Beam Space-Time of a MIMO OFDM-based Wireless LAN System for 2-D Spreading Channels

We consider the performance analysis of beam space-time of a multiple-input multiple output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems for high data rate wireless transmission. The tools of density evolution with mixture Gaussian approximations are used to compute minimum operational signal-to-noise ratios (SNRs) for ergodic MIMO OFDM channels. With the flexibility offered by space-time block coding (STBC), the spectrum of system configurations are available with tradeoff among the bit-error-rate (BER), transmission rate, and implementation complexities. Due to the operations of spreading and dispreading combined with MMSE-based frequency domain equalization, the adverse effects of the low-SNR subcarriers on the average BER performance are potentially improved. This is a direct consequence of spreading, because even if the signal corresponding to a specific chip is obliterated by a deep frequency domain channel fade, after dispreading its effects are spread over the Walsh-Hadamard Transform (WHT) length. Hence there is a high chance of still recovering all the partially affected subcarrier symbols without errors. Also, the MIMO detection algorithm is discussed. Key-Words: OFDM, STBC, MIMO, Walsh-Hadamard Transform (WHT ), MMSE