Empirical Evaluation of Overlap Requirements of Adjacent Radio Cells for Zero Delay Handover

Transcription

1 Empirical Ealuation of Oerlap Requirements of Adjacent Radio Cells for Zero Delay Handoer Marc Emmelmann and Hyung-Taek Lim Tecnical Uniersity Berlin Telecommunication Networks Group TKN Berlin, Germany Abstract Velocity as a non-neglectable influence on te andoer delay experienced by a user if te andoer is triggered on a radio-signal-measurement (RSM) based andoer sceme. Preious publications ae teoretically deried te minimal required oerlap of adjacent radio cells for a zero delay andoer if signal aeraging (low-pass filtering) and a ysteresis margin are employed in te andoer decision process. Tis paper presents for te first time a practical erification of tose results by employing real world cannel traces from a ig-speed train scenario and considering te effects of an existing radio front-end due to te latter s receier sensitiity. Also, in addition to related work, we adapt a time-discrete analytical model for comparing teoretical findings wit cannel trace based results wic is closer to an implementation sampling te radio cannel s quality at distinct time interals. TABLE I ACHIEVABLE DELAY PER HANDOVER PHASE STATE OF THE ART Ref. Detec- Network Deci- Link Retion Discoery sion Establisment [10], [11] [1], [13] 0 ms n/a 0 ms n/a [14] [15], [16] 3.5 ms (plus 10 ms for cannel switc) [17] 10 ms n/a (only single frequency networks; low user mobility) I. INTRODUCTION Next generation wireless local area networks (WLANs) proide trougput rates of seeral 100 Mbit/s by improing te efficiency of te MAC and incorporating adanced MIMO system. Een toug te IEEE standardization body is still working on te final ratification of 80.11n [1] incorporating tese features, te IEEE working group in conjunction wit oter researc projects ae already started working on furter enancements proiding een iger trougput [] [4]. Apart from exploring multi-user diersity including dynamic OFDM scemes [5] [8], deices will also operate witin te 60 GHz frequency band to aciee ery ig trougput. As emitted radiation power is limited, te coerage area of eac WLAN radio cell will srink especially for te iger frequency band. Along wit te fact of extremely reduced cell sizes comes anoter callenge: andoer is not longer a seldom eent. As attenuation limits te cell size of an office enironment to only a few meters, te dwell time of a mobile user in a cell may be in te order of only a few seconds empasizing te need for an efficient sceme proiding a seamless andoer. Accordingly, researcers intensiely focused on approaces reducing te time spent on te andoer. Tabl I [9] classifies te best, currently publised approaces according to te different andoer pases. Accordingly, te delay associated wit andoer detection and decision may be reduced to 0 ms. Te underlying system model assumes tat te decision wen to switc from one AP to te oter is based on a radio-signal-strengt-based (RSM-based) algoritm. Te latter employs a low-pass filter to equalized sort term fading as well as an ysteresis margin to aoid oscillation of te station between two APs. Te autors clearly sow ow te mobile s elocity affects te andoer delay finally yielding to requirements regarding te minimal oerlap of adjacent radio cells in order to enable a zero-delay andoer. Neerteless, te work presented in [10] [14] as two sortcomings: Te used attenuation model only accounts for log-normal pat loss but not for sort term fading. Also, te presented analysis models te low-pat filter as a timecontineous process wic does not correspond to te discrete sampling of te radio cannel used as an input for te filtering process. Extending te findings of tis preious work, tis paper s noelties are: accounting for a casual, discrete non-recursie low-pass filter applicable to existing implementations; te utilization of real cannel traces for te performance ealuation; as well as te consideration of te effects of aing existing radio front-ends on obtaining signal strengt samples. Hereby, tis paper presents for te first time a practical erification of te teoretical findings in [1] and [14]. Te remainder of te paper is structured as follows: Section II introduces te system model in accordance to [1]. Afterwards, Section III focuses on ow to derie te minimal requirements regarding te oerlap of adjacent radio cells in order to guarantee a zero delay andoer. Terein, we firstly

