TECHNOLOGY RESEARCH REPORT:

A Paper Study of WuR for Low Power Wireless Sensor Networks, with Observations, Tentative Conclusions and Recommended Next Steps

Prepared by Ed Spence Managing Director THE MACHINE INSTRUMENTATION GROUP

Revised February, 2020

Survey of the Literature for Wake Up Radio

Contents PROBLEM STATEMENT ...... 2 EXECUTIVE SUMMARY ...... 4 SCOPE ...... 5 SURVEY ...... 5 PROTOCOLS ...... 12 WuR RECEIVER DESIGN ...... 13 RECTIFIERS ...... 13 ENERGY HARVESTING ...... 16 ID DECODING ...... 19 CONCLUSIONS ...... 21 RECOMMENDED NEXT STEPS ...... 22 REFERENCES ...... 25 APPENDIX ...... 27 UHF Frequency Channels in the United States ...... 27

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PROBLEM STATEMENT

The goals for this project are shared with many wireless IoT applications – maximize battery life with minimized system cost and complexity while meeting application goals for network range, data transmission rates, sensor application support, etc. From previous studies and from the literature, we assume that radio communication (transmission as well as reception) dominates the power consumption of the node. Any technique to minimize the time that the primary radio MCU is ‘on’ is the focus of much of the discussion.

From an architecture point of view, there seem to be a few fundamental options to maximize battery life beyond careful circuit design and ultra-low power (ULP) component selection –

1. Operate the radio node and gateway (or hub) on a duty cycled basis, minimizing the Standby or Off time of the radio node, with added system timing complexity and/or necessity for RTC’s (additional BOM and power consumption). Note that in this scenario, node Standby time now dominates the power consumption. 2. Minimize radio transmission time (ie with a high data rate). 3. Trade off error correction coding to avoid re-transmission vs choice of frequency band to avoid collisions, while maintaining acceptable BER. 4. Apply energy harvesting techniques, if practical, to trickle charge the battery. 5. Leverage Wake-up Radio (WuR) approaches for ‘on demand’ data transmission, replacing a duty cycling protocol. 6. There is also the potential for self-wake up at the node based on a detected condition1.

Early papers discuss WuR as a replacement for duty cycling MAC protocols. Ansari [10] provide power consumption and latency results for their WuR design (with node ID) compared to a simple MAC protocol with varying duty cycles.

1 Discussion of data management power implications such as whether to process data locally in the node to determine whether a condition exceeds a threshold, transmitting smaller packets such ‘health indicators’, in order to save power by reducing transmit time is beyond the scope of this report.

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Figure 1. Power consumption comparisons for their Radio Triggered Wake-up with Addressing Capabilities (RTWAC) versus a Low-Power-Listening MAC protocol (Ansari 2009). Low duty cycles to reduce power consumption quickly increase latency an unacceptable amount of time for many applications.

The current application is for machine condition or status monitoring. In this use case as currently defined, there is less concern for transmission latency. In addition, there is at least one use case anticipated where an ‘on-demand’ wake up feature is desirable. This opens the door to consideration of a WuR topology, perhaps with an energy harvesting component to the architecture, by assuming that time is available to accumulate enough energy to start a transmit process.

Also, since large data packets are not necessarily needed for a wake-up signal (WuS), the trade off between smaller data samples and slower bit rates can be explored, reducing transmission time and hence transmission power.

For the WuR approach to be of any benefit, performance may meet one or more of the following criteria:

• The WuR receive has to perform reliably over the same ranges expected for the main data transmit radio. Transmitting at a significantly lower data rate and /or a lower frequency band than the main radio may help meet this criteria. • Use simpler protocols (such as OOK), which include little or no error correction, putting the onus on the WuR receiver performance parameters such as sensitivity and signal-to-noise ratio to achieve the necessary reception range.

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• If a node ID addressing feature must be included to reduce overall system power consumption, the WuR should still use a much simpler data packet than a full measurement data transmission by the primary radio MCU. • Overall power consumption of the WuR circuit has to be comparable to or lower than the main radio standby power, operating on a defined duty cycle, to be considered as a replacement for duty cycled prototol. • If an on-demand wake-up and report mode is required of the application, then the power matching target of a duty cycling protocol does not necessarily apply, instead simply minimizing the WuR power consumption for the use case , frequency or condition where it is exercised may be sufficient to justify adding the additional circuitry.

EXECUTIVE SUMMARY

Fundamental issues regarding the use of WuR are discussed below. Degrees of freedom for WuR design and implementation include circuit technology used (ie discrete or solid state, active components), carrier frequency band, rectifier design, antenna impedance matching circuit design and integration with energy harvesting techniques.

Some observations from the literature and discussions with some authors include:

• Relatively recent surveys of the literature (Piyare 2017) as well as others, identify several designs that achieve 10’s of meters of communication range consuming less than 5W’s of power. • In general, many designs use lower channel frequencies (ie ISM band) to improve transmission distance for a given amount of power. An out-of-band (O-O-B) WuR channel using a different frequency band than the main radio introduces additional complexity and BOM cost. • WuR receiver designs are a key determinant of sensitivity, hence range. Design and architecture choices often trade off power consumption (passive circuits) vs lower sensitivity or longer range (active components). • Passive rectifier topologies have been described in the literature integrated with, and solely powered by, RF energy harvesting techniques to produce a WuS. These have been described as ‘zero power’ proposals. • The overwhelming majority of cited designs use On-Off Keying (OOK), a simplified form of Amplitude Shift Keying (ASK). • The designs described in the literature generally start with the assumption that high data rates (shorter transmission times) and low latency are desirable for most IoT applications. • 100-200ksps data rates appears to be a ceiling for power consumption below 2mW. • Relaxed latency requirements open the door to integrating energy harvesting circuits with the WuR, triggering a transmit sequence once sufficient energy is acquired to do so2.

