The design of a channel-access protocol for a wireless ad hoc network with sectored directional antennas
Introduction
Directional antennas have drawn much attention in recent years as a means to increase the link capacity, the link range, and the spatial reuse of spectrum in wireless communication systems. Introduction of a (properly aligned) directional antenna at either the transmitter or the receiver results in an increased signal power at the receiver for a given transmission power. Introduction of a directional antenna at a receiver can provide improved interference rejection, and the use of a directional antenna for transmission can reduce the interference to third-party receivers.
Directional antennas are used widely in commercial cellular networks and wireless access networks, but their use also holds the potential for improvement in the performance of mobile ad hoc packet radio networks [1]. Not only can directional antennas improve the quality of a given link, they can create a viable link when none might otherwise exist. Thus in a multiple-hop wireless network, the use of directional transmit or receive antennas can also increase the connectivity of the network. Each of the benefits of directional antennas can be realized in a mobile ad hoc network only if protocols are designed to exploit their capabilities effectively, however. Of the components of the radio’s protocol stack, the design of the medium access control (MAC) sub-layer protocol has the most direct impact on the ability of the network to utilize directional antennas.
Research on MAC protocols for wireless ad hoc packet-radio networks with directional antennas has focused primarily on adaptations of carrier-sense multiple access with collision avoidance (used in the IEEE 802.11 Distributed Coordination Function [2]), which employs a ready-to-send (RTS)/clear-to-send (CTS) collision-avoidance mechanism [3]. Most of the work considers a network sharing a single radio channel in which each node includes a single half-duplex radio transceiver with a directional-antenna system that implements a steered beam or switched beam capable of providing directional coverage in one direction at a time [1], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. (In some of these papers, the antenna system is described as a sectored, multiple-beam system, but the functionality that is modeled corresponds more closely to a steered-beam or switched-beam system.) Other research concerns nodes with a more sophisticated multiple-input/multiple-output (MIMO) antenna/transceiver system that provides a combination of spatial diversity and interference rejection for the link in which the node is currently participating [15], [16]. Most of the protocols are designed for nodes that also include an omnidirectional antenna, in which case the node’s radio cannot use its directional antenna and its omnidirectional antenna concurrently.
A variety of undesirable conditions can arise if an RTS/CTS MAC protocol is used in conjunction with directional antennas, including unique forms of the hidden-terminal problem [17], deafness [17], the exposed-terminal problem [14], and the receiver blocking problem [18]. In a single-channel network with nodes using a single-beam directional antenna (as in the works cited above), the first three of the problems are more significant factors in limiting performance than is the receiver blocking problem [19]. Numerous approaches have been considered that mitigate one or more of these conditions in such a network, including various combinations of directional transmission, omnidirectional transmission, and sequential directional (“circular”) transmissions of the RTS and/or CTS packets [4], [5], [7], [8], [10], [11]. Other approaches include the use of higher power for transmissions from omnidirectional antennas than for transmissions from directional antennas (e.g., in [4]) and the use of additional steps in the MAC dialog beyond the basic RTS/CTS/DATA/ACK sequence [20], [21], [12], [14], [22], [23].
An alternative form of directional-antenna system employs multiple directional antennas (sectors), each with a pattern that is fixed relative to the orientation of the node. Each antenna is utilized with a separate radio transceiver, and any two antennas have nominally non-overlapping coverage regions so that the antennas together provide full coverage in the azimuth. If the sectored antenna system is employed with a properly designed MAC protocol that uses multiple frequency channels, it is possible to utilize multiple sectors concurrently at the same node (with certain restrictions discussed below). Concurrent multiple-sector operation at nodes can be exploited to provide increased network throughput. Moreover, the ability to exercise concurrent idle-channel monitoring in all sectors permits a simple mechanism for neighbor-location discovery that does not require adaptive-array beamforming, scanning of switched beams, or externally supplied location information. All-sectors monitoring also removes the need for a separate omnidirectional antenna (and associated range asymmetries), and it alleviates several of the MAC-protocol design challenges that arise with nodes that use a steered-beam or switched-beam directional antenna.
The benefits of a sectored antenna system at the nodes of an ad hoc network can only be exploited within the inherent constraints of the approach, however. The presence of multiple transceivers at a node gives rise to the potential for co-site interference in which a transmission from one sector at the node couples sufficiently with the antenna in another sector to cause a low signal-to-interference ratio for an arriving signal in the second sector. Co-site inteference is most pronounced if the transmitting signal and the arriving signal occupy the same frequency channel. The MAC protocol for a network containing nodes with a sectored antenna system must be designed to avoid problems due to co-site interference.
Multiple transceivers and multiple antennas with beam patterns of sufficient quality for a sectored antenna system (such as horn or corner-reflector antennas [24]) are likely to be impractical for handheld or manpack radios; they may be limited instead to nodes with greater resources, such as vehicle-mounted radios [9]. Thus nodes with a sectored antenna system will likely be part of a heterogeneous network that also includes many nodes using only an omnidirectional antenna. The MAC protocol for a network containing a subset of nodes with a sectored antenna system should be designed to provide good performance regardless of the mix of node capabilities in the network.
