Future application and technical challenge of reverse scattering communication *
Cui Ziqi 1, Wang Gong servant 1, Wei Xu Sheng 2, Jiang Dabin 2, Qin Fei 2, Ai 3
(1. School of Computer and Information Technology, Beijing Jiaotong University, Beijing 100044, China; 2. Vivo Mobile Communications Co., Ltd., Beijing 100083, China; 3. School of Electronic Information Engineering, Beijing Jiaotong University, Beijing 100044, China)
* Fund Project: National Natural Science Foundation of IoT Basics Based on Passive Reverse Scattering Communication Technology (NSFC-61871026); National Natural Science Foundation of China "New Generation Mobile Communication Theory and Application of Typical Scenes for High Speed Railways Key Technologies Research (NSFC-U1834210)
[Abstract] Reverse scattering communication technology is one of the key technologies of "green" Internet of Things and 6G network design, which enables low-power, low cost, and easy deployment "green" communication. Summary of the origin and principle of traditional reverse scattering communication technology, reviewing new reverse scattering communication technology and the latest research progress, predicting the future potential application of reverse scattering communication technology and analyzing its key issues.
【Key words】 Reverse scatter communication; high rate transmission; Internet of Things; passive sensor; wearable device
CLC number: TN929.5 Document Sign Code: a
Article number: 1006-1010 (2021) 04-0029-08
Quote format: Cui Ziqi, Wang Gong servant, Wei Xusheng, et al. Future application and technical challenge of reverse scattering communication [J]. Mobile Communications, 2021,45 (4): 29-36.
IoT, Internet Of Things is the third wave of world information industries after the computer and the Internet. The Internet of Things connects all kinds of objects together with "all-in-one" ideas, get rid of the limitations of traditional communications to access networks, convenient for people to work and improve production efficiency in various industries. my country IMT-2020 (5G) promotion group puts forward "information, all things, tentacles and" 5G vision, everything is one of the core goals of 5G; IMT-2030 (6G) promotion group further proposed "Wan Zhi Zhi, Digital Twin "6G vision . Literature  pointed out that the Internet of Things is the technical cornerstone of the construction of industrial network, and is an important empowerment for manufacturing digital transformation. Literature  indicates that the Internet of Things is one of the key technologies that promote wireless communications from "All things" to "intelligent interconnection".
Reverse scattering communication (BACKSCATTER Communications) is one of the key technologies that build green energy saving, low cost and flexible deployment, and is an important means of achieving "10,000 Zhiyi". Traditional reverse scattering techniques are mainly used in RF Identification (RFID, RADIO FREQUENCY IDENTIFICATION) systems. A typical RFID system consists of a reader and a plurality of tags (shown in Fig. 2 (a), communicating with reverse scattering techniques. However, the traditional reverse scattering technology requires a particular radio frequency source as an excitation signal, and the wireless signal in the communication process will undergo a double path decline of the round-trip, so the path loss is large, and the effective communication distance is short [4-6].
For traditional reverse scattering techniques, a specific radio source, a short effective communication distance is short, and the academic community has proposed a range of new reverse scattering communication technologies to reduce the energy consumption and operating costs of communication, and extend the service life of the equipment. Since the passive device of the new reverse scattering technology can transmit its own information using a third party signal (eg, cellular wireless signal, broadcast television signal, Wi-Fi signal or Bluetooth signal, etc.), so the new reverse scattering technology is also known. For "symbiosis communication". The world's first 6G white paper  pointed out that the new reverse scattering technology provides a possibility of achieving ultra-low power communications, with 6G "ubiquitous" design goals. At the same time, the new reverse scattering communication technology can meet the "green" Internet of Things, low-energy, sustainable design goals, and become the focus of 6G network design and "green" Internet of Things. This article will introduce the principles, research progress and potential applications and technical challenges of new reverse scattering communication technology.
1 New reverse scatter communication technology
1.1 Reverse scatter communication technology principle and origin
Reverse scattering technology originated in World War II, the military in order to identify the mounting label on the body in the fierce, according to the radar signal scattered in the label . Subsequently, a large number of applications based on reverse scattering techniques are mainly used in RFID systems, including the electronic non-parking fee system (ETC, Electronic Toll Collection) is a classic case of large-scale commercialization of reverse scattering technology .
