Introduction to Smart Antenna Design in Mobile Communication

The antenna is an indispensable component of mobile communication and plays an important role. It is located between the transceiver and the electromagnetic wave propagation space, and achieves effective energy transfer between the two. By designing the radiation characteristics of the antenna, it is possible to control the spatial distribution of electromagnetic energy, improve resource utilization, and optimize network quality. Especially in the development of 3G, Smart Antenna has become a hot spot in the research of the international mobile communication community.

1. Symmetrical oscillator and antenna array

At present, the antenna form used in mobile communication is mainly a line antenna, that is, the length l of the antenna radiator is much larger than its diameter d, and the basis of the line antenna is a symmetric vibrator. When the wavelength determined by the frequency of the high-frequency current change of the wire is much larger than the length of the wire, the amplitude and phase of the current on the wire can be considered to be the same, but only its

The value changes sinusoidally with time t. This short wire is called a current element or a Hertzian dipole. It can be used as a stand-alone antenna or as a component of a complex antenna. The electromagnetic field of a complex antenna in space can be seen as the result of the superposition of electromagnetic fields generated by many current elements. The radiant power of the current element is the average value of the electromagnetic energy radiated outward through the ball in a unit time. The energy of the radiation field will no longer return to the source, so it is an energy loss for the source. Introducing the concept of circuit, we use the equivalent resistance to represent this part of the radiation power, then this resistance is called the radiation resistance, and the radiation resistance of the current element is

RΣ=80π2(l/λ)2(1)

The directional pattern of the current element can be obtained by integral calculation: when l/λ<0.5, the directional pattern becomes sharp with the increase of l/λ, and only the main lobe, the main lobe is perpendicular to the vibrator axis; When l/λ>0.5, the side lobes appear. With the increase of l/λ, the original side lobes gradually become the main lobe, and the original main lobe becomes the side lobes; when l/λ=1 The main lobe disappears. This change in directionality is mainly caused by changes in the current distribution on the vibrator.

A plurality of symmetric vibrators are combined to form an antenna array. According to the arrangement of symmetric vibrators, the antenna array can be divided into a linear array, a planar array and a stereo array, and different arrays have different array factors. According to the principle of directional multiplication, the same symmetrical vibrator is used as the unit antenna of the antenna array, and different directional characteristics can be obtained by changing the arrangement position or the feeding phase. The base station high-gain omnidirectional antenna in mobile communication is to arrange the vibrators coaxially, compress the beam width of the vertical plane, and concentrate the radiant energy in the direction perpendicular to the vibrator to improve the gain of the antenna.

2. Directional characteristics and gain of the antenna

The directional characteristics of the antenna can be described by a directional pattern, but the directionality coefficient D is often used to indicate the degree of concentration of the radiated electromagnetic energy of the antenna. It is defined as the power flux density (the electric field power passing through a unit area, proportional to the square of the electric field strength) of a directional antenna at a certain point in the far direction of the maximum radiation direction at the same radiated power, and the non-directional antenna The ratio of power flux density at this point:

Since the loss of the antenna itself is very small, it can be considered that the radiated power of the antenna is equal to the input power, that is, the antenna efficiency η=100%, then the antenna gain G=η·D=D, that is, the antenna gain and the directivity coefficient of the antenna are Numerically equal.

To increase the gain of the antenna, it is mainly necessary to reduce the lobe width of the radiation in the vertical plane while maintaining the same radiation characteristics on the horizontal plane. The effect of the change in the length of the vibrator on the gain is very limited. The antenna array is currently the main means to achieve high gain. The linear array is the simplest and most practical omnidirectional antenna array. On the same axis consistent with the vibrator axis, a plurality of radiating elements are arranged at a certain distance, and an enhanced radiation field can be obtained on a plane perpendicular to the axis. However, to get the best results, the spacing between the vibrators and the phase of the feed must be properly selected. As the radiating element, a half-wave vibrator or other radiation source having omnidirectional performance in a horizontal plane, such as a folded vibrator or various coaxial antennas, or the like can be used. Coaxial antenna array is a commonly used high-gain antenna for base stations. It requires each radiating element to receive equal-amplitude in-phase feeding. There are two types of feeding modes: feed and cross feed. Another high-gain omnidirectional antenna directs multiple directional antennas to different orientations to form approximate omnidirectional radiation. However, when the antenna is to be placed in the middle section of a large iron tower, the directionality of the coaxial antenna array will be destroyed due to the reflection of the tower body. At this time, the directional antenna array arranged around the tower body can solve this problem. . More importantly, when frequency multiplexing is performed in a cellular communication system, the directional antenna can better reduce the same and adjacent frequency interference and improve the frequency reuse rate. A 120° corner reflector or a 120° plane reflector can be used in a 120° sector cell, and a 60° angle reflector can be used in a 60° sector cell.

