Analysis of Working Principle of Smart Antenna Technology in Mobile Communication System

Smart antennas (SmartAntenna or IntelligentAntenna) were originally used in radar, sonar and military communications. In recent years, modern digital signal processing technology has developed rapidly, the processing power of DSP chips has been continuously improved and the price of chips has been declining. It has become feasible to form antenna beams in baseband by using digital technology, and the smart antenna technology has begun to be widely used in wireless communication. Since smart antennas can significantly improve the performance and capacity of the system and increase the flexibility of the antenna system, this technology will be adopted in almost all advanced mobile communication systems in the future.

Smart antennas fall into two broad categories: multi-beam antennas and adaptive antenna arrays. The multi-beam antenna covers the entire user area with multiple parallel beams, the orientation of each beam is fixed, and the beam width is also determined by the number of antenna elements. When the user moves in the cell, the base station selects among different corresponding beams to make the received signal strongest. Because the user signal is not necessarily at the center of the beam, when the user is located at the edge of the beam and the interference signal is at the center of the beam, the reception effect is the worst, so the multi-beam antenna cannot achieve optimal reception of the signal, and is generally only used as the receiving antenna. However, compared with the adaptive antenna array, the multi-beam antenna has the advantages of simple structure and no need to determine the direction of arrival of the user signal. The adaptive antenna array generally adopts a 4-16 antenna array structure, and the array element spacing is half a wavelength. Antenna array elements are distributed in a straight line, a ring shape, and a flat type. The adaptive antenna array is the main type of smart antenna, which can complete user signal reception and transmission. The adaptive antenna array system uses digital signal processing techniques to identify the direction of arrival of the user signal and form the main beam of the antenna in this direction.

Now, briefly describe how smart antennas overcome delay spread and multipath fading in wireless communications to improve system performance and capacity. Different antenna elements of the antenna array are given different weights to the signals, and then added to generate an output signal. If the definition of "antenna gain" is the reduction of the input signal power required for a certain output signal-to-noise ratio, the "diversity gain" is the reduction of the input signal-to-noise ratio required for a given bit error rate in the presence of fading, then In general, an M-ary antenna array can provide M times the antenna gain plus a diversity gain, and the specific increase is determined by the correlation between the antenna elements.

First we consider a multi-beam antenna. A multi-beam antenna is to place multiple antennas in one sector to cover the entire sector, and each antenna covers only a part of the angular range. Another advantage of the sector antenna is that the downstream beam direction can be utilized in the downstream beam direction, so that M times the antenna gain can be obtained in the downstream direction. However, due to its own form of configuration, the sector antenna has the following disadvantages: the overlapping portion between adjacent antennas has a 2dB reduction due to the relatively small antenna gain; when the antenna beam is erroneously locked due to multipath or interference, The beam antenna can provide a small gain.

In the adaptive antenna array, each antenna element can be placed in multiple forms, and the adjacent antenna element spacing is a specific value. When the received signal arrives at the antenna array, the signals on each array element are weighted differently and then superimposed to produce an output signal. The weighting coefficients and superposition can be based on different criteria. It should be noted that the array elements of the adaptive antenna must have similar beams, but there is no such requirement in multi-beam antennas. Similar to a multi-beam antenna, an M-ary adaptive antenna array can provide M times the antenna gain, but it has no disadvantage of partial overlap between beams. The M-ary adaptive antenna array can completely suppress N(N<M) interference users, and a large gain can be obtained even when N>M. Although adaptive antennas have more advantages for more beam antennas, the adaptive antennas must calculate the weighting coefficients at a rate comparable to the fading rate to track the user (note that the fading rate in 3G will reach 1000 Hz). However, switching between multiple beam antenna beams can be done only once in a few seconds.

A key issue in adaptive antenna arrays for use in wireless systems is the difference in performance between line-of-sight (LOS) and non-line-of-sight (NLOS) conditions. In the case of line-of-sight, the received signals are weighted and combined to produce an antenna pattern whose main beam direction is directed to the target user and interferes with the user in other directions. At this time, if the number of antenna elements is greater than the number of signals arriving, the modern spectrum estimation algorithm such as MU-SIC and ESPRIT can be used to estimate the angle of arrival of the signal, so that an array can form directions at least in M-1 directions. Figure zero point. In the case of non-line-of-sight, the arrival of wireless signals passes through multiple reflections and multiple paths (the relative delay between two different paths must be two independent paths). An important feature of adaptive antennas in multipath environments is the ability to suppress interference to the user regardless of the direction in which they arrive, that is, the adaptive antenna array can suppress interference to the user even if the interfering user and all users are only a few inches apart. This is because in a multipath environment, the object around the antenna is a huge reflective antenna, so that the receiving antenna array can distinguish signals from different users. In particular, if the receiving element spacing is sufficiently large, the array can form a beam that is smaller than the extended angle. The number of signals that can be distinguished increases with the number of antenna elements and the angle, and increases with the increase of the multipath reflection density within the extended angle. In this sense, multipath is a favorable factor. However, due to the delay spread, the adaptive antenna suppresses the delay signal as an interference signal. The M-element adaptive antenna can suppress the M-1 delay signal, which is a very unfavorable factor. In order to make full use of the received signal, we can add a time domain processing to each antenna element, such as adaptive equalization or RAKE reception, and then adaptive beamforming.