# Spectrum Slicing for Multiple Access Channels with Heterogeneous Services
###### Abstract  
Wireless mobile networks from the fifth generation (5G) and beyond serve as platforms for flexible support of heterogeneous traffic types with diverse performance requirements. In particular, the broadband services aim for the traditional rate optimization, while the time-sensitive services aim for the optimization of latency and reliability, and some novel metrics such as Age of Information (AoI). In such settings, the key question is the one of spectrum slicing: how these services share the same chunk of available spectrum while meeting the heterogeneous requirements. In this work we investigated the two canonical frameworks for spectrum sharing, Orthogonal Multiple Access (OMA) and Non-Orthogonal Multiple Access (NOMA), in a simple, but insightful setup with a single time-slotted shared frequency channel, involving one broadband user, aiming to maximize throughput and using packet-level coding to protect its transmissions from noise and interference, and several intermittent users, aiming to either to improve their latency-reliability performance or to minimize their AoI. We analytically assessed the performances of Time Division Multiple Access (TDMA) and ALOHA-based schemes in both OMA and NOMA frameworks by deriving their Pareto regions and the corresponding optimal values of their parameters. Our results show that NOMA can outperform traditional OMA in latency-reliability oriented systems in most conditions, but OMA performs slightly better in age-oriented systems. Age of Information; Non-Orthogonal Multiple Access; reliability; heterogeneous access; slotted ALOHA 20212336866 [1]  
and data generation processes [2]. AoI is measured at the point of reception, as the time elapsed since the moment of generation (at the transmitter) of the last successfully received message. A related metric is Peak Age of Information (PAoI), which represents the AoI measured immediately before a new message is successfully received [3]. AoI and PAoI are particularly relevant in control systems and similar setups with (quasi) periodic message exchanges [4]. The underlying assumption is that users send updates of an ongoing process, such that the most recent update provides all the necessary information about the state of the process. In such scenario, the reliability and latency of individual packets are of secondary importance [2]. We refer the interested reader to a recent survey [5] for a thorough review of AoI and its properties, and to our previous work for a discussion on the differences between AoI and latency and reliability as timeliness metrics [6; 7]. The concept of network slicing has been widely investigated in recent years, mainly motivated by the need for accommodation of heterogeneous services in the network. The idea is to allocate (i.e., slice) the network's resources among the different coexisting services, such that each service has the experience of meeting the performance requirements while being isolated from the other service types [8; 9]. In our previous work [7], we introduced the concept of spectrum slicing to refer to the allocation of shared wireless resources among coexisting heterogeneous services in the Radio Access Network (RAN). Those resources can be defined, for example, in time, frequency, or spatial domains. So far, spectrum allocation, rather than slicing, has been widely studied in the form of diverse Orthogonal Multiple Access (OMA) and Non-Orthogonal Multiple Access (NOMA) techniques in the presence of multiple users with the same type of service [10; 11; 12]. OMA techniques assign dedicated resources to individual users and/or services: Orthogonal Frequency-Division Multiple Access (OFDMA), Code Division Multiple Access (CDMA), and multi-user multiple-input multiple-output (MU-MIMO) are examples of OMA that achieved widespread implementation in 3 GPP cellular systems, including 5G [13; 14; 15]. On the other hand, the NOMA concept refers to the allocation of shared (i.e., non-orthogonal) resources in the time and/or frequency domains to multiple services or users.