Space-time block codes versus space-frequency block codes

In wireless broadband systems the available time, frequency and spatial diversity can be exploited using complex space-time-frequency codes. However, in order to retain a flexible scheme with low complexity we prefer a concatenated system where a channel code picks up time and frequency diversity and a simple orthogonal design is used to obtain spatial diversity. The channel needs to be constant during transmission of one orthogonal design. In a 4G OFDM system the elements of the orthogonal design can be distributed as space-time block code over successive OFDM symbols which might cause problems in fast fading or as space-frequency block code over neighbouring subcarriers which causes problems in severe frequency-selective channels. We analyse the suitability and performance of both schemes under 4G conditions and propose a space-time-frequency mapping which might be a good compromise in terms of performance and delay.