Omnidirectional antennas are generally used in networks with a small number of mobile users, or in areas with low user density, such as suburban and rural areas. The horizontal plane pattern should be 360°, and the vertical half-power beam width is different depending on the gain of the antenna. It can have 13° or 6.5°. Directional antennas are generally used in areas with high mobile user density, such as urban areas, stations, commercial centers, etc., and their horizontal half-power beamwidths are generally 65°, 90°, 105°, 120°, and vertical half-power beamwidth. Depending on the gain of the antenna, there may be 34°, 16° or 8°.

3. Use diversity techniques to increase gain

Due to the poor propagation environment, the wireless signal will generate deep fading and Doppler shift, etc., so that the receiving level drops to the vicinity of the thermal noise level, and the phase also randomly changes with time, resulting in a decline in communication quality. In this regard, we can use diversity reception technology to mitigate the effects of fading, obtain diversity gain, and improve reception sensitivity. Diversity antennas have spatial diversity, direction diversity, polarization diversity, and field component diversity. Spatial diversity is achieved using multiple receive antennas. At the origin, one antenna is used for transmission, and at the receiving end, multiple antennas are used for reception. The distance between the receiving antennas is d ≥ λ/2 (λ is the operating wavelength) to ensure that the fading characteristics of the output signals of the receiving antenna are independent of each other, that is, when the output signal of a certain receiving antenna is low, The output of other receiving antennas does not necessarily have a low amplitude at the same time. The corresponding combined circuit selects one channel with a larger signal amplitude and the best signal-to-noise ratio, and obtains a total receiving antenna output signal. This reduces the impact of channel fading and improves the reliability of the transmission. The technology is used in analog frequency division mobile communication systems (FDMA), digital time division systems (TDMA), and code division systems (CDMA).

The advantage of spatial diversity reception is that the diversity gain is high, and the disadvantage is that a separate receiving antenna is needed. In order to overcome this drawback, a directional dual-polarized antenna has recently been produced. In the mobile channel, two signals from two antennas that are orthogonal to each other at the same location exhibit mutually independent fading characteristics. Using this feature, two pairs of vertically polarized and horizontally polarized transmitting antennas are installed at the same location at the same end, and two pairs of receiving antennas, vertically polarized and horizontally polarized, are mounted at the same end of the receiving end, and two fading characteristics can be obtained. Uncorrelated polarization components Ex and Ey. The so-called directional dual-polarized antenna integrates two pairs of vertical polarization and horizontal polarization receiving antennas into one physical entity, and achieves the effect of spatial diversity reception through polarization diversity reception. Therefore, polarization diversity is actually a special space diversity. Happening. The advantage of this method is that it requires only one antenna, is compact and saves space. The disadvantage is that its diversity reception is lower than that of the spatial diversity receiving antenna, and since the transmission power is distributed to two antennas, it will cause 3 dB. Signal power loss.

The diversity gain depends on the uncorrelated characteristics between the base station antennas, and spatial diversity is achieved by separating the antenna positions in the horizontal or vertical direction. The spatial separation of the positions ensures that the two-side receiving antennas respectively receive the mobile station signals from different paths, and also satisfies the requirement of a certain isolation between the two-sided antennas. If a cross-polarized antenna is used, this isolation requirement also needs to be met. For polarization-diversity dual-polarized antennas, the orthogonality of the two cross-polarized radiation sources in the antenna is a major factor in determining the uplink diversity gain of the wireless signal. The diversity gain depends on whether the two cross-polarized radiation sources in the dual-polarized antenna provide the same signal field strength in the same coverage area. The two cross-polarized radiation sources are required to have good orthogonal characteristics and maintain good horizontal tracking characteristics throughout the 120° sector and switching overlap region, replacing the coverage achieved by the spatial diversity antenna. Most cross-polarized antennas have good electrical characteristics in the direction of the main lobe of the antenna field map, but for the base station antenna, it is also required to maintain good cross-polarization characteristics in the edge of the cell and in the switching overlap region. In order to achieve good coverage, the antenna is required to have high cross-polarization resolution over the entire sector. The orthogonality of the dual-polarized antenna over the entire sector, that is, the uncorrelation of the signals of the two diversity receiving antenna ports, determines the overall diversity effect of the dual-polarized antenna. In order to obtain better signal-unrelated characteristics at the two diversity receiving ports of the dual-polarized antenna, the isolation between the two ports is usually required to be more than 30 dB.

The diversity antenna separates the multipath signals so that they are not correlated with each other, and then combines the separated signals by combining techniques to obtain the maximum SNR gain. Commonly used merge methods include selective merge, switch merge, maximum ratio merge, equal gain merge, etc., which are not discussed in detail in this paper.