2 The so called ‘zero power’ WuR.

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• Large radio IC vendors are in the processing of integrating WuR functionality into the SoC3, generally based on a standards such as BLE or WIFI. In general, the highest performing designs use dedicated or specialized ICs and other active components, rather than passive, discrete designs, with associated increase in power consumption and BOM cost.

SCOPE The discussion below is restricted to the implementation of a WuR receive element to any wireless node. Implications for the network and the MAC are discussed only as necessary to aid understanding of implementation, or to enable comparisons of the effectiveness of the WuR in terms of figures-of-merit such as sensitivity and range.

Detailed comparison of the WuR with duty cycling MAC schemes are not considered, other than to highlight architectural considerations necessary for a WuR to be effectively implemented.

Finally, this discussion is restricted to receiver designs – transmission circuits or modulation details are only discussed as necessary, with some comparisons of the efficacy of various frequency choices. Similarly, a detailed discussion of error correction or the merits of various communications protocols are not discussed.

SURVEY Several papers take the form of a survey of available work on this subject, the most recent and comprehensive of which was the work by Piyare [1]. Piyare and team also constructed a useful taxonomy to sort the various implementations in terms of hardware design. Passive vs active circuit topologies are discussed in terms of impact on power consumption vs the sensitivity (range) of the receiver. Other features such as node ID vs broadcast, or use of Out-of-Band frequency channels different from the primary radio are also design considerations.

3 LG, Samsung, Huawei, Ericsson, etc will integrate WuR functionality into the future WiFi modems, with standards work underway.

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Figure 2. Taxonomy of WuR hardware design categories (Piyare 2017).

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Table 1. The table of WuR prototypes with enough published data for comparison in Piyare [1]. This does not encompass a complete list of papers cited (190 total).

In terms of performance, Piyare [1] helpfully tabulates key figures-of-merit such as power consumption versus sensitivity, range, power consumption as well as tracking other features such classifying the design as active vs passive, including a node ID feature and whether the circuit operates ‘in-band’ or ‘out-of-band’.

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Figure 3. Figure 12 from Piyare plots power and sensitivity with respect to signal modulation technique. Out of the 75 prototypes surveyed, only 23 achieved power below 10µW. The circled positions at (A) denote radios with power and sensitivity low enough for WBAN applications. Those positions located at (B) are positioned for Smart City or Metering applications.

Data rates of prototypes reviewed reach a ceiling of 100 to 200ksps, regardless of power consumption, with 5/14 designs achieving 200ksps with less than 10W. In general, higher data rates are considered desirable to reduce the transmission time.

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Figure 4. Piyare figure 14. The green circle (A) highlights a cluster of prototypes with ranges between 30 and 50m. Additional options open up for range requirements as low as 10m.

Table 2. Papers identified from Piyare and elsewhere that describe prototypes with a range of at least 10m, and power less than 5µW.

Year Authors S.V [v] Freq[GHz] D.R [kbps] Sens [dBm] R[m] Pwr.[µW] 2019 Moody et al 0.428 -106 0.033 2016 Magno et al.~\cite{magno2016} 1.8 0.868 10 -55 50 1.2 2015 Zgaren et al.~\cite{zgaren2015} 1.2 0.915 100 -53 - 0.2 2015 Prete et al.~\cite{prete2015} 1.8 0.868/2.4 - -53 - 1.27 2014 Oller et al.~\cite{Oller2014} 3 0.868 2.7 -53 41 26.4 2014 Spenza et al.~\cite{Spenza} 1.8 0.868 100 -55 45 1.276 2014 Kamalinejad et al.~\cite{kamalinejad2014high} - 0.868 100 -33 - 0.5 2013 Oh et al 1.2 0.405 12.5 -45.5 0.116 2013 Oller et al.~\cite{Oller2013} 5 0.868 1 -45 13.5 2.67 2013 Milosiu et al.~\cite{Milosiu2013} 2.5 0.868 0.128 -83 1200 4.75 2013 Oh et al.~\cite{oh2013} 1.2 0.402/915/2.4 12.5 -43.2 - 0.116 2012 Roberts et al.~\cite{Roberts2012} 1.2 0.915 100 -41 1.2 0.098 2011 Hambeck et al.~\cite{hambeck2011} 1.2 0.868 20-200 -71 304 2.4 2011 Marinkovic et al.~\cite{Marinkovic2011} 1.5 0.433 5.5 -51 10 0.27 Piyares data supports our sense that, consistent with calculated over the air energy transmission losses (Friss), lower modulation frequencies should result in reduce power consumption (as well as longer ranges), all else being equal. For the prototypes show in Table 2, 915MHz and 868MHz are in the ISM band4 in United States and Europe, respectively, while 4xxMHz is in the UHF band (US)5 (225-420MHz

4 There is a considerable amount of lawful unlicensed activity (cordless phones, wireless networking) clustered around 900 MHz and 2.4 GHz, regulated under Title 47 CFR Part 15. These ISM bands – frequencies with a higher unlicensed power permitted for use originally by Industrial, Scientific, Medical apparatus – are now some of the most crowded in the spectrum because they are open to everyone (Wikipedia). 5 Television broadcasts, microwave oven, microwave devices/communications, radio astronomy, mobile phones, wireless LAN, Bluetooth, ZigBee, GPS and two-way radios such as land mobile, FRS and GMRS radios, amateur radio, satellite radio, Remote control Systems, ADSB (Wikipedia).

