Energy-efficient communication technologies are a key enabler for many IoT applications. Many existing communication protocols are based on duty-cycling techniques, that have an inherent tradeoff between delay and energy consumption. In the field of sensor networks, wake-up receivers have been investigated to overcome these problems and to further reduce energy consumption. We now go one step further and investigate the use of wake-up receivers in combination with IEEE 802.11 WLAN. We extend the protocol used to communicate between the access point and the client to introduce a wake-up signal. This can be implemented in a way that is fully compatible with existing wireless LAN (WLAN) standards, thus, it can be deployed gradually with little effort and no need to change existing systems. As a proof of concept and to perform first lab experiments, we developed a hardware prototype using a selective wake-up receiver and off-the-shelf USB-WLAN dongles. All experimental results are verified using an analytical model and a detailed simulation study. We show that our wake-up WLAN can provide connectivity for low-power devices with low delays and low energy consumption at the same time.

Ultra-low power communication is critical for supporting the next generation of battery-operated or energy harvesting battery-less Internet of Things (IoT) devices. Duty cycling protocols and wake-up receiver (WuRx) technologies, and their combinations, have been investigated as energy-efficient mechanisms to support selective, event-driven activation of devices. In this paper, we go one step further and show how WuRx can be used for an efficient and multi-purpose low power downlink (LPD) communication channel. We demonstrate how to (a) extend the wake-up signal to support low-power flexible and extensible unicast, multicast, and broadcast downlink communication and (b) utilize the WuRx-based LPD to also improve the energy efficiency of uplink data transfer. In addition, we show how the non-negligible energy overhead of conventional microcontroller based decoding of LPD communication can be substantially reduced by using the low-power universal asynchronous receiver/transmitter (LPUART) module of modern microcontrollers. Via experimental studies, involving both a functioning prototype and larger-scale simulations, we show that our proposed approach is compatible with conventional WLAN and offers a two-orders-of-magnitude improvement in uplink throughput and energy overheads over a competitive, IEEE 802.11 PSM-based baseline. This new LPD capability can also be used to improve the RF-based energy harvesting efficiency of battery-less IoT devices.

We present a flexible real-time software-based multi-band channel sounder, which can flexibly be used in a variety of measurement campaigns, and fills the gap of expensive commercial channel sounders with limitations of operation frequency, sounding bandwidth, and ease of use. In general, channel sounding is a very valuable tool for designing wireless communication systems. Even though commercial channel sounders allow for thorough and detailed measurements, they are typically very expensive. In contrast, existing software-based solutions are often limited in scope and complexity of measurements. Our system closes this gap. Using GNU Radio and USRP radio front-ends, the system provides a high degree of freedom with respect to supported carrier frequencies and bandwidth. We also support features such as automatic spectrum-sweeping and transmitter gain management for dynamic channel characterization. We can use the system for both live visualization of the channel characteristics, e.g., Channel Impulse Response, Channel Frequency Response, Power Delay Profile, and Channel Phase Response, as well as for fully automated and repeatable experiments.

Advances in miniaturization of sensor nodes enable a wide range of novel application scenarios. At the same time, however, this miniaturization drastically reduces the energy available for communication. We focus on wildlife monitoring applications for bats, which set a weight limit of 2 g for the sensor node including the battery. Here, the protocol design is complicated by the need to recharge a capacitor before each communication attempt. For communications with ground stations, wake-up receivers are used that inherently help mitigate synchronization demands and to provide a superframe structure. We study the not obvious choice of transmission slots within these synchronized superframes. Our findings clearly indicate that slotted access outperforms simple random channel access. Well-planned TDMA schedules only bring little gain compared to random slot selection.

Duty cycling has been the main concept for saving energy in sensor networks for a long time. Yet, additional overhead for synchronization and the fact that overhearing and idle listening cannot completely be prevented, motivated further research. Wake-up receiver, i.e., additional ultra-low power radios that are not switched off and can receive a so called wake-up signal, aim to fill this gap. The wake-up signal can either be a simple energy burst that wakes up all nodes in reception range or can include an address to wake up individual nodes. In this paper, we introduce a novel addressing scheme for wake-up receivers that supports broadcast, multicast, and unicast wake-ups, and which can be realized in hardware with little additional complexity at the receiver. We implemented a prototype based on a commercially available wake-up receiver to show the feasibility of our approach. We discuss the tradeoff between the length of the wake-up signal and the possible energy savings and show an application example for sending software updates to sensor nodes in an energy saving way.