) [6]. With LT, LTE etwork coteds with co-located Wi-Fi statios for the medium access, with the goal to esure fair coexistece. However, it is still a ope questio how this cotetio might ifluece the RACH performace, ad, hece, whether LTE i ulicesed spectrum ca support M2M. A. Cotributios I this paper, we aalyse ad quatify the impact of Wi- Fi cotetio with LTE o the performace of RACH. ur sceario, ispired by M2M use case, is a stadaloe LTE ulicesed etwork with a large umber of coected devices, colocated with a Wi-Fi etwork as depicted i Fig. 1. oth Wi- Fi ad LTE share the same 5 GHz ulicesed spectrum. Core

2 cotributios of our paper are: (i) aalytical approximatio model for computig the burst resulutio time: total time to coect a burst of M2M UEs to the etwork. ur aalysis merges a model of the cotetio betwee LTE ad Wi- Fi, ad LTE RACH performace model. (ii) Comprehesive simulatios, quatifyig the implicatios of coexistece o RACH performace.. Related Work May studies have bee coducted to ivestigate the limitatios of covetioal LTE RACH for massive M2M [7]. Multiple potetial solutios have bee proposed: dyamic adjustmet of cotetio parameters (barrig factor, backoff) [4], [8], load-adaptive ad quality-of-service-aware RACH resource allocatio [4], [9]. Also, methods for chael utilizatio improvemet ad fast collisio resolutio, such as tree algorithms or distributed queuig have bee proposed i [3], [4]. However, all of the related works have cosidered oly classical LTE deploymet i a licesed bad. Secodly, the itroductio of LTE ulicesed ad LTE- LAA has created a broad iterest i schemes to esure fair coexistece betwee ulicesed LTE ad other techologies usig the 5 GHz spectrum. Multiple studies aalysig differet outdoor ad idoor scearios, ad various coexistece methods have bee performed [6], [10], [11]. Most of them cosider the coexistece as a medium access problem, ad aalyse the performace by the meas of the Markov chai aalysis, typical for cotetio-based access [12] [14]. Additioally, spectral efficiecy-based approach to defiig the fair coexistet has bee studied i [15]. I summary, while there exist studies addressig LTE ad Wi-Fi coexistece i ulicesed bads, all of them are focusig o the impact of coexistece o LTE ad Wi-Fi data chaels. To the best kowledge of the authors, this is the first work cosiderig how the coexistece impacts a cotrol chael performaces, i particular Radom Access Chael. The paper is outlied as follows. We preset our aalytical system model ad explai the uderlyig techology assumptios i Sec. II, III. I Sec. IV, we validate the aalytical model, ad preset simulatio results. Fially, we coclude with the discussio ad outlook i Sec. V II. CEXISTENCE F SLTE-U AND WI-FI We cosider a sceario with N UEs deployed withi oe stadaloe LTE Ulicesed (slte-u) 1 small cell spaed by a sigle en. UEs are colocated with Wi-Fi statios ad operate o the same frequecy i 5 GHz, see Fig. 1. To esure fair coexistece of LTE ad Wi-Fi, en utilizes Liste efore Talk (LT) i a Wi-Fi like fashio [6]. We assume a cat. 4 LT (recommeded by 3GPP), with expoetial back-off ad variable cotetio widow (CW) [6], [11, ptio ]. 1 We refer to stadaloe LTE ulicesed with a o-covetioal abbreviatio slte-u to emphasize that MulteFire is oly a example ad our approach is geeralizable beyod a particular techology. the same time, we use slte-u to avoid cofusio with LTE-U stadard based o 3GPP rel. 12. M2M burst arrival a) b) c) ICCA Activatio Period usy Couter freeze Subframe Sigals RACH No-RA LTE TX Medium occupatio: urst Resolutio Time Last UE coected DeCCA Resume decrease ecca... Couter = 0 LTE- U Frame No-RA LTE TX MCT TXP Wi-Fi en UE DeCCA MSG2 MSG3 MSG4 Fig. 2: Exemplary timelie: (a) RACH ad burst arrivals; (b) en LT procedure; (c) slte-u frame. Details of a exemplary LT procedure are depicted i Fig. 2b. A trasmissio is iitiated