Performance Evaluation of Multiple Access Protocols for Ad hoc Networks Using Directional A

Performance Evaluation of Multiple Access Protocols for Ad hoc Networks Using DirectionalAntennasTamer ElBatt,Timothy Anderson,Bo RyuInformation Sciences LaboratoryHRL Laboratories,LLC3011Malibu Canyon Rd,Malibu,CA90265,USA{telbatt,cellotim,ryu}@Abstract—In this paper,we introduce a novel reservation-based multiple access protocol for ad hoc networks using di-rectional antennas.First,we investigate the limitations of the extreme reservation schemes,namely omni-directional and directional reservations.We highlight the trade-offbe-tween spatial reuse(favors directional reservation)and con-trol/data packet collisions(favors omni-directional reserva-tion).Next,we show that the so-called hybrid reservation schemes fail to balance the trade-offas well.Therefore, we introduce a novel algorithm that balances the afore-mentioned trade-offvia sending reservation messages,that carry information about the required direction of transmis-sion,in all unblocked directions.In addition,we introduce candidate techniques for handling new types of collisions inherent to directional antennas.Finally,we conduct a sim-ulation study that shows considerable performance gains of the proposed scheme over the omni-directional,directional, and hybrid reservation paradigms.I.IntroductionThe broadcast nature of omni-directional antennas is one of the major causes of excessive multi-user interfer-ence,which limits the spatial reuse of the shared wireless medium.The distribution of energy in directions other than the direction of intended receiver not only causes con-siderable levels of interference,but also limits the trans-mission range.This constitutes a major hurdle towards ef-ficient utilization of the precious resources of wireless sys-tems,namely bandwidth and energy.Directional anten-nas has a fundamental impact on the design of CSMA/CA multiple access protocols.This can be seen from the an-swers to these questions:What information is necessary for a neighbor to decide whether to attempt reservation in a specific direction or not?Is it based on hearing/not hear-ing RTS/CTS messages as in the omni-directional case? We attempt to answer these questions in the paper. The cellular capacity improvement reported in[1],[2]is due to the following unique features of directional anten-nas.First,focusing the electromagnetic energy towards the intended receiver and minimizing it in the directions of unintended receivers.Second,focusing the energy in a specific direction increases the range of the transmitter compared to the case of radiating the same amount of en-ergy in all directions.Several MAC protocols for ad hoc networks using directional antennas have been proposed in the literature[3]-[10].The work in[3]introduces a perfor-mance analysis study that shows the impact of directional antennas on the performance of slotted aloha.In[4],the authors introduce a multiple access protocol that mini-mizes interference by using a group of tones to identify ac-tive neighbors.The reservation schemes in[5]-[8]attempt to modify the RTS/CTS handshake mechanism to exploit the interference reduction feature of directional antennas. These algorithms utilize combinations of omni-directional and directional reservation messages,and hence are clas-sified as hybrid schemes in this paper.However,they fail to balance the trade-offbetween spatial reuse and colli-sion rate as discussed in section III.In[10],very narrow beam-widths are assumed.This assumption may not be feasible in practice due to frequency band and data rate constraints.Our main contribution in this paper is two fold:i) Balance the trade-offbetween omni-directional and direc-tional reservations and ii)Resolve new type of collisions inherent to ad hoc networks using directional antennas. Unlike previous work,we believe that the focus should be shifted from sending reservation packets to a subset of neighbors(due to their geographical location)to send-ing reservation packets carrying”directional antennas in-formation”(rmation about the directional anten-nas used during reservation and data/ACK transmission phases)to as many neighbors as possible.This is essential to better assist them in knowing their locations relative to the transmitter-receiver pair and hence decide whether to transmit on a specific beam or not.The paper is organized as follows:in section II we in-troduce the system model.Next,we illustrate the trade-offvia showing the