2 present a pure analytical solution assuming an ideal AWGN, log-normal cannel model (Section III-A). Secondly, we describe ow we use a empirical cannel traces in a simulation to obtain equialent oerlap requirements (Section III-B). Finally, Section IV exemplarily compares te results from te analysis and simulation. II. SYSTEM MODEL In accordance to [1], we consider two access points AP 1 and AP separated by distance D (c.f. Figure 1). Bot APs regularly announce at a frequency f teir existence by a pilot tone or frame wic also enables acquiring samples of te receied signal strengt indicator (RSSI) µ 1 (d) and µ (d) from eiter AP giing te distance d between mobile to te corresponding AP. Te mobile traels in a straigt line at elocity from one AP to te oter and may retriee samples of te receied power from eiter AP witout being able to successfully decode te receied information as documented in [18]. Te mobile excanges user data wit te AP aing te larger signal. Hence, a andoer from AP 1 to AP occurs if µ (d) µ 1 (d) + were is a ysteresis margin aoiding predominant oscillation between te APs and µ( ) denotes to a receied signal after applying a non-casual, discrete lowpass filter aing N delay elements (c.f. Fig. ). Suc filtering aoids false andoers due to sort term fading. In order to enable a zero-delay andoer, te later as to occur wile te mobile is witin te coerage are of bot APs. For te analytical part, we assume for µ( ) a log-normal pat loss model (ideal AWGN cannel) wereas for te simulation, we use a empirical cannel model based on cannel traces as measured between a ig-speed train moing at seeral undred km/ and a trackside AP operating in te 3.5 GHz band. Details for eiter of te two cannel models are gien in te following sections. Also, it sould be noted tat tis system model only focuses on te andoer decision pase and does not account for andoer delay associated by te network discoery pase nor for any required signal between mobile and AP a prior (association pase) wic may be reduced to zero as sown in [9]. III. DERIVATION OF REQUIRED OVERLAP FOR ZERO-DELAY HANDOVER A. Analytical Solution 1) Cannel Modelling: Te signal power µ in [db] receied by te mobile at distance d is gien by µ(d) = K 1 log(d) (1) were K 1 represents te transmission and reception antenna gain, te enironment-specific attenuation of te cannel, and d te distance of te mobile to te AP emitting µ [19]. ) Low-Pass Filtering: As samples of te receied signal strengt can only be obtained for eac transmission of a beacon frame or pilot, we describe tis beaior by a sequence Fig. 1. Fig.. Handoer Scenario Low Pass Filtering of delta impulses δ n (n) = δ(n k n) µ[n] = µ(d) δ d (d) () d>0 = µ(k )δ(n) f (3) k=0 Te time required to propagate receie power samples from te input of te filter (µ[ ]) to its output ( µ[ ]) is gien by te group delay τ g ( ) τ g (Ω) = ϕ(ω) Ω = N 1 N 1 N were ϕ(ω) = (1 N)Ω/ is te pase sift of te transfer function of te gien filter aeraging te last N radio signal strengt alues [0]. Hence, for a sample frequency f, te andoer decision employing a low-pass filter σ LPF is delayed by (4) (5) σ LPF = 1 f τ g(ω) (6) = N 1 f N 1 N f (7)

