Interference mitigation in code-domain non-orthogonal multiple access systems via codebook and receiver designs

Abstract

Advanced multiple access techniques have been attracted much attention since the future wireless communication systems are faced with new challenges to support various usage scenarios such as massive machine type communications and ultra-reliable and low latency communications. The code-domain non-orthogonal multiple access (CD-NOMA) has been intensively studied from industry and academia as promising solution to meet the requirements of diverse usage scenarios, and various physical-layer techniques should be further studied to realize the grant-free CD-NOMA systems in the future wireless communication systems.

In the CD-NOMA systems, different codebooks are assigned to user equipments (UEs) as the multiple access signatures. Since the transmitted codewords are non-orthogonally multiplexed over the same time-frequency resources, the most important issue on the grant-free CD-NOMAs may be how to control the interferences to maintain reliable performance. Therefore, the primary objectives of this dissertation are the interference mitigation to enhance the CD-NOMA systems by the design of an optimal set of codebooks and the development of the effective multi-user receiver.

We first study the performance of the CD-NOMA system equipped with multiple receiver antennas to drive the guidance on the transceiver design. We analyze the upper bounds on the error rates, and show that the diversity order equals to the product of the spreading gain and the number of receiver antennas. Based on the analysis, we drive the fundamental distance measures, namely the dimension-wise distance and the product distance of the codebooks, which dominantly affect the performance of the system.

The second concern is to development of flexible codebook occupation and efficient transmission scheme. Because the key feature of the CD-NOMA in uplink is a codebook-based transmission, we propose a code-domain superposition transmission by an efficient utilization of codebooks as multiple access resources. For a certain instance, if there are unoccupied codebooks of inactive UEs, they are allowed to be exploited as extra resources for active UEs. Then, an active UE can simultaneously transmit superposed codewords without additional use of physical resources. We analyze the increasing effect of the inter-layer interferences due to the application of the code-domain superposition transmission, and propose a codebook design algorithm that improves the throughput of the systems by mitigating the inter-layer interferences.

In the last part of this dissertation, we study on the practical multi-user receiver to support a number of connected devices in the CD-NOMA systems equipped with multiple receiver antennas. We propose two-stage MUDs based on the Gaussian approximation principle, and analyze the effect of the first stage (i.e., linear filtering such as matched filter, zero-forcing filter or minimum mean square error filter) on the performance of the proposed MUD. To support the system when channel state information is not available at the receiver, we further develop an low-complexity channel estimation method suitable for the proposed MUD. By estimating matched filtered channels (rather than explicitly estimating actual channels), the proposed estimator efficiently support the second-stage Gaussian approximation-based belief-propagation detector with negligible increment of complexity.