Radar 101: Modulation techniques for automotive radar application and why FMCW is winning the race?
Modulation techniques play a key role in in radar technology. The mode of transmission makes a huge difference in the performance of radar and hence the technique will change as per the application.
The two most commonly used techniques are Frequency Continuous Modulated wave (FMCW) and the Pulsed Doppler technique. FMCW is commonly used in industrial as well as automotive applications, while in military applications, the Pulsed Doppler is widely accepted.
Frequency Continuous Modulated wave (FMCW)
In the FMCW technique for automotive industry, the starting frequency is usually around 76-77 GHz and it is linearly modulated to reach a maximum of 81Ghz over a given time period. This waveform is called a chirp. A frame consists of ‘N’ such chirps, each lasting for a given chirp time ‘Tc’. The bandwidth of each chirp and the slope are also crucial to the performance of the FMCW radar. These parameters have a direct influence on the maximum range, maximum velocity and their corresponding resolutions. A general FMCW can be seen in the below figure.
In the FMCW technique, the distance of the object is calculated by the difference in frequency observed between the received signal and the transmitted signal at the same time instant. This is achieved by mixing the transmitted and received signals and an intermediate frequency signal is generated. A Fourier transform on this intermediate frequency signal (also called as beat frequency) yields information on the distance of the object with a good degree of accuracy.
Now the pulsed doppler on the other hand sends a signal for a short duration and waits for the return signal. The duration of the signal and the ‘wait time’ are parameters that must be taken into consideration while designing such a system. A general pulsed doppler signal looks like the below image.
However, in the pulsed doppler, the time difference between the received signal and transmitted signal is used to measure the distance of the object. Now measuring frequency changes is much easier than measuring time difference in electronic systems. Thus, this leads to a better resolution in FMCW and hence is preferable in automotive applications.
Moreover, Pulsed doppler technique requires that the minimum time difference must be above a certain limit. This leads to a blind spot and hence minimum observable distance in pulsed doppler is larger than in the case of FMCW making it disadvantageous in automotive applications.
Pulsed doppler however sends its pulse over a relatively shorter period of time and hence the power of the signal sent can be larger. Thus, the power of the received signal from an object farther away is likely to have more power than in the case of FMCW. Thus, maximum range with a given power constraint is going to be higher in the case of Pulsed doppler.
Faster real-time processing is mandate for automotive application, new processors are able to solve complex math required by FMCW. These processors are also inexpensive as a result FMCW is best suited for automotive application. Furthermore, the doppler velocity can be calculated with better accuracy in the Pulsed doppler technique. As a result, the pulsed doppler method seems to be a clear winner when it comes to military based applications.