Advancing from 24-GHz to 77-GHz radar for mid- and short-range applications

For short-range radar, the 24-GHz narrow and ultrawide bands have been used in legacy automotive sensors. For simple applications such as basic blind-spot detection, you can use the industrial, scientific and medical (ISM) band, but in most cases, including ultra-short-range radar applications, the need for high-range resolution dictates use of the ultrawide band. This 24-GHz ultra-wide band will be phased out soon, however, given spectrum regulations and standards developed by the European Telecommunications Standards Institute and the U.S. Federal Communications Commission. The 24-GHz ultra-wide band became unavailable as of January 1, 2022, known as the “sunset date,” in both Europe and the U.S.; only the narrow ISM band will be available in the long term. The lack of wide bandwidth in the 24-GHz band, coupled with the need for higher performance in emerging radar applications, makes 24 GHz unattractive for new short-range radar implementations.

There is a 76- to 77-GHz band available for vehicular long- and mid-range radar applications. This band has the benefit of high allowed equivalent isotropic radiated power, which enables front long-range radar applications such as adaptive cruise control. The 77- to 81-GHz short-range radar band has recently gained significant traction, both from a worldwide regulation perspective as well as industry adoption.

One of the key benefits of 77 GHz frequency band for automotive is its wide bandwidth. Compared to the 200-MHz ISM band, which is available at 24 GHz, the 77- to 81-GHz short-range radar band offers up to 4 GHz of sweep bandwidth, significantly improving range resolution and accuracy. The range resolution of a radar sensor signifies its ability to separate two closely spaced objects, whereas the range accuracy represents the accuracy in measuring the distance of a single object. Since range resolution and accuracy are inversely proportional to the sweep bandwidth, a 77-GHz radar sensor can achieve 20 times better performance in range resolution and accuracy compared to 24-GHz radar. The achievable range resolution is 4 cm (versus 75 cm for 24-GHz radar) as shown in Figure 2. Because the 24-GHz band will be restricted to narrow bandwidth, moving forward, most 24-GHz automotive radar sensors will likely shift to the 77-GHz band.

Smaller sensor size is another advantage of a higher radio frequency. For a chosen antenna field of view and gain, the size of the antenna array can be approximately three times smaller each in the X and Y dimensions when comparing 77 GHz to 24 GHz (see Figure 3). This size reduction is particularly useful in the context of automotive applications (where sensors need to be mounted in tight spots behind the bumper); in other spots around the car, including doors and trunks for some proximity applications; and inside the car for in-cabin applications.