In this paper we first present the design of a packet scheduler and a collision-avoidance (RTS/CTS) random-access MAC protocol that supports the use of directional antennas in a mobile ad hoc packet radio network. The protocol is designed for a heterogeneous network in which an arbitrary subset of the nodes contain sectored directional antennas with a half-duplex radio transceiver for each sector and the rest of the nodes use only an omnidirectional antenna and a single half-duplex radio transceiver. An instance of the same scheduler and MAC protocol is employed in each node regardless of the type of its antenna subsystem. It exploits the presence of sectored directional antennas to provide improved spatial reuse of the network’s allocated radio spectrum. We consider a network in which the radios employ direct-sequence spread-spectrum modulation, though the MAC protocol can be employed effectively (with adjustments to some parameters and minor changes in the protocol) if a different modulation format is used.
The MAC protocol described in the first part of this paper is a generalization of the unslotted, multiple-channel MAC protocol described in [25], where the generalization is the inclusion of support for nodes with directional antennas and management of co-site interference. The available radio-frequency bandwidth is divided into multiple frequency channels. One channel is designated the control channel, and it is used exclusively for the transmission of reservation packets. The other channels carry information packets and are referred to as traffic channels. The protocol is designed to mitigate the effects of co-site interference in a node with multiple sectors. Multiple-channel MAC protocols for a network of radios with an omnidirectional antenna are considered in many recent papers (e.g., [26]), and multiple-channel MAC protocols are considered in conjunction with radios using a single steered-beam or switched-beam directional antenna in [9], [23] (though in the former a separate instance of the MAC protocol operates on each frequency channel).
The use of sectored directional antennas inherently mitigates the forms of the hidden-terminal problem, deafness, and the exposed-terminal problem that arise as the result of using a single directional antenna. The same is not true of the receiver blocking problem, however, and the vulnerability of the network to receiver blocking is investigated in the second part of this paper. The receiver blocking problem occurs even in a network of nodes with omnidirectional antennas that use a collision-avoidance MAC protocol [27], and various approaches address its effect with omnidirectional antennas using a modified channel-access protocol [42] or routing algorithm [41]. The problem is exacerbated markedly by the presence of nodes with directional antennas, however, as we demonstrate in this paper.
In the third part of the paper, we present a modification of the MAC protocol that incorporates a “negative CTS” (NCTS) response into the collision-avoidance technique. The concept of the “negative CTS” has been employed in earlier work in the context of a network of nodes with omnidirectional antennas [27], [28], [29]. The protocol we present includes features that represent refinements to the earlier approaches even beyond its ability to support directional antennas. It was described by us previously in summary form in [18] along with a limited investigation of the link-level performance. (A similar mechanism was later employed in the work of others concerning a single-channel network of nodes with a single directional antenna [14].) In this paper, we show that our MAC protocol using the NCTS mechanism substantially mitigates the receiver blocking problem and thus leads to a significant improvement in the performance of a multiple-channel network that includes nodes with sectored directional antennas. It includes a more detailed description of the protocol than in [18], it provides new insights into the receiver blocking problem in the context of directional antennas, and it includes an investigation of the performance in terms of network-level performance metrics.
The remainder of the paper is organized as follows. In Section 2, the system design we consider is described and includes a definition of the first (“baseline”) MAC protocol we introduce. The Monte Carlo simulation used for evaluation of the network performance is described briefly in Section 3. In Section 4 the receiver blocking problem is examined, and its occurrence in the presence of sectored directional antennas is illustrated. The modified MAC protocol employing the negative-CTS mechanism is defined in Section 5, and its impact on link-level performance is considered. In Section 6, the performance evaluation is extended to a comparison of the baseline and NCTS MAC protocols in the context of multiple-hop routing. Finally, the important insights that can be drawn from the work are summarized in Section 7.