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reserved for government use). A breakdown of UHF bands in the US is in the Appendix. Available frequency band options and optimum selection is recommended as an area for further study.

Figure 5. Piyare Figure 15 showing a distribution of power consumption for specific transmission frequencies.

Another survey (Oller 2014) engages in a useful qualitative discussion and performance comparison of various WuR architectures in the literature, compared to their own sub-carrier modulated WuR prototype (SCM-WuR).

Using a commercially available WuR chip6 as a sub-carrier modulation to the signal from an 868MHz receiver, Ollers team achieves -53dBm sensitivity and up to 43m range. Testing for reception was performed with a BLE radio signal transmitted with various power levels (-10dBm to +10dBm), a 2.73ksps bit rate and wake-up delay of 13ms.

6 AS3932 115kHz demodulator, with integrated address correlator

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Figure 6. WuR reception range of sniffed BLE Advertising Frames, per Oller [12] SCM-WuR design with +10dBm BLE radio transmission. “Zone 1 denotes a consistent reception of the WuCs and is represented by white color; Zone 2 denotes the zones with certain WuCs are detected, but reception is not 100% guaranteed and is represented by gray color; in Zone 3 the WuRx is not activated at all by any WuC, which is represented by black color.”

Figure 7. Theoretically calculated reception power at 868MHz versus range for the SCM-WuR conditions proposed by Oller [12].

These results with comparisons to prior art provide a helpful and encouraging summary of the potential effectiveness of WuR. WuR approaches discussed include RFID-based, heterodyne architectures, MCU- based designs, ‘low complexity’ (ie passive) topologies, correlator-based and alternatives to RF communication such as Free Space Optical (FSO) and Ultrasonic signal transmission.

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Figure 8. A summary of WuR proposals as of 2014 per Oller [12].

PROTOCOLS WuR design and implementation must be considered with the application and overall network architecture in mind.

The architectural question to answer seems to be:

At whatever measurement latency can be tolerated by the application, does an on-demand wake up scheme based on WuR add enough value to be put in parallel with synchronously and periodically duty- cycled connection, particularly given the low standby power and ease of duty cycling implementation for today’s radio chips?

Communication protocols to reduce power through duty cycling of the communication links require more complex, system wide timing systems. Early literature poses that the additional sensor node BOM such as MCU’s to control the system timing, RTC’s at both ends to synchronize wake up may add power and cost, but one author contacted suggests that today’s standards based radio chips (ie BLE) may render this a non-issue. That said, minimizing timing margin needed to ensure the node is woken up sufficiently to avoid packet collision may require more expensive high accuracy RTCs7 or, simply live with the timing margin needed to ensure acceptable BER at the cost of longer radio on-time.

7 As an example, an RTC with a ±5ppm accuracy using a MEMS resonator is less than $4, while achieving ±2ppm requires a crystal oscillator, and may cost more than $8 (Maxim Integrated Products).

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It is assumed for the purposes of this discussion that a powered gateway is always on and listening, hence the WuR would be implemented at the sensor node only, with the WuS initiated by the gateway8. Connection initiation may be to execute a request for status, request for data, or to initiate an over-the-air (OTA) firmware update (this operation is not necessarily urgent and can also be scheduled).

If the sensor node is to monitor a machine condition, other wake up modes may be adopted, such as a periodic data sample and signal processing cycle to determine if further action should be taken, and if so, transmitting an alert message to the gateway.

In any case, the RF energy available for waking up the receiver comes from the transmitter (gateway) as a dedicated source, via a narrowband signal through the tuned impedance matching network of the WuR receiver. Note that this frequency may or may not be the same carrier frequency used for the main radio.

Figure 9. A timing diagram for a WuR powered by an energy harvesting scheme highlights some of the issues with WuR control. In this case, the sensor node (Receiver) sleeps until enough energy is converted from the Transmitter (gateway) to power itself on and generate an interrupt, initiating a connect / disconnect cycle. The latency required in such a scheme may be unacceptably long, but may also minimize power consumption of the sensor node radio.

WuR RECEIVER DESIGN

RECTIFIERS At the heart of the RF energy receiver is a Rectifier circuit. Rectifier design and topologies to convert AC RF signals into DC voltage (envelope or peak detecting) is a well developed and documented art. In the

8 An ‘ad hoc’ use case is also proposed, where the sensor node status transmission (a new measurement is not necessarily performed) is initiated by the user with a portable WuS transmitter on a walk-by or route based regime.

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last ten years, additional work has been done to maximize the efficiency, improve the sensitivity and integrate the rectifier within a WuR scheme with enough range to match the receive distances of the main communication radio.

Receiver front ends can be active or passive, the former generally trading off sensitivity performance for lower power consumption of the latter. The rectifier circuit itself is generally a combination of discrete components (ie diodes and capacitors).

The basic operation is to detect voltage available at the output of a tuned impedance matching circuit, designed to maximize power transferred from the antenna. This voltage is ‘multiplied’ up by subsequent stages of switches and charge caps until the voltage is high enough to power on the following stage. This transition generally involves an active comparator component to determine when the voltage crosses a threshold and generating a wake-up pulse.

