Sparse code multiple access (SCMA) is a promising non-orthogonal multiple access scheme in order to fulfill the stringent 5G use cases requirements. SCMA can be seen as an improvement of low density signature orthogonal frequency division multiple access that takes advantage of codes sparsity to yield users overload. Generally, SCMA is implemented with message passing algorithm (MPA) at the decoder to benefit from the strong iterative decoding performance. On the other hand, it suffers from high complexity due to the large number of involved exponential operations to compute the log likelihood ratio (LLR). Hence, simplified sub-optimal versions of MPA known as MAX-Log-MPA and Log-MPA were proposed in order to ensure a trade-off between system complexity and performance. This paper presents a comprehensive comparison between those three algorithms in Gaussian and fading channels and precisely explains algorithms performance through a study of the extrinsic information behavior.

Multi-user shared access (MUSA) is one of the most promising non orthogonal multiple access (NOMA) schemes to accommodate the stringent requirements of massive machine type communications (mMTC). MUSA adopts a grant-free access strategy and uses non-orthogonal spreading sequences to accommodate a large number of users in the same radio resources. The inter-user interference is subsequently handled using successive interference cancellation (SIC) algorithm, which allows a low decoding complexity. However, sequential decoding can be affected by error propagation when the power difference between simultaneous transmissions is not significant. In this paper, we propose a power allocation algorithm in order to avoid the error propagation problem by guaranteeing sufficient power difference between concurrent received signals. We formulate an optimization problem which aims at minimizing the system average bit error rate. We show that the optimization problem has an optimal global solution and an algorithm is proposed for its resolution. Simulation results show the impact of performing power allocation for SIC decoding on the bit error rate.

Non-orthogonal multiple access schemes with grant free access have been recently highlighted as a prominent solution to meet the stringent requirements of massive machine-type communications (mMTCs). In particular, the multi-user shared access (MUSA) scheme has shown great potential to grant free access to the available resources. For the sake of simplicity, MUSA is generally conducted with the successive interference cancellation (SIC) receiver, which offers a low decoding complexity. However, this family of receivers requires sufficiently diversified received user powers in order to ensure the best performance and avoid the error propagation phenomenon. The power allocation has been considered as a complicated issue especially for a decentralized decision with a minimum signaling overhead. In this paper, we propose a novel algorithm for an autonomous power decision with a minimal overhead based on a tight approximation of the bit error probability (BEP) while considering the error propagation phenomenon. We investigate the efficiency of multi-armed bandit (MAB) approaches for this problem in two different reward scenarios: (i) in Scenario 1, each user reward only informs about whether its own packet was successfully transmitted or not; (ii) in Scenario 2, each user reward may carry information about the other interfering user packets. The performances of the proposed algorithm and the MAB techniques are compared in terms of the successful transmission rate. The simulation results prove that the MAB algorithms show a better performance in the second scenario compared to the first one. However, in both scenarios, the proposed algorithm outperforms the MAB techniques with a lower complexity at user equipment.