A novel autonomous wireless sensor node for IoT applications

A novel wireless sensor network node (WSNN) is presented in this paper where the solar energy harvester system is used as an autonomous power solution for endless battery lifetime. In this sensor node, the meander-line Inverted-F-Antenna (MIFA) is proposed and integrated in a single-CC2650 chip of Texas Instrument. The simple structure, low cost, compact size, high efficiency and low power consumption are advantages of this single-chip WSNN. The experimental results show that MIFA antenna is promising solution to enhance communication performance in WSN. In addition, the investigated single-chip WSNN with multi-wireless technologies including Bluetooth Low Energy and Zigbee as well as 6LowPAN is an attractive device for internet of thing (IoT) applications


The Proposed Antenna Integrated in WSNN 2.1. Wireless Sensor Network Node Design
In the first step, the planar Inverted-F Antenna (IFA) antenna for WSNN is studied. This initial antenna consists of a thin arm or wire shorted at one of the ends to the ground plane [12][13][14][15][16], as shown in Figure 1 (a) is designed. The length of the arm is nearly λ∕4. The input impedance is controlled by turning position of the feed. As the feed becomes closer to the shorting pin, the input impedance is reduced. Thus, to obtain a 50-ohm input impedance, the feed should be closer to the shorting pin than to the open end. The shorting pin introduces to the input impedance an inductance while the open end introduces a capacitance Cs. To obtain the impedance at resonance frequency, the two reactive components should cancel out, leaving only the radiation resistance . The width W of the shorting pin is very small compared to the wavelength (W ≪ λ), usually W ≤ (0.05−0.1) λ.
As mentioned in the introduction, the WSNN must be small size and compact. It leads to miniaturize antenna dimensions for hardware design. In [17][18][19][20] several methods have been proposed for antenna miniaturization such as: folded antennas, lumped elements, using substrates with high dielectric constant, and multi-layer meta-material. With onboard antenna, the simplest structure is using a meander line or folded antenna. In [21], the simulated and evaluated meander line antenna geometry have studied. The idea is to fold the conductors back and forth to make the overall antenna shorter, it is a smaller area, but the radiation resistance, efficiency and bandwidth decrease. A meander-line antenna can be realized by bending the conventional IFA antenna to decrease the size of antenna. The influence of the meander part of the antenna is similar to a load and the meander line sections are considered as shorted-terminated transmission lines. The meander line sections can be modeled as an equivalent inductor. In the far-field pattern, the result of the cancellation of magnetic fields, the transmission lines of a meander line antenna do not radiate fields. The radiation fields will be radiated from the vertical pars of MIFA. After bending the arm of the IFA antenna (MIFA), we obtained the new antenna with much smaller size of 25% compared with the initial IFA antenna, the total antenna size is only 33.6*42.6 mm. The parameters dimensions of MIFA are shown in Table 1.

2.2
After completing the MIFA design, this antenna is integrated in WSNN, the reflection coefficient of the proposed MIFA in free space (the red line) and in the sensor node circuit (the black line) are presented in Figure 2. These results show that the resonant frequency is  Figure 3, the novel single-chip WSNN uses only one chip SOC of CC2650. The CC2650 chip is a wireless MCU targeting Bluetooth, ZigBee and 6LoWPAN. This SOC chip is a member of the CC26xx family of cost-effective, ultralow power, 2.4 GHz RF devices. Very low active RF and MCU current and low-power mode current consumption provide excellent lifetime. The CC2650 device contains a 32-bit ARM Cortex-M3 processor that runs at 48 MHz as the main processor and a rich peripheral feature set that includes a unique ultralow power sensor controller. This sensor controller is ideal for interfacing external sensors and for collecting analog and digital data autonomously while the rest of the system is in sleep mode. Thus, the CC2650 device is ideal for WSN applications in a whole range of production including industrial, consumer electronics, and medical. shows 3D radiation pattern of IC integrated MIFA model with the peak gain of 1.74 dBi, total efficiency of 81%. A comparison between the proposed CC2650 WSNN and related work is shown in Table 2. It can be seen that this design achieves the compact size, simple structure, high performance and low cost, especially the power consumption of the WSNN is lower than some nodes in the market. The layout of proposed WSNN and the prototype is illustrated in Figure 5.

