Zhiqiang Liu

A 60-GHz waveform-sharing joint radar-communication (JRC) transceiver with phase-locked loop (PLL)-based chirp spread spectrum (CSS) modulator is presented for seamless integrated sensing and communication (ISAC). Unlike existing communication-centric JRC transceivers that prioritize high throughput, the proposed sensing-centric transceiver employs unified CSS waveforms to perform uncompromised sensing performance while enabling robust long-range communication. To this end, a PLL-based CSS modulator is implemented, simplifying the transceiver architecture while generating wideband CSS signals with high chirp slope, superior linearity, and low phase noise. Within this modulator, a multi-bank capacitor array is integrated into a two-point modulation (TPM)-based sub-sampling (SS) PLL, enabling fast wideband chirp generation and rapid frequency hopping while achieving excellent phase noise. Moreover, an adaptive binary-search (BS) overlap compensation scheme is proposed to mitigate the discontinuity issues in the multi-bank capacitor array while ensuring robust and orderly convergence. Furthermore, building on the TPM-based sub-sampling PLL (SSPLL), a digital CSS modulation engine is developed to enable wideband CSS signal generation without compromising the chirp linearity. Fabricated in a 65-nm CMOS process, the CSS transceiver core occupies 6.11 mm² and consumes 568 mW. For the over-the-air (OTA) test, broadband high-gain horn antennas with a gap waveguide (GW)-based transition are integrated with the chip through a low-loss wire-bonding matching scheme, achieving a low-cost, compact, and reliable assembly. OTA measurements demonstrate that the measured range resolution is approximately 5.5 cm, while achieving a communication range exceeding 500 m at a 4.138 Mbps data rate with a bit error rate (BER) below $10^{-5}$ .

In this letter, a method of designing quasi‐elliptic bandpass filter with ultrabroad reflectionless bandwidth and high selectivity is proposed. It is implemented on the basis of a quasi‐elliptic absorptive lowpass filter (ALPF) prototype that features both a low‐reflection response over all normalized frequencies and two controllable transmission zeros near the passband. A distributed‐element absorptive bandpass filter (ABPF) scheme using coupled‐line units is derived from this quasi‐elliptic ALPF prototype. With this distributed‐element ABPF scheme, both ultrabroad reflectionless bandwidth and compact size can be realized. Equations for closed‐form design are derived. A microstrip ABPF operating at 2.0 GHz (i.e., f₀) is designed for validation. The measured roll‐off rates of the upper and lower transition bands are 116.44 and 157.41 dBc/GHz, respectively. The return loss is better than 10 dB from direct current to 6.25f₀. The size of the overall circuit is 0.12λ_g².

A 60-GHz joint radar–communication (JRC) transceiver is presented in this article. To achieve a compact JRC transceiver architecture with highly reused RF modules, a dedicated reconfigurable dual-mode Gilbert cell is proposed. Specifically, in the communication mode, the dual-mode Gilbert cells operate as conventional upconversion mixers to modulate the baseband (BB) communication data, while it could also be configured as an amplifier to strengthen the LO signal in the radar mode. Compared with the quadrature IF chirp modulation scheme in which the chirp bandwidth is constrained by the narrowband IF devices, in this work, a wideband chirp generated directly from the LO chain could be applied for radar sensing, significantly improving the range resolution in the radar mode. Besides, the direct RF dechirping could also relieve the hardware burden during radar signal processing compared with the digital dechirping utilized in the IF chirp modulation scheme. Fabricated in a 65-nm CMOS process, this JRC transceiver realizes an ultracompact chip size, and a 16-dBm saturated TX output power, an 11-dBm OP $_{\text{1\,dB}}$ , and a 5.8-dB minimum RX noise figure (NF) are also achieved. Experiments demonstrate that the proposed JRC transceiver supports $>$ 4-GHz transmitted chirp bandwidth with $<$ 3.75-cm range resolution in the radar mode, and a mechanical vibration with 1- $\mu$ m displacement is successfully detected. Besides, a $>$ 7-Gb/s data rate in the 16-QAM over-the-air (OTA) communication is also demonstrated.

