Improving Spatial Reuse of IEEE Based Ad Hoc Networks

Transcription

1 mproving Spatial Reuse of EEE Base A Hoc Networks Fengji Ye, Su Yi an Biplab Sikar ECSE Department, Rensselaer Polytechnic nstitute Troy, NY 1218 Abstract n this paper, we evaluate an suggest methos to improve the performance of EEE base a hoc networks from the perspective of spatial reuse. Since employs virtual carrier sensing to reserve the meium prior to a packet transmission, the relative size of the spatial region it reserves for the impening traffic significantly affects the overall network performance. We show that the space reserve by for a successful transmission is far from optimal an epening on the one hop istances between the sener an the receiver, we can have three scenarios with very ifferent spatial reuse characteristics. We also introuce a new quantitative measure, the spatial reuse inex, to evaluate the efficiency of the meium reservation accomplishe by virtual carrier sensing. We also propose an improve virtual carrier sensing mechanism for wireless LAN scenarios an using analysis an simulation results, show that it can significantly increase the spatial reuse an network throughput.. NTRODUCTON n EEE base a hoc networks, concurrent transmissions an channel contentions are manage by the istribute coorination function (DCF) [1]. DCF employs Virtual Carrier Sensing () to etermine channel access rights an reuce collisions among the stations competing for the share meium. While using physical carrier sensing an backoff mechanisms to istributely etermine channel access, also uses the exchange of RTS an CTS messages to avoi the well known hien terminal problem [1]. To avoi interfering with the ongoing transmission, all noes which hear the RTS or CTS message efer their transmission till the ongoing transmission is over. Consequently, the geographical area over which the hanshake can be hear etermines the overall throughput achievable in the network. To evaluate the effectiveness an efficiency of the meium reservation accomplishe by the mechanism an consequently the, spatial reuse can serve as an important benchmark. We note that there is a traeoff between higher spatial reuse an increase chances of collisions. Due to its tight connection with the network throughput an latency, spatial reuse serves as one of the key factors in wireless network capacity analysis. This paper analyzes the spatial reuse characteristics of an proposes a protocol to significantly improve the reuse characteristics. Recently, extensive research efforts have been evote to the performance evaluation of a hoc networks, most of which focus on the capacity analysis [3][4][5]. To the best of our knowlege, the influence of interference range on the network performance was first explore in [6] which examines the interaction of the MAC an a hoc forwaring from the perspective of spatial reuse. t is shown that for the total capacity to scale with network size, the average istance between source an estination noes must remain small as the network grows. n [7], spatial reuse characteristics are use to examine the performance of EEE MAC in multihop networks. The paper presents several serious problems encountere in EEE multihop networks an their unerlying causes an conclues that current wireless LAN protocols o not function well in multihop a hoc networks. n [8], the hanshake is analyze an it is shown that this mechanism is not always effective ue to the fact that the power neee for interrupting a packet reception is much lower than that for elivering a packet successfully. Thus the virtual carrier sensing implemente by hanshake cannot prevent all interferences as expecte in theory. Although some of the previous work evaluates the capacity an effectiveness of virtual carrier sensing, none of them has stuie the spatial reuse characteristics thoroughly. n this paper, we evaluate the performance of EEE base a hoc networks in terms of the spatial reuse an provie a quantitative measure of spatial reuse which reflects the effectiveness of meium reservation conucte by virtual carrier sensing. Our analysis shows that there are three ifferent cases with respect to the effectiveness of meium reservation, which we escribe as uneractive, moerate an overactive, respectively. We also show that the the space reserve by for a successful transmission oes not always match the space in which interference may occur. The paper further examines the overactive scenario an proposes an improve virtual carrier sensing scheme to enhance spatial reuse. We focus our analysis an simulations on Wireless LANs, where noes harly move uring packet transmission an have relatively small one-hop istance between them. Experimental an analytic results show that our propose scheme achieves significantly higher throughput over the virtual carrier sensing mechanism use in The rest of the paper is organize as follows. n Section we analyze the three scenarios with respect to the effectiveness of spatial reuse an introuce the spatial reuse measure. An improve scheme is propose an evaluate in Section. Section V summarizes our conclusions.

