Earticle

Mmillimeter-wave (mmWave) massive multiuser multiple-input multiple-output (MU-MIMO) system usually utilizes hybrid precoding to reduce complex hardware and high energy consumption. However, most hybrid precoding scheme consider the fully connected architecture, which may result in a significant loss of energy efficiency. In this paper, we propose a hybrid precoding scheme based on the overlapped subarray architecture (OSA), where the analog part of the MU-MIMO system is realized by a OSA network. Based on the OSA architecture, a modified generalized low rank approximation of matrices (GLRAM) based hybrid precoding scheme is designed to maximize the achievable sum-rate, where the analog precoding algorithm apply the phase alignment technology and GLRAM approach to obtain RF precoder/combiner with high array gain provided by massive MIMO system. Then, the block diagonalization (BD) technology is performed to obtain the digital precoding matrix for the multiplexing gain. Simulation results show that the modified GLRAM based hybrid precoding algorithm for OSA architecture can reduce the number of iterations. Compared with the existing hybrid precoding algorithm, it achieves higher spectral efficiency and lower complexity.

Most studies on symbol-level (SL) precoding have assumed that channel estimation is perfect. However, because interference signals or additive white Gaussian noise exist in the received signal for channel estimation, channel estimation errors always exist. In this paper, we propose an SL precoding scheme that is robust to channel estimation errors. First, using the characteristics of the channel estimation errors, we derive an equation for the worst-case mean squared error (MSE) which is the maximum of the MSE. Then by designing the SL precoding to minimize the worst-case MSE, it has robust characteristics against channel estimation errors.

The high peak-to-average power ratio (PAPR) is one of the major drawbacks of the orthogonal frequency division multiplexing (OFDM) systems that make non-linear distortion in practical implementation of high power amplifier (HPA). In this study, the performance of discrete Hartley matrix transform (DHMT) in precoding based OFDM system is analyzed to minimize the high PAPR. We demonstrate that the DHMT precoding method eliminates the multicarrier assumption in a special circumstance. The remarkable results are achieved and compared with conventional OFDM, Walsh?Hadamard matrix transform (WHMT), discrete cosine matrix transform (DCMT) and Vandermonde-like matrix transform (VLMT) in precoding based OFDM signals.

Peak signal reduction is a critical problem in orthogonal frequency-division multiplexing (OFDM) systems. Herein, we focus on Zadoff?Chu matrix (ZCM) precoding as a peak signal reduction method and analyze its integration using the discrete Fourier transform process. The analysis reveals that the integrated process can be substituted by a sparse matrix multiplication and the influence of ZCM parameters on the sparse matrix is mathematically clarified. The results also show that the ZCM precoding works to produce a single-carrier signal and explains how the ZCM precoding reduces the peak of the OFDM signal.

One of promising transmission technologies for beyond-5G wireless communication is cell-free massive MIMO where all the access nodes (ANs) cooperate to coherently serve all the user equipments (UEs). Based on the benefits from massive MIMO, the novel notion of cell-free network architecture is adequate to overcome the inter-cell interference limitation in ultra-dense network (UDN) environments, owing to the full cooperation among ANs. In this paper, we develop synergetic operations between the cell-free massive MIMO and mmWave ultra-wide frequency band, which is able to provide a genuine high-quality and robust wireless pervasive service by fully utilizing the basic three pillars for wireless capacity, such as bandwidth, spectral efficiency and network densification. For this goal, we study the impacts of analog precoding schemes to be combined with zero-forcing digital precoding on cell-free mmWave massive MIMO UDN systems. In addition, we investigate the effect of correlated shadowing and pilot assignment strategies on the average spectral efficiency performance for the proposed hybrid precoding schemes in both outdoor and indoor UDN environments.