A key insight into rectifier circuits is that they respond to peak voltage levels – not necessarily power levels. The voltage from an RF signal must initially exceed a diode threshold by enough to turn it on, dumping current onto a holding or charge capacitor. This charge is then increased for each negative going cycle of the input signal as well, with charge dumped via another input switch. The AC RF signal itself is filtered out by the series cap at the input.

Figure 10. Basic rectifier operation (Spenza 2015).

More complicated rectifier designs can multiple the voltage higher, at the expense of efficiency due to energy losses at each stage. Building a rectifier with Schottky diodes can reduce the necessary voltage (Chen 2015). These devices can turn on with voltage as low as 0.15V, operating in a low power region between -20dBm and 0dBm9. Also, achieving a higher output voltage also results in less charge current available, a factor in the use of an energy storage cap. For a rectifier circuit driving an active comparator with a high impedance input, this would present less of an issue.

9 Schottky diodes are built with a specialized metal-silicon junction, to enable lower turn on voltages and faster switching time – necessary to operate at radio frequencies in the GHz range. Chen [5] rectifier design used the Avago Technologies’ HSMS-2852.

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Figure 11. Multi-stage ‘Voltage Multiplier’ (Oller 2013), demonstrating rectification of the AC RF input voltage to produce a DC voltage output (V0), which is pumped with each successive cycle of the RF input signal until is reaches a level sufficient to generate a wake-up pulse to the next stage (generally a comparator). Note that in this design, the voltage divider used to bias the op amp uses standing current (526nA in this case), and that the Op Amp, although a low current consumption model, has to be powered by the battery. The optimum number of stages depends on the output voltage (or current) required by the next stage and the losses of each stage.

Multi-stage designs are discussed by Nintanavongsa [8], with associated trade offs in efficiency depending on the amount of input power. The designs described by this team places more than one rectifier in series, to expand the range of input power levels for higher efficiency conversion. Two stage rectifier circuits discussed show efficiency vs input power and choice of load resistance, with data presented suggesting that efficiencies higher than 10% are difficult to achieve at power levels near - 20dBm.

Figure 12. From Nintanavongas [8] discussion of multiple stage rectifier topologies, optimized for input power from the antenna impedance matching circuit.

Nintanavongsa [8] also apply their rectifier design to multiple antennas, increasing the available power at the cost of multiple signal paths with associated increase in the BOM and increased layout area. All

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channels would use separate impedance matching networks and rectifiers, the outputs of which would feed the same energy storage circuit.

Figure 13. A multiple antenna design proposed in Ninatanavongsa [8], to increase the amount of energy delivered to subsequent stages.

Finally, rectifiers designs in the literature describe a means to self-bias and self-power the interrupt generating comparator with voltage derived as part of the rectifier circuit (see Energy Harvesting).

ENERGY HARVESTING

The rectifier is generally following by a low power comparator to generate the Wake-up pulse. The pulse generated from this stage can wake up either an auxiliary ultra low power (ULP) MCU used for power management (and in some cases node ID decoding), or to the primary radio MCU itself. Minimizing the pulse level required to turn on the power management CPU is another design consideration to help save power (at the expense of latency), with one author correlating this voltage threshold to receive range.

As the active component would normally need to run off the battery, but a technique has been proposed to leverage the rectified voltage to power the comparator as well, described in one paper as the so called ‘Zero Power’ radio (Kamalinejad 2015). Strictly speaking, a rectifier simply converts RF signals at high frequencies into a DC voltage proportional to some aspect of the input signal (ie amplitude), transferring information with some level of efficiency. Tapping that voltage also to power an active component can be thought of as energy harvesting.

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Figure 14. Basic functionality of the ‘zero power’ receiver proposed by Kamalinejad [3,4]. The rectifier converts the voltage peaks from the RF signal, pumping until the signal exceeds a self-referencing threshold for a low power comparator. The output interrupts wake up a power management MCU, which determines how and when to engage the sensor battery, enable sensor measurement and wake up the main radio MCU.

To enable the zero power operation described in Kamalinejad [3,4], the rectifier provides enough voltage to power the following comparator as well as provide the signal needed to generate interrupt pulses (Fig 14). Note also that the comparator power also generates it’s own reference, to help reject interferers. A dedicated, battery powered ULP MCU is woken up by the zero power receiver to manage the use of the battery, determine when available power can allow sensor measurement, and wake up the main radio processor for transmission.

Figure 15. Proposed PMU from Kamalinejad controlling the interactive between RF energy harvesting circuit, the battery and the radio / sensor processor.

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An earlier Kamalinejad paper on this subject with (Magno 2014) simulates a fully passive design with energy harvesting at 868MHz, achieving -33dBm on only 0.5µW, with 100ksps data rate and range unspecified. This design, as discussed in the previous article, also self- powers the comparator responsible for generating the interrupt pulse. The rectifier design achieves high power conversion efficiency (PCE) for low voltage inputs with a quasi-floating-gate architecture on 0.13µm CMOS IC. Sufficient information is not provided to fully duplicate this design.

The idea of waking up an auxiliary ULP MCU dedicated to power management (and ID decoding) also appears to have merit. Data from Nintanavongsa [8] suggests that rectifier output voltages on the order of 100’s of mV’s might be produced for input power levels approaching -20dBm. Nintanavongsa cite availability of ULP MCU’s such as the “Texas Instruments’ MSP430L092 can operate at the voltage as low as 0.9 V and consumes 3µA in LPM4 mode, which translates to 2.7W” [8].

