Ultra-wideband (UWB) system has the advantages of high transmission rate, low power consumption, high detection accuracy, strong penetration and high security, and has a wide range of applications in military, radar, biological detection, short-range communication and indoor and outdoor high-precision positioning. And with the development of semiconductor technology, CMOS-based UWB radar chip has become a research hotspot. Many scholars and commercial companies at home and abroad have proposed UWB chips and systems with their own advantages. The research team from Xidian University and the Academy of Military Sciences published the latest article in the Journal of Electronics and Information, and reviewed the research status and development of key circuits and key technologies in UWB system and UWB chip architecture.

Research Status and Development of UWB Radar Chip

Luo Peng ① Hu Zhenfeng ① Tian Shiwei * ② Liu Maliang * ①

① (Xidian University, Xi'an 791000)

② (National Institute of Science and Technology Innovation, Academy of Military Sciences, Beijing 100097)

Cite this article: Luo Peng, Hu Zhenfeng, Tian Shiwei, Liu Maliang. Research Status and Development of UWB Radar Chip [J]. Journal of Electronics and Information. doi: 10.11999/JEIT211082

　　Citation: LUO Peng, HU Zhenfeng, TIAN Shiwei, LIU Maliang. The Status and Trends of UWB Radar Integrated Circuit [ J ] . Journal of Electronics & Information Technology. doi: 10.11999/JEIT211082

Keywords: ultra-wideband; radar; receiver; transmitter; chip

What is Ultra Wideband Radar (UWB)?

The concept of Ultra-Wide Band (UWB) was first proposed in the "time-domain electromagnetics" in the 1960 s, using a narrow pulse signal without carrier for communication. Because of its good security, high transmission rate and high range resolution, it has important application value in military and radar fields.

In February 2002, the U.S. Federal Communications Commission (Federal Communications Commission,FCC) formally approved UWB for civilian use, stipulating that the operating frequency of UWB is 3.1~10.6 GHz and the transmission bandwidth is greater than 500 MHz. However, in order to prevent UWB from interfering with other communication bandwidths, the transmission power of the transmitter is limited, that is, the effective omnidirectional radiation power is less than -41.2 dBm/MHz. Therefore, the high-speed transmission rate of the ultra-wideband technology is at the cost of a very wide bandwidth, and the ultra-wideband pulse radar technology is a transmitter that transmits a pulse signal with a very short duration, and the repetition period of the transceiver is long, so the unit time The power consumption is extremely low, which is suitable for the requirements of low-power application scenarios in the future.

In addition to the application of military radar, UWB systems are important in commercial application scenarios such as biological detection and indoor positioning. Figure 1 shows the advantages and application scenarios of UWB systems.

1 Advantages and application scenarios of UWB system

Key Technology of UWB Radar Chip

The key technologies of UWB radar chip mainly include signal generation technology, ultra-wideband power amplifier, ultra-wideband low noise amplifier, high-speed quantization technology and so on. The author's team made a major review and comparison of the advantages and disadvantages of the above key technologies.

Signal Generation Technology of UWB System

Because the Gaussian pulse can adjust the function parameters, so that the bandwidth and peak frequency of the signal can be changed by simple settings, the transmission power and performance of Gaussian pulse are more suitable for the application of ultra-wideband system under the condition of FCC. Gaussian pulse signal is more simple and insensitive to channel fading, so it is often used in UWB transmission system. At present, the Gaussian pulse generation circuit includes the following structure: pulse generation is realized by charging and discharging the passive filter network through the charge pump, and the pulse signal can be configured by controlling the clock pulse width and current, and the circuit structure is shown in Figure 2.

Fig. 2 (a) unit circuit of pulse generation; (B) node output waveform

Alternatively, the direct radio frequency synthesis technology is adopted, the buffer tail current is programmed, and the smoothness of the output pulse is improved by pseudo-random coding of the transmission pulse control sequence, and the center frequency of the Gaussian pulse is adjusted by controlling the PLL output frequency, so as to realize a smoother programmable frequency shift Gaussian pulse waveform. The circuit structure is shown in Figure 3.

