A Design of Peak to Average Power Ratio Based SWIPT System in 180 nm CMOS Process for IoT Sensor Applications

In this paper, we propose a peak-to-average-ratio (PAPR) based simultaneous wireless information and power transfer (SWIPT) system for Internet-of-Things (IoT) sensor applications. Conventional SWIPT system is based on power-hungry transmitter and receiver modules. The presence of such blocks directly impact the power consumption in IoT devices. This problem can be solved by proposing an ultra-low-power communication mechanism. Recently SWIPT with multi-tone is under consideration because of its increased power conversion efficiency (PCE). We proposed a PAPR based SWIPT receiver that utilizes the multi-tone waveform for information decoding in SWIPT system. An adaptive power splitter (APS) smartly regulates the distribution of received radio frequency (RF) signals between the energy harvesting (EH) path and the information decoding (ID) path. A digital controller was designed to control the demodulation of the PAPR based modulated signal and retrieves the digital information. Back-scattering modulation has been opted for up-link data transfer. For the EH path we design an RF-DC converter with an adaptive matching to increase the dynamic range of input power. The propose SWIPT system based on the PAPR demodulator is implemented on the 180 nm CMOS process. The digital clock frequency at the SWIPT receiver is 64 kHz, which can provide a data rate of 8 kbps with power consumption of $7.3~\mu \text{W}$ with area utilization of 0.4 mm2.


I. INTRODUCTION
The rapid development of 5G communication and modern devices have highly eased our lives and improved the qualityof-service (QoS). The demand of devices is increasing exponentially with advancement of the technology. But the uninterrupted and prolonged power supply is a major concern of billions of these battery-operated, low-powered Internet-of-Things (IoT) devices and sensors nodes [1], [2]. Sometimes, these sensor devices are installed at critical locations, such as The associate editor coordinating the review of this manuscript and approving it for publication was Yuh-Shyan Hwang . the inside of any living body, or in the sensitive and hazardous environment, where recharging with conventional methods is not a viable solution [3]- [5]. These devices require an alternate source of energy instead of traditional methods like solar [6] or vibration [7]. Energy harvesting (EH) from radio frequency (RF) has emerged as a strong choice for power-limited or even battery-less nodes, without affecting data transmission. Hence wireless power transfer (WPT) has emerged as a potential solution due to its wireless power handling feature for sensors and IoT devices [8]- [11]. However, WPT has several constraints like distance, safety, high frequency needed for transmission, and initial setup cost [12]. In [13] it VOLUME 10, 2022 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ is revealed that not only power but information can also be transmitted on a single waveform, which is termed as simultaneous wireless information and power transfer (SWIPT). Having a power efficient SWIPT system is challenging due to the usage of power-hungry RF and communication blocks. A non-linear circuit for efficient power distribution between information decoding (ID) path and EH path is proposed in [14], [15]. In many cases, planning SWIPT waveforms on custom communication modules is still a challenging assignment. In particular, it is vital to plan transmit waveforms such that IoT devices can handle remote data without utilizing active devices [16]. An efficient SWIPT system depends on an efficient antenna design also. Recently a hybrid coupler based antenna design was proposed in [17], [18], as well as a dual-band, dual-mode and dual-polarized wearable antenna designs are proposed in [19], [20] respectively. But our focus will be on receiver side specially information modulation and demodulation. Recent development in efficient waveform design proposes the use of multi-tone waveforms in the EH path that is very handy in terms of power conversion, compared to the single-tone waveform [21], [22]. The sum of multi-tone waveforms generates a high peak-to-average ratio (PAPR) [23], which results in improved power conversion efficiency (PCE) at the receiver end. In the literature variety of SWIPT receiver architectures are proposed, like power splitting (PS) [25], time switching [26], and antenna switching and integrated receivers. In the TS scheme, the receiver toggles between ID and EH path based on the time period and works well when simultaneous operation is not possible due to an insufficient amount of harvested energy. On the contrary, in the PS scheme, the received RF signal is divided into two separate paths, the EH path, and ID path, with a certain splitting ratio, in order to process the received RF signal simultaneously.
