Energy efficient indoor wireless communication techniques based on BLE technology

. The paper presents the design of a wireless communication system for short-range, high-speed, and multi-node information acquisition based on BLE5.0 technology, targeting indoor wearable applications. The system employs the Nordic nRF52832 microcontroller, which integrates BLE5.0 transceivers, as the core, and includes designs for master-slave node communication, data acquisition, hardware circuitry, embedded and upper computer programs. The system is validated through upper computer software for device networking, multi-node data acquisition, and communication speed. The power consumption and communication distance of the devices are tested using a measurement device. The designed multi-node BLE5.0 communication system in this paper provides a technical foundation for the design of relevant short-range IoT applications and has broad application prospects.


Introduction
In the market, there are many products that use Bluetooth technology. Currently, there are 8.2 billion mobile devices using Bluetooth. According to a study by ABI, by 2023, 70% of smart devices will be using Bluetooth. Therefore, the update of Bluetooth technology in the context of widespread demand is an inevitable outcome. As new standards are introduced, the communication mode has evolved from one-to-one communication to one-to-many broadcasting and later to many-to-many communication, with lower power consumption, which will be suitable for more applications, such as network communication, building a small local communication network, or designing portable devices, etc. There is a huge market potential for these applications [1][2][3].
With the progress and development of Bluetooth technology, especially after many industries and manufacturers joined the Bluetooth technology alliance, new application modes such as space positioning, image transmission, automotive applications, industrial production, and information security have emerged. On the one hand, new application modes are continuously being researched and developed, while on the other hand, existing application mode products are becoming more diverse. The low cost and high compatibility with the mobile internet have provided tremendous application prospects for home office automation, e-commerce, industrial process control, and modern intelligent social networks. Therefore, many experts recommend that Bluetooth devices will be ubiquitous in the future, and Bluetooth low energy wireless technology will have immeasurable development in the future [4][5][6].
Wearable devices refer to a category of devices that utilize a variety of sensors, detection and identification technologies (such as speech, gesture, eye movement, etc.), global positioning systems, displays, cloud services, and other interactive and database storage technologies to exchange information with users, monitor human life and entertainment, and directly attach them to the body or integrate them into various portable mobile terminal devices such as clothes and mobile phones. The concept and definition of portable devices are also quite broad. They are not only a new type of hardware device, but also an intelligent one that is realized with software support and has more complete and efficient functions [7][8][9]. Currently, the portable device industry market is very hot, and more and more young people have begun to pay attention to the future development of this industry. One of the main reasons is that portable devices make it possible for devices to be worn near the body for a long time, allowing users to conveniently monitor different data at any time. From the current situation of products available in the market, intelligent portable products are mainly used in medical care, information consultation, body sensing monitoring, environmental impact and monitoring, including heart rate and speed control belts, smart bracelets, body side and visual control devices, smart watches and AI glasses, body sensing and environmental monitoring devices, among others [10][11][12].
The main purpose of this design is to provide communication design ideas for data collection of low-power, short-distance, multi-node wearable devices. In addition, this technology can also be applied to other IoT development applications and has great application value.

Methods and Materials
A high-speed, multi-node information collection application scenario is designed for the wearable field, based on the low-power Bluetooth (BLE 5.0) communication protocol, using a low-power ARM controller and BLE 5.0 transceiver chip or a low-power ARM controller integrated with a BLE 5.0 transceiver, to design a multi-node wireless communication system with sleep and work modes. Through BLE5.0 communication, data transmission between up to 8 nodes can be achieved, with a communication speed of not less than 1Mbps, a communication distance of not less than 30 meters, and a sleep mode power consumption of not more than 20uA.
Specific design work includes: 1.Hardware selection 2.Circuit design 3.Wireless communication design 4.Microcontroller embedded program design The design challenges are: 1.Achieving data transmission between 8 nodes 2.Communication speed not less than 1Mbps 3.Sleep mode power consumption not more than 20uA 4.Communication distance up to 30m. This design system includes a master device node as the core of the communication system, supporting 8 peripheral slave devices as data collection nodes, and adding a host computer to save and manage the collected data. The Nordic company's nRF52832 BLE chip is used as the core of the Bluetooth communication design. This chip supports Bluetooth 5.0 protocol, which can support multiple device interconnection, meeting the communication design of 1-to-8 and high-speed communication. In terms of low power consumption, this design adopts an LDO voltage stabilization circuit scheme, which has a good practical effect. In terms of hardware connection system structure, a single nRF52832 BLE chip is used as the host server, and a communication serial port is configured to enable data transmission between the host server and the PC host computer. Eight slave client chips are used as 8 node slave clients, and the slave and host communicate with each other through Bluetooth 2.4G wireless communication connection. In addition, the slave can also open other communication ports (IIC, UART, SPI, etc.) to connect with external application devices. This can satisfy the data from the PC host computer being controlled to the external device through the master device to the slave device, and the external device can also send data to the master device through the slave device, and finally display the data on the PC. The theoretical feasibility of bidirectional data flow is met, which meets the development needs. The hardware structure diagram is shown in Figure 1.

