# NOMA or OMA in Delay-QoS Limited Satellite Communications: Effective Capacity Analysis
## 1 Introduction  
Due to the explosive growth of various satellite applications and services, which range from smart grid in remote locations to file transfer and environmental monitoring, future satellite networks are expected to offer much higher data rates and resource utilization efficiency for much more users. However, the limited deployed spectrum resource of satellite communications can hardly meet the quality-of-service (QoS) requirements of these increased resource-consuming multimedia applications. To accommodate the expanding satellite users within the limited spectrum, based on an orthogonal multiple access (OMA) scheme, flexibility resource assignment (RA) has been proposed to alleviate the scarcity of spectrum for satellite networks by enabling on-demand resource allocation [1, 2]. Moreover, cognitive radio (CR), with which a licensed user can share the spectrum with an unlicensed user if the interference caused by an unlicensed user is under some certain constraint, has also been introduced in satellite networks to overcome spectrum scarcity by dynamically sharing spectrum between satellites in different orbits, i.e., the CR scheme was introduced to alleviate the spectrum scarcity of LEO satellite by sharing the incumbent spectrum of GEO [3], or between satellite and terrestrial networks, such as the authors in [4, 5] studied the performance enhancement introduced by the CR strategy and further applied artificial intelligence to optimally manage spectrum resource. However, although RA and CR strategies can improve the spectrum utilization efficiency, the adopted OMA scheme limitsthe potential of this improvement due to only one user having access within one time slot/spectrum block. Different from the OMA scheme, the non-orthogonal multiple access (NOMA) scheme, which can realize multiple access in the same time/frequency block by superposing signals in the power domain and providing increased transmit rate and fairness [6], has been regarded as an attractive and promising technology for future satellite communications. Until now, several works have been investigated to demonstrate the potential benefits brought by introducing the NOMA in satellite networks, such as enhanced ergodic capacity [7; 8; 9; 10], maximized energy efficiency [11], decreased outage probability [12; 13], and reduced power consumption [14] in a single spot beam scenario. Extension works to multi-beam satellite networks with massive multiple input multiple output and beam-hopping schemes were conducted in [15; 16], respectively. Moreover, some works explored the performance of the NOMA-based satellite networks from the perspective of power allocation or relay selection with various objectives. The works [17; 18] were to maximize the sum rate of satellite-terrestrial relay networks by using joint reinforcement learning as well as Karush-Kuhn-Tucker algorithm and \(Q\)-learning algorithm, respectively. Taking the transmit reliability as the target, the authors in [19] proposed a relaying protocol and a three-stage relay selection strategy to minimize the system outage probability. However, no matter what access scheme was adopted, most of these aforementioned works mainly focused on Shannon's capacity without taking users' delay quality-of-service (QoS) guarantees into consideration. Actually, in the satellite communications, users' delay-QoS guarantees are significantly varied even if they are located in the same spot beam; for example, smart grid monitoring is a representative delay-sensitive application while file transfer is a delay-tolerant service. In this respect, the concept of effective capacity was proposed as a delay-QoS guaranteed capacity metric to represent the user's various delay requirements [20]. Based on this, the authors in [21; 22] investigated the negative effects of users' delay-QoS requirements of OMA-based satellite networks from the perspective of analyzing the system's achieved effective capacity and maximizing effective energy efficiency, respectively.