LESSON 4
Memorize the following basic vocabulary and terminology to text 4A:
inhibit v — задерживать, тормозить disrupt v — разрушать
din — шум
pursue v — наступать, следовать за subtract v — вычитать
wave trough — минимальная амплитуда огибающей бегущей волны
Text 4A
Adaptive antenna arrays
Each of us is immersed in a sea of radio-frequency waves. The invisible electromagnetic energy comes from many sources: broadcast towers, cellular-phone networks and police radio transmissions, among others. Although this radiation may be harmless to our bodies, it can severely inhibit our ability to receive and transmit information. Excess radio energy is a kind of pollution, because it can disrupt useful communications. As the intensity of radio-frequency interference in our environment grows we have to raise the volume of radio signals so that they can be heard over the electromagnetic background noise. And as our electronic communications become more intense, they simply add to the din of radio interference.
One solution to this problem lies in a new class of radio antennas that could dramatically reduce man-made interference.
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Instead of wastefully broadcasting personal communications such as cell-phone calls — in all directions, these innovative antennas track the positions of mobile users and deliver radio signals directly to them. These antenna systems also maximize the reception of an individual cell-phone user's signal while minimizing the interference from other users. In effect, the antennas create a virtual wire extending to each mobile phone.
These systems are generically referred to as smart antennas, but the smartest members of the class are called adaptive antenna arrays. In 1992 ArrayComm, a San Jose, Calif., company focused on developing adaptive arrays that can be incorporated into both new and existing wireless networks. Each of our arrays consists of up to a dozen antennas and a powerful digital processor that can combine and manipulate the incoming and outgoing signals. The technology, which is also being pursued by Lucent Technologies, Nortel Networks and other firms, promises to decrease the cost and improve the quality of wireless communications. Adaptive antenna arrays are already providing these benefits to millions of cell-phone users. Moreover, these smart antennas may become the linch-pins of the wireless Internet because they are ideally suited to transmitting and receiving large amounts of data.
What makes the adaptive array so smart? The key step is processing the information received by its antennas. An adaptive antenna array can pinpoint the source of a radio signal and selectively amplify it while canceling out competing signals. The array's brain is a digital processor that can manipulate the signals coming down the wires from the antennas. A typical adaptive array contains 4 to 12 antennas, but for simplicity's sake let us consider an array of two antennas, separated by a distance equal to half the wavelength of the radio signal. In an ordinary array the signals from the two antennas are just added together, but in an adaptive array the signals are sent to the adjoining processor, which can for many number of mathematical operations on them.
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For example, suppose that the array is aligned north to south and a signal from a cell-phone user comes in from the east. The processor can quickly determine the direction of the signal because the radio waves reach both antennas at the same time, they must be coming from a direction perpendicular to the array. To maximize reception the processor adds the signals together, doubling their intensity. When transmitting back to the user, the array emits identical signals from both antennas.
But now suppose that another cell-phone user sends a signal from the south. Because the radio waves hitting the north antenna are 180 degrees out of phase from antenna, the processor can tell that the signal is coming from a direction parallel to the array. So the processor now subtracts one signal from the other — that is, it inverts the signal from the north (or south) аntenna, turning wave peaks into wave troughs and vice versa, and adds this mirror image to the signal from the south (or north) antenna. Again, the signal's intensity is doubled. And when the array transmits back to the cellphone user, the processor sends an out-of-phase signal to one of the antennas, generating a radio beam that runs from north to south.
Notice that in both these examples the radio beam generated for one cell-phone user does not reach the other. The two users could be communicating with the adaptive array at the same time and on the same frequency channel, but their signals would not interfere with each other. The array's processor can create radio beams pointing in other directions as well by performing more complex mathematical operations on the signals from the two antennas . The task of selective transmission and reception is thus reduced to solving a series of simultaneous equations. To direct beam at users who are moving around, the processor must repeatedly solve the equations with constantly updated information from the antenna array.
(4133)
Answer the following questions.
1. What are the sources of electromagnetic energy? 2. Why is it necessary to raise the volume of radio signals? 3. What led to
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the appearance of a new class of radio antennas? 4. What does each of adaptive antenna arrays consist of? 5. What are the significant features of adaptive antenna arrays? 6. What is the difference between the signals in an ordinary array and the ones in an adaptive antenna array? 7. What do the examples in the text show? 8. In what way is the signal intensity doubled in both examples?
Task 1. Tell about the benefits of a new class of radio antennas.
Task 2. Explain what makes the adaptive array so smart. Task 3. Discuss the role of the processor in adaptive
antenna array with your fellow students and explain your point of view to the class.
Memorize the following basic vocabulary and terminology to text 4B:
faint adj — слабый, неясный juggle v — изменять
trigger v — запускать, вызывать discard v — отбрасывать
leap n — скачок
erroneous adj — ложный, ошибочный bounce v — ударяться
Read text 4B and give the title to the text.
Text 4B
Adding more antennas to the adaptive array increases the locating precision and the gain of the signal. An array with 12 antennas can hear signal a dozen times as faint as those that can be heard by a single antenna. The array can also transmit 12 times as loudly and much more directly. And the processor can juggle the antenna signals to create beam patterns that ensure the
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greatest possible gain for a desired signal and the greatest possible rejection for other signals on the same frequency.
Because the processor is fast enough to perform this task many times a second, the array can continually readjust the radio beam as the cell-phone user walks or drives across the array's coverage area. The system is designed so that stray reflections of the user's signal against vehicles or buildings do not trigger abrupt changes in the direction of the beam. By tracking the user's route, the array can estimate the like hood of future movements and discard erroneous information indicating sudden leaps in position.
Furthermore, the most advanced adaptive arrays can take advantage of the multipath phenomenon to focus radio signals still further. The processors in these arrays are so powerful that can handle information from all the reflected signals that bounce along various routes between cell phone and the adaptive array. By including the multipath reflections in the mathematical equations, the processor can extrapolate not only the direction of the signal, but also the exact position of the user's cell phone. In an urban environment where there are numerous reflections the adaptive array can receive numerous reflections from and transmit to a small area surrounding the phone. Instead of generating a radio beam, the array creates a “personal cell” that can be only centimeters in radius. And because the array is constantly recalculating the phone's position this personal cell can follow the user as he or she moves about.
(1624)
Task 1. Make questions to the text and address them to the other students.
Task 2. Give the proper name to the text.
Task 3. Tell about the advantages of an array with 12 antennas.
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