MULTIUSER DETECTION METHOD ROBUST AGAINST FREQUENCY OFFSETS IN UPLINK MIMO-OFDM

The increasing pupularity of mobile computing has pushed the demand for a support for a larger number of users and a very high sprectrum effi-ciency must be achieved in the futre wireless access systems. Multiuser multiple input multiple output (MU-MIMO) systems are attracting attention due to their frequency efficiency. However, in uplink MU-MIMO systems, different frequency offsets among multiple mobile stations (MSs) significantly degrade the transmission quality, especially when orthogonal frequency division multiplexing (OFDM) is used. In this paper, the influence of these frequency offsets is analyzed in a frequency selective fading environ-ment. Numerical analysis shows that an error floor occurs in the bit error rate and the influence of the frequency offset becomes larger in short delay spread environments. To overcome this problem, a new multiuser detection method (MUD) is proposed to compensate for the frequency offset by introducing an auto frequency controller after frequency-space equalization in each data stream. To evaluate the effect of the proposed method in an actual indoor propagation environment, the measurement system was developed. The effect of the proposed method is evaluated in a frequency selective fading environ-ment by computer simulations and measured results