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Two of the most challenging goals for the fifth generation (5G) and beyond communication systems are massive connectivity and higher capacity. The use of traditional orthogonal multiple access techniques limits the number of users that can be served using available resources due to orthogonality constraint. Moreover, the available resources may not be utilized effectively by alloted users thereby resulting in inefficiency and user unfairness. This imposes a severe drawback in cases where the number of users to be served are high, like in the Internet of Things or ultra-dense 5G networks. Hence, introducing non-orthogonality to multiple access scheme is advocated as a superior methodology to serve multiple users simultaneously, thereby achieving multi-fold enhancement in connectivity. In scenarios with massive number of users, non-orthogonal multiple access scheme (NOMA) increases the number of active connections by superimposing signals of multiple users on the same resource block, thereby also utilizing the available resources efficiently. This article presents an overview of the integration of NOMA with several other leading technologies for 5G and beyond networks to enhance the connectivity.

Non-orthogonal multiple access (NOMA) is envisioned as the solution for increasing capacity and serving exponentially increasing number of devices (users) in beyond 5G networks. Breaking orthogonality, with NOMA users having different channel gain (preferably a cell edge user (CEU) and a cell center user (CCU)) are selected and served on the same resource block. In urban areas, impact of blockages may limit the system performance. Impact of blockages is more prominent for the CEU because the CCU's close proximity to the base stations (BS) lowers probability of CCU being blocked. In order to overcome performance degradation due to blockages, the research interest have move towards intelligent reflecting surfaces (IRS). One of the prime utilization of IRS is to improve connectivity of the blocked users. Since IRS comprises of low-cost passive elements which reflect a signal incident on them using a certain phase shift, hence, are capable of effectively directing the signal from the BS towards the blocked CEU. Intuitively, the location of IRS from the BS, and from the user should play a vital role in directing the intended signal to the desired user. In the current literature, such phenomenon based on the location of IRS is not considered, and mostly fixed/dedicated IRS are considered for the users, which is not a practical approach, since, the users are mobile. The novel numerical results from the study of proposed NOMA-IRS network suggest strong relation between the distance of IRS from the BS and users on the performance of CEU using NOMA.

Rapid increase in number of cellular users and high demand for data has lead to the formation of multi-tier networks. Non-orthogonal multiple access (NOMA) has proved to be an efficient method to cater to the paradigm shift from 4G to 5G. This paper employs NOMA in an heterogeneous cellular network consisting of a macro base station (MBS) tier underlaid with femto base station (FBS) tier and device-to-device (D2D) tier, where NOMA is employed in FBS and D2D tier only. The congestion at the MBS tier is relieved by offloading macro users (MU) to the FBS tier. The offloaded MU are further supported by the D2D tier when the FBS tier fails to find a corresponding pairing user for the incoming offloaded MU. Since, absence of pairing user means outage for offloaded MU, D2D cooperation is employed, which decreases the rate outage probability by $86.87\%$ for the MU offloaded as cell edge user (CEU) in comparison to no cooperation. Also, a three times increase in ergodic rate and four times increase in sum ergodic rate for MU offloaded as CEU is achieved using cooperation from D2D tier. Verification of the results is done using Monte Carlo simulations.

This paper studies the outage performance of heterogeneous network consisting of ultra high frequency base stations (UHBSs) and millimeter wave base stations (MWBSs). Offloading to MWBS is performed based on the average received power from the base stations of the two tiers, i.e., UHBS and MWBS tier. Both the UHBS and MWBS employ non-orthogonal multiple access (NOMA) to support the users. Furthermore, the impact of the channel gain difference (CGD) between the users paired at the MWBS tier is investigated in detail. The numerical results show that offloading to MWBS achieves significant improvement in the outage probability of the offloaded user. In addition, offloading to MWBS tier also improves the outage performance at the UHBS tier. Besides, it is observed that the CGD between the paired users at the MWBS tier plays a vital role in the outage performance of the offloaded user. The analytical results are verified using Monte Carlo simulations.