SWIPT with multi-tone is created by summing frequency, amplitude, and varying the number of tones [27]. Among all proposed approaches, number of tones based modulation technique is in practice. In this method, the PAPR value of the transmitting signal is controlled by changing the tone number for summation [23], [24]. Literature shows that a higher number of tones results in higher PAPR of RF signal which results in increased rectifier's output voltage. But, it also degrades the bit rate. It is a matter of fact that more the number of tones in limited bandwidth will eventually result in shortening of channel spacing. Having more number of tones means lower frequency spacing, and lower frequency spacing directly affects the bit rate, thus it degrades the communication quality [29]. On the contrary, lower the number of tones means higher the frequency spacing but poor will be PCE performance. Thus achieving high PCE and communication performance with the same conventional receiver hardware is quite challenging. A dual-mode SWIPT was proposed in [25], to control the communication mode and energy status of IoT devices for energy-neutral operations, which supports the dual-mode operation. Dual-mode includes single-tone and multi-tone methods for energy harvesting and communication. This is implemented on PCB having commercial devices that are not reliable for optimal operation. RFID works quite similarly to SWIPT in terms for information communication and RF energy harvesting. But the energy source in RFID is a dedicated transmitter unlike the ambient environment in SWIPT [32]- [35].
In this paper, we designed a PAPR based receiver for SWIPT system that uses multi-tone waveform for information decoding. The PAPR estimator estimates the ratio of Powerpeak and Poweravg. After recovering the data bits, a synchronization pattern is detected, which results in successful information decoding. The key contributions of this work are given as: • A design of the SWIPT system, which utilizes the PAPR modulated multi-tone waveform, was presented.
• A PAPR based demodulator was implemented at the chip level to recover the data and validated the proposed idea through simulation and experiments.
• A digital controller was designed to control the SWIPT system operations. We verified the smooth operation of digital controller.
The rest of the paper is structured as, in section II, PAPR based SWIPT system is presented, section III presents the proposed model. Simulations and experimental results are given in section IV, and the paper is concluded in section V.

II. PAPR BASED SWIPT SYSTEM
The concept of (SWIPT) is presented in Fig. 1. From the base station, an energy signal x(t) is transmitted carrying the modulated information. The channel fading is taken ideal at the moment as ideal adaptive matching is considered. The receiver receives the signal y(t) having channel and system noise η i (t). This received signal is used for both EH and ID blocks. In the SWIPT system, the transmitter's power and modulation waveform is generated as per the requirement of the receiver. Transmitter and receiver are designed for multitone transmission for PAPR modulation and demodulation respectively. The PAPR maps to fine constellation points associated with every energy level. Because of the non-linear rectification process, the PCE is optimized using multi-tone waveforms. Moreover, PAPR based information transmission facilitates low-power ID using simple PAPR measurement [23]. The receiver architecture has two primary paths, the EH and ID path as shown in Fig. 1. In order to harvest energy and decode information from the same RF signal at the receiver with self-powering, an adaptive power splitter splits the RF signal with the ratio of θ , where 0 ≤ θ ≤ 1. Initially, the EH path is selected to charge the super capacitor. Once enough energy is harvested for self-power up and also for energy-neutral operation, the PAPR (ID) path is selected. To meet the minimum signal-to-noise ratio (SNR) for receiver operation, it is assumed that even a small θ is sufficient. Then the EH circuit can harvest maximum signal power [25]. The received EH signal after splitting mathematically can be give as: where, x t (t) is transmitted signal, η i (t) is channel noise which is considered as additive white gaussian noise (AWGN). The ID path is used to decode the received signal with distinct energy levels. As the single waveform is prone to channel fading, we assume it consumes more circuit power compared to that of the PAPR based SWIPT system. The PAPR (ID) path serves for information decoding. For this, the PAPR estimator simply estimates the PAPR value of the RF received signal envelop. The received PAPR can be evaluated as equation 2. Note that, multi-tone waves form consumes less power as compared to the conventional ID path because PAPR based modulation does not require power-hungry devices, such as mixer, voltage controlled oscillator (VCO). Furthermore, the PCE in the low power region is enhanced, due to the multi-tone waveform. Hence the PAPR demodulator is suitable for a low power consumption circuit while increasing the operational range with a low rate [23], [24].
