Future wireless communications require higher reliability, lower latency, and greater connectivity. In this paper, we mainly investigate a multi-dimensional resource-based hybrid quality of service (QoS) massive grant-free random access (GFRA) in the massive MIMO system. First, inspired by the advanced access protocol [1], we develop an extended multi-dimensional resource-based hybrid QoS massive GFRA scheme to address the differentiated requirements of various services, which not only meets the stringent reliability and low latency demands of URLLC users, but also supports the massive concurrent connectivity of mMTC users. Then, based on the extended multi-dimensional resource access scheme, we analyze the access failure probability of URLLC users by comprehensively considering the pilot collision and decoding error in the finite blocklength regime. To achieve massive concurrent connectivity and reliable access for mMTC users, we further propose a multi-slot replica access scheme that leverages time diversity, and investigate the access failure probability of mMTC users by considering the pilot collision and outage probability across multiple slots. Furthermore, by integrating GFRA mechanism and multi-slot replica access scheme, we model the massive access of mMTC users as a Markov state transition process and analyze the access latency of mMTC users from a frame-level perspective. Finally, the effectiveness of the proposed scheme is demonstrated through simulations. The proposed scheme not only meets the differentiated requirements of URLLC and mMTC users, but also further reduces latency of mMTC users compared with repetition transmission.

Next generation wireless network is expected to provide higher rate, more reliable access and lower latency. Cell-free massive multiple input multiple output (CF-mMIMO) has been considered a potential enabler for massive access. In this paper, we primarily investigate massive grant-free random access (GFRA) in CF-mMIMO ultra-reliable and low latency (URLLC) systems. First, we give a massive GFRA model based on CF-mMIMO URLLC system, focusing on analyzing access reliability and access latency. Next, we derive approximated closed-form expression for the decoding error probability and outage probability of active users in the finite block-length regime by leveraging the approximated distribution of the signal to interference plus noise power. Based on this, by comprehensively considering the random access procedure, we derive the expression for access success probability of the attempted access user. We also jointly optimize the pilot length and data block-length by the access success probability to enhance access reliability. Furthermore, by utilizing the macro diversity of the CF-mMIMO system, we propose a scalable user centric-based multiple access points scheme to improve access reliability. In addition, based on access procedure and the associated GFRA protocol, we analyze the components of access latency. Subsequently, considering the access success probability and the number of retransmissions, we analyze and evaluate access latency for massive GFRA in the CF-mMIMO URLLC system. Finally, simulation results demonstrate the rationality and effectiveness of the proposed massive access model for analyzing the access reliability and access latency.

In this paper, the uplink sparse code multiple access over universal filtered multicarrier (SCMA-UFMC) in rapidly time varying channels is studied. In SCMA-UFMC, each subband of UFMC is occupied by multiple users through SCMA. It is observed that the inter-band-interference can be neglected and both the inter-carrier interference and inter-user-interference are small from the equivalent frequency-domain (FD) channel of even component of received signal. Based on this observation, message passing algorithm is first employed to perform multiuser detection (MUD) by utilizing only even-component of received signal. Next, to improve the performance, a two-stage detection is further presented, where the unreliable estimates from the first-stage detection using even-component are refined by utilizing additional odd component of received signal. Finally, simulation results are given to demonstrate the performance of the proposed methods.

Universal filtered multicarrier (UFMC) modulation has attracted much attention recently due to its potential advantages over orthogonal frequency division multiplexing. In this paper, the signal detection of UFMC in rapidly time varying channels is stuided. It is first observed that the frequency domain (FD) channel associated with even component of received signal can be well approximated by a band matrix centered along main diagonal and that associated with odd component can be approximated by block-diagonal matrix with some inter-sub-band-interference (IBI) between adjacent blocks. Based on this observation, an initial estimation is first acquired by Viterbi algorithm using the even component. Then, this estimation is refined block-by-block by utilizing the low out-of-band emission using the odd component. Moreover, considering the IBI between adjacent blocks, a sliding-window strategy is introduced to further improve the detection performance. Finally, simulation results are given to demonstrate the effectiveness of the proposed detection.