Figure 16. Output voltage comparisons between prototype performance of both low power design (LPD) and high power design (HPD) rectifiers of Nintanavongsa [8], simulation, prototypes and a commercially available Powercast EH device.

Chen (2015) topology also uses a low power regulator in tandem with a low voltage threshold to wake up the radio mote. They improve the range of their REACH2 receiver IC by designing two rectifier circuits in series, the 2nd with a separate antenna in order to increase the amount of energy in the RF signal made available to be rectified. This team also developed a model to simulate the amount of energy converted (harvested) by measuring the wake-up delay for the circuit. By extension, reducing the voltage necessary to wake up the Aux MCU reduces the wake-up time as well as conserving power.

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Figure 17. Chen [5] two rectifier circuit. This design increases the range by multiplying the energy harvested from two antenna channels. They also reduce the wake up voltage to the radio mote with more carefully chosen voltage regulation components, shortening wake up time and conserving power. ID DECODING

If the WuS needs to include an ID as part of the packet so as not to wake up adjacent nodes unnecessarily, additional transmit time will be required, as well as a means to decode incoming ID information.

Spenza (2015) describe a simple solution where a short preamble to the data packet transmitted at a specific bit rate serves as a preliminary wake up code for the auxiliary power management controller. Any transmission at a lower rate is filtered by a passive RC low pass filter. An acceptable preample then stimulates the wake up the MCU, which then decodes the subsequent wake up packet with the sensor node address. This simple 2-step scheme provides a level of selectivity to address the node while achieving 32m reception range.

This design also makes use of the fact that at a lower bit rate, more energy per bit can be transmitted (and received), increasing reliability and range of the WuS.

Figure 18. Spenza [11] describes a passive demodulation scheme which essentially rectifies an incoming bit stream ‘preamble’ packet, transmitted at a specific bit rate. Bit rates that are lower than chosen for this connection are filtered out by the low pass filter (LPF). This design consumed less than 1.3µW of power in the 868MHz band.

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Figure 19. Wake probability with the selective wake up preamble per Spenza [11] at 868MHz. The highest value of sensitivity in the lab simulating distance with transmitter attenuation was -55dBm. The maximum distance achieved for selective wake ups in their experiments was 32m (45m without selectivity feature). Note that a lower bit rate improves transmission distance, possibly because of the higher energy per bit that can be transmitted.

A similar approach to sensor ID by filtering the enveloped bit stream with analog filters, but targeting WBAN applications is discussed by Marinkovic [9]. Power from the rectifier is also used to actively set the threshold of the comparator, maintaining the set point at the 50% point of the rectified signal, helping to reject interferers and reducing static power. In this case, the output PWM signal is converted to an interrupt and SPI output signal with discrete logic.

Figure 20. Signals produced by the rectification circuit discussed in Marinkovic [9]. Signal (a) is the rectified OOK signal, and (b) is the self-adjusting comparator threshold extracted from the signal.

Finally, multiple papers describe addressing schemes using correlators or MCU’s (logic devices to match stored node addresses with transmitted signals) to wake up specifically targeted nodes (Blobel 2016).

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CONCLUSIONS

Our current standby power estimate for the first prototype BLE radio is less than 10µW10. Achieving this power consumption for a WuR while still enabling reliable reception at distances in the 15m range seems feasible. As previously stated, several (mostly passive) designs in the literature achieve distances greater than 10m with less than 5W.

Calculating the ideal power incident at a receiver antenna per the Friis equation, power of at least -20 dBm at 15m distance for a variety of transmit frequency bands and standards seems possible.

Table 3. Received power expected by the Friis equation at 15m, for various frequency bands and maximum regulated transmit power. Both transmit and receive antenna gains (Gt and Gr) are assumed to be 1.

The 2nd generation REACH2 device field tested by Chen [5] achieves up to 44 ft transmission distance at 915MHz with less than 1.3µW power, evaluation data cited for their 2-antenna passive receiver design.

Figure 21. Chen [5] test results for their 2-rectifier design with optimized energy harvesting circuit. Receivers that didn’t respond within a 100s limit were considered unreachable. These results were obtained indoors in dry air. Precipitation was expected to limit the transmission and receive distances, but tests on a rainy day achieved almost the same results.

Chen [5] and team also present simulation results comparing a 100 node/single basestation air pollution monitoring network with various receiver architectures at various packet rates, comparing simple duty cycled communication protocols and WuR prior art ‘WISP-Mote’ (Ba 2010) to their REACH2 WuR. It

10 A sensor node designed to measure and transmit surface temperature.

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should be noted that the REACH-mote design does not have an ID decoder option, resulting in some false wake ups, and is flagged as an area for future improvement.

Figure 22. Some interesting simulation results from Chen [5]. WISP-Mote is prior art used for comparison of a passive receiver design [6]. Packet generation rates from 0.2pkt/min to 2pkt/min (100 sensor nodes, one base station, limited buffer).

Higher performing circuits in the literature achieve the results mostly with active designs and at lower transmission frequencies.

For example, Wang [2] describes a receiver that achieves -69dBm sensitivity with 4.5nW power consumption (0.4V power supply) and tuned to 113.5MHz11. Receiver range is unspecified, but the radio is described as targeting LPWAN applications. Performance was achieved by limiting the data rate to 300bps, exercising a custom code sequence to add coding gain, an off chip high Q filter, an oversampled digital baseband correlator, and fabricated on a specialized 0.18µm SOI CMOS process.

