Method for selecting switched orthogonal beams for downlink diversity transmission

#The invention relates in general to wireless communication systems. More particularly, it relates to a system for improving the downlink transmission performance of wireless communication systems.

#Wireless communications may suffer from four major impairments, namely path loss, multipath fading, inter-symbol interference and co-channel interference. Adaptive antenna systems may be used to suppress the effects of such impairments to improve the performance of wireless communication systems. There are two types of adaptive antenna systems, namely diversity antenna systems and beamforming antenna systems. In a diversity antenna system, multiple low-correlation or independent fading channels are utilised in order to compensate multipath fading, thus achieving diversity gain. Beamforming antenna systems, on the other hand, achieve beamforming gain by applying spatial directivity, thus limitedly compensating for path loss and suppressing co-channel interference.

#In a diversity antenna system, antenna spacing is usually required to be sufficiently large, for example 10λ, in order to achieve low-correlation or independent fading channels. This is especially true for small angular spread environments. However, a beamforming antenna system need to achieve spatial directivity, so signals received at and/or transmitted from all beamforming antennas must be correlated. This means that for a beamforming antenna system, antenna spacing is usually small, for example half wavelength for a uniform linear array (ULA). Because of the conflict between the required antenna spacing for diversity and beamforming antenna systems, there exists a technical prejudice among those skilled in the art that diversity and beamforming gains cannot be achieved simultaneously.

#Embodiments of the invention are described hereinafter with reference to drawings, in which:

#FIG. 1 is a block diagram of a conventional downlink transmission system using the STBC technique;

#FIG. 2 is a block diagram of a conventional downlink transmission system combining an adaptive antenna system with the STBC technique;

#A system according to an embodiment of the invention is disclosed hereinafter to address the need for a system that provides the performance of an adaptive antenna system while possessing the implementation simplicity of a switched beam system for achieving diversity gain and beamforming gain simultaneously

#Such a system involves a practical, inexpensive, and robust downlink transmission scheme for achieving both diversity and beamforming gain based on some simple uplink measurements. In particular, the system approximates the maximum beamforming gain and achieves full diversity transmission as well by combining the STBC technique with a switched beam antenna system. The switched beam antenna system preferably has the same simple physical structure as that employed for uplink in a base station.

#The system involves a downlink diversity transmission scheme to combat fading effect for wireless communication systems where multiple antennas are employed in base stations while only a single antenna is employed in mobile terminals. A number of advantages are associated with such a system, which are described hereinafter.