Performance Evaluation of Multiple IEEE b WLAN Stations in the Presence of Bluetooth Radio Interference

Transcription

1 Performance Evaluation of Multiple IEEE b WLAN Station in the Preence of Bluetooth Radio Interference Jung-Hyuck Jo and Nikil Jayant Georgia ech Broadband Intitute, Georgia Intitute of echnology, th St., NW, Atlanta, GA 30318, U.S.A. Phone: , fax: , {jhjo, Atract here are everal paper publihed in the pat few year about the evaluation of IEEE (b) WLAN performance degradation due to Bluetooth radio interference. he network topologie ued for WLAN in the previou literature are point-to-point, i.e., one AP and one WLAN tation. In thi paper we evaluate a performance of IEEE (b) ytem compoed of an AP and multiple WLAN tation uing the ame 2.4GHz frequency band in the preence of Bluetooth radio ytem within a BSS. Our reult focu on the impact of Bluetooth interference on IEEE (b) WLAN ytem. he throughput of IEEE (b) i etimated a a function of not only WLAN parameter but alo Bluetooth radio ytem parameter. Further, we derived theoretical maximum throughput of IEEE (b) ytem with perfect channel condition in order to compare with other cenario that conidering interference and competition with other WLAN tation. A a reult, with light and heavy B uage cenario, the IEEE (b) ytem throughput are degraded by 25% and 66%, repectively.. Index erm IEEE , Bluetooth, interference, wirele LAN I INRODUCION HE evaluation of interference in the 2.4 GHz band ha been receiving more attention lately. Kamerman [1] report on tolerable interference level between Bluetooth and device for variou cenario and device poition. Hi analyi i baed on a imple path lo model and Signal to Interference (SIR) requirement for Bluetooth and receiver. Furthermore, the probability of an packet error in the preence of a Bluetooth piconet ha been derived by Enni, and then extended by Shellhammer [2]. Golmie and Mouveaux [3] evaluated the impact of WLAN and other Bluetooth interference on the Bluetooth acce control layer performance. he previouly publihed literature [1][2][3][4][5] dealt with a point-to-point network topology for IEEE (b) WLAN ytem. In thi paper, we evaluate the performance of IEEE (b) ytem compoed of an Acce Point (AP) and multiple WLAN tation (SA) in the preence of Bluetooth (B) radio ytem. Analyi of interference between the IEEE (b) and Bluetooth i not new. Early attempt to ae interference effect have been baed on imple geometric model [2][5]. Our goal in thi paper i to evaluate the performance of IEEE (b) DSSS ytem not imply compoed of one AP and one WLAN tation pair but compoed of one AP and multiple WLAN in the preence of. hi work wa upported by NCR and Georgia ech Broadband Intitute B radio ytem within a baic-uet (BSS). he latter cenario i a more realitic ituation that can be found in emerging office or home environment. We therefore conider the performance of IEEE (b) ytem that i affected not only by multiple WLAN tation but alo by uncoordinated Bluetooth radio interference within a BSS. he remainder of the paper i organized a follow. In Section II the IEEE (b) MAC i modeled, and in Section III the interference model between IEEE (b) WLAN ytem and Bluetooth radio ytem i preented. Finally, numerical reult of imulation baed on the interference model are preented in Section IV and concluding remark are given in Section V. II MODELING OF IEEE (B) MAC Performance evaluation of IEEE (b) MAC protocol poe everal challenge. he preence of hidden tation, ue of carrier ening, and decentralized nature of multiple acce method, are ome of the key factor that impact on ytem performance. In thi tudy we ued H. S. Chhaya [6] model to evaluate the performance WLAN ytem within a ingle BSS. Before proceeding further we need to introduce the following notation: A i (d): a et of tation in a circular area of radiu d around tation i. X i : a location of tation i D : a ditance between tation i and tation j α: capture parameter, α 1 d : αd H j (d): a et of tation that are hidden from tation j and are in a circle of radiu d around j H (d): a et of tation