Reverse scattering communication devices use radio frequency signals in other devices or environments to transmit their own information. As shown in FIG. 1, the device controls the reflection coefficient of the circuit by adjusting its internal impedance, thereby changing the amplitude, frequency, or phase of the incident signal, analog or digital modulation of the signal. The formula (1) represents the circuit reflection coefficient:
Where Z0 is antenna characteristic impedance, Z1 is a load impedance. Assume that the incident signal is SIN (T), the output signal is。The analog modulation adjusts the built-in analog circuit to change the impedance Z1, and the digital modulation uses the controller to change the impedance Z1.
Passive reverse scattering communication device first needs to acquire energy (Energy Harvesting) from an external RF signal, and supplies internal circuit modules, and then reverse scattering RF signals to communicate, and zero power communication.
1.2 New reverse scatter communication technology
Since 2013, the industry has proposed a range of new reverse scattering communication technology, and eight cases are given below.
2014 Kimionis et al. Proposed a bistatic backscatter, which sets a carrier generator near the tag (as shown in Figure 2 (b)), which can effectively reduce path loss, expand the label and read and write. The communication distance between the units, when the supply carrier power is 20 mW, the tag communication distance is about 130 m .
LIU, Parks and others proposed ambient backscatter [11-12] in 2013 and 2014. This technology does not require carrier generators, using wireless signals such as TV broadcasts and Wi-Fi around the label. To trigger communication (as shown in Figure 2 (c)). Currently relevant research has designed prototypes [11, 13] and communication protocols [11, 13] and communication protocols  of signal energy detection, showing the potential of commercial applications of this technology.
In 2015, Bharadia et al. Proposed a full-duplex-based reverse scattering technology, using multi-antenna Wi-Fi gateways to assist users and labels to communicate with the label  to achieve efficient full-duplex communication (as shown in Figure 2 (d) . The Wi-Fi gateway supports a variety of modulation methods. When the communication distance is 1 m, the data rate can reach 5 Mbit / s.
In 2016, the transformation reverse scattering technology (Inter-Technology Backscatter) moves the frequency band of the Bluetooth signal or the zigbee signal (as shown in Figure 2 (e)), in the signal by changing the impedance of the label. When the rate is 2 mbit / s, the power required for the required signal is only 28 %& micro; W, expands the application range of reverse scattering communication .
2017 LORA-based reverse scattering technology (Lora Backscatter) utilizes LORA signal high sensitivity (-149 dBm) and spread spectrum coding technology to achieve long-distance reverse scattering communication (as shown in Figure 2 (f)), experiment The communication distance is up to 475 m .
2017 and 2018 proposed inverse scattered intelligent surface auxiliary communication technologies can intelligently reconstruct and enhance the wireless channel environment [18-20]. The intelligent surface is composed of a plurality of reflective units, each of which can interact with the incident signal (as shown in Figure 2 (g)). A smart surface designed by Southeast University, with 8PSK modulation by adjusting the phase of the reflected signal, with a communication rate of 6.144 mbit / s .
Mehrdad et al. In 2019, the reverse scattering large-scale access mechanism (NetScatter) uses a combined switch-related distributed 啁啾 (chirp) spreading coding mechanism (as shown in Figure 2 (h)), can support Multi-device concurrent access, when accessing 256 devices, the communication bandwidth is only 500 kHz .
TAEKYung et al. Presented in 2020, the phase difference between the non-coherent channel antenna is constant according to the phase difference between the non-coherent channel antenna, and the symbol information is received by the phase difference of the received signal (as shown in Figure 2 (I). ), The dependence on the reverse scattering technique for a particular type of RF signal is eliminated .
1.3 Advances in research on new reverse scattering communication technology
The new reverse scattering communication technology has become a research hotspot, and the related research has grown rapidly. We will exemplify the technical and application of low-power wearable device related technologies and applications for system transmission design and performance analysis.
（1）Design and Analysis of New Reverse Scatter Communication System
The new reverse scattering communication system is rich in research, and only part of the system performance analysis, channel estimation, signal detection, and encoding modulation.
In terms of system interrupt and capacity analysis, literature  studied the channel capacity of the reverse scattering communication system, giving a progressive value of the system interrupt probability in the case of high signal-to-noise ratios, and the result indicates that the capacity of the reset Signal is not entirely Both the Gaussian signal capacity twice. Similarly, ZHAO et al. Derived a reverse scattering channel distribution expression and analyzed the interrupt performance of the system in real Gaussian channel .
In terms of channel estimation, considering practical channel information acquisition, Qian et al. Analyzed the feasibility of differential detection without pilot , MA et al. Proposed a desired maximum Blind channel estimation plan for algorithm .
In signal detection, Chen et al. Studied the problem of signal detection in the full-duplex reflection communication system, and the maximum likelihood detector is designed, and the corresponding error rate expression  is designed. Document  proposed a maximum likelihood detector for a symbiotic reverse scattering communication system, and a linear detection method of low complexity was designed.