2 SPATAL REUSE ANALYSS As escribe in the previous section, spatial reuse is a key parameter that etermines the performance of wireless networks. n this section, we present a thorough analysis of the spatial reuse in virtual carrier sensing. We also introuce a metric to evaluate the spatial reuse. A. The Signal to nterference Ratio Moel Successful reception of a packet at the physical layer epens on the signal to noise ratio at the receiver. The basic raio propagation moel that we assume in this paper is the two-ray groun reflection moel [2]. Accoring to this moel, the receive power at istance is given by P r () = P tg t G r h 2 t h2 r (1) 4 L where P t is the transmitte power, h t an h r are the heights of the transmitter an receiver antennas respectively, G t an G r are the antenna gains an L is the system loss. As will be seen shortly, it is not ifficult to exten our analysis to the free space moel [2]. Now we escribe the three ranges that we use heavily in this paper, following the efinitions of [8]. Since we use ns-2 [9], [1] as our simulator, whose PHY layer is moele on the 914MHz Lucent WaveLAN DSSS raio interface, the relevant ata in ns-2 an the WaveLAN car are also provie. ffl Transmission Range ( ): The range within which a MAC frame can be successfully elivere an its type/subtype (RTS, CTS, Data, etc.) fiel can be correctly ientifie, assuming there is no interference from other raios. Accoring to Eqn. (1) an for WaveLAN an ns-2, the transmission range in is equal to 25 meters. ffl Carrier Sense Range (R s ): The range within which the power from the transmitter can be sense, inicating the busy state of the meium. Using Eqn. (1) an the carrier sensing power threshol of WaveLAN, we have 55 meters as the common carrier sense range. ffl nterference Range ( ): The range within which stations in receive moe will be interfere with by other transmitters an thus suffer a loss. As will be shown later, it oes not take a fixe value. To obtain the value of, we nee to introuce the moel of signal to interference ratio (SR), which irectly follows the Physical Moel in Gupta an Kumar s work [3]. Suppose that noe B is receiving packets from noe A, which is at an onehop istance of s meters, an concurrently another noe C, i meters away from B, is sening packets to a fourth noe D. We also assume all noes transmit at the same power P t an have the same raio parameters. To etermine whether there is a collision at B, we nee to compare the power receive at B from both A an C, enote by P s (signal power) an P i (interference power), respectively. Neglecting ambient noise, from Eqn. (1), we have [8] SR = P s P i = i s ff CPThresh (2) where CPThresh enotes the Capture Threshol, set to 1 in ns-2, an ff is the signal attenuation coefficient, which is equal to 4 in the two-ray groun reflection moel. Thus the interference range is given by = s (CPThresh) 1=ff = k SR s (3) We use k So enote the multiplier, which epens on the SR moel. n ns-2, k SR = 4p 1 = 1:78. Thus there is no fixe relation between an ; is proportional to the one-hop istance s. Note that an R s are merely raio ranges, which implies they only apply in the PHY layer. However, is involve with the SR moel an the comparison of the receive power, so it must be hanle at the MAC layer as well. We now show how the relative magnitues of an affect the performance of virtual carrier sensing an the effectiveness of the hanshake. Since in Wireless LANs it is common that all noes are within R s meters of each other, in the following sections we focus our iscussions to the other two ranges. B. Effectiveness of Virtual Carrier Sensing n [1], noes efer any impening transmission by the appropriate intervals whenever an RTS or CTS is overhear. This virtual carrier sensing mechanism, together with physical carrier sensing, etermines the busy/ile state of the meium. ts unerlying justification can be escribe by the conitions below. 1) Sufficient conition: f a noe can overhear an, then it is potentially able to interfere with the upcoming transmission. 2) Necessary conition: f a noe is capable of interfering with an ongoing transmission, then it must be able to overhear the preceing RTS or CTS. Obviously, achieves its best performance only when both conitions are satisfie. This happens when the transmission range is equal to the interference range. However, as we mentione earlier, this is not the common case as is not a fixe value. To better illustrate the effectiveness of mechanism, we classify all the situations into three categories with respect to r = =, the ratio of the one-hop istance as compare to. 1) Uneractive Scenario: Fig. 1 shows the scenario where the interference range (otte line circle) is larger than the transmission range (soli line circle). From Eqn. (2) we have = k SR >, so in this scenario, is confine by =k SR <<.nns-2 an WaveLAN, this range is between 141 an 25 meters. As shown in Fig. 1, every location falls into one of the four zones, marke by,, an V. Specifically, Zone, the intersection of the two soli line circles, represents the area in which a noe can overhear both RTS an CTS of the ongoing transmission. n Zone, only RTS can be overhear, while a noe in Zone can only overhear CTS. Zone V, however, is out of an a noe locate in it can sense some energy in the meium but is not able to ientify the signal.

