Modern 5G heterogeneous networks (HetNets) require hybrid multiple access technology for optimal performance. The feasibility of a hybrid power domain sparse code nonorthogonal multiple access (PD‐SCMA) that integrates both power domain nonorthogonal multiple access (PD‐NOMA) and sparse code multiple access (SCMA) for an uplink hierarchical HetNet system is demonstrated. Hybrid schemes namely: Successive Codebook Ordering Assignment (SCOA) for codebook assignment (CA), opportunistic MUE‐SUE pairing (OMSP) for user pairing (UP), and a QoS‐aware power allocation (QAPA) for power allocation (PA) are developed. The SCOA algorithm is based on channel quality ordering metric, OMSP algorithm is based on channel quality diversity and pairing interference metric while the QAPA algorithm features a QoS awareness metric. A joint energy efficiency (EE) resource allocation (JEERA) algorithm that iteratively performs CA, UP, and PA for small cell user equipment (SUE) and the macro user equipment (MUE) to limit interference, improve spectral and energy efficiency is presented. The problem is formulated as a mixed integer nonconvex system EE resource allocation optimization for the small cells under QoS constraints of minimum sum‐rate, interference temperature, maximum power, and SCMA structure for a hybrid low complexity joint SIC‐Log‐MPA receiver. A modified near‐optimal dual decomposition analytical methodology featuring Dinkelbach fractional transformations is utilized to assess the system's performance on an imperfect wireless channel. Through numerical results, the proposed schemes are shown to improve the EE of the small cells in comparison with the prevalent schemes.

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The application of multiple-input multiple-output (MIMO) on power domain sparse code multiple access (PD-SCMA) systems would enhance their performance by increasing the multiplexing and diversity gains. However, this is at a cost of increased detection complexity, as more users and antennas are deployed. This work develops and investigates the performance of spatial multiplexing MIMO based hybrid PD-SCMA system (M-PD-SCMA) transceiver on an uplink heterogeneous network over Rayleigh flatfading channels. The aim is to strike a balance on the number of antennas and capacity/spectral efficiency. The system capacity and outage robustness of the proposed transceiver in imperfect channels is evaluated and the bit error rate (BER) performance analysed. The link-level simulation results demonstrate that M-PD-SCMA achieves performance benchmark with PD-SCMA schemes. The proposed receiver achieves guaranteed BER performance with an increase in the number of transmit and receive antennas. Besides, the results highlight the impact of the codebook size, number of layers and power level distinctiveness on the outage bounds at each receive antenna at different SNR levels. Thus, the feasibility of an M-PD-SCMA system is validated.

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The hybrid power domain sparse code non-orthogonal multiple access (PD-SCMA) scheme multiplexes multiple users on a single resource element (RE) both in power and code domains, thus achieving a superior spectral efficiency. The multiplexing performance of such a scheme can be enhanced not only through deployment of sophisticated resource allocation schemes and low complexity robust multi-user detectors but also prudent multi-user multi-dimensional codebook design. By considering the key codebook design performance indicators (KPIs) in different channel use scenarios, this work assesses various known multi-dimensional constellation (MDC) design criterion on the performance of low-rate turbo-coded PD-SCMA NOMA system. In order to strike a balance with regard to good multi-dimensional modulation (MDM) and low complex detection, a low complex codebook design based on linear error correcting block codes is proposed. Unique user-specific rotation design based on quaternion rotation matrices is proposed with the aim of maintaining users symbol uniqueness for overloaded systems. Simulation results confirm that MDCs that satisfy design KPIs outperform others. Besides, the proposed user-specific rotation design remarkably enhances the BER performance of the block code based codebook design.INDEX TERMS PD-SCMA, Key performance indicators (KPIs), Multi-dimensional constellation, Codebook design, User-specific rotation.