Jingon Joung

To achieve long-range communication, a single-hop long-range wide area network (LoRaWAN) must sacrifice throughput. In contrast, multi-hop LoRaWAN can extend the communication range while maintaining high throughput by introducing relay nodes between a transmitter and a receiver. However, several challenges arise in multi-hop LoRaWAN. First, the hidden-node problem can lead to packet collisions because relay nodes operate autonomously and may not detect other nodes’ transmissions. Second, the acknowledgment (ACK)-based retransmission control process (ACK-RCP) reduces the throughput due to the duty cycle constraint imposed on unlicensed bands. To address these challenges, this paper proposes a monitoring resource activity-based retransmission control process (MRA-RCP) that determines whether retransmission is necessary by overhearing signal transmission in the next hop. Thus, it does not require explicit feedback from the receiving node. The simulation results show that the proposed MRA-RCP can increase the number of successfully delivered packets to 187 % of that of the system with the conventional ACK-RCP. Furthermore, experimental evaluations with commercially available LoRaWAN nodes show that the proposed MRA-RCP achieves 192 % of the packets successfully delivered, compared to the conventional ACK-RCP in a real LoRaWAN environment.

Recently, phase-modulated continuous-wave (PMCW) systems have been gaining attention as a next-generation waveform for autonomous driving systems due to their capability of integrated sensing and communication with high-resolution sensing ability. Because high-resolution PMCW radar systems require a wide array aperture and wide bandwidth, the severe range-angle coupling effect can degrade the sensing performance. To address this issue, we first formulate a signal model for PMCW radar system that accounts for range-angle coupling, also referred to as the spatial-wideband effect, and classify this effect into range migration and beam squint. Based on the formulation, our analysis shows that PMCW radar systems are more vulnerable to range-angle coupling than frequency-modulated continuous-wave radar systems, which highlights the importance of the proposed mitigation technique. To mitigate the issue, we propose a novel range-angle decoupling method for PMCW radar systems that uses a modified correlation process. The simulation results show that the proposed method improves both the peak power and the half-power beam width of the target angle response, even in scenarios with multiple targets. Furthermore, the computational complexity of the proposed method is relaxed by Fourier transform-based interpolation, which enables significantly faster computation with negligible performance degradation.

Wireless communication systems under high mobility and multipath conditions suffer from severe channel variations in time and frequency. To overcome this issue, this study extends a differential space-time code (DSTLC) scheme to an orthogonal frequency-division multiplexing framework, i.e., a differential space–time–frequency line-coded system. In the differential space-time-frequency code (DSTFLC) system, communication performance depends on how the DSTFLC symbols are arranged on the time–frequency resource grid. To address this issue, three resource grid mapping (RGM) schemes are proposed: time-first (TF) RGM, frequency-first (FF) RGM, and generalized Hilbert (Gilbert) RGM schemes. As verified by the numerical results, three RGM schemes exhibit distinct benefits under different channel conditions, namely, time-varying channels with short delay spreads, frequency-selective channels in low-mobility scenarios, and time-varying frequency-selective channels. The proposed RGM schemes would provide practical design guidelines for deploying various Internet of Things (IoT) applications under dynamic channel conditions.

This study considers covert and secure communications in which a transmitter, Alice, sends information to a legitimate receiver, Bob, using a differential space-time line code (Diff-STLC) scheme. A warden, Willie, attempts to detect Alice’s transmission based on signal strength. Upon successful detection, Willie becomes an eavesdropper (Eve) and starts to eavesdrop on the information. Willie and Eve are adversaries against covert and secure communications, respectively. To enhance the limited secrecy of a conventional Diff-STLC system, we propose a code-hopping strategy by randomly switching between two Diff-STLC structures. As a result, only Bob, aware of the hopping pattern, can decode the signals. To quantify both covertness and secrecy, we define a covert secrecy rate (CSR) as the difference between the achievable rates at Bob and Eve. Using the detection probability of Willie and the bit error rates of Eve and Bob, we derive an analytical lower bound for CSR. Both analytical and Monte Carlo results confirm that the proposed Diff-STLC hopping method significantly enhances the performance of CSR, thus improving the overall covertness and secrecy in communications.

This study proposes a direction-of-arrival (DOA)-based phase-shift method for hybrid reconfigurable intelligent surface (RIS)-assisted communication systems. The hybrid RIS can estimate the DOA of the incident signal by utilizing its hybrid elements, which can receive a portion of the incident signal and reflect the remainder. Then, the optimal phase shifts of the hybrid RIS are calculated based on the estimated DOA without any phase-shift information feedback. Here, a tradeoff between the DOA estimation accuracy and the signal-reflecting power can be balanced by optimizing the sensing factor of the hybrid RIS, i.e., the power ratio of the received (or sensing) signal to the total incident signal at the hybrid element. To this end, the optimal sensing factor that maximizes the effective ergodic achievable rate is analytically derived. The simulation results demonstrate that the proposed hybrid RIS system with optimal sensing factor outperforms both conventional passive RIS systems and channel-estimation-based hybrid RIS systems when the number of RIS elements is large, and the line-of-sight channel is dominant.

Conventional differential space-time block coding (DSTBC) and differential space-time line coding (DSTLC) systems require separate transmitter and receiver hardware, leading to redundant radio frequency chains, increased silicon area, and higher power consumption. This paper addresses this hardware inefficiency by presenting a unified transceiver (TRX) architecture compatible with both DSTBC and DSTLC systems. By leveraging the structural symmetry between the transmitter in DSTLC and the receiver in DSTBC—and vice versa—we develop two adaptable, reconfigurable TRXs: TRX1 (single-antenna) and TRX2 (dual-antenna). These TRXs are capable of functioning interchangeably as transmitters or receivers in either coding scheme. The simulation results demonstrate that any combination of TRX1 and TRX2 can communicate effectively, achieving full spatial diversity without requiring channel state information at either end. The proposed architecture is expected to reduce hardware complexity, enhance resource utilization, and support dynamic switching between transmit and receive diversity modes.

This study proposes a novel aerial intelligent reflecting surface (IRS)-aided downlink communication framework to support user equipment (UE) in hotspot coverage areas. Aerial IRSs can establish additional line-of-sight links by bypassing ground blockages but incur high overhead due to channel estimation and control signaling. To reduce overhead while preserving the signal-to-noise ratio performance, we propose an IRS phase mapping method that leverages the geometric knowledge of hotspot coverage. This method involves two phases: i) an offline phase, which partitions the hotspot coverage into multiple disjoint regions and computes an optimal IRS phase-shift vector for each; and ii) an online phase, which maps the appropriate phase-shift vector to the UE based on its position. The generalized Lloyd algorithm is used in the offline phase to optimize the coverage partitions and corresponding IRS phase-shift vectors jointly. Numerical simulations validate the effectiveness of the proposed method, particularly when the UE's position information is known at the base station.

This study proposes a low-complexity, greedy-based scheduler for multiuser space–time line coded (MU-STLC) systems. The proposed scheduler selects one user that was unscheduled in the previous scheduling period and replaces it with the currently scheduled user with minimum proportional fair priority to maximize the sum priority of the scheduled users. Consequently, the proposed greedy-based scheduler can significantly reduce computational complexity compared to existing schedulers. Furthermore, a novel low-complexity matrix inversion (LMI) algorithm has been devised to further decrease computational complexity by leveraging the order-recursive structure of the MU-STLC preprocessing matrix. Simulation results confirm that the greedy-based scheduler utilizing the LMI algorithm significantly reduces the computational complexity of existing schedulers, albeit with a slight degradation in scheduling performance.