limitations of the omni-directional,di-rectional and hybrid reservation paradigms in section III. This is followed by a description of the proposed reserva-tion scheme in section IV.Afterwards,simulation results are presented in section V.Finally,the conclusions are drawn in section VI.II.System ModelA.Antenna ModelIn this paper,we limit our attention to the class of switched beam antennas that consist of several highly di-rective,fixed,pre-defined beams,formed usually with ar-rays[11].Considering more complex and expensive steer-able antennas that support mobile tracking and direction of arrival estimation is out of the scope of this paper and is a subject of future research.We assume that each node is supported by afixed number(B)of switched beams, each of widthθ=2π/B radians.We use an idealized model for transmission on a specific beam.We assume that the transmitted energy is distributed uniformly in a beam of widthθ(thus we ignore the possibility of side lobe interference).In addition,we assume that nodes re-ceive omni-directionally,i.e.signals received by all beams are combined via diversity combining techniques[11],in order to combat multi-path fading and scattering effects. We assume that beams are non-overlapping.Candidate techniques for relaxing this assumption are discussed in section V.work AssumptionsWe consider an ad hoc network consisting of n nodes that communicate only via the wireless medium.In this study,we focus on the performance of MAC algorithms, hence,routing is not considered.Each node generates data packets offixed length according to a Poisson arrival process with rateλpackets/sec.Each generated packet is intended for a single neighbor only according to a uni-form distribution.Each node has a buffer for temporarily storing generated packets awaiting transmission.The size of each buffer is assumed to be arbitrarily large since our main focus in this paper is throughput rather than queuing delays.We assume that a node lying within the coverage of a neighbor’s directional data/ACK transmission,is not allowed to engage in any communications.This is essential to avoid the intolerable interference levels this node may suffer due to the omni-directional reception assumption. On the other hand,nodes lying outside this region may transmit on any directional beam except the one point-ing towards the transmitter.Finally,we assume that each node has the following information via neighbor discovery (ND)schemes:i)The identities of all neighbors and ii) The identities of neighbors that lie within the coverage of each beam.Introducing ND algorithms that efficiently utilize the range extension feature of directional antennas is out of the scope of this paper and is a subject of future research.III.Channel Reservation ParadigmsIn this section,we motivate the work via illustrating the trade-offbetween omni-directional and directional reser-vations.In addition,we show that hybrid reservation schemes proposed earlier in the literature fail to balance the trade-off.This is attributed to the fact that all three reservation paradigms rely on the mere event of hear-ing/not hearing a reservation message,in order to decide whether to proceed with a transmission in a specific di-rection or not.Although this criterion for deciding chan-nel occupancy is efficient for omni-directional antennas,it turns out to be problematic for directional antennas.In case of directional antennas,a node that does not hear a directional reservation message of another node may still be a neighbor of that node and could cause collisions.In the case of omni-directional antennas,a node that does not hear a reservation message of another node is not a neighbor and,hence,is not expected to cause any threat to the ongoing transmission.We denote the set of nodes within the geographical area covered by the radiation patterns of all beams at node x as neighbors N(x).This set can be partitioned into two sub-sets,namely blocked neighbors and unblocked neighbors, depending on their awareness of the ongoing reservation process carried out by node x.A neighbor is said to be blocked if and only if at least one of its beams is blocked from transmission due to overhearing a reservation mes-sage.On the other hand,a neighbor is said to be un-blocked if it does not hear any reservation message,and hence,is completely unaware of the ongoing reservation attempt.Unblocked neighbors could initiate a reserva-tion request independently and hence cause collisions to the ongoing transmission.We