Aiming at improving collaborative learning, the current study builds and tests an LLM-empowered agent to enhance student engagement in group discussion. We introduce a four-module conversational system, providing a user-friendly chat website integrated with an LLM agent, where students can discuss on and learn a specific topic in an online classroom. The LLM agent can continuously monitor the dialogue process and give constructive and reflective responses as a knowledgeable learning peer to engage students in the computer-supported collaborative learning (CSCL) environment. To evaluate the pedagogical performance of the system, three LLMs were tested by prompting. The results showed that LLMs with only prompting were unable to accurately process multi-user dialogue information and lacked pedagogical strategies in their responses.

The spectrally efficient multicarrier faster-than-Nyquist (MFTN) signaling provides an efficient and robust solution for enhancing transmission rate and resisting channel impairments. In this paper, an evolutionary non-orthogonal physical waveform is proposed for simultaneously achieving high spectral and energy efficiency via combining MFTN signaling with index modulation (IM). Exploiting the implicit transmission of IM, the inactivated subcarriers alleviate the inherent two-dimensional interferences imposed by time-frequency packing in MFTN. Then, we develop a pair of iterative equalization algorithms based on vector approximate message passing (VAMP). For the first time-domain equalization (TDE), an extended constellation set is constructed for uniformly characterizing the activated and inactivated subcarriers. To further reducing the computational complexity, we establish a subcarrier-based segment-wise frequency-domain received signal model and accordingly develop low-complexity VAMP-based frequency-domain equalization (FDE) combined with interference elimination. Corroborated by simulations, the novel MFTN-IM waveform achieves superior bit error rate (BER) performance over benchmark waveforms in the high signal noise ratio (SNR) regions. Interestingly, while the proposed VAMP-TDE outperforms the proposed VAMP-FDE in BER performance, the latter is more competitive in balancing demodulation performance and computational complexity.

A low-complexity parametric bilinear generalized approximate message passing (PBiGAMP)-based receiver is conceived for multi-carrier faster-than-Nyquist (MFTN) signaling over frequency-selective fading channels. To mitigate the inherent ill-conditioning problem of MFTN signaling, we construct a segment-based frequency-domain received signal model in the form of a block circulant linear transition matrix, which can be efficiently calculated by applying a two dimensional fast Fourier transform. Based on the eigenvalue decomposition of the block circulant matrices, we can diagonalize the covariance matrix of the complex-valued colored noise process imposed by the associated two dimensional non-orthogonal matched filtering. Building on this model, a PBiGAMP-based parametric joint channel estimation and equalization (JCEE) algorithm is proposed for MFTN systems. In this algorithm, we introduce a pair of additive terms for characterizing the interferences arising from adjacent segments and employ the exact discretea prioriprobabilities of the transmitted symbols for improving the bit error rate (BER) performance. To further enhance the system’s robustness in the presence of ill-conditioned matrices, we develop a refined PBiGAMP-based JCEE algorithm by introducing a series of scaled identity matrices. Moreover, the proposed PBiGAMP-based JCEE algorithms may be readily decomposed into GAMP-based equalization algorithms, when the channel state information is perfectly known. The overall complexity of the proposed algorithms only increases logarithmically with the total number of transmitted symbols. Our simulation results demonstrate the benefits of the proposed PBiGAMP-based iterative message passing receiver conceived for MFTN signaling.

Spectrally efficient frequency division multiplexing (SEFDM) relying on index modulation (IM) has emerged as a promising multicarrier technique. In this paper, we develop a joint channel estimation and equalization method based on factor graphs for SEFDM-IM signaling over frequency-selective fading channels. By approximating the interference in the frequency domain, we reformulate the problem to obey a linear state-space model and construct a multi-layer factor graph. To support a reconfigurable architecture, non-orthogonal demodulation is adopted and the colored noise encountered is approximated by a complex auto-regressive (CAR) model. For deriving a low-complexity parametric Gaussian message passing (GMP)-based method, we exploit an expectation propagation (EP)-based technique for approximating the discrete a posteriori distributions of the transmitted symbols in a Gaussian form. To further simplify the result, variational message passing (VMP) is applied to an equivalent soft node to obtain a Gaussian form. Moreover, we also derive the Cramér-Rao lower bound (CRLB) in closed-form. The overall complexity only grows linearly with the number of subcarriers and logarithmically with the length of the channel's memory. Compared to its Nyquist signaling based counterpart, SEFDM-IM signaling relying on the proposed algorithm exhibits up to 25% higher bandwidth efficiency without any bit error rate (BER) performance degradation.

Faster-than-Nyquist (FTN) signaling with index modulation (IM) is an attractive non-orthogonal transmission scheme characterized by high spectral efficiency and energy efficiency. In this paper, we develop a hybrid belief propagation (BP) and expectation propagation (EP) based iterative receiver for FTN-IM systems. To approach the optimal maximum a posteriori (MAP) receiver, we derive the factorization of marginal posterior probability and construct the corresponding factor graph by ignoring trivial interferences. To address the inherent colored noise imposed by FTN signaling, we employ autoregressive (AR) model to approximate the correlated noise samples. To further design low-complexity parametric message passing receiver, we resort to expectation propagation (EP) to derive Gaussian approximation of discrete transmitted symbols containing specific inactivated zeros. As a result, the overall complexity grows linearly with the number of transmitted symbols. Simulation results show that the coded FTN-IM system relying on the proposed iterative receiver can improve the spectral efficiency up to 43% without performance loss. For identical spectral efficiency with the Nyquist counterpart, FTN-IM signaling achieves 0.80 dB performance gain with proper packing factor and coding rate.

In this paper, we propose a timing synchronization method for orthogonal frequency division multiplexing (OFDM) systems by exploiting the averaged differential correlations of preamble symbol. For the designed simple preamble whose second half-symbol is the complex conjugate of the first one in the time-domain, the proposed timing synchronization method has an impulse-shaped timing metric. Simulation results indicate that the proposed method improves the timing estimate mean square errors (MSE) performance remarkably than conventional methods at low signal-to-noise ratio (SNR) in multipath fading channel. To reduce the computation complexity of the proposed method, we suggest modifying the timing estimation by simplifying the amplitude information of OFDM signals in polar coordinates.