Differential Space-Frequency Group Codes for MIMO-OFDM

In recent years, coherent space-frequency (SF) coded multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems have attracted great attention. However, obtaining perfect channel estimates for frequency-selective channels with lots of taps is challenging and computationally complex. In this paper, we present a transmission scheme that uses differential space-frequency group codes (DSFC) for MIMO-OFDM systems. Our proposed scheme is based on the theory of space-frequency group codes. We use the group properties of space-frequency codes that has recently been investigated and obtain a differential transmission rule incorporated with subcarrier allocation method that allows data to be sent without channel estimates at the transmitter or receiver. Recently proposed noncoherent space-frequency codes using MIMO-OFDM have exponential computational complexity with code rate in frequency-selective channels. Compared to existing noncoherent space-frequency codes, our proposed code design is computationally more efficient and is easier to implement. We also demonstrate that the presence of additional resolvable channel taps does not monotonically improve the performance of DSFCs, which shows that obtaining full space-frequency diversity order for increasing number of channel taps decreases the performance of the receivers with lack of channel state information (CSI). This result is completely different from the coherent case of space frequency group codes in which additional channel taps increase the BER performance due to the exploitation of frequency diversity present in MIMO-OFDM frequency-selective channels.