MIMO Radar Design and Component Selection

Created: November 30, 2021
Updated: October 10, 2024

Today’s commercial radar systems achieve directional control and beamforming with phased array antennas, giving a systems designer the ability to track objects over a solid angle. This technology is not new, phased array antennas have been used since 1979 when the US PAVE PAWS active phased array ballistic missile detection radar system went online in Alaska. Since that time, the useful frequencies have gotten higher, the radar modules have gotten smaller, and the accuracy of these systems has steadily increased.

Today, radar is used in systems with goals beyond object detection or position measurements. Chirped radar is used for simultaneous position and velocity measurements, with some signal processing techniques used to extract accurate targets and track their positions. Today’s automobiles currently use chirped radar modules with a relatively small footprint, with modules operating in the K band for short-range target tracking (~24 GHz) or in the W band for long-range target tracking (~76-81 GHz). The difficulty in current systems is in the need for multiple sensor modules to provide object detection over broad angular sweeps, yet the resolution is low as there is no orchestrated beamforming between these modules.

Other areas like robotics and drones are making use of these or similar radar bands, and there are specialty applications in scientific research and imaging. Over the past 10 years, we’ve seen the integration of another technique from telecom into radar systems: multiple-input multiple-output (MIMO) antenna design and orchestration. Now, further progress in radar systems, including MIMO radar, is being driven by automotive sensor suites and advanced driver assistance systems (ADAS). For electronics designers, we’ll look at the system architecture needed to support more advanced radars and some current chipsets available to support MIMO radar.

Progress in mmWave Radar and Introduction of MIMO

Radar modules and systems operating at mmWave frequencies are currently enjoying widespread use in three principal areas:

Automotive: I mentioned ADAS systems above, which will continue to remain in focus. The primary motivation is to enable more accurate object tracking, but there is also the need to potentially reduce total sensor count while enabling in-situ imaging with radar measurements.
Robotics: There has been some focus on this area over the past 5 years or so, and over that time period, we’ve seen radar sensors integrated into areas like factory automation, speed and position control for small robots, and image segmentation from radar point clouds.
Aerospace: The presence of radar is obvious here, but now radar modules for small autonomous UAVs and drones are coming into focus with live flight tests being conducted by NASA. This reduces the reliance on GNSS/GPS for navigation, something which is not useful for autonomous drone navigation in confined areas.

These systems use chirped broadcast signals to enable simultaneous position and speed tracking, as well as to interpret multiple targets within the field of view. This is done using beamforming in a single phased array, followed by using some standard signal processing steps for target extraction and tracking over time.

Today’s commercially available radar modules do not use MIMO, at least not to my knowledge at the time of writing. However, major component manufacturers are introducing mmWave chipsets to support unique applications of FMCW radar, including MIMO radar and cascaded radar. Commercial radar modules, evaluation boards, and transceivers are still using the tried-and-true center-fed patch antenna architecture, a COTS radar transceiver, and some standard signal processing algorithms. This includes the latest radar modules my company has designed for UAV applications.

Basic phased array antenna architecture for today’s radar. The board section shown here is from the Texas Instruments AWR6843 evaluation module. This design uses a hybrid stackup with a Rogers laminate on the top layer to support low-loss signal propagation to the antennas.

The signal processing steps involved in distinguishing and tracking targets is somewhat complex, and there are many guides in signal processing textbooks on these topics. In these systems, the drawback of the current approach is in the limited field of view and rough resolution. For this reason, many systems use tight beamforming with low divergence for targeted position and speed measurements, and they just broaden the field of view using multiple modules. Current automobiles use multiple short-range and long-range radar modules with the architecture shown above to provide a wide field of view as part of the ADAS system, as shown below.

As long as these systems continue to rely on short-range radar while also being designed for greater autonomy, then the entire suite of sensors on these products will need to be improved to have a higher resolution. MIMO radar is one of the major advances in this area that is yet to be heavily commercialized.

Why integrate MIMO into a radar system when we can just use more modules to expand the field of view to track more targets? It’s a fair question and it might not seem obvious how MIMO techniques are useful in radar design. Obviously, increasing the number of antenna arrays enables tracking of more targets, but the resolution in these systems is still lacking. MIMO radar solves this problem without major increases in complexity.

MIMO Radar Provides Higher Angular Resolution

There are some compelling reasons to build out MIMO radar systems for the above applications. The reason to use MIMO is not focused on increasing the number of tracked targets, but on the resolution of target tracking, specifically in angular resolution within the field of view.

