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Actualités de l'école

06/04/2021

Doctorate thesis defense of Salifou MOUNCHILI




Doctorate thesis defense on April 6th 2021 at 11H00 ,in Sup’Com Amphitheater Ibn Khaldoun.


Entitled :Improving User Pairing Techniques in NOMA For 5G systems

Presented by :Salifou MOUNCHILI


Committee


President :

Sami TABBANE

Professor at SUP'COM, Tunisia

 

 

 

Reviewers :

Inès KAMMOUN

Professor at ENIS, Tunisia

 

Robert SCHOBER

Professor at FAU University, Germany

 

Examiner :

Fethi CHOUBANI

Professor at SUP'COM, Tunisia

 

 

 

Thesis Supervisor :

Soumaya HAMOUDA

Associate Professor at FSB, Tunisia


Abstract


The Orthogonal Multiple Access (OMA) techniques also termed Multiple Access (MA) techniques are well-designed systems that meets the requirements of 4G cellular networks. But faced with growing needs for massive connectivity, OMA for 4G could no longer be adoptable by numerous next generation operators due to its limited resource sharing constraint (time-frequency). On the other side, Non-Orthogonal Multiple Access (NOMA) techniques and more specically the Power-Domain NOMA (PD-NOMA) is attracting much attention for 5G systems due to its relevant features such as spectral efficiency and its ability to exploit the channel gain difference in order to superimpose multiple users into the same OMA Resource Block (RB). These users are said to be paired. However, when this channel gain gap narrows the performance of NOMA can be greatly affected. Conversely, when the latter is large, some users are deprived of participating in NOMA. In our work, we investigate upon the NOMA user pairing techniques in the new dimension known as the power domain. Thanks to this pairing scheme, a distance difference could be guaranteed between a so-called near and far user from the base station selected to participate in NOMA. Thus, the co-channel interference will be limited consequently improving the massive connectivity as well as the system performance. The main critical issue to be addressed in order to achieve a proper match between a far and near user is the area identification where the potential NOMA candidates for any given far user can be located. We proposed an original user pairing approach related to targeted throughputs and the distance difference between paired users; called Minimum Distance-NOMA (MD-NOMA), it is based on the resolved pairing distance threshold, the one that separates the far user from the area where their appropriate NOMA candidates are located. Numerical results showed that, the proposed MD-NOMA approach can pair almost 50% more users than the Uniform Channel Gain Difference-NOMA (UCGD-NOMA) and Conventional-NOMA (C-NOMA).


NOMA is also a promising solution due to its ability to enhance the cell edge users throughputs and sum throughput via NOMA pairing methods which consider more than two users within a pair/cluster (multiple user pairing). This presents an interesting solution for massive connectivity. Thus, in the second part of our work, we started by studying a Distributed NOMA (D-NOMA) pairing scheme to tackle the problem of fair clusters, and protect user pairs having low channel gain difference of small performance, and also to increase their chances of accepting unpaired users. In fact, it is a NOMA scheme in which, users are divided into multiple groups and an intergroup NOMA is applied. The results showed that the performance of the low channel gain difference pairs are improved when the division introduces a large intergroup interval. Next, after providing a comprehensive analysis on the impact of bad NOMA clustering on the NOMA pairs throughput of more than two users, we defined the admission condition of unpaired users in NOMA pairs with two users. Finally, in order to better manage intra-cluster interference affecting low channel gain users and increase the users number to be admitted into a NOMA cluster, we have proposed a user clustering and joint power allocation scheme. More specifically, the Joint Power Allocation (JPA) formulation considers the user's Signal to Interference plus Noise Ratio (SINR) of lowest channel gain in the cell, the noise power spectral density and user channel gains to be included in the cluster to reduce the complexity of Successive Interference Cancellation (SIC) on the receiver side. This study was carried out frst in a context where users have separate channel gains, then in a context of similar channel gains. The case of similar channel gains precisely gives the chance to assign powers separately to users within a cluster to mitigate intra-cluster interference. The results of the evaluation show that with the proposed method we can improve cell edge throughput, control the users number to be included in a NOMA cluster and increase massive connectivity.




Keywords :


Power-Domain NOMA (PD-NOMA), Orthogonal Multiple Access (OMA), Throughput, Massive Connectivity, User Pairing, Pairing Distance Threshold, User clustering, Joint Power Allocation (JPA), Signal to Interference plus Noise Ratio (SINR).