Energy Consumption Behavior and Performance of ... - CiteSeerX

Energy Consumption Behavior and Performance of Directional Virtual Carrier Sensing Schemes Chavalit Srisathapornphat

Chien-Chung Shen

Department of Computer and Information Sciences University of Delaware

Abstract— Energy drainage from a node is mainly caused by the power consumption of the node’s communication device. A MAC protocol is the main mechanism to provide efficient access to the shared wireless channel. Directional antennas provide several benefits over omnidirectional ones including the reduced energy consumption in frame transmissions. In this paper, we study and compare different generic directional virtual carrier sensing schemes (i.e., directional-RTS/CTS) in terms of their energy efficiency and performance. Both analytical and simulation results show that the schemes using directional-RTS and/or directional-CTS are more energy efficient than other schemes.

I. I NTRODUCTION Ad hoc networks, where mobile nodes autonomously establish connectivity via multihop wireless communications, are constrained by the energy supply from the internal batteries of the nodes. Moreover, nodes in the network need to share the wireless communication channel with other nodes located in the same vicinity. Due to the lack of centralized coordination, medium access control is one critical component that affects the performance and the energy consumption behavior of the nodes. Recently, several MAC protocols [1], [2], [3], [4], [5] have been proposed to take advantage of the many benefits of directional antennas. For instance, directional antennas can reduce interference and overhearing of messages by non-target nodes. Directional antennas can also improve the quality of wireless communication for the higher gain and longer communication range in a specific direction [6]. In terms of energy consumption, fewer overheard messages result in lower power drainage from the nodes. In addition, directional transmissions require less transmitting power than omnidirectional transmissions to reach nodes at the same distance. The objective of this paper is to study the performance and energy efficiency of directional MAC protocols in unicast packet transmission when different RTS/CTS (request-to-send/clear-to-send) exchange schemes are employed to perform virtual carrier sensing. The remainder of the paper is organized as follows. Section II describes the directional antenna model and the energy dissipation model of directional-antenna-based communication devices. Section III briefly describes possible alternatives of utilizing directional antennas in unicast packet transmissions, their considerations, and recently proposed MAC protocols using directional antenna. Analysis of probability of successful transmission for different directional virtual carrier sensing schemes is presented in Section IV. In Section V, we present the simulation results of evaluating the energy consumption behavior and performance of MAC protocols with different RTS/CTS schemes using direcPrepared through collaborative participation in the Collaborative Technology Alliance for Communications & Networks sponsored by the U.S. Army Research Laboratory under Cooperative Agreement DAAD19-01-2-0011.

tional antennas. Conclusion and future work are discussed in the last section. II. P RELIMINARIES A. Directional Antenna Model The directional MAC schemes considered in this study are based on the switched beam antenna model [7], [8]. We assume a perfect radiation model in which a directional antenna is capable of generating and transmitting radio signals in non-overlapped sectors. Each sector covers a two dimensional area with an angle span of radians. Nodes are able to transmit through one or all sectors at one time; thus, creating a unicast or broadcast communication, respectively. In the receiving mode, all sectors of the antenna are used to detect incoming radio signals, but only the sector which detects the strongest signal will be locked to receive the desired signal. Therefore, the direction of the sender can be determined by the sector that receives the strongest signal. We also assume that in the receiving mode, directional antennas have the ability to lock on to a signal being received [4], which protects the receiving signal from other interference and reduce the chance of packet drop due to interference in the reception process. Although a directional antenna consists of multiple antenna elements, our model assumes that a node has only one transceiver unit. This forbids concurrent transmissions and receptions of different signals. In the case of mobile nodes, they are assumed to have the capability to maintain a fixed orientation of their antennas at all times, regardless of their own orientation. B. Energy Dissipation Model To compare the energy consumption of different RTS/CTS exchange schemes, our energy dissipation model for omnidirectional communications is based on the WaveLAN PC Card specification [9], which specifies the power consumption rate of 900 and 1400 mWHr for the receiving and transmitting modes, respectively. However, we do not use the fixed power consumption rate for omnidirectional transmissions, but calculate the amount of energy consumed, , using the following equation ! #"%$'&)(+*-,