3 3) Hysteresis Margin: Te optimal andoer decision inoling a ysteresis margin sould occur wen obtaining te n opt t sample of te receied power suc tat n opt = n : µ [n] µ 1 [n] (8) wic simplifies using Eqs. (3) and (1) to [ ] Df µ n opt µ[n opt ] (9) yielding to = log Df 10 Df n opt (10) n opt = n opt (11) assuming tat te decision occurs as early as possible, i.e., for te smallest possible n opt. Tis corresponds to te distance of te mobile to te originating AP gien by n opt /f at wic te mobile as to be witin te oerlap of adjacent cells in order to enable a zero andoer delay. According to [1], we assume a neglectfully small oerlap of adjacent cells requiring te andoer to occur in te middle of te two APs at D/. Passing te latter point witout triggering a andoer results in a andoer delay gien by n opt f D σ HM = = 1 Df 10 f D wic simplifies for /fd Df/ 1 to (1) (13) σ HM D D (14) ) Oerlap Requirements for Zero Delay Handoer: Te total andoer delay σ total = σ LPF + σ HM describes te interim between te mobile passing D/ and te point at wic te andoer from te old to te new AP is triggered. Te latter as to be witin te oerlapping region to aoid loss of connectiity, ence: O σ total = (σ LPF + σ HM ) (15) Defining te oerlapping ratio p of adjacent cells as O p = (16) R were R is te radius of eac AP s coerage area, we can use te relation R = D + O (17) in combination wit Equations (7), (13), (15) to obtain te minimal required oerlap ratio p min for zero delay andoer ( ) N 1 f + 1 Df 10 K f D 1+10 K p min = ( )(18) N 1 D + f + 1 Df 10 K f 1+10 D wic simplifies for N 1 and /fd Df/ 1 to p min N f + D D + N f + D B. Simulation Employing Empirical Cannel Model (19) 1) Empirical Cannel Model: Te attenuation pattern imposed on te receied signal at a gien distance is aried according to cannel traces. Te latter are obtained from te Transrapid ig speed (bullet) train passing access points operating at 3.5 GHz and being positioned along te track. For proprietary reasons, Fig 3(a) sows only te moing aerage of obtained RSSI samples. Te magnification in Fig 3(b) reeals tat te receied signal is actually experiences ery seere, sort-term fading. In order to allow a comparison wit te preiously presented analysis, we use te caracterization of te receie antenna to conert RSSI samples in db alues. Hereby, we employ te 10t-degree approximation of te empirical reception caracteristics as illustrated in Fig 3(c). ) Simulation Set-Up: Two access points are positioned D apart and emit eery 1/f seconds a frame allowing te mobile to obtain a sample te receie signal strengt. If te mobile cannot obtain a sample witin 1/f + ɛ seconds, a alue of 10 db is assumed wic is well below te receier sensitiity. Te mobile traels at a gien, constant elocity directly form one AP to te oter. Only packets receied from te AP wose low-pass-fltered receie signal µ is by db larger tan te oter s are decoded wic allows a quantification of te experienced andoer delay in accordance to te access point transition time metric defined by te IEEE draft recommended practice on wireless performance prediction as te time between te last successful reception of a (user) data frame ia te originating AP and te first successful reception ia te destination AP [1]. Accordingly, te smallest possible (measurable) andoer delay depends on te inter-packet arrial time wose expected mean in tis particular set-up sould not be below 1/f. 1 Te distance between APs D is increased from a small alue (resulting in a large oerlap of te APs coerage) until te expected mean of te access point transition time starts to exceed 1/f wit a statistic significance. At tis point, te resulting oerlap is te one required for a zero delay andoer (witin te limits of te measurement granularity of 1/f). To determine te oerlap, we erein define te radius R of te coerage area to be te distance, were te expected mean of te receied radio signal falls below te radio receier s reception sensitiity. IV. COMPARISON OF ANALYSIS AND EMPIRICAL RESULTS Figure 4 sows te minimal required oerlap deried from te analysis (using an ideal AWGN cannel) and te simulation (using empirical cannel traces) for parameters gien in 1 Een toug we derie te metric caracterizing te andoer delay from te IEEE context, system caracteristics and assumptions do not limit te results presented erein to tis specific tecnology as suc.

