Adaptive cross-layer MAC design for improved energy-efficiency in multi-channel wireless sensor networks☆
Introduction
Wireless sensor networks (WSNs) have recently been used for numerous applications, including environmental monitoring, smart spaces, data collection, robotic exploration, etc. [1], [18]. The sensing devices in these applications are characterized by a limited battery lifetime, making energy efficiency a critical factor in the design of communication protocols [5] for WSNs. Current channel access protocols for WSNs can be divided into scheduling-based and contention-based protocols. Scheduling-based protocols, which include TDMA-, FDMA-, and CDMA-based schemes, are collision free. Among these protocols, TDMA-based designs are considered the most appropriate for WSNs [13]. However, many factors limit the use of TDMA protocols in WSNs, including scalability and adaptivity to network dynamics [15], [17]. For contention-based (random access) protocols, the most mature channel-access approach is the one that follows the carrier sense multiple access (CSMA) paradigm. CSMA is characterized by its simplicity, flexibility, robustness, and adaptivity to changes in the number of active nodes. No clock synchronization or global topology information are needed. Essentially, there are two variants of CSMA: p-persistent and non-persistent. As shown in [3], the MAC protocol used in the IEEE 802.11 standard can be well modeled by a p-persistent CSMA scheme. In contrast, many other MAC schemes proposed for WSNs are variants of non-persistent CSMA. In non-persistent CSMA, a node senses the carrier only when it is about to transmit. This limits the time spent on monitoring the medium, and hence conserves energy [1], [8]. Both variants of CSMA have been extensively studied over the past three decades. Stationary throughput and delay characteristics were derived for slotted and unslotted channels, under finite- and infinite-population models [8], [16]. Analytical results related to the energy efficiency were reported for a slotted CSMA system with a finite population size [2], [4], [3]. In these works, the system consists of a small number of stations (usually less than 100), and each station is assumed to operate under heavy traffic, i.e., each station always has data packets to transmit. The finite-population and heavy-traffic assumptions best describe the situation in a WLAN, but do not adequately characterize that of a WSN. In contrast to a WLAN, a WSN may consist of a large number (thousands) of nodes. Each individual node contributes only a small amount of traffic to the network through sparse access to the channel (i.e., low duty cycle). Such a setup makes a model with an infinite-population and moderate traffic load more appropriate for analyzing random channel access in a WSN.
In this paper, we investigate the energy efficiency of a multi-channel non-persistent CSMA MAC protocol for a WSN with an infinitely large node population. To improve the energy efficiency, defined as the energy consumption for successfully transmitting a bit, we consider the joint optimization of the modulation scheme (physical layer) and packet retransmission probability (MAC layer). We assume that at the physical layer, a node is capable of adjusting its modulation order according to the instantaneous traffic load of the system. By using adaptive modulation, the system can control the transmission duration of each packet, leading to a controllable traffic load. The key advantage of using a multi-channel scheme is that the traffic load in the network can be distributed over different channels, which leads to fewer collisions and improved capacity. As we show later, this allows for more energy saving and higher network utilization. It should be noted that multi-channel CSMA protocols for wireless networks have been previously considered [12], [11], [10], [14], [7]. It was shown that such protocols are more efficient than their single-channel counterparts. However, these previous works have not considered the joint optimization of the physical layer and the MAC layer, and thus leave the room for further energy efficiency improvement.
The remainder of this paper is organized as follows. In Section 2, the system model is presented. An analytical expression for the transmission delay is derived in Section 3. In Section 4, the energy efficiency is optimized. Section 5 describes the proposed protocol. In Section 6, numerical and simulation results are presented. Conclusions are presented in Section 7.
Section snippets
System model
We consider the system in Fig. 1. The available bandwidth R (in symbols/s) is divided into J + 1 non-overlapping additive white gaussian noise (AWGN) channels. One channel is used for control, while the remaining J channels are used for data. Each data channel has a transmission rate Ri symbols/second. The functional abstraction of a node contains three components: a packet generator, an M-ary quadrature amplitude modulation (MQAM)-based physical layer, and a multi-channel non-persistent
Delay analysis
Before deriving the minimum per-bit energy efficiency, we first express the distribution of the packet transmission delay D in closed form.
Minimum per-bit energy for a successful transmission
To evaluate the effectiveness of the proposed adaptive cross-layer design, we derive the minimum per-bit energy consumption that guarantees the delay requirement. Let be the normalized delay bound. The packet loss probability due to delay is given byTo satisfy an upper bound δ on the packet loss probability, the minimum success probability must satisfyTypical WSNs applications are characterized by low-power, low-rate, and
Protocol design
In this section, we describe the proposed multi-channel non-persistent MAC protocol. This protocol is based on RTS–CTS-data-ACK handshaking. We first summarize our main assumptions:
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The control channel has a bandwidth Wc, which is determined off-line and is fixed. The remaining bandwidth (the major part) is evenly divided among the J non-overlapping data channels.
