Regulations and standards for automotive millimeter wave radar (IV)

In the first article, Regulations and Standards for Automotive Millimeter-Wave Radar (Part 1), we mentioned the news that “an empty cemetery showed a large number of human figures on the automotive radar.” This is a case of radar “false alarm”, which can be seen everywhere in the laboratory.

01

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False alarm and missed alarm problems

The Ministry of Industry and Information Technology's document No. 2021 [181] defines "false alarm" as follows: a false alarm refers to an event in which the radar detects a target when the actual target does not exist under specified conditions. False alarms are related to false signals. The false alarm phenomenon in the figure below is caused by the power of the interference signal exceeding the detection threshold.

When the automotive radar is interfered by other automotive radars, false signals or noise floor rise will occur. In addition to false alarms, the impact on target detection is also missed alarms or a shorter detection distance (i.e., decreased sensitivity). Missed alarms refer to events where, under specified conditions, when there is a target, the radar detection result is judged as no target. Missed alarms are related to noise power. The missed alarm phenomenon in the figure below is caused by the noise power being higher than the target power after the noise floor is raised.

The shortening of detection distance (reduction of sensitivity) means that under certain observation environments or conditions with the probability of false alarm or missed alarm, the maximum distance at which the radar can detect the target becomes shorter.

We know that one of the most important functions of radar is to detect the existence of various targets and report them effectively. For example, the targets to be detected by weather radar are rain and snow, so other buildings, the ground and the like are useless targets and will interfere with it; the radar used for terrain mapping or height measurement detects the ground, so rain, snow, flying birds and the like are useless targets and will also generate clutter or interference.

Let's take a look at an example of a test in a laboratory (note: laboratory test samples are not the same as commercial products, this is just an example). The red frame in the figure below shows a useful target given to the car radar by the target simulator (a target with a distance of 60m and a speed of 10m/s). The following table is a list of target results detected by the car radar. While detecting this useful target, many other useless targets are also detected. The figure shows 9 useless targets:

Even useless targets are detected, and useful targets are not displayed at all, as shown below:

Then, by simply adjusting the placement of the absorbing materials in the lab, the number of useless targets was reduced to 3, as follows:

"A deserted cemetery shows a large number of human figures on the car radar." This is because the radar mistakenly displays false targets on the screen, and fails to correctly filter out and eliminate useless targets. These so-called "human figures" appear because there are various interferences and clutters in the external environment. We can see that even in the darkroom environment above, they cannot be completely avoided. We cannot change the real environment like in the laboratory, and can only process and avoid it through the radar's own algorithms. Almost all radars have the ability to automatically track and detect targets, including various thresholding technologies for false alarms and target suppression, as well as algorithms for estimating target positions and distinguishing targets, etc.

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Radar receiver and its blocking test

Therefore, the design of radar receiver is very critical. Compared with the wireless communication system we are familiar with, radar is also composed of a transmitter and a receiver. The difference is that the radar receiver needs to amplify, filter, down-convert and digitize the radar echo in a way that provides the maximum discrimination rate between useful echoes and useless interference. In other words, when designing a radar receiver, it is necessary not only to consider the traditional problems of the most basic wireless communication system such as system noise, dynamic range, RF front end, amplifier, mixer, local oscillator, frequency synthesis, but also to add more algorithms to distinguish useful and useless targets, such as a variety of gain control technologies.

Sensitivity Time Control (STC) is one of them, which can make the sensitivity of the radar receiver change over time. Let's imagine that different radar cross-sections, different weather conditions and different distances will cause different echo strengths. Among them, distance may have the greatest impact on radar echoes, so the farther the useful target that the radar is searching for, the smaller the echo strength will be. And useless targets may produce strong echoes nearby, so strong that they even exceed the dynamic range of the receiver. Therefore, the sensitivity time control (STC) technology can alleviate the adverse effects of radar echo strength caused by distance.

There is also a clutter map gain control. In addition to the noise generated by the radar receiver itself, nearby radars, communication equipment, etc. also generate interference energy. The energy radiated by the radar itself that is scattered by useless targets (such as rain, snow, birds, insects, atmosphere, metal, etc.) can also be classified as interference or clutter. The clutter map is controlled by a digital map, which records the average amplitude of clutter in each clutter map unit during multiple scans, and adjusts the receiver gain where necessary to ensure that the clutter echo is below the receiver saturation level.

In the previous documents and regulations of our country, there was no test item for radar receiver blocking. In the new document No. 181, the test item of radar receiver blocking has been added. Although the performance of the transmitter has always been the most important concern of the radio regulatory authorities, if the radar receiver cannot guarantee performance and work effectively, then the corresponding radar transmitter is equivalent to creating useless signals, becoming interference and clutter for other radar equipment. In the regulations, CW interference within and outside the radar working frequency band is used as blocking signals to examine whether the radar receiver can still detect the target normally in the presence of interference.