Some problems about radar.

1. Smart antenna interferes with Martin Cooper adaptive antenna array to move users through virtual line connection, which greatly improves wireless communication.

We are immersed in the ocean of radio frequency waves every day, and the invisible electromagnetic energy comes from different sources: broadcasting towers, cellular telephone networks and wireless communication of police. These radiations may be harmless to human body, but they will seriously affect our sending and receiving information. Excessive wireless energy is also a kind of pollution, because it will destroy useful communication. With the increasing frequency

of electronic communication, radio interference is becoming increasingly noisy. With the increase of the intensity of radio interference signals in our environment, we must increase the intensity of radio signals in order to distinguish useful signals from background electromagnetic noise.

one solution to this problem is to adopt a new type of RF antenna, which can greatly reduce human interference. Taking cellular telephone communication as an example, after adopting this brand-new antenna, we don't need to broadcast the user's call omni-directionally, but instead track the mobile user's position and send the wireless signal directly to him. This antenna system not only minimizes the interference of other users, but also maximizes the received signal strength of the target users. In fact, this is equivalent to establishing a virtual wired connection for each mobile user.

These systems are usually called smart antennas, and the most intelligent one among them is also called adaptive antenna array. In 1992, I co-founded Airy Company in San Jose, California, USA, and devoted myself to developing adaptive antenna arrays that can be applied to existing and new wireless networks. Each array contains as many as 12 antennas and a powerful digital processor (for combining and processing input and output signals). Lucent, Nortel and other companies are also developing this technology. Our goal is to reduce the cost and improve the quality of wireless communication. Now adaptive antenna arrays have provided these benefits to millions of cellular phone users. In addition, because they are very suitable for the transmission and reception of large amounts of data, they are likely to become a key part of the wireless Internet.

the physical principle of antenna

to understand the working principle of smart antenna, we must first understand the "clumsy" common antenna. The RF antenna converts the current and voltage signals generated by the transmitter into electromagnetic waves and emits them. At the same time, the antenna can intercept these electromagnetic waves and convert them into current and voltage signals that can be processed by the receiver. The simplest and most commonly used antenna is dipole, which is just a rod with a specific length that can emit energy to space in all directions. In the process of radio wave propagation in the air, the intensity gradually weakens and is absorbed by obstacles such as air, trees and buildings.

Commercial radio and TV stations must provide services to geographically dispersed users, so it is natural to broadcast omni-directionally, while a cellular telephone communication is usually aimed at only one user. In a cellular network, users communicate with the nearest base station, and there is a set of antennas in the base station to handle all wireless service signals in the surrounding area (called cell). Base stations are set up according to certain rules, so that the whole coverage area can be divided into multiple cells; When the user moves from one cell to another, the system can automatically switch the call to other suitable base stations. In this case, if wireless energy can be concentrated on a single user, it will be much more efficient, just as a flashlight concentrates light into a beam through a mirror. With the same power, the bundled signal can travel much farther than the omni-directional signal. The beams sent by the base station to different users are separated in space, so the mutual interference is also reduced.

Reflectors can focus radio waves into beams, but they are really bulky and expensive. So engineers have come up with many ways to generate wireless beams without reflectors. If we place two antennas side by side, and the distance between them is half the wavelength of the wireless signal, then the pattern of energy emitted by this simple array is 8-shaped when viewed from above. In two directions perpendicular to the array (that is, perpendicular to the connecting line between two antennas), the transmission distance of radio waves will reach the maximum, because users can receive the signals sent by two antennas at the same time in these two directions (in other words, the two signals are in phase). However, in the direction parallel to the array, the user will receive two signals with a phase difference of 18 degrees. When the peaks and valleys of the two signals meet, they cancel each other out, thus creating a null zone where no signal will be detected.

