# Spectrum Slicing for Multiple Access Channels with Heterogeneous Services
###### Abstract
Such allocation intrinsically implies collisions of users' transmissions in the shared domain(s), and NOMA techniques generally rely on more complex receivers, capable of Multi-Packet Reception (MPR) to resolve collisions. The benefit of this techniques is potentially a higher resource efficiency than OMA, and less need for strict coordination among users. On the downside, implementation of MPR techniques is usually complex; a typical example is Successive Interference Cancellation (SIC) [11; 16]. In scenarios with broadband services only, resource efficiency is easily defined and the trade-offs are clearly characterized by the achievable data rates and/or throughput [11; 12]. However, further research is needed on novel slicing mechanisms in scenarios with heterogeneous services, for example, broadband and intermittent, since the efficiency cannot be simply measured in terms of throughput or data rates [9; 17]. We illustrate this through a toy example presented in Figure 1, where there are (i) 3 intermittent users following an ALOHA-based protocol, and (ii) a broadband user. With OMA, orthogonal resources are defined for each service type. This limits the frequency of resources for the intermittent users, which increases the probability of collision among them, as shown by the cross-mark in Figure 1; these collided packets cannot be recovered. In contrast, with NOMA all resources are available for the intermittent and broadband users, and SIC is used to recover the packets lost due to collision between the broadband and intermittent users. In this example, NOMA obtains greater throughout for the broadband user and a lower latency and greater reliability for the intermittent users than OMA. This insight motivates the work presented in this paper. In particular, in this paper we investigate orthogonal and non-orthogonal slicing mechanisms in the case where a broadband user shares a wireless channel with multiple intermittent users share. Specifically, we explore the performance of slicing implemented via multiple access schemes standardly used in the cellular access, which are Time Division Multiple Access (TDMA) and slotted ALOHA, and a scheme representing their combination. The broadband user implements a \(K\)-out-of-\(N\) erasure code, which allows the user to counteract the packet losses due to channel and potential collisions with the intermittent users transmission in the case of non-orthogonal slicing. In the later case, once the block of \(N\) broadband users' packets becomes decoded, the receiver uses SIC to attempt recovery of the intermittent users' packets. The performance parameters of interests are throughput of the broadband user and two timeliness metrics for the intermittent users: latency-reliability of individual packets and PAoI. In our previous works [6; 7], we investigated the performance trade-offs of OMA and NOMA in a simple uplink scenario with one broadband user and one intermittent user. The general conclusion was that OMA usually outperforms NOMA when transmissions takes place in a collision channel with packet erasures and without capture, which is a rather conservative channel model. However, NOMA schemes achieved a similar performance as OMA in extreme cases when the single objective is to maximize the throughput of the broadband user or to minimize the latency of the intermittent user [6]. We also evaluated how the capture effect and immediate (i.e., intra-collision) SIC at the receiver enhance the performance of NOMA. Under this scenario we observed that important gains can be achieved with NOMA when the intermittent user aims to minimize latency, but the gains are limited when the objective is to minimize AoI [7]. This paper extends that analysis to the case with multiple intermittent users, showing quite different trade-offs. We derive closed-form expressions for the performance parameters and show that, when the intermittent users aim to minimize the PAoI, OMA with TDMA is the best choice, albeit by a small margin.