In terms of encoding, the literature  uses Manchester encoding in the label, and the corresponding decoding mechanism is designed in the reader side, which effectively reduces the bit error rate. Document  proposes an empty time-time encoding for reverse scattering communication, and analyzes that the performance of the encoded when the reverse scatter communication system is empty is almost the same as the Alamouti code, and the circuit is simple and has good application prospects.
(2) Technical support and application of low-power wearable equipment
Low-power wearable devices based on reverse scattering technology are current research hotspots. Since most of the wearable devices currently applied to the market use Bluetooth or Wi-Fi signal communication, a hot spot in the current academic community is to use existing Wi-Fi or Bluetooth signals to communicate with commercial equipment reliable communication.
To reduce the co-frequency interference of Wi-Fi or Bluetooth signal when the wearable device is communicated, literature  proposes the frequency shift-based reverse scattering technology (FS-BACKSCATTER), installing a 20 MHz oscillator in the label, will reverse The scattering signal moves to a frequency band having a difference of 20 MHz with the original signal frequency and improves communication reliability.
The Bluetooth reverse scattering system proposed by , using an existing Bluetooth signal to communicate with commercial Bluetooth terminals. The modulation process is shown in FIG. 3, first generating a monosone signal as a modulation carrier frequency, and then mover different components to represent symbol "0" and "1". The system can also modulate the local clock frequency for dynamic channel configuration, avoiding a serious interference channel.
Document  utilizes the frame polymer (A-MPDU) characteristic of the Wi-Fi signal MAC layer to achieve reverse scattering communication, and the communication process is shown in FIG. The passive device reverses the data of the subframe (MPDU) transmission according to the data information to be loaded, selectively changing the channel during the subframe transmission. Since only one channel estimate is performed during an A-MPDU transmission, the access point (AP) cannot recover the interference MPDU, at which point the AP returns the ACK information that the MPDU is recovered is information to be transmitted.
The body is attached when wearing equipment, so the impact of assessing and reducing the human body on reverse scattering communications is also one of the key to the actual promotion of low power wearable devices. However, the academic current industry has a relatively related test, and we build a test environment to assess the impact of the human body on the inverse scattering signal strength, as shown in Figure 5. We communicate using General Software Radio Peripherals and WISP  passive tags, and measure label reflection signal strength using spectrum meter. The test results show that when the label is attached to the human body, the signal strength of the reverse scatter will decrease 5 dB to 10 dB.
2 New reverse scattering technology application prospects
Reverse scattering technology enables the device to get rid of the binding of the battery, reduce equipment production and maintenance costs, accomplishing the lower power, lower cost, wider connection of 5G and 6G, in addition to traditional reverse scattering technology has applied warehouse In addition to the scenes such as goods, there is a broader potential application in the future, which gives a typical application example.
2.1 Low-power wearable device
The wearable device can reduce equipment power consumption by reverse scattering techniques, extending battery life. When the periphery radio frequency signal is available to the device, the device transmits the acquired information to the intelligent terminal utilizes the reverse scattering technology. As shown in Fig. 6 (a), the smart watch will use information such as the number of steps, heart rate, etc., and the Wi-Fi radio frequency signal is reversed to the mobile terminal to display to the user. Can wear multi-physiological parameters monitor clothing embedded reverse scattering sensing unit, real-time physiological parameter signals, etc. Indicated.
2.2 Bio-built-in sensor chip
The bioconch is embedded in the organism and operates with external radio frequency sources. As shown in FIG. 7, after the chip collects the brain biological information data, the data is reflected back to the mobile terminal using an external radio source, and the terminal analyzes the received signal and restores the brain information.
2.3 Railway system operation maintenance
Document  The first time the reverse scattering communication technology is used for railway wireless network transmission, which reduces the influence of penetration loss and channel rapid changes in high-speed rail signals. Reverse scattering communication technologies may also be applied to rail intrusion detection, contact network monitoring, freight tracking, and personnel item positioning.
（1）Track foreign body intrusion monitoring. As shown in FIG. 8, the passive tag collection track peripheral state information is deployed in the railway peripheral, and the intelligent terminal is reversed to monitor the animal intrusion, bridge damage, or collapse, water pipe burst, and other hazardous event information.
（2）Contact network intelligent monitoring. As shown in FIG. 9, the passive label is installed in the railway contact network cable, collect cable tension data in real time, transmits the high-efficiency real-time analysis and monitoring of the status of the contact network to avoid electricity accidents caused by abnormal tension.
