In the near future, cellular machine-to-machine (M2M) communications are expected to play an important role to realize the Internet of Things (IoT). Due to the exponentially growing number of machine nodes, however, one of challenging issues is to accommodate their random access (RA) requests without severely degrading the performance of human-to-human (H2H) communications. In this letter, we propose an enhanced RA scheme with spatial group based reusable preamble allocation (ERA-SGRPA), which reuses the preamble resources based on a spatial grouping during the RA procedure. By allocating the identical preamble set to the groups which are far apart from each other, the ERA-SGRPA scheme enables the given preamble set to be efficiently reused. The performance evaluation shows that the ERA-SGRPA scheme significantly lowers the collision probability and reduces the access delay and can accommodate a significantly larger number of machine nodes without degrading the performance of H2H communications.

Silicon nitride (SiN) optical phased arrays (OPAs) have emerged as a potential alternative to their silicon counterparts, owing to their low nonlinearity, easy scalability, and low propagation loss. However, the development of SiN OPAs for light detection and ranging (LiDAR) applications has attracted less attention due to the low thermo-optic (TO) effect of SiN. Moreover, SiN OPAs are considered to be undesirable for transmitters in LiDAR systems because of the limited longitudinal tuning efficiency of SiN grating antenna. We present a hybrid integrated SiN-polymer OPA for realizing an efficient LiDAR. Owing to the large TO effect of polymer, the proposed OPA with polymeric phase modulators resolves the power dissipation drawback of the SiN devices. The polymeric modulators are integrated with a SiN power splitter and grating antenna, and the coupling loss is alleviated via a spot-size converter. Additionally, a steering angle magnifying lens is used to enhance the tuning efficiency of the antenna, expanding the longitudinal beam steering range. The prepared OPA transmitter equipped with the lens exhibits decent two-dimensional beam steering, featuring stable emission characteristics over a 12° × 30° field of view. With the transmitter, efficient time-of-flight based LiDAR is demonstrated to provide a detection range of 10 m under a peak optical power of 55 dBm. The proposed SiN-polymer OPA is highly anticipated to circumvent the challenges of conventional SiN counterparts and potentially spearhead the development of a prominent OPA platform for advanced LiDAR schemes.

We propose an enhanced access reservation protocol (ARP) with a partial preamble transmission mechanism for the narrow band Internet of Things (NB-IoT) systems. The proposed ARP can enhance the ARP performance by mitigating the occurrence of preamble collisions, while being compatible with the conventional NB-IoT ARP. We provide an analytical model that captures the performance of the proposed ARP in terms of false alarm, misdetection, and collision probabilities. Moreover, we investigate a tradeoff between the misdetection and the collision probabilities, and optimize the proposed ARP according to the system loads. The results show that the proposed ARP outperforms the conventional NB-IoT ARP, in particular, at heavier system loads.

We propose an enhanced physical random access channel (PRACH) preamble detector, which efficiently identifies non-orthogonal preambles while suppressing the noise rise. The proposed PRACH preamble detector reconstructs preambles from the received signal and efficiently utilizes them to improve the detection performance. Through simulations, we verify that the proposed detector successfully detects the desired preambles while suppressing the undesired noise rise, and, thus, it can detect ten non-orthogonal preambles simultaneously with an extremely low mis-detection probability lower than $10^{-5}$ , while achieving a false alarm probability of approximately $10^{-3}$ .

The high rotation speeds and mega-constellations of low earth orbit satellites (LEO SATs) cause the inter-satellite frequent handovers (HOs) problem which can lead to substantial performance degradation. This paper proposes a novel distributed multi-agent deep Q-network based SAT HO strategy for the LEO SAT networks to simultaneously minimize the number of HOs and maximize the throughputs and the visible times of UEs while satisfying the quality-of-service (QoS) constraints of all UEs. The proposed HO scheme allows UEs to independently and simultaneously perform the HO decision makings based on their own local information, which enables to immediately adapt to the dynamic changes of the LEO SAT network environments. The numerical results demonstrated that our proposed HO strategy achieves the lowest average HO rate and the highest achievable throughputs compared to other conventional HO strategies, while ensuring a higher QoS guarantee time ratio.

Solid-state light detection and ranging, capable of performing beam scanning without using any mechanical moving parts, requires a phase-modulator array. Polymers facilitate the fabrication of efficient phase modulators with low drive power, owing to their high thermo-optic (TO) effect and low thermal conductivity. We designed and fabricated a polymeric phase-modulator array and analyzed the temporal response of the TO phase modulator. The frequency response of the phase modulator was measured for a Mach–Zehnder interferometer (MZI), and the transfer function was modeled in terms of multiple poles and zeros. The frequency response of a fabricated beam-scanning device incorporating the TO phase modulator was also measured. The temporal response of the beam scanner was confirmed to coincide well with that of the MZI modulator. The device exhibited a fast rise time of 12 ms, accompanied by slight power variations appearing for a long period (over hundreds of seconds), which originated from the inherent viscoelastic effect of the polymer materials.