Absorptive bandpass filters (ABPFs) are highly attractive in modern microwave communication systems due to their ability to internally absorb the harmful stopband RF-power reflections. This paper reports an approach to designing quasi-elliptic ABPFs with ultra-wide reflectionless range, enhanced selectivity, and compact size. The method is realized based on a fourth-order quasi-elliptic absorptive lowpass filter (ALPF) prototype with a simplified structure. This ALPF prototype exhibits both good impedance-matching over the whole normalized frequency domain and an adjustable transmission zero close to the passband. By applying an equivalent impedance transformer model, a coupled-line-based ABPF scheme is devised from the ALPF prototype, which eliminates conventional dispersive transmission line inverters, resulting in an ultra-wide reflectionless range and a compact size. Closed-form equations are derived to support the filter synthesis. A 2.45 GHz microstrip ABPF with 30% fractional bandwidth is designed for verification. The measured minimum in-band insertion loss is 0.83 dB and the reflectionless range of return loss better than 10 dB is from DC to 12.88 GHz. Both the upper and lower stopband suppression exceed 20 dB, with the upper stopband extending up to 6.80 GHz. The upper and lower out-of-band roll-off rates are 93.9 and 121.4 dB/GHz, respectively. The overall circuit size is 0.12 λ_g².

A 60-GHz joint radar–communication (JRC) transceiver is introduced in this letter. Benefiting from a reconfigurable dual-mode Gilbert cell, which could not only operate as an upconversion mixer for the baseband signal but could also be configured as an amplifier for the LO signal, a highly reused JRC transceiver architecture is realized. In the radar mode, instead of utilizing the narrowband IF chirp modulation scheme, which is widely adopted in previous JRC systems, in this work, a wideband chirp generated directly from the LO chain could be applied for both transmitting and direct RF dechirping, thereby significantly improving the range resolution and relieving the hardware burden in radar signal processing. Fabricated in a 65-nm CMOS process, this work achieves a 16-dBm saturated TX output power, an 11-dBm OP $_{1~\mathrm{dB}}$ , and a 5.8-dB minimum RX noise figure (NF). Experiments demonstrate that the proposed JRC transceiver supports $>$ 7-Gb/s data rate in the 16-QAM over-the-air (OTA) communication as well as $>$ 4-GHz transmitted chirp bandwidth with $<$ 3.75-cm range resolution in the radar mode.

A self‐equalized linear‐phase microstrip bandpass filter (BPF) utilizing negative group delay (NGD) parallel‐coupled three‐line (PCTL) units is presented in this letter. The NGD PCTL unit possesses an NGD valley in its frequency response of group delay (GD). By a combination of two or more NGD valleys, the sharp positive GD peaks at passband edges of a conventional coupled‐line BPF are counterbalanced, resulting in a linear phase response over the entire passband. A prototype of linear‐phase BPF in S‐band is designed by replacing two coupled‐line units in a three‐pole conventional BPF with the proposed NGD PCTL units. Measured passband GD fluctuation of the BPF utilizing NGD PCTL units is 0.5 ns while that of the BPF using conventional coupled‐line units is 1.7 ns, a considerable improvement of 70.5% is achieved with no increase in overall time delay. The out‐of‐band suppression is well retained, and the circuit size is 0.18 λ_g².

In this letter, a miniaturized Vivaldi antenna based on fan-out wafer-level packaging (FOWLP) process with a single redistribution layer (RDL) is proposed. A step-by-step design procedure for the proposed antenna is presented. By introducing the slots and semicircular-shaped loads, its low-end cut-off frequency is extended from 35.28 to 24.04 GHz, leading to a size reduction by 31.8% when normalized to the lowest operating frequency. Moreover, a wideband low-pass coplanar waveguide to slotline transition is proposed for antenna feeding, in which a folded split ring resonator and etching periodic slots are added to reduce the transition insertion loss above 40 GHz. Prototypes of the proposed antenna and back-to-back coplanar waveguide to slotline transition were fabricated and measured. The measured return loss of the BTB transition is better than 13.4 dB from DC to 67+ GHz, and the insertion loss of a single transition is below 0.95 dB. The measured |S11| of the proposed antenna is less than -10 dB from 23 to 67+ GHz with 4.5-7.1 dBi measured gain and >84% simulated efficiency.

