On the analysis of WiFi communication and WiMAX network entry over single radios

Transcription

1 On he analysis of WiFi communicaion and WiMAX nework enry over single radios Sven Wiehöler, Marc Emmelmann, Yerong Chen, and Adam Wolisz Telecommunicaion Neworks Group (TKN), Technical Universiy Berlin, Germany Fraunhofer Insiue for Open Communicaion Sysems (FOKUS), Berlin, Germany Absrac Fuure wireless mobile devices will have o suppor a variey of heerogeneous access echnologies while having a limied number of ransceiver chains. One of he mos challenging asks remains how o suppor wireless access in one of he echnologies, while preparing a smooh ransiion o anoher, especially o echnologies wih a lenghy nework enry such as WiMAX. This work proposes an efficien way o organize a WiMAX nework enry process while coninuing a communicaion session wih sringen QoS requiremens via WLAN. We advocae he prioriizaion of he sric WiMAX iming over he conenion-based WLAN channel access. Our resuls show he applicabiliy of his approach over a wide range of mobile WiMAX parameers and idenify bounds on he WiMAX downlink load for which he WiFi communicaion does no suffer qualiy disorions. I. INTRODUCTION The diversiy of heerogeneous wireless access echnologies used by individual erminals (e.g., lapops, smar phones, ables) has been growing coninuously over he pas few years. Blueooh, WLAN, and some wide area wireless echnologies coun nowadays as a sandard mix for hese devices. An exensive se of echnologies would include Blueooh, IEEE802.11a/b/g/n, WiMAX, GSM, GPRS, Edge, HSPA, LTE, and even FM and DVB(-T). This wide deploymen has several implicaions. On he one hand, users depend increasingly on he (preferably high speed) conneciviy for everyday aciviies and express clear dissaisfacion if his conneciviy is no available. On he oher hand, he growh of wireless raffic leads o an increase of inerference and o hroughpu bolenecks. Dynamic decisions abou he access echnology o be used (e.g. pushing he users from a cellular nework o a WLAN) seem o be one of he promising approaches for boh miigaing inerference and overcoming he hroughpu bolenecks. While quie a lo of research is devoed o a proper selecion of he access echnologies [1], i is frequenly aking for graned ha he swiching iself, i.e., an enforced handover from one echnology o anoher, is somehow easily done. Swiching from one echnology o anoher while supporing a coninuous communicaion is, however, by far no rivial. The process of deciding abou he availabiliy of anoher echnology, selecing a proper access poin, and finally compleing he associaion procedure can ake a significan amoun of ime. Thus, i is hardly recommendable o enforce a hard handover, i.e., breaking he ongoing communicaion associaion before assuring ha anoher, beer conneciviy is really available. Realizing a sof handover wih service coninuiy is resriced, however, by he applied communicaion hardware. Even if muliple nework inerface cards (NICs), each supporing one of he echnologies, were available, heir parallel usage would be avoided in order o economize energy. In realiy nowadays, here is a srong rend o limi he number of separae NICs wihin a single device due o space and cos issues. As a resul, muli-sandard NICs become a soluion of choice. Such muli-mode radios can suppor muliple echnologies bu a a given ime only access o one wireless echnology over a specific ransceiver chain is possible. This work deals wih he problem how o enable a usage of heerogeneous links over such muli-sandard NICs while meeing QoS consrains of an on-going ransmission including consrains imposed by real-ime voice connecions. While following a more general concep, we presen in deail a soluion compaible o he family of IEEE and sandards. We idenify he fundamenal limis of his novel approach by considering