Smart antenna technology can effectively improve the transmission quality of wireless channels
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An antenna array is an antenna system consisting of a group of transmitting elements. If these antenna elements are identical, such as in an omnidirectional array, and if they are spaced equidistantly from each other on one axis, then the array antenna is called a linear array. Furthermore, if these elements are excited by current on the same wire, then the antenna elements can be adjusted synchronously, and the individual elements of the linear array are balanced and consistent. Antenna Array Figure 1 shows a four-element linear array, where each element is an omnidirectional antenna, separated by half a wavelength, assuming that the same current is used to drive each element (same amplitude and phase excitation). Using multiple elements to form an antenna array has a larger effective size than a single omnidirectional antenna element, and this antenna has a more directional overall radiation pattern. The radiation pattern of the antenna array is the sum of the radiation patterns of each element, and the radiation pattern of each element is shown as the dotted circle drawn in Figure 1. Directional gain is generated on the midline between these elements. It is worth noting here that the same set of physical array elements can form different antenna radiation patterns, which allows the receiver to adjust the beam direction to achieve correct signal reception. This also makes it possible to produce such antennas in such a way that one antenna size can be suitable for multiple uses. That is, a basic unit structure can be established, and the radiation pattern formed by the unit can be changed by the user by adjusting the unit excitation method. Let's discuss the issue of directivity further. Assume that the array in Figure 1 is actually composed of eight antenna elements, with a spacing of half a wavelength, and half of the elements are not excited. Now, three more elements are excited to form a seven-element array system, and the spacing between these elements is still half a wavelength (Figure 2). Increasing the size of the antenna to increase the directivity of the radiation pattern is achieved by increasing the length of the antenna. The result is a narrower beam and higher directional gain. Since only the number of elements is increased here, the increase in directional gain occurs in the same direction. Accordingly, the gain is greatly reduced at locations deviating from the center of the radiation direction, so the allowable error in directional alignment will be very small. Another advantage of this array antenna is that the system can change the signal transmission direction without changing the structural design. The directional gain in Figure 1 is concentrated on the centerline of the array because, in this case, the excitation phase is the same as the radiation pattern phase (not shown). Zero phase difference means that the superposition of the radiation patterns is consistent, so a system consisting of four or seven elements is intuitive. In the absence of identical excitation currents, the excitation current of each element is intentionally phase-shifted one by one, so that the phase sum will be at an angle to the center of the array. One method of achieving phase adjustment at present is to use a programmable phase shifter, and its implementation principle is very simple, that is, to continuously send instructions to the phase shifter to make the beam change direction quickly. Therefore, if this feature is used reasonably, the system has obvious advantages. Digital Beamforming A beam former is an antenna structure that can control the excitation of a cell in both amplitude and phase. Figure 2 provides a basic example of beamforming. Figure 1 is changed to the system of Figure 2, which adds three excitation cells. The three cells that are "off" in Figure 1 simply program their excitation signals to zero. There are two variables in the excitation signal that can be changed: amplitude and phase. These two quantities increase the flexibility of control, adjust the roll-off characteristics and side-lobes of the main beam, enhance the directivity of the signal, and reduce signal interference. The antenna elements have two variable quantities: amplitude and phase, which can be represented by a complex exponential, commonly known as the complex weight Wk, where the subscript k is the number of linear elements. The above discussion shows that phase shifting can be achieved by programming electronic phase shifters, without having to insert a certain length of cable or using passive circuits to achieve phase shifting. A variable gain amplifier (VGA) is used, which can be driven by a command word that adjusts its gain and has certain switching constraints. The combination of these adjustable variables together defines the structure of the beamforming. Application of digital technology Implementing the VGA and phase shifter units in DSP form opens the door to embedding various intelligent features in beamforming. Complex mathematical algorithms and closed-loop dynamics can provide antenna tolerance and reliability, as well as upgrade flexibility. These mathematical implementations can be faster and more efficient than analog methods. The signals picked up by the antenna are generally RF signals, microwaves and millimeter waves, and it is obvious that these frequencies cannot be processed by current digital processing technology. Therefore, a good digital modulator and demodulator generally have a digital circuit to perform the modulation/demodulation function. In communication systems, this work is done on the demodulator. In advanced communication systems, the receiver has an RF front end to perform the signal down-conversion, converting it to a frequency that can be processed by the A/D converter. The rest of the frequency conversion is achieved digitally. Thus, we have complex weights and powers that produce an antenna radiation pattern with a wide range of variations in both directional gain and direction. If the weights are adaptive, we can maximize their effectiveness by automatically correcting them under closed-loop control and optimize some functional characteristics, such as maximizing the signal-to-noise ratio. The situation discussed above is similar to that of an adaptive equalizer, since it also uses complex weights to reduce inter-symbol interference. In an equalizer, we send a "training sequence" to familiarize the equalizer with the channel characteristics, and similarly, in an adaptive beamformer, we use similar techniques to optimize its radiation pattern. Note that inter-symbol interference is affected by multi-channel propagation. Equalizers and adaptive antennas improve the performance of the system, and adaptive antennas can eliminate the effects of multi-channel interference and prevent it from reaching the receiver. Therefore, it can be seen that the digital algorithms developed to support adaptive equalization can also be used to support beamforming. Conclusion If an antenna can enhance signal strength in the above manner, or by adjusting complex weighting factors to increase the cell segmentation size, generate beam peaks and nulls in the radiation pattern, we call this antenna a smart antenna. The impact of smart antennas on wireless systems exists in two aspects: first, there are many decibels of available signal strength on directional antennas compared to other link budgets; it uses advanced hard-core technology to reduce a few additional decibels of noise from the receiver front-end design, or reduce 1dB of digital modem loss. Although the signal energy in the cell was uniformly radiated when the signal was transmitted before, most of the energy was wasted by reflection between leaves and buildings, and the smart antenna uses a precise beam to aim at the user antenna, which means higher signal strength and less signal interference. This controllable radiation pattern is perfectly suited to the dynamic characteristics of the wireless channel, because the characteristics of the wireless channel are usually in constant change. This technical feature is taken into account in the design process of the third-generation wireless system. By Rob Howald Director of Systems Engineering Motorola Broadband Communications Division Transmission Network Systems Group
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