Multiple Access in Aerial Networks: From Orthogonal and Non-Orthogonal to Rate-Splitting

Recently, interest on the utilization of unmanned aerial vehicles (UAVs) has aroused. Specifically, UAVs can be used in cellular networks as aerial users for delivery, surveillance, rescue search, or as an aerial base station (aBS) for communication with ground users in remote uncovered areas or in dense environments requiring prompt high capacity. Aiming to satisfy the high requirements of wireless aerial networks, several multiple access techniques have been investigated. In particular, space-division multiple access(SDMA) and power-domain non-orthogonal multiple access (NOMA) present promising multiplexing gains for aerial downlink and uplink. Nevertheless, these gains are limited as they depend on the conditions of the environment. Hence, a generalized scheme has been recently proposed, called rate-splitting multiple access (RSMA), which is capable of achieving better spectral efficiency gains compared to SDMA and NOMA. In this paper, we present a comprehensive survey of key multiple access technologies adopted for aerial networks, where aBSs are deployed to serve ground users. Since there have been only sporadic results reported on the use of RSMA in aerial systems, we aim to extend the discussion on this topic by modelling and analyzing the weighted sum-rate performance of a two-user downlink network served by an RSMA-based aBS. Finally, related open issues and future research directions are exposed.Comment: 16 pages, 6 figures, submitted to IEEE Journa

Degrees of Freedom of MIMO Wireless Networks with General Message Sets: Channel Decomposition, Message Splitting and Beamformer Allocation

The physical layer of a wireless network can be used in various settings distinguished by the specific set of messages transmitted in the network. In this thesis, rather than considering each setting in isolation as has been the traditional approach, we study the unified setting of general message sets in which any subset (including all) of messages can be transmitted simultaneously in a network. The total number of possible messages is exponential in the size of the network and the number of settings simultaneously studied is double-exponential. For this reason, the problem quickly would become difficult or even impossible to solve for large networks unless some structure is found in smaller network settings. In this thesis, we begin this journey by settling the approximate capacity regions in the form of exact degrees of freedom (DoF) or the linear DoF regions of selected small, but representative, multiple-input multiple-output (MIMO) wireless broadcast and interference networks. The aim is to not only develop novel approaches to fully obtain the DoF-optimal solutions through tight inner and outer bounds but also to suggest approaches that could be potentially useful in generalizing the results herein to a broader class of problems that may include larger networks and/or channel uncertainty models. In developing novel achievable schemes, we propose a methodology that combines the idea of message splitting and channel decomposition, which notably simplifies the construction of the achievable region for the network. Using channel decomposition, the transmitter beamformer space is partitioned into several linearly independent subspaces, each of which has special properties and is easier to analyze. Message splitting involves expanding the number of message types beyond the original ones by splitting each message into several independent types according to their different impacts to the receivers or which beamformer subspaces they are transmitted through. This enlarges the dimensions needed to specify the achievable DoF region in split-message space. Interestingly though, it also simplifies the analysis and provides what is effectively a high-dimensional description of the achievable DoF region. When projected to the desired dimensions, the achievable region is specified

The Degrees of Freedom Region of Temporally Correlated MIMO Networks With Delayed CSIT

We consider the temporally-correlated Multiple-Input Multiple-Output (MIMO) broadcast channels (BC) and interference channels (IC) where the transmitter(s) has/have (i) delayed channel state information (CSI) obtained from a latency-prone feedback channel as well as (ii) imperfect current CSIT, obtained, e.g., from prediction on the basis of these past channel samples based on the temporal correlation. The degrees of freedom (DoF) regions for the two-user broadcast and interference MIMO networks with general antenna configuration under such conditions are fully characterized, as a function of the prediction quality indicator. Specifically, a simple unified framework is proposed, allowing to attain optimal DoF region for the general antenna configurations and current CSIT qualities. Such a framework builds upon block-Markov encoding with interference quantization, optimally combining the use of both outdated and instantaneous CSIT. A striking feature of our work is that, by varying the power allocation, every point in the DoF region can be achieved with one single scheme. As a result, instead of checking the achievability of every corner point of the outer bound region, as typically done in the literature, we propose a new systematic way to prove the achievability.Comment: Revised to IEEE Trans. Inf. Theory. A new simple and unified framework is proposed, allowing to attain optimal DoF region for general antenna configurations and current CSIT qualities. A striking feature is that, every corner point in the DoF region can be achieved with one single scheme, and hence a new systematic way is proposed to prove the achievability instead of checking every corner poin

Rate-Splitting Multiple Access for 6G Networks: Ten Promising Scenarios and Applications

In the upcoming 6G era, multiple access (MA) will play an essential role in achieving high throughput performances required in a wide range of wireless applications. Since MA and interference management are closely related issues, the conventional MA techniques are limited in that they cannot provide near-optimal performance in universal interference regimes. Recently, rate-splitting multiple access (RSMA) has been gaining much attention. RSMA splits an individual message into two parts: a common part, decodable by every user, and a private part, decodable only by the intended user. Each user first decodes the common message and then decodes its private message by applying successive interference cancellation (SIC). By doing so, RSMA not only embraces the existing MA techniques as special cases but also provides significant performance gains by efficiently mitigating inter-user interference in a broad range of interference regimes. In this article, we first present the theoretical foundation of RSMA. Subsequently, we put forth four key benefits of RSMA: spectral efficiency, robustness, scalability, and flexibility. Upon this, we describe how RSMA can enable ten promising scenarios and applications along with future research directions to pave the way for 6G.Comment: 17 pages, 6 figures, submitted to IEEE Network Magazin