Microwave Journal
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Single-Chip Millimeter Wave Radar

January 14, 2015

In this article we explore the coming consumer radar era, where tiny single-chip radar systems will be available for just a few dollars. These systems will use sophisticated waveform diversity and adaptive signal processing to optimize performance. Automotive radar concepts developed at the University of Melbourne are discussed, including CMOS RF transceivers, waveform and signal processing, and antennas.

The idea of radar is more than 100 years old. In 1900, Nikola Tesla foreshadowed the idea of remotely detecting and locating objects using radio waves. His idea was first demonstrated in Germany in 1904 when Christian Hulsmeyer detected ships using echoes of radio signals.

Figure 1

Figure 1 The single-chip radar transceiver developed by the University of Melbourne.

While the essential idea of radar is simple, the evolution to improve performance and capabilities has been an ongoing technology driver. We believe this trend will accelerate with the emerging capabilities in single-chip millimeter wave radar that are enabled by CMOS scaling (Moore’s law) and recent advances in adaptive waveform design.

Interest in consumer applications for radar is not new, of course. In 1993, the IEE held a symposium on this very topic,1 and low cost, handheld Doppler radar systems are already on the market (e.g., pocket radar). Today, the automotive market is arguably the main opportunity driving innovation in small, low cost millimeter wave radar. Radar systems are installed in many transportation vehicles to assist safety and comfort, providing drivers with information about the distance between vehicles and obstacles on the road. Radar systems can take another step functionally, by automatically controlling acceleration and braking to avoid collisions.

Sensing systems for “micro” unmanned aerial vehicles (UAV) is another application for small, lightweight, yet high performance radar technology. The availability of tiny and cheap radar systems is likely to open up many new applications, mimicking the transformation following the availability of small, inexpensive GPS systems.

The relentless scaling of semiconductor technology is enabling the move to higher and higher RF frequencies and integrating complete radar systems, with both RF and digital processing, on a single IC. Clever new waveform diversity techniques2 with innovations in small antenna technology will lead to highly sophisticated consumer radar systems over the next few years.

Figure 2

Figure 2 Photograph of the single-chip CMOS transceiver die, which contains two transmit and four receive channels and the local oscillator.

Single Chip RF System

The advancement of key techniques and technologies in low cost millimeter wave radio systems has aided the development of automotive radar sensors in the frequency bands from 60 to 95 GHz. Today, with the rapid evolution of SiGe bipolar and CMOS processes, a 77 GHz automotive radar transceiver can be realized in a single IC (see Figure 1). The low cost and compact size of the chip are enabling the adoption by low and medium priced cars.3–7

Compared with a SiGe bipolar process, CMOS is lower cost, better integrates digital circuitry and benefits from technology scaling. However, the maximum available gain (MAG) at millimeter wave frequencies is lower for CMOS, and its low supply voltage limits output power. Therefore, developing a millimeter wave CMOS transceiver is still a challenging task for many researchers.8–13

The radar sensor on a chip developed by the University of Melbourne is a CMOS transceiver (see Figure 2) suitable for both short and long range sensing and operating in the globally harmonized 76 to 77 GHz band.14-15 Design objectives were low cost, low power dissipation, compact size, flexibility and ease of use. As shown in the schematic (see Figure 3), the transmitter comprises two variable gain amplifiers (VGA), sub-harmonic mixers (SHM) and power amplifiers (PA). Four receive channels, supporting eight separate receive antennas, each contain a chain of switched low noise amplifiers (LNA), SHMs, VGAs, and lowpass filters (LPF). The IF outputs from the four receive channels are multiplexed into a single 12-bit, 100 MSPS analog-to-digital converter (ADC). All the RF components – passives, amplifiers, mixers, oscillators – are fabricated on the 7.2 × 5.5 mm chip.

In the 76 to 77 GHz band, the transceiver has demonstrated 10 dBm single sideband output power14 with LO-RF leakage power of -35 dBm. Receiver sensitivity is better than -100 dBm.15 The gain of each receive channel is adjustable in 5 dB steps from 0 to 80 dB.

Figure 3

Figure 3 Schematic showing the functional complexity of the single-chip radar transceiver.

Figure 4

Figure 4 Position error as a function of range.

Waveforms and Signal Processing

Radar design was put on a sound theoretical footing in 1953 when Philip Woodward developed the radar ambiguity function at TRE in England.16 His work was based on the matched filter developed by Dwight North in 1943 at RCA Labs in Princeton, N.J. Woodward’s ambiguity function characterizes the performance of a matched filter radar for any particular transmitted waveform. Unfortunately, it is not possible to precisely synthesize a transmit waveform based on the desired ambiguity function. Significant research has been devoted to the synthesis problem, including the work of Wilcox17 on the group theoretic foundations of ambiguity theory, which enabled synthesis of a certain restricted class of waveforms. Sussman18 and Vakman19 subsequently proposed Hilbert-Schmidt operator approximation techniques to approximately synthesize waveforms with specified ambiguity properties. Work in this direction is continuing.20 Progress on this difficult yet deeply important problem has taken on a new flavor in recent years under names such as waveform diversity, adaptive waveforms and waveform scheduling.2 Digital waveform generation and fast adaptable digital matched filter implementation allow waveform diversity techniques to be implemented even in low cost, single-chip radar systems.