The beam of this antenna array composed of two antennas is quite wide, and it will also emit in two opposite directions. By adding more antennas, the beam width becomes narrower and narrower. Since World War II, this type of phased array antenna has been used to focus radar beams. Although the increase in the number of antennas makes the beam narrower, it also produces more sidelobes beside the main beam. Depending on the user's direction, the beam signal may be stronger ("gain") than the signal transmitted by a single antenna, or it may become weaker ("loss") due to the cancellation effect.

a wireless beam is still of little use if its orientation cannot be directed at a specific receiver. The most obvious solution is to move the antenna array itself, but obviously this method is clumsy and expensive. It will be much easier to manipulate the beam electronically. Through a technique called beam switching, the antenna array can generate a set of overlapping beams, which together can cover the surrounding area. When a cellular phone user makes a call, the wireless receiver first determines which beam direction the user's signal is the strongest, and then the array transmitter "answers" the user according to this incoming wave direction. If the user walks from the original beam to another adjacent beam, the control system will automatically switch both transmission and reception to that new beam.

However, beam switching still can't work well in the real wireless communication environment. The beam is most effective only when the user is in the center of the beam. Just as the direction of light leaving the flashlight will dim, once the user leaves the beam center, the signal will fade. When the user is close to the edge of the beam, the signal strength will decline considerably before the system switches it to the adjacent beam. What if a user in the other direction needs to use the same wireless channel? If the second user is in the zero field, it will not bring interference to the previous user, but once he is in the center of a sidelobe, the signal given to him will block or distort the signal of the previous user.

another problem of beam switching system is that in almost all environments, wireless signals rarely propagate along the direct path. The signal received on our mobile phone is usually a combination of multiple reflected signals. Reflectors may be natural or man-made objects (buildings, mountains, cars and trees, etc.). These signals are constantly changing, especially those caused by large vehicles (such as buses). This so-called multipath phenomenon will also affect the signal sent from the mobile phone to the base station. In the beam switching system, if the user is close to the edge of the beam, his or her transmitted signal may be bounced back to other beams before reaching the antenna array. In this case, the antenna array may send the wrong beam, and the user may not get the response signal at all.

in practical application, it is obviously not enough to only have a beam switching system. A truly intelligent antenna array should be able to directly give a beam to the mobile user, instead of choosing a beam relatively close to the user. The ideal antenna array must also be able to adjust the beam pattern to minimize the interference from other users on the same frequency channel. Finally, this antenna array must be able to respond quickly according to the rapid changes of user position and reflection. These are the reasons why adaptive antenna array should be introduced.

cocktail party effect

what makes adaptive antenna arrays so intelligent? The most critical factor lies in the processing of the signal received by the antenna, which is just like the processing of the sound information received by the ear by the human brain. A person with normal hearing, even if his eyes are blindfolded, can usually locate the sound, because those coiled folds on the external ear will produce different sounds according to the direction of the sound. Unless the sound comes from directly above or directly below the head (or directly before and after), it will arrive at both ears in a different time. Based on these received information, the brain can quickly calculate the position of the sound source.

In addition, people with normal hearing can extract relatively light sounds from noisy environmental noise, such as a conversation he is interested in. This effect is often called cocktail party effect. Researchers have found that the ability to focus on a certain sound comes in part from the ability to locate the sound source. In an experiment to test people's ability to distinguish signals in background noise, compared with the test object with one ear, the test object with two ears can hear much lighter sound. Once the brain can locate the sound source, it can concentrate on the sound and ignore the noise from other different directions.

similarly, the adaptive antenna array can accurately locate the source of wireless signals. By eliminating other interfering signals, useful signals can be selectively amplified. The "brain" of the antenna array is a digital processor that can process the signals received by the antenna. A typical adaptive array contains 4 to 12 antennas, but for simplicity, let's look at an array with two antennas. The distance between antennas is half the signal wavelength. For an ordinary array, the signals from two antennas are just ordinary addition; However, in the adaptive array, two parts of the signal are sent to the processor, where arbitrary mathematical processing can be performed on the signal.

for example, suppose that the array is placed north and south, and the signal from a mobile phone user comes from the east. The processor can quickly determine the direction of the signal: because electromagnetic waves reach two antennas at the same time, they must come from a direction perpendicular to the array. In order to maximize the received signal, the processor adds the two signals to double their intensity. When the signal is sent back to the user, the same signal will be sent on the two antennas of the array.