(1)

where is the transmission power coefficient, & is the constant power offset, . is the power used in transmitting the signal, and * is the transmission time. To determine the amount of energy consumed for directional transmissions, we assume the same power consumption rate as in the case of an omnidirectional transmission. However, the power consumption rate of transmitting into one particular sector using a directional antenna is assumed to be /0 of the power consumption rate in the transmitting mode of an omnidirectional

antenna, while maintaining the same transmission range. Therefore, the energy consumed by directional transmissions, 1 2-3 , is obtained from the following equation: 2-3 456!

"%$7&)(+*-8

(2)

III. D IRECTIONAL MAC SCHEMES Most omnidirectional-antenna-based MAC protocols for ad hoc networks rely on the RTS/CTS handshake to reserve the channel before transmitting a unicast packet. For example, in IEEE 802.11, a node with a buffered data frame first broadcasts a RTS frame. The intended receiver replies by sending a CTS frame to the sender of the RTS, if it is ready to receive the announced data. Other nodes, upon hearing the RTS or CTS being targeted elsewhere (not to themselves), refrain from transmitting that could interfere with the announced communication. To improve the reliability, IEEE 802.11 MAC protocol requires a receiver to acknowledge the successful data frame reception by sending an ACK to the sender. With directional antennas, RTS/CTS schemes can be performed in one of the following four configurations: o-RTS/oCTS, d-RTS/o-CTS, o-RTS/d-CTS, and d-RTS/d-CTS, where the prefixes 9 - and : - refer to directional and omnidirectional, respectively. Figure 1 illustrates these four possible schemes. The use of a d-RTS frame can relieve the exposed terminal problems and allows better channel reuse. Similarly, with the directional antenna’s capability of locking on to only a receiving signal, the use of d-CTS reduces the channel reservation area and allow more nodes (previously considered as hidden nodes when using o-CTS) to transmit. Therefore, the performance improvement in a MAC scheme with d-RTS and/or d-CTS is expected. However, one disadvantage of using d-RTS, as also pointed out in [2], is the increased collision probability of control frames. Although this probability is small due to the fact that the size of most control frames is small compared to the size of data frames, it is still higher than when an o-RTS is exploited. Several MAC protocols using directional antennas have been developed based on the mentioned general schemes with various improvements. For instance, [4] studied directional virtual carrier sensing in ad hoc networks but focused only on the case of directional-RTS/directional-CTS. The proposed scheme provides interoperability between nodes equipped with directional antennas and nodes equipped with only omnidirectional antennas. Its simulation model considered the capability of locking on to a signal being received (beam locking), AOA (angle of arrival) determination, and DNAV (directional NAV). [2] introduced a scheme to use d-RTS with o-CTS to improve the performance of the network. The scheme relies on neighbors’ location information obtained prior to the transmission of a RTS. Since this work did not assume the beam locking capability, to reduce the collision probability of control frames, a companion scheme is proposed to use o-RTS whenever possible. [10] considered similar directional antenna schemes as in [2]. However, the schemes do not require explicit knowledge of neighbors’ location information. Instead, it uses o-RTS, AOA determination capability, and AOA caching to determine the direction of the communication party on-demand. Other differences from [2] are that [10] transmits CTS directionally and does not rely on acknowledgments to confirm successful reception of data packets. In [11], another variation of directional MAC protocol is presented. The proposed protocol relies on the o-RTS/CTS to search

for the direction of the parties and the transmission power adjustment to control the transmission power to meet target SNR (signal-to-noise ratio). It requires two different NAV intervals: SHORT NAV for protecting the following CTS frame and a regular NAV for protecting the data and ACK frames. The protocol also assumes the beam locking capability to protect the data reception. The main objective of this paper is to evaluate the network performance and energy efficiency of the various directional virtual carrier sensing schemes mentioned above. IV. A NALYSIS ON PROBABILITIES