Antenna Gain Measurement
As the antenna is integrated in the sensor node circuit, the antenna gain measurement method is based on the Friis transmission formula, two sensor nodes are separated at a distance R of 4 m shown in Figure 6. where: -A wireless sensor node using an external antenna which actived as a peripheral for advertising using BLE communication technology, the transmit power of this node is known. External antenna is omnidirectional dipole antenna with the peak gain of 2.01 dBi at 2.45 GHz.  [16]. Ideally, the losses due to impedance loss, impedance matching should be ignored and assumed that the transmitting and receiving antenna of WSNN are matched to their lines or loads, then Friss formula is reduced to [7].
Transmitting power of WSNN using external antenna is 0 dBm, then receiverd power measured by CC2650 WSNN is -60 dBm at the distance of 4 m. Therefore, the 0.087 dBi of measured gain of MIFA integrated WSNN is calculated by (1): Figure 6. Gain measurement scenario of MIFA antenna integrated WSNN

Power Consumption Measurement
The power consumption measurement set up of the WSNN is presented in Figure 7 to calculate its lifetime. To measure current consumption in each operating mode of the device, using the oscilloscope to analyze the operation of the device in each mode is essential. Since the oscilloscope only has voltage transducers, a resistor connected in series with test equipment (DUT) is required. A 10 Ω resistor is an appropriate value so that it does not affect the circuit and can provide a voltage value that is large enough to be visible on the oscilloscope as shown in Figure 8. The power consumption is listed in Table 2. 1 + 2 + 3 + … + 14 (2) with power supply of 3.3 V. We can calculate the device power in each mode as follows:

Autonomous WSNN using Solar Energy Harvester System 3.1. Power Supply Based on Solar Energy Harvester System
The proposed sensor node is powered by a battery with the determined lifetime. A solar energy harvester system is added as in Figure 9 to obtain endless lifetime, this power supply solution makes wireless sensor node becomes an autonomous WSNN. In this article, the power supply includes a DC-DC converter circuit and energy storage. The problem is finding ways to manage the input voltage of the charger circuit. This leads to the idea of storing energy in the super capacitor before it is introduced into the charger circuit. Figure 10 presentes a voltage  Figure 11 show that the output voltage of the solar charging circuit is stable at 3.4 V, enough to provide power for stable wireless sensor node operation.

Wireless Sensor Node using Multi-wireless Technologies
WSNN supports three wireless communication technologies: Bluetooth Low Energy, 6LoWPAN, ZigBee. In this work, we conducted a test scenario to test the transceiver distance between two WSNNs using BLE technology with extremely low power consumption of only 25.37 mW in active mode. At the same time, we also built a wireless sensor network monitoring room temperature using 6LoWPAN technology with active mode power consumption of 50.66 mW. The process of data exchange between two BLE devices is described in Figure 12. The flowchart of the WSNN BLE and 6LoWPAN is presented in Figure 13.

WSNN Experimentation
In this simple scenario, the proposed WSNN is used in environment monitoring system showing its advantages in terms of computation, storage, time life and omni-directional communication. We conducted two scenarios with CC2650 WSNN: (1) read-range measurement between the two nodes using the Bluetooth Low Energy communication and; (2) an indoor temperature monitoring using wireless sensor network based on 6LoWPAN communication technology.

Scenario 1: Read-range Measurement
The distance of communication between two WSNNs using BLE communication standard is tested and measured in this experimentation as in Figure 14: -Setting one WSNN in BLE Peripheral mode, one WSNN in BLE Central mode -Setting the transmission power is 0dBm -Transmitting 1000 packets between two nodes and calculating packet error rate (PER) -Moving the WSNN to determine the antenna's multi-directional radiation -The number of packets transmitted and received between the two sensing nodes will be monitored by the computer. Table 3 illustrates the result of read-range measurement between two WSNNs using BLE communication standard. Two nodes can communicate together at a maximum distance of 64 m, the PER is 0.5%. Figure 14 The measurement configuration of the read-range between two WSNNs operating in BLE mode In the indoor temperature monitoring WNS as in Figure 15, the CC2650 WSNNs measure the temperature in a room and then transmit the measured date to the Raspberrypi3-Gateway. The CC2650 WSNNs send data to the gateway, from which the gateway sends data to the Internet via the MQTT protocol. Measuring cycle of each WSNN is set to 10 s for one measurement. The 6LoWPAN technology uses the IPv6 address of the device. In order to transport data from the wireless sensor node to the Internet, we need to use NAT64 to switch between IPv6 and IPv4. To do this, we used the software package provided by Wrapsix, installed the software package directly on the Rasberry Pi embedded computer. The temperature WSN prototopye is presented in Figure 16 with the website interface.

Conclusion
The proposed WSNN using MIFA antenna is working at the frequency band of Zigbee, Bluetooth low energy ad Wi-fi band. The simple structure and quasi-omnidirectional, compact size and low cost are advantages of this novel node. Besides, the planar structure, low power consumption and multi-wireless communication technologies allow this node to be an excellent wireless sensor network node in wireless sensor network and IoT applications. The WSNN is tested in a real scenario giving long read-range, very low power consumption and high PER. The limited battery lifetime challenge is solved using solar energy harvester system and store energy as well as voltage monitoring and battery charging. This solution helps WSNN to become an autonomous wireless sensor network node. ISSN: 1693-6930 ◼