Cooperation and Directionality: A Co-opdirectional MAC for Wireless Ad Hoc Networks

Transcription

1 1 Cooperation and Directionality: A Co-opdirectional MAC for Wireless Ad Hoc Networks Zhifeng Tao, Thanasis Korakis, Yevgeniy Slutskiy, Shivendra Panwar, Leandros Tassiulas Mitsubishi Electric Research Laboratories, Cambridge, MA 2139 Department of Electrical and Computer Engineering, Polytechnic University, Brooklyn, NY 1121 Department of Electrical Engineering, Columbia University, New York, NY 127 Computer Engineering and Telecommunications Department, University of Thessaly, Volos, Greece tao@merl.com, korakis@poly.edu, ys216@columbia.edu, panwar@catt.poly.edu, leandros@inf.uth.gr Abstract Advances in cooperative communications and directional antenna design so far have taken place in parallel, if not in isolation from each other. To explore the role they may play in the next generation of wireless networks, it is important to design and evaluate protocols that can provide co-opdirectionality, the capability of tapping into the combined potential of both cooperation and transmission directionality. Inspired by the protocols presented in [1] and [2], we propose a novel co-opdirectional MAC that fully leverages both cooperation diversity and transmission directionality 1. Special attention has been paid to the protocol design so that the new MAC would not only inherit the advantages of two previous protocols, but also avoid their weaknesses in the wireless ad hoc environment. The protocol thus delivers a superior performance, as our extensive simulations confirm. To the best knowledge of the authors, this paper represents the first effort to deal with the challenges of integrating cooperation and directional capabilities at the MAC layer. I. INTRODUCTION Both cooperative communications and directional antenna have recently gained tremendous traction in the research community, as they have the potential of reshaping the landscape of future wireless system design. Upon its inception, the notion of cooperative communications has been prodominantly interpreted in the form of innovations at the physical layer [3] [7] (e.g., cooperative coding, modulation, etc.). By intelligently utilizing overheard information from surrounding stations and retransmitting it to a destination, physical layer innovations provide spatial diversity as well as improvement in capacity and reliability. However, cooperation can be implemented in various forms and at different protocol 1 This work is supported in part by National Science Foundation (NSF) under award 525, and the New York State Center for Advanced Technology in Telecommunications (CATT). The work is also supported by Wireless Internet Center for Advanced Technology (WICAT), an NSF Industry/University Research Center at Polytechnic University. layers. For instance, a cooperative MAC protocol has been devised in [8], which, by exploiting physical layer cooperation capability, reduces the number of retransmissions and ultimately leads to an impressive system performance improvement. G. Jakllari et al. propose a MAC protocol [9] in which a set of neighboring stations can synchronize with the source and beamform together to the intended destination. Used in conjunction with a modified DSR routing protocol, the proposed MAC can significantly increase the network capacity. A different cooperative approach has been pursued in [1], wherein an intermediate helper station is enlisted to expedite the transmission from a source to a destination. This new protocol called CoopMAC requires minimal changes to the existing distributed coordination function (DCF) [1], and thus is backward compatible with the legacy IEEE standard. Directional antenna is another key lower-layer technology that holds the promise of extending the coverage range and of delivering higher capacity for the next generation of wireless networks. However, directionality of communication poses numerous challenges across multiple protocol layers, without the resolution of which the full power of directional antennas cannot be fully harnessed and unleashed. Several sophisticated MAC protocols have been recently proposed in [2], [11] [13] to address the issues of destination positioning, effective channel reservation, as well as the deafness problem, among others. Advances of cooperative communications and directional antenna design so far have taken place in parallel, if not in isolation from each other. To explore the role they may play in the next generation of wireless networks, it is important to design and evaluate protocols that can provide co-opdirectionality, the capability of tapping into the combined potential of both cooperation and transmission directionality. Inspired by the protocols presented in [1] and [2], we propose a novel co-opdirectional MAC that fully