As several passive WuR topologies are cited with acceptable results, further investigation is warranted.

RECOMMENDED NEXT STEPS The passive designs in the literature are relatively simple analog circuits implemented with readily available discrete components. Discussion in the literature is sufficiently detailed to enable duplication of their results.

Further, the operation of the various stages are sufficiently independent that the best ideas for each stage of the signal chain can be integrated into a single topology.

11 113.5MHz falls into the VHF spectrum in the US, which includes FM, television broadcasts, line-of-sight ground-to-aircraft and aircraft-to-aircraft communications, land mobile and maritime mobile communications, amateur radio, weather radio. 108–118 MHz: Air navigation beacons VOR and Instrument Landing System localizer.

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Figure 23. An example ‘Frankenstein’ topology might look like that shown above, replacing or adapting stages of the topology proposed by Spenza [11] with proposed solutions from other authors.

The topology can then be further optimized for component selection (ie selecting the highest performance components available, such as an ultra-low power comparator12, AUX MCU, etc) before being built and tested.

Chen’s team also equates turn on voltage required to wake up the power management MCU with distance over which WuR is effective, offering a potential figure of merit as well as another potential design degree of freedom using the latest in available ULP MCU products.

Several operational and performance aspects of the topology can be simulated before final design, build and test, particularly with the target application in mind. At this concept design stage we might also consider the impact of novel technologies, such as solid state, chip scale 50µAh batteries (Cymbet) as a means to power the active components.

12 Comparator IC’s with supply voltages down to 0.9V and quiescent currents as low as 75nA are commercially available (ie TI’s TLV3691).

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Behaviors of the circuit better understood as an outcome of this design and simulation stage might include:

• Rectifier design • Power consumption trade-offs, limitations and dependencies • Input sensitivity • Wake up time • Transmit power, packet size, bit rate, bit duration impact on latency and range • Wake-up Signal threshold as a proxy for receive distance • Response to various choices of frequency band (ISM, UHF, etc)13 • Evaluating Node ID schemes (ie Preample and decode [9,11]) • RF EH techniques to power the comparator • Available component choices

The next steps will be to simulate the operation of WuR building blocks, validating the potential of this approach to power management.

13 Due diligence regarding the choice of available frequency band can be explored at this stage – available bands, regulations for use and maximum available transmit power can be better understood and applied.

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REFERENCES

[1] Rajeev Piyare, Amy L. Murphy, Csaba Kiraly, Pietro Tosato, and Davide Brunelli, “Ultra Low Power Wake-Up Radios: A Hardware and Networking Survey”, IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 19, NO. 4, FOURTH QUARTER 2017 [2] Po-Han Peter Wang , Haowei Jiang, Li Gao, Pinar Sen, Young-Han Kim, Gabriel M. Rebeiz, Patrick P. Mercier , and Drew A. Hall, “A Near Zero Power Wake Up Receiver Achieving -69dBm Sensitivity”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 53, NO. 6, JUNE 2018 [3] Pouya Kamalinejad, Chinmaya Mahapatra, Zhengguo Sheng, Shahriar Mirabbasi, Victor C. M. Leung,and Yong Liang Guan, “Wireless energy harvesting for the Internet of Things”, IEEE Communications Magazine, June 2015 [4] Pouya Kamalinejad1, Kamyar Keikhosravy1, Michele Magno2, Shahriar Mirabbasi1, Victor C.M. Leung1, and Luca Benini, “A High-Sensitivity Fully Passive Wake-Up Radio Front-End for Wireless Sensor Nodes”, 2014 IEEE International Conference on Consumer Electronics (ICCE) [5] L. Chen, J. Warner, P. L. Yung, D. W. Zhou, W. Heinzelman, I. Demirkol, U. Muncuk, K. Chowdhury, and S. Basagni, “REACH2-Mote: A Range-Extending Passive Wake-Up Wireless Sensor Node”, ACM Transactions on Sensor Networks, Vol. 11, No. 4, Article 64, Publication date: December 2015 [6] He Ba, Ilker Demirkol, and Wendi Heinzelman. 2010. Feasibility and benefits of passive RFID wake- up radios for wireless sensor networks. Proceedings of the IEEE Global Telecommunications Conference (GLOBECOM’10). IEEE, Los Alamitos, CA, 1–5. [7] J. Oller, I. Demirkol, J. Casademont, J. Paradells, “Design, Development, and Performance Evaluation of a Low-Cost, Low-Power Wake-Up Radio System for Wireless Sensor Networks”, ACM Transactions on Sensor Networks, Vol. 10, No. 1, Article 11, Publication date: November 2013 [8] P. Nintanavongsa, U. Muncuk, David Richard Lewis, K. R. Chowdhury, “Design Optimization and Implementation for RF Energy Harvesting Circuits”, IEEE JOURNAL ON EMERGING AND SELECTED TOPICS IN CIRCUITS AND SYSTEMS, VOL. 2, NO. 1, MARCH 2012 [9] Marinkovic and Popovici, “NANO-POWER WIRELESS WAKE-UP RECEIVER WITH SERIAL PERIPHERAL INTERFACE”, IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 29, NO. 8, SEPTEMBER 2011

[10] J. Ansari, D. Pankin and P. Mahonen, “Radio-Triggered Wake-ups with Addressing Capabilities for Extremely Low Power Sensor Network Applications”, 978-1-4244-2644-7/08/$25.00 ©2008 IEEE

