Introduction to the application of smart antennas in 3G systems
In the development of the 3rd generation mobile communication system (3G), the contradiction between rapidly increasing traffic and limited spectrum resources has become increasingly prominent. Operators are eager to improve the spectrum utilization of the system to provide greater capacity. Smart antennas are one of the core technologies to resolve this contradiction. Starting from the principle of smart antenna, combined with the complexity and cost of use in practical applications, the application of smart antenna in 3G system is introduced, and its advantages and existing problems are analyzed in detail. Finally, aiming at the current situation of the domestic mobile communication market, it is concluded that in the upcoming third generation mobile communication system, smart antennas will have a bright application prospect.
With the rapid development of mobile communications, the number of users has increased rapidly, and spectrum resources have become increasingly tight. How to use existing spectrum resources to further expand capacity has become a key issue in the development of mobile communications. Smart antenna technology has attracted wide attention in the industry because it can reduce system interference and improve system capacity and spectrum efficiency.
1. Overview of smart antenna principle A smart antenna is also called an adaptive antenna. It consists of multiple antenna elements. Each antenna is followed by a complex weighting device, and finally the combined output is performed by the adder. The smart antenna of this structure can only perform spatial domain processing. At the same time, the smart antennas with airspace and time domain processing capabilities are relatively complex in structure, and each antenna is followed by a delay tap weighting network (the same structure as the time domain finite impulse response equalizer). The main meaning of adaptive or intelligent means that these weighting coefficients can be adaptively updated and adjusted according to certain adaptive algorithms.
The basic principle of a smart antenna is that the antenna dynamically tracks multiple desired users with multiple high-gain narrow beams. In the receive mode, signals from outside the narrow beam are suppressed, and in the transmit mode, the desired user can receive the signal. The power is maximized while minimizing the interference experienced by undesired users outside the narrow beam illumination range. Smart antennas use different user space locations to distinguish different users. Unlike traditional frequency division multiple access (FDMA), time division multiple access (TDMA) or code division multiple access (CDMA), smart antennas introduce a fourth type of multiple access method: space division multiple access (SDMA), ie in the same time slot. In the case of the same frequency or the same address code, it can still be distinguished according to the different intermediate propagation paths of the signals. SDMA is a channel capacity enhancement method that is fully compatible with other multiple access methods, thus enabling a combined multiple access method, such as air separation? Code Division Multiple Access (SD-CDMA). The smart antenna is essentially different from the traditional antenna concept. Its theoretical support is signal statistical detection and estimation theory, signal processing and optimal control theory. Its technical basis is adaptive antenna and high-resolution array signal processing.
Europe, Japan, the United States and other countries attach great importance to the position and role of smart antenna technology in future mobile communication solutions. A large number of theoretical analysis studies have been carried out, and some technical test platforms have also been established. The European Communications Commission (CEC) implemented the first phase of smart antenna technology research in the RACE (research into advanced communica TIon in Europe) program, which was completed by Germany, the United Kingdom, Denmark and Spain. Japan's ATR Photoelectric Communication Research Institute has developed a multi-beam smart antenna based on beam space processing. ArrayComm of the United States and China Telecom Science and Technology Xinwei Company have also developed smart antenna systems for wireless local loop (WLL). China has also included research on smart antenna technology in the personal communication technology sub-item of the national 863-317 communication technology theme research, and many experts and universities are conducting relevant research.
2.1 Applications in WCDMA and CDMA2000
The 3rd generation system was designed as a system that can provide fairly high speed data services. However, they are also subject to air channel quality limitations as in the second generation systems. Standardization organizations have recognized the role of smart antennas in improving this paradox and have developed relevant provisions in the upcoming 3G standard. Both WCDMA and CDMA2000 allow for the assignment of dedicated pilot channels for each mobile user in the uplink and downlink, but will require the use of a smart antenna system.
For WCDMA and CDMA2000 systems, although smart antennas are the recommended configuration, some WCDMA and CDMA2000 base station products have begun to support smart antennas.