Fig.3 Realization circuit of direct RF synthesis Gaussian pulse

Alternatively, the delay time is changed by voltage control, and then the pulse signal required for transmission is generated by combining logic and a pulse combiner, and the circuit structure is as shown in FIG. 4.

Fig.4 Digital pulse generating circuit

Modulation mode

In the current IR-UWB system, the common modulation methods are on-off keying (OOK), pulse amplitude Amplitude Modulation (PAM), pulse position modulation (PPM), phase keying (BPSK) and other modulation methods. With the increasing demand for data rate and miniaturization and low power consumption performance of communication systems, on-off keying OOK usually has a higher modulation data rate, but it will inevitably consume too much power. Pulse position modulation PPM can greatly improve the data rate of communication because it can realize a repetition period to send multi-bit data, and can adopt a fully digital design scheme to further reduce the area and power consumption of the chip, PPM will show the advantages of digital modulation.

ultra-wideband power amplifier

UWB transmitter bandwidth is wide, so the power amplifier bandwidth requirements are higher, and even some need to cover 3.1~10 GHz. The wider the bandwidth of the power amplifier, the more difficult it is to ensure the performance of efficiency and power. Therefore, the output of some transmitters does not pass through a special power amplifier and is output through an ordinary buffer. Because in the application of power amplifier, the input signal amplitude is not fixed, in a single type of power amplifier, the efficiency corresponding to different input amplitudes may vary greatly, so the power amplifier often requires 6 dB back-off efficiency, Doherty the power amplifier can still have good output efficiency at the input power corresponding to the efficiency peak back-off of 6 dB. Therefore, in some applications requiring higher efficiency, such as IOT, in some low-power transceiver applications, a power amplifier with higher efficiency is often applied. Figure 5 shows a digital Doherty power amplifier, which uses multi-channel digital power combining technology to divide 16 power amplifiers into two parts in the output and combine them into Doherty power amplifiers, achieving 29.5% efficiency and 24.4 dBm output power.

Figure 5 Digital Doherty power amplifier

UWB Receiver System Architecture

In the UWB system, different system structures are proposed according to different signal types and sampling methods. Transmitter (TX) structure is often composed of signal generation module, power amplifier module, and antenna 3 parts. Due to different signal types and quantization methods, scholars have proposed many structures for receivers. Among them, the more common structures include superheterodyne structure, zero intermediate frequency structure, direct RF sampling structure, time-based extended sampling structure, equivalent time-based sampling structure, STsampling, energy detection, etc.

Superheterodyne structure: The receiver includes low noise amplifier (LowNoise Amplifier,LNA), mixer (Mixer), filter, analog-to-digital converter (ADC) and other modules. In this structure, the radio frequency signal amplified by the LNA is first mixed down, and then the frequency-multiplied signal is filtered out by a low-pass filter, so as to obtain an intermediate frequency signal with a lower frequency, thereby reducing the requirement on the ADC bandwidth.

Time expansion: in the common IR-radar system, the transmitted radio frequency signal has the characteristics of narrow pulse, there is a large number of dead time in the cycle of the signal, in view of the characteristics of the transmitted signal, some scholars put forward the time expansion sampling structure, the schematic diagram is shown in Figure 6 below. First, the pulse is sampled by a high-speed sampling unit, and the pulse width of the signal obtained is Δds, and then the pulse width of the sampled signal is amplified by a time expansion amplifier GDTE to obtain a low-speed signal with a pulse width of Δde, thereby reducing the speed and performance requirements of the post-stage digital quantization analog-to-digital converter (ADC).

6 Principle of time-extended sampling

Equivalent time sampling: Like the time extension sampling method, when the dead time of the pulse signal is longer and the moving speed of the detected target is smaller, the received pulse signal can be considered almost unchanged within a certain period of time. The schematic diagram of equivalent time sampling is shown in Figure 7. Assuming a repetition period of 10 ns, then sampling the signal with a clock with a period of 5 ns yields two sampling points per period. Then, five clocks with different phases and the same period are introduced to sample the signal respectively, and 10 sampling points per signal period can be obtained through synthesis, that is, 10 times the equivalent sampling rate of the signal is realized.