OF SUCCESSFUL

TRANSMISSION

In this section, we perform an analysis on the probabilities of successful transmission of unicast data packets for the four RTS/CTS exchange schemes introduced in Section III. We assume that all nodes ;=< are equipped with -sector directional antennas with . However, for the ease of analysis, we assume all directional antennas are steerable, in which both senders and receivers are able to align their directional antennas directly toward each other before each transmission. Their transmission ranges are the same for both directional and omnidirectional transmissions, which is > . Nodes are uniformly distributed over a terrain with the density of ? . All nodes have the same traffic model in which all data packets are of the same length. Each packet requires a transmission time @ , and is randomly destined to a local neighboring node. Data packets are generated every time interval A , where @CB A . For simplicity, we also assume that RTS and CTS frame size is very short compared to that of data frames, and the nodes either transmit a successful RTS which is always followed by a CTS reply from its receiver, or not transmit any RTS at all. In this study, the receive threshold and the sensing threshold of a node are assumed to be the same. In unicast transmission, data transmission occurs after the successful exchange of RTS/CTS frames between a sender and a receiver. Therefore, in a static network or a dynamic network where the transmission interval of a packet is considered very short compared to the topology changing rate, we may assume that data packet transmissions are always successful if they follow successful RTS/CTS exchanges. Based on these assumptions, we can approximate the probabilities of successful transmission of data packets for different RTS/CTS exchange schemes as follows. A. o-RTS/o-CTS In this case, all RTS/CTS frames are transmitted omnidirectionally as depicted in Figure 1 (a). For any node D to transmit a RTS or to send a CTS reply, D must not be within other nodes’ RTS and CTS coverage. Let E represent the situation when none of the nodes within D ’s transmission range (or Area R as depicted in Figure 2 (a)) transmits, F represent the situation when none of the nodes inside Area 2R whose receivers’ CTS covers D (as depicted in Figure 2 (b)) transmits, and G represent the situation when the channel will be free for D (i.e. D does not detect any RTS or CTS from its neighbors). The probability that D can transmit without interfering with other nodes’ transmission can be defined as . ) GH"IJK 6EL"KMNK 6FO"-,

(3)

U

V

U

(a)

V

U

(b)

V

U

(c)

V

(d)

Fig. 1. Four possible schemes of deploying RTS/CTS using directional antennas: (a) o-RTS/o-CTS, (b) o-RTS/d-CTS, (c) d-RTS/o-CTS, and (d) d-RTS/d-CTS, where U is the sender who transmits a RTS, V is the receiver who replies with a CTS, solid lines indicate areas covered by corresponding transmissions, and dashed lines indicate the areas that would be covered if omnidirectional transmissions are used.

where subscript oo represents o-RTS/o-CTS 1 . According to the traffic model and the assumption of nodes distribution, P 6EL" or the probability when none of the nodes within Area R transmits is @ (4) K ) 6EL"QSRT/VU 8 AXWYZ[ \ Let ] be the situation when a node inside Area 2R is transmitting and its receiver is in Area R, then

6a " ya 9a8 [ b %>1exw < 1 > e{z

(7)

Based on the traffic model, [the probability that none of the nodes inside Area 2R whose receivers’ CTS covers D will transmit can be obtained by @ A

. ) ^]t"~

e

Y Z# [ d\

" Z[ \

8

(8)

Therefore, the probability that the channel will be free and D can communicate, ) 6GH" , can be obtained from Equation (4) and (8). B. o-RTS/d-CTS In this case, the area covered by a virtual carrier sensing from an o-RTS/d-CTS exchange is always equal to the transmission coverage of the o-RTS from a sender node as illustrated in Figure 1 (b). Therefore, the probability that the channel will be free for any node D to communicate, represented by )2 6GH" , is equal to the probability that all nodes within the coverage area of D do not transmit, which is determined by )2 6GH"Q

R /VU

@ 8 AXW YZ[ \

Subscript xy represents x-RTS/y-CTS, where directional) or (directional).

