What Is MIMO?

MIMO is a wireless RF technology characterized by a unique method of data transmission. With MIMO technology, the same data signal is sent through multiple paths to improve reliability. MIMO technology achieves this by transmitting the data through numerous antennas. Even though these antennas send data using the same direction in the same bandwidth, receiving antennas get the signals through slightly different paths due to interference and reflection.

These signals are collected in the receiver, which accounts for any time differences, noise, interference, or lost signals. This process results in a more stable and reliable signal.

The unique signal integrity provided by MIMO routers makes these devices highly desirable for various technologies. For example, MIMO is a unique selling point for high-performance broadband radio communication for unmanned mobile systems because this technology ensures the robot stays connected despite its orientation. MIMO broadband routers can improve channel capacity and provide excellent signal strength even without a clear line-of-sight by using bounced and reflected signals in addition to direct transmissions. This quality makes MIMO systems excellent at transmitting high-quality video and audio even in challenging circumstances, like automated vehicles operating in mines, or long-range connectivity to a drone with obstacles between the drone and the operator.

The History of MIMO

MIMO technology represents the culmination of decades of research in multi-channel transmission systems. Up until the 1990s, radio systems were limited to those that passed signals between pairs of antennas or combined basic signals. However, research from those decades set the stage for MIMO technology.

The earliest research related to MIMO started in the 1970s, though this primarily focused on multi-channel systems and interference. This research was used in the 1980s to develop the theory behind cross-coupled networks, which was essential for the development of the first MIMO systems. By the 1990s, basic beam switching and signal propagation were available, but further advancements were limited by processing power.

As processing power improved in the early 1990s, initial work began for MIMO systems. These studies focused on using MIMO systems in multi-path propagation, initially to limit signal degradation. As time passed and processing power improved, MIMO systems were altered to leverage additional signal paths to carry more data and improve the quality of the signal.

Arogyaswami Paulraj and Thomas Kailath were credited as the first to propose spatial multiplexing and were granted the patent in 1994. It was Bell Labs, however, that first demonstrated the technology with V-BLAST in 1998.

Why MIMO Was Needed

MIMO presented a solution to a long-standing issue with point-to-point radio transmission. When two radio terminals are sending information to one another, the signal can reflect or follow multiple communication paths. The arrival of these duplicated signals at the receiving antenna causes an effect called fading. There are two types of fading, both of which result in errors:

• Selective fading: Selective fading occurs when a frequency segment of a signal is attenuated relative to another frequency segment. When this happens, the signal fragment is distorted.
• Non-selective fading: Non-selective fading happens when two frequency segments of a signal are dynamically attenuated by the same amount. When this occurs, the signal fragments are lost.

Before MIMO, the only solution to these issues was to analyze the signal and adjust the type of antenna or signal strength. With MIMO, receiving antennas are designed to collect these varying signal reflections and fragments and process them while accounting for any time differences, noise, or interference. This ability made MIMO radio for Defense applications an essential, as it could improve signal quality significantly in front-line applications.

How MIMO Radio Communication Technology Works

MIMO works by transmitting the same data through multiple antennas to multiple receiving antennas. The premise behind MIMO is to use the principle of diversity. Different signals sent along different signal paths may be affected in various ways, but it is unlikely that they will be affected in the same ways.

By collecting a diverse range of signals, the receiver can combine those signals to create a sum that is as close to the original as possible. This phenomenon results in a more stable link, along with reduced error rates and improved performance.

MIMO Functions

MIMO technology utilizes three essential functions to transmit signals and maximize signal quality and integrity. These are described in detail below:

• Precoding: Precoding is the spatial processing that occurs at the transmitter. Precoding involves beamforming, which is when the transmit antennas each emit the same signal with appropriate phase and gain to maximize the signal at the receiver input. This function only works when the system knows the channel state information (CSI) for the transmitter and receiver, as this is required to alter the signal for maximum effect.
• Spatial multiplexing: The spatial multiplexing function involves splitting a signal into multiple smaller streams. Each stream is transmitted on the same frequency channel, but from a different output antenna. These signals arrive at the receiver antenna array, each with different spatial signatures, and the receiver then separates the streams into parallel channels. If the transmitter has CSI, it can combine this technique with precoding to further improve the signal quality.
• Diversity coding: Diversity coding is a method used by the MIMO system when it has no CSI. Diversity coding involves transmitting a single stream using different diversity modes. The signal is then emitted from each transmit antenna, leveraging signal fading to improve signal diversity. If the transmitter has some knowledge of the channel, diversity coding can be used with spatial multiplexing to enhance the signal quality.