In a MIMO system, you have an array of Tx antennas that broadcast orthogonal signals. The set of Tx antennas is not orchestrated to produce beamforming like you would in a phased array. However, if you want to supercharge your MIMO radar system, you can use individual phased arrays for Tx emitters, where each Tx array broadcasts with one of the signals in your orthogonal set. If each Tx emitter is an array, then beamforming is accomplished using standard phase control between each element in the array, where the phase between each emitter in a Tx array is delayed to control beamforming and steering.

The signal chain in MIMO radar is very similar to that in phased-array radar.

There is also an Rx array, which receives all signals from the Tx antennas, requiring multiplexing to separate each orthogonal signal emitted from the Tx antennas. Because the received signals from each Tx broadcast are orthogonal, you have a set of measurements each linked to a specific broadcast angle. This is what gives the higher resolution: the interfered beam at the Rx array can be interpreted using the same signal processing steps as in a typical phased array, where the reception angle is extracted along with the speed and position parameters from a Doppler measurement. However, you are now sensing multiple orthogonal signals that are linked to a specific transmitter. It’s then a simple trigonometry calculation to link each received signal back to the specific transmitter and extract the target position with very high accuracy.

You can also layout the arrays in two dimensions, giving a set of patch antennas on the PCB that provides azimuthal and polar beam interpretation. This is useful for newer 4D radar modules that are being developed for state-of-the-art ADAS systems, which can then be used to sense object elevation.

What’s important to realize here is that we are using an array of M by N emitters that operate with a set of orthogonal signals (multiple frequencies, or multiple chirps), which is key to the increased broadcast resolution. Consider the case where we want to take a typical FMCW automotive radar and double its resolution. We could do this by simply doubling the number of antenna elements. However, we could get the same result by just adding 1 more Tx antenna. It is this multiplexing between multiple antennas that increases resolution on the Rx side and the Tx side.

Components for MIMO Radar

Modern radars use linearly chirped signals (known as FMCW radar) to enable simultaneous position and speed tracking, as well as highly accurate target extraction. The signal processing block and the transceiver block are the critical components to select here as they will be responsible for generating and interpreting signals used in the system. It’s also possible to take a software-defined radio approach to signal generation for each Tx emitter.

How you choose to generate FMCW radar pulses is up to you, but there are COTS components you can use to help design your system. Because these systems are not yet highly integrated, you’ll have to piece together multiple mmWave components to create MIMO radar systems.

Texas Instruments, AWR1642

The AWR1642 radar transceiver from Texas Instruments targets automotive applications and is one of their standard transceiver components for FMCW radar systems. This component can be used in a spatially multiplexed manner, where multiple center-fed patch antenna arrays are used to broadcast and receive signals. MIMO radar is then implemented by broadcasting with different subcarriers from each Tx section.

Block diagram for the AWR1642 transceiver, from the AWR1642 datasheet

The IWR6843 radar transceiver from Texas Instruments targets industrial applications and imaging applications operating at 60 to 64 GHz. This newer transceiver expands radar options for systems designers, and MIMO radar can still be implemented with spatial multiplexing with multiple transceivers. This particular component provides extremely low phase noise of -93 dBc @ 1 MHz, high Tx power of 12 dBm, an integrated PLL, and an integrated ADC. Configuration is implemented over standard low-speed digital interfaces. Functionally, the component is much the same as the automotive transceiver shown above.

More Innovation With MIMO Radar Systems

As I mentioned earlier, one of the upcoming areas in MIMO radar is 4D radar. These systems used dozens of radiating elements operating at mmWave frequencies, where Tx beams are scanned over polar and azimuthal angles. Azimuthal scanning is used to detect the vertical span of a target and vertical motion. 4D radar is currently being developed for the newest ADAS systems, but these radars will also be useful for robotics operating in complex environments, as well as small drones that require very precise object recognition. With high angular resolution and azimuthal scanning, it may be possible to reduce reliance on visual cameras and optical image processing and rely totally on the radar.

The new components to support these areas are highly integrated SoCs, where the transceiver, ADC, signal processing block, and MCU are fabricated on the same die. This will help reduce total system size and make newer MIMO radar systems much more competitive with standard phased-array radars.

Other Components to Support MIMO Radar Systems

MIMO systems need more than transceivers for individual arrays because current radar chipsets are not highly integrated for MIMO applications. Because of the lack of integration, designers need to link together multiple components into a larger system to support MIMO radar applications. Some other important components you might need include:

As new transceivers for MIMO radar designs become available, you’ll be able to find these and other important components using the advanced search and filtration features in Octopart. The electronics search engine features in Octopart give you access to updated distributor pricing data, parts inventory, parts specifications, and CAD data, and it’s all freely accessible in a user-friendly interface. Take a look at our integrated circuits page to find the components you need.

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