Automotive Radar

SaberTek IP offering

SaberTek has developed a rich IP portfolio for automotive radar systems. Our design services and 76-81GHz IP offering will enable our customers with low power market winning products.

Motivation

Over a million fatalities occur in road accidents every year around the world. Majority of these accidents occur due to human error. While several technologies are now used to reduce the impact of accidents (airbags, ABS, etc.), a new class of safety systems, called advanced driver assistance systems (ADAS), is now being introduced to reduce human error. These systems are enabled by smart sensors based primarily on millimeter-wave automotive radars.

Automotive Radars: Applications and Benefits

Automotive radar sensors are responsible for the detection of potential collisions or hazardous situations. A positive detection can be used to warn/alert the driver or to intervene with the braking and other controls of the vehicle in order to prevent an accident.

In an automotive radar system, one or more radar sensors detect obstacles around the vehicle and their speeds relative to the vehicle. Based on the detection signals generated by the sensors, a processing unit determines the appropriate action needed to avoid the collision or to reduce the collateral damage.

Automotive radar systems are capable of detecting:

  • Objects and obstacles surrounding  the vehicle
  • Their position relative to the vehicle
  • Their speed relative to the vehicle

And by means of a decision-making unit, they can:

  • Alert the driver about any potential danger
  • Prevent collision by intervening with the control of the vehicle in hazardous situations
  • Take over partial control of the vehicle (e.g., adaptive cruise control)
  • Assist the driver in parking the car

Performance Parameters

The key aspects of a radar system are:

  • Detection range
  • Speed detection Range
  • Range precision
  • Velocity precision
  • Angular resolution
  • Angular width of view

Based on the above characteristics, automotive radar systems can be divided into three sub-categories: short-range, mid-range and long-range automotive radars. For short-range radars, the main aspect is the range accuracy, while for mid-range and long-range radar systems the key performance parameter is the detection range.

Short-range and mid-range radar systems (range of tens of meters) enable several applications such as blind-spot detection and pre-crash alerts. It can also be used for implementation of “stop–and-go” applications in city traffic.

Long-range radars (hundreds of meter) are utilized in adaptive cruise-control systems. These systems can provide enough accuracy and resolution for even relatively high speeds (~120mph).

Millimeter-Wave Radar Systems

Mm-wave devices play a key role in automotive radar systems. Through advancements in semiconductor technology, cost-effective solutions for high-resolution radar applications will soon be available for even economy-class vehicles.
The 24-29GHz (and more recently, 77-81GHz) frequency band has been allocated for short/mid-range automotive radars. The wide bandwidth enables high range resolution at short ranges. Short-range detection provided by the radar systems operating at this frequency can be used for:

  • Blind-spot detection
  • Pre- crash warning and intervention (impact mitigation)
  • Stop-and-go
  • Lane departure warning

The 76-77GHz band is used for long-range detection which is typically utilized in adaptive cruise control (ACC) systems. Recently, the 77-81GHz (sometimes identified as 79GHz) band has also been allocated for short-range, high-resolution radar systems. Therefore, this band also can be used for stop-and-go, pre-crash alarm and blind-spot detection.

Millimeter-Wave Radar Systems

Long-Range and Short-Range Radars

Both short-range radars and long-range radars are capable of measuring the relative velocity with high accuracy. Long-range radar is ideally suited for ranges longer than 30m and can typically detect objects 200m away. The angular resolution is almost the same for both radar systems, but long-range systems, usually have a higher tangential distance error. On the other hand, short-range radar is capable of providing a wide view, usually greater than 30 degrees, with good spatial resolution (~10cm).

In contrast with other safety systems, such as LIDARs, mm-wave radars can operate in severe weather conditions such as snow or fog, and even when dirt has covered the sensor.

It should be noted that long-range radar is based on narrow-band operation, with a narrow beam-width suitable for adaptive cruise control and can operate even in high speed condition (around 200km/h), while short-range radar is based on wide-band operation, which can provide a wide view of the surroundings of the car at moderate speeds.

Radar Fundamentals

Radar operation can be divided into two major tasks:

  • Distance detection
  • Relative speed detection

Distance detection can be performed by measuring the round-trip duration of a signal. Based on the wave speed in the medium, it will take a certain time for the transmitted signal to travel, be reflected from the target, and travel back to the radar receiver. This time interval indicates the distance that the signal has travelled.