A bandpass filter (BPF) using dual-band negative group delay equalizers (NGDEs) for extending linear-phase bandwidth is reported in this letter. This dual-band NGDE is synthesized based on a coupling-routing-diagram approach and implemented by a parallel-coupled three-line structure. By cascading two NGDEs to the input and output ports of a conventional BPF, the dual-band NGD characteristic associated with NGDEs is applied to counteract the group delay (GD) peaks at the transition bands of the conventional BPF. A microstrip linear-phase BPF prototype in the S -band is designed. Compared with the conventional BPF, the GD fluctuation of the equalized BPF is decreased by 75% from 0.4 to 0.1 ns within the passband of 1.50–2.50 GHz, while no obvious degradation of out-of-band suppression is observed and the overall time delay is not increased. The devised BPF achieves a linear-phase characteristic in the whole passband with a compact circuit size of 0.32 $\uplambda_{\text{g}}^{2}$ .

A compact scalable gap waveguide (GW) phased array horn antenna with efficient decoupling network (DN) is proposed. An H-plane GW horn antenna with transverse slots-based slow-wave (SW) structures is designed as the array element. By compensating the phase error along the horn aperture, the antenna length is greatly reduced while the gain is enhanced. To suppress the strong mutual coupling between adjacent elements due to the small spacing, an efficient DN is proposed by introducing a through slot inside the intermediate sidewall between adjacent antenna elements. With this DN, an extra wideband equal-magnitude and anti-phase signal can be generated to cancel the coupling signals. Based on the contactless characteristic of the GW, the presented antenna can be extended to large-scale arrays with high reliability. An 8-element phased array antenna operating at 24-30 GHz is implemented for verification. The antenna length is reduced by 62.2% compared with the optimum horn antenna, and the mutual coupling is reduced by up to 32.7 dB with the DN. The measured array gain is higher than 19.8 dBi and the scan angle reaches ±60° with a scan loss of less than 4.5 dB.

This paper presents a wideband dual-polarized L-shaped probe-fed stacked patch antenna element and a 16-element phased array antenna prototype for 60 GHz antenna-in-package (AiP) applications. The primary objective is to overcome the fabrication restrictions and achieve high-performance, cost-effective AiP module using an organic multi-layer industry standard HDI process (100/230 μm minimum via/land size). A new L-shaped probe-fed structure, which functions as a monopole antenna, is implemented to enhance the bandwidth and gain performance of the stacked patch antenna. With help of the comprehensive mode analysis method, the polarization coupling mechanism is highlighted, and an isolated patch is introduced, improving the antenna polarization isolation by up to 7dB. To connect RFIC, considering limited fabrication accuracy and the AiP structure features, a broadband vertical transition structure is designed using the half-wavelength impedance transformation technique, and its working mechanism is analyzed theoretically through a distributed equivalent circuit model. Furthermore, impact of variations in key dielectric properties on important AiP characteristics is analyzed, and the process control guidelines are provided. Thermal simulations using a heatsink solution are performed to address thermal issues. For demonstration, an antenna element and a 16-element antenna array prototype are designed, fabricated, and measured. For both polarizations, measurements indicate that the proposed antenna element has a fractional impedance bandwidth (|S11|<-10 dB) of 23.1%, and its polarization isolation is better than 20 dB. Moreover, their peak gain is above 5.45 dBi. For the 16-element antenna array, each element achieves a |S11| below -10 dB from 51 to 64 GHz with 4±1 dBi gains.