Physical Layer Driven Protocol and Algorithm Design for Energy Efficient Wireless Sensor Networks Eugene Shih Seong Hwan Cho Nathan Ickes Rex Min Amit Sinha Alice Wang Anantha Chandrakasan Department of Electrical Engineering and Computer Science Massachusetts Institute of Technology Cambridge MA USA 02139 4307 eugene chosta nickes rmin sinha aliwang anantha mtl mit edu ABSTRACT network sensors can collaborate to perform high quality sensing and form fault tolerant sensing systems With these advantages in mind many applications have been proposed for distributed wireless microsensor networks such as warehouse inventory tracking location sensing machine mounted sensing patient monitoring and building climate control 1 3 4 5 The potential for collaborative robust networks of microsensors has attracted a great deal of research attention For the most part this is due to the compelling applications that will be enabled once wireless microsensor networks are in place location sensing environmental sensing medical monitoring and similar applications are all gaining interest However wireless microsensor networks pose numerous design challenges For applications requiring longterm robust sensing such as military reconnaissance one important challenge is to design sensor networks that have long system lifetimes This challenge is especially difficult due to the energyconstrained nature of the devices In order to design networks that have extremely long lifetimes we propose a physical layer driven approach to designing protocols and algorithms We first present a hardware model for our wireless sensor node and then introduce the design of physical layer aware protocols algorithms and applications that minimize energy consumption of the system Our approach prescribes methods that can be used at all levels of the hierarchy to take advantage of the underlying hardware We also show how to reduce energy consumption of non ideal hardware through physical layer aware algorithms and protocols 1 While the applications enabled by wireless microsensor networks are very attractive in order to build well functioning robust systems there are many system challenges to resolve Furthermore because the proposed applications are unique wireless microsensor systems will have different challenges and design constraints than existing wireless networks e g cellular networks and wireless LANs For instance since the number of sensors will be large node densities will be high up to 20 nodes m3 and large amounts of data will be produced Thus large scale data management techniques will be needed Secondly user constraints and environmental conditions such as ambient noise and event arrival rate can be time varying in a wireless microsensor network Thus the system should be able to adapt to these varying conditions In addition to these challenges the energy consumption of the underlying hardware is also of paramount importance Because applications involving wireless sensor networks require long system lifetimes and fault tolerance energy usage must be carefully monitored Furthermore since the networks can be deployed in inaccessible or hostile environments replacing the batteries that power the individual nodes is undesirable if not impossible INTRODUCTION In recent years the idea of wireless microsensor networks has garnered a great deal of attention by researchers including those in the field of mobile computing and communications 1 2 A distributed ad hoc wireless microsensor network consists of hundreds to several thousands of small sensor nodes scattered throughout an area of interest Each individual sensor contains both processing and communication elements and is designed to monitor the environment for events specified by the deployer of the network Information about the environment is gathered by the sensors and is delivered to a central basestation where the user can extract the desired data Because of the large number of nodes in such a This need to minimize energy consumption and to maximize the lifetime of a system makes the design of wireless sensor networks difficult For example since packets can be small and data rates low low duty cycle radio electronics will be used in the system However designing such circuits to be energy efficient is technically challenging As we will show current commercial radio transceivers such as those proposed for the Bluetooth standard 6 are not ideal for microsensor applications since the energy overhead of turning them on and off is high Thus innovative solutions in transceiver and protocol design are required to achieve efficient transmission of short packets over short distances Another challenge arises due to the remote placement of these nodes and the high cost of communication Since sensors are remotely deployed transmitting to a central basestation has high energy cost Thus the use of data aggregation schemes to reduce the amount of redundant data in the network will be beneficial 7 Finally since environmental conditions and user constraints can be time varying the use Permission to make digital or hard copies of part or all of this work or personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page To copy otherwise to republish to post on servers or to redistribute to lists requires prior specific permission and or a fee ACM SIGMOBILE 7 01 Rome Italy 2001 ACM ISBN 1 58113 422 3 01 07 5 OO 272 is relatively new many energy efficient network protocols for adhoc wireless networks have been presented In 11 12 13 techniques and metrics to evaluate and design energy efficient routing and MAC protocols for wireless networks are presented Energyefficient protocols that adapt transmit output power and or error correction control parameters have been explored extensively by a number of researchers 14 15 16 17 In 14 the authors use an adaptive radio designed for wireless multimedia communications over ATM as a model In that paper frame length and forwarderror correction parameters are adapted to lower energy consumption of the radio and improve throughput as conditions of the channel change A similar study is performed by 16 in the context of a cellular style network but the output transmit power is also considered In 17 an energy efficient protocol that adjusts both RF transmit power and error control strategy is examined for 802 11