Architectures for MIMO-OFDM Systems in Frequency-Selective Mobile Fading Channels

Orthogonal frequency-division multiplexing (OFDM) is used in third and fourth generation and is being planned for fifth-generation (5G) systems. OFDM requires that the received subcarriers be orthogonal. However, various factors, such as residual carrier frequency offset due to Doppler shift in mobile fading channels, can lead to a loss in orthogonality between subcarriers, which results in intersymbol interference (ISI), intercarrier interference (ICI), and bit error rate (BER) degradation. In developing the conjugate cancelation (CC) scheme for mitigating these ISI and ICI of OFDM systems, we integrate the CC scheme together with a space-time (ST) system and form an STCC multiple-input-multiple-output (MIMO) scheme. This STCC system improves the BER without expending power or bandwidth or complexity. Additionally, a Walsh-Hadamard (WH) transform is employed as a precoder for this STCC system and forms a novel WHSTCC scheme. It provides a low peak-to-average power ratio (PAPR) at the transmitter and a significant diversity gain at the receiver. Simulations indicate that both STCC and WHSTCC schemes outperform the regular CC and ST systems in mobile fading channels. Both schemes are simple and are backward compatible, and they can serve as the baseband building block for reconfigurable 5G multiuser MIMO systems.