Introduction and testing of TD-SCDMA smart antenna system
This paper briefly introduces the principle of smart antenna, the physical characteristics of smart antenna array and beamforming, and the realization of smart antenna algorithm. Finally, the field test of TD-SCDMA smart antenna was analyzed, and the matters that should be paid attention to during the test were pointed out.
A smart antenna is generally defined as an antenna array mounted on the base station side of a mobile radio access system. The directional characteristics of each link between the base station and the mobile station are obtained through a set of fixed antenna units with programmable electronic phase relationships. The principle is to direct the radio signal to a specific direction, and generate a spatial directional beam, so that the main beam of the antenna is aligned with the direction of arrival of the user signal (DOA), and the side lobes or nulls are aligned with the arrival direction of the interference signal, so as to achieve efficient use of the mobile. User signal and eliminate or suppress the purpose of the interference signal. At the same time, the smart antenna technology utilizes the difference in signal space characteristics between mobile users, and receives and transmits multiple mobile user signals on the same channel through array antenna technology without mutual interference, making the use of radio spectrum and signal transmission more efficient. .
The basic idea of ​​a smart antenna in a TD-SCDMA system 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 be enabled. The received signal power is maximized while minimizing the interference experienced by undesired users outside the narrow beam illumination range. Smart antennas use the difference in user space to distinguish users, in the same time slot, the same frequency or the same address code. It can still be distinguished according to the different spatial propagation paths of the signals.
TD-SCDMA uses the same carrier frequency for uplink and downlink wireless links, and the wireless propagation characteristics are similar. It can support smart antenna technology well. The use of smart antennas increases the capacity of TD-SCDMA wireless interfaces.
The TD-SCDMA smart antenna mainly implements two kinds of beams: a broadcast beam and a service beam. The broadcast beam is formed in the broadcast time slot to realize the broadcast to the entire cell. Therefore, the beam width is required to be wide, and the cell is covered as much as possible. The service beam is formed after a specific call link is established, that is, a tracking beam is formed, which forms a narrow beam for each user, and these beams closely track the user. Since the beam is narrow, the energy is concentrated. At the same power, the smart antenna can increase the useful signal strength while reducing the interference to users in other directions. Since the smart antenna can concentrate the signal well, the transmitter can appropriately reduce the transmission power.
2. Physical characteristics and beamforming of smart antenna arrayCommon smart antenna arrays are generally classified into a 360° omnidirectional array and a 120° planar sector array. The omnidirectional antenna array is mainly suitable for rural areas and remote mountainous areas with low user density, and can be covered by 360° omnidirectional cells. The planar antenna array mainly covers a sector of 120°. Usually a three-sector base station can cover a 360° range. Planar antenna arrays have become the mainstream of TD-SCDMA smart antennas because of their better beamforming performance, the ability to form narrower lobe widths, stronger sidelobe suppression and higher shaping gain. It is applied to the coverage of the user-intensive urban environment.
The number of array elements of a smart antenna array is usually 4-16. At present, more antenna arrays composed of eight array elements are used in the system. The more the number of antenna elements, the higher the gain, the stronger the beamforming ability, but the complexity of the cost and implementation will be greatly increased. In the future, an estimated four-element smart antenna will be gradually put into use, which can provide a more economical choice based on lowering the system implementation cost. of course. Specific applications need to consider the balance between economy and performance.
The smart antenna has exactly the same physical characteristics of each antenna element, so the single antenna lobe pattern has very similar characteristics. A plurality of antenna elements are arranged in an antenna array at a certain pitch (typically 1/2 λ). The signals (including amplitude and phase) of each antenna element are controlled by an algorithm to finally form a directional downlink beam.
Figure 1 and Figure 2 show the vertical and horizontal lobe of a single array of 8-element omnidirectional smart antennas.
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