Doctor's Theses (authored and supervised):

B. Tahir:
"Towards Massive Connectivity via Uplink Code-Domain NOMA";
Supervisor, Reviewer: S. Schwarz, M. Rupp, Db. da Costa, Y. Liu; Institute of Telecommunications, 2022; oral examination: 04-12-2022.

English abstract:
Future mobile networks are envisioned to provide wireless access to a massive number of devices. The substantial increase in connectivity comes mainly from machine-type communication (MTC), for which a large of number of low-rate transmissions take place. Accommodating access for such a large number of user equipments (UEs) can be inefficient if applied to current network architectures, which are mainly based on orthogonal multiple access (OMA) and scheduling-based transmissions. This is due to the resulting control overhead and increased access-delay. The framework of non-orthogonal multiple access (NOMA) has attracted attention recently as a promising solution to tackle these issues. It allows multiple UEs to access the network simultaneously over the same resources, and provides naturally, the support for grant-free access, in which no explicit scheduling of the UEs is required.

Motivated by the potential benefits of NOMA in enabling massive connectivity, this dissertation focuses on studying uplink code-domain NOMA, where multiple UEs access the network via short non-orthogonal spreading signatures. The dissertation consists of three major parts corresponding to the three building-blocks of the NOMA communication chain: the transmitter, the receiver, and the channel. In the first part, we consider the codebook design problem, that is, the design of the spreading signatures across the different UEs. The formulation we consider leads us to constructing codebooks of a Grassmannian nature. We propose an iterative algorithm for constructing such codebooks, with close-to-optimal correlation properties, leading to enhanced performance under low-complexity suboptimal detection. We then extend the codebook design problem to the case where the UEs are available as groups, such as in cells, or spatial clusters. We propose to jointly design the codebooks across the different groups via an alternating projection algorithm, and show that such a joint design can improve the performance of UEs suffering from strong inter-group interference.

The second part of the dissertation is then concerned with the receiver side, aiming to reduce the complexity of the detection procedure. We consider the user activity detection in the context of grant-free access under a practical frame-structure. We formulate the activity detector based on subspace methods, and address the influence of the channel. Namely, we show that strong time-frequency correlation of the channel can prevent successful detection of the active set of UEs. To address that, we propose overlaying the pilot sequences with user-specific masking sequences, which results effectively in a decorrelation of the channel. We also consider, on the other hand, the influence of strong time-frequency selectivity, for which we investigate different pilots' allocation strategies. We then focus on reducing the detection complexity of the data part of the transmission. We utilize the time-frequency correlation of the channel to reduce the number of calculated filters for the spreading blocks over the time-frequency grid. Also, by assuming the base station (BS) is equipped with a sufficient number of receive antennas, we show how that combination of the code- and spatial-domains allows us to replace exact minimum mean square error (MMSE) filtering with a low-complexity approximation requiring no inverse calculation, while resulting only in a small performance loss.

The last part investigates the controllability of the channel via reconfigurable intelligent surfaces (RISs). We consider first a RIS-assisted two-UE NOMA uplink, where part of the surface elements are configured to boost the signal of the first UE, while the other part is used to boost the second one. By approximating the receive powers as gamma random variables, tractable expressions for the outage probability under interference cancellation (IC) are derived. We show how the optimization of the RIS impacts the NOMA detection performance, and identify robust operation points that guarantee reliable link quality for both UEs. Finally, we consider the combination of a K-UE code-domain NOMA uplink with RISs, in the context of a cluster-based massive multiple-input multiple-output (MIMO) deployment. We investigate the optimization of the RIS under such a setup, and show a solution based on a semi-definite relaxation of the problem. The results show that our proposed approach can substantially increase the number of UEs supported by the system.

non-orthogonal multiple access; massive connectivity; code-domain; grant-free; reconfigurable intelligent surface; NOMA; RIS

"Official" electronic version of the publication (accessed through its Digital Object Identifier - DOI)

Created from the Publication Database of the Vienna University of Technology.