if the chael has bee sesed idle for a Iitial Clear Chael Assessmet (CCA) (ICCA) of duratio similar to Wi-Fi s Distributed Iter-Frame Space (DIFS). therwise, if the chael is busy, a radom back-off couter is draw, ad the chael has to be sesed idle agai for a defer period D ecca of similar legth to ICCA. For every exteded CCA (ecca) duratio that the chael is sesed idle, the couter is decremeted. If the chael is foud to be busy, the couter freezes ad is oly resumed after aother D ecca. ce the couter reaches zero, the statio obtais the Trasmissio pportuity (TXP), ad captures the medium by trasmittig for up to the Maximum Chael ccupacy Time (MCT) of T max. If multiple statios reach the ed of their back-off couter at the same time, a collisio occurs, ad both respective trasmissios are lost (we assume o recovery is possible). We cosider all Wi-Fi statios to use the classical Distributed Coordiatio Fuctio (DCF) with CSMA/CA ad biary slotted expoetial back-off [16]. Moreover, we assume a sceario of fair coexistece betwee Wi-Fi ad slte- U. While, i geeral, fair co-existece ca be defied i differet ways, as equal spectral efficiecy or equal crossimpact of techologies [15], [17], here, we adopt the defiitio of fairess as equal steady-state shares of medium access time [12]. This implies that the cotetio parameters of slte-u ad Wi-Fi are cofigured similarly [17], ad, hece, we ca approximate the set-up by treatig Wi-Fi statios ad oe en as a homogeeous set of + 1 statios. Furthermore, we further assume a saturatio coditio, where all statios cotiuously coted for medium, ad fully utilize respective TXPs. Note that both homogeeity ad saturatio usy

3 assumptios are commo i the literature ad the model ca be easily exteded to relax them [12], [18]. III. SYSTEM MDEL I cotrast to LAA, stadaloe LTE, e.g., MulteFire, trasmits both data ad sigalig i the ulicesed bad. Naturally, the uderlyig LTE Wi-Fi cotetio impacts LTE sigalig procedures, ad delays the coectio establishmet. I this sectio, we describe our model for slte-u coectio establishmet, which is based o the works of iachi [18] ad Wei et. al [19]. I III-A, we explai the Markov chai model for the Wi-Fi/sLTE-U cotetio, ad i III-, we outlie the performace model of preamble cotetio o RACH. After that, we merge the two models i III-E. A. Markov Chai Medium Access Model Give the prelimiaries stated above, the behavior of a idividual statio (either Wi-Fi or en) ca be modeled as a Markov Chai [18]. I the followig we outlie the model ad its usage, without goig ito the details, sice the approach is well kow ad commoly used i the literature for modelig of cotetio-based access [12], [18]. The behavior of a statio is comprised of discrete states (i, j), represetig differet stages of the back-off, where i [0, m] is the trasmissio attempt (back-off stage), ad j [0, W i 1] is the back-off couter value. W i deotes the cotetio widow size at the back-off stage i. Startig from the mth back-off stage, back-off widow remais costat. Trasitios betwee states occur at every slot. A slot is defied by two cosecutive decreases of the back-off couter, thus, its legth δ is a radom variable (e.g., whe a statio seses the chael busy, the back-off couter is froze). We defie τ as the expected chael access probability, ad p as the expected collisio probability i a give slot. I the steady state, both p ad τ are idepedet of the back-off stage. Trasitio probabilities for all the states i a Markov chai ca be computed as a fuctio of the back-off stage i, back-off couter j, iitial back-off widow size W 0, ad collisio probability p [18]. Next, these probabilities are used to obtai the steady-state probabilities p i,j for all states, ad, usig ormalizatio coditio, we obtai the system of equatios: τ = 2(1 2p) (1 2p)(W 0 + 1) + pw 0 (1 2p) m, (1) p = 1 (1 τ). (2) After solvig the equatios