The radar-on-a-chip developed at the University of Melbourne employs adaptive digital matched filtering and scheduling of advanced multi-frequency coded waveforms to reduce clutter, mitigate against interference and reduce the computational load in the digital signal processing. Waveforms employed include sections of fast ramping linear FM (LFM), long random stepped frequency waveforms and bursts of CW.

Figure 5

Figure 5 Doppler estimation error as a function of range.

Typical radar performance is demonstrated with an automotive scenario consisting of 25 oncoming point scatterers, centered on, and five targets, centered around the field of view. All scatterers and targets have unity radar cross section (RCS), and the speed of each target and scatterer is chosen randomly within the range of typical automobile speeds. The performance of the radar is shown in Figure 4, the range estimation error as a function of target range, and Figure 5, the Doppler estimation accuracy.

As radar sensing becomes ubiquitous in consumer applications, the spatial density of users utilizing a common frequency band will increase vastly. While it is fortunate that a range of techniques for mitigating interference has been developed for use in wireless communications, the requirements for radar pose distinct new challenges.

Antenna System

The antenna is a critical element in all radar systems. For automotive applications, the requirements are especially challenging: high gain and low loss for best performance traded off with small size, low cost and aesthetically pleasing for vehicles. The additional need for the radar to have wide angle visibility requires the use of multi-beam or scanning antennas and solutions based on digital beam forming with multiple antenna elements.

The design of the radar antenna depends on the specifications for directivity/gain, angular resolution and accuracy. These parameters determine the ability of a radar sensor to detect a target at the required distance, resolution and accuracy. The radar equation defines the required antenna directivity/gain necessary to detect an object at distance R

Math 1

Figure 6

Figure 6 Photograph of the microstrip series-fed patch array and lens.

where P0 is the feeding power of the antenna, σ is the radar cross section of the object, λ is the wavelength and the receiving power PR is determined by the minimum signal-to-noise ratio. The ability of a radar system to determine the exact position of an object is based on the half power (3 dB) antenna beamwidth. For a linear array, this can be estimated by

Math 2

where M is the number of antenna elements, d is distance between the antennas and θ is the beam angle towards the normal of the air-dielectric interface. The angular accuracy can be determined with the signal-to-noise ratio of the system as

Math 3

Mid and long-range radar systems have smaller beamwidth than short-range antennas. For 77 GHz long-range radar, 3 to 4 degree beamwidths are typically required. This results in very large arrays and gains around 30 dB, which led the first automotive radar systems for long-range detection to adopt parabolic reflector or lens antennas.21 The advantage of lens over reflector antennas is that the feed is not in the path of the secondary rays; however lens antennas are typically thicker, heavier and more difficult to construct. The need for small sensor depth in vehicles limits their use.

The University of Melbourne radar sensor uses a dielectric lens antenna illuminated by series-fed patches, achieving both low cost and high gain for mid and long-range applications. Figure 6 shows the lens antenna with cascaded microstrip patch elements interconnected by half-wavelength high impedance transmission lines. The patch element design is based on the transmission line model and the equivalent circuit concept.22–23 The lens is hemispherical (plano-convex) and made of a low dielectric constant material such as Teflon (εr = 2.2), which is low loss and easy to manufacture. The lens provides a support structure for the substrate containing the patch antenna elements.

Figure 7

Figure 7 Measured antenna gain of one channel at 77 GHz.

The antenna gain and the 3 dB beamwidth of the dielectric lens depend on its size. The correlation between the radius, r, of the lens and the 3 dB beamwidth (in the elevation plane) for a uniform illuminated circular lens is given by 24

Math 4

where λ is the operating wavelength.

The measured elevation (H plane) and azimuth (E plane) radiation patterns of the antenna are shown in Figure 7. The maximum gain at 77 GHz is 18 dB, and the 3 dB beamwidth is 10 degrees in elevation and 18 degrees in azimuth.

INTEGRATED RADAR

Our vision of the integrated radar system is shown in Figure 8. An RF printed circuit board (PCB) contains the RF transceiver IC, digital signal processor (DSP) and printed antenna array. The lens extends above the PCB, attached to the board and providing a protective cover for the complete assembly. The ICs are flip-chip attached to the RF PCB.

Figure 8

Figure 8 Concept design of the integrated radar system. The single-chip RF transceiver, DSP, patch antenna array and lens are integrated on a PCB to minimize cost.

Antenna scanning is achieved with phased arrays and digital beam forming. The phase shifting method connects antenna elements or sub-arrays to phase shifters, enabling beam scanning. At millimeter wave frequencies, this approach can be complex, lossy and costly. With digital beam forming, several antenna elements or sub-arrays are successively switched to a receiver or transmitter or, alternatively, connected to multiple transmit–receive circuits.

CONCLUSION

This article describes the concept and development of a low cost, 77 GHz automotive radar system, including a single-chip CMOS millimeter wave transceiver. The architecture shown in Figure 8 will enable a consumer radar revolution as a result of advances in CMOS semiconductor processes and radar algorithms.

The performance achieved from this radar matches standard radar theory, with the potential for further improvement in the information that can be extracted from a radar system. Our approach to this 25 builds on Lebedev’s work 26 on the information carrying capacity of a bosonic field.

References

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