Time synchronization is critical for time-sensitive cell-free massive MIMO systems, as packet loss from mobility or poor channels degrades performance. In this paper, we propose robust coordinated time synchronization (Co-TS) schemes for the cell-free massive MIMO to combat the impact of packet loss. Specifically, a multi-criteria based AP association mechanism is first proposed where in addition to the large-scale fading factor, the effects of received signal strength and packet loss probability are also considered during the AP-device association process. In addition, the fuzzy logic theory is utilized to balance among the three criteria to achieve higher robustness against packet loss over the conventional single-criterion based AP association approaches. After that, a Co-TS model considering both the coordination among APs and the effect of packet loss is developed, where the unsynchronized node can receive a set of timestamps from its associated APs during each synchronization cycle. We use maximum likelihood estimation (MLE) to jointly estimate the clock offset and skew under Gaussian-distributed random delays, and derive their closed-form Cramér-Rao Lower Bounds. Simulation results demonstrate that our proposed schemes are robust against packet loss and can significantly enhance time synchronization performance over the existing methods.

Sparse vector coding (SVC) is a promising short-packet transmission method for ultra reliable low latency communication (URLLC) in next generation communication systems. In this paper, a dual-mapping SVC (DM-SVC) based short packet transmission scheme is proposed to further enhance the transmission performance of SVC. The core idea behind the proposed scheme lies in mapping the transmitted information bits onto sparse vectors via block and single-element sparse mappings. The block sparse mapping pattern is able to concentrate the transmit power in a small number of non-zero blocks thus improving the decoding accuracy, while the single-element sparse mapping pattern ensures that the code length does not increase dramatically with the number of transmitted information bits. At the receiver, a two-stage decoding algorithm is proposed to sequentially identify non-zero block indexes and single-element non-zero indexes. Extensive simulation results verify that proposed DM-SVC scheme outperforms the existing SVC schemes in terms of block error rate and spectral efficiency.

In this article, we focus on the resource allocation and autonomous aerial vehicle (AAV) 3-D trajectory design for the AAV-ground base station (GBS) coordinated multipoint nonorthogonal multiple access (CoMP-NOMA) system to maximize the sum-rate of CoMP users while maintaining users’ high Quality of Service requirements. The main contributions of this article are summarized as follows: 1) with the assistance of closed-form power allocation result, a generalized joint user scheduling and power allocation (G-USPA) algorithm is proposed to derive the optimal user scheduling solution; 2) by revealing the monotone increasing relationship between the sum transmit power and the transmit rates of non-CoMP users, the optimal rate of each non-CoMP users turns out to be its inherent minimum required rate, consequently, the optimal transmit rates and power allocation of all users can also be derived; and 3) moreover, considering the Line of Sight (LoS) and non-LoS factors in the air-ground channel, the 3-D trajectory of AAV is designed based on successive convex approximation to provide a flexible user-centric service. The proposed G-USPA algorithm is compatible with the AAV trajectory design, which is optimized alternatively and can lead to fast convergence. Numerical results verify that the 3-D AAV-GBS CoMP-NOMA model and the G-USPA scheme have a superior performance in terms of total system sum rate and the sum rate of CoMP users over the non-CoMP AAV assisted nonorthogonal multiple access (NOMA) systems.

We study an unmanned aerial vehicle (UAV)-enabled uplink heavy non-orthogonal multiple access (NOMA) system in this paper, where the UL communication becomes extremely important in some hot spot areas such as live concerts or soccer stadiums, and investigate the joint optimization of bandwidth assignment (BA) and power allocation (PA) with UAV three-dimensional (3D) trajectory design to maximize the minimum average rate among all ground users, while meeting their heterogeneous rate requirements. More specifically, we propose a joint BA and PA algorithm by revealing that the inter-user interference in each NOMA group can be eliminated naturally while deriving the sum rate of users. The algorithm is proposed to get the optimal BA and PA solutions with the help of closed-form results and ellipsoid method. After that, in order to solve the UAV 3D trajectory design problem we introduce the elevation angle as a supplementary variable, and the successive convex approximation method is adopted to obtain the UAV 3D trajectory. The joint BA and PA algorithm and the UAV 3D trajectory design can be optimized alternatively, which leads to fast convergence and demonstrates a superior minimum average rate among users and user fairness performance over the previous methods.