denote the sets of blocked and unblocked neighbors as BN(x)and UBN(x)respec-tively.The objective is to optimally partition N(x)into blocked and unblocked groups depending on:i)Their lo-cation with respect to the transmitter,ii)Their required direction of transmission and iii)The direction to be used by the transmitter for sending data or ing this framework,we discuss the limitations of the three reser-vation paradigms.A.Omni-directional ReservationIn the communication scenario shown in Figure1,we consider the conventional CSMA/CA protocol where the reservation messages are exchanged omni-directionally(O-RTS/O-CTS)between S and D.It is straightforward to notice that the neighbor sets of nodes S and D are given by N(S)={1,2,3,4,5,6,D}and N(D)={1,4,5,6,7,8,9,S}re-spectively.Furthermore,BN(S)=N(S),UBN(S)=φ, BN(D)=N(D)and UBN(D)=φ.This is due to the fact that the reservation packets are sent in all directions and hence all neighbors become aware of the ongoing trans-mission and refrain from transmission.Although omni-directional reservations minimize the possibility of colli-sions,they turn out to be too conservative due to the possibility of blocking neighbors unnecessarily.B.Directional ReservationIn this section,we examine the other reservation ex-treme where reservation messages are exchanged betweenFig.1.Omni-directionalReservation Fig.2.Directional ReservationS and D in a directional manner (D-RTS/D-CTS)[7],[8]as illustrated in Figure 2.In this case,we do not consider the range extension feature of directional antennas,i.e.the set of neighbors reached omni-directionally is assumed to be equal to the union of the sets of neighbors reached by each beam individually.This is mainly due to the as-sumption that omni-directional transmissions are achieved by transmitting on all beams simultaneously.In this case,it is straightforward to notice that BN(S)={4,5,6,D },UBN(S)={1,2,3},BN(D)={4,5,S },UBN(D)={1,6,7,8,9}.Clearly,this scheme is more aggressive since it initiates more simultaneous reservation attempts,yet,they are un-aware of each other and are highly subject to collisions.Thus,there is a fundamental trade-off,between collisions caused by unblocked neighbors and the number of un-necessarily blocked neighbors which limits spatial reuse.Thus,a neighbor knowing about an ongoing transmission may degrade performance (e.g.node 2sending to node 10,or node 8sending to 11),while another neighbor not know-ing about an ongoing transmission could degrade perfor-mance as well (e.g.node 2sending to node S,or node 6sending to D).C.Hybrid ReservationThe reservation schemes proposed in [5],[7]can be clas-sified as hybrid schemes.The common aspect among those schemes is that they use different combinations ofomni-Fig.3.Hybrid Reservationdirectional and directional reservation messages.How-ever,none of them attempt to balance the above trade-off.In this paper,we consider the reservation scheme in [5]as an example of hybrid schemes.It utilizes D-RTS and O-RTS packets,along with O-CTS packets.Ac-cording to Figure 3,BN(S)={4,5,6,D },UBN(S)={1,2,3},BN(D)={1,4,5,6,7,8,9,S },UBN(D)=φ.Clearly,the pro-posed RTS solution does not strike a balance between min-imizing control packet collisions and number of neighbors that back-offunnecessarily.Furthermore,sending omni-directional reservation messages (whether O-RTS or O-CTS)to all neighbors,without distinguishing neighbors that lie within the coverage of the directional data/ACK transmission,may lead to further collisions.Therefore,we introduce a new concept that is motivated by the follow-ing key observations:1.All neighbors of the source and the destinationshould be aware of the intended transmission,if pos-sible.2.Antenna blocking decisions should be based on the information included in the RTS/CTS packets,not on the mere event of hearing/not hearing a reserva-tion packet.IV.Channel Reservation based on DirectionalAntennas Information In this section,we attempt to balance the aforemen-tioned trade-offin three steps.First,we propose to send the RTS/CTS packets over all unblocked beams,even of one or more beams are blocked.This incorporates some aggressiveness to the reservation scheme.In addi-tion,it notifies all neighbors,covered by unblocked beams,of the ongoing reservation.Second,we propose to add two fields,to the RTS/CTS messages,that carry the in-dex of the directional beam currently being used dur-ing the channel reservation phase and the index of the directional beam intended to be used during data/ACK transmission.This