7 Principle of equivalent time sampling

Scanning threshold sampling structure (Swept threshold sampling ) : Similarly to the equivalent time sampling, using the repeat frequency cycle for a certain period of time, some scholars have proposed a scanning threshold sampling (Swept threshold sampling ) , the schematic diagram is shown in Figure 8. It consists of a comparator, a counter, and a threshold generation module. By scanning VT from 0.1 to 0.9 scanning, step 0.1V, when the VT is greater than the amplitude, the sampling is 1. When VT is less than the amplitude, the sample is 0. As shown in the figure, if the amplitude is 0.65V, the first 6 times in the 9 scanning sampling results are 1, and the last 3 times are 0. Finally, the counter accumulates 1 of the scanning period to obtain 6, and a quantized value of 6/9 can be obtained. Other ranges are similarly available.

　　图 8 Swept threshold sampling 原理

Energy detection structure: The structure for incoherent energy detection is generally shown in FIG. 9. The receiver includes a low noise amplifier (LNA), a square (squarer), an integrator, an analog-to-digital converter (ADC), and a digital part. After Squarer, the signal envelope is obtained, and then the time position of the integration window is adjusted. Finally, the ADC quantizes the integration result, and the pulse signal is restored through the digital module. At the same time, the demodulation of modulation methods such as PPM can be realized.

9 Principle of energy detection

The circuit architecture of the superheterodyne can achieve direct frequency reduction and has a simple structure, thereby reducing the pressure of a subsequent analog-to-digital converter. However, due to the need for a module such as a mixer, both signal quality and overall linearity will be lost. Time expansion technology requires a high-speed sampling circuit to sample the signal first, and then expand the sampling signal pulse width through the time expansion amplifier, this structure requires a higher sampling circuit, and the time expansion amplifier will have corresponding system errors and random errors, affecting the sampling accuracy.

Equivalent time sampling can achieve a sampling rate that is multiple times the sampling clock by using multi-phase clock sampling and then synthesizing, thus designing a high-speed analog-to-digital converter that can be applied to a direct RF sampling system, which simplifies the RF signal chain and reduces the cost and channel density of each channel. The scanning threshold sampling structure only requires a 1-bit quantizer, which simplifies the design and increases the inherent linearity of the system. The energy detection method can reduce the accuracy requirement of the comparator and has better stability. By comparing the performance of the above receiver system architecture, the equivalent sampling structure will be more applied to the UWB receiver system in the future due to its unique advantages.

Ultra-Wideband Low Noise Amplifier

UWB LNA, as the first stage in a UWB receiver, will determine the performance of the entire receiver chain. Therefore, UWBLNA is particularly important in UWB systems. In the UWB system, the antenna is often a single-ended input signal, but the differential circuit in the post-stage circuit has better even-order harmonic suppression and common-mode suppression capabilities, and the input terminal uses an on-chip transformer (balun) to achieve input matching, to achieve the single-ended signal is converted into a differential signal. The adaptive bias ADB ( ADaptive Biased) circuit realizes an adaptive gain low noise amplifier to improve the dynamic range of the UWB receiver. The circuit diagram of the adaptive gain low noise amplifier is shown in Figure 10.

Figure 10 Adaptive gain low noise amplifier

However, due to the large area of the on-chip transformer, the active balun structure is used to realize the conversion from single-ended to differential, which further saves area. As shown in FIG. 11 below, two opposite-phase signals are generated through the output of the second-stage two-stage common-source amplifier to realize single-ended to differential conversion.

11 Two-stage LNA structure with active balun

In the above structure, since VON adds one stage of common source amplification, there will be a phase delay, so the output differential signal has a phase and gain error, and is greatly affected by PVT. By combining the common gate (CG) and the common source amplifier (CS), the gain and phase error of the output differential signal can be reduced, and the circuit structure is as shown in FIG. 12.

12 Modified active balun structure

As the first stage of UWB receiver system, UWB low noise amplifier plays a very important role in amplifying the weak signal received from the antenna and reducing the noise figure of the whole system. The low noise amplifier realizes the gain adaptive function by adding the feedback module, and improves the dynamic range of the system through the reconfigurable function. By cascading with active balun, the system can realize the function of single-ended to differential, and the differential signal can improve the system's common-mode rejection ratio and anti-interference characteristics. Multi-functional integration of ultra-wideband low noise amplifier will continue to be studied.

