IM-based Pilot-assisted Channel Estimation for FTN Signaling HF Communications

This paper investigates doubly-selective (i.e., time- and frequency-selective) channel estimation in faster-than-Nyquist (FTN) signaling HF communications. In particular, we propose a novel IM-based channel estimation algorithm for FTN signaling HF communications including pilot sequence placement (PSP) and pilot sequence location identification (PSLI) algorithms. At the transmitter, we propose the PSP algorithm that utilizes the locations of pilot sequences to carry additional information bits, thereby improving the SE of HF communications. HF channels have two non-zero independent fading paths with specific fixed delay spread and frequency spread characteristics as outlined in the Union Radio communication Sector (ITU-R) F.1487 and F.520. Having said that, based on the aforementioned properties of the HF channels and the favorable auto-correlation characteristics of the optimal pilot sequence, we propose a novel PSLI algorithm that effectively identifies the pilot sequence location within a given frame at the receiver. This is achieved by showing that the square of the absolute value of the cross-correlation between the received symbols and the pilot sequence consists of a scaled version of the square of the absolute value of the auto-correlation of the pilot sequence weighted by the gain of the corresponding HF channel path. Simulation results show very low pilot sequence location identification errors for HF channels. Our simulation results show a 6 dB improvement in the MSE of the channel estimation as well as about 3.5 dB BER improvement of FTN signaling along with an enhancement in SE compared to the method in [1]. We also achieved an enhancement in SE compared to the work in [2] while maintaining comparable MSE of the channel estimation and BER performance.

Pilot Design and Doubly-Selective Channel Estimation for Faster-than-Nyquist Signaling

Being capable of enhancing the spectral efficiency (SE), faster-than-Nyquist (FTN) signaling is a promising approach for wireless communication systems. This paper investigates the doubly-selective (i.e., time- and frequency-selective) channel estimation and data detection of FTN signaling. We consider the intersymbol interference (ISI) resulting from both the FTN signaling and the frequency-selective channel and adopt an efficient frame structure with reduced overhead. We propose a novel channel estimation technique of FTN signaling based on the least sum of squared errors (LSSE) approach to estimate the complex channel coefficients at the pilot locations within the frame. In particular, we find the optimal pilot sequence that minimizes the mean square error (MSE) of the channel estimation. To address the time-selective nature of the channel, we use a low-complexity linear interpolation to track the complex channel coefficients at the data symbols locations within the frame. To detect the data symbols of FTN signaling, we adopt a turbo equalization technique based on a linear soft-input soft-output (SISO) minimum mean square error (MMSE) equalizer. Simulation results show that the MSE of the proposed FTN signaling channel estimation employing the designed optimal pilot sequence is lower than its counterpart designed for conventional Nyquist transmission. The bit error rate (BER) of the FTN signaling employing the proposed optimal pilot sequence shows improvement compared to the FTN signaling employing the conventional Nyquist pilot sequence. Additionally, for the same SE, the proposed FTN signaling channel estimation employing the designed optimal pilot sequence shows better performance when compared to competing techniques from the literature.