The increase in number of cellular users had lead to the evolution from the traditional cellular networks to the more efficient heterogeneous cellular networks (HetNet) to handle the traffic. Offloading plays a vital role in handling the traffic from the congested macro base station by handing users to the less congested femto base stations (FBS). Further, non-orthogonal multiple access (NOMA) has proved to be efficient for the future generation networks. In this work, we study the offloading in HetNets, where the FBS tier serves the users using NOMA. Imperfect channel state information (CSI) is considered to analyze the outage probability of offloaded user, since, in practice CSI estimation requires significant system overhead, especially when the number of users are large. Hence, for practical systems, the analyses with imperfect CSI is more relevant. Some important observations regarding offloading to FBS tier with NOMA based on imperfect CSI are drawn. The analytical results are validated using Monte Carlo simulations.

Surge in number of cellular users and data-intensive applications require the traditional cellular networks to evolve towards a more promising heterogeneous cellular networks (HetNets). Offloading in HetNets alleviates congestion on the overloaded cellular network where the femto base stations (FBS) acts as offloading spots. The offloaded user (OU) is paired with the user available at the FBS (called as pairing user (PU)) and served using non-orthogonal multiple access. Two parameters namely, power allocation factor (PAF), i.e., the power to be allocated to the OU, and channel ratio (CHR), i.e., the ratio of channel gain between the OU and the PU play a crucial role in determining the ergodic rate achieved by the OU. If the PAF and the CHR are not optimally selected, it results in degradation in overall rate performance. This work jointly optimizes the PAF and the CHR to maximize the ergodic rate performance of the OU. The impact of optimized and non-optimized parameters on the performance of OU is studied to prove the advantage of using optimized scenario for the OU. An increase of upto 3 times is observed in the ergodic rate and sum ergodic rate by using the optimized parameters.

The current literature characterizes the performance gain achieved by non-orthogonal multiple access (NOMA) using stochastic geometry based on the assumption of Poisson Point Processes (PPP) to model the spatial distribution of base stations. This paper develops an analytical framework to analyze the gain achieved by NOMA in a heterogeneous cellular network (HCN) consisting of a macro base station (MBS) tier and a femto base station (FBS) tier modeled using repulsive point process namely modified hard core point process (MoHCPP). It is observed that modeling NOMA network using MoHCPP approach decreases the outage probability by $ 74.04 \% $ for a cell center user (CCU) and by $48.65 \%$ for a cell edge user (CEU) as compared to NOMA using PPP. This improvement is attributed to decrease in dominant interferers due to thinning process of MoHCPP. It is also observed that with the increase in FBS density the advantage of NOMA with MoHCPP modeling is more for CEU as compared CCU due to additional benefit of dense FBSs for CEU. Hence, it is inferred that high density FBS network with NOMA modeled using MoHCPP proves to be better suited for CEU as compared to PPP modeling. The results for the proposed NOMA with MoHCPP model are also verified using simulations.

The explosion of cellular users and high demand for data has evolved the single-tier cellular network to multi-tier network. This paper analyses the gain of non-orthogonal multiple access (NOMA), an enabling technique for 5G, in a heterogeneous cellular network (HCN) consisting of a macro base station (MBS) tier and femto base station (FBS) tier. Offloading users to the FBS tier is performed to relieve the MBS tier of over congestion. In order to support offloaded user, the FBS tier employs NOMA. The results are plotted for different channel conditions while offloading the user to FBS tier. Benefits of NOMA (for e.g., higher ergodic rate, user fairness, performance enhancement at cell edge user) are observed for different channel conditions for the offloaded user. It is also observed that offloading to FBS tier with NOMA gives better performance as compared to offloading without NOMA.

Femtocell deployment improves the data rate and indoor coverage of the heterogeneous cellular network (HCN). However, it also increases the cross-tier interference of the system which results in performance degradation of the macro base station (MBS) tier. In this work, a mathematical framework is developed to mitigate the cross-tier interference of MBS tier using a power control scheme (PCS). This PCS works on path loss inversion and location-based power level rule for femto base stations (FBS) to improve the coverage of the MBS tier. Furthermore, a cooperation scheme and association policy are proposed to improve the performance of the FBS tier. Stochastic geometry is considered for deriving the signal to interference and noise ratio (SINR) outage probability, total outage probability, and area spectral efficiency (ASE) for MBS and FBS tiers. The impact of the PCS, proposed cooperation scheme with association policy on outage probability, and ASE are numerically evaluated. Numerical results show that considerable improvement in outage performance of MBS tier is obtained using proposed PCS. Moreover, applying PCS results in degradation of outage performance of FBS tier. This loss in performance can be compensated by using the proposed cooperation scheme and association policy.