for τ umerically, probabilities of at least oe ad exactly oe trasmissio o the chael, deoted respectively P tx ad P s, are computed as: P tx = 1 (1 τ) +1, (3) P s = ( + 1)τ(1 τ). P tx (4) Eqs. (3), (4) are used to compute the expected slot legth: E[δ] = (1 P tx )σ + P tx P s T s + P tx (1 P s )T c. (5) UE AC check roadcast SI2 PR CH co guratio MSG1: RA Preamble MSG2: Preamble Reply MSG3 slot allocated MSG3: RRC Coectio Request MSG4: RRC Coectio Reply UE coected en Fig. 3: Protocol ad messages of LTE Radom Access Procedure. Here, σ is the empty slot legth, ad the values for T s ad T c deote the time duratio that the medium is sesed busy due to a successful trasmissio or a collisio o the medium, respectively. I additio to the duratio of chael capture, T s also icludes the overhead of Iter-Frame Spaces ad ACKs: T s = T max + DIFS + SIFS + ACK. (6) This accouts for the fact that prior to every successful trasmissio the chael eeds to be sesed idle for DIFS. Also, it ca oly be sesed idle agai from aother statio after Short IterFrame Space (SIFS) ad ACK have bee trasmitted (i the case of Wi-Fi). the other had, i case of a collisio, we have T c = T max + DIFS, (7) because the collidig statios are assumed to cotiue trasmissio for a full duratio T max. Furthermore, i case of collisio o ACK is trasmitted such that the chael ca be sesed idle directly from the ed of the TXP o.. Coectio Establishmet i slte-u Havig stated the modelig prelimiaries of the Wi-Fi ad slte-u cotetio, we ow proceed by describig the RACH performace model. The steps for establishig a coectio of a UE to the etwork are iitial cell search ad radom access procedure [20]. MultiFire RA procedure is based o the 3GPP LTE four-step procedure [2] as illustrated i Fig. 3. ce en obtais a TXP, oe LTE frame cosistig of multiple sub-frames is set. We assume that en fully occupies the medium for MCT T max. Compliat with the MulteFire Discovery Referece Sigals (DRS) scheme, we assume that at the begiig of the first sub-frame, sigals to provide UEs with the ecessary iformatio to coect to the etwork are set. Amog them. the Physical Radom Access Chael (RACH) (PRACH) cofiguratio idex: preamble cotetio parameters, such as Access Class arrig (AC) factor p acb, ad the locatio of the PRACH sub-frame. UE proceeds with sedig a radomly chose preamble (codeword) i MSG1, ad receives MSG2 as a en reply, cotaiig the timig ad locatio of the sub-frame for MSG3 for every received ( activated ) preamble. The en is oly able to detect whether a particular preamble has bee activated, but ot how may UEs have set it. Hece, if two or more UEs choose the same preamble at MSG1, en assigs them the same uplik grat, ad their coectio requests (MSG3s)

4 will collide. If the coectio requests collide, o MSG4 is received from the en, ad the UEs will re-attempt sedig the preambles after a radom back-off time. We assume that all RA procedure hadshake, MSG1 to MSG4, occurs withi oe TXP of a en (exemplary frame structure i Fig. 2c), ad there are eough resources i Physical Dowlik Cotrol Chael (PDCCH) for MSG2. For tractability, we also assume throughout the aalytical model that o LT for UEs i UL (RACH ad MSG3) is ecessary. This assumptio is later relaxed i Sec. IV-D. C. urst Coectio Requests Arrival As a RACH traffic model, we cosider a burst arrivals sceario for coectio establishmet requests, i.e., ear sychroous activatio of a large umber of UEs i a cell. This sceario is commo for M2M commuicatio, e.g., etwork recovery after a power outage, or alarm reportig i emergecy situatios [3], [7], [8]. All N M2M UEs are activated i a simultaeous maer over activatio time accordig to a beta distributio [8]: g(t i a) = (ti a) α 1 ( t i a) β 1 T α+β 2, 0 t i a, (8) A (α, β) where t i a is the activatio time of UE i. urst arrivals [4] ca cause overload i the chael, ad result i very