Duty-cycled Wake-up Schemes for Ultra-low Power Wireless Communications

In sensor network applications with low traffic intensity, idle channel listening is one of the main sources of energy waste.The use of a dedicated low-power wake-up receiver (WRx) which utilizes duty-cycled channel listening can significantlyreduce idle listening energy cost. In this thesis such a scheme is introduced and it is called DCW-MAC, an acronym forduty-cycled wake-up receiver based medium access control.We develop the concept in several steps, starting with an investigation into the properties of these schemes under idealizedconditions. This analysis show that DCW-MAC has the potential to significantly reduce energy costs, compared to twoestablished reference schemes based only on low-power wake up receivers or duty-cycled listening. Findings motivatefurther investigations and more detailed analysis of energy consumption. We do this in two separate steps, first concentratingon the energy required to transmit wake-up beacons and later include all energy costs in the analysis. The more completeanalysis makes it possible to optimize wake-up beacons and other DCW-MAC parameters, such as sleep and listen intervals,for minimal energy consumption. This shows how characteristics of the wake-up receiver influence how much, and if, energycan be saved and what the resulting average communication delays are. Being an analysis based on closed form expressions,rather than simulations, we can derive and verify good approximations of optimal energy consumption and resulting averagedelays, making it possible to quickly evaluate how a different wake-up receiver characteristic influences what is possible toachieve in different scenarios.In addition to the direct optimizations of the DCW-MAC scheme, we also provide a proof-of-concept in 65 nm CMOS,showing that the digital base-band needed to implement DCW-MAC has negligible energy consumption compared to manylow-power analog front-ends in literature. We also propose a a simple frame-work for comparing the relative merits ofanalog front-ends for wake-up receivers, where we use the experiences gained about DCW-MAC energy consumption toprovide a simple relation between wake-up receiver/analog front-end properties and energy consumption for wide ranges ofscenario parameters. Using this tool it is possible to compare analog front-ends used in duty-cycled wake-up schemes, evenif they are originally designed for different scenarios.In all, the thesis presents a new wake-up receiver scheme for low-power wireless sensor networks and provide a comprehensiveanalysis of many of its important properties

Comparing Analog Front-Ends for Duty-Cycled Wake-Up Receivers in Wireless Sensor Networks

Using ultralow-power wake-up receivers (WRxs) can reduce idle listening energy cost in wireless sensor networks with low traffic intensity. This has led to many WRx analog front-end (AFE) designs presented in literature, with a large variety of trade-offs between the sensitivity, the data rate, and the power consumption. Energy consumed during wake-up in a network depends on many parameters and without a unified energy analysis, we cannot compare performance of different AFEs. We present an analysis of duty-cycled WRx schemes which provides a simple tool for such a comparison based on the energy consumed in an entire single-hop network during a wake-up. The simplicity is largely due to the fact that all network and communication parameter settings can be condensed into a single scenario constant. This tool allows us to both compare AFEs for specific scenarios and draw more general conclusions about AFE performance across all scenarios

Improving practical sensitivity of energy optimized wake-up receivers: proof of concept in 65nm CMOS

We present a high performance low-power digital base-band architecture, specially designed for an energy optimized duty-cycled wake-up receiver scheme. Based on a careful wake-up beacon design, a structured wake-up beacon detection technique leads to an architecture that compensates for the implementation loss of a low-power wake-up receiver front-end at low energy and area costs. Design parameters are selected by energy optimization and the architecture is easily scalable to support various network sizes. Fabricated in 65nm CMOS, the digital base-band consumes 0.9uW (V_DD=0.37V) in sub-threshold operation at 250kbps, with appropriate 97% wake-up beacon detection and 0.04% false alarm probabilities. The circuit is fully functional at a minimum V_DD of 0.23V at f_max=5kHz and 0.018uW power consumption. Based on these results we show that our digital base-band can be used as a companion to compensate for front-end implementation losses resulting from the limited wake-up receiver power budget at a negligible cost. This implies an improvement of the practical sensitivity of the wake-up receiver, compared to what is traditionally reported.Comment: Submitted to IEEE Sensors Journa

