The Research Of Switched-Beam Smart Antenna

Due to the enormous performance gains associated with the use of smart antenna in wireless communications, smart antenna has received intense attention. There exist many adaptive algorithms those can be used in smart antenna systems, but with the limitation of the complexity of the algorithms and the processing speed, it is difficult to find a universal and practical algorithm for designing the smart antenna system with acceptable complexity and exellent real-time processing ability in various adverse signal environments. Thus, people pay their attention to the realization of the switched-beam smart antenna gradually. For the tracking-beam smart antenna, adaptive algorithm is used in the downlink beamforming, while for the switched-beam smart antenna, an optimum beam is selected out of a set of previously fixed beams in the base-station according to a given principle. As the adaptive algorithms are not necessary to be used in beamforming, the real-time processing can be guaranteed. In the thesis, the key technologys of the switched-beam smart antenna and its influence on the capacity of the cellular CDMA are discussed.First of all, the impact of radio environments on signals are discussed, especially the time delay and spatial spread caused by multipath propagation are researched. Also, radio channel models that are suitable for smart antenna are given and analyzed.Secondly, a simple comparison between the two types of smart antenna is discussed in terms of performance,processing speed,hardware complexity and the degree that be affected by the environment. Thirdly, several approaches of the selection of the narrow beams for switched-beam smart antenna are presented. In the approach that based on the detection of DOA(Direction-Of-Arrival) of users, the spatial smooth MUSIC,L times rotate TLS ESPRIT and spatial smooth MUSIC based on ILSP-CMA algorithms are compared in terms of the performance in the environment of multipath time delay spread,spatial spread and performance effected by SNR. This approach can only be used in macrocellular because of the restriction of the performance of the DOA algorithms in the environment of multipath spatial spread. In the approach that based on the detection of energy per chip per interference density(), a method of combined beam is introduced to solve the problem that the received signal will degrade rapidly when the mobile user is located at the edge of the beams. It can be proved in the simulation that SNR of the received signal can be improved by using the method.Fourthly, beamfoming algorithm based on the Chebyshev multinomial is used in the downlink beamforming. When the direction of the beam has to be changed with the wide of the beam keeping as the same, it only needs to alter the phase parameters of the weight vectors, while the amplitudes keep the same. It can be see that this algorithm is advisable for the multiuser and time critical application.Fifthly, the influence on the capacity of cellular CDMA by applying smart antenna technique to the CDMA system is explored. The system BER and Carrier-to-Interference-Ratio()are calculated and compared according to different beam width. The analysis shows that comparing with the omni antenna technique, the smart antenna technique can substantially increase the capacity of the CDMA system. Finally, the summary is made and main results are given.