∆T

sine4.tif

The underlying concept in the theory of speed detection is the Doppler frequency shift. A reflected wave from a moving object will experience a frequency change, depending on the relative speed and direction of movement of the source that has transmitted the wave and the object that has reflected the wave. If the difference between the transmitted signal frequency and received signal frequency can be measured, the relative speed can be measured.


Types of Automotive Radar Systems

Bi-static and Mono-static

Every radar system can be divided into two categories:

  • Bi-static Radars: In these systems, two separate antennas are used for transmitter and receiver. Transmitter-toreceiver leakage can be mitigated by physical separation of the antennas.
  • Mono-static Radars: In this type of radars, a single antenna is used for Transmitter and receiver. The main advantage of these radars is their compactness.

Pulsed Radar

In pulsed radar architectures, a number of pulses are transmitted and from the time delay and change of pulse width that the transmitted pulses will experience in the round trip, the distance and the relative speed of the target object can be estimated.

Pulsed radars typically suffer from blind speed and ambiguous range issues. In addition, transmitting a narrow pulse in the time domain means that a large amount of power must be transmitted in a short period of time. In order to avoid this issue, spread spectrum techniques may be used.

Continuous-Wave Radar

In continuous-wave (CW) radars, a high-frequency signal is transmitted and by measuring the frequency difference between the transmitted and the received signal (Doppler frequency), the speed of the reflector object can be estimated. These systems are incapable of detecting the target range and they cannot distinguish between objects moving toward or away from the transmitter. The direction ambiguity can be eliminated by extracting the sign of the Doppler frequency using quadrature architecture.

Frequency-Modulated Continuous-Wave (FMCW) Radar

In FMCW radars, a ramp waveform or a saw-tooth waveform is used to generate a signal with linearly varying frequency in time domain. The variation of the instantaneous frequency is proportional to the ramp waveform. The generated signal is then transmitted and by measuring the round-trip delay and the frequency difference, the detector can estimate the velocity and the distance of the moving object.





24-GHz and 77-GHz Automotive Radars

There are 4 major frequency bands allocated for radar applications, which can be divided into two sub-categories: 24-GHz band and 77-GHz band.

The 24-GHz band consists of two bands, one around 24.125GHz with a bandwidth of around 200MHz and, the other around 24GHz with a bandwidth of 5GHz. Both of these bands can be used for short/mid-range radars.

The 77-GHz band also consists of two sub-bands, 76-77GHz for narrow-band long-range radar and 77-81GHz for short-range wideband radar.

As frequency increases, smaller antenna size can be employed. As a result, by going toward higher frequencies angular resolution can be enhanced. Furthermore, by increasing the carrier frequency the Doppler frequency also increases proportional to the velocity of the target; hence by using mm-wave frequencies, a higher speed resolution can be achieved. Range resolution depends on the modulated signal bandwidth, thus wideband radars can achieve a higher range resolution, which is required in short-range radar applications.

Recently, regulatory agencies are pushing for migration to mm-wave range by imposing restrictions on manufacturing and power emission in the 24GHz band. It is expected that 24-GHz radar systems will be phased out in the next few years (at least in the EU countries). This move will help eliminate the issue of the lack of a worldwide frequency allocation for automotive radars, and enable the technology to become available in large volumes.

By using the 77-GHz band for long-range and short-range applications, the same semiconductor technology solutions may be used in the implementation of both of them. Also, higher output power is allowed in this band, as compared to the 24-GHz radar band.

76-81-GHz Radar Systems

76-77-GHz and 77-81-GHz radar sensors together are capable of satisfying the requirements of automotive radar systems including short-range and long-range object detection. 

For short-range radar applications, the resolution should be high; as a result, a wide bandwidth is required. Therefore, the 77-81-GHz band is allocated for short-range radar (30-50m).

For long-range adaptive cruise control, a lower resolution is sufficient; as a result, a narrower bandwidth can be used. The 76-77-GHz is allocated for this application.

freqAlloc.tif

Silicon Technology

The advancement of high-speed bipolar transistors in silicon-germanium technology has made automotive radar products commercially available, suitable not only for luxury vehicles, but also for economy-class cars. Integrated SiGe transistors with cutoff frequencies (ft, fmax) exceeding 250GHz are now commercially available. Automotive radar system requirements, such as high resolution, low phase noise and high linearity can be achieved at much lower price and higher integration level in SiGe than III-V MMIC devices.