Section snippets
System description
A subset of the nodes in the network employ multiple directional antennas at the physical layer, and each antenna has a corresponding half-duplex radio transceiver. Each of the remaining nodes has an omnidirectional antenna and a single radio transceiver. In the following, the coverage area of each directional or omnidirectional antenna is referred to as a sector. Hence there is a one-to-one correspondence between transceivers and sectors at each node. The network uses two or more frequency
Simulation of network performance
An event-driven simulation created using Opnet is used to investigate the performance of the MAC protocols and the scheduling algorithm in a distributed direct-sequence packet radio network. Each transmitting node in the network generates data packets for each of its destination nodes according to a Poisson process. The network simulation includes accurate models of physical-layer performance, including models of packet acquisition and detection performance similar to those used in [25]. The
The receiver blocking problem and its effect on performance
In this section we illustrate the receiver blocking problem and demonstrate its impact on performance if the network includes directional antennas. Two eight-node networks are considered. Network I consists only of nodes with omnidirectional antennas. In network II, nodes 0–3 each have multiple directional antennas that form three sectors and nodes 4–7 have omnidirectional antennas. The two networks have the same node locations, which are shown in Fig. 1 along with the sector orientations of
Some insight into mitigation of the problem
An idealized modification of the baseline MAC protocol suggests how the protocol might be modified in practice to reduce the frequency of occurrence of the receiver blocking problem. Specifically, consider an enhancement of the protocol by the inclusion of a “genie” which provides side information to a node/sector that has transmitted a RTS as the prelude to the transmission of a data packet. If the destination node/sector has no available traffic channels and one or more of the channels are
Comparison of network-level performance with the baseline and NCTS MAC protocols
In this section we consider the performance with the baseline and NCTS MAC protocols for a 30-node network in which multiple-hop routing is enabled. The network dimensions and the mix of node capabilities are the same as in the last example of the previous section. The performance measures of interest are the aggregate end-to-end throughput of the network, the distribution of end-to-end throughputs achieved for the individual network-layer source–destination pairs, and the percentage of
Conclusions
A packet transmission scheduling algorithm and two MAC protocols are presented in this paper for use in a DS mobile ad hoc packet radio network in which an arbitrary subset of the nodes employ directional antennas. It is shown that the characteristics of link-level communications using directional antennas results in a greater probability of occurrence of the receiver blocking problem than with omnidirectional antennas. It is also shown that this has a detrimental effect on network throughput
Acknowledgments
This work was supported in part by the US Army Research Laboratory and the US Army Research Office under Grant numbers DAAD19-00-1-0156 and W911NF-05-1-0328 and in part by the DoD Multidisciplinary University Research Initiative (MURI) program administered by the Office of Naval Research under Grant N00014-00-1-0565. A portion of this work was presented at the 2004 IEEE Military Communications Conference, Monterey, CA.
Arvind Swaminathan received the B.Tech degree in Electrical and Communications Engineering from the National Institute of technology, Calicut, India and the M.S. and Ph.D. degrees in electrical engineering in from Clemson University, South Carolina.
From 2001 to 2006 he was a Research Assistant in the Wireless Communications and Networks Group in Clemson University where he worked on research projects related to adaptive link and network layer protocols for handheld mobile ad hoc networks. From
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Arvind Swaminathan received the B.Tech degree in Electrical and Communications Engineering from the National Institute of technology, Calicut, India and the M.S. and Ph.D. degrees in electrical engineering in from Clemson University, South Carolina.
From 2001 to 2006 he was a Research Assistant in the Wireless Communications and Networks Group in Clemson University where he worked on research projects related to adaptive link and network layer protocols for handheld mobile ad hoc networks. From 2006 to 2008, he was a Systems Engineer in Texas Instruments’ wireless terminals group working on the design of algorithms for DVB-H, WLAN and Bluetooth products. He joined the Systems Engineering group of Qualcomm in 2008 were he is currently a Staff Engineer.
His research interests are in distributed protocols for wireless ad hoc networks and design of algorithms for cellular modems. The primary focus is on system selection algorithms, power-efficient communication, mobility mechansims between cellular technologies and optimizing the performance of smartphone applications.
Daniel L. Noneaker received the B.S. degree (with high honors) from Auburn University in 1977 and the M.S. degree from Emory University in 1979, both in mathematics. He received the M.S. degree in electrical engineering from the Georgia Institute of Technology in 1984, and he was awarded the Ph.D. degree in electrical engineering from the University of Illinois at Urbana-Champaign in 1993.
He has industrial experience in both hardware and software design for communication systems. From 1979 to 1982 he was with Sperry-Univac, Salt Lake City, Utah, and from 1984 to 1988 he was with the Motorola Government Electronics Group, Scottsdale, Arizona. He was a Research Assistant in the Coordinated Science Laboratory, University of Illinois, Urbana, Illinois from 1988 to 1993. Since August 1993 he has been with the Holcombe Department of Electrical and Computer Engineering at Clemson University, Clemson, SC, where he currently holds the rank of Professor and the position of Associate Department Chair.
He is engaged in research on wireless communications for both military and commercial applications, including cellular networks and ad hoc mobile radio networks. He has published numerous papers on design and performance analysis for spread-spectrum multiple-access communications, error-control coding for fading channels, signal acquisition, and protocols for packet radio networks. In 2008 he received the IEEE Military Communications Conference Award for Sustained Technical Achievement, which is awarded for outstanding, sustained technical contributions to military communications.
Harlan B. Russell received the B.S. degree in Computer Engineering in 1986 and the M.S. and Ph.D. degrees in electrical engineering in 1989 and 1993, respectively, all from the University of Illinois, Urbana-Champaign.
From 1993 to 1999 he was a Senior Research Engineer for Techno-Sciences, Inc., Pendleton, SC. While at Techno-Sciences he worked on several joint projects with industrial, government, and university partners related to adaptive link and network layer protocols for handheld mobile ad hoc networks. He joined the faculty at Clemson University in December 1999 were he is currently an Associate Professor in the Electrical and Computer Engineering Department.
His research interests are in distributed protocols for wireless ad hoc networks. The primary focus is on link, network, and transport layer protocols that provide different quality-of-service levels for multimedia traffic. On-going research interests include energy-efficient routing, adaptive-transmission protocols, and transmission scheduling for multiple-hop radio networks.