Automotive Imaging Radar Waveform Selection

The choice of automotive radar imaging radar waveform is TDM, BPM, DDM, or TDMA+DDMA, or others. Let’s take a look at the introduction of this paper.

1. MIMO waveform introduction

Today's emerging 4D imaging radar (4D-radar) uses multi-chip cascade MIMO technology to achieve high resolution in azimuth and elevation dimensions, providing high-quality 3D point cloud imaging. General automotive radar chips have 3 transmit channels and 4 receive channels, so a 4-chip cascade MIMO solution can be used at the same time to expand the virtual array to 12×16=192 channels.

Virtual array synthesis in automotive radar using MIMO technology relies on the separation of transmission signals emitted by different antennas. There are three modes of MIMO radar technology currently used, namely TDMA-MIMO, DDMA-MIMO, and BPMA-MIMO.

When the transmission signals of different antennas are orthogonal, separation is easier. The simplest way to achieve waveform orthogonality is TDMA-MIMO. Therefore, TDMA-MIMO waveform has become the most widely used method due to its simple implementation and high orthogonality. However, the disadvantage of TDMA-MIMO is that the detection distance is limited due to low transmission power, and there is a contradiction between the maximum unambiguous speed, distance resolution and the number of orthogonal signals. Although there are many subsequent algorithms to solve the speed ambiguity problem, the difficulty of target detection caused by spectrum aliasing is still serious, and the situation with more transmitting antennas is even worse.

BPMA-MIMO (Phase Coded) radar utilizes Code Division Multiple Access (CDMA) technology to efficiently achieve low cross-correlation waveforms without sacrificing transmit power, bandwidth, or chirp duration. Since there is no ideal orthogonal code sequence with perfect autocorrelation and cross-correlation characteristics, phase coded waveforms sometimes only approximately meet the orthogonality requirement.

The main problem with phase-coded MIMO radar is that the Doppler FFT of interference will spread across the entire Doppler spectrum, as shown in the figure below. The most serious impact is that weak target signals will be submerged in interference signals, for example, when a truck and a pedestrian are in the same range resolution cell, the pedestrian is likely to be undetectable.

RD spectrum of phase-coded MIMO with four antennas transmitting simultaneously

(a) Gold sequence, length 128

(b) Chu sequence, length 128

DDMA-MIMO can improve the radar detection range. Each transmit channel simultaneously transmits a frequency modulated signal with the same slope with a small and unique frequency offset, which effectively separates each transmit signal in the Doppler spectrum, equivalent to an orthogonal waveform. However, due to the small Doppler shift, there is coupling between the transmit channels, and the additional extended targets are easy to overlap.

DDMA-MIMO RD spectrum

(a) Simultaneous transmission from two antennas

(b) Simultaneous transmission of three antennas

Although waveform orthogonality is achieved by modulating each transmitting element with a different Doppler frequency shift in the Doppler domain, DDM-MIMO radar technology has some disadvantages that limit its application in automotive radar.

In a multi-target scenario, each target will generate a real position in the range Doppler spectrum, but there is interference from multiple transmitting antennas between the same range cells. When there are multiple targets at the same distance but different speeds, the real target and the interference will be confused. Therefore, how to mitigate the false alarms caused by interference in multi-target scenarios is a key issue.

The technical characteristics of the three waveforms are shown in the figure below:

2. TDM+DDM-MIMO technology

According to the above introduction and analysis, it is almost impossible to complete the design of 4D high-resolution imaging automotive radar by using only one of the waveform orthogonality technologies, DDMA or TDMA. Therefore, a comprehensive solution must be considered. This paper designs an integrated TDM-DDM-MIMO framework to obtain a compromise performance.

TDM-DDM-MIMO technology takes advantage of the best waveform orthogonality of TDMA and DDMA, and multiple transmit antennas transmitting signals simultaneously can also reduce energy loss. When using the same chip cascade MIMO technology as ARS540, with a maximum of 12 transmit antennas, the TDM-DDM-MIMO framework can be set to 2 or more transmit antennas for DDMA encoding, but it is not recommended to use more than 4 transmit antennas for transmission at the same time, because interference will seriously reduce target detection performance. Compared with TDM-MIMO technology, the maximum unambiguous speed represented by the distance Doppler spectrum in TDM-DDM-MIMO will be several times higher than the number of antennas transmitting in a single Chirp cycle, but the impact of interference will be more serious.

The waveform of a TDM-DDM-MIMO car with 2 transmit antennas is shown in the figure above. Using Doppler FFT on the data of two different channels, the interference and target will be distributed in the Doppler domain, but the interference is on different sides of the target between the channels, so the target and interference can be distinguished.

There are two special cases. Once the coordinate distance between two targets in the Doppler domain is Na/4 or Na/2, where Na is the slow time sampling point, the above target detection method will result in inaccurate estimation of false targets and real targets. Therefore, more information needs to be added to distinguish targets from interference, such as signal amplitude and phase information.

Personal experience: Waveform design is one of the key technologies of 4D imaging radar. It can be combined with antenna layout and optimization, super-resolution algorithm and tracking to form the four core technologies of 4D imaging radar!