In this blog post, drawing inspiration from the principle by which our two eyes utilize information obtained from different vantage points, we’ll explore the principles of antenna diversity—a technique that enhances the reliability and performance of wireless communications using multiple antennas.
Why Two Eyes Determine the Position of Objects More Accurately
Why do humans have two eyes? The reason is simple: we can determine the position of objects more accurately by using both eyes. First, close one eye and look at an object. Next, close the other eye and look at the object. Finally, open both eyes and look at the object. In each case, the position of the object will appear slightly different. The most accurate information is obtained when both eyes are open. So, why is it possible to determine the exact position of an object when both eyes are open? This is because the positional information transmitted by a single eye always contains a certain margin of error. In other words, since it is difficult for humans to perfectly determine the position of an object with just one eye, they combine the different information obtained from both eyes to determine the object’s position more accurately.
This same principle is applied in the field of wireless communications. If you look at the wireless routers commonly found in homes, you’ll often see that they have two or more antennas. In wireless communication systems that use multiple antennas, the signal conditions received by each antenna can vary; therefore, by comparing, selecting, or combining the information received from multiple antennas, it is possible to enhance the reliability and performance of the communication. While performance does not simply improve in direct proportion to the number of antennas, using multiple antennas provides an advantage in coping with situations where signals weaken or become distorted. This technology is known in technical terms as “antenna diversity.”
Noise in Wireless Communications and Antenna Diversity
Let’s take a closer look at the scientific principles behind antenna diversity. Just as we cannot determine the exact location of an object with a single eye, information received in wireless communication is always subject to some degree of distortion. No matter what information a transmitter sends, the radio waves may be affected by the surrounding environment as they travel; the signal strength may weaken due to great distance; and the original signal may become distorted as the waves reflect off and overlap along multiple paths. Furthermore, other transmitted signals or electromagnetic interference may mix with the received signal. These factors, which interfere with communication or have an undesirable effect on the original signal, are collectively referred to as “noise.” The various methods we use to improve communication performance aim to reduce the effects of this noise and interference and to more accurately recover the desired information from the received signal. Antenna diversity can also be described as a technology that compares or combines the states of signals received by multiple antennas to obtain relatively stable information and reduce the effects of distortion and fading that occur during communication.
So, which information should be selected or combined from the signals received by multiple antennas? There are various ways to utilize signals received from multiple antennas. The simplest example is to take the arithmetic average of the signals. In actual antenna diversity, in addition to such simple methods, techniques such as combining signals by applying different weights based on the quality of the signal received by each antenna or the level of noise, or selecting the signal with the best quality from among the multiple signals, can be used.
Problems to Solve When Using Multiple Antennas
Several important assumptions are required when utilizing antenna diversity. First, the signals received by the multiple antennas must have different characteristics. If the signals received by each antenna are completely identical—including the noise levels—there is little benefit to using multiple signals.
This is because, whether using the arithmetic mean or selecting the signal with the least noise, if all antennas distort the same information to the same degree, it is ultimately difficult to escape the original distorted information. Therefore, it is important to maintain an appropriate spatial separation between the antennas so that each antenna experiences a different propagation environment. Doing so increases the likelihood of receiving a relatively good signal from one antenna even when the signal from another antenna is significantly weakened.
Second, time alignment and synchronization are necessary to compare or combine signals received from multiple antennas. Let’s assume a broadcaster is airing the line “The waters of the East Sea and Mount Baekdu…” from the national anthem. At a given moment, one antenna might be receiving “The waters of the East Sea,” while another might be receiving “Mount Baekdu.” This can occur because the physical distance between the transmitter and each antenna, as well as the paths the radio waves travel, differ. If information from different points in time is simply combined without synchronization, not only would it be impossible to find an “information equilibrium point,” but the original information could actually be further distorted. Therefore, the receiver aligns the signals by accounting for the time difference between the signals received from each antenna and, if necessary, uses memory and buffers to align the information received at different times before comparing or combining them.
Distance Between Antennas and Signal Synchronization
Antenna diversity utilizes the spatial differences between antennas to solve these problems. In other words, by placing the antennas at an appropriate distance from one another, each antenna experiences different propagation paths and fading environments, which can reduce the correlation between the signals. As a result, even if one antenna is significantly affected by noise or fading, the other antenna is more likely to receive a relatively stable signal. By utilizing these differences, it is possible to select the best signal from among several or to combine multiple signals appropriately to enhance communication reliability.
Additionally, storage devices such as memory can be utilized in the receiver’s signal processing to synchronize the time differences in the information received by multiple antennas. An antenna that receives information slightly earlier can temporarily store that information and, once the information from another antenna—which received it later—arrives, align the signals in time to compare or combine them. In the previous example, the antenna that first receives “Baekdusan-i” stores it in memory, waits until another antenna receives the same segment of “Baekdusan-i,” and then aligns the timing of the two signals. Thus, rather than simply combining signals from multiple antennas simultaneously, processing them while considering the time differences and states of each signal is a crucial aspect of antenna diversity.
Why Antenna Diversity Is Important in Wireless Communications
Antenna diversity is a wireless communication technology that can be understood by drawing a parallel to how the human eyes utilize information obtained from different positions. Using multiple antennas increases the likelihood of securing a relatively stable signal through other antennas even in situations where the signal from a single antenna is significantly weakened or affected by fading. It also offers the advantage of improving communication reliability by enhancing the receiving end’s antennas and signal processing methods. However, in modern wireless communications, multiple antennas are not used merely to receive the same information and calculate an average. Technologies such as MIMO utilize multiple antennas to combine signals while transmitting multiple spatial streams, or employ techniques like beamforming to increase signal strength and transmission efficiency.
These multi-antenna technologies continue to play a crucial role today in various wireless communication systems, including Wi-Fi and mobile communications. Antenna diversity and MIMO were utilized even in the previous 4G LTE environment, and in today’s 5G and the latest Wi-Fi environments, technologies that use multiple antennas to enhance signal reliability, transmission efficiency, and processing capacity continue to be employed. Ultimately, multiple antennas do not exist merely to receive more signals. Rather, they are designed to transmit information more accurately and reliably—even in unstable wireless environments—by leveraging the differences between signals received from different locations. Just as humans use their two eyes to obtain different visual information and thereby determine the location of objects more accurately, wireless communication can achieve more stable connections by gathering information from different radio wave environments through multiple antennas, compared to using a single antenna.
