IEEE Communications Magazine • August 2009 92 0163-6804/09/$25.00 © 2009 IEEE INTRODUCTION The conventional sensor network communica- tion model assumes the deployment of low-cost, multifunctional sensor nodes operating on limit- ed power capacity of their batteries, which can- not be recharged due to dense and random wireless sensor network (WSN) deployment. Research efforts thus far have sought new meth- ods to prolong the limited lifetime of WSNs through efficient computing and communication techniques [1]. However, the finite-capacity bat- teries eventually deplete, and the WSN runs out of energy. In conclusion, a battery powered WSN is a disposable system, the use of which is strictly limited by the life span of the batteries. An alternative source of power, particularly a source without limited capacity, should be con- sidered for WSNs. External radio frequency (RF) power, in this regard, stands as a promising source for WSNs. The problem to be investigat- ed here is whether it is practical to remotely feed the sensor nodes with this new power source. Considering remote feeding with RF power, RFID emerges as a progressing technology for a number of applications [2]. In passive RFID tags, the whole system is run on the power from an external RF source. The RF power incident on the tag is converted to DC power, which, in turn, operates the internal circuitry of the tag. The tag transmits the information back to the source by modulated backscattering (MB), which is basically modulating the incident RF signal by passively switching the reflection characteristics of the tag [3]. Switching is also accomplished by DC power converted from the incident RF sig- nal. Since no active transmission is involved, the power consumption for communication is very low on passive RFID tags. However, the range of these systems is very short, usually not exceed- ing 10 m [2]. Passive RFID tags generally transmit stored identification information. In a sensor network, the data obtained by the sensors should be trans- mitted. The development of wireless, remotely powered telemetry systems [4], in this regard, is an encouraging process. In [4] RF power can be stored on the node and consumed to run a tem- perature sensor with a transmitter. In the literature currently there is only one passive sensor implementation with RF transmit- ters, with which sensors communicate using MB at a range of approximately 15 m indoors, with 5 mW transmission power at a rate of 10 b/s [5]. The implemented system is designed for simulta- neous reception of signals that are backscattered from several ultra-low-cost sensors. While there are some preliminary studies that aim to inte- grate RFID with sensor networks in order to improve sensing capabilities [6], to the best of our knowledge, there has been no effort intend- ed to address the energy limitation problem of WSNs from a fundamentally different approach. Rather than enhancing the lifetime of the network within the conventional WSN approach, a completely new sensor networking paradigm, wireless passive sensor networks (WPSNs), free of battery lifetime constraint, is introduced in this article. The objective of this work is to investigate the potential of eliminating the life- time constraint of WSNs and point out the chal- lenges for efficient and reliable communication in WPSNs and related open research issues to the research community. A WPSN is non-dispos- able, more functional and cost efficient, runs as long as power is delivered in, and remains idle but ready to operate when no power is incident on the network. The rest of the article is organized as follows. In the next section an overview of the WPSN system model, the communication architecture, and several possible topologies are introduced. We then present the theoretical background of WPSNs. Discussions on alternative communica- tion schemes and protocol related issues for WPSNs are presented in the following section, along with open research challenges. The final section concludes the article. ABSTRACT The primary challenge in wireless sensor net- work deployment is the limited network lifetime due to finite-capacity batteries. Hence, the vast majority of research efforts thus far have focused on the development of energy-efficient commu- nication and computing mechanisms for WSNs. In this article a fundamentally different approach and hence completely new WSN paradigm, the wireless passive sensor network, is introduced. The objective of the WPSN is to eliminate the limitation on system lifetime of the WSN. In a WPSN power is externally supplied to the sensor network node via an external RF source. Modu- lated backscattering is discussed as an alternative communication scheme for WPSNs. The feasibil- ity is investigated along with the open research challenges for reliable communication and net- working in WPSNs. ACCEPTED FROM OPEN CALL Ozgur B. Akan, M. Talha Isik, Buyurman Baykal, Middle East Technical University Wireless Passive Sensor Networks This work was supported in part by the Turkish Scientific and Technical Research Council (TUBITAK) Career Award under grant #104E043 and by Turkish National Academy of Sciences Distinguished Young Scientist Award Program (TUBA-GEBIP).