2 2 leverages both cooperation diversity and transmission directionality. Special attention has been paid to the protocol design so that the new MAC would not only inherit the advantages of two previous protocols, but also avoid their weaknesses in the wireless ad hoc environment. The protocol thus delivers a superior performance, as our extensive simulations confirm. To the best knowledge of the authors, this paper represents the first effort to deal with the challenges of integrating cooperation and directional capabilities at the MAC layer. The rest of this paper is organized as follows. An essential background on directional antennas and its impact on transmission range extension is first provided in section II. The proposed MAC protocol is then elaborated in section III, and its performance evalauted in section IV, respectively. Finally, the conclusion and discussion of future work are presented in section V. A. Directional Antennas II. BACKGROUND A directional antenna consists of multiple element arrays [1] [15]. By sophisticated combining of the transmitted/received signals in the elements, the electromagnetic waves are enhanced in certain directions and are attenuated in others, resulting in an amplified signal that is directed to or received by certain directions. Thus, the transmission/reception is beamformed toward certain preferred transmission and reception directions. Based on the signal possessing method for the element s signal combination, the implementation of directional antenna can be roughly categorized in two different types, namely switched beam and steerable beam [16] [17]. 1) Switched Beam: It is also known as a beamforming antenna, which forms fixed and predefined beams. This type of antenna is able to switch its transmission toward a specific direction using one of these beams. In such antenna the signal that is sent to each of the elements is weighted in both magnitude and phase before being transmitted. The specific set of weights that are applied to the different antenna elements is responsible for the antenna pattern formed. In the case of switched beam antennas, a predetermined set of weights is used, each of which results in a beam pointing to a particular direction with a high gain. 2) Steerable Beam: As compared to the switched beam antenna, it works in a more sophisticated way. In this case the main lobe can be pointed virtually in any direction, by dynamically adapting the weights for the antenna elements. This adaptation is feasible due to a signal processing mechanism that takes the feedback from the received signal into consideration. Although steerable beam antennas are more efficient than switched beam antennas, their implementation complexity is higher since now a sophisticated signal processing mechanism is necessary for the weight adaptation procedure. B. Propagation Model The two-ray pathloss model is used for the wireless channel, and the operation is assumed to be in an outdoor environment at 2. GHz (e.g., IEEE 82.11b/g). For sake of simplicity, a pathloss exponent of will be used throughout the simulation and the actual reception power P r becomes: P r G t G r h 2 t h 2 r d P t (1) Where G t and G r are the transmit and receive antenna field radiation pattern in the line-of-sight (LOS) direction, h t is the transmitter height, h r is the receiver height and P t is the transmission power. We use the superscripts d and o to represent the quantity for directional and omnidirectional antenna, respectively. For instance, d d and d o represent the range within which a certain modulation and coding scheme can be sustained by directional antenna and omnidirectional antenna. Using the equality P d r = P o r, the relation between d d and d o can be derived as follows: G d t G d r (d d ) Go t G o r (d o ) (2) We further assume the reception is not directional (i.e., G d r = G o r), and the directional antenna at the transmitter side has N beams. In addition, the relation between the antenna gain for the directional and the omni transmission can be approximated as follows. G d t N G o t (3) Thus, the range extension factor F RE, which is defined as d d /d o, can be written as: F RE = dd d o N () The range extension factor F RE for the antenna setting used in the simulation is listed in Table I. Number of Beamwidth Range extension antenna beams θ m factor F RE 9 C 1.1 TABLE I: Range extension factor

