A novel simplified opportunistic beamforming method for wide-band systems

Opportunistic beamforming, also called random beamforming, has been proved to be an effective way to utilize spatial resource for downlink of multiuser scheduling systems (P. Viswanath et al., 2002). The basic idea is to artificially create large channel fluctuations among different users by transmitting random weighted signal copies from multiple transmit antennas. These fluctuations can then be exploited by a smart user scheduler to enhance the performance in system level perspective. However, for wide-band systems, these weighted transmitted signals can not form a single beam pointing to certain direction due to multipath propagation. In this case, orthogonal frequency division multiplexing (OFDM) is suggested to slice the whole bandwidth into many narrow band flat fading sub-carriers so that random beamforming concept can be applied on each of them. Conventional wide-band random beamforming needs to generate a large number of random transmit vectors which is equal to number of sub-carriers. This induces high computational complexity and make the problem of optimizing probability density function (pdf) of the these random vectors hard to be followed. Moreover, it also needs as many number of IFFT operation as number transmit antennas. This paper presents a novel structure for random beamforming of wide-band systems by using cyclic delay operation where only two random vectors and one IFFT operation are needed. Compared with traditional method, our proposal can significantly reduce the system complexity. Furthermore, the simulation results show that our proposal points out a direction for how to optimize the pdf of random vectors