S

Area_R

V

(5)

where > is the transmission range and >pqarps> . By unconditioning a , we obtain

K 6FO"I}|^/VU

u V

Area_R

where da " is the unsafe area or the area that a CTS from any b nodes inside this area can interfere with D (equivalent to the area illustrated in Figure 2 (b)). da" is a function of the distance beb b tween D and a node whose receiver can interfere with D (equivalent to the distance a between D and f in Figure 2 (b)) and can be calculated by m a a mn da"QJg6> eihkj !!:l U > e Uoa e "!, (6) b >

e

S

Area_2R

d a" . ) ^]`_ a"Pcb , %>1e

]t"Qvu

x D

(9)

and can be either (omni-

(a)

(b)

Fig. 2. (a) All transmissions within Area R interfere with . (b) The transmission from , located in Area 2R, will interfere with only if its receiver is in Area R and the area is the unsafe area for .

C. d-RTS/o-CTS In this case, a node D can only transmit when none of the nodes within its transmission range are receiving since receivers always response to a RTS with an o-CTS. Let E be a situation when a receiver is within the transmission range of D , then K2- 6EO_ a"P=b

6a " , %>1e

(10)

where da" is the unsafe area obtained in the similar manner as b (6), a is the distance between D and a node whose reEquation ceiver can interfere with D , and Opapq> . By unconditioning a , we obtain e da " a 9a8 2- 6E"Pvu [ b R %> e > e W

(11)

Let F represent the situation when a node does not communicate with any receiver within D ’s transmission range. Given the traffic model described previously, we have 2- 6FO"Q/VU A

@

2- E"-8

(12)

Therefore, the channel will be free for any node D to communicate when none of the nodes within D ’s transmission range is receiving and its probability, represented by 2- 6GH" , is defined as e 8 (13) 2- 6GH"QJ 2- 6F" YZ# [ d\ D. d-RTS/d-CTS In this case, a node D is not able to communicate if either there is another node whose d-RTS covers D or the other node’s d-RTS does not cover D but its corresponding d-CTS covers D .

2-2 6EL"Q

@ A

8

(14)

Then, the probability that a node within the transmission range of D transmits a d-RTS but the d-RTS does not cover D , Q2-2k6FO" , is @% U7/0" 8 .2-2kF"I (15) A However, a node that transmits a d-RTS which does not directly cover D may have its receiver whose d-CTS interferes with D . Let ] be the situation when a node, within the transmission range of D , whose receiver’s d-CTS interferes with D , then 2-2 ^]`_ a"(

da" b , > e %

(16)

where da " is the unsafe area obtained in the similar manner as b (6), a is the distance between D and a node whose reEquation ceiver can interfere with D , and pJaopJ> . By unconditioning a , we obtain K2-2^]t"Q

6a " u [ b %> e

ya 9Ta8 w > e z

(17)

Therefore, the channel will be free for any node D to communicate with the probability, 2-2 GH" , which can be obtained by 2-2 6GH"I/VUq 2-2 E".$J 2-2 6FO"KMi 2-2 ]t""(T8

(18)

Figure 3 depicts the relationship between the probability of successful transmission and the number of nodes in the network of size 1000 1000 m e obtained from the analysis of all the possible four RTS/CTS exchange schemes, where > m, m (for d-RTS/d-CTS @4{8 {/ second, A/ second, and scheme, an additional result using is also presented). Notice that for two nodes to communicate, a successful RTS/CTS exchange must be accomplished. Therefore, the probability of successful transmission initiated by any node D is defined as O successful transmission ¡Ls`6GH" e 8

(19)

The results show that the use of directional antennas increases the chance of successful unicast packet transmission. The benefit is the highest when both RTS and CTS are directionally transmitted. Moreover, the smaller beam width (higher ) gives better result than the larger one. The most important reason of the benefit from directional antennas is the reduction of the area covered by d-RTS and/or d-CTS. However, the results show that d-RTS/oCTS has better communication probability than o-RTS/d-CTS. V. P ERFORMANCE

EVALUATION

In this section, we describe the simulation study to validate the analytical results of the successful transmission probability and the prediction of the energy consumption behavior of the four RTS/CTS exchange schemes presented in Section IV. We also conduct further simulation experiments for multi-hop traffic to study the behavioral differences in a more realistic situation.