3 3 (a) Control plane operation (b) Data plane operation Fig. 1: Illustration of the proposed co-opdirectional MAC. III. THE PROPOSED PROTOCOL The new co-opdirectional MAC is based on the existing IEEE DCF protocol, with necessary modifications made in both the control and data planes to leverage the cooperation and transmission directionality, as illustrated in Figure 1(a) and 1(b), respectively. For the sake of simplicity, we assume in the ensuing protocol description that all stations can beam a signal in different directions. However, note that the proposed coopdirectional MAC is able to handle an arbitrary number of antenna beams. A. Control Plane When a station starts to handle a new packet, it should follow a binary exponential backoff scheme similar to that defined in IEEE One key difference for a station equipped with a directional antenna is that each direction has its own network allocation vector (N AV ) value, which is updated by the ready-to-send (RT S) and clear-to-send (CT S) messages received in that direction. Two different approaches can be used with the protocol to handle the backoff procedure. In the first approach, a single backoff counter is used within each station for all antenna beams. The backoff counter value can be decremented if and only if the physical medium around a station is determined to be idle and the NAV values for all directions are zero. As an alternative, a separate backoff counter is maintained for each possible direction, and it counts down only if both the physical channel and the NAV value for the associated direction are clear. The proposed co-opdirectional MAC protocol can adopt both backoff designs, while the results presented in section IV are based on the single backoff counter approach. 1) When the backoff counter reaches zero, and the wireless channel is not occupied, the source station (N s ) attempts to transmit by first sending out a RTS message in all directions sequentially, in a circular manner, with a short interframe spacing (SIF S) between two consecutive circular RTS frames. The primary purpose of duplicating an RTS in all directions is to locate the intended destination, and to mitigate the hidden terminal problem that is intrinsic to directional antenna networks. N s shall also look up a special local data structure called the cooperation table (CoopT able) to identify a candidate helper N h, and the perceived physical transmission rate on the link between station N s and N h (i.e., R sh ), and on the link between station N h and the intended destination station N d (i.e., R hd ). The detailed information about CoopT able will be provided in section III- C. If a potential helper station N h is found, N s inserts the MAC address of the selected N h and the proposed rate information R sh and R hd into the circular RTS message so that the selected helper station N h can be properly informed. 2) Upon a successful decoding of a RTS message, the station N d should check its own channel status and NAV values. It should only agree to engage in communication with N s, if no interference will be caused to any ongoing transmissions in its neighborhood and if the proposed rate R hd can be sustained. A CTS message is sent by the N d in the direction from which a RTS was received to signal its willingness to join the dialog. In addition, a CTS should also be separately transmitted to the selected helper station, if N h and N s have to be covered using different beams. Since the identity of N h has been indicated in the RTS message, N d can consult the location table (L-Table) to determine the beam through which this CTS should be directed. The creation and maintenance of the L-Table will be discussed in section III- C. Similarly, the two consecutive CTS messages

4 Neighbor (8 bits) Beam (2 bits) MAC address of neighbor N d The beam at N s through which N s can transmit to N d TABLE II: Location table (L-Table) at station N s. Destination (8 bits) Helper (8 bits) R sd (6 bits) R sh (6 bits) R hd (6 bits) MAC address of MAC address of Rate between Rate between Rate between destination N d potential helper N h N s and N d N s and N h N h and N d TABLE III: Cooperative table (CoopTable) at station N s. Location table at N s Neighbor Beam N h Mbps Cooperative table at N s RTS(11 Mbps) 3 Destination Helper R sd R sh R hd N d N h 1 Mbps 5.5 Mbps 11 Mbps N s Mbps 3 11 Mbps N d CTS N h Fig. 2: Illustration of table establishment. should be separated by a SIFS interval. 3) The helper station N h, upon receiving a RTS and a CTS from N s and N d respectively, should send a helper-ready-to-send (HTS) message to both N s and N d, if it is able and also willing to participate in the cooperative transmission. Again, there is a SIFS interval between the two HTS messages. Station N h is said to be able to help, if it can support rates R sh and R hd, and if its participation in the cooperation does not interrupt any other ongoing communication. Certainly, various incentive schemes can be devised to encourage or discourage N h to cooperate. However, this subject is beyond the scope of this paper, and thus will not be discussed hereafter. If this three-way handshake is successfully completed, the data exchange can start. Otherwise, a failure to receive any feedback (i.e., CTS or HTS) would result in a timeout at N s, which should treat it as a transmission failure and thus return to a new backoff. When the number of such failures exceeds a threshold T 1 (e.g., 2), N s may consider switching to a different helper station, if possible at all. If N s continuously experiences such failures for over T s (e.g., 5) times, N s shall fall back to the non-cooperative mode, and transmit directly to N d. B. Data Plane The data exchange procedure is relatively straightforward, wherein N s should first transmit data at the rate R sh to the selected helper N h, which after a SIFS time forwards the received packet(s) to the intended destination N d at the rate R hd. These correspond to steps 1 and 2 depicted in Figure 1(b), respectively. To confirm a successful reception of data, N d shall transmit an acknowledgment (ACK) message to the source station N s. Otherwise, a timeout will result at N s, which then again enters a new backoff process, as long as the corresponding retransmission limit has not been exceeded. C. Miscellaneous Discussion It is evident from the preceding description that two special data structures, namely the CoopT able and the L-Table play a critical role in enabling cooperation and directional transmission. The detailed format of CoopTable and L-Table are specified in Table II and III, respectively. 1) L-Table: When a station initially joins a directional network, it can only transmit directly to a destination. Such a direct transmission of data packets is always preceded by a handshake, as illustrated in Figure 2. During

