Direct sequence code division multiple access technology is becoming the main access method for modern mobile communication, but its system performance is limited by multipath fading, multiple access interference (MAI) and inter-symbol interference (ISI) of wireless channels. How to eliminate these Impact is a major issue in improving the performance of wireless communication systems.
Space-time processing technology is one of the current research hotspots. Joint processing of received signals in the spatial domain and time domain can make full use of the advantages of spatial signal processing technology and time signal processing technology, effectively resist ISI, reduce MAI, increase diversity gain, and improve array. Gain, the effect achieved is not achievable with a single-time processing of a single antenna.
It is well known that equalization techniques and RAKE reception techniques can suppress channel fading and resist ISI in the time domain. The equalization technique is an equalization of channel propagation characteristics and is used to cancel the ISI caused by the time-varying multipath propagation characteristics of the wireless channel, and is suitable for the case where the multipath signal is inseparable. If the received multipath signals can be separated, the RAKE diversity reception techniques can be used to combine the separated multipath signals to increase the diversity gain. Therefore, the difference between space-time processing techniques is mainly in space signal processing technology. Currently, terms describing spatial processing techniques are smart antennas, adaptive antennas, switched beam antennas, and spatial diversity receiving techniques.
The smart antenna technology in this paper is defined as an antenna array with beamforming capability, which can form a specific antenna beam for directional transmission and reception. Smart antennas can take advantage of the spatial characteristics of the signal to separate user signals, MAI, and multipath interference signals. The smart antenna includes an adaptive antenna and a switched beam antenna: the adaptive antenna array adaptively recognizes the direction of arrival of the user signal, and continuously adjusts its own pattern through feedback control; and the switched beam antenna predetermines a plurality of fixed beams, With the user's movement in the cell, the base station selects the corresponding beam that makes the received signal strongest. Spatial diversity reception is the use of diversity combining techniques to combine multiple uncorrelated received signals in space, which can effectively resist spatial selective fading of signals and improve system performance.
Smart antennas and spatial diversity reception techniques are essentially different, but so far there has been no literature comparing the two techniques in detail. There are even references in the literature that the terms "optimized combining" and "adaptive beamforming" are interchangeable. of. So what is the difference between these two technologies, and how to choose different space-time processing technologies according to the communication environment has been plaguing people.
This paper has done some work in this aspect, specifically analyzing the working principle of spatial diversity receiving technology and smart antenna (mainly adaptive antenna array), the difference between the two and the applicable occasions. The author first studies the wireless propagation characteristics between the mobile station and the base station, the space-time channel model and the multipath characteristics of different environments, and puts forward the main problems of the CDMA communication system. On this basis, the two spatial processing techniques are analyzed and compared. The basis and difference for solving the problem, and the application of each technology. Since it is not practical to place multiple antennas on the mobile station, this paper mainly analyzes the space-time processing technology of the uplink from the perspective of the base station.
1. Wireless channel propagation characteristics
Radio wave propagation in mobile communication environment has the characteristics of free space propagation loss, shadow fading and multipath fading. Multipath fading has a serious impact on the signal transmitted on the wireless channel. The reflection, scattering and diffraction of the radio wave make the received signal Delay spread, frequency (Doppler) expansion, and angle expansion.
(1) Delay spread
It is assumed that the transmitted signal is a pulse signal with a very narrow time width. After passing through the multipath channel, the received signal is a series of pulses due to the delay of each channel, so the waveform of the received signal is wider than the original pulse, resulting in ISI. .
(2) Frequency expansion
Due to the relative motion of the mobile user and the base station, each multipath will have a significant frequency shift, causing time-selective fading, ie the signal amplitude changes over time.
(3) Angle expansion
The angular spread is the broadening of the angle of arrival of the multipath signal to the antenna array, resulting in spatially selective fading, ie the signal amplitude is related to the position of the antenna.
2. Multiple access interference
Direct Drive Motor,Ex Direct Drive Motor,Direct Drive Motor Sewing Machine,Overlock Sewing Machine Motor
LISHUI SHUANGZHENG MOTOR CO.,LTD. , https://www.szservomotor.com