where, yID is the received signal at information decoding path and T m is the symbol time of a multi-tone waveform. The power management and digital controller (PMDC) monitors the received power and energy is stored in super capacitor. When the stored energy in super capacitor is greater than a threshold, it activates the backscattering module block to transmit the data through backscattering. Fig. 2 shows the block diagram of the proposed PAPR demodulator-based SWIPT system. The proposed architecture has two parts; the EH and the PAPR (ID) path. An antenna gathers information from the surrounding environment in the form of an RF signal. Antenna matching is performed with the help of a network analyzer to ensure the maximum possible power transfer from the antenna to the power splitter as the signal strength of the incoming signal varies greatly. Based on the input power level, the designed adaptive power splitter (APS) distributes the received power between EH and PAPR (ID) blocks as the input RF signal is not fixed and received power from RF signal changes over time. The distribution of received RF power to EH and PAPR (ID) path with a splitting ratio of θ and (1 − θ ) respectively. This adaptive logic allows the SWIPT system to cover a wide range of input power, with high PCE efficiency and decodes the information in parallel. The algorithm-1 describes the overall operation of APS.  [22] into optimal signal design for energy harvesting has found that employing a multi-toned modulation waveform can result in significant PCE gain instead of the single-tone waveform. This gain comes from the nonlinear behavior of RF rectifiers which results in higher DC output with a signal of higher PAPR. However, the modulation technique used for the single-tone waveform cannot be used for multi-tone waveform [21], [22]. To address this issue, a PAPR demodulator is proposed in the SWIPT system to yield high PCE and low power consumption in the ID path. Initially, maximum power is harvested and stored in a super capacitor by the EH path. When sufficient power is accumulated in the super capacitor, then it is supplied to the PAPR (ID) path. At the low input power, a multi-tone signal results in increased PCE performance when the input power is low. While multi-tone operation, the EH path harvests energy. The PAPR path demodulates the multi-tone waveform.

B. ENERGY HARVESTING (EH) PATH
The EH receiver path is consists of an RF-DC converter, a DC-DC boost converter which is followed by a low-dropout voltage regulator (LDO) and a super capacitor. The EH receiver starts with an RF-DC converter which receives the RF signal and rectifies it. As the output from RF-DC is very VOLUME 10, 2022 low, not enough to charge super capacitor independently. An efficient design of reconfigurable RF-DC converter is given in [32]. Fig. 4 represents circuit description of one of the rectifier blocks. The rectifier circuit employs internal threshold voltage cancellation (IVC) technique for threshold voltage (V th ) compensation of the transistor used as rectifying devices. The main rectification body is composed of one NMOS transistor and two PMOS transistors. An auxiliary block is made-up of two PMOS transistors. Fig. 3(a) shows the positive phase of input power. The back compensated transistor reduces V th of the forward-biased transistors and increase harvested power in the main rectification chain. Fig. 3(b) shows the negative phase of input power, the rectifying devices are reversed-biased. This reduces source-gate voltages to zero, consequently minimizes the leakage current in the rectification chain. The output of DC-DC boost converter is given to LDO in order to keep constant output voltage. A super capacitor is connected at the output of the LDO to store energy and provides power for the ID path. Self-power and energy-neutral operations in the SWIPT system is very essential to provide power to the ID transceiver through the EH receiver but this task is very challenging. Energy harvested through the EH path is small as compared to the energy required for the continuous operations of the device, especially in battery-less applications. To overcome this situation, two modes of device operation was designed: energy harvesting mode and active mode. In the harvested energy mode, the SWIPT system will continue its normal communication operation, and when the store energy drops below the operation threshold voltage, the ID path is disabled and SWIPT system is switched to the harvesting mode. The quantity of RF energy present in the ambient environment and the amount of energy need for the device operations determine how frequently the frequency is switched between these two modes. A higher ambient energy from the RF signal indicates a faster charging of the super-capacitor, thus allows more energy harvest in active mode. This will shorten the time taken by the SWIPT system in energy harvesting mode, when the voltage across the super capacitor falls below the operation threshold voltage. As a result, the device's total switching frequency between two modes is reduced.