now let's assume the following situation: another mobile phone user sends a signal from the south. Because there is a 18-degree phase difference between the electromagnetic wave reaching the north antenna and the electromagnetic wave reaching the south antenna, the processor knows that the signal comes from a direction parallel to the array. At this time, the processor subtracts the two signals, that is, changes the polarity of the signal received by the north (or south) antenna, turns the peak into a trough (or vice versa), and then adds the obtained mirror image signal to the signal received by the south (or north) antenna. Similarly, the strength of the signal is doubled. When the array transmits a signal to the user, the processor sends an inverted signal to an antenna, thus generating a beam from north to south. Please note that in the above two examples, the beam for one mobile phone user will not reach another mobile phone user. These two users can communicate with the adaptive array on the same frequency at the same time, and their signals will not interfere with each other. The array processor can also generate beams in other directions by using more complex mathematical operations on the signals from two antennas. The problem of selective transmission and reception is transformed into the problem of solving a series of simultaneous equations here. For those mobile users, the processor must solve these equations repeatedly according to the constantly updated information.

Adding more antennas to the adaptive array will increase the positioning accuracy and signal gain. An array with 12 antennas can hear a signal 12 times weaker than that of a single antenna. The array can send signals at 12 times the intensity and has much greater directivity. The processor can process the signal received by the antenna to generate a beam pattern, so as to maximize the gain of a desired signal and ensure the maximum blocking effect on other signals in the same frequency band.

because the processor is fast enough to handle such tasks many times in one second, the array can continuously adjust the beam when the mobile phone user walks or drives through the coverage area of the antenna array. The design of the system ensures that the stray resonance of vehicles and buildings to user signals will not cause drastic changes in beam direction. By tracking the user's path, the array can estimate the user's next movement direction and eliminate the wrong information indicating the sudden change of the user's position.

In addition, the more advanced adaptive antenna array can further focus the wireless signal by using the multipath phenomenon. The capabilities of these processors are so powerful that we can make use of the signals transmitted in various paths between the adaptive antenna array and the mobile phone. By introducing multipath components into the mathematical equation, the processor can not only calculate the direction of arrival of the signal, but also calculate the exact position of the user. In an urban environment with abundant reflectors, the adaptive array can receive and send signals from a small area around the mobile phone. In this case, the antenna array produces no longer a beam, but a "personal cell" with a radius of only a few centimeters. Because the array can repeatedly calculate the position of the mobile phone, the personal community can move with the mobile phone users.

advantages and applications

compared with traditional cellular networks, wireless networks with adaptive antenna arrays have many advantages. For the same power, the coverage of base stations equipped with adaptive arrays is much larger than that of ordinary base stations, thus covering the same area, and the number of required base stations is correspondingly reduced. Although adaptive arrays may be more expensive than traditional antennas, reducing the number of base stations can dramatically reduce the cost of setting up and operating wireless networks. Adaptive array enables cellular service companies to make better use of scarce resources: the spectrum allocated to the company. Many cellular networks are overloaded due to the increase in the number of users: in some crowded areas, sometimes the signal volume of generate at the same time exceeds the limited number of wireless channels in the system. Users can feel this tension when the call is dropped or the signal quality is degraded. Since the adaptive array allows some users within the coverage of the same base station to use the same wireless channel at the same time, it increases the capacity of the spectrum. Compared with the common antenna, this improvement is remarkable: for voice service, the user capacity of the base station equipped with adaptive array is increased by 6 times; For data services, this figure is as high as 4 times. The result of adopting adaptive antenna array is better service and less interference, in addition to less energy waste and radio frequency pollution.

in this way, we won't be surprised by the commercial application of adaptive antenna. In Japan, China, Thailand and other parts of Asia and Africa, more than 15, base stations have been equipped with antenna arrays using Airui technology, providing telephone services to more than 15 million people in total. The commercial application of adaptive array in the United States and Europe is relatively slow, which is partly due to the reduction of new investment in cellular networks caused by the sluggish telecommunications industry. But there is still an American manufacturer (Airnet Company in Montreal, Florida) that is producing cellular base stations using the technology of Airy Company. At the same time, Marconi, a British telecommunications company, is also developing an advanced base station with adaptive array.

adaptive arrays are also a great boon for wireless data networks. Because this array can minimize interference, more data can be transmitted and received in a given frequency range. A base station equipped with an adaptive antenna array can serve 4 users at the same time.