Massive MIMO opens up new avenues for enabling highly efficient random access (RA) by offering abundance of spatial degrees of freedom. In this paper, we investigate the grant-free RA with massive MIMO and derive the analytic expressions of success probability of the grant-free RA for conjugate beamforming and zero-forcing beamforming techniques. With the derived analytic expressions, we further shed light on the impact of system parameters on the success probability. Simulation results verify the accuracy of the analyses. It is confirmed that the grant-free RA with massive MIMO is an attractive RA technique with low signaling overhead that could simultaneously accommodate a number of RA users, which is multiple times the number of RA channels, with close-to-one success probability. In addition, when the number of antennas in massive MIMO is sufficiently large, we show that the number of orthogonal preambles would dominate the success probability.

Grant-free random access (RA) with massive MIMO is a promising RA technique that provides significant benefits in increasing the channel reuse efficiency with low signaling overhead. Since user equipment (UE) detection and channel estimation in grant-free RA rely solely on the received preambles, preamble designs that enable high success rate of UE detection and channel estimation are very much in need to ensure the performance gain of grant-free RA with massive MIMO. In this paper, a super preamble consisting of multiple consecutive preambles is proposed for high success rate of grant-free RA with massive MIMO. With the proposed approach, the success of UE detection and channel estimation for an UE depends on two conditions: 1) it is a solvable UE, where we define the UE whose super preamble is not a linear combination of the other UEs’ super preambles as a solvable UE; 2) its super preamble is detected. Accordingly, we theoretically analyze the solvable rate of the UEs with multiple preambles and propose a reliable UE detection algorithm to obtain the super preambles of the UEs by exploiting the quasi-orthogonality characteristic of massive MIMO. Theoretical analysis and simulation results show that turning a preamble into a super preamble consisting of two or three shorter preambles, the success rate of UE detection and channel estimation could be significantly increased using the proposed approach.

Fifth-generation mobile communication systems aim to support massive access for future wireless applications. Unfortunately, wireless resource scarcity in random access (RA) is a fundamental bottleneck for enabling massive access. To address this problem, we propose a virtual carrier sensing (VCS)-based RA scheme in massive MIMO systems. The essence of the proposed scheme lies in exploiting wireless spatial resources of uplink channels occupied by assigned user equipments (UEs) to increase channel resources for RA. With the proposed scheme, RA UEs are able to exploit the spatial resources that are approximately orthogonal to those of assigned UEs, thus sharing the uplink channel resource with assigned UEs without causing significant interference to them. Specifically, to ensure RA UEs avoid serious interference with assigned UEs, a base station (BS) sends tailored virtual carriers to RA UEs on behalf of assigned UEs. The RA UEs then conduct VCS to determine whether or not the uplink channel resource is available for RA. The closed-form approximations for probability of channel availability and uplink achievable rate with the proposed scheme are derived. Theoretical analysis and simulation results show that the proposed scheme is able to significantly increase channel resources of RA for massive access.

With the development of 5G new radio (NR) applications in high-speed scenarios, such as 5G non-terrestrial networks (NTN), the Doppler shift in the systems is significant. In this paper, an estimation scheme for frequency offset with large range in orthogonal frequency division multiplexing (OFDM) systems is proposed. The proposed scheme firstly takes advantage of the $2\pi$-periodicity of the phase offset between two pilot OFDM symbols to estimate a set of candidate frequency offsets. It then uses the autocorrelation of the pilot sequence to determine the final estimated frequency offset. This method allows for a large estimation range, independent of the symbol gap between the two pilot OFDM symbols. Moreover, the low-complexity implementation of the scheme is provided. The simulation results based on 5G NR physical uplink shared channel (PUSCH) show the effectiveness of the proposed scheme in both single-user and multi-user scenarios, where various Doppler shifts and numbers of configured resource blocks (RB) are considered. The simulation results also show that the proposed frequency-domain method outperforms the conventional time-domain method with additional computation complexity.

Massive multiple input multiple output (MIMO) enabled grant-free random access (mGFRA) stands out as a promising random access (RA) solution, thus effectively addressing the need for massive access in massive machine-type communications (mMTCs) while ensuring high spectral efficiency and minimizing signaling overhead. However, the bottleneck of mGFRA is mainly dominated by the orthogonal preamble collisions, since the orthogonal preamble pool is small and of a fixed-sized. In this paper, we explore the potential of non-orthogonal preambles to overcome limitations and enhance the success probability of mGFRA without extending the length of the preamble. Given the RA procedure of mGFRA, we analyze the factors influencing the success rate of mGFRA with non-orthogonal preamble and propose to use two types of sequences, namely Gold sequence and Gaussian distribution sequence, as the preambles for mGFRA. Simulation results demonstrate the effectiveness of these two types pf non-orthogonal preambles in improving the success probability of mGFRA. Moreover, the system parameters’ impact on the performance of mGFRA with non-orthogonal preambles is examined and deliberated.

Recently, results have shown that full-duplex (FD) system based on orthogonal frequency division multiplexing (OFDM) is potential for wireless sensor networks. However, extending FD to further improve spectral efficiency by removing cyclic prefix (CP) with optimal canceling self-interference (SI) remains a challenge. In this paper, we analytically study the conventional FD OFDM system and proposed a CP-free system on this basis. The key point of the proposed scheme is the symbols transmitted in a unit of two identical symbols without CP. To eliminate the intersymbol interference, the disturbed part are replaced by the repeated signal at the receiver, which has the same effect as CP. Finally, an SI cancellation is down to restore the transmitted symbols. As a key of analysis, a multiple-input multiple-output (MIMO) FD simulation model is carried to show that our proposed scheme is effectiveness in terms of the bit error ratio (BER).