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. In this paper, we investigate the use of space-time coding in OFDMbased broadband systems where both spatial and frequency diversity are available. We consider a strategy which basically consists of coding across OFDM tones and will therefore be called space-frequency coding. For a spatial broadband channel model taking into account physical propagation parameters and antenna spacing, we derive the design criteria f o r 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 scheme across tones fails to exploit frequency diversity. For a given set of propagation parameters and given antenna spacing, we establish the m a x i m u m 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. 1. I N T R O D U C T I O N A N D O U T L I N E Two of the major impairments of wireless communications systems are fading caused by destructive addition of multipaths in the propagation medium and interference f r o m other users. Diversity provides the receiver with several (ideally independent) replica of the transmitted signal and is therefore a powerful means to combat fading and interference. Common forms of diversity are time diversity (due to Doppler spread) and frequency diversity (due to delay spread). In recent years the use of spatial (or antenna) diversity has become increasingly popular. Spatial diversity is particularly attractive since it can be provided without loss in spectral efficiency. Receive diversity, i.e., the use of multiple antennas on the This work was supported in part by FWF grants J1629-TEC and J1868-TEC. H. Bolcskei is on leave from the Institut fiir Nachrichtentechnik und Hochfrequenztechnik, Technische Universitat Wien, Vienna, Austria. A. Paulraj is on part-time leave at Gigabit Wireless Inc., 3099 N. First Street, San Jose, CA. 0-7803-6596-8/00/$10.00