In this study, the authors propose a low-complexity frequency-domain turbo equalisation scheme for doubly-selective high-frequency (HF) channel. The performance of time- and frequency-domain turbo equalisers (TD/FD-TE) by applying them to the waveforms of STANAG 4539 is compared. Compared with TD-TE, FD-TE has reduced computational complexity. However, its performance degrades for time-varying channels. To improve the performance of FD-TE, they divide the received blocks into consecutive sub-blocks where the channel impulse response remains fixed for each sub-block. Due to the absence of cyclic prefix in the sub-blocks, the overlap frequency-domain equalisation (FDE) is used in the first iteration. In the following iterations, the authors remove the interference term, therefore the channel matrix becomes circular and low-complexity FDE is used. Simulation results show that compared with the TD-TE, the performance loss of FD-TE equipped with the authors' proposed scheme at the bit error rate of 1x10(-5) is <1dB with much lower computational complexity. By real-time software defined radio (SDR) implementation of the proposed receiver on a general purpose processor (GPP), the authors ensure real-time feasibility of their algorithm. They use GNU radio tool for SDR implementation. It is shown that with GNU radio capabilities such as multi-threading and pipelining, further reduction in processing time can be achieved.