2.2 Application in TD-SCDMA system
The high efficiency of TD-SCDMA (Time Division Synchronous Code Division Multiple Access) smart antennas is obtained based on the symmetry of the uplink and downlink wireless paths (the same wireless environment and transmission conditions). In addition, smart antennas can reduce inter-cell interference and reduce intra-cell interference. These characteristics of smart antennas can significantly improve the spectral efficiency of mobile communication systems.
The smart antenna of the TD-SCDMA system is composed of a concentric array of 8 antenna elements and has a diameter of 25 cm. It achieves higher gain than an omnidirectional antenna. The principle is to arrange and excite a group of antennas and corresponding transceivers in a certain way, and use the interference principle of waves to generate a strong directional radiation pattern, and use DSP (digital signal processor) to make the main lobe adaptively Pointing to the direction of the mobile station, the purpose of improving the carrier-to-interference ratio of the signal and reducing the transmission power can be achieved. The above performance of the smart antenna allows for more dense frequency reuse, resulting in a significant increase in spectral efficiency.
Since each user's location within the cell is different. This aspect requires the antenna to be versatile, and on the other hand requires the system to track individual users in each individual direction. The above requirements can be achieved by controlling the direction measurement of the user by the DSP. Each user's tracking is measured by the angle of arrival. In the TD-SCDMA system, since the length of the wireless subframe is 5 ms, at least 200 times per second can be measured, and the uplink and downlink transmissions of each user occur in the same direction, and the directionality and tracking of the smart antenna can be obtained. The best performance.
A further advantage of TD-SCDMA in TDD (Time Division Duplex) mode is that both the transmission and reception of user signals occur at exactly the same frequency. Therefore, the transmission conditions in the uplink and downlink directions are the same or symmetric, so that the smart antenna can minimize inter-cell interference, thereby obtaining the best system performance.
3.1 Smart antenna improves system performance
Smart antennas can significantly improve the performance of wireless communication systems and increase the capacity of the system. Specifically embodied in the following aspects:
1) Improve spectrum utilization. The smart antenna is used instead of the common antenna to improve the spectrum reuse rate in the cell, and the system capacity can be increased and the operator cost can be reduced without building or building a base station as little as possible.
2) Quickly solve the bottleneck of dense urban capacity. The smart antenna should be able to allow any wireless channel to be paired with any of the beams, so that channels can be allocated on demand to ensure that more areas of the call are blocked, which is equivalent to increasing the capacity of the wireless network in such areas.
3) Suppress interference signals. Smart antennas spatially filter beams from all directions. It adjusts the excitation of each antenna element, optimizes the antenna array pattern, aligns the zero point with the interference direction, greatly improves the output signal-to-interference ratio of the array, improves the system quality, and improves the system reliability. For soft-capacity CDMA systems, an increase in the signal-to-interference ratio also means an increase in system capacity.
4) Anti-fading. The main problem of high-frequency wireless communication is the fading of the signal. Ordinary omnidirectional antennas or directional antennas will cause large signal distortion due to fading. If the smart antenna is used to control the receiving direction and adaptively form the directivity of the beam, the gain in the direction of the delayed wave can be minimized, and the influence of signal fading is reduced. Smart antennas can also be used for diversity to reduce fading.
5) Realize mobile station positioning. A base station employing a smart antenna can obtain a spatial feature matrix of the received signal, thereby obtaining a power estimate and direction of arrival of the signal. With this method, the user terminal can be located to a smaller area with 2 base stations. Since the current cellular mobile communication system can only determine the cell in which the mobile station is located, the implementation of the mobile station positioning can facilitate the launch of many new services related to the location, and the development of the new service is currently the mobile operator to improve the ARPU (average revenue per User) value, the inevitable means to strengthen their own competitiveness.
3.2 Problems to be solved in practical applications
Smart antenna technology has tremendous benefits for wireless communications, especially for CDMA systems. However, when using smart antennas in CDMA systems, the issues that must be addressed must be addressed and addressed in standards and product design.