high delay ad coectio request drop probabilities [4], [9]. Stadardized LTE method to mitigate overload effects is AC. Prior to every trasmissio a device draws a radom umber from a set X ad compares it to a broadcasted value p acb called AC factor. If the umber draw is smaller tha p acb X, it proceeds to access the medium, otherwise it retries i the ext slot, goig through AC agai (geometric back-off). D. Preamble Cotetio Model To aalyze the performace of preamble cotetio, we apply a simplified drift approximatio model proposed by Wei et al. [19]. We cosider discrete time divided ito PRACH slots. I cotrast to [19], i our sceario PRACH slots have variable legth T RA because of the cotetio betwee en ad Wi-Fi. Assumig the activatio patter give by (8), expected umber of UEs activated i a PRACH slot i is give by λ i = N ie[t RA] (i 1)E[T g(t)dt. RA] The curret backlog of the system, i.e., the umber of activated but ot yet coected UEs is described by its expected value q i, represetig the expected umber of backlogged UEs at the slot i. We further subdivide the state trasitio ito a activatio ad a trasmissio step. The activatio step (additio of the ewly activated UEs) is computed by addig the ew arrivals q i = q i 1 + λ i. Next, the expected umber of successful UEs is computed as a fuctio of the barrig factor p acb,i ad the umber of available preambles per slot M [19]: ( q i = p acb,i q i 1 1 ) pacb,iq i 1. (9) }{{} M o-barred UEs Heceforth, the trasmissio step is modeled as a additio q i = q i + q i. Fially, we aim at determiig the burst resolutio time T s R, i.e., the time eeded to coect all N UEs to the etwork (expressed i uits of PRACH slot legth). Give the backlog state of the system q i, we ca compute the expected T s R with arbitrary precisio ɛ iteratively with t : E[T s R] = i [PRACH slots] if q i ɛ 0. (10) E. Mergig Models: urst Resolutio Time For slte-u, PRACH slot legth T RA, ad, hece, the absolute periodicity of PRACH sub-frames, is a radom variable, ad its expected value depeds o the medium access cotetio betwee en ad Wi-Fi statios, outlied i Sec. III-A. The steady-stated medium access parameters derived from the Markov chai model, ca be used to obtai the umber of slots betwee two successful chael captures of en, deoted as T tx. The probability that there is o slot betwee two trasmissios of the same statio, i.e., it draws a zero backoff couter ad retrasmits directly, equals s = τ(1 p). The probability mass fuctio of T tx is f T tx (x) = (1 s) x s, which is a geometric distributio, whose expected value is foud as: E[T tx] = x=0 f T tx x = 1 s. (11) s Assumig a PRACH cofiguratio correspodig to oe PRACH per en frame, we obtai expected RACH slot legth E[T RA ] usig Eqs. (5), (11) as: 1 τ(1 p) E[T RA ] = E[δ] + T max. (12) τ(1 p) Fially, i order to fid the expectatio of the absolute value of the burst resolutio time (i secods), deoted as T R, we use the expectatio of it i PRACH slots E[TR s ] ad the expected PRACH slot legth E[T RA ] obtaied via Eqs. (10) ad (11) respectively. Assumig TR s ad T RA are idepedet, we get 2 : ( ) E[T R ] = E[TR] s 1 τ(1 p) E[δ] + T max [s]. (13) τ(1 p) A. Evaluatio Setup IV. PERFRMANCE EVALUATIN We have implemeted a detailed MAC-layer simulatio model for the Radom Access (RA) procedure i a slte-u Network with the evet-based MNeT++ framework (C++) [21]. Processig of statistics has bee doe with SciPy [22] libraries. Results are plotted with a 95% cofidece iterval. If ot stated otherwise, the simulatio follows the assumptios as preseted i II, III. The parameters of the simulatios are summarized i Tab. I. We simulate with both a static ad optimal dyamic AC factor (p acb ) p acb,i = mi(1, M q i ) with q i beig the umber 2 I geeral, T s R ad T RA are correlated because of λ i. However, the idepedece assumptio is accurate for typical bursts with T R.