In addition to these methods, MIMO technology can further increase the throughput of a channel by adding antennas to the system. Signal throughput increases linearly with each pair of receive and transmit antennas added to the system.

MIMO Diversity Modes

There are several different diversity modes MIMO systems use in diversity coding. Each mode provides unique advantages, which are described in more detail below:

• Time diversity: Time diversity involves transmitting a message at different times.
• Frequency diversity: Frequency diversity involves transmitting signals using different frequencies. A system may achieve this by using various channels or technologies to send the signal.
• Space diversity: Space diversity is the primary basis for MIMO and utilizes the different positions of antennas on a router to leverage various signal paths.

MIMO uses space diversity but may also use one or both of the other modes to increase signal diversity based on the available resources.

MIMO Configurations

While MIMO technology is based on the principle of utilizing multiple inputs and outputs, other configurations can be used. Each format is defined by the number of antennas available and offers different advantages and disadvantages, making them optimal for various applications. The forms of antenna links are described below:

• SISO: SISO is the simplest form of a radio link. SISO operates as a standard radio channel, with one transmitter antenna sending a signal to a single receiver antenna. There is no diversity or additional processing involved with this system, giving it the advantage of simplicity. The disadvantage, however, is its limited performance — SISO systems are more prone to interference and fading, and their capacity is limited by channel bandwidth.
• SIMO: In this format, the transmitter has a single antenna, while the receiver has multiple. This format is also known as receive diversity. SIMO systems are used when a receiver system needs to receive signals from several independent sources and is used for short wave receiving stations. There are two forms of SIMO — switched diversity SIMO works by switching to the antenna receiving the strongest signal, while maximum ratio combining SIMO combines all received signals. SIMO is easy to implement, but significant processing is required on the receiver end.
• MISO: This form involves multiple transmitter antennas and a single receiver antenna. This format is also called transmit diversity. MISO works by transmitting the same data from the two transmitter antennas. The receiver then collects whichever signal is optimal. This system is also simple to implement, but the processing power is needed at the transmitter end instead of the receiver.
• MIMO: Finally, MIMO systems involve more than one antenna at either end of the radio link. Out of all forms, MIMO provides the best signal quality and data capacity. Simultaneously, it is the most expensive to implement, as it requires more robust processing and more antennas.

When choosing a MIMO radio system configuration, stakeholders must balance performance against cost, size, available processing, and battery life.

MIMO Technology on the Smart Radio

When MIMO technology first came out, only a few access points supported it, but now MU-MIMO systems are supported by a wide range of endpoint devices. These include smartphones, tablets, computers, and more. These devices also include modern radio systems.

Advanced radio systems are often used in Industrial and Defense applications to provide high-quality signals for communication. MIMO technology has proven to be highly beneficial for this industry, allowing for higher-quality signals and robust connectivity in environments that often prove challenging for traditional radio connections. For this reason, MIMO mesh radios are a staple of military and rugged applications. Today, MIMO radios, like the Smart Radio, are essential for drones, UGVs and other mobile robotics.

The Smart Radio offers a MIMO radio system designed to be the most advanced mesh router available. The Smart Radio leverages numerous types of antenna polarization schemes to improve signal diversity and maximize connectivity. For example, a Smart Radio can be configured with two linearly polarized antennas with a 90-degree orthogonal polarization. On the receiving end, the antennas can be configured to match that 90-degree position to receive the optimal signal. 

The flexible antenna configuration makes the Smart Radio ideal, for example, as a MIMO radio for unmanned vehicles, drones, and ground robots. These vehicles stream video back to users, meaning they rely on high-quality signals that can be achieved through MIMO with signal diversity.

Doodle Labs offers embedded, external, and pocketable versions of the Smart Radio, delivering MIMO capabilities to a range of applications. Key features of the Smart Radio include:

• Easy to use Graphical User Interface (GUI): This feature enables local and remote radio configuration for enhanced device management capabilities.
• Low SWaP-C: Low Size, Weight, Power, and Cost are essential elements of airborne applications, as drone manufacturers must ensure that their unmanned vehicles can fly efficiently without being weighed down. The Embedded Smart Radio is one of the lowest SWaP-C models in the industry.
• Advanced Mesh: The Smart Radio’s mesh is self-forming and self-healing. Multi-hop relay and multi-frequency mesh enhance network stability, simplify fleet coordination and management, and can expand signal range.
• Encryption: Protectingtransmitted data is critical, which is why the Smart Radio has built-in 256-bit Advanced Encryption Standard (AES) encryption. This protects information over air traffic. Most notably, the encryption engine complies with Federal standards from the National Institute of Standards and Technology (NIST), including NSA Suite B and FIPS 140-2 Level 2.