he iming issues for boh WLAN communicaion and he lenghy WiMAX nework enry [2]. The res of his paper is srucured as follows. Secion II summarizes he relevan relaed work. Secion III specifies our sysem under consideraion and gives he problem formulaion, while our approach ogeher wih is analysis for WLAN and WiMAX is highlighed in Secion IV. Nex, Secion V presens and discusses resuls regarding he general feasibiliy of our approach. Finally, Secion VI concludes our work. II. RELATED WORK In order o enable heerogeneous wireless access anywhere and anyime by jus a limied number of NICs wihin a device, a hybrid approach is common oday: muli-mode, reconfigurable radios are able o do MAC as well as some PHY (base-band) processing in pure sofware, bu sill apply ransceiver chains wih specific analog pars (amplifier, filer) ha are specifically designed and adoped o heir purposes and frequency bands (e.g. join WLAN/WiMAX and 2G/UMTS/LTE ransceiver chains) [3]. Vendors like Infineon or Inel [4], [5], for example, have been developing dual-mode WLAN/WiMAX ransceiver chips ha have been incorporaed ino odays mobile devices. A rivial soluion may simply swich he ransceiver chain from one access echnology o he oher, imposing a hard

2 verical handover. Choi e al. [6] propose a more sophisicaed heerogeneous handover scheme, however for muliple NICs, in which i is possible o have only a single NIC acive during each ime insance o accoun for he limied baery power and radio resources. The auhors uilize he muual silence periods of ON/OFF raffic, such as VoIP, for he execuion of handovers. Boh, hard verical handovers as well as Choi s scheme, may be promising approaches for echnologies wih quick Layer 2 associaions, bu may suffer in case of handovers o echnologies such as WiMAX, which are known o have a very lenghy nework enry process [2] hus exceeding he lengh of muual silence periods by magniudes. Insead, we consider o alernae he ongoing communicaion in he firs radio access echnology (RAT) wih he link seup in he second RAT on properly chosen ime scales. The only work addressing simulaneous, QoS-consrained ransmissions and neighbor discovery over he same NIC on small ime scales has been presened in he conex of homogeneous WLANs [7]. This approach, denoed as opporunisic scanning, uilizes IEEE Power Save mode (PS) o shorly pause he on-going communicaion and scan oher WLAN channels passively wih inervals in he area of jus some milliseconds. The heoreical limis for finding anoher WLAN cell wih a given probabiliy have been presened in [7], he follow-up work [8] shows he surdiness of his approach even wih background raffic; a paen applicaion is pending for his homogeneous WLAN approach [9]. Sill, i remains an open poin how o realize a similar approach in a sandard-complian way for neighbor discovery or even handovers o a second, heerogeneous echnology in which iming consrains and requiremens regarding medium access differ remendously from III. SYSTEM AND PROBLEM FORMULATION As shown in Fig. 1, we consider a user wih a porable device like a noebook which is associaed wih an IEEE WLAN access poin (AP) and which is wihin he coverage of an IEEE e base saion (BS) addiionally. User s device is equipped wih several muli-mode radios, whereby WLAN and WiMAX share he same dual-mode ransceiver. This WLAN/WiMAX ransceiver chain is able o swich among he RATs wihin insignifican ime of a single clock cycle [10]. We focus on raffic from a user wih nomadic mobiliy, i.e., he remains saionary during his communicaion sessions, as i is expeced ha more han 80 percen of he mobile daa raffic will appear indoor in office or home scenarios, only [11]. For WLAN, we focus on IEEE g ERP OFDM wih daa raes ranging from 6 o 54 Mbps. For WiMAX, we consider he TDD mode and he parameers of he mobile profiles