In order to reduce mode-switching, the quantity of energy required for device operation is also crucial. When the necessary energy rises, the super capacitor is quickly discharged, causing the SWIPT system to switch to the harvesting mode. Devices that require a very small amount of energy to operate take relatively small energy from the super capacitor. The device can function continuously without switching to the harvesting mode if the quantity of energy required for device operation is less than or equal to the amount of energy harvested. The sensitivity of proposed system is −5 dBm, and the measured peak power efficiency of the energy harvesting system is 69%.

C. PAPR (ID) PATH
The current research still lacks in achieving higher harvesting efficiency while using RF signal of higher PAPR. There have been attempts to design modulation specific schemes, particularly multi-tone based excitation and nonlinear signal amplification in order to enhance the SWIPT efficiency [28]- [32]. Conventional communication design is being used to transfer the information in SWIPT system like OFDM, QAM, QPSK, etc., and such communication designs are not optimized for higher energy harvesting in the SWIPT system. PAPR based SWIPT system provides a simple and efficient way to transmit/receive information. Digital information is modulated on a multi-tone waveform having different PAPR values. At the receiver end PAPR values are calculated for each symbol and digital information is retrieved using the lookup table. Fig. 4 depicts the proposed PAPR demodulator. The peak power (Power peak ) and average power (Power avg ) must be calculated to obtain the PAPR, for which two dedicated modules were designed. The switching between Power peak calculation and Power avg calculation is controlled through a mux. Initially, Power peak is selected by default. The PAPR values of the symbol can be calculated as: The finite-state machine-based digital controller controls the overall operation of the SWIPT receiver. The selection of analog-to-digital converter (ADC) reference voltage for both peak selection and average selections are adaptive. The digital controller controls the switching of mux logic to ADC control. Different PAPR values can be produced for a given number of tones and the variation in each tone yields a specific PAPR value. Estimation is done for modulation order that is required to obtain to communication rate. In order to communicate, the SWIPT transmitter and receiver need to be synchronized. A synchronization sequence frame (SSF) is initiated from the SWIP transmitter to receiver. SSF consists of a Preamble, SYNC, data frame (byte1, byte 2), and cyclic redundancy check (CRC). The frame format is given in Fig. 5. The overall flow of the PAPR demodulator is given in algorithm 2. As the RF signal arrives from APS to ID path, VOLUME 10, 2022 the PAPR demodulator calculates the Peak value and average value respectively, and switching is done through mux logic controller, once peak and average values are determined from RF signal, PAPR estimator ratio is calculated from the peak and average values. Estimated value is provided to corresponding PAPR symbol. Digital symbol values are extracted based on the PAPR lookup table, and then the algorithm checks that whether bits are recovered, if not the algorithm again starts form selecting peak and average values. If bits are recovered than SYNC is detected, after successfully detection of SYNC, data frames are extracted and final information is recovered. The uplink communication between transmitter and receiver is handled using back-scattering modulation. In case of limited power for transceiver operations, a back-scatter modulated signal is sent to the receiver. The power management and digital controller modules control the switching operation which diverts the antenna output to carry out modulation. Since back-scattering modulation doesn't involve any oscillator hence power consumption is comparatively very low.