that are hidden from tation i but not from tation j in a circle of radiu d around tation j β: propagation delay including carrier ening delay (in wirele network the propagation delay i dominated by the carrier ening delay given the mall diameter of BSS, i.e., le than 100m) l: the length of a pay load thi doe not include PHY and MAC header. l type i the length of a type frame. A in [6], the implifying aumption are ued. We ignore the effect of frame error due to bit error introduced by channel noie. Limited tation mobility i aumed, which mean that all tation that are currently tranmitting or receiving a data or a control frame remain tationary until the completion of the tranaction /03/$ IEEE 1163

2 Conider a cae where tation i tart the tranmiion of a data frame of length l from tation i to tation j at time t uing baic acce method. We begin by auming that all data frame are of identical ize. he tranmiion will be ucceful only if the following condition are atified: i) the x and the Rx are not hidden from each other; ii) no tation in the Rx capture area that i not hidden from the Rx, tranmit in the time interval (t - β, t + β); iii) no tation in the x capture area that i not hidden from the x, receive uccefully a data frame whoe tranmiion wa initiated in the time interval (t - β, t + β); iv) no tation in H (d ) tranmit during the interval (t, t + l); v) no tation in H ji (d ) tranmit in the interval (t + l + DIFS, t + l + max{difs, SIFS + l ack }). Fig. 1 illutrate the above condition of ii, iii, iv, and v for a pair of tation to tranmit a data frame uccefully. We aume that once the channel i ened idle and a time interval DIFS ha elaped, the time until a data frame i generated at tation i i aumed to be exponentially ditributed with rate i). For notational convenience we define G:=Σ i Ai). For implicity we aume that tranmiion from any tation that are detined for ditinct detination are independent. Let S be the normalized ytem throughput, defined a the fraction of time the channel i ued to uccefully tranmit payload bit. Let be the renewal interval, i.e., the average time the channel i ened buy becaue of ucceful tranmiion, and c be the average time the channel i ened buy by each tation during colliion. he upercript tand for baic acce method and rc for RS/CS acce method. For the renewal interval of ucce tranmiion and failure, we obtain c rc c = PHYHD + MAC + β + 1/ G = PHY + MAC = PHYHD + MACHD + l + β + l + SIFS + β + l = l RS HD HD HD + DIFS + β + 1/ G theor _ max + l + SIFS + β + l + l + DIFS + β + 1/ G RS ACK S = ACK + DIFS + SIFS + β + lcs + SIFS + DIFS + β + 1/ G l ( 1 ) ( 2 ) where, 1/G i the time until the initiation of the firt tranmiion once the channel ha been ened idle for more than DIFS. Baed on thee renewal interval we can alo calculate theoretical maximum throughput, S theor_max, of baic acce method and RS/CS acce method in (2) and the Fig. 2 how thi reult in plot. In thi theoretical maximum throughput, we aume perfect channel condition, no interference, no competing tation, and compatibility among 1Mbp, 2Mbp, 5.5Mbp, and 11Mbp mode. he compatibility mode with variou network data rate mean that A j - H j iv ii A i - H i A i & A j v iii H H ji Fig. 1. IEEE : Illutration of ucceful tranmiion of data frame from SA i to SA j for baic acce method. Among the 5 condition, ii, iii, iv, and v condition are hown in thi figure. Black olid arrow line repreent tranmiion of a data frame, black dotted arrow line repreent tranmiion of an ACK, and red dotted line repreent undeired reception by SA j, which reult in colliion. A circle denote capture area. hroughput Pay Data [byte] RS/CS 2Mbp RS/CS 11Mbp Baic 2Mbp Baic 11Mbp Fig. 2. heoretical maximum throughput of IEEE (b) ytem for baic acce method and RS/CS acce method. hroughput i normalized to it raw data rate. the PHY layer header operate at the peed of lowet rate,1mbp, and the data rate change from MAC header according to the information from PHY layer header. Let u define C(i, to be the et of x-rx pair (m, uch that, C( i, {( m, : m A ( d' ) H ( d' ) or ( 3 ) n A ( d' ) H ( d' )} i i j j 1164

