Aspects of MIMO Channel Estimation for LTE Uplink

The use of MIMO (Multiple Input Multiple Output) in current and future mobile communications relies on accurate channel estimates. In OFDM based systems such as LTE (Long Term Evolution), the channel estimation can be successfully designed with processing in a transform domain, both for uplink and downlink. One such popular transform is to use the IDFT (Inverse Discrete Fourier Transform), window the channel estimate in the time domain and then use DFT to transform back to the frequency domain again. This channel estimate is distorted in the band edges leading to a biased channel estimate. One suggested solution to this problem is to use a DCT (Discrete Cosine Transform) based channel estimator, such that this distortion is reduced. However, as discussed in the current paper, the DCT based channel estimate suffers more from cyclically shifted reference signals, as used for MIMO in LTE for uplink, than the DFT based channel estimator. In order to overcome band edge problems with the DFT based channel estimate, a method is proposed of debiasing over-sampled DFT based channel estimate by using digital sinc functions. Simulation results indicate that the de-biased DFT based channel estimate is more robust against interference from cyclic shifted reference signals than DCT based channel estimates.