Smart antenna technology is one of the key technologies in TD-SCDMA (Time Division Synchronous Code Division Multiple Access). The article mainly introduces the proposal, working principle and classification of smart antennas, and analyzes the technical advantages of smart antennas in TD-SCDMA.
TD-SCDMA (TIme DivisiON Synchronous Code Division MulTIple Access), an instant division and synchronization code division multiple access technology, has officially become an integral part of the International Telecommunication Union (ITU) third-generation mobile communication standard IMT2000 recommendations. my country has independent intellectual property rights. TD-SCDMA, European WCDMA and American CDMA2000 have become the most mainstream technologies in the 3G era. TD-SCDMA integrates code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA) and other technical advantages into one, using smart antennas, joint detection, relay switching, synchronous CDMA, software radio, low code Chip rate, multi-slot, variable spread spectrum system, adaptive power adjustment and other technologies, mobile communication technology with the advantages of large system capacity, high spectrum utilization, and strong anti-interference ability.
In recent years, with the rapid development of global mobile communication services, the requirements for signal transmission strength, coverage, and transmission capacity have become higher and higher, and how to use wireless spectrum more efficiently has attracted widespread attention. Smart antenna technology studies the spatial separability characteristics of unlimited resources, which is an effective way to further increase the system capacity.
1 Proposal of smart antennaSmart antennas are developed on the basis of adaptive filtering and array signal processing technology. They are an antenna that can enhance antenna performance by adjusting reception or transmission characteristics in communication systems. Smart antennas use space division multiple access technology to distinguish signals with the same frequency or at the same time slot and the same code channel by using the difference in signal transmission direction, so as to maximize the use of limited channel resources. It uses the spatial characteristics of signal transmission to distinguish between the desired signal and the interference signal from the spatial position and angle of incidence, thereby controlling the pattern of the antenna array to achieve the purpose of enhancing the required signal and suppressing the interference signal; at the same time, it can also control the interference signal according to the required signal. The direction pattern of the antenna array is automatically adjusted according to the position of the interference signal and the incident angle, so as to realize the purpose of intelligently tracking environmental changes and user movement, to achieve the best transmission and reception of signals, and to achieve the effect of dynamic "spatial filtering". Compared with the non-directional antenna, the gain of the uplink and downlink is greatly improved, the transmission power level is reduced, the signal-to-noise ratio is improved, and the influence of channel transmission fading is effectively overcome. At the same time, because the antenna lobe points directly to the user, it reduces the interference with other users in the cell and with users in adjacent cells, and also reduces the multipath of the mobile communication channel. The CDMA system is a power-constrained system. The application of smart antennas achieves the two major goals of improving antenna gain and reducing system interference, thereby significantly expanding system capacity and improving spectrum utilization. The earliest smart antennas were side-lobe cancellation antennas that appeared in the 1950s. This antenna includes a high-gain antenna for receiving useful signals and one or several low-gain, wide-beam antennas for suppressing sidelobes. Combine several such loops into an array antenna to form an adaptive antenna. With the development of array signal processing technology, there are more and more terms related to smart antennas, such as smart antennas, phased arrays, space division multiple access (SDMA), and spatial processing (spaTIal processing). ), DIGItal beam forming, adaptive antenna system (adapTIve antenna sySTem), etc., reflect many different aspects of smart antenna system technology.
2 Principle of smart antennaThe working principle of TD-SCDMA smart antenna is to make a group of antennas and corresponding transceivers arranged and excited in a certain way, using the principle of wave interference to generate a strong directional radiation pattern, and controlling the antenna beam through an adaptive algorithm Direction and shape: aim the high-gain narrow beam in the direction of serving users, and the nulling in the direction of interference to achieve beamforming and achieve the purpose of directional transmission and reception.
Adaptive algorithm is the core of smart antenna, it is divided into non-blind algorithm and blind algorithm. Non-blind algorithm refers to an algorithm that requires reference signal (pilot sequence or pilot channel). At this time, the receiver knows what is being sent, and determines or gradually adjusts the weight according to certain criteria to maximize the output of the smart antenna and the known input Correlation, commonly used correlation criteria include MMSE (minimum mean square error), LMS (least mean square), LS (least squares), etc. The blind algorithm does not need to send the known pilot signal. It generally uses the inherent characteristics of the modulated signal and has nothing to do with the specific information bits carried, such as constant modulus, subspace, finite symbol set, cyclic stability, etc., and adjust the weight Value so that the output meets this characteristic. Compared with blind algorithms, non-blind algorithms usually have smaller errors and faster convergence speed, but some system resources are wasted. Combining the two to produce a semi-blind algorithm, that is, the non-blind algorithm is used to determine the initial weight, and then the blind algorithm is used to track and adjust. This can combine the advantages of the two, and it is also consistent with the actual communication system.
3 Classification of smart antennasSmart antennas are divided into two categories: multi-beam antennas and adaptive antenna arrays. The multi-beam antenna uses multiple parallel beams to cover the entire user area, the direction of each beam is fixed, and the beam width is also determined with the number of antenna elements. Multi-beam antennas cannot achieve optimal signal reception and are generally only used as receiving antennas. The adaptive antenna array generally adopts a 4-16 antenna element structure, and the distance between the elements is half a wavelength. The antenna array elements are distributed in linear, circular, and plane. Adaptive antenna array is the main type of smart antenna, which can complete user signal reception and transmission.