3 For baic acce method, we can define p (i) a the probability that a tranmiion of a data frame from tation i to AP i ucceful. p ( i) [ 1 p ( i) ] + l h k H ( d ' ) k) exp{ [ β + max{0, SIFS + l For RS/CS acce method, p rc ( i) [ 1 p ( i) ] + l h RS k H ( d ' ) ack exp{ [ β k)]} ( m, C ( i, DIFS} ( m, C ( i, m, k H ji ( d ' ) m, k)]} ( 4 ) ( 5 ) B DSSS τ ΒΤ B x y p Fig. 3. ime coincidence between a B piconet lot and a IEEE b packet. For example, if x < p < y, then the IEEE b packet could collide with 2 or 3 B lot db where a tation j i regarded a AP, and p h (i) i the probability of tation i being hidden from AP. hen the throughput of tation i, S(i) i, S( i) = i) p ( i) + p ( i)(1 e G p c c β i) + (1 p ) ( m, C ( i, c ) m, G ( 6 ) 11MHz 22MHz 11MHz Fig. 4. Simplified WLAN receiver filter frequency repone where p c i a probability of colliion with a competing SA. Since l ack + β << l we aume that c, then the denominator reduce to. he overall ytem throughput i given by, S = S i) i A ( ( 7 ) III INERFERENCE MODELING A. Channel Model For the indoor channel model, we apply a propagation model coniting of two part. Line-of-ight (LOS) propagation i aumed for the firt 8 meter. Beyond thi point, path lo increae a function of r, where r i range and n = 3.3. hi can be expreed in term of decibel: L path = FSL + 20 log(4πr / λ), r 8m = nlog( r /8), r > 8m ( 8 ) where, λ i free pace wavelength at 2.45 GHz, and r i range. FSL i the free pace path lo in db determined in the far field of the antenna. Uually for indoor environment, thi i calculated to be 1 or 10 meter. he n factor ha been determined from empirical data collected and can be found in the reference by [8]. For line of ight application in hallway the n factor i le than 2. hi i due to the wave-guide effect provided by the propertie of hallway or corridor. hi model i imilar to the one ued by Kammerman [1]. A Bluetooth FH ytem interfere with the IEEE (b) ytem a a kind of an interference tone. A long a the IEEE (b) receiver get a deired ignal that i 10dB tronger than the in-channel interference tone, the activity of a B device doe not harm. B. Probability of ime Coincidence he time coincidence analyi i baed the model developed in [4][7]. he B piconet wa aumed to be uing ingle time lot packet only, a hown in the Fig. 3. It i aumed that thi i the wort-cae cenario, ince the ue of multiple time lot packet effectively reduce the B hop rate and increae throughput. hi reduce tranmiion time and reult in longer gap in B interference, thereby increaing the chance of ucceful reception of WLAN packet. Under thi aumption, a ingle B packet i tranmitted at each frequency hop with the B time lot duration, B = 625 µ, and the tranmiion time within the time lot, τ B = 366 µ. he IEEE b frame duration p can be up to 1210 µec including PHY and MAC layer header for a tranmiion of 1500 byte packet at the peed of 11Mbp. he tart of tranmiion relative to the B hop time i modeled a a uniform random variable. Under thi aumption, the number of B time lot overlapping in time with the IEEE b packet i either n or n-1 a hown in Fig. 3. he probability of overlapping with n B lot, P n i calculated baed on [4]. ( n 1) + τ FD ( 9 ) B B B n 1 =, Pn = 1 Pn 1 B P 1165

4 where FD i frame duration that include PHY and MAC layer header a well a pay load data. he maximum number of B lot, n max that could collide with IEEE b packet i 3 given that the maximum length of b frame duration i 1210 µec. C. Probability of Frequency Coincidence Frequency coincidence occur when the tranmit frequencie of a B piconet reult in a packet error. he method ued in [4] doe not take into account adjacent channel interference. he method ued in thi tudy follow [7] that incorporating out-of-band interference model. Baed on empirical data [1][4], the IEEE (b) can provide reliable ervice in the preence of narrow-band interferer uch a B tranmitter falling within the pa band given that the Signal-to-Interference Ratio (SIR) in (10) i greater than 10dB. he out-of-band interference model ued in [7], i modified. Intead of uing