Smart antenna application background
The original smart antenna technology is mainly used for radar, sonar, military anti-jamming communications, used to complete the spatial filtering and positioning. In recent years, with the development of mobile communication and the rapid development of modern digital signal processing technology, the processing capability of digital signal processing chip has been improved with the development of mobile communication, networking technology and antenna theory. Baseband antenna antenna formation possible, the reliability and flexibility of the antenna system continues to increase.
Smart antenna technology can be used for mobile communication environments with complex radio propagation. The principle of the smart antenna is to direct the radio signal to the specific direction, generate the spatial directional beam, so that the antenna main beam at the user signal arrival direction DOA (DirectionofArrinal), sidelobe or zero-aligned interference signal arrival direction, to achieve full and efficient use Move the user signal and delete or suppress the interference signal. At the same time, the smart antenna technology utilizes the difference of the signal space characteristics among the mobile users, and receives and transmits multiple mobile user signals on the same channel through the array antenna technology without interfering with each other, making the radio spectrum utilization and signal transmission more effective The Without the need to increase the complexity of the system, the use of smart antennas to meet the quality of service and network expansion needs. In fact, it makes the communication resources no longer confined to the time domain, frequency domain or code domain and extended to the spatial domain, belonging to the space division multiple access (SDMA) system. The third generation mobile communication standards organizations have recognized the important role of smart antennas in reducing network interference. Therefore, in 3G standards such as WCDMA and cdma2000, the terms of supporting smart antennas have emerged. Smart antennas have become an important part of 3G.
At present, in addition to a very small number of countries have a formal 3G commercial network, China and most areas of 3G is still in the experimental network stage, the next two to three years is expected to GSM network is still the main carrier network traffic. In China, large and medium-sized cities GSM network users to an annual rate of about 20% increase in some hot spots, per square kilometer carrying traffic even reached 1500erl. The capacity of the GSM network is limited to the frequency resources of the network in the environment with sufficient hardware configuration. Under the limited bandwidth, it is necessary to increase the network capacity and only reduce the frequency reuse distance. The closer the frequency reuse distance is, the network The higher the interference.
According to the current GSM network operation experience, in order to ensure the use of the appropriate cell signal under the premise of maintaining the quality of the network in the customer acceptable voice quality (clear no noise) range, you must follow the load ratio of 18dB or more in the wireless network planning. Whether in accordance with the traditional 4/12 frequency multiplexing or MRP or 1 * 3 frequency reuse, in the existing bandwidth are up to 12dB load-carrying ratio of frequency planning, the remaining load-bearing ratio can only rely on Dynamic power control, frequency hopping, discontinuous transmission and other functions of the gain to compensate. Reducing the interference level of the network and improving the system load-keeping ratio is very important in the GSM high-speed growth period, especially after the formal business of the GPRS service. Therefore, many GSM manufacturers and operators are considering the use of the smart antenna in the 3G standard GSM network, some manufacturers such as Ericsson, Medford and other companies in the high traffic area to carry out the corresponding research and testing, and achieved the desired results.
Second, the principle of smart antenna
The intelligent switching antenna with high gain, narrow beam phase array antenna is replacing the traditional sector antenna. A multi-beam antenna panel includes four 30? (Or 22.5?) Beams, so that the received signal can receive less than the standard sector antenna interference signal, thus improving the quality of the received signal. According to the theoretical calculations, an average C / I value of four tunable narrow beam antennas can be increased by 6 dB over conventional 3-sector antenna systems.
The smart antenna system continuously performs beam selection on a time slot basis to ensure the voice quality of the user during the call.
The smart antenna used for the test consists of two parts, a multi-beam array antenna and a dual-polarized antenna, which can be integrally packaged in an antenna plate or separately. The multi-beam array antenna is a four-level 3dB lobe width formed by an antenna array of 6-8 antenna sources. The multi-beam array antenna is generally called a narrow beam antenna whose main function is to transmit the signal of the traffic channel (TCH) and receive uplink signals; dual polarized antenna contains two horizontal 3dB lobe width 90? Antenna, commonly known as wide beam antenna, its function is the launch control channel signals (such as: BCCH, SDCCH and CBCH, etc. ).
Third, the smart antenna in the GSM network application examples
1. The introduction of smart antenna cell selection
As the introduction of smart antennas need to do the corresponding hardware transformation of the network, taking into account its price, in the current GSM network is more suitable for the strategy is the introduction of smart antennas in the local site. The purpose of the introduction of smart antenna is mainly to use its characteristics to reduce the frequency interference within the network, it is recommended to introduce smart antenna wireless base stations to select those sites that are high, the other cells interfere with serious sites. Such as the Shenzhen test site selected a district of the urban area, the site is higher than the height of the surrounding site about 10 meters, with the surrounding station average station distance of 500 meters, in the area coverage area there are more overlapping signal coverage.
2. Improvement of Downward Interference in Road Test
In order to more clearly compare the introduction of the smart antenna before and after the interference of the situation, in the course of the test, we artificially with the test area in the opposite direction of the district frequency with the test area with the frequency. Theoretically, the smart antenna concentrates the downstream signal strength in the traffic area, thus reducing interference to other cells, where (a) is the coverage of the cell using the normal directional antenna, (b) the coverage of the cell using the smart antenna Happening.
(1) Comparison of downstream signal strength
In the practical application, by comparing the changes of the downlink signal strength before and after the introduction of the smart antenna, it can evaluate the improvement of the downlink interference. The smart antenna can reduce the interference to other cells by reducing the downstream field strength. A continuous field strength test (more than 15 minutes) was performed during the test (10: 00-12: 00) for the different beam directions of the smart antenna.
The signal field strength reduction in the range of 1.4-8.1dbm changes, the signal strength changes in different reasons, on the one hand the antenna fan cover, on the other hand is the traffic in different beam direction of the distribution of different.
(2) load-bearing ratio of the comparative measurement
As the original network frequency reuse distance is very far, with the frequency interference is very small, in order to strengthen the contrast effect we will test the district with the right side of the neighborhood to the same frequency cell (this situation in the current sector antenna will be Causing serious quality should be avoided). The selected neighborhood is most likely to be interfered with by the pilot cell, especially in a main road in the area, the selected cell is the serving cell, and the test cell is the first neighborhood. Select the C / I value of the 7 points on the main road, and use TEMSinvestgation to test the C / I value of frequency 1 to compare the C / I values before and after the replacement of the smart antenna.
Replace the antenna before and after the interference signal intensity curve consistent, indicating that before and after the replacement of the wireless environment is similar. Comparison of interference before and after the replacement of cell signal strength, the application of smart antenna after the downlink interference can be reduced by 3.3dB, equivalent to 53% lower downlink interference. Test points 1, 2, 3 of the downlink interference did not improve, it is estimated that the test point around the block caused by the test point to receive more reflection signal.