1 Probability of successful transmission

Let E represent the situation when a node transmits a d-RTS into the direction that covers D . Given the traffic and directional antenna model, we have

o-RTS/o-CTS o-RTS/d-CTS d-RTS/o-CTS d-RTS/d-CTS (M=8) d-RTS/d-CTS (M=4)

0.8

0.6

0.4

0.2

0 0

200

400 600 Number of nodes

800

1000

Fig. 3. Probability of successful unicast data packet transmission at various node density using different RTS/CTS schemes obtained from the model

A. Simulation Environment Setup and Metric Simulations are conducted using QualNet 3.1 [12]. The radio model is modified to support directional transmission and reception using the switched beam antenna model with 8 perfect (non-overlapping) sectors ( ). The radio interface module transmits omnidirectionally with the power ( ) of 7.87 dBm at the frequency of 2.4 GHz, and the shared bandwidth of 2 Mb/s. The propagation path loss model is the two-ray model. The sensing and receiving thresholds are set to be the same at -81 dBm so that the results can be compared with the results from the analysis in Section IV. The reception model is BER-based. This radio setting is equivalent to the transmission range of approximately 250 m. The simulation time for all experiments is 120 seconds and the results are the average of 10 simulation runs. To compare the performance and energy consumption behavior of the four directional RTS/CTS exchange schemes, the following metrics are considered. First, throughput and delay are used to capture the ability to resolve communication conflicts when nodes communicate using CBR (constant bit rate) traffic. Hop-by-hop throughput and delay will be measured in the case of local traffic experiment (in Subsection V-B), while end-to-end throughput and delay will be measured in the case of multi-hop traffic experiment (in Subsection V-C). Finally, we calculate the number of packets successfully received per unit of energy (Joule) consumed by the entire network in order to show the efficiency of each scheme in utilizing node’s energy. In all experiments, the amount of energy consumed is calculated based on the energy dissipation model described in subsection II-B, where the transmitting mode consumption is based on the Equation (1) and (2), with 1c/N¢ and &L4 , which is the power consumption rate in the receiving mode. [ B. One-hop traffic experiment and discussion To verify the results of successful communication probabilities obtained in Section IV, we set up a 1000 1000 m e simulated network consisting of 100 to 400 static ad hoc nodes, which are equivalent to the density of 19.63 to 78.54 nodes per transmission coverage area. Each node generates 512 byte CBR packets with the mean arrival rate of 10 packet per second. The data packets are randomly targeted to one of its local neighbors. We assume that nodes are able to obtain information about their local neighbors through some mechanisms, such as GPS or beacon message exchanges. Figure 4 (a) shows the overall throughput of the entire network obtained from the four different RTS/CTS exchange schemes as a

