A space-time coded multiantenna OFDM scheme over frequency-selective fading channels

We introduce a novel space-time coding scheme, called space-time cyclic-delay coding (STCDC), for multiple input multiple output orthogonal frequency-division multiplexing (MIMO-OFDM) systems over frequency-selective fading channels. The scheme exploits the redundancy introduced by channel coding. The transmitter antennas send the cyclically delayed copies of OFDM symbols, and the receiver uses the maximum likelihood estimation method to estimate the transmitted sequence. We show that STCDC-OFDM can achieve the maximum diversity, which is the product of the number of transmitter antennas (N/sub t/), the number of receiver antennas (N/sub r/) and the channel length (L), if the error control code has at least a free distance of N/sub t/L. Compared with the existing space-time coding and space-frequency coded MIMO-OFDM techniques, the proposed scheme has the advantages of being independent from the number of transmit antennas and having a lower implementation complexity.

1. INTRODUCTION

Space-time (ST) coding offers an attractive means of achieving high data rate wireless transmissions with diversity and coding gains [1], [2]. However, space-time codes are originally designed for flat-fading channels. In order to maintain decoding simplicity, most works on ST coding for frequency selective channels employ orthogonal frequency division multiplexing (OFDM) to convert a frequency-selective channel to a set of flat fading channels [3], [4]. Some ST-OFDM uses codes designed for flat channels but does not exploit the multipath diversity. Some combine space-time coding, space-frequency coding, and space-time-frequency coding across multiple antennas with OFDM technique to provide full diversities (both spatial and multipath diversities) over frequency-selective channels [5], [6], [7]. However, their computational complexity can be very high or the full diversity gain cannot be guaranteed, especially for systems with a large number of antennas. In practice, some existing redundancies, such as channel coding, can also be exploited in the space-time codes design, but they are ignored in the existing literature.