Radar Waveforms Bring Safety to Modern ADAS Vehicles

Learn how both pulsed and continuous-wave waveforms are being used at millimeter-wave frequencies in modern ADAS radar systems to make roads safer.

By Jack Browne, Technical Contributor
[Sponsored by Rohde & Schwarz]

Radar waveforms have long been associated with defense electronics systems and early-warning systems in military applications. But radar technology is also bringing new levels of safety and comfort to commercial users as part of advanced driver assistance systems (ADAS). Different types of signal waveforms are being employed at high frequencies, such as 24 and 77 GHz, as part of radar-based safety systems designed to minimize the numbers of collisions and accidents. These commercial radar systems are enabling a growing number of automatic safety functions, including adaptive cruise control (ACC), blind-spot detection (BSD), collision avoidance, collision mitigation, collision warning, back-up assistance, lane-changing support, and parking aid.

These are not simple waveforms, as they require precision, stability, and repeatability to achieve the reliability demanded by ADAS systems. Understanding ADAS waveform characteristics (link to download page) and how to generate them is also essential for properly testing these microwave/millimeter-wave vehicular radar systems. This article provides an overview of how both pulsed and continuous-wave (CW) waveforms are being used at millimeter-wave frequencies in modern ADAS radar systems to make roads safer, whether vehicles are handled with or without a driver.

To provide some background, the major difference between ADAS and military radar systems is in the types of radar waveforms used. Military radar systems transmit pulsed radar waveforms, in which transmit signals are turned on and off and transmit and receive periods are essentially interleaved. The echoes from the transmitted signals bouncing off a target are measured by the receiver after the transmitter has been turned off. Changes in the signal characteristics of the received echoes, such as shifts in phase or Doppler shifts in frequency, provide information about the target.

The range from a transmitter to a target is measured by transmitting a single pulse and measuring the time to receive the echo signal reflected by the target. The radial velocity of a target can be measured by a pulse Doppler radar when changes in the Doppler modulation are used to determine the radial velocity of the target relative to the transmitter. The radial velocity of a target can also be determined by transmitting and receiving reflected versions of CW signals.

Military radar systems typically use high-power pulsed signals to detect targets. Radar range can be boosted by increasing the transmit power and, to a certain extent, by increasing the radar receiver sensitivity. But while higher radar transmit power levels make it possible to increase the radar range, they also make it possible for other receivers to detect the radar transmitter.

Radar Waveforms for A&D and Automotive Radar
Radar Waveforms Bring Safety to Modern ADAS Vehicles

This White Paper provides a detailed review of radar waveforms for Aerospace and Defense and commercial radar systems, as well as commercial radar sensors such as those used in automotive safety applications. Waveforms such as pulse and pulse-Doppler signal, continuous wave and frequency shift keying waveforms are described.

Download free white paper

Radar Review

In a pulsed radar, the time, τ, for a single transmitted pulse to reflect off a target and return to a radar receiver is measured to determine the range, R, from the transmitter to the target. By knowing the speed of light, c, which is the expected speed of the transmitted pulse, R can be found from the simple relationship R = (c/2)τ . The duration of the pulse, Tp, is the main variable that establishes a radar system’s range resolution, ΔR, which is the minimum difference in range by which two different targets can be separated by the same radar system.

In contrast, ADAS systems operate with continuous-wave signals at much lower average power levels, where the transmitter and receiver operate constantly without off times. An ADAS radar receiver monitors specific signal characteristics, such as Doppler shifts in frequency-modulated CW (FMCW) signals, to detect objects around a vehicle, such as automobiles in adjacent traffic lanes for BSD. ADAS radar sensors are being made smaller and with increased functionality by means of semiconductor integrated-circuit (IC) technology. Such technology is also helping to simplify ADAS system integration while reducing system power consumption, which is essential for mobile, vehicular applications. In addition, ADAS radar sensors must work with modern electronically steered antenna technologies including multiple-input, multiple-output (MIMO) arrays to provide a full 360-deg. view of the driving environment and avoid any blind spots.

Due to the many safety functions that rely on radar technology in ADAS systems, those systems use several different radar waveforms and frequency bands by means of multiple antennas and sensors (Figs. 1-3).

Chirp (FMCW) waveforms are commonly used in ADAS radar systems to detect surrounding objects, including traffic and pedestrians. (Courtesy of Rohde & Schwarz GmbH & Co.)


Frequency-shift-keying (FSK) waveforms make use of differences in frequency between transmitted CW signals. (Courtesy of Rohde & Schwarz GmbH & Co.)


MFSK waveforms at 77 GHz support many safety functions in ADAS systems. (Courtesy of Rohde & Schwarz GmbH & Co.)

