Application Report SWRA553A–May 2017–Revised February 2020 Programming Chirp Parameters in TI Radar Devices Vivek Dham ABSTRACT This application report provides information on how to select the right chirp parameters in a fast FMCW Radar device based on the end application and use case, and program them optimally on TI’s radar devices. Contents 1 Introduction ................................................................................................................... 2 2 Impact of Chirp Configuration on System Parameters.................................................................. 2 3 Chirp Configurations for Common Applications.......................................................................... 7 4 Configurable Chirp RAM and Chirp Profiles.............................................................................. 7 5 Chirp Timing Parameters ................................................................................................... 8 6 Advanced Chirp Configurations .......................................................................................... 11 7 Basic Chirp Configuration Programming Sequence ................................................................... 12 8 References .................................................................................................................. 14 List of Figures 1 Typical FMCW Chirp ........................................................................................................ 2 2 Typical Frame Structure .................................................................................................... 2 3 Angle Estimation Basics .................................................................................................... 5 4 Example Antenna Gain Pattern............................................................................................ 5 5 Effective Recevier Channels in MIMO Radar ............................................................................ 6 6 Chirp and Profile RAM Memory Allocation ............................................................................... 8 7 Example Usage of the Ramp Timing Calculator ....................................................................... 10 8 Sub Frame Structure Showing Three Bursts Looped Twice (start index having an offset of 2) ................. 11 9 Example of Advanced Frame Configuration of Two Sub Frames ................................................... 12 10 Radar Configuration Sequence........................................................................................... 12 List of Tables 1 Example Chirp Configurations for Typical Applications................................................................. 7 2 Chirp Timing Parameters ................................................................................................... 8 3 Typical Synthesizer Ramp Down Times for Different Modulation Bandwidths ...................................... 9 4 Typical IF/DFE Filtering Latencies as a Function of DFE Mode and Output Sampling Rate .................... 10 SWRA553A–May 2017–Revised February 2020 Programming Chirp Parameters in TI Radar Devices 1 Submit Documentation Feedback Copyright © 2017–2020, Texas Instruments Incorporated Introduction www.ti.com Trademarks All trademarks are the property of their respective owners. 1 Introduction Frequency Modulated Continuous Wave (FMCW) mmWave radar sensors are becoming increasingly popular for multiple automotive and industrial applications. The system requirements and care-abouts in each of these applications could be very different. Range requirement, range resolution, max velocity requirement, sensor field of view, data memory, processor MIPS, and so forth are some of the aspects that need to be analyzed based on the end application. Understanding the relationships between the FMCW chirp configuration and system performance parameters helps in selecting the right chirp configurations. TI’s mmwave radar devices (MMIC) provide large flexibility in configuring chirp parameters and also allow multiple chirp configurations in a single frame. The timing parameters are accurately controlled by the digital timing engine and a built-in radio processor without heavy real-time software interference. This document describes the programming of chirp parameters and explains the various system considerations that determine the values for these parameters. 2 Impact of Chirp Configuration on System Parameters In linear FMCW radars, the transmit (TX) signal is a single tone with its frequency changing linearly with time. This sweep in frequency is commonly referred to as a “chirp”. A set of these chirps form a “Frame” and this can be used as the observation window for the radar processing. The various parameters of the chirp ramp (like frequency slope, sweep bandwidth, and so forth) impact the system performance. Figure 1 depicts a single chirp and the associated timing parameters. Figure 2 shows frame structure that consists of a series of chirps followed by inter frame time. This represents ‘Fast FMCW’ modulation, where each chirp is typically 10’s of µs in duration. Figure 1. Typical FMCW Chirp Figure 2. Typical Frame Structure 2 Programming Chirp Parameters in TI Radar Devices SWRA553A–May 2017–Revised February 2020 Submit Documentation Feedback Copyright © 2017–2020, Texas Instruments Incorporated www.ti.com Impact of Chirp Configuration on System Parameters The following sections list key system performance parameters that are typically considered in any radar application and how the chirp configuration impacts each one of them. 2.1 Measurement Range and Range Resolution The maximum and minimum distance over which a radar sensor can detect objects is an important parameter for a radar sensor. Also, the range resolution (ability to distinguish two nearby objects) is another important metric. 2.1.1 Maximum Range In applications like automotive adaptive cruise control (ACC), it’s important to be able to view a far off object (>150m). Detecting a far-off object can be limited by either the SNR of the received signal or the IF bandwidth supported by the Radar device. The max range relationship with the IF bandwidth is shown in Equation 1. TI’s AWR2243 radar device provides a large 20-MHz bandwidth, and AWR1243 provides 15-MHz bandwidth, allowing more flexibility in the slope that can be used, which indirectly helps increase the max velocity as will be seen later. IFmax u c Rangemax 2Su (1) IFmax → Maximum IF bandwidth supported c → Speed of light S → Slope of the transmitted chirp Note that the IFmax is also dependent on the ADC sampling frequency (ADCsampling) used. In a complex 1x sampling mode, the IF bandwidth is limited to 0.9* (ADCsampling). In case of complex 2x and real sampling modes, the IF bandwidth is limited to 0.9* (ADCsampling)/2 . The maximum ADC sampling frequency in the TI’s radar devices is 45 MHz (AWR22xx) and 37.5 MHz (AWR1xxx). The other aspect that could limit the max range is the signal to noise ratio (SNR) of signal received by the receiver. This depends on: • RF performance of the Radar device, like TX output power, RX noise figure, as well as chirp parameters like chirp duration and number of chirps in the frame. • Antenna parameters like the TX and RX antenna gain in the direction of interest. • Object characteristics like Radar Cross Section (RCS). RCS is a measure of the amount of energy the object reflects back. This decides how detectable the object is with a radar sensor. • Minimum SNR required by the detection algorithm to detect an object. PtG G c2s NT Range based on SNR 4 Rx Tx r max 2 3 fC() 4p u kT u NF u SNR det (2) Pt → Tx output power GRx ,GTx → RX and TX Antenna gain σ → RCS of the object N → Number of chirps Tr → Chirp time NF → Noise figure of the receiver SNRdet → Minimum SNR required by the algorithum to detect an object k → Boltzman constant Tdet → Ambient temperature SWRA553A–May 2017–Revised February 2020 Programming Chirp Parameters in TI Radar Devices 3 Submit Documentation Feedback Copyright © 2017–2020, Texas Instruments Incorporated Impact of Chirp Configuration on System Parameters www.ti.com 2.1.2 Range Resolution In many applications it is important to be able to resolve two closely spaced objects as two separate objects, rather than detect them as one. The smallest distance between two objects that allows them to be detected as separate objects is referred to as range resolution. This primarily depends on the chirp sweep bandwidth that the radar sensor can provide. The larger the sweep bandwidth, the better the range resolution. TI’s radar devices support a 4 GHz sweep bandwidth that allows a range resolution of as low as approximately 4cm. c Rangeresolution 2Bu (3) c → Speed of light B → Sweep bandwidth of FMCW chirp Better range resolution also helps in detecting very close by objects, hence, improving a minimum detection range. 2.2 Measurement Velocity and Velocity Resolution 2.2.1 Maximum Velocity Along with the distance, the relative velocity of the object is another critical parameter of interest. The maximum measurable velocity in Fast FMCW modulated radars depends on the chirp cycle time, that is, the time difference between the start of two consecutive chirps. This in turn depends on how fast the frequency sweep can be performed and the minimum inter-chirp time allowed. The faster the MMIC can ramp the frequency, the higher the maximum unambiguous velocity. TI’s MMIC allows a fast ramp of 100 MHz/µs. Also the closed loop PLL is designed to