Gerd Ulrich Gamm

Low power wake-up receiver for wireless sensor nodes

Duty cycling is a common method to reduce the energy consumption in wireless sensor nodes. Since all the nodes turn on their radio in a time slot, but only few of them actually exchange messages, energy is wasted. This can be avoided by using a separate wake-up radio. We present a new approach for a wake-up receiver that consumes 2.78 µA of current and integrates a 16 bit address coding for a selective wake-up. We use a low frequency wake-up signal that is modulated on a high frequency carrier in the main radio of the transmitting node. In the receiving node a passive demodulation circuit regains the low frequency signal and feds it to a low power low frequency wakeup IC. Four sample boards were manufactured and measured that operate at 868 MHz in the European ISM band. By using coils with high Q factors in the demodulation circuit we reached a sensitivity of −52 dBm which resulted in a wake-up distance of up to 40 meters at an output power level of +10 dBm in the transmitter. Compared to van der Doorn et. al, “A prototype low-cost wakeup radio for the 868 MHz band” Int. J. Sensor Networks, Vol.5, No.1, 2009, we achieved a 20 times farther wakeup range and a 100 times lower current consumption.

Low-power sensor node with addressable wake-up on-demand capability

The ambition to equip more and more consumer products with sensor intelligence and communication capability entails two major demands for wireless sensor nodes. First, they have to operate for several years. Second, they must be permanently accessible for communication requests. These two demands are in contrast to each other. Regarding a transceiver in permanent receiving mode, the power consumption of a few milliwatts implicates a lifetime in the order of days for a common battery. Addressing the challenge to solve these conflicts, we present a new wake-up receiver approach in this paper. Our solution consumes 5.6 μW of power while listening for a communication request. Furthermore, it possesses a 16-bit address coding for specific WSN wake-up. And moreover, an improved impedance matching with a high sensitivity improves the wake-up distance significantly to 40 m at +10 dBm output power. This approach empowers wireless sensor nodes for the first time to be usable for a wide range of applications.

Performance evaluation and comparative analysis of SubCarrier Modulation Wake-up radio systems for energy-efficient wireless sensor networks

Energy-efficient communication is one of the main concerns of wireless sensor networks nowadays. A commonly employed approach for achieving energy efficiency has been the use of duty-cycled operation of the radio, where the nodes transceiver is turned off and on regularly, listening to the radio channel for possible incoming communication during its on-state. Nonetheless, such a paradigm performs poorly for scenarios of low or bursty traffic because of unnecessary activations of the radio transceiver. As an alternative technology, Wake-up Radio (WuR) systems present a promising energy-efficient network operation, where target devices are only activated in an on-demand fashion by means of a special radio signal and a WuR receiver. In this paper, we analyze a novel wake-up radio approach that integrates both data communication and wake-up functionalities into one platform, providing a reconfigurable radio operation. Through physical experiments, we characterize the delay, current consumption and overall operational range performance of this approach under different transmit power levels. We also present an actual single-hop WuR application scenario, as well as demonstrate the first true multi-hop capabilities of a WuR platform and simulate its performance in a multi-hop scenario. Finally, by thorough qualitative comparisons to the most relevant WuR proposals in the literature, we state that the proposed WuR system stands out as a strong candidate for any application requiring energy-efficient wireless sensor node communications.

Smart metering using distributed wake-up receivers

Readout of meters installed in private buildings often need a technician to be physical on-site. The readout data is manually stored in a Laptop. This is very inconvenient for the habitants of the building and very time consuming for the technician. When using a conventional radio to transmit the readout data, a battery powered device would be depleted after a few days. Listening for incoming orders is nearly as energy consuming as sending data itself. We are using wake-up receivers in the metering devices to listen for a wake-up request. Our addressable wake-up receiver consumes 2.8 μA of current and enables a coin cell driven device to be operable for several years. A handheld device connected to a notebook is used to wake-up the sleeping devices from outside the building. After wake-up, the devices turn on their main radio and send the readout data to the handheld device. With our system it would be possible to collect all metering data of a whole street by just driving by in a car and waking up one device after the other.

Range extension for wireless wake-up receivers

Wireless Wake-up receivers are used whenever a long lifetime and a permanent accessibility are required. The main disadvantage they have is their short range of communication of few meters. We therefore present in this work an infrastructure for a single hop network that makes use of a powerful gateway node that can send out wake-up signals with up to +20 dBm. The end point nodes have an integrated wake-up receiver and consume 3.5μA of current. The wake-up range was measured up to 90 meters in an open air field.