IV. SIMULATION AND EXPERIMENTAL RESULTS
The chip micro-photograph of the proposed PAPR based SWIPT system is shown in Fig. 6. The chip is implemented and fabricated in 180 nm CMOS technology with     power management unit (PMU), serial peripheral interface (SPI), PAPR estimator, decoder, digital controller, and backscattering blocks. The PAPR based SWIPT system consumes 7.3 µW. Fig. 7 presents the measurement environment created for measuring the actual design unit. The PAPR modulated RF signal is generated through a vector signal generator. In which multiple tones can be combined to generate the required PAPR waveform. A network analyzer is used for impedance matching between antenna and chip input. PAPR decoded information and SWIPT system control registers can be accessed by PC through the SPI interface. The RF signal is received by both EH and PAPR (ID) paths through APS. The harvested energy is stored on the super capacitor, which is used to power up the PAPR (ID) path. Fig. 8 shows the performance measurement of RF-DC converter at different number of tones. It shows that PCE measured against the RF input power with respected to number of FIGURE 11. The measured PCE and data rate with respect to the power splitting ratio at different input power levels power.
tones. For low power, all tones performs almost similarly in terms of PCE. As the input power increases the multi-tone performance increases. The breakeven point is observed at -3 dBm input power level among all tones. The achieved throughput of our proposed design is 2 bit/sec/Hz as comparted to [31] for 4 number of tones is 50 bit/s/Hz. However the rectifier output voltage in [31] at 0 dBm is 0.57 V as compared to 0.68 V which is achieved in our proposed design.
The simulated result of PAPR based modulate data frame and its counter demodulated data at receiver end is shown Fig. 9. Fig. 10 shows the relation between the multi-tone and harvested DC power. The higher the number of tones, the more the harvested DC power. The communication pattern is implemented and verified in Verilog HDL. Fig. 11 show the PCE and data rate versus PS ratio at different input power VOLUME 10, 2022    Fig. 12 shows the extracted peak and average waveform of the RF PAPR signal. Peak and average waveform are digitized to calculate the PAPR values of the data symbol. Fig. 13 shows the mapping of PAPR values to the data symbols in LUT. Based on received PAPR values of RF signal, data bit stream is recovered. Fig. 14 shows the structure of the frame in the PAPR modulated signal. The structure of the PAPR data frame is consists of preamble, SYNC, data byte1, data byte2 and CRC which comprises 16 bit, 8 bits, 8 bits, 8 bits, and 8 bits correspondingly. The preamble is used to align the data frame which helps in the extraction of data bytes. Two data byte fields are used in the frame structure. CRC field is used to check the error in the frame. The performance summary of proposed stateof-the-art PAPR based SWIPT with IC implementation and experimental results and the comparison summary with Swipt systems is given in Table 1.

V. CONCLUSION
In this paper, a PAPR based SWIPT system is presented for sensor applications. Since the SWIPT system is different from other conventional systems in terms of power supply, it relies on RF energy. In the proposed architecture, the SWIPT system harvests energy from RF signals using the PAPR technique. This results in improved PCE. The proposed architecture performs its operations with the help of a digital controller implemented in the system. Furthermore, an adaptive RF-DC converter converts the RF signal to DC voltage. A boost converter and LDO both assure smooth and interruption-free operation. The ID path is dedicatedly designed on PAPR logic. Power peak and Power avg are calculated from the input RF signal. And a PAPR estimator estimates the PAPR values. The simulated and experimental using different signals demonstrates the validity of proposed design. The proposed PAPR based SWIPT system is implemented in 180 nm CMOS process and consumes very low power (7.3 µW). This occupies 0.4 mm 2 of die area. The clock frequency is 64 kHz and provides an 8 kbps of data rate.  From 2001 to 2016, he was with GCT Semiconductor Inc. From 2016 to 2021, he was with Celfras Semiconductor Inc., Seoul, South Korea. In 2021, he joined SKAIChips Company, Suwon, as a Chief Design Officer. He is currently a University-Industry Collaboration Professor also with Sungkyunkwan University. His research interests include analog backend design and mass-production management, power management integrated circuits (PMICs) implementation, and CMOS RF transceiver and SOC integrated implementation.