Challenge of Channel Estimations and Its Way Out in MIMO OFDM Systems for Mobile Wireless Channels

In current common channel estimation schemes for MIMO OFDM systems, channel state information is usually achieved by estimating channels according to frequency domain pilot sequences. However, when the length of MIMO OFDM symbols is larger than that of wireless channel delay, there are two intractable issues in the case of cellular fast fading channel scenarios with large numbers of users, i.e., the bandwidth overhead of channel estimation and the difficulty to construct large numbers of orthogonal training sequences. Inspired from the Steiner channel estimation method in multi-user CDMA uplink wireless channels, we proposed a new design scheme of training sequence in time domain to conduct channel estimation in MIMO OFDM systems. According to the proposed schemes, training sequences of different transmit antennas can be simply obtained by truncating the circular extension of one basic training sequence, and the pilot matrix assembled by these training sequences is one circular matrix with good reversibility. Furthermore, when the length of channel profiles is less than that of MIMO OFDM symbols, more bandwidth resources can be saved, as the training sequence only occupies a part of MIMO OFDM symbols. At last, we analyze the classical common time and frequency domain schemes in terms of channel overheads and computation complexity, and the results disclose the proposed scheme can be one way out for current common frequency domain channel estimation issues. For typical ITU indoor, pedestrian and vehicular channel profiles, the corresponding numerical results indicate the proposed method can save abundant bandwidth and achieve good channel estimation accuracy when compared with classical frequency and time domain approaches, respectively. Key-Words: channel estimation, wireless communications, MIMO, OFDM, pilot sequences