（3）Freight real-time remote tracking. As shown in Figure 10, the train freight business scenario shows the passive label. The reader is read and refreshed in the cars. The console can read the real-time location and status of the goods on the mobile app or online.
（4）Equatorial and dining car positioning. As shown in Figure 11, in the passenger train, each flight service is attached to the passive label of loading its related information. The reader reads the label position in real time, and the user can obtain the flight attendant through the train. Dining car real-time location.
2.4 Wisdom agriculture
Document  designed an insect network platform, the platform physical and circuit diagrams are shown in Figures 12 (a) and 12 (b), respectively. Passive sensor is attached to the living insect, the sensor uses the reverse scattering technology to communicate between the fixed site. As shown in Fig. 12 (c), the intelligent agricultural scene is shown in the insect, the temperature and humidity, lighting sensor, obtains information such as a crop, pollination state, etc. Implementation of fine agricultural production control.
2.5 Industrial sensor network
Future Smart Factories will deploy a large number of wireless sensors, which make up a wireless sensor network, and its goal is to monitor relevant processes and parameters in industrial environments . Such an environment is usually monitored using various types of sensors, such as microphones, carbon dioxide sensors, pressure sensors, light sensors, and the like. Various types of sensors are collected and transmitted to the central control nodes, and the rate requirements are generally less than 2 Mbit / s. The battery life requirements are several years, and there are also small equipment size and low cost. The related needs of the industrial sensor network can be well met by reverse scattering technology.
Document  applies a piezoelectric material to a passive label, and uses the piezoelectric effect of the material to achieve underwater reverse scattering communication. As shown in FIG. 13, the passive sensor with temperature and humidity, acid-alkalinity, and the like is deployed underwater, collecting water data, transmitting the information back to the terminal using reverse scattering technology to achieve low energy consumption, easy to deploy water monitoring.
3 The challenges of new reverse scattering technology and future research direction
The new reverse scattering communication technology is also facing a series of challenges, and the following is given six challenges and corresponding research directions.
（1）Theoretical performance analysis of new reverse scattering communications, such as the theoretical performance analysis of sensitivity limited. Most of the existing reverse scattering studies have not considered circuit sensitivity, and in practice, their internal circuits are activated when the passive device receives the radio frequency signal energy exceeding a certain threshold. Therefore, there is an important research value and engineering guiding significance in combination with the constraints of passive equipment sensitivity.
（2）Channel estimation and complex signal detection. The ability to transmit the pilot frequency in the reverse scatter communication system, the signal received by the receiving end is superimposed in the reflected signal and the radio frequency source signal, especially when multiple user access, how to model, channel parameters How to extract, how to detect the reflection signal is an emerging research direction.
（3）Large-scale user access. Passive device storage and computing power is limited, and the anti-conflict algorithm in the traditional network is difficult to apply to large-scale passive device access. The anti-conflict algorithm designed to design a large number of users access scenarios is the research direction of practical value.
（4）Self-interference. The received signal in the passive reverse scattering communication system is a useful reflection signal and the overlay of the disclosed self-interference signal, and the strength of the interference signal may be much larger than the reflection to the scattered signal strength. The traditional method takes radio frequency and baseband self-disturbance cancellation, which is an important challenge from a strong self-interference signal.
（5）Wide area coverage. Due to the influence of channel attenuation and external signal interference, the distance of reverse scattering transmission is limited. Currently, communication distance can be improved by relay, spread spectrum and LORA technology, and the future combined with large-scale reflection arrays, honeycomb network and multi-layer heterogeneous networks, and realizes wide coverage and is one of the network evolution directions, coverage and power time. The trade-off is also a theoretical issue worth studying.
（6）Design of close - distance high rate transmission scheme. Passive devices generally use low-order modulation due to energy limits, and are subject to challenges of crystal stability and synchronization and interference, and therefore its communication rate is low. The high-speed green communication in the near-distance is one of the sixg demand, how to further improve the short-distance transmission rate, such as by introducing MIMO, high-order modulation, promoting self-interference cancellation, etc. to increase the transmission rate, is reverse scattering communication One of the practical key challenges.
Reverse scattering communication equipment uses other equipment or radio frequency signals in the environment to transmit its own information, low-power, low cost, easy maintenance, good deployment, etc. of new reverse scattering communication technology, can effectively meet the future 6G network Application requirements. This paper introduces the principles and development history of new reverse scattering communication technology, which expounds the latest research progress, and gives the application prospects and future research direction. I hope that the industry will drive new reverse scattering communications technology maturation.