A brief discussion on the application of UWB in autonomous driving

If RFID PEPS is the dominant player in the era of fuel vehicles, and BLE PEPS is the leader in the era of Internet of Vehicles, then UWB PEPS will surely be the new star in the era of autonomous driving. With its advantages in precise ranging and positioning, UWB will not only be the first to be implemented in the intelligent upgrade of the PEPS system in the body domain, but will also gradually become the key technical support for autonomous driving in specific scenarios and specific functions. In the second issue of the body domain jargon, the author first introduces the characteristics, development history, and ranging and positioning principles of UWB technology, then presents the UWB PEPS system solutions of mainstream players, and finally briefly discusses the application prospects of UWB in autonomous driving.

1. Technical features

UWB (Ultra Wide Band) technology is a wireless communication technology that uses nanosecond pulses for data transmission. UWB is named ultra-wideband because the pulse signal it transmits occupies a very wide spectrum range (>1GHz).

UWB's distance measurement method is the same as that of most current laser radars, which are both ToF (Time of Flight) methods. The transmitter sends a pulse signal, which hits the object and returns. The receiver receives the transmitted signal and calculates the reception time difference between the two, and measures the distance between objects by multiplying it by the speed of light.

The following figure summarizes the main differences between UWB and the RFID/BLE/Wi-Fi wireless communication technologies introduced above. RFID/BLE/Wi-Fi technologies use a carrier (modulated sine wave) to transmit information over a standard narrowband and determine the distance between devices based on signal strength. Compared with them, UWB has the following typical advantages.

(1) High security. The distance measurement based on the ToF principle measures the reflected signal of the real object. In this way, hackers cannot use an absent device to forge a signal to communicate with the UWB device (BLE is based on the principle of signal strength value measurement and can be easily deceived by a strength signal forged by hackers). It is a magic that can avoid relay attacks. The IEEE 802.15.4z standard adds protection mechanisms such as encryption and random numbers to the PHY packet of the signal, further enhancing the security of UWB communication;

(2) High positioning accuracy. Compared with standard narrowband signals, the rise and fall times of UWB pulse signals are shorter, and the arrival time of the reflected pulse can be measured more accurately. Currently, centimeter-level positioning accuracy can be achieved, which is about 100 times higher than BLE.

(3) Large bandwidth. The theoretical transmission rate can be very high, but due to the power density limitation, the transmission rate is usually between tens of Mbps and hundreds of Mbps. Currently, it can reach 27Mbps. With the improvement of standards, it is expected to be further improved. At the same time, due to the low power density of the pulse-per-second signal, the transmission distance is usually limited to 10m;

(4) Strong anti-interference capability. UWB has very narrow pulses in the time domain, so it has greater resolution in time and space and is basically unaffected by noise. In addition, its ultra-wideband determines its strong multipath resolution capability, which can distinguish and eliminate the influence of most multipath interference signals.

2. Development History

In the 1960s, UWB technology first appeared in the research of time-domain electromagnetics in the military and radar fields, and has been shining in the military field. In 2002, the FCC (Federal Communications Commission) announced that under strict restrictions, the public communication frequency band of 3.1GHz~10.6GHz, a total of 7.5GHz, would be opened to UWB. At the same time, the radiation power was limited to -43.1dBm, which is much lower than BLE/Wi-Fi. So far, UWB has been officially opened to the civilian field and ushered in its first development peak. Based on the characteristics of large bandwidth and low power consumption, everyone initially envisioned how to use UWB to build a short-range high-speed wireless LAN within 10m, but because the technical path has never been agreed upon, and the WI-Fi technology as a competitor has developed rapidly, UWB eventually withdrew from the stage of high-speed wireless LAN transmission. After years of hard work, UWB finally ushered in a turnaround in the field of positioning. The characteristics of high bandwidth determine the high positioning accuracy.

