Abstract-Use of a passive wake-up radio can drastically increase the network lifetime in a sensor network by reducing or even completely eliminating unnecessary idle listening. A sensor node with a wake-up radio receiver (WuRx) can operate in an extremely low power sleep mode until it receives a trigger signal sent by a wake-up radio transmitter (WuTx). After receiving the trigger signal, the attached WuRx wakes up the sensor node to start the data communication. In this paper, we implement and compare the performance of three passive wake-up radio-based sensor nodes: 1) WISP-Mote, which is a sensor mote that employs an Intel WISP passive RFID tag as the WuRx; 2) EH-WISP-Mote, which combines a novel energy harvester with the WISP-Mote; and 3) REACH-Mote, which uses the energy harvester circuit combined with an ultra-low-power pulse generator to trigger the wake-up of the mote. Experimental results show that the wake-up range and wake-up delay for the EH-WISP-Mote are improved compared with the WISP-Mote, while providing the ability to perform both broadcast-based and ID-based wake-ups. On the other hand, the REACH-Mote, which can only provide broadcast-based wake-up, can achieve a much longer wake-up range than any known passive wake-up radio to date, achieving feasible wake-up at a range of up to 37f t.

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The effect of the pinning field (Hp) on the distribution of the average magnetization direction in the pinned layer (PL) and the reference layer (RL) of spin valve (SV) sensors with synthetic antiferromagnetic layers was investigated with respect to the magnetic fields that can cause canting in the PL and RL magnetization. Hp was controlled by annealing temperatures and seed layers. It was found that the direction of the average magnetization in the PL and RL is largely governed by shape anisotropy in the patterned SV sensors with a low Hp. Both experimental and modeling results support the idea that the canting of the PL and RL can occur in both SV sheet films and patterned SV sensors, when Hp is not large enough to overcome the interfacial magnetostatic coupling (H1) between the free and reference layer.

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