The reconfigurable holographic surface (RHS) designs precoding by controlling the amplitude of its radiation elements. The existing RHS-aided precoder treat multiuser interference as a harmful element. However, if the multiuser interference is constructively aligned to the desired signal, it becomes a beneficial element for improving signal’s receive-quality. In this work, we exploit the knowledge of the data to be transmitted and channel state information (CSI), for the purpose of interference utilization. A novel hybrid precoding is formulated for RHS multiuser systems, aiming for maximizing the lower bound of signal-to-interference-plus-noise ratio (SINR) of multiple users. To strike the balance between the complexity and system performance, the optimization problem is decoupled into two convex subproblems i.e., one for optimizing digital precoder and the other for optimizing holographic precoder. Finally, a novel constructive interference (CI) based holographic hybrid precoding algorithm is proposed, which iterates its digital and holographic precoders with a fast convergence rate. Simulation results demonstrate the superiority of the proposed algorithm for enhancing receiving performance against the benchmarks.

In this paper, we propose an uplink non-orthogonal multiple access (NOMA) based hybrid long-packet and short-packet frame structure for data collection in industrial wireless sensor networks (IWSNs) with heterogeneous quality of service (QoS) requirements of sensors. The proposed NOMA-based hybrid frame structure allows the superimposition between a number of short packets and a long packet during data collection, where the short packets can be firstly decoded to maintain the low-latency requirement, and the long packet is decoded later to achieve a high signal-noise-ratio (SNR). In addition, a joint short packet scheduling and pilot and block length optimization (JSLO) algorithm is proposed to minimize the maximum block error probabilities among short packets while maintaining a high SNR of long packet, with the assistance of optimal closed-form short packet scheduling results and pilot and block length expressions. The JSLO algorithm achieves near-optimal performance and a dramatic complexity reduction compared to exhaustive search and can lead to fast convergence. Numerical results demonstrate the proposed NOMA-based hybrid frame structure and JSLO algorithm can significantly reduce the maximum error probability among short packets and maintain a high level of fairness.

Industrial Internet of Things (IIoT), which contains different types of devices with heterogeneous Quality-of-Service (QoS) requirements, has encountered significant challenges on guaranteeing the needs of heterogeneous data collection utilizing limited resources. In this article, we investigate the joint frame structure and resource optimization for the hybrid long- and short-packet nonorthogonal multiple access (NOMA)-based data collection with imperfect successive interference cancellation (SIC) in IIoT, where a number of short and long packets can multiplex the same time-frequency resource simultaneously to guarantee their respective heterogeneous QoS requirements. Specifically, the short packet is first decoded to guarantee low latency, afterward the superposed long packet can be decoded to maintain high signal-to-interference-plus-noise-ratio (SINR) performance. A joint short-packet scheduling, pilot length, blocklength, and dynamic power allocation (JSLP) algorithm is proposed to minimize the maximum block error probability among short packets and mitigate the impact of SIC error propagation in NOMA transmission while maintaining a high SINR of long packet, with the assistance of the derived optimal closed-form short-packet scheduling results and pilot and block length expressions. Thanks to the closed-form expressions, the proposed JSLP algorithm demonstrates near-optimal performance and a significant complexity reduction compared to the exhaustive search, leading to fast convergence. Numerical results demonstrate that the designed hybrid NOMA-based frame structure and JSLP algorithm are robust against the SIC error propagation, and can maintain a high level of fairness by significantly mitigating the maximum block error probability gap among short packets.

In this paper, we propose a cooperative time synchronization (CTS) scheme for cell-free massive multiple-input multiple-output (MIMO) systems, where synchronization packets are jointly broadcast by a set of coordinated access points (APs). Thanks to multiple timestamps available at each target node, the proposed CTS scheme enables a much higher accuracy in clock parameters estimation than the conventional non-cooperative synchronization approaches. In addition to the clock parameters between the target nodes and their associated APs, the clock deviations among APs are also jointly estimated, where the clock deviations can be fed back to their associated APs via acknowledgment for further clock adjustment. Furthermore, to mitigate the impact on synchronization performance under the packet loss scenario, a matrix completion-based CTS (MC-CTS) algorithm is proposed that complements the lost timestamp information. Simulation results demonstrate that the proposed CTS scheme presents a robust performance against the clock deviations among coordinated APs with packet overhead of approximately 50% less than that of the non-broadcast synchronization approaches, and the proposed MC-CTS algorithm effectively enhances the estimation performance of clock parameters, compared to just utilizing the CTS algorithm. Hence, the proposed algorithms are particularly suitable for time-sensitive cell-free massive MIMO systems.