3 V the necessary conition. Thus the hanshake an consequently the has reasonable performance in this case an we call it the Moerate Scenario. V sener receiver V V Fig. 1. Scenario : Uneractive, :56 << Fig. 3. Scenario : Overactive, <:36 n this scenario, we can see the sufficient conition aresse previously is satisfie, as any noe locate in Zone, or is able to interfere with the ongoing traffic. However, while a noe in Zone V can not successfully receive packets, it is still able to interrupt the ongoing transmission since it is within the interference range. Note that noes in Zone V fail to obtain the transmission uration information from the packets an thus o not efer from accessing the channel. Therefore in this scenario mechanism might fail to prevent a hien noe from interfering with the transmission, an we call this the Uneractive Scenario. Fig. 2. Scenario : Moerate, :36 <<:56 2) Moerate Scenario: Fig. 2 epicts the scenario where is smaller than an part of the otte line circles (interference region) lies within Zone /. n this scenario the range of is =(k SR +1) << =k SR, which is between :36 an :56 an 9 < < 141 meters in Lucent s WaveLAN an ns-2. n Fig. 2, observe the interference circle (otte line) an the transmission circle (soli line). As can be seen, Zone,, all intersect with the interference circle, which implies that the noes in the three zones have a chance (equal to 1% when = ) to interfere with the ongoing traffic. On the other han, all the noes within the interference circle are able to receive the preceing, which satisfies 3) Overactive Scenario: n Fig. 3, is small enough so that there is no intersection between the interference circle an Zone / an both interference circles are locate within Zone. n this case, the range of is < =(k SR +1)an for Lucent s WaveLAN an ns-2, is below :36 (9 meters). We call this the Overactive Scenario since the sufficient conition is no longer satisfie while the necessary conition still hols. t is seen that, although the noes in Zone / can still receive, they are not capable of interrupting the ongoing transmission. n this case gives false alarms that reuce the spatial reuse. t is also interesting that the power level neee for a successful packet elivery is much smaller than that of interrupting a transmission, a scenario which might not have been consiere when the was first propose. C. Evaluation of Spatial Reuse The effectiveness of is closely relate to the efficiency of spatial reuse, which in turn has a strong influence on the throughput an elay characteristics. Since the necessary an sufficient conitions mentione above are rarely satisfie simultaneously, the spatial reuse achieve in general is far from ieal. n Fig. 3, the spatial reuse is low since noes outsie the interference circles coul actually transmit/receive espite the etection of. n this scenario the hanshake claims much more space than necessary for a successful transmission, thereby reucing the spatial reuse. On the other han, in Fig. 1, mechanism unerestimates the space require for a successful transmission an thus incurs potential collisions by excessive spatial reuse, which is also unesirable. To evaluate spatial reuse quantitatively, we introuce the Spatial Reuse nex (SR) as follows: area of the region where interference may occur SR = area of the region reserve by to avoi interference (4) n a single hop scenario, this inex is equivalent to the ratio of the area of the interference region (union of the two otte line circles) to that of the transmission region (union of the