1. Limitations of traditional antennas

In recent years, with the continuous development of communication requirements, smart antenna technology has become the focus of attention, which has helped wireless network operators achieve two valuable purposes: to provide higher data transmission rates and increase network capacity. In GPRS, EDGE and 3G networks, operators are beginning to use wireless networks to provide packet data services to users. As with voice services, data traffic requires a certain quality of wireless signal to achieve the specified transmission rate, depending on the carrier-to-interference ratio (C/I) of the network. If the carrier-to-interference ratio is too low, the transmission rate and quality of service will be seriously affected. In the middle and later stages of the GSM network, the system capacity will continue to increase, the cell will continue to split, and the increased interference will hinder the further increase of the system capacity. The traditional omnidirectional Antennas and directional antennas are no longer sufficient. The smart antenna uses digital signal processing technology to generate spatially oriented beams, providing each user with a narrow directional beam, enabling signals to be transmitted and received in an effective direction region, making full use of the effective transmit power of the signal and reducing the omnidirectional emission of the signal. The electromagnetic pollution and mutual interference increase the carrier-to-interference ratio, and the carrier-to-interference ratio is improved, which can provide higher data transmission rate and larger network capacity.

Interference is an important factor in the performance and capacity limitations of cellular systems. It causes crosstalk, lost calls, or dropped calls and distracts users. The most important thing is that interference limits the tightness of the frequency that operators can reuse. The extent to which the communication bearer capacity is extracted from the fixed RF spectrum. Interference may come from another mobile terminal, other cellular sites operating at the same frequency, or out-of-band RF energy leaking into the allocated spectrum. The most common types of cellular interference are co-channel interference and adjacent channel interference. Co-channel interference is caused by the transmission of non-adjacent cells using the same frequency. This interference is most pronounced when approaching the cell boundary, where the physical separation from neighboring cells using the same frequency is at a minimum. Adjacent channel interference is caused by leakage of user channels from neighboring cells using adjacent frequencies. This can occur on adjacent channels when the user is operating very close to the telephone user receiver, or when the user signal is significantly weaker than the signal of the adjacent channel user. The carrier-to-interference ratio is an important indicator of call quality. For users, a higher C/I ratio is lower interference, less dropped calls, and improved audio quality. For operators, a higher C/I The I ratio allows for signal range extension and tighter frequency reuse, thus increasing the overall system capacity.

2. Multi-beam smart antenna

A smart antenna is an antenna array. It consists of N antenna elements, each of which has M sets of weights, which can form M beams in different directions, and the number of users M can be greater than the number N of antenna elements. According to the shape of the antenna pattern used, smart antennas can be divided into two 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 depends on the number of elements. As the user moves in the cell, the base station selects different beams accordingly, so that the received signal is strongest. However, since its beam is not arbitrarily pointed, it can only partially match the current transmission environment. When the user is not at the center of the fixed beam and 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. However, compared with the adaptive antenna array, it has the advantages of simple structure, no need to judge the arrival direction of the user signal, and fast response speed. More importantly, the same beam of the uplink can also be used for the downlink, thereby providing gain on the downlink as well. However, due to the sector distortion, such as the difference between the inter-beam patterns, the gain and angle obtained by the multi-beam antenna are non-uniformly distributed. It sometimes differs between beams by 2 dB, and it may be locked on the wrong beam due to multipath or interference, because they cannot suppress interference signals with useful signals in the same beam. Multi-beam antennas, also known as beam-switched antennas, can actually be viewed as a technique between a sector-oriented antenna and a fully adaptive antenna. Among the multi-beam antennas, the following are worth studying: how to divide the airspace, that is, to determine the beam problem, including the number and shape; the criteria for selecting the beam; the implementation of the beam tracking mainly refers to the implementation of the fast search algorithm; Adapt to the theoretical relationship of beamforming.

3. Adaptive antenna array

Adaptive antenna array (AdapTIve Antenna Array), originally used in radar, sonar, military, mainly used to complete spatial filtering and positioning, such as phased array radar is a simple adaptive antenna array. The adaptive antenna is an antenna array that continuously adjusts its own pattern through feedback control. Its pattern is similar to that of the amoeba. It has no fixed shape and changes with signal and interference. Generally, 4 to 16 antenna array elements are used, and the array elements are separated by 1/2 wavelength. When the spacing is too large, the correlation degree of each received signal is reduced. If the spacing is too small, unnecessary side lobes are formed in the pattern. The smart antenna uses digital signal processing technology (DSP) to identify the direction of arrival of the user signal, and forms the main beam of the antenna in this direction to provide a spatial channel. Since the adaptive antenna can form different antenna patterns, and the adaptive algorithm update can be completed by software design, and the direction map can be adaptively adjusted, the system flexibility can be increased without changing the hardware configuration of the system, so it is called For the software antenna. The disadvantage of the adaptive antenna array is that the algorithm is more complicated and the dynamic response speed is slower.