5 5 the handshake, a source, which happens to be station N h in Figure 2, sends out directional RTS messages circularly in all directions it supports, and the destination station N d replies with a CTS if it is ready to receive data from N h. The beam from which station N h (N d ) receives the message CTS (RTS) should be the direction it can use to reach N d (N h ). For example, station N d learns that it should use beam 3 to communicate with N h in the future, since it hears a CTS from N h through beam 3. Thus, every wireless station in the end can establish a L-Table, which records the relative direction via which each of the station s neighbors can be reached. The L- Table can be constantly updated, as long as there is traffic in the network. 2) CoopTable: Directional transmission can not only pinpoint the relative location of each neighbor, but also acquire the sustainable physical rate on the corresponding link. Specifically, N s can determine the rate R sd directly by using any of the rate adaptation algorithms proposed for IEEE [18] [2]. Similarly, N h can decide what rate to use on the link between N h and N d for its direct transmission. N h will include the value of rate R hd in the circular RTS message associated with the data packets that subsequently will be directly transmitted from N h to N d. Since this RTS will sweep through the entire neighborhood of N h, all nearby stations can obtain this rate information. For instance, upon overhearing the circular RTS, station N s can immediately learn the rate R hd. Finally, the rate R sh can be estimated by N s based on the signal-to-noise-ratio (SNR) of the packet N s overhears from N h. In the end, every station should store the learned rates R sd, R sh and R hd into the CoopTable along with the MAC addresses of the corresponding stations N h and N d. IV. PERFORMANCE EVALUATION In order to evaluate the performance of the proposed protocol and gain deeper understanding of its behavior in a large scale ad hoc network, extensive simulations have been conducted. The results of these simulations provide a performance comparison of our protocol with three other closely related protocols: IEEE DCF [1], CoopMAC [1] and the directional antenna protocol proposed in [2]. CoopMAC is a cooperative protocol, wherein a relay helps slow stations speed up the entire data exchange session. Designed for omnidirectional transmission, CoopMAC replaces wherever possible onehop slow transmissions with two-hop fast transmissions, thereby achieving an improvement in network performance. The directional MAC [2] that we use in our comparison aims at leveraging the directional antenna capability in an ad-hoc network. It uses a circular directional RTS transmission to initiate the communication, and a directional transmission to deliver a CTS, data packets, and an ACK. Relying on directional transmission for control as well as data packets, the protocol can significantly increase the network capacity and extend the coverage range. Interested readers are encouraged to refer to [1] and [2] for more details. A. Simulation Settings To quantify the performance of the proposed coopdirectional MAC, and to assure a fair comparison with other protocols, we have developed an event-driven simulator partially based on the simulators used in [1] and [2]. Four possible rates, namely 1 Mbps, 2 Mbps, 5.5 Mbps and 11 Mbps, which constitute the permissible set of rates defined in IEEE 82.11b, are used in our simulations. For each simulation, stations are randomly placed in a circle of radius R = 35 m. The number of stations varies from 15 to 3. The coverage area of an omnidirectional antenna is a dish of radius r omni = 1 m, while that of a directional antenna is a dish of radius r directional = 1 F RE m. The F RE here is the range extension factor, which has been provided in Table I for a given antenna setting. In our simulations, we have implemented an antenna array model that produces N non-overlapping lobes of 36 N degrees each. In this paper, however, we will only present results for an antenna array producing nonoverlapping lobes, as it is one of the the most typical settings for directional antenna. The destination of each packet has been chosen randomly from all of the neighbors that are within the direct transmission range of a source station. Key statistics such as aggregate network throughput and delay distribution have been collected and examined. Each simulation result discussed hereafter has been averaged over 1 runs, each of which has a different random initial seed and runs for a period of time that is long enough to ensure convergence. B. Simulation Results Figures 3(a) and 3(b) reveal the relation between the network capacity and the number of stations deployed in the network. It is apparent that our co-opdirectional MAC significantly outperforms all three other protocols. Indeed, the CoopMAC protocol is anticipated to deliver more throughput than the legacy IEEE DCF, as it accelerates the slow transmission by replacing it with a faster two-hop transmission. The directional MAC