4 Spatial Reuse nex Overactive Moerate spatial reuse curve saturation point Uneractive One hop istance as compare to transmission range: / Fig. 4. Spatial Reuse nex (SR) two soli line circles). Assume r = =, the ratio of the onehop istance to the transmission range, we can compute the one-hop SR as p SR one hop = (ßk2 k 2 cos 1 1 2k ß cos 1 r 2 + r 4 4k 2 1)r 2 p4 r 2 (5) where k = k SR as specifie in (2). Using k SR = 4p 1 = 1:78, we obtain the SR curve in Fig. 4. n Fig. 4, the reuse inex achieves its optimal (saturation) value of 1 as r =1=k SR =:56. This happens when = an hanshake elivers its best performance. For SR above 1, uneractive causes an excessive spatial reuse, which may introuce unforeseeable collisions. We also see that overactive has a low spatial reuse with its SR below.5. t can be seen that the SR serves as a goo measure for evaluating both spatial reuse an hanshake efficiency.. AN MPROVED SCHEME AND EXPERMENTAL RESULTS n generic Wireless LAN settings, one-hop istances less than 1 meters are one of the most common scenarios since such environments are primarily expecte to span rooms in a builing, a few houses or builings. Accoring to the WaveLAN an ns-2 settings, these woul correspon to the overactive scenario. Also, while the uneractive scenario is not esirable ue to the potential collisions incurre, an alternative CTS reply scheme to eliminate such collisions has been propose in [8]. For these reasons, we only consier the overactive scenario in our propose scheme. A. Aggressive Virtual Carrier Sensing (A) We see in Fig. 3 that spatial reuse is inaequate in most wireless LAN scenarios since prohibits eligible noes in Zone / from sening or receiving. To enhance spatial reuse, we propose a simple new scheme name Aggressive Virtual Carrier Sensing (A). ts basic iea is escribe in Table -A, where we use an event-action table. To illustrate the working of A, consier a noe in Zone with ata to sen. Uner 82.11, the noe has to Zone etection Sener s action Normal Aggressive Normal Receiver s action Aggressive RTS&CTS hol hol hol hol RTS only hol sen RTS hol respon with CTS CTS only hol sen RTS hol respon with CTS TABLE NORMAL VS.AGGRESSVE : EVENT-ACTON TABLE efer/hol its RTS till the en of the ongoing transmission an then use the backoff mechanism. n our propose scheme, however, a noe in Zone woul sen its RTS immeiately whenever it wants to sen a packet, or respon instantly with a CTS if it is an intene receiver. n short, a noe which overhears either a RTS or a CTS but not both woul not consier the meia as busy. The unerlying justification for A is that in the overactive scenario, if a sener or receiver can hear either a RTS or a CTS but not both, it is guarantee that such a noe cannot interfere with the ongoing transmission. B. Evaluation of A From the iscussion in the Section -A, it is clear that A increases the spatial reuse in wireless LANs by allowing noes in Zones an to transmit. This improvement can be quantifie if we re-evaluate the spatial reuse inex of Eqn. 5. Accoring to its efinition, the -reserve region (enominator in Eqn. (4)) shrinks to the intersection of the two transmission circles when we apply A. Therefore we can re-erive the SR as p SR AV CS = (ßk2 k 2 cos 1 1 2k cos 1 r 2 r 4 4k 2 1)r 2 p4 r 2 (6) n Fig. 5, we compare the new SR curve with the original one, with the X-axis set as the real-worl one-hop istance. We see that the spatial reuse improves significantly with A in wireless LAN settings, specially as the one hop istance increases. n fact, the spatial reuse can be further improve in the overactive scenario if we exten our attention to Zone. n Fig. 3, the noes in Zone which are not within the interference circles, can also sen an receive packets without Fig. 5. Spatial Reuse nex Normal Aggressive One hop istance (meters) Comparison of A with the normal in terms of SR

