An advanced radar method for spatial resolution.

In their search for the right radar solution, many people stumble over the term ‘MIMO radar’. The MIMO antenna design is a recent radar technology development and is particularly popular with more complex radar applications. It is based on the knowledge of phased array antennas, also known as SIMO radar. MIMO technology boasts improved spatial resolution. In this article, you will find out what special features the advanced radar method boasts and how it works.

What is MIMO?

The term MIMO stands for multiple-input, multiple-output. The radar method describes the arrangement of numerous antennas. These means of grouping antennas combines several transmitting and receiving antennas, which significantly improves the angular resolution. The special thing is that for each transmitting module, there is also a corresponding receiving module.

And each transmitting antenna is equipped with its own waveform generator to emit individual signals. This way, a received response signal can be allocated to a specific transmitting antenna (the source). It is thus possible to make more precise statements about the position of the target, since there is not only the information about the path between the object and the receiver but also the path between the sender and the object. The systems then systematically evaluate all combinations of transmitting and receiving antennas.

If, in addition, each individual transmission signal is different (adaptive waveform), then the individual signal-to-noise ratio can also be optimised. This function has a positive effect on downstream radar signal processing, because the data quality is better.

At a glance – advantages of MIMO antenna design:

• • High spatial resolution due to better angular resolution
  • Reduced susceptibility to interference
  • Optimisation of the signal-to-noise ratio
  • Better positioning and discovery of targets

MIMO technology in comparison

 For one thing, other radars use just one transmitter and one receiver.

They therefore have no angular selectivity, because due to the limited number of antennas they can only provide data about the speed and distance of an object. This also limits the number of objects that can be detected at the same time, since the targets are only distinguished over a maximum of two dimensions.

MIMO radars are capable of detecting multiple objects at the same time, even if they are moving at the same speed and are at the same distance from the antenna. Such systems also distinguish the targets based on angle information.

SIMO radars (single-input, multiple-output), also referred to as phased array antennas, are also group antennas. They emit the pulses via a central transmitting antenna and receive the reflection via multiple- receivers.

As with MIMO, a high resolution can be achieved with a transmission module and any number of receivers. Because it can be influenced by the number of reception modules. However, the more receivers are added, the bigger the aperture, and numerous side lobe occur. This is where MIMO technology is helpful in counteracting the issue with multiple transmitting antennas.

In addition, SIMO radars require more development, since they have a complex design due to the necessary phase shifter, calculations, and thinning-out of the antenna field. This is less cost-effective.

Spatial resolution with MIMO radars

The resolution corresponds to the radar’s capability to distinguish objects. It indicates how well objects can be distinguished from each other. The resolution is often confused with the measurement accuracy. However, while the latter focuses on the quality of the measurement (deviation rate, measurement errors), the capability to distinguish objects indicates how well an object can be positioned in space.

In order to clearly distinguish nearby targets and determine their position, the radar must have a high resolution. Objects are distinguished via different measured dimensions. This includes the speed of the object, the distance, and the angular position in azimuth and elevation of the target with respect to the antenna. Radars with just one transmitter and antenna have just one distance and speed resolution. While object distinction via speed depends on the measurement time and modulation, the ability to distinguish objects via distance is determined by the available bandwidth.

MIMO radars generally have a three- or four-dimensional resolution. Because they always distinguish objects based on distance and speed, but also angle. Because the angular resolution is generated by combining several antennas.

How does angle measurement work with MIMO radars?

In order to conduct the simplest type of angle measurement for an object, 2 receivers are required, which should optimally be positioned half a wavelength away. The phase shift is used to attain the angle information, since the path to the antenna suddenly changes in the event of a diagonal angle of incidence. The radar uses this difference to determine the angular position of an object.

When three antennas are combined, already two objects can be distinguished from each other at the same time, since all combinations of antennas output data to be analysed.

The more antennas are available, the more objects the radar can distinguish (multi-target capability).

Application examples

One challenge presented by MIMO radars is installation size. Since as many antennas are put in place as possible, the systems are generally larger. But through the possibility of using higher frequencies, antenna designers can counteract this, because the antenna patches are smaller. The wavelength and the distance are shorter. Thus, at higher frequencies, not only is there a greater bandwidth available (and therefore a better distance resolution), but the 60 or 77 GHz radar systems are significantly smaller.

But many MIMO radars are used for more complex applications for more reasons than these frequencies. The systems’ high resolutions boast advantages for applications in challenging environments, such as factory buildings or road traffic. If many objects have to be distinguished from each other, such as for counting people, or tracking or positioning in high-traffic rooms, it is hardly possible to achieve a satisfactory result without the use of MIMO technology. Such systems can also be found in the security sector at 24 GHz. Infrastructures with numerous static objects likewise benefit from the higher resolution and improved performance of MIMO radars.

Header Picture Source: © kras99 by adobestock.com

Information on this advanced sensor technology

The demand for sensors is increasing due to trends such as digitalisation, automation in industry & logistics, smart homes & cities, and autonomous driving. But the development and integration of radar units is a complex topic, and the technical terms and functions raise many questions for users. Our radar experts have put together comprehensive information to help you get started in the world of radar.