In 2019, Apple released the iPhone 11 system, which was pre-installed with the Apple U1 chip equipped with UWB technology. UWB has entered the mainstream consumer electronics field. On August 25, 2020, the IEEE 802.15.4z was finalized. The standard has made improvements to positioning security, theoretically further reducing the probability of being hacked and tampered, and further paving the way for the application of UWB PEPS. In July 2021, CCC (The Car Connectivity ConsorTIum, International Car Networking Alliance) released the 3.0 specification, defining the interconnection solution for the third-generation digital key. UWB, BLE, and NFC will work together in different scenarios to achieve more intelligent and secure identity recognition, access control, and ignition control. Among them, BLE is used for remote vehicle wake-up and transmission authorization, UWB is used to accurately locate the user's position after wake-up, and NFC is used as a backup solution when the mobile phone is out of power. The NIO ET7, which will be delivered in Q1 2022, will be the world's first new car equipped with UWB PEPS, marking the beginning of the installation of the UWB PEPS system.

3. Distance measurement method

Depending on the different requirements for ranging accuracy in application scenarios, UWB defines two ranging implementation methods: SS-TWR (Single Sided - Two-Way Ranging) and DS-TWR (Double Sided - Two-Way Ranging).

In the SS-TWR method, as shown in the figure above, device A sends a request pulse signal at time t1 and records the sending timestamp. After the transmission delay, device B receives the pulse signal at time t2, and after internal processing, sends a response pulse signal at time t3. The response pulse contains the timestamps t2 and t3 recorded at the time of receiving the request pulse and sending the response pulse. After device A receives the response pulse signal from device B, it records the timestamp t4 at this time. After the local clocks of device A and device B complete precise time synchronization, the distance D between device A and device B can be obtained by the following formula.

From the implementation principle of SS-TWR, it can be seen that the accuracy of time synchronization between two devices directly affects the accuracy of ranging. According to calculations, a synchronization accuracy error of 1ns will lead to a ranging error of about 0.3m, while nanosecond-level time synchronization accuracy is simply unattainable between many current UWB devices. In order to reduce the dependence on time synchronization accuracy, the DS-TWR method came into being.

In the DS-TWR method, as shown in the figure above, the first step of the request pulse is the same as the SS-TWR method, except that device B returns a response + request pulse signal. Device A does not stop after receiving this signal, but immediately sends another request pulse signal after internal processing. Device B records the receiving timestamp after receiving this request pulse signal, and tells device A this timestamp through a response pulse. At this point, a complete DS-TWR process is considered complete, and the distance D between device A and device B can be calculated by the following formula.

4. Positioning method

UWB currently has three relatively mature positioning algorithms, TOA (Time of Arrival), TDOA (Time Difference of Arrival) and AOA (Angle of Arrival). In the specific implementation process, a hybrid positioning solution that integrates the three positioning methods is generally used to achieve the best positioning performance. TOA uses a circular positioning method to achieve positioning by measuring the distance between the mobile terminal and three or more UWB base stations. The location of the mobile terminal can be determined by the intersection of three circles at one point. However, due to the existence of multipath, noise and other phenomena, multiple circles may not intersect or intersect not at a point but at an area, so TOA positioning is rarely used alone.

TDOA is an improvement based on TOA, which allows accurate time synchronization of base stations, which is easy to implement, without worrying about the time synchronization between the mobile terminal and the base station. First, the distance difference between the mobile terminal and base stations A and B is calculated, and the mobile terminal must be on a hyperbola with base stations A and B as the focus and a constant distance difference from the focus. Then, through the distance difference between the mobile terminal and base stations A and C, another set of hyperbolas can be obtained, and the intersection of the hyperbolas is the location of the mobile terminal. Within the vehicle space, the distance difference can also reduce the impact of multipath, noise, etc.

AOA positioning calculates the arrival angle based on the principle of phase difference, and only two base stations are needed to achieve positioning. Due to the problem of angle resolution, the positioning accuracy decreases as the distance between base stations increases, and it is mostly used for medium and short distance positioning.

5. System Solution

(1) Interconnected PEPS Solution