Space-frequency coded broadband OFDM systems

Space-time coding for fading channels is a communication technique that realizes the diversity benefits of multiple transmit antennas. Previous work in this area has focused on the narrowband flat fading case where spatial diversity only is available. We investigate the use of space-time coding in OFDM-based broadband systems where both spatial and frequency diversity are available. We consider a strategy which basically consists of coding across OFDM tones and is therefore called space-frequency coding. For a spatial broadband channel model taking into account physical propagation parameters and antenna spacing, we derive the design criteria for space-frequency codes and we show that space-time codes designed to achieve full spatial diversity in the narrowband case will in general not achieve full space-frequency diversity. Specifically, we show that the Alamouti (see IEEE J. Sel. Areas Comm., vol.16, p.1451-58, 1998) scheme across tones fails to exploit frequency diversity. For a given set of propagation parameters and given antenna spacing, we establish the maximum achievable diversity order. Finally, we provide simulation results studying the influence of delay spread, propagation parameters, and antenna spacing on the performance of space-frequency codes