This paper introduces a new multiple-input multiple-output (MIMO) communication system, referred to as "pseudo MIMO". Unlike conventional MIMO systems, the pseudo MIMO technology enables the transmission of a greater number of parallel data streams than the available radio frequency (RF) chains at the receiver. This is achieved by connecting multiple antenna elements to each receiving RF chain and applying a sequence of analog combining patterns for signal reception within the orthogonal frequency division multiplexing (OFDM) sampling interval. The entire signal processing procedures are formulated using matrix representations, leading to a compact expression of the equivalent channel matrix for each OFDM subcarrier. This matrix is further decomposed into the product of the frequency domain wireless channel matrix and the analog combining matrix. Numerical results and prototype testing demonstrate the benefits of pseudo MIMO in terms of both spectral and energy efficiency.

Unsourced multiple access (UMA) is a promising technology for the sixth generation (6G) mobile networks to fulfil the requirements of massive random access or massive small data transmission. However, the performance evaluation of UMA for non-orthogonal data transmission with real user active detection and real channel estimation is not thoroughly considered for fading channels. In this paper realistic user active detection and realistic channel estimation is carried out for unsourced sparse multiple access (USMA) scheme where sparse interleaver division multiple access is employed. The estimated channels of active users from preamble part are applied for multi-user signal detection for data transmission part. It is found that the multi-user detection performance loss of USMA compared to the ideal channel estimation is marginal in both additive white Gaussian noise (AWGN) channel and flat fading channel under the reasonable system parameter setting.

Multiple-input-multiple-output (MIMO) optical wireless communication (OWC) is a promising technology capable of providing high data rates. However, interuser interference can negatively impact system performance. Existing studies employ CVX-based beamforming designs to mitigate this issue, but these approaches suffer from high computational complexity. To tackle this challenge, this article studies the low-complexity beamforming design to maximize the sum rate. Specifically, we first utilize the fractional programming (FP) method to decompose the original beamforming problem into a series of convex subproblems and propose a block coordinate descent (BCD)-based beamforming framework. Then, we provide closed-form or semi-closed-form solutions for these subproblems. For the general approximate sum rate maximization, we drive the optimal semi-closed-form beamforming structure based on the Karush-Kuhn–Tucker (KKT) conditions. Furthermore, we propose a closed-form solution for the upper bound maximization by taking advantage of the separability of optical power constraints. Finally, numerical results indicate that the proposed beamforming designs reduce the computational complexity significantly while keeping the sum rate performance.

This paper presents a new multiple-input multiple-output (MIMO) communication system. The system, termed as “pseudo MIMO”, facilitates the transmission of more parallel data streams than the number of receiving radio frequency (RF) chains. The key principle involves connecting multiple antenna elements to each receiving RF chain and employing multiple analog combining patterns for signal reception within the orthogonal frequency division multiplexing (OFDM) sampling period. Through detailing the entire transmission and signal processing procedures by matrix manipulation, the equivalent channel matrix for each OFDM subcarrier is derived in a unified matrix form, which can be further represented by the product of the analog combining matrix and the frequency domain wireless channel matrix. As per the equivalent channel matrix, an optimization problem is formulated and solved to maximize the spectral efficiency by jointly designing the digital precoding and analog combining matrices. Numerical results demonstrate that the performance of a pseudo MIMO system can closely approach that of a fully digital (FD) MIMO system with the same antenna configuration but more RF chains. It is also revealed that a low-precision discrete analog combining scheme is sufficient to achieve nearly optimal performance. Further, the effectiveness of pseudo MIMO technology is validated through prototype testing.

Orthogonal Time Frequency Space (OTFS) is a two-dimensional modulation technique designed in the delay-Doppler (DD) domain. Each symbol experiences a sparse channel with low variability in the DD domain rather than the time-varying multi-path channel in conventional orthogonal frequency division multiplexing systems. In practical applications, fractional Doppler shifts is unavoidable due to the limited frame duration. Channel dispersion caused by fractional Doppler shifts makes the channel estimation more difficult and causes data aliasing. In this paper, we propose an adaptive scheme to reduce fractional Doppler shifts. Doppler shift compensation is performed on time-domain signals and the number of time-domain symbols is changed to approximate the quantized Doppler shifts integers. With the proposed adaptive scheme, the overhead of the reference signal can be significantly reduced with the fractional Doppler shifts eliminated. Simulation results show that block error rate performance is not degraded and system throughput is improved with the proposed scheme.

In this paper we propose a novel non-orthogonal multiple access (NOMA) scheme via non-binary factor graphs by introducing operations over Galois field GF(q). By utilizing sparse GF(q) spreading, the proposed scheme benefits from an improved diversity of the superimposed symbols and accordingly a lower probability of constellation overlapping. Meanwhile, the coordination with high-order modulation helps to reduce the interference at the receiver, and therefore a better error performance can be expected. Iterative belief-propagation (BP) detection based on log-domain probabilities is derived for the GF(q) NOMA. The upper bounds of maximum likelihood (ML) detection and simulation results in terms of symbol error rate are provided, which demonstrate the superiority of the proposed scheme to its binary counterparts.

Multi-user multiple-input multiple-output (MU-MIMO) is one of the key techniques to meet high spectral efficiency requirements for the fifth generation (5G) and beyond 5G wireless network. To achieve the high capacity of uplink MU-MIMO, a generalized polar precoding scheme based on rotation and permutation (GP-RP) is proposed in this paper, which can enhance the reliability distinctions between users’ channels. Via the three-stage channel transformation, the binary polar coding, signal modulation and the original multi-user channel are successively partitioned into multiple bit polarized channels. To further amplify the polarization effect, an unitary precoding scheme is proposed for the uplink MU-MIMO system without changing the communication capacity. The simulation results show that the proposed precoding scheme can further improve the polarization between the synthesized user channels and achieve lower block error ratio (BLER).