function of number of nodes. We observed that the d-RTS/d-CTS scheme is the most scalable scheme. Its throughput increases with the highest rate among all schemes when number of nodes or node density increases. This is the result of (i) the considerable decrease of the area suppressed by virtual carrier sensing, and (ii) the capability of locking on to only a signal being received when directional antennas are used in a reception mode. The one-hop delay depicted in Figure 4 (b), which shows that the d-RTS/dCTS scheme has the shortest delay, also confirms the results. However, there is a significant performance difference between the schemes that utilize directional antennas only on either d-RTS or d-CTS. As shown in Figures 4 (a) and 4 (b), d-RTS/oCTS results in much higher throughput and shorter delay than o-RTS/d-CTS. In fact, the performance of o-RTS/d-CTS is almost the same as o-RTS/o-CTS scheme, which is different from the result obtained by the analysis in Section IV. This difference can be accounted to the simplified model used in the analysis. The analysis in Section IV assumes that a sender initiates packet transmission by sending a RTS to its receiver only if the receiver is able to reply with a CTS. However, in the simulation, there could be situations when a node initiates a RTS but its receiver cannot reply with a CTS because the receiver may either be suppressed by virtual carrier sensing from other nodes or involve in another communication. The consequence is that all the nodes exposed to the unsuccessful RTS are suppressed from their communication activities for one frame transmission interval. The RTS-suppression when using o-RTS is more severe than d-RTS, because of the larger coverage area when omnidirectional transmission is used. Thus, this situation brings down the performance in the case of o-RTS/d-CTS to the same level as o-RTS/o-CTS. Notice that, at high number of nodes, packet delay is in the order of ten seconds as depicted in Figure 4 (b). From the experimental setup, all nodes generate packets with identical rate of 10 packets per second, which means the total arrival rate linearly increases with the number of nodes. Therefore, the contention for the shared channel is very high when node density increases. This causes each outgoing packet to be kept in the MAC layer buffer for a longer period waiting to be retransmitted. This situation incurs a very long delay in the network layer queue 2 , and finally results in an extremely large delay when packets get to their destinations. Consider the energy efficiency result shown in Figure 4 (c), the d-RTS/d-CTS scheme is the most efficient scheme. It can deliver more data packets while consuming the same amount of energy as other schemes. For the schemes using o-RTS and/or o-CTS, the larger the area covered by virtual carrier sensing, the lesser the number of nodes can simultaneously utilize the channel. Nodes that are being kept idle lower the amount of traffic the network can deliver. Moreover, the idle nodes still consume significant amount of energy, almost the same consumption rate as when they are transmitting or receiving. In combination, these factors reduce the energy efficiency of the schemes that use omnidirectional transmissions. Notice that the efficiency of all schemes decreases when the number of nodes (density of nodes) in the network increases. This is the result from more energy wasted from all nodes when they are idle due to the high contention in the network. From the observation above, the deployment of d-RTS improves both performance and energy efficiency of the network, £

The network layer queue is FIFO with queue size equal to 50,000 bytes.

while the deployment of d-CTS without d-RTS does not show any benefit gain. Note here that the capability of locking on to only a signal being received is required for d-CTS to help improving network performance and efficiency. Without this capability, hidden nodes can interfere with a receiving node and render d-CTS useless. C. Multi-hop traffic experiment and discussion To study the effect of different RTS/CTS exchange schemes in a more realistic scenario, the same network configuration used in Subsection V-B is set up with 100 nodes. Five pairs of nodes are randomly chosen to carry CBR traffic of various rates using the AODV protocol. The resulting graphs showing the average throughput per flow and average end-to-end delay are depicted in Figure 5 (a) and 5 (b), respectively. At low traffic where the load is less than 100 Kb/s per flow, all schemes perform equally well. We can only notice a slight difference in end-to-end delay in which d-RTS/d-CTS shows the shortest delay when the load is close to 100 Kb/s per flow. However, when the load is higher than 100 Kb/s, the level of contention is high enough to differentiate the performance of various schemes. All four schemes show a similar performance consistent with the results obtained from the analysis in Section IV. The d-RTS/d-CTS scheme achieves the highest performance (high throughput/low delay), while oRTS/o-CTS the lowest. In contrast to the one-hop scenario, where the performance of o-RTS/d-CTS is very close to that of o-RTS/o-CTS, o-RTS/dCTS shows a significant higher performance than o-RTS/o-CTS in the multi-hop scenario. The performance difference can be accounted for the difference in traffic models and number of exposed nodes between the two scenarios. In the one-hop scenario, there is no relationship between the traffic generated at each node, since nodes only randomly transmit packets destined to one of their neighbors at a certain rate. However, in the multihop scenario, all five CBR flows are more than one-hop apart, and only the source nodes will generate packets. If we consider the difference between schemes, by considering the area covered by virtual carrier sensing (as depicted in Figure 1), o-RTS/d-CTS has smaller area than o-RTS/o-CTS. Therefore, in the multi-hop scenario, when there is no contention from nodes located outside the flows, o-RTS/d-CTS can achieve better channel spatial reuse, which leads to better performance than o-RTS/o-CTS. In terms of energy efficiency, as depicted in Figure 5 (c), the average numbers of packet received at the destination per unit of energy for all RTS/CTS exchange schemes follow the same trend as the throughput result. When the channel is almost saturated at the traffic load more than 150 Kb/s, d-RTS/d-CTS helps improving the efficiency of the network. The improvement is the result of low energy dissipation rate when directional antennas are employed, the reduction in the nodes’ idle time when the traffic is high, and the better channel spatial reuse when the area suppressed by RTS/CTS is small if directional transmission is deployed. By considering these results, we are able to conclude that directional virtual carrier sensing helps improving the performance and energy efficiency of wireless networks, especially in the case of high load and/or high interference from local transmissions. The channel reservation scheme also has an important effect to the performance. The use of d-RTS/d-CTS scheme in conjunction with the capability of locking on to the received signal yields the best performance and energy efficiency among different schemes at high load in a realistic traffic scenario.

