With the vigorous development of mobile communications, the number of users is increasing rapidly, and spectrum resources are becoming 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 uses array antennas to replace conventional antennas, which can reduce system interference and improve system capacity and spectrum efficiency. Therefore, smart antenna technology has received widespread attention in the industry.
The original smart antenna technology was mainly used for radar and sonar phased array antennas to complete spatial filtering and positioning. In recent years, with the development of modern digital signal processing technology, the improvement of digital signal processing chip processing power and the reduction of price, and the increasing maturity of ASIC technology, smart antenna technology will soon be applied to mobile communication systems.
1. Principle of Smart Antenna
Distinguish from the directional pattern, the antenna mainly has two kinds of omnidirectional antenna and directional antenna: the omnidirectional antenna has the same transmission and reception in all directions, and is applied to the 360 ​​° coverage cell; when the cell splitting technology is adopted, only the coverage part The directional antenna of the cell. Compared with the former, the latter improves the channel reuse rate. The shape of the coverage area in the above two ways is fixed. The smart antenna can generate multiple spatial directional beams, dynamically change the shape of the coverage area, align the antenna main beam with the direction of the user signal, and the side lobes or nulls to the direction of the interference signal, and automatically track changes in the user and application environment. Effectively suppress interference, extract user signals, and improve link performance and system performance.
Corresponding to time division multiple access, frequency division multiple access or code division multiple access, the smart antenna is a space division multiple access SDMA technology. It cooperates with other multiple access methods to increase the degree of freedom, so it can effectively increase system capacity, reduce interference and fading, and reduce system cost. Without increasing the complexity of the system, the use of smart antennas can meet the needs of service quality and network expansion.
Second, the classification of smart antennas
Beamforming is a key technology for smart antennas and a guarantee for improving the signal-to-interference ratio and increasing user capacity. Beamforming controls the beam amplitude, beam pointing and beam zero position of the array antenna. While ensuring high-gain beam pointing in the desired direction, the beam zero is formed in the interference direction and is changed by adjusting the weighted amplitude and phase of each array element Direction shape. Smart antennas can be divided into two categories: pre-multibeam and adaptive beam.
1. Pre-multibeam forming
Pre-multibeam generates multiple fixed beams in advance to cover a certain cell. According to the received user signal, the beam where the user is located is determined. When the user moves in the cell, the user and the beam are switched. The downlink beam adopts the weight corresponding to the uplink beam. The key to the pre-multibeam switching class is to determine the beam corresponding to the user.
Pre-multibeam can suppress the interference in the non-main lobe area by controlling the side lobes. For interference in the area of ​​the main lobe, the system will not be able to suppress it.
Calculations and simulations in TDMA and FDMA systems show that the 30 ° pre-multibeam smart antenna system has an average gain of 7.6dB. When the system frequency reuse factor is 4, the capacity of the frequency reuse factor of 7 can be achieved. The beam processing system is composed of a pre-multibeam forming subsystem and a pre-multibeam switching subsystem. The traditional baseband processing part includes: Rake combining, power control instruction generation, deinterleaving, channel decoding and so on.
2. Adaptive beamforming
Adaptive beamforming changes the pattern of the array by adjusting the weighted amplitude and phase of each array element, so that the main lobe of the array antenna is aimed at the desired user; at the same time, the zero point and side lobe of the array antenna are aimed at other users, thereby improving reception The signal-to-noise ratio meets the best reception under a certain criterion. The difference between it and pre-multibeam is that the beam of a certain user should move as the user moves. There are many types of adaptive beam structures.
The main adaptive algorithms are as follows:
a. Estimation based on direction of arrival
Classical DOA estimation methods include well-known MUSIC, ESPRIT and their improved algorithms, as well as maximum likelihood estimation, high-order cumulants, and sub-optimal estimation based on eigenvalue decomposition. The problems to be solved by this kind of algorithm are complex calculation, small degree of freedom, matrix factorization and so on.
b. Non-blind adaptive processing
The expected signal in adaptive processing has a great influence on the result of adaptive processing. In the CDMA system, since the pilot channel is provided, non-blind adaptive processing is fully conditionally performed. LS-DRMTA and LS-DRMTCMA are concrete examples of this algorithm.
c. Blind adaptive beamforming
Blind adaptation is unable to provide the desired signal required in the adaptive algorithm, and can only use the characteristics of the transmitted signal for beamforming to implement the blind adaptive algorithm. This method is not the optimal estimate, and the typical representative is the CMA constant modulus algorithm. There are many variants of CMA, such as MT-LSCMA, MT-DD, etc.
3. The impact of smart antennas on mobile communication systems
1. Advantages of smart antennas
Smart antennas improve the system performance as follows:
a. Increase system capacity
Smart antennas use narrow beams to receive and transmit mobile user signals, reducing the interference of other users. Therefore, for self-interference systems such as CDMA systems, the system capacity can be effectively increased; at the same time, the use of space division technology to reuse channels also increases the system capacity .
b. Increase coverage
Beamforming is a vector superposition of multiple antennas, which is equivalent to an increase in antenna gain, that is, the sensitivity of the base station receiver and the effective omnidirectional radiation power of the base station transmitter are improved. This means that under the same receiving and transmitting conditions, a longer communication distance can be achieved, thus increasing the coverage.
c. Reduce system interference
The main beam of the narrow beam is used to receive and transmit signals, and the side lobes and zeros suppress the interference signals, which can reduce system interference and improve the output signal-to-noise ratio of the array, that is, improve the anti-interference ability of the system. In addition, it also has a certain weakening effect on multipath interference in mobile systems.
d. Reduce system cost
Because the gain of beamforming can reduce the requirements for the power amplifier, reduce the cost of the base station, and improve reliability; at the same time, it can reduce the volume and weight of the mobile station, extend the battery life of the mobile station, and reduce the cost of the mobile station.
e. Increase value-added services
The smart antenna can obtain the position information of the mobile user, and cooperate with other technologies to realize the wireless positioning of the mobile user. Wireless positioning is currently a hot technology in the field of mobile communications, and the future market potential is huge. This is a very practical value-added service.
2. Problems with smart antennas
a. Increased system complexity
Smart antennas require efficient algorithms and high-speed DSP devices to meet real-time processing requirements. The algorithm structure of the smart antenna should be compatible with the conventional processing structure as much as possible to facilitate the flexible configuration of the system and reduce costs. Base stations using smart antennas must be able to mix with conventional base stations and be compatible with management and maintenance on the network side.
b. Added channel correction
If you want to complete beamforming in baseband, you need to perform channel correction, which increases the channel requirements.
4. Conclusion
Smart antenna technology has been a research hotspot in the field of mobile communications in recent years. It has been commercialized in PHS, and it has received more attention in the third-generation mobile communication system. Both WCDMA and CDMA2000 support the smart antenna technology to varying degrees, and TS-SCDMA clearly expresses the use of smart antennas. It can be said that smart antennas are a key technology for future mobile communications, and this technology also has bright development prospects in other wireless systems.
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