The core of adaptive antenna research is adaptive algorithm. At present, many well-known algorithms have been proposed. Generally speaking, there are two categories of non-blind algorithms and blind algorithms. The non-blind algorithm refers to an algorithm that needs to rely on a reference signal (a pilot sequence or a pilot channel). At this time, the receiving end knows what is sent. When performing the algorithm processing, the channel response is first determined according to certain criteria, such as optimal forced. Zero Forcing determines each weighted value, or directly determines or gradually adjusts the weight according to certain criteria, so that the smart antenna output is most correlated with the known input. Commonly used correlation criteria are MMSE (Minimum Mean Square Error), LMS. (minimum mean square) and LS (least squares), etc. The blind algorithm does not need to send the known pilot signal at the origin, and the decision feedback algorithm (Decision Feedback) is a special type of blind algorithm. The receiver estimates the transmitted signal and uses this as the reference signal for the above processing, but it should be noted. It should be ensured that there is a small error between the decision signal and the actual transmitted signal. Blind algorithms generally use some features inherent in the modulated signal that are independent of the specific information bits, and adjust the weights so that the output satisfies this characteristic. Commonly, various gradient-based algorithms using different constraints are used. Compared with the blind algorithm, the non-blind algorithm usually has smaller error and faster convergence speed, but it wastes certain system resources. There is a semi-blind algorithm that combines the two, that is, the initial weight is determined by the non-blind algorithm. The blind algorithm is used for tracking and adjustment. On the one hand, the advantages of the two can be combined. On the one hand, it is consistent with the actual communication system, because usually the pilot symbols are not transmitted from time to time but are time-divided with the corresponding traffic channels. use.

It should be noted that the smart antenna uses a shaped beam for each user's uplink signal, but when the user does not transmit, only when in the receiving state, it moves in the coverage area of ​​the base station (idle state), and the base station is impossible. Knowing the location of the user, only the omnidirectional beam can be used for transmission (such as synchronization, broadcasting, paging, etc. in the system), that is, the base station must be able to provide omnidirectional and directional shaped beams. As a result, for omnidirectional channels, much higher transmit power will be required, which must be considered in system design.

4. Smart antenna application examples

At present, some smart antennas have been put into commercial use. For example, Shanghai Unicom has used the SpotLight GSM smart antenna system from Metawave of the United States, which has achieved good results. The SpotLight GSM antenna is a multi-beam smart antenna that replaces a 120° sector antenna with four 30° antennas. The system relies on a patented optimal beam selection algorithm to convert the transmit and receive beams. The RF energy is downlinked in each time slot within a specified 30° beam rather than the entire 120° sector, so co-channel interference is greatly reduced in adjacent cells. Similarly, the open beam that receives co-channel interference is effectively reduced from 120° to 30°. Thus, the 30° antenna effectively reduces the co-channel interference by 4 times compared to a single 120° sector antenna, theoretically equivalent to a 6 dB C/I improvement. This gain improves both the upstream (cell-base) and downlink (base-cell) of the communication channel. On the uplink side, the carrier-to-interference ratio of the cells in which the smart antenna system is installed is increased; on the downlink side, the carrier-to-interference ratio of the same-frequency cells in the original visible range is increased. If you want to maintain the original C / I value, you can use more dense frequency reuse, which increases the system capacity. SpotLight GSM performs beam switching without additional communication with the base station, so the installation of the SpotLight GSM system does not increase the base station communication load. In fact, the load on the base station processor is also reduced due to invalid call attempts and reduced redials due to interference or poor coverage. In addition, it is found in the test that in the cell using the smart antenna, not only the network capacity and quality in the cell are effectively improved, but the average receiving power and the transmitting power of the mobile phone in the cell are reduced by 2 to 3 dB, especially The mobile phone's transmit power dropped to 54%, while the mobile phone's rate of full-power transmission dropped from 22% to 8%. The SpotLight GSM smart antenna reduces the radiation of the mobile phone's electromagnetic waves to the human body by reducing the transmission and reception power of the mobile phone, and reduces the capacity and quality of the network, thereby reducing the number of new base stations that need to be established in the cell. Therefore, it has the reputation of “green antenna”.

As an important part of mobile communication, antenna plays an important role in improving network performance and improving network quality. The antenna technology is developing rapidly. The antenna diversity technology is an important means to improve the system gain. The diversity method includes space diversity and polarization diversity. For the convenience of engineering and maintenance, an antenna with electrical tilt angle appears. The pattern is not deformed and distorted, and a built-in tilt antenna has been developed. Especially in recent years, smart antennas represent the development direction of mobile communication antenna technology. It has shown great advantages in practical applications, but further research and improvement are needed in speeding up beamforming response speed and switching. .