The big controversy about sensors in autonomous driving is whether lidar is essential. Radar has improved significantly in resolution and so I like to phrase the question as to whether radar is getting better faster than lidar is getting cheaper. Today's focus is on radar since the technology is playing an increasingly important role, driven by automotive ADAS applications. These applications require higher performance and more capabilities from the radar module to determine distance, direction, and speed of targets in a multi-target scenario.

The radar technology used is known as frequency modulated continuous wave (FMCW), typically in the 77GHz band. Instead of putting out individual radar pulses and measuring the time-of-flight for the echo to return, the radar is transmitted continuously but with frequency varying, typically in a linear sawtooth wave.

The chirps illuminate the objects around the vehicle and depending on their distance and speed, the returning signals (from multiple chirps) can be analyzed to determine the distance and size of the objects, how fast they are moving, and what direction.There may be just a single transmitter antenna, or digital beamforming can be used there to steer the radar beam. Of course, the critical function is that all the targets of interest are illuminated. This all requires a lot of signal processing for receiver digital beamforming (DBF) to start with. If you know what steerable beams are in 5G, then this is the opposite. Instead of doing phase array transmission on lots of antennas, it is doing the same thing in reverse, using multiple antennas to work out where signals came from. Then there is the 2D range-Doppler FFT processing with windowing and 2D constant false alarm rate (CFAR) kernels.

A typical automotive application will have multiple receiver antennas. A simplified block diagram of a radar unit appears below.

In linear sawtooth FMCW, the transmitted waveform is a sequence of linear chirp signals, and the reflected received signals are mixed with the transmitted signals to generate the beat signal. The beat signal is the baseband signal, which is sampled by the ADCs and sent to the DSP. The total duration of the multiple chirp sequence transmission is called the coherent processing interval (CPI). For one CPI, the ADCs generate a data sequence that is sampled along three dimensions: range dimension (time sampling of one chirp), Doppler dimension (multiple chirps), and channel dimension (each antenna element). This three-dimensional data sequence for one CPI is the received radar data cube, which is processed by the signal processing algorithm chain on the DSP. The sizes of the various dimensions are chosen based on required range, speed, and direction resolutions. The chirp rate might be as little as 10us, whereas the CPI used for analysis might be 25ms. An example of the datacube is shown below.

Sign up for Sunday Brunch, the weekly Breakfast Bytes email.