The multipath effect of communication channels can be efficiently addressed by using the orthogonal frequency-division multiplexing (OFDM) technology. But this technology is highly sensitive to Doppler Effect.
In the case of large-Doppler-spread environments, there could be possible degradation of the orthogonality between the subcarriers of OFDM systems, and inter-carrier interference could even emerge.
The symbol detection and subsequent channel decoding process could be affected by this phenomenon, which would, in turn, increase the bit error rate (BER) of the system.
To overcome the above-mentioned challenges, scientists at the Institute of Acoustics of the Chinese Academy of Sciences (IACAS) have developed a so-called bidirectional soft-decision feedback equalizer (BiSDFE) for OFDM systems.
Traditional single-direction soft-decision feedback equalizer (SDFE) was employed by the research team to equalize the received signal. The researchers then derived the frequency-reversed SDFE by carrying out the frequency-reversal operation of the demodulated symbols and predicted channel matrix.
Lastly, the output of soft symbols from both the equalizers was weighted by the team and combined to achieve the final equalizer output.
The correlation between the error position and noise in the outputs of both the equalizers was found to be low. The correlation of the predicted symbols was found to be high.
Therefore, by using the weighted combining operation, the team was able to achieve bidirectional diversity gain and minimize the error propagation problem in single-direction SDFE. They thus enhanced the bit error rate (BER) performance of the system.
The researchers then performed simulations to compare the BER performance of the proposed BiSDFE with that of the current OFDM equalizers. From the results, it was evident that it could achieve better BER performance under all values of signal-to-noise ratio (SNR).
Furthermore, its advantage was more evident in channels that involved large-Doppler-spread environments, for example, underwater acoustic channels. With respect to computational complexity, the BiSDFE falls on the same order of magnitude as the single-direction SDFE.
The BiSDFE not only demonstrates the potential of OFDM technology but also offers a potential solution for its use in large-Doppler-spread environments, for example, mobile underwater acoustic communication.
The study, which was financially supported by the National Natural Science Foundation of China, was published in the IEEE Wireless Communications Letters journal.
Zhao, S., et al. (2020) Bidirectional Soft-Decision Feedback Equalization for OFDM Systems. IEEE Wireless Communications Letters. https://doi.org/10.1109/LWC.2020.2988659.