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All the data channels have the same backoff distribution; the traffic load is equally distributed over these channels.
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Since there is
Numerical examples and simulation results
In this section, we present numerical examples and simulation results for our proposed scheme. We conduct numerical experiments using MATLAB to evaluate the efficiency of the proposed joint backoff-modulation optimization. We also perform simulations using CSIM (CSIM is a C-based process-oriented discrete-event simulation package) [19] to validate our assumptions. In our numerical examples, we set L = 1000 bits, R = 250 Ksymbols/s, Tlimit = 500 ms, δ = 0.01, and the largest distance in the network dmax = 300
Conclusions
In this work, we proposed a novel cross-layer design for multi-channel non-persistent CSMA, typically used in wireless sensor networks. Our design combines bandwidth partitioning and adaptive modulation at the physical layer with adaptive backoff at the MAC layer for the purpose of maximizing the energy efficiency. The modulation order and the backoff probability at each node are periodically adapted according to the traffic load. Numerical results demonstrate the significant improvement in the
Haythem A. Bany Salameh received the B.Sc. and M.S. degrees in electrical engineering (with the highest distinction) from Jordan University of Science and Technology, Irbid, Jordan, in 2003 and 2005, respectively. Yarmouk University, Irbid, awarded him a scholarship to pursue his study. He is currently working toward the Ph.D. degree in electrical and computer engineering at the University of Arizona, where he is a Research Assistant. His current research interests include cognitive radio
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Haythem A. Bany Salameh received the B.Sc. and M.S. degrees in electrical engineering (with the highest distinction) from Jordan University of Science and Technology, Irbid, Jordan, in 2003 and 2005, respectively. Yarmouk University, Irbid, awarded him a scholarship to pursue his study. He is currently working toward the Ph.D. degree in electrical and computer engineering at the University of Arizona, where he is a Research Assistant. His current research interests include cognitive radio networks, cross-layer energy minimization for wireless and sensor networks, channel access protocols, and general communication theories.
Tao Shu received the B.S. and M.S. degrees in electronic engineering from the South China University of Technology, Guangzhou, China, in 1996 and 1999, respectively, and the Ph.D. degree in electronic engineering from Tsinghua University, Beijing, China, in 2003. He was a postdoctoral research associate at the electrical and computer engineering department at the University of Arizona, Tucson, USA, from May 2004 to August 2006. He is currently a researcher and a project leader at DoCoMo Beijing Labs. His research interests include resource allocation in wireless cellular and sensor networks, optimization of physical and MAC layers in wireless communication systems, and queuing theory.
Marwan Krunz is a professor of electrical and computer engineering at the University of Arizona and the director of the advanced networking and wireless communications group. He received his Ph.D. degree in electrical engineering from Michigan State University in 1995. From 1995 to 1997, he was a postdoctoral research associate with the Department of Computer Science at the University of Maryland, College Park. He joined the University of Arizona in January of 1997. He previously held visiting research positions at INRIA (Sophia Antipolis, France), HP Labs (Palo Alto, California), Paris VI, and US West (now Qwest) Advanced Technologies. His research interests lie in the fields of computer networking and wireless communications. His recent interests include power/rate control in wireless and sensor networks, channel access and routing protocols, media streaming, quality of service routing, and optical networking. He previously worked on traffic analysis and performance evaluation, packet video modeling, and QoS provisioning in high-speed networks. He is a recipient of the National Science Foundation CAREER Award (1998–2002). He currently serves on the editorial board for the IEEE/ACM Transactions on Networking, the IEEE Transactions on Mobile Computing, and the Computer Communications Journal. He was a guest co-editor for special issues in IEEE Micro and IEEE Communications Magazines. He served as a technical program chair for the IEEE WoWMoM 2006, the IEEE International Conference on Sensor and Ad hoc Communications and Networks (SECON 2005), Santa Clara, September 2005; the IEEE INFOCOM 2004 Conference, Hong Kong, March 2004; and the 9th Hot Interconnects Symposium, Stanford University, August 2001. He has served and continues to serve on the executive and technical program committees of many international conferences and on the panels of several NSF directorates. He gave several tutorials at premier wireless networking conferences. He is a senior member of the IEEE and a member of the ACM.
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An abridged version of this paper was presented at the IEEE GLOBECOM Conference, San Francisco, November 2006. This work was supported in part by the National Science Foundation under Grants CNS-0627118 and CNS-0313234.