Experimental characterization of the body-coupled communications channel

Body-coupled communications (BCC), in which the human body is used as a communications channel, has been shown to be a promising solution for wireless body-area networks (WBANs). For successful deployment of these BCC-based WBANs, it is necessary to develop a clear understanding of the channel behavior. Therefore, this paper presents the key characteristics of the capacitively-coupled on-body channel used for BCC. This is based on an experimental study, which was carried out with a specifically designed measurement system. The goal of the study was to reveal the influence of electrode design, electrode position and body motion on the propagation loss and to characterize the experienced interference. It is concluded that the maximum propagation loss for the whole body channel is below 80 dB. Moreover, the frequency dispersion and the influence of body movement on channel attenuation are shown to be much smaller than for radio frequency (RF) WBAN channels. From the results we conclude that BCC can result in a simpler, more robust, and lower-power WBAN than what is achievable with traditional RF solutions

Knowledge diversities and group dynamics within international master programs

Although international students are allowed to study at Swedish universities since 1985, the Swedish university landscape has undergone a strategic internationalization mainly during the recent few years. With a steadily growing number of international master programs and thereby more international students being part of the Swedish education system, a variety of problems related to cultural-diverse teaching and learning environments and their corresponding methodologies arises. Focusing in particular on knowledge diversity and heterogeneous group work, reasons for facing such problems and suitable approaches will be discussed. Amongst others, more detailed course descriptions, profiling of newly-admitted students, or more individual pre-arrival programs are proposed. Suitable ways how to actively encourage students to interact within cultural-diverse, heterogeneous working groups are discussed, highlighting the necessity of reflection and feedback

Influence of Duty-Cycled Wake-Up Receiver Characteristics on Energy Consumption in Single-Hop Networks

In sensor network applications with low traffic intensity, idle channel listening is one of the main sources of energy waste. The use of a dedicated low-power wake-up receiver (WRx), which utilizes duty-cycled channel listening, can significantly reduce the idle listening energy cost. Extreme low-power design typically leads to performance losses, indirectly increasing energy costs. Striking the right balance is, therefore, very important when introducing WRxs. We present a system analysis and heuristic parameter optimization for an existing duty-cycled WRx medium access control scheme. First, we introduce a framework for analysis of energy consumption and delay of an entire single-hop network where we include WRx characteristics. The WRx characteristics are condensed into two parameters: reduction in power consumption and associated loss of performance, compared with the main receiver. The analysis framework is used to find optimal wake-up beacon and protocol parameters for different WRx characteristics to minimize the total network energy consumption per data packet. The importance of the optimization is that it provides information on if, and how much, we can save in terms of energy by introducing a WRx with a certain characteristic. We also present accurate approximations of both optimal energy savings and resulting delays

DCW-MAC: An energy efficient medium access scheme using duty-cycled low-power wake-up receivers

In this work we present a new low-power medium access scheme for sensor-type networks specifically with low traffic intensity. We call the proposed scheme DCW-MAC where ultra-low-power wake-up receivers are combined with optimal duty-cycled listening. First we introduce a framework for the analysis of energy consumption of the studied network type, then we use it to optimize the MAC scheme to achieve very low total energy consumption per transmitted data packet. It is shown that even with large sacrifices in terms of wake-up receiver detection performance, required to achieve ultra-low-power consumption with a limited form factor, we can achieve very competitive total energy consumption and outperform other MAC schemes for scenarios with low traffic

Performance Analysis and Energy Optimization of Wake-Up Receiver Schemes for Wireless Low-Power Applications

The use of duty-cycled ultralow-power wake-up receivers (WRxs) can significantly extend a node lifetime in low-power sensor network applications. In the WRx design, both the low-power operation of the WRx and the wake-up beacon (WB) detection performance are of importance. We present a system-level analysis of a duty-cycled WRx design, including an analog front end, a digital baseband, the WB structure, and the resulting WB detection and false-alarm probabilities. We select a low-power WRx design with about two orders of magnitude lower power consumption than the main receiver. The associated cost is an increase in the raw bit error rate (BER), as compared with the main receiver, at the same received power level. To compensate, we use a WB structure that employs spreading. The WB structure leads us to an architecture for the digital baseband with high address-space scalability. We calculate closed-form expressions for detection and false-alarm probabilities. Using these, we analyze the impact of design parameters. The analytical framework is exemplified by the minimization of the WB transmit energy. For this particular optimization, we also show that the obtained re- sults are valid for all transmission schemes with an exponential relationship between the signal-to-noise ratio and the BER, e.g., the binary orthogonal schemes with noncoherent detection used in many low-power applications