About Radar Design in Driver Assistance Systems

A 25-year-old male driver was checking his smartphone when the traffic jam began to move. Just as he stepped on the accelerator, the car in front of him braked suddenly due to the congestion. A collision was about to happen, but fortunately the on-board radar system intervened at lightning speed. The adaptive cruise control system with automatic stop and start function detected that the car in front was slowing down and decided to stop immediately in order to maintain the pre-set distance and avoid a collision.

In the two decades since airbags became standard safety features, automotive safety technology has made great progress. "Passive safety" represented by seat belts, airbags and collision detection systems has developed into "active safety" - anti-lock braking (ABS), electronic stability control, adaptive suspension and steering angle/roll control. The latest stage is "driver assistance" safety, including adaptive cruise control (ACC), blind spot detection (BSD) and lane change assist (LCA). These systems are gradually beginning to merge with in-car communication systems to make cars more autonomous and intelligent.

Radar is a very promising driver assistance technology. Radar systems can significantly reduce the frequency and severity of accidents, especially those related to driver distraction. In many countries, automotive safety legislation has promoted safer driving, reduced driver fatalities to historic lows, and promoted the development of smart cars. Until recently, radar was limited to aircraft and luxury cars, but now it has become a technology of interest in ordinary cars.

The challenge for designers is how to integrate multiple safety features while meeting the demanding quality and cost requirements of the automotive industry. These goals are not necessarily in conflict. For the first time ever, a steady stream of highly integrated systems are being produced that integrate adaptive cruise control (ACC) and other radar-based detection and avoidance applications all in a tiny package the size of a smartphone. Advances in on-chip signal conditioning technology allow designers to program the required settings for different driving conditions, whether it is city driving or highway cruising, all in an affordable package.

Therefore, radar system designers now need to make a choice: use discrete components or use an integrated solution. Electronic integration has been adopted in many industries, such as medical imaging, communication infrastructure, consumer electronics, and now it is finally the turn of automotive radar. Regardless of the solution, designers must consider its pros and cons.

Size and cost

Radar is moving from standard equipment on luxury cars to optional equipment on mid-range cars and is expected to become a universal safety feature in cars in five years. The pace of adoption will accelerate with the introduction of cheaper radars, which will provide better target classification and higher range resolution. The design method of the analog front end (AFE) is critical.

It is possible to build a top-notch custom solution using discrete components, and there are always some perfectionists who want to optimize every parameter. However, building a radar system with discrete components takes more time, takes up more space, and costs more. Integrated ICs can provide most of the functions that automakers want, even for multiple applications such as ACC and BSD, at a fraction of the size and cost of discrete solutions.

It is now possible to integrate signal conditioning and data acquisition circuits all on a single IC. Size is important because radar sensor modules must fit into small areas, such as behind a shock absorber, that are not designed to accommodate such electronics. With an integrated solution, the mounting area can be reduced by at least half. Integrated devices are cost-effective while providing the high performance levels required by radar system designers.

You could certainly build a discrete system yourself that does exactly what you need, but it would probably be too expensive to scale. An integrated solution means radar systems can be put into more cars at a lower cost, improving safety for all vehicles overall.

Ease of use

Integrated devices can add many built-in innovative features such as programmable gain and flexible filters. These features support a platform design approach that not only speeds up the time to market for the first system, but also reduces the development time of all subsequent systems using the same platform.

For example, filters need to be fine-tuned for different driver safety applications. Discrete designs make it difficult to reprogram filters, and designers must manually swap resistors and capacitors to change filter characteristics. Integrated devices with tunable filters easily solve this problem, and all adjustments are made by reprogramming the chip through a serial port. This adjustment can even be done on the fly, allowing designers to quickly make multiple adjustments, thereby reducing design time.

Multiple channels on the same chip also help simplify the design because the channels are well matched, and it also benefits the driver because the sensor has a wider detection range. An ideal radar system can detect objects within a 180-degree field of view around the car, just like human vision. A receiving system with six channels can do this, but with better angular resolution because it can receive more transmitted signals. This means more time to locate the target and the radar's ability to resolve the approximate size of the target. Although designers can use discrete components to accomplish the same task, the structure may be bloated.

The latest integrated solutions can automatically adjust the characteristics of the radar sensor according to the distance. Designers must prevent the system from overloading when the radar signal is reflected back to the car. If the target is directly in front of the car, the amplitude of the return signal will be very high and must be attenuated. If the target is 120m away from the car, the return signal will be weak. The microprocessor will constantly try to optimize the signal-to-noise ratio to help identify the target, determine its position in the field of view, and how far away it is from the car.

Many of these functions are implemented with programmable gain amplifiers. Designers can use discrete PGAs, but not as easily or economically as one that is controlled through the same serial port as the programmable filter.

Considering the variability of radar system requirements, flexibility is another attractive feature of the integrated solution. Highway ACC requires a wide dynamic range, while the ACC automatic stop-and-go function requires a narrower range, a larger field of view, and a faster response time to quickly adapt to the traffic conditions ahead. With the user-programmable settings of the integrated products, one platform can provide enhanced performance under different working conditions, adapting to both highway and severely congested driving conditions.

It is clear that a platform design approach using ASICs can simplify the design process, reduce the size of radar systems, and more importantly, make more affordable cars safer.