A hybrid beamforming structure is suggested as one of the solutions to reduce implementation costs and energy consumption in millimeter-wave massive multiple-input, multiple-output (MIMO) systems. In this study, an optimization problem of resource allocation was formulated to minimize the total system transmission power on downlink under a certain quality-of-service (QoS), such as bit/block error rates and data rates for each user, and the solution was proposed therein. Our proposed stream incremental algorithm can dynamically adjust the number of data streams for each user according to the channel state information. Precoding and combining schemes need to be developed to solve the formulated problem and also are proposed in this paper to be paired up with the stream incremental algorithm. Our proposed algorithms consider the practical modulation and coding scheme for transmission in various data stream allocation and beamforming designs. The proposed algorithms provide beamforming solutions for millimeter-wave massive MIMO systems, which achieve comparable performance to that of a fully digital block-diagonalization (BD) algorithm with a lower implementation cost and outperform those of modified existing hybrid beamforming algorithms. Simulation results demonstrate the efficacy of the proposed schemes by allocating the different numbers of data streams for each user according to the channel state information. The simulation results verified that the proposed method can achieve a good trade-off between complexity and performance on comparison with the modified existing schemes and the full digital solutions.

Recently, the use of unmanned aerial vehicles (UAVs) or airship platforms to improve the performance of cellular networks has received growing interests. It is expected that the next generation cellular networks will enable the low-latency and high-reliability communications with the support of aerial-terrestrial heterogeneous networks (HetNets) composed of flying and ground base stations (BS). In addition, with the emergence of various delay-sensitive applications (e.g., autonomous vehicles), the end-to-end delay becomes a crucial quality of service (QoS) metric. However, how to design an efficient and delay-guaranteed aerial-terrestrial HetNets remains an open research challenge. In this paper, we develop an analytical approach to characterize the end-to-end delay performance of aerial-terrestrial HetNets by using network calculus. A stochastic model that takes into account the successful transmission and resource utilization in the radio access network is proposed to evaluate the delay performance within a target delay-violation probability. In addition, the effect of background traffic in the core network are carefully examined to present a tight upper bound for the end-to-end delay. Simulation results validate the proposed analysis under different network settings such as available bandwidth, the number of UAVs, and the trajectory of UAVs. Also, it is revealed that the trajectory of UAVs plays a more significant role in the delay performance compared to the number of UAVs.

In this paper, we investigate hybrid beamforming and resource allocation design problems for uplink millimeter wave (mmWave) multi-user (MU) massive multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. We propose three hybrid beamforming designs with dynamic streams assignment (DSA) integrated into the solution for the formulated resource allocation problem to maximize spectral efficiency. In the analog beamforming stage, three different designs are developed to maximize the channel gains of the equivalent baseband channel at each subcarrier, while the number of data streams is also determined for each mobile station (MS). Specifically, for the first design, the analog beamformers are designed to match the corresponding channel across the entire band for each MS. In the second one, to significantly reduce the complexity of the first design, we can completely avoid the matrix decomposition by selecting the appropriate set of array response vectors. The last design only performs singular value decomposition (SVD) on the channel of the center subcarrier to find the analog beamformers. In the digital beamforming stage, for the interference cancellation, the digital precoder and the combiner are obtained at each subcarrier using the block diagonalization (BD) and the coordinated transmit-receive processing method. We also develop fully-digital beamforming and another three hybrid beamforming schemes based on the existing algorithms as performance benchmarks. By being verified through numerical simulations, the spectral efficiency of the proposed schemes can be significantly improved after dynamically assigning the number of data streams for each MS. Moreover, simulations indicate the first and the third proposed hybrid beamforming designs can achieve a very competitive performance and even outperform the fully-digital design in some situations.

Aggregating a lot of subcarriers will generate high peak‐to‐average power ratio (PAPR) which makes the overall system performance degradation. Among the existing PAPR reduction methods for orthogonal frequency‐division multiplexing (OFDM)‐based systems, iterative clipping and filtering (ICF) may be easy to implement, but ICF may cause signal distortion, so that the system cannot approximate the processed signals to the original signals which degrades error rates at receivers. Here, the authors proposed a new method based on ICF with smoothed clipping signals, and find the optimised filter to achieve the minimum error vector magnitude and efficiently reduce PAPR. Owing to the lack of a wide range of continuous bandwidth, the Third Generation Partnership Project proposed carrier aggregation (CA) technology in LTE‐advanced systems. CA can aggregate many contiguous or non‐contiguous fragment spectrums for supporting larger bandwidth and higher peak data rate. Here, the authors also evaluate the developed PAPR reduction scheme for the use in CA scenarios. The efficacy of the proposed scheme is demonstrated in the simulation results.