Physical Layer Design for Multi-Service Communications in Future Wireless Networks

Future wireless networks will require the flexibility to accommodate an extremely diverse set of use cases each with different performance requirements. The physical layer is a key consideration for supporting the wide variety of services in future wireless systems. Previous generations of wireless communications systems considered a one-size-fits-all physical layer design which is unsuitable for accommodating services with a wide variety of quality of service (QoS) requirements. Additionally, designing a separate radio infrastructure for each service type is unfeasible due to the high cost and complexity involved. Therefore, this thesis investigates the problem of physical layer design for accommodating services with differing QoS requirements. In particular, this thesis focuses on the physical layer design aspects of waveforms, waveform numerologies and multiple access techniques. This thesis investigates a mixed numerology NOMA (MN-NOMA) scheme using successive interference cancellation (SIC) as an enabler for coexistence of users with with different numerologies. Analytical expressions for the inter-numerology interference (INI) experienced by each user at the receiver are derived, where mean-squared error (MSE) is the metric used to quantify INI. Using the MSE expressions, achievable rates are derived analytically for each user in the MN-NOMA system. These expressions are then evaluated and used to compare the spectral efficiency performance of MN-NOMA with that of its single-numerology counterpart. The proposed scheme can achieve the desired flexibility in supporting diverse use cases in future wireless networks. As with conventional single numerology NOMA, in order to achieve the full performance benefits of a MN-NOMA system, resource allocation is paramount. However, the coexistence of mixed numerologies changes the nature of the interference that each user experiences. This thesis investigates the problem of optimizing subcarrier and power allocation for maximizing the spectral efficiency of MN-NOMA. In particular, a two-stage sub-optimal approach is utilized to solve this problem. Numerical results show that the proposed approach provides performance gains over existing benchmark schemes of up to 14\% and 12\% in spectral efficiency and fairness, respectively. The work presented on MN-NOMA in this thesis considers orthogonal frequency division multiplexing (OFDM) as the base waveform. However, OFDM is well known to perform poorly in high-mobility environments due the Doppler effect. This is a significant problem for future wireless systems which are expected to accommodate high-mobility services such as high-speed rail, vehicle-to-everything (V2X) and unmanned aerial vehicle (UAV)communications. Orthogonal time frequency space (OTFS) is an alternative potential waveform which provides improved performance in high Doppler spread scenarios. However, while OTFS is a promising potential technology, it requires substantial further research with regard to multiple access, channel equalization and detection. This thesis therefore investigates the topic of OTFS signaling with NOMA. In particular a novel low-complexity receiver is proposed for downlink OTFS-NOMA. The proposed method uses an iterative process together with a modified version of the least squares with QR factorization (LSQR) algorithm for equalization and a novel reliability zone (RZ) detection scheme with optimized RZ thresholds. The numerical results presented demonstrate the superiority of the proposed method, in terms of symbol error rate (SER) performance, with respect to an MMSE-SIC benchmark scheme and with respect to a corresponding scheme with naive, pre-determined RZ threshold design.