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[11] D. Spenza, M. Magnoz, S. Basagnix, L. Beniniz, M. Paoli, C. Petrioli, “Beyond Duty Cycling: Wake-up Radio with Selective Awakenings for Long-lived Wireless Sensing Systems”, 2015 IEEE Conference on Computer Communications (INFOCOM) [12] J. Blobel, J. Krasemann and F. Dressler, “An Architecture for Sender-based Addressing for Selective Sensor Network Wake-Up Receivers”, 978-1-5090-2185-7/16/$31.00 ©2016 IEEE [13] J. Oller, I. Demirkol, J.Casademont, J. Paradells, G. U. Gamm and L. Reindl, “Performance Evaluation and Comparative Analysis of SubCarrier Modulation Wake-up Radio Systems for Energy- Efficient Wireless Sensor Networks”, Sensors 2014, 14, 22-51; doi:10.3390/s140100022

January 2020 The Machine Instrumentation Group 26 Survey of the Literature for Wake Up Radio

APPENDIX

UHF Frequency Channels in the United States (Wikipedia) UHF channels are used for digital television broadcasting on both over the air channels and cable television channels. Since 1962, UHF channel tuners (at the time, channels 14- 83) have been required in television receivers by the All-Channel Receiver Act. However, because of their more limited range, and because few sets could receive them until older sets were replaced, UHF channels were less desirable to broadcasters than VHF channels (and licenses sold for lower prices). A complete list of US Television Frequency allocations can be found at North American Television Frequencies. There is a considerable amount of lawful unlicensed activity (cordless phones, wireless networking) clustered around 900 MHz and 2.4 GHz, regulated under Title 47 CFR Part 15. These ISM bands – frequencies with a higher unlicensed power permitted for use originally by Industrial, Scientific, Medical apparatus – are now some of the most crowded in the spectrum because they are open to everyone. The 2.45 GHz frequency is the standard for use by microwave ovens, adjacent to the frequencies allocated for Bluetooth network devices. The spectrum from 806 MHz to 890 MHz (UHF channels 70–83) was taken away from TV broadcast services in 1983, primarily for analog mobile telephony. In 2009, as part of the transition from analog to digital over-the-air broadcast of television, the spectrum from 698 MHz to 806 MHz (UHF channels 52–69) was removed from TV broadcasting, making it available for other uses. Channel 55, for instance, was sold to Qualcomm for their MediaFLO service, which was later sold to AT&T, and discontinued in 2011. Some US broadcasters had been offered incentives to vacate this channel early, permitting its immediate mobile use. The FCC's scheduled auction for this newly available spectrum was completed in March 2008.[8] The FCC has allowed Americans to connect any device and any application to the 22 MHz of radio spectrum that people are calling the 700 MHz band. The FCC did not include a wholesale condition, which would have required the owner of the band to resell bandwidth to third parties who could then service the end user. Google argued that the wholesale requirement would have stimulated internet competition. As of 2007, 96% of the country's broadband access was controlled by DSL and cable providers. A wholesale condition could have meant a third option for internet service.[9]

• 225–420 MHz: Government use, including meteorology, military aviation, and federal two-way use[10] • 420–450 MHz: Government radiolocation, amateur radio satellite and amateur radio (70 cm band), MedRadio[11] • 450–470 MHz: UHF business band, General Mobile Radio Service, and Family Radio Service 2- way "walkie-talkies", public safety • 470–512 MHz: Low-band TV channels 14–20 (shared with public safety land mobile 2-way radio in 12 major metropolitan areas scheduled to relocate to 700 MHz band by 2023[12]) • 512–608 MHz: Medium-band TV channels 21–36 • 608–614 MHz: Channel 37 used for radio astronomy and wireless medical telemetry[13] • 614–698 MHz: Mobile broadband shared with TV channels 38–51 auctioned in April 2017. TV stations will relocate by 2020.

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o 617–652 MHz: Mobile broadband service downlink o 652–663 MHz: Wireless microphones (higher priority) and unlicensed devices (lower priority) o 663–698 MHz: Mobile broadband service uplink • 698–806 MHz: Was auctioned in March 2008; bidders got full use after the transition to digital TV was completed on June 12, 2009 (formerly high-band UHF TV channels 52–69) • 806–816 MHz: Public safety and commercial 2-way (formerly TV channels 70–72) • 817–824 MHz: ESMR band for wideband mobile services (mobile phone) (formerly public safety and commercial 2-way) • 824–849 MHz: Cellular A & B franchises, terminal (mobile phone) (formerly TV channels 73–77) • 849–851 MHz: Commercial aviation air-ground systems (Gogo) • 851–861 MHz: Public safety and commercial 2-way (formerly TV channels 77–80) • 862–869 MHz: ESMR band for wideband mobile services (base station) (formerly public safety and commercial 2-way) • 869–894 MHz: Cellular A & B franchises, base station (formerly TV channels 80–83) • 894–896 MHz: Commercial aviation air-ground systems (Gogo) • 896–901 MHz: Commercial 2-way radio • 901–902 MHz: Narrowband PCS: commercial narrowband mobile services • 902–928 MHz: ISM band, amateur radio (33 cm band), cordless phones and stereo, radio- frequency identification, datalinks • 928–929 MHz: SCADA, alarm monitoring, meter reading systems and other narrowband services for a company internal use • 929–930 MHz: Pagers • 930–931 MHz: Narrowband PCS: commercial narrowband mobile services • 931–932 MHz: Pagers • 932–935 MHz: Fixed microwave services: distribution of video, audio and other data • 935–940 MHz: Commercial 2-way radio • 940–941 MHz: Narrowband PCS: commercial narrowband mobile services • 941–960 MHz: Mixed studio-transmitter fixed links, SCADA, other. • 960–1215 MHz: Aeronautical radionavigation • 1240–1300 MHz: Amateur radio (23 cm band) • 1300–1350 MHz: Long range radar systems • 1350–1390 MHz: Military air traffic control and mobile telemetry systems at test ranges • 1390–1395 MHz: Proposed wireless medical telemetry service. TerreStar failed to provide service by the required deadline[14]. • 1395–1400 MHz: Wireless medical telemetry service • 1400–1427 MHz: Earth exploration, radio astronomy, and space research • 1427–1432 MHz: Wireless medical telemetry service • 1432–1435 MHz: Proposed wireless medical telemetry service. TerreStar failed to provide service by the required deadline[14]. • 1435–1525 MHz: Military use mostly for aeronautical mobile telemetry (therefore not available for Digital Audio Broadcasting, unlike Canada/Europe) • 1525–1559 MHz: Skyterra downlink (Ligado is seeking FCC permission for terrestrial use[15]) o 1526–1536 MHz: proposed Ligado downlink o 1536–1559 MHz: proposed guard band • 1559–1610 MHz: Radio Navigation Satellite Services (RNSS) Upper L-band o 1563–1587 MHz: GPS L1 band o 1593–1610 MHz: GLONASS G1 band o 1559–1591 MHz: Galileo E1 band (overlapping with GPS L1[16])

