Spatially Interpolated OFDM with Channel Estimation for Fast Fading Channels

OFDM is well suited for data-transmission over frequency-selective channels. If the cyclic prefix is chosen sufficiently long, intersymbol interference is avoided. Thus, the linear convolution becomes circular and the channel is rendered into a set of flat-fading channels, which can be easily equalized. But this holds no longer in the case of large Doppler spread, i.e., when the channel is rapidly changing. Then the subcarriers lose their orthogonality giving rise for so-called intercarrier interference which is detrimental for the overall system performance. Among previous proposals to cope with this problem was the use of multiple receive antennas. These are positioned in the moving direction of the receiver such that the individual antennas are receiving delayed replica of the transmit signal via identical paths. By means of interpolation this enables to fabric a virtual non moving point of reception which seemingly experiences no Doppler spread. So far, this technique has been applied in a purely noncoherent setting resorting to differential modulation. We will demonstrate how this approach can be exploited by coherent reception. Our emphasis thereby lies on the use of higher-level modulation as well as least-square (LS) and minimum mean square error (MMSE) channel estimation.