6 Legacy DCF 35 3 Legacy DCF Network throughput (Mbps) 15 1 Network throughput (Mbps) Number of stations Number of stations (a) MPDU = 12 bytes (b) MPDU = 2312 bytes Fig. 3: Throughput as a function of the number of stations ( beams) Network throughput (Mbps) Legacy DCF Network throughput (Mbps) Legacy DCF MPDU size (bytes) Traffic load (Mbps) (a) Impact of MPDU size (b) Impact of traffic load: MPDU = 12 bytes Fig. : Throughput with respect to MPDU size and traffic load (1 stations, beams). protocol achieves a throughput higher than that of the CoopMAC and legacy 82.11b protocols, primarily due to the spatial reuse it leverages. The co-opdirectional MAC protocol that we propose is similar to CoopMAC in that it also fully exploits the cooperative diversity and the multirate capability. Thus, the co-opdirectional MAC not only improves the performance of slow stations, but also makes it possible for fast stations to access the channel earlier, as the data transmission from slow stations takes significantly less time. Meanwhile, the directional antenna enables each station in our protocol to take advantage of spatial reuse. Multiple transmissions may occur in parallel now, even when the parties involved are in a direct transmission range of each other and would cause unacceptable interference to each other, were they operating in an omnidirectional mode. In addition, the directionality of transmission can extend the range, thereby increasing the possibility of finding a qualified helper by a source. One phenomenon that is particularly interesting in Figures 3(a) and 3(b) is the fact that as the number of stations increases, the throughput for legacy 82.11b network decreases, while that achieved by co-opdirectional MAC continuously grows. This highly desirable feature of co-opdirectional MAC seems to defy the conventional wisdom at first sight, as it is widely known that a

7 CDF.5 CDF Legacy DCF Channel access delay (s).2 Legacy DCF Channel access delay (s) (a) A network of 2 stations (b) A network of 3 stations Fig. 5: Distribution of channel access delay (MPDU = 12 bytes, beams) crowded network would witness excessive collisions and thus yield a lower throughput. Nevertheless, a thorough analysis of the simulation traces reveals that as the number of stations increases, the likelihood of a low rate station finding a high rate two-hop path also increases. The growing availability of helping stations not only cancels the throughput degradation caused by increasing collisions, but also results in a substantial net increase in the aggregate network throughput. Figure (a) provides a more detailed examination of the impact of MPDU size on the network throughput. First of all, it is evident that a larger packet size would lead to an appreciable increase in the protocol efficiency. Secondly, when the packet size is relatively small, the performance gain enabled by co-opdirectional MAC is somehow offset by the signaling overhead (e.g., circular RTS, multiple CTSs, addition HTS) introduced therein, and thus all four protocols yield more or less similar throughput. But note that as the packet size grows, the throughput improvement that co-opdirectional MAC can achieve becomes more pronounced, which is primarily due to the fact that the signaling overhead in coopdirectional MAC can then be amortized over more payload bits. To obtain the system capacity in Figure 3(a), 3(b) and (a), saturation load has been applied on the network, which implies that each station is always in a backlogged state. In Figure (b), however, we evaluate the protocol performance under a wide vareity of loading conditions. Under light load, since none of the four protocols have reached its capacity, they again can deliver similar throughput. The throughput of each protocol continues climbing and then remains relatively flat on a plateau, as the load enters the corresponding saturation region. Finally, the delay statistics collected under saturation loading are reported in Figure 5(a) and 5(b), which depicts the cumulative distribution of service delay for a network with 2 and 3 stations, respectively. The service delay, which is also known as channel access delay, of a successfully transmitted packet is the interval from the time the packet first reaches the head of the buffer (i.e., head-of-line), until it leaves the system. Figure 5(a) and 5(b) indicate that directional MAC [2] and co-opdirectional MAC suffer much less service delay than legacy DCF and CoopMAC. Between these two MAC protocols that adopt directional antenna, an appreciable improvement in delay performance yielded by co-opdirectional MAC can still be clearly noticed. V. CONCLUSIONS In this paper we have proposed a novel medium access control protocol for ad-hoc networks, which leverages cooperative diversity, exploits directional antenna functionality, and takes advantage of multi-rate capability. Comparison with three other related MAC protocols reveals that the proposed MAC protocol exhibits a far better performance in terms of throughput and delay. To the best knowledge of the authors, this represents the first effort to coherently integrate cooperation with transmission directionality in a single MAC protocol for the next generation wireless networks. As for the future work, the possibility of combining the physical layer cooperation techniques (e.g., cooperative modulation, coding, etc) with the MAC protocol

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