Ultra-Low-Power Short-Range Radios

Design of radios with ultra-low-power consumption can enable many new and exciting applications ranging from wearable healthcare to Internet of Things devices and beyond. Achieving low power operation is usually an exercise in trading-off important performance metrics with power. This chapter presents an overview of state-of-the-art narrowband architectures and techniques that achieve ultra-low-power operation, and concludes with a section that benchmarks recent state-of-the-art designs in order to illustrate power-performance trade-offs.

Wireless body area networks (BANs) are the latest generation of personal area networks (PANs) and describe radio networks of sensors, and/or actuators, placed in, on, around and some-times near the human body. BANs are motivated by the health-care application domain where reliable, long-term, operation is paramount. Hence understanding, and modeling, the body-area radio propagation channel is vital. In this chapter we describe channel models for wireless body area networks, in terms of operating scenarios—including on the human body, off the body, in the body, and body-to-body (or interfering); carrier frequencies from hundreds of MHz to several GHz; and bandwidth of operation, including narrowband and ultra-wideband. We describe particular challenges for accurate channel modeling such as the absence of wide-sense-stationarity in typical on-body narrowband BANs. We describe results following from a large amount of empirical data, and demonstrate that the BAN channel is dominated by shadowing with slowly-changing dynamics. Finally two particularly challenging scenarios for BAN operation are described: sleep-monitoring and also where there is a large number of co-located BANs.

As low power radio circuits are enabling new technology avenues such as health monitoring, efforts to tackle the design challenge of making extremely reliable yet lost cost, low power CMOS radios have gained prominence. Despite advancements in battery technology and energy harvesting, there remains a wide gap between the available and desired performance. Innovative system architectures and circuits need to be explored to bridge this gap. Over the last few years, there have been numerous sub-mW integrated system offerings that trade off performance and reliability to achieve low power consumption. At the same time, an ever growing list of applications has led to a number of commercial products that guarantee robust operation with power consumption in 10′s of mWs. However, a vast number of applications demand sub-mW power consumption or complete energy autonomy while demanding robust operation and high performance over a highly variable environment. Implantable systems are a prime example of this. In this chapter, we will begin with an overview of system considerations, application driven challenges, and proceed to discuss the existing design approaches to make the reader aware of the practical limitations of many of these techniques. We will then identify the fundamental design challenges and explore fresh angles to approach the problem.

There is a growing class of event-driven devices that require instant-on wireless connectivity, but only use the radio to communicate intermittently throughout their lifetime. Home automation devices and most wellness monitors fall into this class, only using or needing their radios when prompted by an event. In these applications, the radios dominate the amount of energy consumed from the batteries. More specifically, the energy spent synchronizing the radios, or maintaining a connected state, dominates, as opposed to the energy spent communicating data.

This chapter looks at various practical aspects of architecting and designing low power wireless radios and systems-on-chip for applications such as consumer wearables, industrial automation etc. The chapter starts with a discussion on the need for industry accepted protocols for low power wireless and aspects in these protocols that lend themselves to low power implementations. With these protocols in place, we then look at practical design techniques of the RF/analog components, followed by a look at the Physical layer and the MAC and conclude the section by looking at the overall SoC design techniques for proper energy management. The chapter concludes by looking at the upcoming IEEE 802.11ah standard and discuss how this is adapted in an advantageous manner for low power wireless applications.

An effective method to reduce radio power consumption in applications whose average data rates fall below the maximum data rate capabilities of the underlying radio is to duty-cycle the radio front ends between bursts of data transmission. Doing so in a network, however, requires careful synchronization amongst cooperating radios, often requiring ultra-low-power, yet precise and stable clocks. Such synchronization clocks can be implemented as low frequency crystal oscillators, temperature compensated crystal oscillators, MEMS oscillators, or integrated oscillators. Each of these options has advantages and disadvantages. It is important to understand both the expected system duty cycle and temperature variation as well as the required form factor, cost, and power consumption to know which clock source is most appropriate for the application. This chapter discusses these trade-offs along with several design examples.

Ultra-wideband (UWB) radios offer tremendous promise in terms of achievable data rates due to the large capacity afforded by their inherently large occupied bandwidth. While achieving ultra-high data rates may have been one of the original intents of UWB radios, pulsed-UWB radios have another potential advantage over their narrowband counterparts: energy. By exploiting the large available bandwidth in conjunction with non-coherent signaling, low-complexity and ultra-energy-efficient transmitters can be designed using all-digital architectures that do not require the use of a PLL. Similarly, energy-detecting receivers can receive pulses with low energy-per-bit at high data rates and can be rapidly duty-cycled to minimize overall power consumption. This chapter outlines the main challenges in UWB design, while discussing several representative receiver and transmitter implementations in detail.

Interest on the low energy wireless connections among humans, sensors and mobile devices is getting grown with the start of the phrase, "Internet of Things". The establishment of the Wireless Body Area Network (WBAN) standard by IEEE Standard Association in February 2012 is representing those demands. In the standard, a Human body communication or HBC is newly added and it is regarded as a new promising solution with its low energy consumption and high reliability. The research history of the HBC from the proof of the concept to the high-end standard compatible system with the network consideration will be introduced in this chapter.

This chapter describes the fundamental principles of cm-range wireless telemetry through inductive links and provides insight in regards to the methods of analysis, choice of modulation schemes, carrier frequencies, and coil design. After presenting simplified models for the inductance and mutual coupling of conductive loops, the inductive link equivalent network is derived to be used for analysis of inductive data links. Different carrier-based modulation schemes such as amplitude-shift keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK) are discussed for near-field simultaneous data and power transmission in different applications such as implantable microelectronic devices (IMDs), radio frequency identification (RFID), and smart cards. Data communication through load-shift keying (LSK) is also discussed followed by presenting the pulse-based schemes for low-power communication. Finally, new pulse-harmonic modulation (PHM) and pulse-delay modulation (PDM) schemes that offer high data rate in IMDs without dissipating much power on the implantable side are presented.

Wireless power transfer links are becoming increasingly important for consumer, industrial, and medical electronic devices. There are two principal applications for wireless power transfer that require different optimization criteria: continuous power deliver (e.g., cochlear implant) and periodic charging (e.g., cellular phone). In the former case, optimizing power transfer efficiency is a metric of great importance, while in the latter case, minimizing charging time by maximizing power transfer is important. This chapter presents analytical equations that predict optimal conditions for both applications through first-principals step-by-step reflected load analysis. These equations are then used as the basis for the design of a rapid wireless ultra-capacitor charging circuit that speeds up time-to-charge by 3.7×.

Advancements in integrated circuit design have made it possible to have ultra-low-power wireless sensor nodes for health monitoring, smart buildings, industrial automation and for the automotive industry. These low power circuits generally have an Analog Front End (AFE) to sense weak signals, ADCs to digitize the sensed signals, microcontrollers for processing and low power radios for transmitting the low data rate information to a base station. These wireless sensors may be deployed in remote locations or may be in large numbers making battery replacement challenging. By harvesting the ambient energy, it is possible to power these systems and achieve near perpetual operation making battery replacement unnecessary. However, in order for these systems to extract energy from harvesters, these circuits need to not only be ultra-low-power themselves but they also need to ensure maximum available power is always extracted from the energy harvester. In this chapter, the basics of energy harvesting systems will be discussed with a focus on low power design techniques, maximum power extraction and battery management in these systems.