4 RSSI Satturation of Transceier Distance [m] Magnified Area Signal falls below Receier Sensitiity (a) Moing Aerage of RSSI Samples RSSI Distance [m] (b) Magnification sowing actual RSSI samples Signal Strengt [db]!50!55!60!65!70!75!80!85 Emperical transfer function 10t degree approximation Receied power dries transceier into satturation Receied power below sensitiity of transceier! RSSI (c) Receie Caracteristic of Transceier Fig. 3. Empirical Radio Signal Strengt oer Distance TABLE II PARAMETERIZATION OF ANALYSIS AND SIMULATION 15.5 required (min) oerlapping for zero HOD [HM=3,LP N=500] Parameter Value applicable Analyzed parameter for presented results range Hysteresis Margin 3 db 1 5 db Filtered Samples N Sampling frequency f 1 khz dito Table II. Results differ by less tan 1%; for large elocities, analytical results een fall witin te 95% confidence interal of simulation based oerlap requirements. Neerteless, we obsered te general trend tat oerlap requirements obtained from te simulation using real cannel traces are sligtly below tose gained from te analysis wic can be explained by te beaior of te underlying cannel model. For bot results, larger elocities require a larger oerlap enabling a zero delay andoer. Te sligt difference in te analytical and empercal results can be explained as follows: Te trace-based cannel model does not allow te reception of packets once te mobile leaes te coerage area of te AP. Hence, a sample of te RSSI alue cannot be retrieed witin 1/f + ɛ and we feed a alue of 10dB in te low pass filter. Tis alue is actually well below te receiers sensitiity tresold (at approximately 80dB and results tat te output signal of te low pass filter decreases rater fast once te mobile leaes te AP s coerage area. In contrast, te analytical, AWGN cannel does not model te beaior resulting from suc a receier sensitiity tresold and ence continuously considers cannel samples fading oer distance at log-normal scale. Hence, te slope of te lowpass filtered signal is lower at large distances and te andoff condition µ (d) µ 1 (d) + is reaced at a later time. Tis results a sligtly larger oerlap. Spoken from an operator s deployment point of iew, tis difference can be ignored as it is practically in te order of a few meters. Witout loss of generality, we erein present only te results of one parameter set as results resemble qualitatiely. required oerlapping[%] simulation results (trace CH) analytical calc elocity [km/] Fig. 4. Minimal required cell oerlap for zero-delay andoer V. SUMMARY AND FUTURE WORK Tis paper presented te influence of te mobile s elocity on te oerlap requirement of adjacent radio cells if te andoer decision is based on radio signal leel measurements. Comparing te results from a discrete, analytical model and tose of a simulation employing real world cannel traces, tis paper alidated for te first time te mere teoretical findings in [1] assuming a time-continuous acquisition of te cannel s attenuation caracteristic. In our future work, we plan to extend te analysis and simulation to a system model were te mobile does not moe directly form one AP to te oter but instead moes at a random trajectory between multiple APs. Suc results will allow to assess oerlap requirements for scenarios oter tan typical eicular enironments for wic mobiles are bound to trael along te road / railway track. REFERENCES [1] IEEE 80.11n/D8.0 Enancements for Higer Trougput, Draft Amandment to Standard for Information Tecnology Telecommunica-

