For massive multiple-input multiple-output (MIMO) systems, linear precoding is preferable to nonlinear precoding for better performance-complexity trade-off. However, linear precoding is still difficult to implement in practice for such large systems, because the precoding matrix involves the complicated matrix inversion that must be rapidly computed in real-time. In this paper, we use the large-scale property of massive MIMO systems, the excellent characteristics of the weighted Neumann series (WNS) matrix, and steepest descent (SD) method to devise a new iterative precoding for massive MIMO systems. First, by exploiting the characteristics of WNS iteration, we proposed a weighted Neumann series-steepest descent (WNS-SD) iterative algorithm to perform precoding with low-complexity, and the convergence condition is always met in underloaded scenarios. Second, by devising a novel first iterative step of the aforementioned WNS-SD iterative algorithm, we proposed an accelerated iterative algorithm, named the accelerated weighted Neumann series-steepest descent (AWNS-SD) algorithm. Furthermore, in the first iterative step of the AWNS-SD algorithm, we designed a promising preconditioning matrix for the SD algorithm based on the WNS matrix. Subsequently, via merger with the WNS iterative method, the AWNS-SD iterative precoding not only greatly improves the convergence rate of WNS-SD and other competitive precoding algorithms while maintaining low-complexity, but also guarantees a wide range of convergence. Finally, simulation results verify the validity of the theoretical analysis and show that without additional iteration steps, the proposed AWNS-SD precoding achieves near-optimal performance of the exact zero forcing (ZF) precoding in just one step for typical massive MIMO system configurations. Furthermore, the diagonal matrix concept is applied to the preconditioning technique to further reduce the overall complexity.

Sparse code multiple access (SCMA) and wireless multicast are viewed as two effective techniques to improve spectral efficiency. Due to the diversity of channel gains for different receivers in the multicast group, the bit error rate (BER) analysis of the unicast SCMA is not suitable for the multicast SCMA systems. Motivated by this, in this letter, the BER performance of multicast SCMA system with randomly deployed receivers is investigated. More specifically, we derive a tight upper bound expression for the BER of the system. To gain more insights of the expression, asymptotic analysis is conducted. Both theoretical and simulation results show that the BER increases as the number of receivers increases in the multicast group.

The orthogonal frequency division multiple access (OFDMA)-based cooperative multicast (CM) technology realizing the intra-cooperation of multicast group (MG) has improved the performance in throughput and coverage. In the MG scenario, users are uniformly distributed. Due to the small effective transmission range (ETR) of relay user (RU), the cooperation is hindered, which limits the performance of CM. Therefore, non-orthogonal multiple access (NOMA)-based two MGs joint cooperative opportunistic multicast scheme (termed as COM-NOMA) is proposed. In COM-NOMA scheme, two MGs are served as one quasi-MG. Firstly, on the same frequency band, base station (BS) opportunistically multicasts the superposition signal, which is formed by two MGs’ signals, to the users with good channel conditions in the two MGs using NOMA way. Secondly, some successfully receiving users are selected as RUs. The superposition signal is forwarded to the other users by Device-to-Device multicast (D2MD) way. Then, according to the coverage ratio of BS and the decode condition of NOMA, the signal to interference and noise ratio (SINR) threshold for RU selection is given. Finally, the spectral efficiency of COM-NOMA is analyzed and the approximate expression of system coverage ratio is derived. The simulation results show that, spectral efficiency at unit transmit power (spectral efficiency per watt) and system coverage ratio is improved in comparison with other OFDMA-based CM schemes.

Content sharing among users through Device-to-Device (D2D) multicast communication is an attractive solution to offload the mobile traffic. To achieve stable and effective sharing of content among nearby users, how to select D2D multicast cluster heads (DMCHs) is important. In this paper, a D2D multicast cluster head selection strategy based on Chinese Restaurant Process (CRP) is proposed. Firstly, in order to establish stable D2D multicast links, some requesting users (RUs) are classified as D2D multicast users (DMUs) taking account of their mobility. Secondly, to help the base station (BS) distributing data to DMUs effectively, DMCHs are selected from DMUs jointly considering DMUs' social attributes, available energy of their devices and the transfer rate from the BS to DMUs. At last, simulation results show that the proposed strategy can not only improve throughput, but also reduce the energy consumption and the transmission delay.

This paper studies a multi-user network model based on sparse code multiple access (SCMA), where both unicast and multicast services are considered. In the direct transmission scheme, the communication between the base station (BS) and the users is completed with one stage, in which the relay is inexistent. In the two-stage cooperative transmission scheme, any number of relays are placed to improve the reliability of wireless communication system. The BS broadcasts the requested message to users and relays in the first stage, and the successful relays forward the message to unsuccessful users in the second stage. To characterize the performance of these two schemes, we derive the exact and approximate expressions of average outage probability. Furthermore, to take full advantage of the cooperative diversity, an optimal power allocation and relay location strategy in the high signal-to-noise ratio (SNR) regime is studied. The outage probability reaches the minimum value when the first stage occupies half of the total energy consumed. Simulation and analysis results are presented to demonstrate the performance of these two schemes. The results show that the two-stage cooperative scheme effectively reduce the average outage probability in SCMA network, especially in the high SNR region.

The original user relay (UR) selection scheme of non-orthogonal multiple access-based cooperative opportunistic multicast scheme, which realizes inter-group cooperation between two multicast groups, ignores the distribution trend of candidate UR in the cell and adopts fixed efficient relay selection range (ERSR) to select UR. It results in high UR selection ratio. Then the coverage efficiency, defined as the ratio of successfully received users to URs, is low. To tackle this problem, a range-division user relay (RDUR) selection scheme is proposed in this paper. Firstly, it divides the circular coverage range of base station into several continuous annular areas (AAs). Secondly, different ERSRs are assigned to unsuccessfully received users in different AAs. Under different ERSR assignments, the performances of UR selection ratio and coverage ratio are analyzed. Lastly, the radius set of ERSR that optimizes system coverage efficiency is used to perform UR selection. From simulation results, with different radius sets, analytical results of UR selection ratio and coverage ratio match well with their simulated ones. It is proved that ERSR allocation affects UR selection ratio and coverage ratio. With RDUR scheme, coverage efficiency increases by at least 14% and capacity efficiency has also been improved.