Reconfigurable holographic surface (RHS) exhibits low energy consumption and low cost, which attracts extensive attention. Since unmanned aerial vehicle (UAV) is limited by power and volume, holographic beam is suitable for UAV Internet of things (IoT) systems. However, UAV will encounter complex airflow, causing jitter, which indicates necessary suppression method for holographic beam disturbance. To address the challenge, we explore the problem of robust rotary holographic beam, consisting of angle controlling and hybrid holographic beamforming. The problem is formulated as minimizing the sum mean squared error (SMSE) under the beam disturbance. Resolving such non-convex issues poses considerable difficulty in achieving optimality. A heuristic scheme is proposed, which decouples the angle controlling and hybrid holographic beamforming. Specifically, angle controlling aims to maximize the sum of the mathematical expectations of the disturbed holographic beamforming matrix to mitigate disturbances. Meanwhile, hybrid holographic beamforming minimizes SMSE under beam disturbances to manage multi-user interference. The suppression gain and computational complexity are analyzed theoretically to validate the effectiveness of the proposed scheme. Furthermore, simulation results show that our scheme performs better than benchmark schemes in sum-rate and bit error rate (BER).

Cell-free integrated sensing and communication (ISAC) offers simultaneous high-quality communication and robust target sensing, boasting superior spectral efficiency. This approach has emerged as a crucial technique in meeting the demanding requirements of 6G systems. For such a system, we investigate Voronoi-cluster multi-resolution assemblages, which involves the creation of a multi-resolution hierarchical codebook designed to support robust channel estimation and accurate latency estimation. Compared with the classical channel estimation method, the proposed scheme achieves a balance between search efficiency and detection accuracy. Leveraging on the benefits of the cell-free structure, we also propose a joint correlation multiple targets sensing algorithm to acquire the round-trip latencies of multiple targets. Moreover, we evaluate the performances of normalized mean square error, detection rate and latency estimation rate. Simulation results demonstrate that the proposed cell-free ISAC architecture provides superior detection performances than the classical centralized ISAC counterpart.

Multiple-input–multiple-output (MIMO) is a promising technique in Internet of Things (IoT) Networks, which can effectively multiplex more IoT users and improve the spectrum efficiencies (SEs) of the users. However, when the number of IoT users is large, the complexity becomes enormous. Moreover, perfect channel state information (CSI) is a basic assumption in MIMO systems, which is impractical with a large number of IoT users. In order to reduce the complexity and break the CSI limitation, an opportunistic-beamforming (OBF)-based IoT network is proposed, which can achieve high SE with low complexity and feedback under a large number of IoT users’ conditions. Then, to solve the unfairness problem and further improve SE, a downlink multiple-weight and multiple receive antenna OBF (MW-OBF-MRA) system with the proportional fairness (PF) strategy is proposed, where the transmitter and receiver are jointly designed. Moreover, a joint resource allocation scheme is provided to achieve the maximum SE with PF constraints. Numerical results show that our proposed system can achieve fairness with the minimum SE loss and provide better SE than the other beamforming schemes with the limited feedback information.

Millimeter-wave cell-free and user-centric massive multiple input multiple output (MIMO) systems are promising, due to the reliable ultra-high data rate services. Thus, it is a significant issue to design low-complexity hybrid precoders. In this paper, a simplified sum-mean-square-error (SMSE) is used to design hybrid precoders, based on the array gain and spatial macro-diversity. Then, a heuristic hybrid pre-coding scheme is proposed, with two analog design algorithms that are with or without the knowledge of statistical information. The influence of the imperfect state information of beams is theoretically analyzed, from the instantaneous and ergodic aspects, verifying the robustness of the proposed scheme. Simulation results show that our proposed low-complexity hybrid precoder can be easily realized with acceptable bit error rate (BER) and sum-rate performances, especially when the total transmit power is small.

