Radar 101: FMCW CHIRP CONFIGURATIONS FOR SRR, MRR and LRR
Radar configurations can be broadly categorized into three categories namely short-range radar with a maximum distance of about 30 meters, medium range radar with about 60 meters and long-range radar with about 250 meters.
Chirp configuration for short range radar (SRR)
Short Range Radar can be used in blind spot detection, rear and forward collision mitigation, parking assist, lane change assist etc. While, Medium range Radar can be used for forward collision warning, cross traffic alert, stop & go etc. The industry has not formulated the specific definitions and distinctions between SRR and MRR and neither about their use case.
In FMCW, the chirp configurations control all the basic requirements like maximum range, range resolution, maximum velocity and velocity resolution. The maximum range is given by the equation:
Max range = IFmax * Tc * c/ (2 * Bw)
Range resolution = c/ (2 * Bw)
Where IFmax = maximum supported intermediate frequency. This is directly related to the sampling rate of the chirp, S = slope of the chirp, c = speed of light and Tc = chirp time.
For detailed explanation on the formulas of FMCW system, please refer to the “Basic Radar system for Automotive ADAS” white paper. To achieve different maximum distances in SRR and MRR, with sufficient range resolution, it can be seen that the slope (Bw/Tc) is to be controlled. By similar formulas, it can also be shown that the chirp time and the active frame time control the maximum velocity and velocity resolution. It can be seen that all the parameters are entwined and adjusting one chirp configuration like say chirp time, would affect almost all the required parameters. Added to this, the hardware chip limitations also put lot of restrictions. Hence, the radar chirp configuration is a tedious step that has to be performed carefully, with the help of an expert if possible.
Chirp configuration for medium range radar (MRR)
While moving from SRR to MRR, for the added distance, either the velocity resolution (assuming same range resolution) or the range resolution (assuming same velocity parameters) has to be compensated. If the radar chip supports sufficient higher sampling rate, then MRR can be achieved with the similar configuration as SRR but with more amount of data in radar cibe.
Chirp configuration for long range radar (LRR)
Now the case of LRR is extremely different. In general scenarios, while scanning at 150 meters, the range resolution is not so crucial. A range resolution of 1m is sufficient. This helps in reducing the bandwidth to 1GHz and there by reduces the radar data cube size, which results in lesser data for processing. It is also known that the noise level from both the chip and attenuation from atmosphere is lesser for lesser frequency. Hence it is preferred to use 76-77 GHz band for LRR, even though, literally this doesn’t fall within the 77-81 GHz band.