3.2.1 Omnidirectional beam and shaped beam
The functions of the above smart antenna are mainly realized by adaptive transmit and receive beamforming, and the receive and transmit beamforming is based on the base station antenna geometry, system requirements and received user signals. In the mobile communication system, the smart antenna uses a shaped beam for each user's uplink signal, which is very straightforward to improve the system performance; but it is moved within the coverage area of ​​the base station when the user is not transmitting, only in the receiving state. At the time (idle state), it is impossible for the base station to know the location of the user, and only use the omnidirectional beam for transmission (such as physical channels such as pilot, synchronization, broadcast, paging, etc. in the system). An omnidirectionally covered base station whose transmit beams of different code channels are different, 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.
3.2.2 Calibration of Smart Antenna
When using a smart antenna, it is necessary to have a technique for real-time automatic calibration of the smart antenna. When the smart antenna is used in the TDD system, the downlink beamforming is directly used by the uplink beamforming coefficient according to the reciprocity principle in the electromagnetic field theory. However, for an actual wireless base station, the wireless transceiver of each path may not be identical, and its performance will vary depending on factors such as time of day, operating level and environmental conditions. If real-time auto-calibration is not performed, the downstream beamforming will be severely affected. In this way, not only the advantages of the smart antenna are not obtained, but even communication is impossible at all.
3.2.3 Combination of smart antennas and other anti-jamming technologies
At present, there must be a compromise between the complexity of smart antenna algorithms and the possibility of real-time implementation. In this way, the practical smart antenna algorithm can not solve the multipath interference with a delay longer than one chip width, and can not overcome the channel deterioration caused by the high-speed mobile Doppler effect. In a high-speed mobile environment with severe multipath, it is necessary to combine smart antennas with other anti-jamming digital signal processing techniques to achieve the best results. These digital signal processing techniques include joint detection (jointdetecTIon), interference cancellation, and Rake reception. At present, a combination of smart antennas and joint detection or interference cancellation has a practical algorithm, and the algorithm combined with the Rake receiver is still under study.
3.2.4 Consideration of equipment complexity
Obviously, the performance of smart antennas will increase as the number of antenna elements increases. However, increasing the number of antenna elements will increase the complexity of the system. This complexity is mainly due to the fact that the amount of baseband digital signal processing will increase geometrically. Nowadays, the CDMA system is developing in the direction of broadband, the chip rate is already high, and the complexity of baseband processing has put higher and higher requirements on microelectronic technology, which limits the number of antenna elements to be too much. At the current level, the number of antenna elements is between 6 and 16.
Considering the current status of different mobile communication networks in China, different operators will adopt different 3G standards and different evolution paths for the operators that will receive 3G licenses, but smart antennas will be the common choice of operators.
For operators that did not have a 2G mobile communication network, it is very suitable to build a new network. If you choose to build a TD-SCDMA system, as its core technology, smart antennas will effectively reduce the operator's investment and improve its economic benefits by virtue of its advantages of increasing system capacity and reducing mobile station transmit power. For operators that have already built GSM or CDMA networks, the transition to WCDMA and CDMA2000 systems respectively will be the best choice. For these operators, although the smart antenna is only a recommended configuration, it can significantly increase the capacity of the wireless network. This is especially important for operators in China because the population density of large cities in China is generally high. . At the same time, due to the use of smart antenna technology, the signal quality of the cell is improved, and the interference of neighboring cells is reduced, thereby expanding the coverage. The interference mitigation mechanism of smart antenna technology also improves the system: due to the reduction of the overall noise level, the signal power can be concentrated on a specific user terminal, and the base station and the user terminal can achieve the same signal quality level only by requiring a small transmission power. Although the smart antenna technology requires multiple antennas to be added, the number of power amplifiers is increased, but more importantly, the power of the power amplifier is greatly reduced, and the unit price of the power amplifier is greatly reduced. Due to the complicated manufacturing process of the high-power broadband amplifier, The cost is high, so the use of multiple low-power amplifiers saves investment and increases the reliability of the entire power amplifier subsystem.
In summary, in the upcoming 3rd generation mobile communication system, most operators will adopt smart antenna technology to improve the performance and capacity of the system. There are good reasons to believe that smart antennas will have a bright application prospect.
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