Energy efficiency (EE) performance of millimeter-wave (mmWave) large-array systems attracts a lot of attention. And insertion loss is an inherent characteristic of hybrid precoding systems, which greatly reduces the system performance. EE analysis considering the insertion loss based on the dynamic array architecture is still an open issue. This paper researches the ergodic EE of mmWave hybrid pre-coding system with the insertion loss, based on the adaptive overlapped subarray (OSA) architecture. The ergodic EE is a more valuable indicator than the instantaneous EE for the system under dynamic architecture. However, it is difficult to directly calculate the precise ergodic EE due to the intractable relationship between EE and the channel matrix. Instead, we simplify the precise ergodic EE to get a lower bound, and analyze the ergodic EE performance based upon the lower bound. By this way, we analyze the ergodic EE performance of two typical hybrid pre-coding schemes taking the dynamic architectures. Simulation results verify the effectiveness of analyses.

In large-array millimeter-wave (mmWave) systems, hybrid pre-coding is one of the most attractive research topics. This paper first presents a flexible adaptive overlapped subarray (OSA) architecture for the analog precoder, whose architecture can be adjusted by a connection network. An objective function is formulated to maximize the energy efficiency (EE) in consideration of the insertion loss for the proposed adaptive OSA based hybrid precoder. The optimal scheme is intractable to achieve, so that we present a heuristic hybrid pre-coding scheme, where the digital precoder is designed based on the zero-forcing (ZF) rule and the analog precoder is designed based on simplified EE optimization in consideration of the insertion loss. We discuss the effect of non-ideal factors on the EE performance, such as quantized phases, imperfect channel state information (CSI), and faulty switches. Simulation results show that, when the initial transmit power is large, our proposed scheme offers a better EE performance than the fully digital pre-coding scheme and many other popular hybrid pre-coding schemes. Moreover, it is found that faulty switches make the double-edged effect on the EE performance when the numbers of activated phase shifters and combiners are pre-defined.

Hybrid pre-coding design is a promising research direction in large antenna array millimeter wave (mmWave) systems. The insertion loss is an inherent and significant feature of hybrid pre-coding, resulting in lower energy efficiency and inferior bit error rate (BER) performance. This paper takes the minimum sum-mean-square-error (Min-SMSE) considering the insertion loss as the optimization objective function, which aims at increasing the sum-rate and improving the BER performance. Then a hybrid pre-coding is designed based on this criterion with an adaptive overlapped subarray (OSA) architecture. It is proved that the optimization problem is non-convex. Thus, we decompose the optimization problem into two sub-optimum ones. One is to design the digital pre-coding based on the Min-SMSE criterion under the equivalent channel condition. The other one is to design the analog pre-coding based on the simplified Min-SMSE with the insertion loss objective function. Theoretical analyses of the proposed scheme are conducted, including the upper bound of the average BER, the lower bound of the average sum-rate, and the computational complexity. Simulation results show that our proposed scheme outperforms three representative hybrid pre-coding schemes with different architectures in both BER and sum-rate, when the numbers of the phase shifters and combiners are relatively small.

This paper proposes a new frequency-domain pre-coding algorithm for the downlink multi-user MIMO system in a frequency-selective fading channel, where each user is equipped with multiple antennas. The proposed algorithm is based on the minimum sum-mean-square-error (Min-SMSE) criterion, which is able to eliminate both the multi-user and multi-antenna interferences, resulting in an improved bit error rate (BER) performance. Compared with the traditional optimal Min-SMSE algorithm, the proposed algorithm can reduce the computational complexity through simplifying the iterative and power allocation processes. Furthermore, the theoretical BER and ergodic sum-rate are derived in detail. In addition, we define and analyze the complexity of the pre-coding algorithms. Simulation results show that the proposed algorithm compares favorably with the optimal Min-SMSE and traditional block diagonalization (BD) algorithms in the sense of the ergodic sum-rate due to the approximately equal power allocation. The BER performance of the proposed algorithm is close to that of the optimal one and much better than that of the BD algorithm in both the uncorrelated and correlated channels. Moreover, the computational complexity of the sub-optimal Min-SMSE algorithm is significantly less than that of its optimal counterpart, especially in the low SNR region.