Table 11 NOMA-aided cell-free massive MIMO

[87]

This survey focuses on the capacity limit of a power-domain NOMA technique for CF massive MIMO systems. The authors emphasize that OMA would be hugely inadequate in satisfying the high demands of next-generation wireless standards, having almost reached its fundamental SE limits. To this end, the work aims at meeting the high demand for massive connectivity in future wireless systems.

▪ The research revealed that CF massive MIMO-OMA outperforms the proposed model in terms of the achievable sum rate when the number of users is low, owing to intra-cluster pilot contamination and imperfect SIC.

▪ Nevertheless, NOMA can serve a large number of users than its counterpart by grouping users into clusters and at the same time outperform OMA considering the sum rate when the number of users within a cluster grows large.

▪ Guaranteeing high reliability with user-fairness is quite complex.

2018

[29]

The authors present the performance of a NOMA-aided CF massive MIMO under stochastic AP and user locations. The goal is to maximize the achievable rate of CF systems. The imperfect SIC is explored to develop the achievable rates and the probability of successful SIC.

▪ NOMA outperforms its OMA counterpart under a low path loss environment and networks with high AP density in terms of rate performance.

▪ The rate gain in NOMA diminishes as the density of AP becomes smaller.

▪ The overall rate of NOMA is generally reduced while providing reduced latency for higher path loss exponents.

2019

[128]

The work considers the performance of a NOMA-aided CF massive MIMO with three linear precoders. The goal is to maximize the SE of the system. A closed-form expression for the achievable DL sum rate with MRT and fpZF is presented considering the effects of inter-cluster interference, intra-cluster pilot contamination, and imperfect SIC.

▪ The proposed hybrid CF massive MIMO-NOMA permits more users to be supported at the same time-frequency resource than its OMA counterpart.

▪ Pilot contamination and imperfect SIC degrades the performance of NOMA considerably.

2019

[160]

A NOMA-enabled CF massive MIMO with CB and multiple clustered users is investigated. A closed-form expression for the bandwidth efficiency (BE) assuming no DL training is presented. The work also formulates a max-min BE optimization problem, and a bisection search method is proposed to address the non-convexity of the max-min fairness problem.

▪ The max-min BE is significantly enhanced with NOMA compared to OMA.

▪ Selecting the optimal mode from the set mode is mainly dependent on the length of the channel coherence time and the total number of users.

2019

[161]

The achievable rate of a NOMA-aided CF massive MIMO underlaid below a primary massive MIMO is investigated. The goal is to address the physical challenges presented by MIMO-OMA. The work considers the effects of intra-cluster pilot contamination, inter-cluster interference, imperfect SIC, and statistical DL CSI at secondary users to develop the closed-form secondary DL sum rate.

▪ The proposed underlay CF massive MIMO improves the sum rate considerably by exploiting the channel gain differences.

▪ The adverse effects of imperfect SIC and intra-cluster pilot contamination impact the system’s performance severely.

2019

[130]

A CF massive MIMO-NOMA employing underlay spectrum sharing is proposed. The goal is to enable massive connectivity in both primary and secondary networks. The work considers the adverse effects of primary/secondary pilot contamination, inter/intra-cluster interference, imperfect SIC, and partial CSI to develop the achievable rates of the secondary system.

▪ The proposed system model enhances the number of concurrent connections significantly.

▪ Moreover, all users are served with an improved QoS compared to the equal power allocation scheme.

▪ The detrimental effects of imperfect SIC and error propagation degrade the achievable rate substantially.

2020