5 Throughput per connection (Kbps) Normal Aggressive One hop istance (meters) Fig. 6. Ranom connection patterns for one hop istance of 85 meters Fig. 7. Throughput in Aggressive an Normal interrupting the ongoing transmission. However, in orer to juge whether such a noe is within the interference circle, the istance information between the noes is also require. With a more sophisticate MAC protocol that can hanle istance measurements an istribution, even better spatial reuse can be achieve. Finally, we note that there are some traeoffs when using the A. Since noes in Zone an transmit while the original transmission is going on, the original source an estination noes will not upate their NAV which might lea to potential collisions. However, the probability of such collisions epens on the spatial istribution of the traffic patterns an are likely to be small. C. Simulation Results Using ns-2 base simulation results, we now show that the improve SR results in increase network throughput. The raio parameters assume correspon to Lucent s WaveLAN. For evaluating the scheme, we use a test topology wherein we ranomly generate two pairs of noes with the same one-hop istance. Due to the assumption of wireless LAN settings, we confine this one-hop istance to less than 9 meters, the upper boun for the overactive RTS scenario. The locations of the pairs are generate ranomly so that either pair has a chance to be place in any zone of the other pair. UDP is use as the transport protocol for both the connections an each source is a CBraffic generator with a rate of 448Kbps. The generate topology is then simulate an the per-connection throughput is measure. This experiment is then repeate 2 times with ranomize placement of the two source estination pairs. From these 2 runs, the average per-connection throughput is compute for both A an the normal. The 2 ranom connections pairs generate for the case of one hop istance of 85 meters is shown in Figure 6. Fig. 7 plot the achieve per session throughput for ifferent one-hop istances. t can be seen from the figure that A achieves about 2% greater throughput that normal in the overactive RTS scenario. n Fig. 7, the throughput achieve by normal stays at a constant level an has little fluctuation, while for A, the throughput goes up slightly as we increase the one-hop istance. This matches our intuition that as Zone an Zone grow larger, A gains a better chance to enhance the spatial reuse, consequently improving the overall throughput. V. CONCLUSONS The performance of wireless networks in general an EEE base a hoc networks in particular, in terms of both the throughput an elay characteristics, epen on the spatial reuse characteristics. n this paper, we present a thorough analysis of the spatial reuse characteristics of base a hoc networks. We showe that epening on the one hop istances between the sener an the receiver, we can have three scenarios with ifferent spatial reuse characteristics. We also introuce a new metric, spatial reuse inex, an emonstrate its effectiveness in evaluating the spatial reuse. The paper also propose an improve virtual carrier sensing scheme for wireless LAN scenarios. This protocol is specifically esigne to enhance the spatial reuse in the overactive scenario. Through both analytic an simulation results, we show the effectiveness of our scheme in increasing the spatial reuse an also the network throughput. REFERENCES [1] ANS/EEE St 82.11: Wireless LAN Meium Access Control (MAC) an Physical Layer (PHY) Specification, [2] T. Rappaport, Wireless Communications: Principle an Practice, Prentice Hall, New Jersey, [3] P. Gupta an P. R. Kumar, The Capacity of Wireless Networks, EEE Transaction on nformation Theory, vol. 46, no. 2, pp , March 2. [4] M. Gastpar an M. Vetterli. On the capacity of wireless networks: The relay case, n Proc. NFOCOM, New York, NY, June 22. [5] Y. C. Tay an K. C. Chua, A Capacity Analysis for the EEE MAC Protocol, Wireless Networks, vol. 7, no. 2, pp , 21. [6] J. Li, C. Blake, D. Couto et. al., Capacity of A Hoc Wireless Networks, n Proc. ACM MobiCom 21, Rome, taly, July 21 [7] S. Xu an T. Saaawi, Does the EEE MAC Protocol Work Well in Multihop Wireless A Hoc Networks? EEE Communications Magazine, vol. 39, no. 6, pp , June 21. [8] K. Xu, M. Gerla, an S. Bae, How Effective is the EEE Hanshake in A Hoc Networks? n Proc. GLOBECOM 22, Taipei, November 22. [9] K. Fall an K. Varahan, Es., ns manual/ns Notes an Documentation, [1] CMU Monarch Group, CMU Monarch Extensions to ns,