directional information is critical for each neighbor to know its relative location with respect to the source-destination pair and hence take appropriate antenna blocking decisions accordingly.The prime mo-tivation for adding the previousfields is to decouple the event of hearing a reservation packet from the decision of beam blocking.The decision to transmit on a specific beam,or not,depends solely on the required direction and the information included in the reservation packets. According to the proposed scheme,node S sends differ-ent RTS packets on all unblocked beams to inform neigh-bors that it wishes to initiate a transmission with node D. Thus,in Figure1,node5would know that it lies within the intended direction of data transmission from S to D and therefore should not engage in any communications in order to avoid interference caused by S.On the other hand,node2would know that it is out of the intended direction of data transmission from S to D,and hence it blocks only the beam pointing towards node S.Moreover, node2would know that it can initiate a transmission with node10since it would use a different beam from the one pointing towards S,and thus it will not cause any inter-ference to S and will not suffer any interference from S.In this case,the proposed scheme decides that node2belongs to the set UBN(S).Otherwise,if node2wishes to transmit towards node S,it should belong to the set BN(S). Finally,the following new type of collisions inherent to directional antennas remains as a challenge to the pro-posed scheme:a neighbor may miss reservation messages due to lying in the coverage of a blocked beam.This neighbor,who may be active or inactive,would be left unaware of the attempted reservation and may cause col-lisions later.One way to circumvent this problem is to use auxiliary channel(s)to transmit special reservation pack-ets on blocked beams.Under such proposition,blocked beams transmit special RTS/CTS packets,on a differ-ent frequency,whereas regular RTS/CTS packets are still transmitted on the unblocked beams at the same time. This scheme eliminates the above type of collisions caused by inactive neighbors lying in the coverage of the blocked beam.This is gained at the expense of using more than one channel,adding more complexity to the radio trans-ceivers,and the possibility of having relatively rare colli-sions among the special reservation packets on the aux-iliary channel.However,the threat of suffering collisions from active users,after completing their ongoing trans-mission,still prevails.Therefore,we propose to send ”pending”RTS/CTS packets once the blocked beam be-comes unblocked.The transmission from source to desti-nation could be temporarily halted,such that the source can transmit the pending packet on the beam recently unblocked.Once the packet is sent,the source node re-sumes its original data transmission on the beam pointing towards the destination.This approach is only feasible due to the short time needed to send a control packet, which can be tolerated by the longer data packet trans-mission.However,it involves synchronization complexity due to the possibility of having multiple pending reser-vation packets.Another variation of this approach ex-ploits the underlying radio’s ability to transmit different packets on multiple beams at the same time.Therefore, the pending packet can be sent on the recently unblocked beam while at the same time continue sending data on the beam pointing towards the destination.In this paper, we employ the auxiliary channel technique for handling inactive neighbors and pending packets for handling ac-tive neighbors.Clearly,this problem involves a trade-offbetween protocol complexity and control overhead in one hand and throughput on the other hand.Finally,a third approach,that is undergoing research,involves adapting the aggressiveness of the proposed reservation scheme de-pending on the number of blocked beams per node.V.Results and DiscussionIn this section,we present performance results obtained using the ns-2simulator.The results show the perfor-mance gains of the proposed scheme over the omni-direc-tional,directional,and hybrid reservation schemes under a wide variety of network loads.In addition,we show the impact of varying the number of beams on performance.A.Simulation SetupIn this paper,we consider small networks consisting of n=50nodes since it sufficiently captures the trade-offunder investigation.We consider a rectangular area of di-mensions500meters x500meters