specified by he WiMAX Forum [12]. QPSK-1/2 is applied as Modulaion and Coding Scheme (MCS) for DL-/UL-MAP, and DL/UL channel descripor (DCD, UCD) messages. Via he WLAN access cell, he user has an acive VoIP communicaion session (ITU-T s G.711 voice codec wih 20 ms packeizaion). Thus, he join WLAN/WiMAX ransceiver is WLAN/WiMAX GSM/UMTS/ HSPA Wireless device wih muli-mode ransceiver chains Fig. 1. WLAN AP WiMAX BS Nework Scenario Inerne VoIP connecion VoIP peer blocked, such ha no simulaneous access o WiMAX may be possible. The VoIP call has he usual hard QoS consrains (maximum packe loss of 1 o 3 percen and a maximum olerable one-way end-o-end delay of 150 ms, cf. [13]). There exis manifold reasons for los or delayed VoIP packes. They may sem from he IEEE access cell or he wired par, e.g., a DSL-provider, he Inerne side or any combinaion of he involved eniies. Even he wired par can significanly conribue o variaions in he VoIP QoS. Several measuremens over backbones pahs have shown a emporal diverse behavior beween differen pahs, whereby some of he pahs have even periodic paerns wih respec o emporal higher delays [14]. As a basis of his work, we assume ha he end user wih he wireless device perceives some variaions in he VoIP QoS due o jiering delay in he wired par. Alhough hey do no bring he overall QoS below he accepable level immediaely, he user has perceived a sligh degradaion. To circumven a poenial sronger impairmen, a handover is imminen, alhough he wireless device iself perceives a good and lowloaded WLAN channel. For his scenario, we consider a soluion ha alernaes he ongoing VoIP over WLAN communicaion wih he nework enry process in WiMAX, which is he basis for he esablishmen of anoher pah o he VoIP peer. In order o allow a fas seup of he alernaive pah, he WiMAX nework enry process has o be conduced as quickly as possible. Thereby, he fundamenal quesion appears how o sill suppor he VoIP communicaion over he WLAN pah addiionally in a sandard-complian way. IV. HETEROGENEOUS OPPORTUNISTIC APPROACH A. Principle Following he spiri of [7], [8], we pause WLAN by means of he power save mode and swich o WiMAX in he gaps (Fig. 2). Le us analyze he quana in which he WiMAX enry process has o proceed. The firs sep consiss of finding he WiMAX downlink channel and o adap o he sric iming of he WiMAX frames aferwards. This is followed by he seps of obaining DL/UL parameers, iniial ranging, capabiliy negoiaion, auhorizaion and key exchange, and nework regisraion [2]. Lasly, he esablishmen of he IP conneciviy and he service flow finally prepares WiMAX for he VoIP ranspor.

3 WiMAX WLAN downlink TFrame uplink Fig. 2. REQ RSP TFrame RSP a) b) Aciviy of NIC in each access echnology WLAN-WiMAX alernaion principle In order o allow for a fas seup of he second pah via WiMAX, we have o give sric iming prioriy o WiMAX in order o keep he duraion of he nework enry process small. Neverheless, for oher applicaions differen from his work, one may furher radeoff he prioriy of WiMAX iming and WLAN channel access. In case of srongly flucuaing WLAN channels, for example, i may be imporan o give more channel access ime o WLAN (i.e., prioriize he ranspor of VoIP daa) and pospone subsequen seps of he WiMAX nework enry process (in he valid range of he IEEE sandard) insead. Since his work limis us o keep he duraion for he WiMAX nework enry process as small as possible, we prioriize he WiMAX iming, i.e., he Mobile Saion (MS) has o be presen during he downlink (DL) par for all WiMAX frames. In case here is no pending acion for he uplink (UL), MS swiches o WLAN and reurns back for he sar of he nex WiMAX frame. Beside he neighbor discovery, all furher seps of he nework enry process for WiMAX [2] are based on reques (REQ) / response (RSP) wo-way handshakes, where MS issues he REQ and wais for