Making preamble shorter in OFDM real-time wireless control systems

Real-time and robust communication is essential for wireless control systems (WCS). Preambles are considered as significant communication overheads in WCS where payloads are typically small. To shorten the overhead in OFDM systems, previous works have considered fixed subcarrier sampling to obtain subcarrier-fixed short preambles (SFSP) and adopted frequency-direction interpolation (FDI) for channel estimation. Despite of advantage in fast fading channel, it results in poor performance in frequency-selective fading channel. In this work, we propose a subcarrier-selectable short preamble (SSSP) which provides full sampling coverage of all subcarriers with several preamble transmissions. By exploiting the periodicity of transmissions in WCS, the SSSP scheme obtains the channel estimation of non-selected subcarriers by not only FDI but also the time-direction extrapolation (TDE), which is robust in frequency-selective fading. In addition, in order to make the channel estimation robust in both fast fading and frequency-selective fading we adopt an adaptive algorithm altering between FDI or TDE mode. The feasibility of the proposed method is validated by evaluating symbol error rate (SER) in fast frequency-selective fading. Simulation results verify that unlike the SFSP the adaptive SSSP scheme shows no error floor in SER curve when the channel is frequency-selective fading, and performs similar to the SFSP scheme when the channel is fast fading.