Smartphone remote control for home automation applications based on acoustic wake-up receivers

In most home automation scenarios electronic devices like shutters or entertainment products (Hifi, TV) are constantly in a standby mode that consumes a considerable amount of energy. The standby mode is necessary to react to commands triggered by the user. To reduce the standby current we present a node that can be attached to the plug of electronic devices and that can turn them on and off. The node contains a wake-up receiver module that reacts to an acoustic 18 kHz tone and that switches the node from active to passive mode. In active mode the node can turn on or off the respectively power source for the device under control. The acoustic wake-up signal can be sent out by any kind of speaker which enables a commercial smartphone to act as an universal acoustic remote control without line-of-sight requirement. Our wake-up receiver consists of an 18 kHz LF receiver and an MEMs-Microphone. A wake-up range of 7.5 m using a smartphone as a sender was achieved. The overall power consumption was measured to 56 μW in standby mode. Using a 230 mAh coin cell as the energy supply a theoretical lifetime of 500 days is possible.

Low power wireless sensor node for use in building automation

In modern buildings the majority of sensors and actuators used for controlling temperature, roller shutters or alarm systems are cable wired. The use of wireless sensor nodes in such a scenario would facilitate the installation and would reduce costs significantly. However the main problem is the energy consumption of the nodes. Real time behavior and lifespans of several years seem to be conflictive. In this work we present a wireless sensor node with attached wake-up radio, that consumes as few as 2.78 µA current in sleep mode. The wake-up signal that triggers the node from sleep to active mode consists of an 125 kHz square signal modulated on an 868 MHz carrier. For a selective wake-up of different nodes the 125 kHz signal is additionally modulated with an 16 bit address information. In a sleeping node the incoming wake-up signal is passively demodulated and fed to a separate 125 kHz wake-up receiver IC. The sensitivity of the node was measured to −50dBm which means a wake-up distance of 40 meters at +10dBm output power in an open space scenario. Inside buildings the signal was able to pass through two concrete walls.

Wake-up receiver operating at 433 MHz

Sensor nodes often have to work for a long time with a single battery. A change of the power source is sometimes not possible or involves effort and costs. If the node needs to be accessible for communication at any point of time it must have a radio in permanent receive state. This depletes the battery in a few days. The contradiction between long lifetime and permanent accessibility can be solved by using a separate wake-up receiver on the node. In this work we present a sensor node with included wake-up receiver working in the 433MHz ISM band. Our solution consumes 2.8 μA of current in sleep state while still maintaining a realtime behaviour. The low current consumption is achieved by modulating a 125 kHz wake-up signal on the 433MHz carrier in the sender. In the receiving node a passive demodulation circuit extracts the wake-up signal and feeds it to an 125 kHz low frequency receiver IC. An additional 16 bit address coding is used for an selective wake-up of nodes.

Slot antenna packaging for infrastructure monitoring based on wake-up transceiver strategy

The use of the package as a radiating element is the optimal solution in term of robustness for wireless sensor network (WSN) applications such as infrastructure monitoring. A robust, 83 % efficient and 6.5 dBi gain slot antenna packaging (simulation values in air) for wireless sensor nodes working at 868 MHz is presented in this paper. A low power wake-up radio transceiver strategy allowing a current consumption of 3.8 μA is associated to the radiating package. The theoretical battery life time for a 3 V input voltage and a 220 mA/h battery capacity is more than 6 years using the proposed system. Therefore the combination between the antenna packaging and wake-up approach increases the wireless system life time. Finite Element Method (FEM) simulation and experimental results are presented to show the effects of different concrete supports on the antenna performances. Moreover, indoor and outdoor wake-up rate measurements show the coverage efficiency of the complete device. Using a transmission power of +10 dBm and a wake-up sensitivity of -51.8 dBm, a distance of 55 meters is reached in comparison to more than 250 meters for data transmission.

Using quartz resonators for maximizing wake-up range in wireless wake-up receivers

Separate wake-up receiver circuits for wireless sensor nodes combine the advantage of permanent accessibility with a very low current consumption, enabling nodes to work for several years. In a sleeping node incoming signals are passively demodulated and low-pass filtered. Till now we used an R-C circuit for filtering. In this work a quartz resonator is used between its parallel and serial resonance as an inductance with high Q-factor. This way higher voltages after demodulation compared to the R-C circuit could be reached. Since a higher voltage results in an increased wake-up signal detection, the wake-up range of the node can be extended.