J (f offet ) of inc function, the narrow band ignal falling into out-of-band are attenuated 30dB more than the interference ignal falling into in-band (22MHz) a hown in Fig. 4. Note that the out-ofband region i 11 MHz wide on each ide of the pa band. I ( f o SIR = S I I ( f offet ) = 30, = 0, f f o offet offet offet ) > 11MHz < 11MHz [ db] and f offet < 22 MHz ( 10 ) where f offet i frequency offet from (b) channel center and I o (f offet ) i 0dB if the narrow band interferer fall into the pa band of 22MHz, and 30dB if within the 11MHz of the out-of pa band. Otherwie we aume that the narrow band interferer doe not affect IEEE (b) performance. D. Probability of Colliion A tudied in the model [1], even if a nearby B piconet i active, there i till a ignificant probability that the IEEE b tation receive uccefully a packet. For a B tranmiion to dirupt an b packet, there mut be an overlap in time and in frequency. he analyi preented here i baed olely on the ue of ingle time lot packet by the B piconet. It i aumed that thi i the wort-cae cenario, ince the ue of multiple time lot packet effectively reduce the B hop rate and increae throughput. hi reduce tranmiion time and reult in longer gap in B interference, thereby increaing the chance of ucceful reception of (b) packet. he probability of colliion model ued in thi tudy i baed on [3]. he probability of colliion with n lot overlap i P ( )) coll where L B i piconet load factor. Overall probability of colliion i P n = 1 (1 ( Phop L ( 11 ) B total = n coll n ( ( P 1 P ( n 1)) + ( P P ( ) coll ( 12 ) ABLE I NEWORK PARAMEERS: IEEE (B) AND BLUEOOH Parameter Value Probability of hidden tation 0.08 or 0.16 Capture parameter 1.5 Propagation delay 1[µec] WLAN data rate 2, 11[Mbp] Bluetooth data rate 1[Mbp] Slot duration 20[µec]] PHY header 128[bit] MAC header 272[bit] ACK 112[bit] CS 160[bit] RS 160[bit] SIFS 10[µec] DIFS 50[µec] WLAN EIRP 20[dBm] B EIRP 0[dBm] Noie floor -95[dBm] Max # of wall between SA 3 Max # of wall between B device 0 Max ditance between SA 60 2[m] Max ditance between B device 10[m] Lo per wall 15[dB] ABLE II B PICONE PARAMEERS FOR HE LIGH AND HEAVY NEWORK ACIVIY SCENARIOS L AND H, RESPECIVELY Scenario D B L B L % H % For more accurate etimate of IEEE (b) network throughput in the preence of interference, the B lot colliion with ACK packet i conidered. here can be multiple B piconet that are cloe enough to caue interference, and there i the poibility that multiple piconet could be active imultaneouly. he probability of interference from m B piconet i P ) m mult( m) = 1 (1 P total ( 13 ) IV NUMERICAL RESULS For the purpoe of obtaining numerical reult we conider a finite number of tation ditributed randomly throughout the BSS. We conidered one AP, 25 SA and B piconet with location a hown in Fig. 5. he tation location were picked to be uniformly ditributed in a 2-dimenional BSS of 60 m by 60 m quare. For IEEE (b) cenario, we conider the cae where the offered load due to all tation, i,, i, j A, i identical. hroughout it i aumed that the probability of hidden terminal, p h (i, i the ame for all tation pair i, j A. he propagation delay between tation i alo conidered contant for all tation pair. hi aumption i valid ince the maximum ditance between a pair of tation i m (diagonal ditance of a quare of a ide length of 60m, i.e., 1166

5 60m 30m hroughput Baic RS/CS Baic low_if RS/CS low_if Baic high_if RS/CS high_if AP 30m SA Fig. 5. Illutration of ditribution of AP, SA, and B piconet. AP i located center of BBS. SA and B piconet are uniformly ditributed over 60m by 60m quare. B 60m (a) Baic RS/CS Baic low_if RS/CS low_if Baic high_if RS/CS high_if hroughput Baic RS/CS Baic low_if RS/CS low_if Baic high_if RS/CS high_if hroughput (b) Fig. 7. IEEE b WLAN data rate of 11Mbp. p h = 0.16 (a) payload ize of 1000 byte and (b) payload ize of 1500 byte hroughput (a) (b) Baic RS/CS Baic low_if RS/CS low_if Baic high_if RS/CS high_if Fig. 6. IEEE WLAN data rate of 2Mbp. p h = 0.08 (a) payload ize of 100 byte and (b) payload ize of 1000 byte = 60 2). he IEEE (b) parameter ued for