5 tions and Information Excange Between Systems LAN/MAN Specific Requirements Part 11: Wireless Medium Access Control (MAC) and pysical layer (PHY) specifications, IEEE Std n, Re. D8.0, February 009. [] E. Peraia, Vt 60 gz par plus 5c s, IEEE VHT SG Very Hig Trougput Study Group, Dener, CO, USA, doc /806, July 008. [3], Vt60 tutorial, IEEE VHT SG Very Hig Trougput Study Group, Dener, CO, USA, doc /814, July 008. [4], Vt below 6 gz par plus 5c s, IEEE VHT SG Very Hig Trougput Study Group, Dener, CO, USA, doc /807, July 008. [5] M. de Courille, M. Kamoun, A. Robert, J. Gosteau, R. Fraccia, and S. Gault, Anoter resource to exploit: multi-user diersity, IEEE VHT SG Very Hig Trougput Study Group, San Fransico, CA, USA, doc /187, July 007. [6] J. Gross, M. Emmelmann, O. Punal, and A. Wolisz, Dynamic singleuser ofdm adaptation for IEEE systems, in Proc. of ACM/IEEE International Symposium on Modeling, Analysis and Simulation of Wireless and Mobile Systems (MSWIM 007), Cania, Crete Island, October 007. [7], Dynamic multi-user ofdm for systems, IEEE VHT SG Very Hig Trougput Study Group, San Francisco, CA, USA, doc. 07/06, July 007. [8], Performance comparison of dynamic ofdm wit 80.11n, IEEE VHT (Very Hig Trougput) SG, Atlanta, GA, USA, doc /860, Noember [9] M. Emmelmann, T. Langgärtner, and M. Sonnemann, System design and implementation of seamless andoer support enabling real-time telemetry applications for igly mobile users, in Proc. of te 6-t ACM International Symposium on Mobility Management and Wireless Access (MobiWac 008). Association for Computing Macinery (ACM), October , pp [10] N. Zang and J. Holtzman, Analysis of andoff algoritms using bot absolute and relatie measurements, Veicular Tecnology Conference, 1994 IEEE 44t, ol. 1, pp ol.1, Jun [11] N. Zang and J. Holtzmann, Analysis of andoff algoritms using bot abosute and relatie measurements, Veicular Tecnology, IEEE Trans. on, ol. 45, no. 11, pp , February [1] M. Emmelmann, Influence of elocity on te andoer delay associated wit a radio-signal-measurement-based andoer decision, in Veicular Tecnology Conference Fall 005, Proc. of IEEE 6nd. Dallas, TX, USA: IEEE, September, , extended ersion. [13], Influence of elocity on te andoer delay associated wit a radio-signal-measurement-based andoer decision, TU Berlin, Berlin, Germany, Tec. Rep. TKN , April 005. [14], Velocity effects on rsm-based andoer decision, IEEE TGt Wireless Performance Prediction, Plenary Session, Atlanta, GA, USA, doc /33, Marc 005. [15] Y.-S. Cen, M.-C. Cuang, and C.-K. Cen, Deucescan: Deuce-based fast andoff sceme in ieee wireless networks, Veicular Tecnology, IEEE Transactions on, ol. 57, no., pp , Marc 008. [16] Y.-S. Cen, C.-K. Cen, and M.-C. Cuang, Deucescan: Deuce-based fast andoff sceme in ieee wireless networks, Veicular Tecnology Conference, 006. VTC-006 Fall. 006 IEEE 64t, ol. 1, pp. 1 5, Sept [17] Y. Amir, C. Danilo, M. Hilsdale, R. Musǎloiu-Elefteri, and N. Riera, Fast andoff for seamless wireless mes networks, in MobiSys 06: Proceedings of te 4t international conference on Mobile systems, applications and serices. New York, NY, USA: ACM, 006, pp [18] IEEE 80.11k/D7.0 Radio Resource Measurement, Draft Amandment to Standard for Information Tecnology Telecommunications and Information Excange Between Systems LAN/MAN Specific Requirements Part 11: Wireless Medium Access Control (MAC) and pysical layer (PHY) specifications, IEEE Std k, Re. D7.0, January 007. [19] A. Aguiar and J. Gross, Wireless cannel models, Tecnical Uniersity of Berlin - Telecommunication Networks Group, Einsteinufer 5, Berlin Germany, Tec. Rep. TKN , April 003. [Online]. Aailable: ttp:// [0] A. V. Oppeneim, A. S. Willsky, and W. Hamid, Signals and Systems, nd ed., ser. Signal Processing. Prentice-Hall, August [1] IEEE Recommended Practice for te Ealuation of Wireless Performance, IEEE Std , Re. D1.01, 008.