5 TALE I: Simulatio Parameters. Parameter Rage/Value Variable Parameters Number of Wi-Fi statios 0-25 Number of UEs N Activatio period s Number of preambles M 25, 54 (default) Fixed Parameters MCT for a specific sceario (T max) 7 ms σ / ecca slot duratio 9 µs [6], [16] DIFS / ICCA Defer period ecca (D ecca ) 34 µs [6], [16] SIFS 16 µs [16] ACK legth 14 bytes W 0 32 [18] m 5 [18] Air propagatio delay 1 µs of backlogged UEs at PRACH slot i. Simulatio rus with a static factor are used for model validatio. We choose to use the optimal factor to study coexistece, because there exist schemes which approximate it also for practical scearios [8].. Model Validatio First, we compare the aalytic model proposed i Sec. III with the simulative results. For model validatio, we set the umber of preambles withi the etwork to M = 25 ad use a static AC factor. We compare aalytical ad simulatio results for burst resolutio time T R vs. umber of Wi-Fi statios i Fig. 4a ad observe the values predicted by the model match with the simulatio withi the 95 % cofidece itervals dow to = 5s. For small activatio times 1s ad large umber of Wi-Fi statios > 15, we observe that the aalysis is overly pessimistic. C. Impact of Coexistece o Radom Access Performace Now we show the impact of the medium access cotetio betwee oe en ad Wi-Fi statio o RACH performace. Fig. 5a shows burst resolutio time as a fuctio of the umber of UEs N for varyig umber of Wi-Fi statios. Clearly, T R rapidly icreases with. For example, at N = 1000 UEs, T R grows from 0.37s for = 0 (o cotetio) to 13.7s for = 25, which is a 37 times icrease. Dividig by 25 Wi-Fi statios, oe fids a average icrease of almost 50 % per additioal statio. However, icreasig oly has a clear effect o T R whe the system already operates at maximum i terms of itesity of access requests preamble. We itroduce the otio of a stressed system. If T R > 0, we have a stressed system, if T R 0, the system is ustressed. We plot the differece betwee T R ad i Fig. 5b, to show the ifluece of o burst resolutio time. For a stressed system, icreasig directly traslates ito higher T R. The reaso lies i a icreased PRACH slot legth due to cotetio (see Fig. 4b), hece, icreasig decreases the umber of available PRACH slots per secod. T R (a) urst resolutio time T R vs. for various. Static p acb = 0.2, T max = 7ms, N = T RA (b) PRACH slot legth T RA vs. ; N = 1000, = 0.1s. Fig. 4: Model validatio: compariso of simulatio ad aalysis. If, however, a system is i ustressed state, ca be icreased without great effect o T R, as the behavior of cya curve for = 10s illustrates i Fig. 5b. This is related to the amout of preamble collisios withi oe PRACH slot. To quatify this amout, we ca look at the Collisio Probability of Preamble (CPP) i Fig. 5c, defied as the ratio betwee the umber of preambles activated by more tha oe UE ad the total umber of available preambles M. As we use optimal AC factor, which maximizes the umber of successfully trasmitted preambles, we observe a asymptotic limit for CPP. Wheever CPP gets close to that value, the system is stressed; otherwise the system is ustressed ad, hece, UEs i a cell ca be supported efficietly. We further provide a overview o the Empirical Cumulative Distributio Fuctio (ECDF) of idividual UEs service time i Fig. 5d. A icrease i leads to a decreased slope ad a icrease of the maximum service time, i.e., the service time where the probability reaches oe. This further cofirms our previous observatio o T R. We observe that all plots i Fig. 5d show liear behavior up to a specific probability, which iitially lies close to oe for = 0 ad the starts to decrease with growig. D. Liste-efore Talk prior to MSG3 Uplik Fially, sice LT for UL trasmissios is likely to be the regulatory requiremet LTE Ulicesed realizatios [2], [5], we ivestigate the ifluece of LT before uplik MSG3 trasmissio by UEs.