imulation are hown in able I. For the B topology, we aumed that B piconet are randomly ditributed a SA are without co-locating with SA. In [4], it i however aumed that there i one B piconet co-located with each IEEE WLAN SA and the former one i more practical ituation. For the B piconet parameter and B uer cenario, the ame value and cenario are ued a in [7]. D B i B piconet denity in piconet/m 2. he B piconet parameter value for imulation are in able II. wo B piconet cenario are conidered: L i light B piconet cenario and H i heavy B piconet cenario. he light B piconet traffic activity wa baed on etimate for typical uage for a B piconet in an enterprie etting provided by Bluetooth SIG a reported in [4][7]. For the heavy B piconet cenario, the B piconet denity, D B wa doubled from 0.04 piconet/m 2 to 0.08 piconet/m 2, and the data tranmitted per day were increaed from 150 Kbyte/day to 10 Mbyte/day. hroughout the imulation throughput and offered load are normalized to raw data rate of IEEE (b) ytem. he imulation reult with the WLAN data rate of 2 Mbp, p h = 0.08, i hown in Fig. 6 (a) with payload of 100 byte and (b) with payload of 1000 byte. A hown in Fig. 2, at a WLAN data rate of 2 Mbp, the maximum throughput achievable theoretically i 30% with baic acce method and 20% with RS/CS acce method auming a perfect channel condition. hu, the maximum throughput in Fig. 6 (a) i 23% with baic acce method and 17% with RS/CS acce method due to channel imperfection and competing tation even without B interference. Due to the mall payload ize of 100 byte, baic acce method perform better than RS/CS method with light or heavy B uer cenario. However, in Fig. 6 (b) with payload ize of

6 byte, RS/CS acce method perform better than baic acce method with light and heavy B uer cenario. With a light B uer cenario in Fig. 6 (b), the throughput degraded 25% and with a heavy B uer cenario the throughput degraded 66%. With IEEE b WLAN ytem at a data rate of 11 Mbp in Fig. 7, we aumed the p h i doubled from 0.08 to 0.16 due to the horter range of 11 Mbp WLAN ytem than 2 Mbp WLAN ytem. Under thi aumption with a payload ize of 1000 byte in (a) and 1500 byte in (b), offered load of up to 50% and 40% repectively, the baic acce method perform lightly better than RS/CS acce method. With a light B uer cenario in Fig. 7 (b), the throughput degraded 23% and with high B uer cenario the throughput degraded 66%. V. CONCLUSION In thi paper we evaluated the throughput of multiple IEEE (b) WLAN SA ytem within a BSS in the preence of multiple B radio interference. he WLAN parameter a well a B piconet parameter affected on the throughput of multiple IEEE (b) WLAN SA. With a light and a heavy B uer cenario, the throughput of multiple WLAN SA ytem degraded 25%, and 66% repectively regardle of the WLAN data rate of 2 Mbp and 11 Mbp. In potential future work, a real building databae with interior wall could be ued intead of uing a normalized quare hape a a building databae. In thi cae we could ue a more realitic, ite-pecific ray-tracing model intead of log-normal ditribution channel model. REFERENCES [1] A. Kamerman, Coexitence between Bluetooth and IEEE CCK olution to avoid mutual interference, Lucent echnologie Bell Laoratorie, Jan [2] S. Shellhammer, Packet error rate of an IEEE WLAN in the preence of Bluetooth, in IEEE P /133r0, Seattle, Wahington, May [3] N. Golmie and F. Mouveaux, Interference in the 2.4 GHz ISM band: Impact on the Bluetooth acce control performance, in Proceeding of IEEE ICC, Helinki, Finland, June [4] J. Zyren, Reliability of IEEE WLAN in preence of Bluetooth radio, in IEEE P /073r0, Santa Roa, California, Sept [5] N. Golmie, R. E. Van Dyck, and A. Soltanian, Bluetooth and b Interference: imulation model and ytem reult, in IEEE /195R0, Apr [6] H. S. Chhaya, Performance evaluation of the IEEE MAC protocol for wirele LAN, Mater hei, Illinoi Intitute of echnology, Chicago, Illinoi, May [7] I. Howitt, WLAN and WPAN coexitence in UL band, IEEE ran. Veh. echnol., vol. 50, no. 4, pp , July [8]. S. Rappaport, Wirele Communication. Prentice Hall,