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Faster-than-Nyquist signaling (FTNS) has emerged as a promising technique to increase communication capacity in bandwidth-limited channels. However, the presence of FTN-induced inter-symbol interference (FTN-ISI) in the received observations, is detrimental to channel estimation (CE) and data detection in terms of computational complexity and performance. This paper copes with these problems by incorporating linear pre-equalization (LPE) and composite precoding formed by linear spectral precoding and Tomlinson-Harashima precoding (THP), into the FTNS. Specifically, LPE completely pre-equalizes the FTN-ISI, while spectral precoding resolves the LPE-caused signal spectral broadening by introducing proper artificial ISI, which is pre-equalized by THP. Channel-induced ISI, as the only ISI component in the observations, is estimated and equalized using the classical frequency-domain low-complexity schemes. We show that there are four advantages of the LPE-aided CE over the CE designed for the FTN transmissions without FTN-ISI pre-equalization, namely lower pilot overhead, simpler yet optimal pilot sequence design, lower mean-squared error of CE, and more robust against the FTN-ISI. Simulation results show that our scheme improves the performance of CE and detection, compared to existing FTN frequency-domain CE and equalization schemes. [ABSTRACT FROM AUTHOR]

Orthogonal time frequency space (OTFS) has shown to be a promising modulation technology that achieves the robust wireless transmission in high-mobility environments. The high mobility incurred Doppler effect in OTFS system, is represented as a continuous and relatively large band in the Doppler frequency. It yields the equivalent channel responses (ECRs) in the system change significantly within one symbol block, posing a challenge to channel estimation (CE) or tracking. In order to tackle this issue, in this paper, a set of transform-domain basis functions is designed to span a low-dimensional subspace for modeling the OTFS channel. Then, the CE can be performed by estimating a few projection coefficients of ECRs in the developed subspace, with training pilots. According to the individual transmission characteristic of OTFS signal, we propose a corner-inserted pilot pattern, which targets the low pilot overhead and satisfactory CE performance. Moreover, an OTFS signal detector, leveraging the time-domain channel equalization, linear-complexity interference cancellation and delay-Doppler domain maximal ratio combining detection, is developed to retrieve the transmitted data symbols. The simulations show the precisely estimated ECRs enable the detector to ideally demodulate 256-ary quadrature amplitude modulation signaling, under a velocity of 550 km/h at 5.9 GHz carrier frequency. [ABSTRACT FROM AUTHOR]

Orthogonal time frequency space (OTFS) is a two-dimensional modulation scheme realized in the delay-Doppler domain, which targets the robust wireless transmissions in high-mobility environments. In such scenarios, OTFS signal suffers from multipath channel with continuous Doppler spread, which results in significant inter-symbol interference and inter-Doppler interference (IDI). In this article, we analyze the interference generation mechanism, and compare statistical distributions of the IDI in two typical cases, i.e., limited-Doppler-shift channel and continuous-Doppler-spread channel (CoDSC). Focusing on the OTFS signal transmission over the CoDSC, our study firstly indicates that the widespread IDI incurs a computational burden for the element-wise detector like the message passing in the state-of-the-art works. Addressing this challenge, we propose a block-wise OTFS receiver by exploiting the structure and characteristics of the OTFS transmission matrix. In the receiver, we deliberately design an iteration strategy among the least squares minimum residual based channel equalizer, reliability-based symbol detector and interference eliminator, which can realize fast convergence by leveraging the sparsity of channel matrix. The simulations demonstrate that, in the CoDSC, the proposed scheme achieves much less detection error, and meanwhile reduces the computational complexity by an order of magnitude, compared with the state-of-the-art OTFS receivers. [ABSTRACT FROM AUTHOR]

Similar to orthogonal frequency division multiplexing receiver, the frequency-domain (FD) channel estimation (CE) and equalization are indispensable parts in the coherent single carrier frequency division multiplexing (SC-FDM) receiver. When the channel varies slowly, the FD processing is cost effective. For the applications with high mobility, the traditional implementation of the SC-FDM receiver causes significant performance degradation. In this paper, we propose to estimate time-varying pieces of channel taps for pilot symbols based on basis expansion model (BEM), and subsequently to reconstruct time-domain (TD) channel response for data symbols by utilizing the Slepian sequences-based piece-wise interpolation. Furthermore, two simplified schemes, i.e., the Slepian sequences-based multiple-point interpolation and the segmented BEM, are developed to reduce the computational complexity of the TD-CE. The Cramer-Rao lower bound (CRLB) of channel impulse response (CIR) estimation is also analyzed. In the light of the sparsity of TD channel gain matrix, we design an iterative least-squares QR decomposition algorithm to equalize the SC-FDM symbols. In the simulated SC-FDM system, when considering the carrier frequency of 5.9 GHz and the velocity of about 510 km/h, we observe that the traditional FD methods cause the demodulation failure, while the proposed TD processing schemes achieve ideal error-probability performance and preserve a relatively low complexity. [ABSTRACT FROM AUTHOR]

When orthogonal frequency-division multiplexing (OFDM) is used in high-speed mobile scenarios, the traditional implementation of the OFDM receiver causes significant performance degradation. This letter proposes a subband superposed oversampled OFDM (SS-OOFDM) scheme for accommodating the diverse scenarios, including the scenarios with high mobility. To track the fast-varying mobile channel, a time-domain channel estimation method is developed to first obtain the channel impulse response (CIR) values at pilots by means of the basis expansion model. Subsequently, by utilizing Slepian sequences-based interpolation, the CIR values at data symbols are reconstructed. Furthermore, we design a subband decision feedback and feedforward equalizer for exploiting the Doppler and multipath diversity gains. Simulation results show that the SS-OOFDM, especially in high-speed mobile channels, outperforms the traditional OFDM receiver in terms of the bit-error-rate performance. [ABSTRACT FROM AUTHOR]

In OFDM system, channel estimator with fixed-coefficient interpolation gives rise to significant estimation errors when the estimator mismatches the experienced time-varying channel. This paper presents a pilot-aided channel estimator adopting a double one-dimensional (1-D) polyphase subfilter structure, in which a few interpolating coefficients are applicable to all subcarriers of an OFDM symbol. By analyzing the channel interpolation for OFDM receiver, we propose a two-dimensional (2-D) adaptation scheme of the interpolating coefficient, based on a block least-mean-square algorithm, which requires none of knowledge on channel statistics. The 2-D adaptation scheme is simplified as two types of 1-D schemes respectively for the time and frequency dimensions, based on the known frequency- and time-direction interpolating coefficients. By simultaneously exploiting channel correlation in the time and frequency dimensions, the adaptive interpolators are trained to match channel properties. Furthermore, the proposed schemes of adaptive interpolation demand lower implementation cost, compared with the existing methods. Simulation results show that the adaptive interpolators converge at a high rate, and that the simplified schemes outperform the conventional methods in the high-speed mobile channel and the single-frequency-networks channel with long-delay echoes, while preserving a low implementation complexity. [ABSTRACT FROM AUTHOR]