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• 1610–1660.5 MHz: Mobile Satellite Service o 1610–1618: Globalstar uplink o 1618–1626.5 MHz: Iridium uplink and downlink[15] o 1626.5–1660.5 MHz: Skyterra uplink (Ligado is seeking FCC permission for terrestrial use[15]) ▪ 1627.5–1637.5 MHz: proposed Ligado uplink 1 ▪ 1646.5–1656.5 MHz: proposed Ligado uplink 2 • 1660.5–1668.4 MHz: Radio astronomy observations. Transmitting is not permitted. • 1668.4–1670 MHz: Radio astronomy observations. Weather balloons may utilize the spectrum after an advance notice. • 1670–1675 MHz: Geostationary Operational Environmental Satellite transmissions to three earth stations in Wallops Island, VA; Greenbelt, MD and Fairbanks, AK. Nationwide broadband service license in this range is held by a subsidiary of Crown Castle International Corp. who is trying to provide service in cooperation with Ligado Networks.[17] • 1675–1695 MHz: Meteorological federal users • 1695–1780 MHz: AWS mobile phone uplink (UL) operating band o 1695–1755 MHz: AWS-3 blocks A1 and B1 o 1710–1755 MHz: AWS-1 blocks A, B, C, D, E, F o 1755–1780 MHz: AWS-3 blocks G, H, I, J (various federal agencies transitioning by 2025[18]) • 1780–1850 MHz: exclusive federal use (Air Force satellite communications, Army's cellular-like communication system, other agencies) • 1850–1920 MHz: PCS mobile phone—order is A, D, B, E, F, C, G, H blocks. A, B, C = 15 MHz; D, E, F, G, H = 5 MHz • 1920–1930 MHz: DECT cordless telephone • 1930–2000 MHz: PCS base stations—order is A, D, B, E, F, C, G, H blocks. A, B, C = 15 MHz; D, E, F, G, H = 5 MHz • 2000–2020 MHz: lower AWS-4 downlink (mobile broadband) • 2020–2110 MHz: Cable Antenna Relay service, Local Television Transmission service, TV Broadcast Auxiliary service, Earth Exploration Satellite service • 2110–2200 MHz: AWS mobile broadband downlink o 2110–2155 MHz: AWS-1 blocks A, B, C, D, E, F o 2155–2180 MHz: AWS-3 blocks G, H, I, J o 2180–2200 MHz: upper AWS-4 • 2200–2290 MHz: NASA satellite tracking, telemetry and control (space-to-Earth, space-to- space) • 2290–2300 MHz: NASA Deep Space Network • 2300–2305 MHz: Amateur radio (13 cm band, lower segment) • 2305–2315 MHz: WCS mobile broadband service uplink blocks A and B • 2315–2320 MHz: WCS block C (AT&T is pursuing smart grid deployment[19]) • 2320–2345 MHz: Satellite radio (Sirius XM) • 2345–2350 MHz: WCS block D (AT&T is pursuing smart grid deployment[19]) • 2350–2360 MHz: WCS mobile broadband service downlink blocks A and B • 2360–2390 MHz: Aircraft landing and safety systems • 2390–2395 MHz: Aircraft landing and safety systems (secondary deployment in a dozen of airports), amateur radio otherwise • 2395–2400 MHz: Amateur radio (13 cm band, upper segment) • 2400–2483.5 MHz: ISM, IEEE 802.11, 802.11b, 802.11g, 802.11n wireless LAN, IEEE 802.15.4- 2006, Bluetooth, radio-controlled aircraft (strictly for spread spectrum use), microwave ovens, ZigBee

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• 2483.5–2495 MHz: Globalstar downlink and Terrestrial Low Power Service suitable for TD-LTE small cells[20] • 2495–2690 MHz: Educational Broadcast and Broadband Radio Services[21] • 2690–2700 MHz: Receive-only range for radio astronomy and space research

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