As research continues for the commercialization of 5G and 6G communications, the study of MU-MIMO systems in the mm-Wave band is one of the most key research areas in the field of communication algorithm research. MU-MIMO systems utilize a very wide bandwidth in the tens to hundreds of GHz frequency band, which has a significant disadvantage that the power consumption of the RF at the Rx end is very high. In this paper, a hybrid precoding algorithm in a hybrid beamforming and combining system is studied to solve these drawbacks. The proposed hybrid precoding algorithm defines a problem formulation that can eliminate interference by setting constraint conditions and designing an unconstrained precoder, and then derives a hybrid precoder by projecting a square circle into the null space. The proposed algorithm reduces the computational complexity by about 40% or more by reducing the size and number of SVDs compared to existing works.

In this paper, we study precoding techniques for co-channel interference suppression in multiuser MIMO systems. DPC is optimal techniques to achieve the system capacity of multiuser MIMO downlink channels. DPC is not proper in practical wireless systems because complexity is very high. So block diagonal precoding for multiuser MIMO downlink channel is studied. The block diagonal precoding is used to suppress co-channel interference between multiuser. Block diagonal precoding method, whose complexity is reduced by modified null space operation, change multiuser MIMO channel to multiple single-user MIMO channel. We also use V-BLAST decoder in receiver. V-BLAST decoder can achieve increased system capacity in proportion to the number of users. We show improved system performance by using computer simulation.

Power line carrier communication, with the widespread application, broadband multi-input multi-output system will have a co-channel interference, the interference is in addition to asynchronous pulse noise and pulse cycle factors influencing the multi-input multi-output (MIMO) system's largest broadband power line carrier. Aiming at this problem, this paper, by using multiple input multiple output channel state information, the sender with the method of block diagonalization, eliminate the interference between the channel, so as to improve the capacity of broadband power line carrier system, at the same time reduce the bit error rate of the system. Through the simulation results and analysis can show that the proposed block diagonalization pre-coding method effectively eliminates the common channel interference, improve the drying ratio, thus improve the maximum mutual information of the system and reduce the bit error performance of the system.

In this work, we develop on average relationship between the Per-Antenna Signal to Interference plus Noise Ratio (PA-SINR) and the Per-Antenna Signal to Leakage plus Noise Ratio (PA-SLNR) performance metrics. Simulation results confirm the equivalence between the two metrics which opens up the door to use PA-SLNR instead of PA-SINR to construct simple Multi-user Multiple Input Multiple Output (MU-MIMO) precoding algorithm.

In this paper, we propose a limited feedback transmission based on precoding mechanism for multiple-input multiple-output (MIMO) systems, referred as to supermode transmission. With a fixed data-rate, it allows different degree-of-freedom (DoF) depending on the current channel state information (CSI) and different transmission scheme with the same DoF. Through numerical examples, we show that the pairwise error probability (PEP)-based criterion outperforms the conventional criterion based on minimum eigenvalue (ME) of the received signal matrices, and the supermode transmission improves performance over both multimode MIMO (MM-MIMO) and multimode space-time block coding (MM-STBC), in particular, at low SNR regime.

Out-of-band (OOB) power emission is a conventional issue in orthogonal frequency division multiplexing (OFDM) system. The spectral leakage results in interference to licensed users (LU) in OFDM-based cognitive radio (CR) system. In this article, an adaptive precoding scheme based on the adaptive orthogonal projection matrix is proposed for OOB power suppression. Low complexity is the main feature of this preceding scheme. Simulation and analysis show that this method reduces the out-of-band power significantly by forcing the power spectrum density (PSD) of several frequency points in OOB to be zero. An advanced precoding scheme proposed later achieves a tradeoff between performance of OOB power suppression and bit error rate (BER) in the receiver. With those schemes, an adaptive precoder is proposed, which has three ways to adaptively adjust precoding matrix to achieve a balance among the complexity of system, the performance of out-of-band power reduction and BER in the receiver.

