virtual LoS link. A promising strategy to address this issue is to install dedicated active sensors on the RIS and STAR-RIS [375], [380], [381] so that the signals can be received with higher SNR due to the reduced path loss. Another effective strategy to counteract the cumulative path loss is leveraging active RIS or STAR-RIS, which incorporate active amplifiers within each element to enhance the amplitude of the reflected or refracted echo signals [382].

3) **ISAC**: ISAC has emerged as a promising technology for providing both communication and sensing functions on the same shared platform [366]. A key design issue for ISAC is how to develop efficient multi-function transceivers. Specifically, for joint communication and sensing, it is necessary to balance the trade-off between improving the communication and sensing performance since they usually compete for the same resources. In addition, new types of waveforms must be designed since communication systems usually employ random signals for carrying information, while sensing systems such as radar use deterministic signals to recover the information to be sensed [383]. This trade-off was studied in [384], which designed the transmitted waveforms to share a deterministic sample covariance for delivering communication information, while assuming the realizations of the random waveform to be perfectly known at the sensing receivers. One application for sensing-assisted communication systems is to exploit sensing information such as echo signals to estimate the communication channels, thus avoiding the need for dedicated pilots and hence achieving enhanced spectral efficiency. For communication-assisted sensing, one could use OFDM waveforms common to communication systems for target detection and tracking. Efficient signal processing techniques must be developed for such ISAC systems and deserve future investigation.

### C. Circuit Theory Based Transceiver Design

To achieve demanding performance requirements in future wireless systems, new cross-disciplinary techniques are needed that better account for the effects of real hardware on communication systems. For example, circuit and EM theory can be used to establish the fundamental relationship between the digital signals processed by the computing hardware of a communication system, and the corresponding EM fields produced by the system's antennas [236], [385]. Compared with traditional simplified models, an end-to-end channel model based on circuit theory that takes into consideration fundamental physical laws such as conservation of energy and the superposition principle will provide more accurate predictions of performance.

For example, in circuit theory, an antenna can be modeled as a wave transformer that converts guided waves at its terminal ports into EM fields. As such, a multi-antenna communication system can be modeled as a multi-port network:

$$ \begin{bmatrix} V_t \\ V_r \end{bmatrix} = \begin{bmatrix} Z_{tt} & Z_{tr} \\ Z_{rt} & Z_{rr} \end{bmatrix} \begin{bmatrix} I_t \\ I_r \end{bmatrix}, \quad (23) $$

where $V_t$ and $I_t$ ($V_r$ and $I_r$) respectively denote the circuit voltages and currents for the transmitter (receiver), $Z_{tt}$ and

$Z_{rr}$ denote the mutual impedance matrix of the transmit and the receive array, respectively, $Z_{rt}$ denotes the mutual impedance between the transmit and receive array, and $Z_{tr}$ represents the back-scattering impedance from the receive antennas back to the transmitter. This latter term can typically be neglected for far-field scenarios since the relatively weak currents received at the users cause negligible effects at the transmitter [226], [236]. Such a multi-port network model based on circuit theory can facilitate in-depth analysis of impedance matching, antenna mutual coupling, structural losses, and both intrinsic and extrinsic sources of noise [386]. For example, [387] proposed a circuit-based EM-compliant communication model for RIS assisted wireless communication systems, which characterizes the end-to-end mutual coupling in RIS elements based on mutual impedance. In [388], a universally EM-compliant framework was proposed for RIS-assisted wireless systems based on impedance, admittance, and scattering parameter analysis, revealing the effects of impedance mismatches and mutual coupling on RIS communication performance. In addition, the authors of [385] showed that the typical scattering models utilized for RIS-aided channels largely ignored the structural scattering from the RIS, hence resulting in an unwanted specular reflection. To address this issue, an iterative algorithm was developed in [385] to optimize the tunable loads of RISs, based on the S-parameter representation in the presence of electromagnetic mutual coupling. Moreover, multi-port network models were used in [230], [237] to reveal the negative and positive aspects of mutual coupling on wireless communication systems. The authors of [389] used multi-port models and accurate antenna characterizations to determine the effect of mutual coupling on the achievable rate of SISO near-field communication systems. However, applying circuit theory to more complex wireless systems, possibly with multi-user interference, is an open problem that deserves more in-depth study in the future.

## VI. CONCLUSIONS

In this paper, we have provided a comprehensive overview of the recent progress on NGAT with an emphasis on three new aspects. Specifically, we first introduced new channel models and transceiver design issues for near-field communication systems, such as beamforming, beam training and channel estimation. Then, three new-form NGAT technologies were presented, including RISs (from metals to metamaterials), FAs (from static to movable), and HMIMO (from discrete to continuous). Furthermore, we elaborated recent advances in semantic-aware NGAT technology and its open challenges. Finally, other promising transceiver technologies were discussed to inspire future research. It is hoped that this paper will provide valuable insights as well as up-to-date guidance for future research on NGAT to unlock its full potential in next generation wireless networks.

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