he RSP of he Base Saion (BS). For each of hese seps, MS has o spend he complee frame plus he following DL subframe in WiMAX mode (in order o send ou REQ and poenially receive RSP, if ransmied by BS immediaely in he subsequen DL subframe, Fig. 2 case a)). If RSP will be sen laer in one of he following frames, i will be received by MS anyway since i always spends he DL-par of he frame wihin WiMAX (Fig. 2 case b). The iming prioriy of WiMAX may lead o UL phases, in which MS canno swich o WLAN because of pending acions. In hese cases, WLAN access is delayed o he nex WiMAX UL subframe. If a VoIP packe is awaiing a ransmission in WLAN, his imposes a marginal addiional delay of anoher WiMAX frame bu ensures imely nework enry in WiMAX. B. IEEE e Timing Issues The duraion of he WiMAX DL subframe limis he available ime for VoIP ransmissions in WLAN and vice versa. The analysis akes ino accoun he maximum duraion of communicaion paerns in each echnology, such ha MS can be sill presen for WiMAX DL subframes and can ransmi VoIP wihou any qualiy disorions in WLAN. For his, we assume as a saring poin an idle channel in WLAN and no oher acive MS in WiMAX. Laer in Sec. V-C we TABLE I PARAMETERS ACCORDING TO IEEE E AND WIMAX FORUM frame duraion [ms] T frame 5 bandwidh [MHz] BW 3.5, 5, 7, 8.75, 10 cyclic prefix raio G 1/8 512 (3.5, 5 MHz), FFT size N FFT 1024 else sampling facor n 28/25 (5, 10 MHz), 8/7 else 15 (512 FFT), #PUSC subchannels N DL-PUSC 30 (1024 FFT) 8 (512 FFT), #FUSC subchannels N DL-FUSC 16 (1024 FFT) code rae c 1/2, 2/3, 3/4 modulaion level m 2, 4, 6 modulaion #Symbols QPSK, 16QAM & 64QAM S preamble 1 S FCH,DL-MAP 2 DL burs #1 [Bye] L burs1 301 DL burs #2 [Bye] L burs2 163 consider resuls for various DL loads in WiMAX, for he load dependency in WLAN he reader is referred o [8]. 1) WiMAX: The duraion of he DL par akes is maximum for he nework enry process if DL/-UL-MAP, UCD, DCD (wihin DL-burs #1) and (he larges) RSP message (DLburs #2) are ransmied ogeher in one DL subframe. Eq. 1 gives he duraion of he DL subframe: WiMAX-DL = symbol {S FCH,DL-MAP +S preamble +S DL-PUSC +S DL-FUSC }, where he number of symbols S DL-PUSC/FUSC, he number of occupied slos N slo, and he symbol duraion symbol are specified below: N slo S DL-PUSC = 2 [symbols], N DL-PUSC N slo S DL-FUSC = [symbols], N DL-FUSC N slo = L bursx [Bye] 8 [bi/bye] c m [bi/daa-sc] 48 [daa-sc/slo] symbol = (1+G) N FFT n BW. Table I summarizes seleced parameers and heir values according o IEEE e OFDMA [15] and he mobile profiles from he specificaion of he WiMAX Forum [12]. C. IEEE Timing Issues The maximum duraion of VoIP ransmissions in WLAN occurs, when AP and saion (STA) apply he mos robus daa rae. Depending on he siuaion wheher packes are awaiing heir ransmission in UL as well as DL or in one of he direcions only, he power save (PS) signaling and is duraion changes., (1)

4 AP STA WiMAX Fig. 3. wai UL VoIP PwrM=0 up More=1 rand(0,cwmin) DIFS DL VoIP DLDaa TWLANmax rand(0,cwmin) DIFS Null Daa PwrM=1 down WiMAX WLAN PS signaling wih up- and downlink ransmission TABLE II WLANmax (ms) FOR IEEE G ERP OFDM Residual ime of WiMAX frame [ms] QAM-2/3, 64QAM-3/4 16QAM-1/2, 16QAM-3/4, 64QAM-1/2 QPSK-3/4 QPSK-1/2 WLAN hreshold, up & downlink raffic WLAN hreshold, downlink raffic only 1 VoIP packe (Mbi/s) Null daa in DL in UL in UL & DL each Fig WiMAX bandwidh [MHz] Available residual ime of WiMAX frame Fig. 3 shows he wors case, which consiss of he wakeup, he exchange of one VoIP packe in UL and DL, and finally he sleep signaling. For his, Eq. 2 specifies he maximum acive duraion in WLAN: where: WLANmax = up + DLdaa + down, (2) up = wai DLdaa = { VoIP pk in UL, Null else, { 0 no pk in DL, DIFS + VoIP + + else, down = DIFS + Null + +. Table II gives he maximum acive communicaion duraion for IEEE g ERP OFDM (parameers as in [7], [16]), for he cases of no raffic, a packe in each direcion only, and for boh up- and downlink. For he cases wih presen VoIP raffic, he highes values for he mos robus MCS wih 6 Mbps have been seleced as hresholds (highlighed in grey). V. DISCUSSION OF RESULTS A. WiMAX Neighbor Discovery The firs sep of he heerogeneous opporunisic approach ackles he neighbor discovery of a WiMAX nework. Hereby, we shorly derive consrains for he selecion of he scanning inerval similar o he work in [7]. On he one hand, he scanning duraion will be maximized in order o speed up he WiMAX discovery process, i.e., o minimize he number of required scanning aemps. On he oher hand, we propose o say below he packe inergeneraion ime for he considered VoIP raffic, which is in his case 20 ms, in order o no induce large addiional delays. Moreover, he scan inerval mus no be equal o one or o muliples of he WiMAX frame sizes, since hen he periodiciy leads o problems in finding he oher nework. In [7] i is described ha inervals wih prime numbers fulfill his las requiremen. Overall, his leads o an opimal scanning inerval size of 19ms. Wih he choice of his inerval value, a WiMAX nework in a specific frequency band will be found in jus one scanning inerval if he WiMAX frame size (T WiMAX ) is equal or smaller han 12.5ms, which is he case for mobile WiMAX wih of 5ms frames. Wih T WiMAX of 20ms, Equaion 4 in [7] holds and resuls in a maximum of 2, 3, and 4 scanning aemps wih 5 percen probabiliy each, while jus a single aemp is required in 85 percen of he cases. Compared o he ask of finding anoher WLAN AP ([7]), he number of required scanning aemps for WiMAX is very low and can be seen as an uncriical par of he nework enry process. B. Single MS: Feasible Parameer Space for Mobile WiMAX Once he neighbor discovery has been compleed, all furher seps of our approach require he MS o say for he DL subframes wihin he WiMAX cell (cf. Secion IV-A). Now, when he MS says in WiMAX for he duraion of he DL subframe, a residual duraion occurs, which is analyzed in he following. Since we assume ha his residual ime span is used for WLAN communicaion, residual has o be greaer han he WLAN hresholds defined in Secion IV-C. residual = T frame WiMAX-DL > T WLANmax (3) The residual ime values were calculaed for all combinaions of channel bandwidhs and MCSs (for DL-burs #2) lised in Table I. Fig. 4 shows he resuls for he case of no oher DL-load in WiMAX: he residual ime of he WiMAX frame says far above he WLAN hresholds for all parameer combinaions. Thus, our approach is feasible if no oher raffic is presen. C. Muli-MS Case: Load-Dependency The las par deals wih he influence of background raffic in he WiMAX DL subframe, i.e., BS serves also oher MSs. This furher reduces he ime span ha is uilized o swich o WLAN. We idenify he performance limis for his duraion as a funcion of presen raffic in he WiMAX DL subframe.

5 forum as well as WLAN such ha QoS consrains even of realime raffic such as VoIP are no violaed. The resuls furher moivae research regarding he suppor of real-ime raffic over wo RATs via single ransceiver chains, hereby focusing on differen reasons for he usage of alernaive pahs. In order o handle much differen siuaions (e.g., regarding wireless channel condiions or bursy background raffic), boh in WiMAX and WLAN, we will consider differen policies for he prioriizaion of he iming beween he wo access echnologies. For example, i may be imporan in oher scenarios o give more channel access ime o WLAN (i.e., prioriize he ranspor of VoIP daa) and pospone subsequen seps of he WiMAX nework enry process (in he valid range of he IEEE sandard) insead. Fig. 5. Residual ime wih differen levels of WiMAX DL load TABLE III MAXIMUM WIMAX DL LOAD (IN PERCENT OF FRAME SIZE) WLANmax Frame Duraion [ms] ms ms We