Backscatter Sensor Network for Extended Ranges and Low Cost with Frequency Modulators: Application on Wireless Humidity Sensing Eleftherios Kampianakis, John Kimionis, Konstantinos Tountas, Chris Konstantopoulos, Eftichios Koutroulis and Aggelos Bletsas ECE Dept., Technical University of Crete, Chania, Greece 73100 {ekabianakis, ikimionis, ktountas, ckonstantopoulos}@isc.tuc.gr, efkout@electronics.tuc.gr, aggelos@telecom.tuc.gr Abstract— Dense monitoring of environmental parameters (e.g. air/soil humidity, ambient temperature) is critical in precision agriculture, urban area monitoring and environmental modeling applications. In this paper, the design of a novel wireless sensor network (WSN) is proposed, consisting of low-power and low- cost sensor nodes, deployed in a bistatic architecture (i.e. carrier emitter in a different location than the receiver) and achieving long-range backscatter communication. The tags modulate sen- sor information using analog frequency modulation (FM) and frequency division multiple access (FDMA) at the subcarrier frequency, even though a single carrier is assumed. In sharp contrast to prior art, the developed backscatter sensor network performs environmental monitoring over a relatively wide area. A proof-of-concept prototype WSN application has been developed for capacitive relative humidity (RH) sensing, with 1.5 mW per tag, 0.9 RMSE and range on the order of 50 m. I. I NTRODUCTION Wireless sensor networks (WSNs) have become a field of tremendous research interest due to a rich and diverse variety of applications, including large-scale environmental sensing [1], [2]. A common WSN platform typically consists of a micro-controller unit (MCU) and an active radio for sensor data transmission. Despite the fact that literature develops algorithms and protocols for ultra large-scale networks, on the order of hundred of nodes, large-scale outdoor demonstration deployments are rare. Work in [3] demonstrated one of the very few examples of a large-scale, outdoor deployment. How- ever each node cost was on the order of 50 e, amplifying the overall network cost and thus, restricting scaling opportunities. Additionally, work in [4] studied the complexity of cross- layer application development in typical WSN platforms and emphasized the difficulty to implement software for robust sensor networks. On the other hand, recent work relevant to backscatter radio has shown significant progress towards low-cost and low- power sensing (e.g. work in [5]–[8]). Backscatter communi- cation can be implemented with a single antenna, a switching transistor and logic that controls that switch, such that the sensor’s antenna is alternatively terminated at different loads. However, inductive coupling and/or monostatic battery-less setups result in short communication range, limiting the use Z1 Z2 Antenna S11 Parameters 1 2 Z1 Z2 Vin RF Frontend F c Frequency Spectrum F c + F 0 F c - F 0 Fig. 1. Backscatter radio principle: a single, low-cost RF switch is utilized that alternates the antenna termination load between two values (left). Different antenna termination loads offer different reflection coefficients (i.e. S11 parameter, middle) that modulate the induced carrier signal (at frequency Fc ) with different amplitude and phase. Switching frequency between the two load states varies with the measured quantity and is called subcarrier frequency, F 0 . Reader Tag Tag Tag Tag Tag Carrier Emitter 1 Tag Tag Carrier Emitter 3 Tag Tag Tag Tag Carrier Emitter 2 Tag Tag Tag Tag Carrier Emitter 4 Tag Tag Tag Fig. 2. Bistatic backscatter sensor network architecture. Low-cost carrier emitters produce the carrier, tags modulate the scattered signals received by a single reader (receiver).