For example, linear FMCW (LFMCW) radar signals at 24 GHz provide BSD while multiple-frequency-shift-keying (MFSK) waveforms at 77 GHz support various other short-range detection and collision-avoidance functions in detection of multiple objects or other vehicles. Frequency-shift-keying (FSK) radar systems are based on CW waveforms, but transmit two or more alternating, unmodulated signals at different carrier frequencies with a precisely known difference frequency, fshift.

An ADAS FMCW radar operates with what are known as “chirp” waveforms. Here, the frequency of a CW signal is swept in a linear fashion as a function of time across a specified bandwidth, such as from a lower frequency to a higher frequency. An FMCW waveform that is increasing in frequency is known as an “upchirp,” while one that is decreasing in frequency is referred to as a “downchirp.”

An ADAS radar system transmits and receives reflected versions of these chirp signals to detect targets like pedestrians and other vehicles. Changes in frequency-modulation characteristics between the transmitted and detected echo signals of these FMCW waveforms provide information about the illuminated objects, such as position and relative radial velocity from the transmitter. Typically, that transmitter is mounted on a vehicle body part, such as a front or rear fender. In addition to the use of LFMCW waveforms for object detection in ADAS systems, they may be used with fast chirps in a form of waveform known as a chirp sequence (CS) to avoid ambiguities during the detection of multiple targets.

Additionally, ADAS radar systems are adopting the use of frequency-hopped signals. The different frequencies and waveform formats help to minimize interference. However, they also place further demands on test signal generators and analyzers to handle the different frequencies and bandwidths required for laboratory and production testing.

The range resolution of a radar system is determined by its bandwidth, with more bandwidth providing finer resolution. The resolution of the radial velocity is a function of the coherent processing interval (CPI), which is the length of the chirp in an FMCW waveform. In a CS waveform, the CPI consists of multiple short chirps, with at least two needed for a measurement. By using CPIs on the order of milliseconds, a 77-GHz ADAS radar system can achieve the radial resolution needed to differentiate even slow-moving pedestrians from non-moving targets, such as traffic signs and lights.

Radar Waveforms for A&D and Automotive Radar
Radar Waveforms Bring Safety to Modern ADAS Vehicles

This White Paper provides a detailed review of radar waveforms for Aerospace and Defense and commercial radar systems, as well as commercial radar sensors such as those used in automotive safety applications. Waveforms such as pulse and pulse-Doppler signal, continuous wave and frequency shift keying waveforms are described.

Download free white paper

How Much Bandwidth?

ADAS systems are currently operating with these various forms of chirp waveforms at several center frequencies and bandwidths, with both narrowband (NB) and ultrawideband (UWB) operating modes. For example, the unregulated NB span at 24 GHz is 200 MHz, extending from 24.05 to 24.25 GHz. The UWB frequency range at that center frequency is 5 GHz wide, from 21.65 to 26.65 GHz. The 77-GHz band has initially designed by the Electronic Communications Committee (ECC) for use by automotive short-range-radar (SRR) applications. It generally refers to the 4-GHz bandwidth from 77 to 81 GHz, with ADAS radar sensor manufacturers each differing somewhat in their use of center frequency and total bandwidth.

For both ADAS radar frequency bands, the ECC established acceptable levels of automotive radar transmit power in terms of equivalent isotropic radiated power (EIRP), with backing from the European Conference of Postal and Telecommunications Administrations (CEPT). Last updated in 2015, the ECC and CEPT determined that to maintain 77-GHz SRR equipment on a noninterference basis, a maximum mean power density of −3 dBm/MHz EIRP was acceptable. It is associated with a peak limit of 55 dBm EIRP. The mean power density outside a vehicle resulting from single SRR equipment should not exceed −9 dBm/MHz EIRP. At 24 GHz (or within 21.65 to 26.65 GHz), the mean power density should be no more than −41.3 dBm/MHz EIRP or an EIRP peak limit of 0 dBm/MHz.

Generating such waveforms and combinations of waveforms for testing ADAS radar systems and their components is not trivial. It requires test signal sources with the programmability and frequency agility to create a variety of waveforms. As ADAS radar systems become a greater part of everyday life for anyone with a modern vehicle, accurately and reliably developing the means of creating the types of waveforms used in those systems will be critical. For both pulse Doppler and FMCW waveforms, waveform generation becomes an essential part of ensuring that present and future ADAS radar systems deliver the reliable performance that lives literally depend on. It also guarantees that ADAS radar systems continue to evolve to eventually support fully autonomous vehicles.

Bibliography

“Radar Waveforms for A&D and Automotive Radar,” White Paper, No. 1MA239_0e1, Rohde & Schwarz GmbH & Co., www.rohde-schwarz.com.

“Automated Measurements of 77 GHz FMCW Radar Signals,” Application Note, No. 1EF88_Oe, Rohde & Schwarz GmbH & Co., www.rohde-schwarz.com.

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