Multiple Carrier Frequency Offset and Channel State Estimation in the Fading Channel

The joint estimation of multiple carrier frequency offsets (CFOs) and channel state information (CSI) in a multibase MIMO downlink system is considered here for the case of a fading channel. The multiple CFOs result from independent local oscillators at each basestation (BS), and cause accumulation of phase errors that compromise beamforming accuracy. Using previously devised multiple CFO estimation techniques for simultaneously transmitted training sequences in a non-fading channel, we begin by examining the effects of fading on a linear channel state estimate. An example illustrates that a fade rate threshold can be defined after which linear channel estimation is infeasible. Because of the dependence of the CFO estimators on the CSI estimate, this threshold applies to the CFO estimates as well. As a way to combat the channel variation, the maximum likelihood metric is modified to accommodate a parameterized fading channel estimate.