where the nodes are uniformly distributed in the area.Data packet size is as-sumed to be500bytes.In this study,we do not con-sider nodes’mobility since the arguments made in this paper are independent of mobility.We believe that the ad-dressed trade-offremains valid under mobility conditions. Throughout the simulations,omni-directional transmis-sions are achieved via using all directional beams.There-fore,the range of omni-directional transmissions(250me-ters)is assumed to be equal to the range of directional transmissions.We assume B=6switched beams per node,each of60o width.In order to simplify simulations, we abstract any aspect of ad hoc networks that is irrel-evant to the main focus of the study.Thus,we do not simulate specific ND algorithms,we compute its output based on the geographical location of neighbors instead. Each simulation run is carried out for the duration of900 sec.B.Simulation ResultsThe performance metric used to compare the perfor-mance of the proposed algorithm to the aforementioned reservation paradigms is the multiple access throughput. It is defined as the long-run average number of bits that reach their respective neighbors successfully per second. This ratio is to be computed at the end of each simula-tion run.First,we compare the long-run average num-ber of data packets transmitted per second under the four schemes.The importance of this experiment stems from the fact that this parameter reflects the”aggressiveness”of the algorithms.It can be easily noticed from Figure4that classical CSMA/CA is the most conservative since it has least attempts of sending data packets.This is a di-rect consequence of unnecessarily blocking neighbors from transmission.On the contrary,the directional reservation scheme is the most aggressive since it sends reservation packets in the intended direction only.This,in turn,leads to initiating excessive number of data transmissions that are subject to frequent collisions with reservation packets transmitted by unaware neighbors.The other two algo-rithms lie in between.Our algorithm is more aggressive than the hybrid algorithm proposed in[5].This is at-tributed to the fact that the proposed algorithm sends reservation packets over all unblocked beams,even if one or more beams are blocked.In addition,it provides the sufficient information for each neighbor to take the appro-priate antenna blocking decision.This,in turn,enables our algorithm to achieve a balance between the two ex-tremes.Next,we compare the long-run average number of data collisions per second under the four schemes as shown in Figure5.This result reflects the price paid for the aggres-siveness.It is straightforward to notice that aggressive algorithms(directional reservation)suffer more collisions than conservative algorithms(plain CSMA/CA).Again, the number of data collisions caused by the other two algo-rithms lie in between.Moreover,the proposed algorithm suffers more collision than the hybrid scheme due to its aggressiveness.However,the excessive number of colli-sions is compensated by the significantly higher number of data packets transmitted as shown later.The interac-tion between these two conflicting requirements is what determines the net MAC throughput.Figure6shows the MAC throughput achieved by the four schemes under in-vestigation.First,we notice that despite the aggressive-ness of directional reservation,it outperforms the omni-directional scheme by a factor of approximately22%at heavy loads.This result implies that aggressiveness is a desirable feature of MAC protocols for directional an-tennas.Second,the hybrid scheme is noticed to achieve an intermediate performance since it is a combination of both schemes.Third,our algorithm gives the best per-formance since it guarantees low levels of data collisions (less than half the data collisions caused by the directional reservation scheme),while being relatively aggressive(al-most twice the number of data packets transmitted by the omni-directional scheme).It is worth noting that using an auxiliary channel was the main contributor to reduc-ing collisions and,hence,improving performance.Trans-mitting pending reservation packets,as described in the previous section,had negligible impact on performance. This is attributed to the fact that the”auxiliary channel”protects against multiple inactive neighbors which could cause collisions at any time.On the other hand,”pending packets”protect against small number of active neigh-bors(typically one)which may cause collisions