Orthogonal time frequency space (OTFS) is a multi-carrier technique where a time-varying channel can be converted into a time-invariant channel in the delay-Doppler (DD) domain through a two-dimensional transformation. In practical wireless systems, due to the non-biorthogonal waveforms, the inter-carrier interference (ICI) will bring up phase shifts related to Doppler shifts, so that the channel in the DD domain is no longer invariant. Moreover, the limited frame durations will lead to channel dispersion, which makes it more difficult to estimate Doppler shifts. However, existing channel estimation schemes for OTFS are unable to accurately estimate the Doppler shifts. In this paper, we propose a novel channel estimation scheme. Firstly, we estimate Doppler shifts by linear fitting based on the channel dispersion in the DD domain. Then we calculate the power of dispersion paths as combining coefficients to obtain channel fading. Simulation results show that the proposed scheme can significantly reduce the normalized mean square error compared with conventional schemes, especially if Doppler shifts are large.

With the development of mobile Internet and the Internet of Everything (IoE) for beyond the fifth generation (BSG)/sixth generation (6G) mobile network, the short-packet communication have attracted much attention. In order to improve the utilization of sporadic spectrum and to reduce the peak-to-average power ratio (PAPR) and out-of-band emission (OOBE), a filtering and windowing based orthogonal frequency division multiplexing (FW-OFDM) waveform is proposed in this paper. An optimization problem is formulated to minimize the power spectral density (PSD) in the stopband, and subject to the constraints of the PAPR of the FW-OFDM signal. Meanwhile, both windowing and filtering operation will not cause energy degradation of the transmitted signal. The suboptimal coefficient vectors of windowing and filtering are obtained by the genetic algorithm (GA). Simulation results show that FW-OFDM and cyclic prefix-orthogonal frequency division multiplexing (CP-OFDM) perform better than filter bank multi-carrier (FBMC) in terms of time efficiency. Besides, because of better OOBE and PAPR performance, the proposed FW-OFDM waveform is more suitable for the IoE scenario.

The amorphous multi-connected networks have e-merged as a new trend to meet the fast increasing wireless capacity and coverage demands of future 5G networks. To realize such amorphous networks, ultra dense network (UDN) technology is expected as a promising candidate for its flexible network architecture and abundant radio resources. Yet, the multi-connection optimization for UDN is still left as an open problem nowadays. This motivates us to explore how many and which access points (APs) should be accessed in multi-connected amorphous UDNs. To solve this problem, we introduce a new definition of the maximum number of accessed APs for help, and propose two algorithms to identify this number and handle the AP selection respectively. Monte Carlo simulations testify the validity of our scheme and prove that our strategy can guarantee the soft handover and overcome the “ping-pong effect” efficiently in various kinds of networks with high sum-rate performance.

5G network is anticipated to meet the challenging requirements of mobile traffic in the 2020's, which are characterized by super high data rate, low latency, high mobility, high energy efficiency, and high traffic density. This paper provides an overview of China Mobile's 5G vision and potential solutions. Targeting a paradigm shift to user-centric network operation from the traditional cell-centric operation, 5G radio access network (RAN) design considerations are presented, including RAN restructure, Turbo charged edge, core network (CN) and RAN function repartition, and network slice as a service. Adaptive multiple connections in the user-centric operation is further investigated, where the decoupled downlink and uplink, decoupled control and data, and adaptive multiple connections provide sufficient means to achieve a 5G network with “no more cells.” Software-defined air interface (SDAI) is presented under a unified framework, in which the frame structure, waveform, multiple access, duplex mode, and antenna configuration can be adaptively configured. New paradigm of 5G network featuring user-centric network (UCN) and SDAI is needed to meet the diverse yet extremely stringent requirements across the broad scope of 5G scenarios.

Non-orthogonal multiple access (NOMA) is expected to be a promising technique for future wireless networks due to its superior spectral efficiency. In this paper, the sum rate optimization problem for multiple-input multiple-output (MIMO) NOMA systems is studied with the total transmit power constraint and the minimum rate constraint of weak user. We first derive a channel state information (CSI) condition in which MIMO NOMA systems can achieve full rate transmission, i.e. the transmission rate of the weak user equals to the channel capacity of weak user. Based on the CSI condition, we propose an optimal power allocation scheme for MIMO NOMA systems, which can achieve the capacity region of MIMO Broadcast channel as dirty paper coding. A low complexity suboptimal scheme is proposed as well for all CSI channel conditions. Numerical results show that the proposed NOMA schemes significantly outperform the traditional time division based single user MIMO scheme and the multi-user MIMO scheme.

Massive multiple-input and multiple-output (MIMO) systems have draw a lot of attention, due to its outstanding performance in energy saving and spectrum efficiency [1]. In massive MIMO systems, the intra-cell interference, uncorrelated noise and fast fading nearly vanish. Further study reveals that for multi-cell massive MIMO systems the performance will be limited by so-called pilot contamination (inter-cell interference) [2], where the transmitted CSI at one BS includes the channel coefficients from all the users using the same pilot. However, the analysis in [2] ignores a fact that the spatial/temporal characters of channel coefficients of different users are different such that they are distinguishable in certain extends. In practice, utterly isolating the user channels is extremely hard or even not feasible in certain cases. This paper firstly presents a practical temporal domain based method against pilot contamination without coordination, which just remains the strongest channel impulse responses (CIR) as the effective channel information. Simulation results show the outperformance of the proposed method with low complexity.