UWB Radar Development Trend

UWB radar is a new system radar with the fastest development at present. Because its system works in a wide frequency bandwidth, has the characteristics of high data transmission rate, high resolution and strong penetration, UWB radar is widely used in the fields of positioning, detection, communication, biological medicine and so on. With the continuous development of silicon-based technology, the frequency of UWB radar is continuously improved, UWB radar chips can already be fully integrated using a low-cost CMOS process. UWB technology distributes the power in a wide frequency band, so that the power of each frequency point is very small, which will avoid interference with other wireless protocols. As spectrum resources become more and more precious, UWB solutions will be more applied to mainstream electronic products in the future.

UWB technology has the advantage of high bandwidth, which determines that UWB radar will have higher positioning accuracy. Because UWB radar uses a principle similar to Time Of Flight (TOF), a signal is sent through the transmitting end, and the signal bounces back to the receiving end after encountering an obstacle. The signal transmission distance can be obtained by calculating the time difference between transmitting and receiving signals multiplied by the speed of light. The geometric position information of the object can be obtained by positioning and scanning through multiple transmitting ends. Compared with the meter-level positioning error of traditional Bluetooth positioning and other technologies, UWB radar technology can achieve centimeter-level positioning accuracy, which makes UWB radar have a larger application market. Moreover, because UWB technology requires field equipment to directly collect and calculate, it is difficult for a third party to break through the information security barrier, thus having higher security. At present, the two most widely used areas of UWB radar are for the medical industry, mainly including high-precision medical monitoring and medical testing, and the other is mainly for high-precision positioning military applications such as military street fighting, anti-terrorism, disaster search and rescue.

Non-contact UWB life monitoring radar is a radar specially applied to medical monitoring at present. Different from the traditional detection form of contact between electrodes and sensors, it can realize long-distance and long-term contactless detection of breathing and heartbeat signals of patients. It can realize real-time detection of vital signs such as breathing and heartbeat of patients without affecting the normal rest of patients, and compare the detection data with the set data, timely feedback to the medical staff, compared with the traditional respiratory and ECG recorder, the non-contact way has the characteristics of more relaxed and comfortable, can better assist the medical staff to carry out the corresponding treatment. At present, the average life expectancy of our population continues to grow, and the trend of population aging is obvious. With the continuous optimization of UWB life detection radar towards smaller and more accurate direction, it will become a more common biomedical device in the home in the future.

The current international and domestic anti-terrorism situation is quite serious, which puts forward an urgent demand for portable UWB through-wall radar, and at the same time provides a huge market. Wearable UWB through-wall perspective radar is studied. The application level mainly includes the internal layout and imaging of buildings, as well as the detection, identification, classification and tracking of people and moving targets. It can realize the detection, positioning, imaging and tracking of targets behind buildings or obstacles. It has wide application prospects and value in military equipment, urban safety, natural disaster search and rescue such as fire and earthquake, rapid response personnel and anti-terrorism.

Conclusion

UWB system based on pulse signal has the advantages of high transmission rate, low power consumption, high detection accuracy, strong penetration and high security. The implementation of UWB chip based on CMOS can realize the further miniaturization and low power consumption of UWB system. The key technologies of UWB radar chip include signal generation technology, ultra-wideband power amplifier, ultra-wideband low noise amplifier, high-speed quantization technology and so on. This paper mainly reviews the above key technologies and compares their advantages and disadvantages. In the past few years, UWB systems have been used in military, radar, biological detection and other fields due to their security and high-precision advantages. In recent years, with the rapid development and rise of 5G and the Internet of Things, short-range communication and indoor/outdoor positioning based on UWB system have developed rapidly. The main advantages of ultra-wideband are low power consumption, insensitive to channel fading (such as multipath, non-line-of-sight and other channels), strong anti-interference ability, strong penetration, and high positioning accuracy and positioning accuracy. It can be used in the intersection of automobiles, mobile devices and consumer devices, such as car keys, warehouse management, staff management, sweeping robots, mobile phone positioning, etc., to realize the interconnection of everything.