frequency modulated continuous wave (radar) (FMCW)
Frequency Modulated Continuous Wave (FMCW) is a modulation technique used in long-range radars( LRR) in automotive applications. FMCW radar is a frequency modulated continuous wave radar. In automotive technology, it operates in the W-band at frequencies of 77 GHz or 79 GHz.
In FMCW technology, radar pulses are transmitted that are linearly modulated and last only a few milliseconds. The frequency deviation of such a radar burst is a few hundred megahertz. The frequency modulation can be done with a sawtooth signal, whereby the frequency increases time-dependently linear like in a Chirp Spread Spectrum( CSS). In addition, triangular signals, square-wave signals for simple frequency shift ke ying( FSK) and signals with a step-like increasing signal level, staircase signals, are used as modulation signals. Which modulation pattern is used depends on the design and application in which the radar is used. For example, the range, detail resolution, and discrimination of objects.
As with CW radar, in FMCW radar the radar burst is reflected off obstacles and objects, received by the radar receiver, and compared with the transmitted radar burst. The distances and velocities of objects are determined from the deviation of the linearity of the frequency deviation. The range resolution with which two objects at approximately the same distance can be detected as separate objects depends on the frequency spectrum of the transmitted radar signal. Moving objects, such as oncoming vehicles, cause frequency shifts due to the Doppler effect, from which the speed and direction of travel can be determined.
Long-range radar systems with Frequency Modulated Continuous Wave transmit and receive radar pulses continuously and do not wait for the reflection signal first. To use the radar bursts for range measurements, they are modulated using frequency shift keying (FSK) for discrimination. This means that each individual radar pulse differs in frequency from those previously transmitted.