40000

o-RTS/o-CTS o-RTS/d-CTS d-RTS/o-CTS d-RTS/d-CTS

Efficiency (Pkt/Joule)

7000

30


20 6000

Delay (s)

Enire network throughput (Kb/s)

8000

5000

10 4000


30000

20000

10000 3000 100

200

300

0 100

400

200

Number of nodes

300

400

100

Number of nodes

(a) Throughput

200

300

400

Number of nodes

(b) Hop-by-hop delay

(c) efficiency

Fig. 4. Overall throughput (a), average delay (b), and efficiency of the network in delivering packet with one unit of energy in a one-hop data transmission scenario with various RTS/CTS exchange schemes as a function of number of nodes 1

50


0 0

100 200 300 Traffic load per flow (Kb/s)

(a) Throughput

Efficiency (Pkt/Joule)

100

10000


0.8

150

End to end delay (s)

Average throughput per flow(Kb/s)

200

0.6 0.4 0.2 0

400

8000 6000 4000 o-RTS/o-CTS o-RTS/d-CTS d-RTS/o-CTS d-RTS/d-CTS

2000 0

0


(b) Hop-by-hop delay

400

0


400

(c) efficiency

Fig. 5. Overall throughput (a), average end-to-end delay (b), and efficiency of the network in delivering packet with one unit of energy (c) in a multi-hop data transmission scenario with various RTS/CTS exchange schemes as a function of traffic load per flow

However, there are some situation when other schemes, which result in inferior performance, are preferred. For example, if nodes are not equipped with GPS or do not have a mechanism to acquire next hop’s location (or direction) information, d-RTS cannot be deployed. In this case, o-RTS is necessary because it allows the sender to perform on-demand searching for its receiver in all directions. Then, d-CTS can be used in reply after the receiver detects the direction of the sender from the incoming oRTS. Another benefit of o-CTS over d-CTS is when the assumption that a receiver can lock on to a signal being received is not valid. In this case, o-CTS is more preferable than d-CTS since o-CTS is more capable of eliminating hidden nodes and lowering the chance of packet drop due to high interference. Therefore, the selection of a proper directional virtual carrier sensing scheme should be based upon the traffic model, the hardware features, and the performance requirement. VI. C ONCLUSION AND FUTURE WORK We have studied the performance and energy consumption behavior of various directional virtual carrier sensing schemes. Both analytical and simulation models were constructed to compare their abilities to deliver user data and their energy efficiency in wireless ad hoc networks. The results show that channel reservation schemes using different combination of directional and omnidirectional RTS/CTS control frames affect the performance and energy consumption, in which directional-RTS/directionalCTS (d-RTS/d-CTS) scheme with the ability to lock on to a signal being received shows the best performance and energy efficiency. While the other schemes are inferior to d-RTS/d-CTS, they are still necessary in certain situations, such as o-RTS is necessary for receiver’s direction determination, and o-CTS provides better protection against interference from hidden nodes. In the future work, we plan to integrate directional MAC protocols with a passive coarse-grain energy conservation mechanism [13], where nodes are scheduled to enter the sleep mode to

conserve energy in a coordinated manner, while sustaining network forwarding performance. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government.

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