Holographic multiple-input multiple-output (HMIMO) systems deploy ultra-dense antennas in confined spaces, yet spatial degrees of freedom (DoF) fail to scale with element count. Only by harnessing evanescent waves can the potential gains of ultra-dense arrays be unlocked. However, in practical scenarios, antenna apertures are typically too small relative to communication distances to generate significant near-field effects for capturing these evanescent waves. This paper proposes a method to alter the dispersion relation via locally resonant metamaterials (LRM), thereby enabling the radiation of near-field evanescent wave components into the far field. This approach leverages the DoF gains offered by ultra-dense elements within a confined aperture. Full-wave simulations validate the effectiveness of the evanescent-propagating wave conversion method, demonstrating an increase in the spatial DoF radiated into the far field by the HMIMO system even with a limited aperture.

An integrated sensing and communication (ISAC) scheme for a millimeter wave (mmWave) multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) Vehicle-to-Infrastructure (V2I) system is presented, in which both the access point (AP) and the vehicle are equipped with large antenna arrays and employ hybrid analog and digital beamforming structures to compensate the path loss, meanwhile compromise between hardware complexity and system performance. Based on the sparse scattering nature of the mmWave channel, the received signal at the AP is organized to a four-order tensor by the introduced novel frame structure. A CANDECOMP/PARAFAC (CP) decomposition-based method is proposed for time-varying channel parameter extraction, including angles of departure/arrival (AoDs/AoAs), Doppler shift, time delay and path gain. Then leveraging the estimates of channel parameters, a nonlinear weighted least-square problem is proposed to recover the location accurately, heading and velocity of vehicles. Simulation results show that the proposed methods are effective and efficient in time-varying channel estimation and vehicle sensing in mmWave MIMO-OFDM V2I systems.

With the development of wireless communication, the 6G mobile communication technology has received wide attention. As one of the key technologies of 6G, terahertz （THz） communication technology has the characteristics of ultra-high bandwidth, high security and low environmental noise. In this paper, a THz duplexer with a half-wavelength coupling structure and a sub-harmonic mixer operating at 216 GHz and 204 GHz are designed and measured. Based on these key devices, a 220 GHz frequency-division multiplexing communication system is proposed, with a real-time data rate of 10.4 Gbit/s for one channel and a transmission distance of 15 m. The measured constellation diagram of two receivers is clearly visible, the signal-to-noise ratio (SNR) is higher than 22 dB, and the bit error ratio (BER) is less than 10^-8. Furthermore, the high definition (HD) 4K video can also be transmitted in real time without stutter.

In view of the existing design challenges for Terahertz (THz) power amplifiers (PAs), the common design methods and the efforts of the State Key Laboratory of Millimeter Wave, Southeast University, China in the development of silicon-based THz PAs, mainly including silicon-based PAs with operating frequencies covering 100–300 GHz, are summarized in this paper. Particularly, we design an LC-balun-based two-stage differential cascode PA with a center frequency of 150 GHz and an output power of 14 dBm. Based on a Marchand balun, we report a 220 GHz three-stage differential cascode PA with a saturated output power of 9.5 dBm. To further increase the output power of THz PA, based on a four-way differential power combining technique, we report a 211–263 GHz dual-LC-tank-based broadband PA with a recorded 14.7 dBm Psat and 16.4 dB peak gain. All the above circuits are designed in a standard 130 nm silicon germanium (SiGe) BiCMOS process.