Results and Discussion
To simultaneously achieve ultra-low power consumption and device cost, this design adopts a single-chip approach, where the device does not include a processor other than the Bluetooth chip, and the software program runs directly on the Bluetooth chip. The Bluetooth chip, which uses NRF52832 as its core, is designed to be a low-power indoor communication system, consisting of a mesh network in a star topology.
The low power consumption design of wearable devices is the basis for ensuring the longterm operation of the device. In terms of hardware, it is essential to improve the power performance of the system by designing an internal power regulator [13][14][15]. Nordic's nRF52832 Bluetooth chip has two types of internal voltage regulators (LDO and DC/DC) [16][17][18][19], and considering that this design uses a common 3.0V button battery for power supply, the voltage difference before and after voltage regulation is not significant, and LDO requires fewer components and has lower cost. Therefore, using LDO voltage regulator is more suitable.
While meeting the design of the internal low-power voltage regulation circuit, the main body of the design is a minimum system board based on the Nordic nRF52832 Bluetooth chip, plus antenna, reset button, crystal circuit, LED light, plug, etc. The design also includes JTAG interface for downloading and debugging UART interface. The design goal is to lay a hardware foundation for functional and performance testing verification of low-power Bluetooth chip operation, and considering the wearable nature of the device, the PCB design selects small-sized packaging components to compress the device volume into a compact design, hardware circuit schematic, PCB, and the physical picture are shown in Figure 2

Software design
The program design directly determines the functions and corresponding performance of the device. In the indoor wireless communication system based on BLE technology, software programming must be carried out for the host device, the slave device, and the PC-based upper computer to achieve system integration. When designing the program, the following aspects are mainly considered: 1.Implementing data interaction between the host device and the slave device. 2.Implementing data interaction between the host device and the PC-based upper computer.
3.Designing an upper computer with basic functions to meet the basic requirements for subsequent functional testing of the system.
The main function of the host device is to transmit the data sent by the slave device to the PC-based upper computer. In addition, as the core of the communication system, the host device also needs to maintain and control the entire system. After booting up, the host device initializes the hardware, starts broadcasting and scanning for slave devices. If no slave device is connected within three minutes, the host device enters a hardware sleep mode, which can only be awakened by an external button press. If a successful connection with a slave device is established within three minutes, the host device enters a software sleep mode to reduce power consumption and waits for the slave device to send data. If the slave device sends data to the host device, the data is transferred to the PC-based upper computer, the flowchart of the host program is shown in Figure 4. The main function of the slave device is to read the data sent by the master device, and execute commands from the master device. After the slave device is powered on, it initializes the hardware devices, starts broadcasting and scans for the master device. If there is no connection with the master device within three minutes, it will enter hardware sleep mode.
At this time, external button operation is required to wake up the device. If the slave device successfully connects with the master device within three minutes, the slave device will enter software sleep mode to reduce power consumption, waiting for the master device to send data and commands. If the master device sends data or commands to the slave device, the slave device executes the relevant instructions or reads the relevant data. The program flowchart for the slave device is shown in Figure 5. Due to the focus of this design on the establishment of the system framework, it is difficult to intuitively view the system's operation status and identify any existing issues after system integration. Therefore, a debugging software for the PC end needs to be designed for system testing purposes. The PC software is intended to monitor the information exchange status between the master and slave devices, and facilitate operation and observation [20][21][22][23].
In practical applications, the indoor wireless communication system based on BLE technology has two most important points. The first is that the host device can accurately and reliably receive data collected by the slave device, which is the basic function of this designed system. The second is that the normal sleep of the slave device ensures the low power consumption of the entire design system, highlighting the design advantages and prominent features of this system for wearable devices. While ensuring the normal operation of the above two functions, the relevant performance of the system is tested [24][25][26][27].
Low power consumption is crucial for portable wearable devices, and achieving low power consumption is the only way to meet the long standby time and make users' wireless experience more comfortable. Table 1 shows the power consumption of the device under different conditions. It is found that due to the absence of other high-energy-consuming devices, the standby power consumption of the device is as low as 0.3uA, and the connection power consumption is 2.59mA. The designed sleep mode power consumption requirement of no more than 20uA is met. Communication speed is a core attribute of system performance and the foundation of user experience [9]. Table 2 below shows the communication speed of the master device with different numbers of slave devices. It meets the design requirement of a communication speed of not less than 1Mbps. For wearable devices, there are two main factors affecting the communication distance, one is the transmission power of broadcasting, and the other is the direct relationship between the communication distance and the selected antenna. Generally, the antenna gain is proportional to the wireless transmission distance. The higher the antenna gain, the farther the transmission distance [28][29][30]. However, the antenna gain cannot be infinitely large. For this design, it is necessary to consider both the transmission power of broadcasting and to select an antenna receiver with a suitable gain. Table 3 below shows the communication distance obtained from testing different types of antennas and transmission power.  -40db  3m  7m  12m  18m  -20db  9m  12m  19m  24m  0db  17m  19m  25m  31m  4db  23m  28m  32m  38m In actual application, there are many factors affecting the communication rate. It can be seen from the table that the design requirement of 30m is met under ideal conditions.