define he maximum WiMAX DL load L max as fracion of T frame, under which he iming consrains of our soluion sill work: L max = 1 WLAN max T frame. (4) Table III gives he resuls for various WiMAX frame sizes and boh WLAN hresholds. If T frame is far below he VoIP packeizaion inerval, i is prey likely ha here is only one packe waiing in UL or DL. In his case, he smaller WLANhreshold applies. For larger T frame above 10ms, he second WLAN hreshold is likely. Overall, for e OFDMA wih 5ms frames, our soluion is applicable if he DL par consumes no more han 63.9 percen of he frame duraion (or 3.2ms). Fig. 5 finally connecs he resuls wih and wihou oher DL raffic graphically: he upper edge of he inclined plane represens he case wih no oher background raffic (and QPSK-1/2 MCS for all messages). Dependen on he WiMAX parameer combinaion, here is sill space for oher 18.1 percen (3.5MHz bandwidh) and 39.9 percen (10MHz) of he WiMAX frame for he DL load. VI. CONCLUSIONS AND FUTURE RESEARCH This paper presened a sandard-complian approach enabling ongoing WLAN communicaion as well as he WiMAX nework enry using only a single, shared ransceiver chain for boh. Thereby, we alernae boh echnologies on small ime scales. The evaluaion of he novel scheme highlighed he iming consrains for WLAN and WiMAX. Our resuls idenify he limis for differen mobile profiles of he WiMAX NOWLEDGMENTS This work has been parially suppored by he projecs EU- COAST (FP7-ICT ), MEVICO (BMBF 01BU1014), as well as EU-TREND (FP7-ICT ). REFERENCES [1] X. Yan, Y. A. Sekercioglu, and S. Narayanan, A survey of verical handover decision algorihms in Fourh Generaion heerogeneous wireless neworks, Compuer Neworks, vol. 54, no. 11, pp , [2] M. Hollick, P. Mogre, C. Scho, and R. Seinmez, Slow and seady: Modelling and performance analysis of he nework enry process in IEEE , in IEEE IWQoS 2007, jun. 2007, pp [3] U. Ramacher, Sofware-defined radio prospecs for mulisandard mobile phones, Compuer, vol. 40, pp , [4] Produc Brief: PMB 2008, SMARTiTM WiMAX Single-chip dualband WiMAX / WLAN RF Transceiver IC wih sandard I&Q inerface, Infineon Technologies AG. [Online]. Available: hp:// [5] Produc Brief: Inel Cenrino Wireless-N + WiMAX 6150, Inel. [Online]. Available: hp:// [6] H.-H. Choi, O. Song, Y.-K. Park, and J.-R. Lee, Performance evaluaion of opporunisic verical handover considering on-off characerisics of VoIP raffic, Vehicular Technology, IEEE Transacions on, vol. 59, no. 6, pp , July [7] M. Emmelmann, S. Wiehöler, and H.-T. Lim, Opporunisic scanning: Inerrupion-free nework opology discovery for wireless mesh neworks, in IEEE WoWMoM, Kos, Greece, Jun. 2009, pp [8], Influence of nework load on he performance of opporunisic scanning, in IEEE LCN, Zurich, Swizerland, Oc. 2009, pp [9] M. Emmelmann, S. Wiehöler, and H.-T. Lim, Coninuous nework discovery using opporunisic scanning, European Paen PCT/EP 2010/ , pending. [10] J. G. Aallah and M. Ismail, Fuure 4G fron-ends enabling smooh verical handovers, Circuis and Devices Magazine, IEEE, vol. 22, no. 1, pp. 6 15, [11] Nokia Siemens Neworks, Improving 4G coverage and capaciy indoors and a hospos wih LTE femocells, Whiepaper, [12] WiMAX Forum, Mobile Sysem Profile Specificaion, Release 1.5, Common Par, WiMAX Forum, Tech. Rep. WMF-T R015v01, Augus [13] S. Shin and H. Schulzrinne, Measuremen and analysis of he VoIP capaciy in IEEE WLAN, Mobile Compuing, IEEE Transacions on, vol. 8, no. 9, pp , Sep [14] A. Markopoulou, F. Tobagi, and M. Karam, Loss and delay measuremens of Inerne backbones, Compu. Commun., vol. 29, no. 10, pp , June [15] : IEEE Sandard for Local and meropolian area neworks, Par 16: Air Inerface for Broadband Wireless Access Sysems, IEEE Sd., May [16] IEEE , Wireless LAN Medium Access Conrol (MAC) and Physical Layer (PHY) Specificaions, IEEE Sd , June 2007.