We consider a three-dimensional (3D) unmanned aerial vehicle (UAV)-terrestrial base station (TBS) coordinated multi-point non-orthogonal multiple access (CoMP-NOMA) scheme where UAV coordinates with TBS to allow joint transmission for the terrestrial users. With the assistance of closed-form power allocation derivations, a joint user scheduling and power allocation (J-USPA) algorithm is proposed to obtain the optimal user scheduling solution, with the consideration of imperfect channel estimation. Moreover, considering the line of sight (LoS) and non-LoS factors in the air-ground channel, the 3D trajectory of UAV is designed to provide a flexible user-centric service. Numerical results verify that the 3D UAV-TBS CoMP-NOMA model and the J-USPA scheme have a superior performance in terms of sum rate of users over the TBS CoMP-NOMA and the UAV assisted NOMA systems without CoMP transmission.

In this paper, we consider a unmanned aerial vehicle (UAV)-enabled uplink (UL) heavy non-orthogonal multiple access (NOMA) system where the UL communication becomes increasingly important in some hot spot regions such as live concerts or football stadiums, and study the maximization of the minimum average rate among all users by jointly optimizing bandwidth assignment (BA) and power allocation (PA) alongside UAV trajectory design, while meeting their specified heterogeneous rate requirements. Specifically, by revealing that the inter-user interference can be naturally eliminated while deriving the sum rate of users in each NOMA group, an one-step BA and PA algorithm is proposed, with the assistance of closed-form results. Afterwards, the elevation angle is introduced as an auxiliary variable to help solve the UAV trajectory design problem. The joint BA and PA algorithm and the UAV trajectory design can be optimized alternatively, which leads to fast convergence and demonstrates a superior minimum average rate among users and user fairness performance over the previous methods.

In this paper, a composite robot aided system is proposed to support the coexistence of enhanced mobile broadband (eMBB), ultra reliable low latency (URLLC) and massive machine type communication (mMTC) traffic in smart factory. The composite robot is deployed to inspect the factory by upstreaming high quality images/videos via eMBB, while collecting information from the mMTC devices and allowing URLLC traffic to overlap upon the scheduled robot transmission. This ensures high energy efficiency of the mMTC traffic and low latency of the URLLC traffic. As the heterogeneous traffic affects each other in a complex manner with shared resources, the objective of this paper is to maximize the minimum average rate of the inspection robot while completing the information collection tasks of all mMTC devices and responding to URLLC requests. In light of dynamic growth of information in mMTC devices and strict delay limits for URLLC traffic, we propose an alternative optimization algorithm of joint optimization of task scheduling, bandwidth allocation and robot trajectory (TSBART). The proposed TSBART algorithm achieves a higher minimum average rate as well as higher quality of service (QoS) than the previous work based on greedy and average resource allocation algorithms, thanks to its higher degree of freedom in optimization.

External passive intermodulation (ePIM), which results from external passive sources around base station (BS), such as rust fence, barn roof and weather may cause different levels of intermodulation power and significant interference in the uplink. In this paper, we propose an adaptive beamforming algorithm based on the general linear combination (GLC) to suppress the ePIM signal, which is the first attempt to design beamforming for ePIM suppression. Rather than the complex and intractable modeling of the ePIM signal, the ePIM and noise correlation (PNC) matrices can be estimated and combined by the GLC to design the beamformer, which is more adaptive and can significantly suppress the ePIM. In addition, The power of the essential factors of the ePIM sources is estimated based on the eigenvalues and eigenvectors of the ePIM signal. Simulation results show that the proposed beamformer provides a near-optimal output signal-to-interference-plus-noise ratio performance and also significantly outperforms the existing beamforming schemes which merely focus on co-channel interference without considering ePIM suppression.

In this paper, we propose a novel cooperative time synchronization (CTS) scheme via broadcasting for time-sensitive cell-free (CF) massive multiple-input multiple-output (MIMO) networks, by allowing distributed access points (APs) to jointly broadcast synchronization messages to the target nodes. With the proposed CTS scheme, more timestamps from distributed APs are available to achieve higher synchronization accuracy over the conventional non-cooperative time synchronization approaches. Parameter estimation is conducted by maximum likelihood estimation of clock offset and clock skew under the Gaussian transmission delay model, and the corresponding Cramer-Rao lower bounds (CRLBs) are derived. In addition, the clock offset and skew deviations among APs are also estimated, which are fed back to APs for further clock adjustment. Thanks to the AP association scheme and broadcast characteristics, the proposed CTS scheme demonstrates robustness against clock parameter deviations, while at a relatively low overhead and computational complexity.

