In this article, we investigate the issue of massive access in a beyond fifth-generation (B5G) cellular Internet of Things (IoT) network. To reduce the overhead of channel state information acquisition and the complexity of transceiver design, an integrated framework of massive beam-division multiple access (BDMA) is proposed according to the characteristics of beamspace propagation. Then, we analyze the performance of the proposed massive BDMA scheme and derive a closed-form expression for the weighted sum rate in terms of channel conditions and system parameters. To improve the overall performance, we propose a massive access algorithm by jointly optimizing transmit power and receive vector. It is found that the optimal receive vector for each IoT device is the base beam corresponding to the arrival of angle of the IoT device's signal in the beamspace, which significantly simplifies the design of the receiver. Considering the constraint of radio-frequency (RF) chains in practical networks, we propose a clustering-based massive access algorithm, which allocates an RF chain for a cluster. Finally, extensive simulation results confirm that the proposed algorithms can provide small overhead and low complexity massive access schemes for B5G cellular IoT.

Active device detection is a precondition of realizing grant-free random access in beyond fifth-generation (B5G) cellular Internet-of-Things (IoT). However, due to the deployment of a large antennas array and the existence of a huge number of IoT devices, activity detection usually has high computational complexity and needs long pilot sequences. To overcome these challenges, we first propose a dimension deduction method by projecting the original device state matrix to a much lower dimension space. Then, we develop an optimized design framework with a logarithmic smoothing objective function and a coupled full column rank constraint. Under that framework, we transform the original interested matrix to a positive semidefinite matrix, followed by proposing a Riemannian trust-region algorithm to solve the problem in complex field. Simulation results show that the proposed algorithm outperforms the state-of-art algorithms in terms of device detection performance.

In order to support massive connections over limited radio spectrum for the cellular Internet-of-Things (IoT) in the fifth-generation (5G) wireless network, we propose a new non-orthogonal beamspace multiple access framework. First, we analyze the performance of the proposed non-orthogonal beamspace multiple access scheme, and derive an upper bound on the weighted sum rate in terms of channel conditions and system parameters. Then, we provide a transmit beam construction algorithm for further improving the overall performance. Finally, extensive simulation results show that substantial performance gain can be obtained by the proposed non-orthogonal beamspace multiple access scheme over the baseline ones.

In this paper, we address a practical but adverse problem that successive interference cancellation (SIC) is imperfect in a massive non-orthogonal multiple access (NOMA) system. The benefits of a multiple-antenna base station are exploited to support massive access through user clustering in the spatial domain and alleviate the impact of imperfect SIC. In particular, transmit beams and powers are jointly optimized to mitigate the intra-cluster and inter-cluster interference, so as to improve the overall performance in the presence of imperfect SIC. Specifically, we design the joint optimization algorithms from the perspectives of maximizing the weighted sum rate and minimizing the total power consumption, respectively. Moreover, in order to reduce the computational complexity, we design the massive NOMA algorithms with zero-forcing beamforming fixedly. The impacts of imperfect SIC on the design of massive NOMA algorithms are revealed, and it is found that the proposed algorithms are still applicable even if SIC is perfect. Finally, simulations results validate the theoretical claims and show that obvious performance gain can be obtained over the baseline algorithms.

This paper considers the security issue of the fifth-generation wireless networks with massive connections, where multiple eavesdroppers aim to intercept the confidential messages through active eavesdropping. To realize secure massive access, non-orthogonal channel estimation and non-orthogonal multiple access techniques are combined to enhance the signal quality at legitimate users, while the inter-user interference is harnessed to deliberately confuse the eavesdroppers even without exploiting artificial noise. We first analyze the secrecy performance of the considered secure massive access system and derive a closed-form expression for the ergodic secrecy rate. In particular, we reveal the impact of some key system parameters on the ergodic secrecy rate via asymptotic analysis with respect to a large number of antennas and a high transmit power at the base station. Then, to fully exploit the inter-user interference for security enhancement, we propose to optimize the transmit powers in the stages of channel estimation and multiple access. Finally, extensive simulation results validate the effectiveness of the proposed secure massive access scheme.

This paper considers the application of relay to enhance the performance of massive non-orthogonal multiple access (NOMA) systems and solve the challenge of channel state information acquisition in the case of a massive number of users. First, we design a general framework for a multiple-relay-aided massive NOMA system. Then, we analyze the performance of the multiple-relay-aided massive NOMA system, and derive a closed-form expression for a lower bound on the spectral efficiency. In particular, we reveal the impact of system parameters on the spectral efficiency via asymptotic analysis in three important scenarios, e.g., a large number of antennas at the base station (BS), a high transmit power at the BS or the relays, and a large number of relays. To further improve the spectral efficiency in the context of massive access, we propose two effective schemes to optimize the transmit power at the BS and relays, respectively. Finally, extensive simulation results validate the effectiveness of the proposed multiple-relay-aided massive NOMA scheme.

To achieve spectral-efficient massive access in future wireless networks, this paper proposes a comprehensive fully non-orthogonal communication framework. First, we design a fully non-orthogonal communication scheme which consists of non-orthogonal channel estimation and non-orthogonal multiple access. Then, we analyze the performance of the proposed fully non-orthogonal communication, and derive a tight lower bound on the spectral efficiency in terms of key system parameters and channel conditions. Meanwhile, several novel insights are provided on spectral efficiency via asymptotic analysis in three important cases, i.e., a large number of base station (BS) antennas, a high BS transmit power, and perfect channel state information (CSI) at the BS. Finally, we optimize the performance of the proposed fully non-orthogonal communication and present two simple but efficient optimization algorithms for maximizing the weighted sum of spectral efficiency. Extensive simulation results validate the effectiveness of the proposed schemes.