In addition, due to the fact that SiGe technology is more integration-friendly than traditional III-V technology, the functionality of the entire radar system can be extended by means of integrated chip design. Moreover, the power consumption of SiGe-based radars can be drastically lower than III-V devices.

In recent research by academia and industry, the possibility of designing sophisticated phased arrays on a single silicon chip has been shown. With the high angular resolution and accuracy provided by a phased-array system, implementation of sophisticated radar algorithms is feasible for automotive radar systems, yielding enhanced system functionality.

Radar Transceiver Architectures

The figure below shows typical transceiver architectures for FMCW and pulsed radar systems. In the FMCW system, a ramp waveform is modulated by the carrier frequency and transmitted. The reflected waveform is then mixed with a replica of the transmitted signal to measure the round-trip delay and frequency shift by means of Fourier transform and digital signal processing.

FMCW Radar

Pulsed Radar




 

 

Spatial Detection and Resolution

Spatial resolution can be achieved by using multiple antennas. A narrow beamwidth can improve the spatial resolution. For a specific type of antenna the beamwidth is proportional to wavelength. As a result, by increasing the operating frequency, the beamwidth will be reduced:

where D is the antenna diameter. The above equation shows that for a given beamwidth, employing higher frequencies leads to a reduced antenna size.

By using two or more antennas with a separation of L, the angular position of the detected object can be determined, based on the phase difference between the signals received at each of the antennas.

Spatial

Automotive Radar Design Challenges

Reliability and accuracy of radar systems must be maintained over a wide range of temperature. Therefore, on-chip temperature compensation techniques must be implemented to minimize performance variation.

As mentioned in Automotive Radar Systems section, for long-range and mid-range radar systems, the main specification is the detection range, which typically requires a high transmitter output power together with a sensitive receiver, which dictates stringent requirements on linearity and dynamic range of the system. Meeting this goal is more difficult for mono-static radar systems. Furthermore, in mono-static radars, the use of a coupler in front of the antenna introduces additional loss in the RF path, which degrades the noise performance of the system, conversion gain of the receiver and output power of the transmitter. In contrast to this, bi-static systems do not suffer from this issue and show a higher SNR, but due to the wide variety of requirements in a radar system, designing a compact bi-static radar system can be difficult. In addition, Tx/Rx isolation is a serious issue and requires careful attention at the design level.

One of the most important performance parameters of the RF front-end of an automotive radar system is the local oscillator (LO) phase noise. In order to meet this requirement, using push-push structures and harmonic LO generation at 76-81GHz has been extensively examined. As a consequence, in order to achieve acceptable phase noise, fundamental oscillators are often designed at lower frequencies, in the range of 20-40GHz and then up-converted to the 76-81-GHz band using frequency multipliers to achieve phase noise less than -90dBc/Hz at 1MHz offset. In some cases, a Dielectric Resonator Oscillator (DRO) has been used to provide a high quality tank circuit for oscillation.

One of the parameters in system design is the slope of the ramp waveform generated for frequency modulation in an FMCW system. With a higher slope a higher resolution can be achieved, but at the same time a wider bandwidth will be occupied by the transmitted signal. Furthermore, among all the saw-tooth and triangular waveforms, the symmetric triangular waveform shows superior bandwidth efficiency. The period of the ramp is also inversely proportional to signal bandwidth.

As mentioned in Transceiver Architecture section, while the spatial resolution is improved by using higher frequencies, the atmospheric attenuation increases and for a given detection range, higher output power is required. 

Regulations

ETSI (Fixed Antenna Structure)

Band

76-77GHz

EIRP (FMCW)

50dBm (mean)

55dBm (max)

EIRP (Pulsed)

23.5dBm (mean)

55dBm (max)

3dB Beam width (Typical)

5⁰

Out-of-Band Emission

73.5-76GHz

0 (dBm/Hz)

77-79.5GHz

0 (dBm/Hz)

ETSI allows -15dBm/MHz to -3dBm/MHz mean spectral density at 79GHz and 46.2-to-55dBm peak EIRP for short range radar at 79GHz.   

FCC

For FCC, the state of the vehicle determines the restrictions on transmitted output power in 76-77GHz band:

  • Vehicle not in motion: 0.2uW/cm2 in any direction
  • Vehicle in motion: 60uW/cm2for looking forward direction and 30uW/cm2 for side looking and rear looking directions.

Based on the general requirements of the FCC, the maximum field strength should be 500uV/m at 3m distance. Also, the EIRP power spectral density shall not exceed -51.3dBm/MHz.