In this paper, we investigate unmanned aerial vehicle (UAV) assisted communication systems that require quasi-balanced data rates in uplink (UL) and downlink (DL), as well as users’ heterogeneous traffic. To the best of our knowledge, this is the first work to explicitly investigate joint UL-DL optimization for UAV assisted systems under heterogeneous requirements. A hybrid-mode multiple access (HMMA) scheme is proposed toward heterogeneous traffic, where non-orthogonal multiple access (NOMA) targets high average data rate, while orthogonal multiple access (OMA) aims to meet users’ instantaneous rate demands by compensating for their rates. HMMA enables a higher degree of freedom in multiple access and achieves a superior minimum average rate among users than the UAV assisted NOMA or OMA schemes. Under HMMA, a joint UL-DL resource allocation algorithm is proposed with a closed-form optimal solution for UL/DL power allocation to achieve quasi-balanced average rates for UL and DL. Furthermore, considering the error propagation in successive interference cancellation (SIC) of NOMA, an enhanced-HMMA scheme is proposed, which demonstrates high robustness against SIC error and a higher minimum average rate than the HMMA scheme.

In this paper, we investigate an unmanned aerial vehicle (UAV) assisted communication system for uplink (UL) heavy scenarios such as a football match, where intensive video uploading from the audience is demanded. Non-orthogonal mul-tiple access (NOMA) is exploited to enhance system throughput, while it suffers a dynamic user rate gap within each NOMA group due to UAV movement. In light of the user fairness issue and the UL heavy demands, we propose joint UL-DL optimization of user scheduling, power allocation (PA) and UAV trajectory (JOSPT). In particular, closed-form optimal solutions are derived for dynamic PA in both UL and DL, which plays a dominant role in system performance. The proposed PA scheme is more effective in maintaining user fairness than the previous work and also computationally efficient and suitable for energy-limited UAV system. The average spectrum efficiency of the proposed NOMA UAV system in UL and DL is also much higher than that of the orthogonal multiple access (OMA) based UAV system, with the heterogeneous rate demands accommodated.

In this paper, we propose a novel dynamic manager selection scheme for vehicles platooning systems and investigate the corresponding resource allocation in the finite blocklength regime, to meet the ultra-reliable and low-latency communication (URLLC) requirements of safety-related data. To the best of our knowledge, this is the first work to investigate the impact of finite blocklength on communications of vehicles platooning systems. By taking into account the factors of communication and the changes in the platoon structure, the proposed dynamic platoon manager selection scheme enables a significant performance enhancement over the conventional fixed manager scheme where the head vehicle in the platoon acts as a manager. Based on the proposed platoon manager selection scheme, a joint resource allocation and coding rate optimization algorithm is proposed to minimize the intra-platoon groupcast latency. Thanks to the closed-form expression of the optimal coding rate and transmission power derived, the proposed optimization algorithm achieves optimal performance in terms of the intra-platoon groupcast latency and converges within only 3 iterations, with a significant complexity reduction over exhaustive search.

In this paper, we propose an unmanned aerial vehicle (UAV) base station (BS) assisted communication system for a special event (e.g., a football game) with heterogeneous traffic demands by all users and the uplink (UL)-downlink (DL) rate balancing requirement. With respect to UAV's high mobility, we propose a hybrid mode multiple access (HMMA) strategy where both orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA) techniques are utilized to meet heterogeneous traffic demands. Specifically, NOMA is utilized to achieve high average data rate, and OMA helps to meet the instantaneous rate demands of users. The proposed HMMA strategy has a high degree of freedom and provides superior minimum average rate across all users and a higher user fairness than the previous work with OMA only or NOMA only, where the instantaneous rate demands of users may not always be guaranteed during UAV's flight time due to dynamic channel changes, the inter-user interference and successive interference cancellation (SIC) error propagation. Furthermore, we investigate joint UL-DL optimization for a UAV assisted wireless system. Based on the channel reciprocity of the air-ground channels, an alternative algorithm is proposed to conduct joint UL-DL optimization of bandwidth assignment and UAV trajectory to accommodate heterogeneous rate demands across users and achieve quasi-balanced average rates in UL and DL.

We propose an unmanned aerial vehicle (UAV) ground base station (GBS) coordinated NOMA scheme where UAV and GBS jointly serve the cell-edge users. To the best of our knowledge, this is the first work to investigate air-ground BSs coordination for UAV-assisted NOMA systems, by taking advantage of the interference between UAV and GBS. Therefore, the proposed UAV-GBS coordinated NOMA scheme achieves much higher sum rate of cell-edge users than the non-coordinated UAV-assisted NOMA schemes where interference is suppressed as much as possible. The proposed scheme also outperforms GBSs coordinated NOMA due to more flexible and cost-effective interference management, thanks to the deployment of low-cost UAV BS. We conduct joint optimization of power allocation, user scheduling and UAV trajectory for the UAV-GBS coordinated system. A closed-form optimal solution to power allocation is derived. In addition, a dedicated successive interference cancellation (SIC) ordering approach is proposed. It is proven that the selection of a cell-center user with higher SIC order contributes to a higher rate of cell-edge user, based on which an SIC order based user scheduling algorithm for both cell-center and cell-edge users is presented.