IEEE 802.11p defines an international standard for wireless access in vehicular environments. It is normally designed for operating at high user mobility. Thus, decreasing the signal bandwidth to 10 MHz in IEEE 802.11p makes the communication more efficient for the vehicular channel. Furthermore, the Orthogonal Frequency Division Multiplexing (OFDM) methodology is commonly used to mitigate the Inter-Symbol Interference (ISI) occurred over a wireless channel. However, the OFDM suffers from certain drawbacks including higher system complexity due to the generation of the peak to average ratio (PAPR). The objective of this study is to reduce the high PAPR resulted from adapting the OFDM system to vehicular networks. In this paper, we propose a novel precoding Zadoff-Chu Sequence (ZCS) combined with the Clipping technique for PAPR reduction in OFDM based on the Discrete Cosine Transform (DCT). The proposed technique which is based on precoding the constellation symbols of the vehicular channels with DCT succeeds to decrease the PAPR ratio up to 4dB for 64 subcarriers.

If there is a certain distance between the primary users and the second, there are asynchronous interferences because of existing nonsynchronous signals. And the interferences do not influence only the primary user system, but they could also affect second user system and increase interference. To solve this problem and to eliminate interference to primary and secondary asynchronous users among which there are influences in wireless communication system, a novel kind of precoding method based on the minimum mean square error is put forward in this paper. In this method, asynchronous interferences are eliminated in the precoding process and this process also satisfies users’ server quality. Analysis and simulation show that the proposed scheme effectively ,under the condition of the limitation of interferences in main user system, enhance the second user system capacity and improve the reliability of the transmission system of second users system.

This paper studies the problem of precoding and post-coding design for multicell multiuser downlink transmissions in the absence of perfect channel state information (CSI). Using statistical information of imperfect CSI, an iterative multiuser multicell transceiver design is formulated by minimizing the mean squared error (MSE) cost function of signal and leakage interference under per-base station power constraint (PBPC). The convergence of the iterative precoding and postcoding algorithm is verified by analytical and empirical results. The proposed precoding and postcoding algorithm offers a low computational complexity and robustness against CSI imperfection.

다중 안테나를 이용한 송수신 기술은 전송 효율을 높이거나 오차율을 낮추기 위해 사용되지만 인접한 셀로부터 강한 간섭의 영향을 받게 되면 그 효용성은 떨어진다. 특히, 셀 가장자리 사용자를 위해서는 다른 셀로부터의 간섭을 고려한 고급의 송수신 처리 기술이 필요하다. 정확한 간섭 채널 정보는 송신단에서 보다 수신단에서 얻기 용이하므로, 본 논문에서는 수신단에서 간섭 채널 정보와 다중 안테나를 이용하여 간섭을 제거하는 수신기의 사용을 가정하였으며, 이러한 수신기에 적합한 코드북 기반 협력 전처리 기법을 제안하였다. 협력 정도에 따라 중앙집중형 협력 기법과 분산형 협력 기법을 제안하였다. 모의실험을 통해서 기존 비협력 기법과 피드백 양이 동일하고 추가적인 셀 간 정보 교환이 필요 없는 분산형 협력 기법도 기존 비협력 기법에 비해 성능이 뛰어남을 확인하였다.

Multiple antenna transmission and reception, whose principal merits are significant increase in spectral efficiency and/or reduction in error rate, lose much of their effectiveness in high levels of interference from other cells. Incorporating the other cell interference into advanced signal processing at transmitter and receiver is one of the key challenges for cell boundary users in cellular system. Since receiver can obtain exact knowledge of interference channels more easily than transmitter, an interference-aware multiple antenna receiver that can significantly attenuate interferences is considered. Based on the receiver, codebook-based coordinated precoding schemes are proposed. According to the level of cooperation, centralized and distributed schemes are proposed. We verified by the simulation results that even the distributed schemes, which have same amount of feedback and no cooperation between cells, have performance gain compared to the conventional non-coordinated scheme.