only when it completes the ongoing transmission.Finally,we noticeB=6,Av.number of neighbors=13.84)that as the offered load increases the MAC throughput in-creases for all reservation schemes.The proposed scheme outperforms the omni-directional scheme by a factor of 64%at heavy loads and decreases to about40%at mod-erate loads.Furthermore,it outperforms the directional scheme by a factor of33%at heavy loads and25%at mod-erate loads.Finally,it outperforms the hybrid scheme by a factor of52%at heavy loads and decreases to35%at moderate loads.Thus,we conclude that the proposed reservation scheme achieves considerable performance im-provement over a wide range of network loads.At light loads,the four schemes achieve almost the same perfor-mance due to the low number of simultaneous transmis-sions attempted,on the average,which rarely causes col-lisions.Finally,the MAC throughput is plotted versus the num-ber of beams per node(B),as shown in Figure7.In this experiment,the parameter B takes the values1,2,4,6,8, where the beam-width is given by2π/B radians.The sig-nificance of this experiment is to compare various reser-vation schemes as the directivity of the beams increases. First,note that the MAC throughput of plain CSMA/CA remains constant for all values of B since it is designed for omni-directional antennas.Second,the MAC through-put of the other three algorithms increase,as the number of beams increases,due to higher interference reduction. Furthermore,the proposed algorithm is shown to outper-form the other two schemes for all values of B.Finally,we note that the performance improvement becomes negligi-ble for more than B=6beams/node which suggests that the performance gain achieved by highly directional beams could be outweighed by the design complexity and high cost associated with developing such antennas.In this paper,we adopted the assumption of non-overlapping beams.However,real switched beam an-tennas experience some overlapping due to coverage con-straints.This,in turn,may degrade the performance of the proposed reservation scheme since it involves trans-mitting different packets over multiple beams at the samesignificantly between non-neighboring beams.=13.84)VI.ConclusionsIn this paper,we introduce a novel CSMA/CA-based multiple access protocol that improves the throughput of wireless ad hoc networks using switched beam anten-nas.First,We showed the limitations of omni-directional,directional,and hybrid reservation schemes proposed in the literature.Our contribution in this paper is three fold.First,incorporating aggressiveness in the reservation scheme.Second,modifying the channel occupancy crite-rion to depend on information carried by the RTS/CTS messages,rather than their mere reception as in IEEE 802.11.Thus,we proposed to include information aboutof neighbors =13.84)ReferencesJ.Ward and pton,”Improving the Performance of a Slotted ALOHA Packet Radio Network with an Adaptive Ar-ray,”IEEE Transactions on Communications ,vol.40,no.2,pp.292-300,1992.F.Shad,T.Todd,V.Kezys and J.Litva,”Dynamic Slot Allo-cation (DSA)in Indoor SDMA/TDMA Using a Smart Antenna Basestation,”IEEE/ACM Transactions on Networking ,vol.9,no.1,pp.69-81,2001.J.Zander,”Slotted ALOHA multihop packet radio networkswith directional antennas,”IEEE Electronics Letters ,vol.26,no.25,1990.T-S Yum and K-W Hung,”Design algorithms for multi-hoppacket radio networks with multiple directional antenna sys-tems,”IEEE Transactions on Communications ,vol.40,no.11,pp.1716-1724,1992.Y.Ko,V.Shankarkumar and N.Vaidya,”Medium AccessControl Protocols Using Directional Antennas in Ad Hoc Net-works,”IEEE INFOCOM ,April 2000.[6] A.Nasipuri,S.Ye,J.You and R.Hiromoto,”A MAC Proto-col for Mobile Ad Hoc Networks Using Directional Antennas,”IEEE WCNC ,2000.[7]M.Sanchez,T.Giles and J.Zander,”CSMA/CA with BeamForming Antennas in Multi-hop Packet Radio,”Swedish Work-shop on Wireless Ad hoc Networks ,March 2001.[8]R.Ramanathan,”On the Performance of Ad Hoc Networkswith Beamforming Antennas,”ACM Mobihoc ,Oct.2001.[9]M.Takai,J.Martin,R.Bagrodia,and A.Ren,”DirectionalVirtual Carrier Sensing for Directional Antennas in Mobile Ad Hoc Networks,”ACM Mobihoc ,June 2002.[10] C.Balanis,J.Aberle,J.Capone,T.Duman,S.El-Ghazaly,A.Spanias and T.Thornton,”Smart Antennas for Future Re-configurable Wireless Communication Networks,”Arizona State University Technical Report ,April 2000.[11]R.Janaswamy,”Radiowave Propagation and Smart Antennasfor Wireless Communications”,Kluwer Academic Publishers ,Boston,2001.。