We investigate a downlink multi-cell non-orthogonal multiple access (NOMA) system with coordinated base stations (BSs) and propose a joint resource allocation (RA) scheme alongside adaptive user association to green the system. To the best of our knowledge, this is the first work to investigate joint allocation of subchannels and power for coordinated NOMA systems, while the previous work on RA for coordinated orthogonal multiple access (OMA) systems is not applicable. A serving channel gain based joint RA (SCG-JRA) algorithm is proposed, based on the theoretical proof that the total transmission power is mono-decreasing with respect to the SCGs of non-coordinated users. As for user association, an adaptive fuzzy logic (FL) based multi-criterion approach is proposed to achieve higher robustness against the combined effect of shadowing, fading and inter-cell interference, compared to the previous single-criterion based approaches. Numerical results show that the proposed SCG-JRA with adaptive FL based user association significantly outperforms the previous RA schemes assisted by single-criterion user association, in terms of energy efficiency (EE) and total transmission power, enabling a greener system.

BACKGROUND: Bamboo is a very important forest resource. However, the prolonged vegetative stages and uncertainty of flowering brings difficulties in bamboo flowers sampling. Until now, the flowering mechanism of bamboo is still unclear. RESULTS: In this study, three successive stages of flowering buds and the corresponding vegetative buds (non-flowering stage) from Lei bamboo (Phyllostachys violascens) were collected for transcriptome analysis using Illumina RNA-Seq method. We generated about 442 million clean reads from the above samples, and 132,678 unigenes were acquired with N50 of 1080 bp. A total of 7266 differentially expressed genes (DEGs) were determined. According to expression profile and gene function analysis, some environmental stress responsive and plant hormone-related DEGs were highly expressed in the inflorescence meristem formation stage (TF_1) while some floral organ development related genes were up-regulated significantly in floral organs determination stage (TF_2) and floral organs maturation (TF_3) stage, implying the essential roles of these DEGs in flower induction and maturation of Lei bamboo. Additionally, a total of 25 MADS-box unigenes were identified. Based on the expression profile, B, C/D and E clade genes were more related to floral organs development compared with A clade genes in Lei bamboo. CONCLUSIONS: This transcriptome data presents fundamental information about the genes and pathways involved in flower induction and development of Lei bamboo. Moreover, a critical sampling method is provided which could be benefit for bamboo flowering mechanism study.

Combining non-orthogonal multiple access (NOMA) with coordination techniques among base stations (BSs) is a promising solution to enhance spectral efficiency and alleviate inter-cell interference of cell-edge users in 5th generation (5G) networks. In this paper, we consider a multi-cell NOMA network with coordinated BSs in the downlink, and investigate its user coordination mode selection and resource allocation to achieve a green system. A fuzzy logic (FL) based multi-criterion scheme is proposed for user coordination mode selection. It is more robust against shadowing and fading than the previous selection schemes that classify coordinated and non-coordinated users by a single criterion. Also, the FL ranking list is fed into subcarrier allocation, leading to significant reduction in searching complexity of subcarrier allocation. A dramatic performance enhancement is achieved over the previous schemes based on single-criterion, in terms of transmission power and energy efficiency.

Time synchronization is a significant component in Wireless Sensor Networks (WSNs) for maintaining synchrony among nodes. Most time synchronization protocols in WSNs utilize dedicated synchronization packets for clock accuracy optimization. However, in practical WSNs, the joint design of time synchronization and Media Access Control layer protocols should be considered. One approach is adding timestamps directly into data packets and the corresponding acknowledgements (ACKs). Therefore, communication and energy overhead could be saved greatly and time synchronization could be seamlessly integrated into networks. In this letter, we investigate the time synchronization scheme of listening nodes overhearing the neighboring two-way timing packet exchange based on periodical ACK mechanism and present an efficient clock skew estimation algorithm with clock correction upon every synchronization. The Maximum Likelihood Estimator (MLE) of clock skew for Gaussian random packet delay is derived, and the corresponding Cramer-Rao Lower Bound (CRLB) is obtained. In addition, the MLE shows that the clock skew could be estimated without any prior knowledge of the fixed packet delay and the time of adjustment. Simulation results verify that the MLE is efficient.