According to the realization form, smart antennas can be divided into 3 categories.
(1) Adaptive zeroing smart antenna
It is based on adaptive antenna technology and adopts adaptive algorithm to form a pattern. According to the input and output characteristics of the antenna, it automatically adjusts the amplitude and phase weight of the antenna element according to a certain algorithm to form a null in the interference direction. Thereby greatly reducing the interference level and improving the signal-to-noise ratio of the system. From the perspective of spatial response, the adaptive antenna array is a spatial filter. The physical position of the antenna is not changed. The signal detection and processing system determines the source of interference and signal, and adaptively changes the antenna pattern and zero The trap direction is aligned with the interference, and the main lobe is aligned with the signal to be received. However, the adaptive smart antenna's ability to suppress interference in the main lobe area is very limited.
(2) Equivalent sidelobe needle beam smart antenna
It is also based on adaptive antenna technology. Its antenna pattern is an iso-sidelobe pattern, and the weighted value of the pattern is pre-calculated.
When the system is working, first determine the direction of arrival (DOA) of the signal through direction finding, select appropriate weights, and then point the main lobe of the iso-sidelobe pattern to the target direction. This type of smart antenna can be suppressed by low equal side lobe levels for interference in the non-main lobe area, but the use of this type of smart antenna will not be able to suppress the interference in the main lobe area, and it is not as good as the adaptive smart antenna . But the sidelobe smart antenna does not require iteration, and the response speed is fast.
(3) Digital beam forming smart antenna
It uses digital beamforming (DBF) technology to combine its beamforming adaptive antenna array with digital signal processing technology. The high-resolution direction finding algorithm is used to obtain the communication reference signal during work. When the reference signal reaches the beam forming adaptive antenna array, it provides a direction information to the signal processor, and converts the received signal of each element to the baseband. The /D converter converts into a digital signal, and then weights the digital signal according to the direction information to form the required beam in this direction.
4 Technical advantages of smart antennasTD-SCDMA smart antennas can improve the quality of the link by using multipath, increase the capacity of the system by reducing mutual interference, and allow different antennas to transmit different data. The advantages of smart antennas are summarized as follows:
(1) Increase system capacity. The CDMA system is a self-interference system, and its capacity limitation mainly comes from the interference of the system. The reduction of system interference and the increase of signal-to-interference ratio means the increase of system capacity. Smart antenna technology using multi-beam plate antennas improves antenna gain and carrier-to-interference ratio (C/I) indicators, reduces co-frequency interference, reduces frequency reuse coefficient, and improves spectrum utilization efficiency without adding new base stations. It can improve system coverage quality and expand system capacity. In the TD-SCDMA system, the use of smart antenna technology can solve the capacity problem of dense urban areas without affecting the call quality.
(2) Reduce signal fading. Signal fading is the main problem of high-frequency wireless communication. In land mobile communications, as the mobile station moves and the environment changes, the instantaneous value of the signal and the delay distortion change very uniformly and irregularly, resulting in signal fading. Smart antennas are used to adaptively construct the directionality of the beam, so that the gain in the delayed wave direction is minimized, and the influence of signal fading is effectively reduced. Smart antennas can also be used for diversity to reduce fading. The radio waves reach the receiving antenna through different paths, and their direction angles are different. Using multiple adaptive receiving antennas pointing at different points to isolate these components, and then combine them to achieve angle diversity and reduce signal fading.
(3) Suppress interference signals. Using smart antennas in CDMA base stations can reduce interference from mobile stations to base stations and improve system performance. The essence of anti-interference technology is spatial domain filtering. Smart antenna beams in TD-SCDMA running in TDD mode have directivity, which can distinguish radio waves with different incident angles, and can adjust and control the excitation "weights" of antenna array units. Adapt to changes in the radio wave propagation environment. Optimize the wireless array pattern and automatically align its "zero point" to the interference direction, thereby greatly improving the output signal-to-noise ratio of the array and improving system reliability.
(4) Realize mobile station positioning. The base station using smart antennas can obtain the spatial characteristic matrix of the received signal, thereby obtaining the power estimation and direction of arrival of the signal. Through this method, two base stations can be used to locate the user terminal in a small area, thereby achieving precise positioning of the mobile station; in addition, in the wireless base station using ordinary antennas, the transmitted signal uses a high-power amplifier, which uses For smart antennas, the beam-forming gain can reduce the requirements for power amplifiers, greatly reduce the transmission power of the base station, and reduce the electromagnetic environment pollution.
In short, smart antennas can reduce the chance of interference and interference, expand system capacity, increase coverage, increase spectrum utilization, reduce the cost of wireless base stations, significantly improve the performance of mobile communication systems, and provide technology for expanding new businesses stand by. With the continuous development of digital signal processing technology and the continuous improvement of integration, smart antenna technology will be more widely used in the field of mobile communications.
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