A detailed explanation of the proposal, working principle and classification of smart antennas and the technical advantages of TD-SCDMA in smart antennas

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A detailed explanation of the proposal, working principle and classification of smart antennas and the technical advantages of TD-SCDMA in smart antennas [Copy link]

Smart antenna technology is one of the key technologies in TD-SCDMA (Time Division Synchronous Code Division Multiple Access). This paper 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), which is a time division synchronous code division multiple access technology, has officially become a component of the International Telecommunication Union (ITU) third-generation mobile communication standard IMT2000 recommendation. my country's TD-SCDMA with independent intellectual property rights, Europe's WCDMA and the United States' CDMA2000 have become the most mainstream technologies in the 3G era. TD-SCDMA integrates the advantages of code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA) and other technical advantages, and adopts smart antennas, joint detection, relay switching, synchronous CDMA, software radio, low chip rate, multiple time slots, variable spread spectrum system, adaptive power adjustment and other technologies. It is a mobile communication technology with the advantages of large system capacity, high spectrum utilization, and strong anti-interference ability.

With the rapid development of global mobile communication services in recent years, the requirements for signal transmission strength, coverage and transmission capacity have become increasingly higher. How to use the wireless spectrum more efficiently has received widespread attention. Smart antenna technology studies the spatial divisibility of unlimited resources and is an effective way to further improve system capacity.

1. The Proposal of Smart Antenna

Smart 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 receiving or transmitting characteristics in communication systems. Smart antennas use space division multiple access technology to distinguish signals of the same frequency, time slot, and 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 the desired signal from the interference signal in terms of spatial position and incident angle, thereby controlling the directional pattern of the antenna array to achieve the purpose of enhancing the desired signal and suppressing the interference signal; at the same time, it can also automatically adjust the directional pattern of the antenna array according to the changes in the position and incident angle of the desired signal and the interference signal, so as to achieve the purpose of intelligently tracking environmental changes and user movement, achieve the best signal transmission and reception, and achieve the effect of dynamic "spatial filtering". Compared with non-directional antennas, the gain of its uplink and downlink links 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, since the antenna lobe points directly to the user, the interference with other users in the cell and with users in adjacent cells is reduced, and the multipath of the mobile communication channel is also reduced. The CDMA system is a power-limited system. The application of smart antennas achieves the two major goals of increasing antenna gain and reducing system interference, thereby significantly expanding system capacity and improving spectrum utilization. The earliest smart antenna was the sidelobe cancellation antenna that appeared in the 1950s. This antenna includes a high-gain antenna for receiving useful signals and one or more low-gain, wide-beam antennas for suppressing side lobes. Combining several such loops into an array antenna constitutes an adaptive antenna. With the development of array signal processing technology, there are more and more terms related to smart antennas, such as intelligent antenna, phased arrays, space division multiple access (SDMA), spatial processing, digital beam forming, adaptive antenna system, etc., reflecting the many different aspects of smart antenna system technology.

2 Principles of Smart Antennas

The working principle of TD-SCDMA smart antenna is to arrange and excite a group of antennas and corresponding transceivers in a certain way, use the principle of wave interference to produce a highly directional radiation pattern, and control the direction and shape of the antenna beam through an adaptive algorithm, so that the high-gain narrow beam is aimed in the direction of the service user and the null is aimed 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, which is divided into non-blind algorithm and blind algorithm. Non-blind algorithm refers to the algorithm that needs the help of reference signal (pilot sequence or pilot channel). At this time, the receiving end knows what is being sent, and determines or gradually adjusts the weight according to certain criteria to make the output of smart antenna correlated with the known input. Commonly used correlation criteria include MMSE (minimum mean square error), LMS (least mean square), LS (least squares), etc. Blind algorithm does not need to transmit known pilot signal at the transmitting end. It generally uses some characteristics inherent in the modulated signal itself that are irrelevant to the specific information bits carried, such as constant modulus, subspace, finite symbol set, cyclostationarity, etc., and adjusts the weight so that the output meets this characteristic. Compared with blind algorithm, non-blind algorithm usually has smaller error and faster convergence speed, but it needs to waste certain system resources. Combining the two produces a semi-blind algorithm, that is, first use the non-blind algorithm to determine the initial weight, and then use the blind algorithm to track and adjust. This can combine the advantages of both, and is also consistent with the actual communication system.

3 Classification of Smart Antennas

Smart antennas are divided into two categories: multi-beam antennas and adaptive antenna arrays. Multi-beam antennas use multiple parallel beams to cover the entire user area. The direction of each beam is fixed, and the beam width is also determined by the number of antenna elements. Multi-beam antennas cannot achieve optimal signal reception and are generally only used as receiving antennas. Adaptive antenna arrays generally use 4 to 16 antenna array element structures with an element spacing of half a wavelength. Antenna array element distribution methods include linear, circular, and planar. Adaptive antenna arrays are the main type of smart antennas, which can complete user signal reception and transmission.