6 T R N (a) urst resolutio timevs. umber of UEs N for various ; =0.1s. (b) Differece betwee burst resolutio time ad activatio time, T R ; N = System is stressed, if T R > 0. (c) Collisio Probability of Preamble (CPP) vs. for various ; N = (d) UEs service times ECDF for = 0 25; = 10s, N = Fig. 5: Impact of coexistece o RACH performace. For that, we relax the saturatio assumptio that en cotiuously occupies its TXP, ad allow a idle pause i the medium occupatio, just before MSG3 sub-frame. Durig the pause, en remais idle, ad, hece, releases the chael ad Wi-Fi statios ca potetially capture it. Ituitively, the pause represets the time whe o DL or UL trasmissio is occurrig i the LTE etwork. The pause ca last up to multiple sub-frames. We preset illustrative results for the en pause T p = 34µs (oe DIFS), ad T p = 68µs i Fig. 6. For values smaller tha T p < 34 µs o chael capture by Wi- Fi statios is possible. Followig MulteFire assumptios, we cosider that UEs oly have to perform a oe shot CCA. If the chael is sesed idle, UEs start to trasmit ad LTE resumes to capture the chael util the ed of en s TXP. If UEs sese the chael busy, because of Wi-Fi capture, MSG3s fail ad UE eeds to restart the RA procedure, sice the uplik grat has to be re-allocated. We observe i Fig. 6 that a legth of T p = 34µs icreases T R by up to 50 %, with the higher icrease for larger umber of Wi-Fi statios. This is a moderate icrease caused oly by collisios betwee Wi-Fi ad UEs. However, with T p = 68µs we observe that the T R is doubled already for = 10 Wi-Fi statios. With = 25, T R drastically icreases 6.88 times, from 40 s for o pause up to 275s. T R TR requiremet Fig. 6: T R vs. for varyig idle pause T p; N = 1000, = 10s. Fig. 7: T R vs. umber of preambles M available per TXP. N = 1000; = 0.1s; exemplary T R requiremet of T R = 20 s.

7 V. DISCUSSIN AND CNCLUSINS I this paper, we have preseted a performace evaluatio of the Radom Access Chael of stadaloe LTE etwork i a ulicesed bad. Up to ow, stadaloe LTE-U techology is oly represeted by recetly specified MulteFire stadard, but our evaluatio methodology is ot boud to a specific techology. We have studied a sceario with oe en coexistig with multiple active Wi-Fi statios, ad evaluated the impact of Wi-Fi en cotetio o the burst resolutio time for a semi-sychroous arrival of a large umber of M2M coectio requests to the en. The evaluatio icluded a aalytical model ad comprehesive simulatios. Most importat fidigs of our evaluatio are: (1) burst resolutio time is heavily impacted by the cotetio with Wi- Fi, icreasig by 50 % per every added Wi-Fi statio. (2) Wi-Fi cotetio icreases collisio probability of a preamble, ad the less sychroous UEs activatio is, the more is the collisio probability icreased. (3) Medium release by en, together with LT for UEs i the UL, leads to a eve more dramatic icrease i the burst resolutio time. E.g., for = 20 Wi-Fi statios, ad a short medium release by en for 68µs, the resolutio time icreases sevefold from 25s to 175s. A. Model Applicatios The results of our model ad performace evaluatio ca be used for dimesioig of the etworks i multiple ways. For istace, M2M applicatio ruig over the slte-u etwork might have fixed requiremets for the re-coectio delay ( bootig time ). I that case, our model ca be used to determie the umber of PRACH resources (preambles) per TXP ecessary to fulfill a give re-coectio delay requiremet. I Fig. 7, we show the delay vs. M depedecy for a exemplary requiremet of T R = 20 s. We observe that for < 5 Wi-Fi statios, M = 5 preambles per TXP are sufficiet, while M 40 preambles are ecessary to keep T R < T R for = 25. Aother possible applicatio, i a case of a cotrolled eviromet (e.g., idustrial site, itraaircraft [3]), where Wi-Fi ad LTE etworks are operated together, our model ca help aswer the questio of how may additioal Wi-Fi statios ca be added to a existig system without violatig burst resolutio time requiremets.. Future Work As our results i Sec. IV poit out, resolutio of a burst of coectio requests ca take uacceptably large time, especially if the en releases the medium prior to uplik, ad UE has to perform LT. This problem has to be addressed by the future work, ad the meas for decreasig coectio delay ought to be developed. For example, it could be techiques for more aggressive medium access of en, triggered i a case of high RACH load. Additioally, preseted framework could be exteded to evaluate the set-up more thoroughly by relaxig a umber of assumptios we made. For example, saturatio assumptio could be relaxed ad the impact of Wi-Fi traffic load ca be cosidered. 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