Smart antennas can be divided into three categories according to their implementation form.

(1) Adaptive nulling smart antenna

It is based on adaptive antenna technology, uses adaptive algorithms to form a directional pattern, and automatically adjusts the amplitude and phase weighting of the antenna array elements according to a certain algorithm based on the input and output characteristics of the antenna, forming a null in the interference direction, thereby significantly reducing the interference level and improving the signal-to-noise ratio of the system. From the perspective of spatial response, its adaptive antenna array is a spatial filter. The physical position of the antenna does not change. The signal detection and processing system determines the direction of the interference and signal, adaptively changes the antenna's directional pattern, and aligns the null direction with the interference, and the main lobe with the signal to be received. However, the adaptive smart antenna has a very limited ability to suppress interference in the main lobe area.

(2) Equal sidelobe needle beam smart antenna

It is also based on adaptive antenna technology. Its antenna radiation pattern is an equal sidelobe radiation pattern, and the weighted values of the radiation pattern are pre-calculated.

When the system is working, it first determines the direction of arrival (DOA) of the signal through direction finding, selects the appropriate weighting, and then points the main lobe of the equal side lobe pattern to the target direction. This type of smart antenna can ensure the suppression of interference in the non-main lobe area through low equal side lobe levels, but it cannot suppress interference in the main lobe area, which is not as good as the adaptive smart antenna. However, the equal side lobe smart antenna does not require iteration and has a fast response speed.

(3) Digital beamforming smart antenna

It uses digital beamforming (DBF) technology to combine its beamforming adaptive antenna array with digital signal processing technology. When working, it uses a high-resolution direction-finding algorithm to obtain a communication reference signal. When the reference signal reaches the beamforming adaptive antenna array, it provides a direction information to the signal processor, converts the received signal of each array element to the baseband, and converts it into a digital signal by the A/D converter. Then, the digital signal is weighted according to the direction information to form the required beam in this direction.

4 Technical advantages of smart antennas

TD-SCDMA smart antennas can improve the quality of links by utilizing multipath, increase system capacity 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. Its capacity is mainly limited by the interference of the system. Reducing system interference and improving the signal-to-interference ratio means increasing system capacity. The use of smart antenna technology with multi-beam plate antennas improves antenna gain and carrier-to-interference ratio (C/I) indicators, reduces co-channel interference, reduces frequency reuse coefficients, and improves spectrum utilization efficiency. It can improve system coverage quality and expand system capacity without adding new base stations. In the TD-SCDMA system, the use of smart antenna technology can solve the capacity problem in dense urban areas without affecting call quality.

(2) Reduce signal fading. Signal fading is the main problem of high-frequency wireless communications. 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, causing signal fading. The use of smart antennas to adaptively construct the directionality of the beam makes the gain in the direction of the delayed wave minimum, effectively reducing the impact of signal fading. Smart antennas can also be used for diversity to reduce fading. Radio waves reach the receiving antenna through different paths, and their directional angles are different. By using multiple pairs of adaptive receiving antennas with different pointing directions, these components are isolated and then synthesized for processing, which can achieve angle diversity and reduce signal fading.

(3) Suppress interference signals. Using smart antennas in CDMA base stations can reduce the interference of mobile stations to base stations and improve system performance. The essence of anti-interference technology is spatial domain filtering. The smart antenna beam in TD-SCDMA operating in TDD mode is directional and can distinguish radio waves with different incident angles. It can adjust and control the excitation "weights" of the antenna array units and adapt to changes in the radio wave propagation environment. Optimize the wireless array radiation pattern and automatically align its "zero point" with the interference direction, thereby greatly improving the array's output signal-to-noise ratio and improving system reliability.

(4) Mobile station positioning. A base station using a smart antenna can obtain the spatial feature matrix of the received signal, thereby obtaining the power estimation and arrival direction of the signal. Through this method, two base stations can locate the user terminal in a smaller area, thereby achieving accurate positioning of the mobile station; in addition, in a wireless base station using an ordinary antenna, the transmitted signal uses a high-power amplifier. With the use of a smart antenna, the gain of the beamforming can reduce the requirements for the power amplifier, greatly reducing the base station's transmission power, while also reducing electromagnetic environmental pollution.

In short, smart antennas can reduce the chances of interference and being interfered with, expand system capacity, increase coverage, improve spectrum utilization, reduce the cost of wireless base stations, significantly improve the performance of mobile communication systems, and provide technical support for the development of new businesses. 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.