# KQI Performance Evaluation of 3 GPP LBT Priorities for Indoor Unlicensed Coexistence Scenarios
Even though 3 GPP-based unlicensed standards and WiFi use the same techniques in their physical layer [5], namely Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output spatial multiplexing (MIMO), they differ in other aspects such as the transmission power, bandwidth, Modulation and Coding Schemes (MCS) or error correction codes, to name a few. Regarding the packet loss recovery mechanisms, although they present similarities, the differences are substantial. While WiFi uses automatic retransmission (ARQ), 3 GPP incorporates the Radio Link Control layer (RLC) with hybrid ARQ [6]. Besides, in terms of scheduling, 3 GPP acts as a synchronous system, via a centralized scheme. At the same time, WiFi operates half-duplex random access where the receiver must confirm all frame transmissions. Although both approaches can achieve similar performance in terms of data rate, the random access used by WiFi produces persistent collisions when there are several devices connected to the network [7]. At present, although several articles evaluate the coexistence of both mechanisms [8], there is still a need to investigate in this area with end-to-end performance assessment purposes especially in dense environments [9], avoiding, as far as possible, the degradation of pre-existing technologies. Most of the collected works mentioned in [8] choose a certain configuration of the access channel parameters to show the benefits of their proposal or to demonstrate the harmful or beneficial effect on WiFi. Many of these chosen parameters are not sufficiently justified or do not fully comply with the standard. In neither case do they cover the entire range of cases proposed by the 3 GPP priorities. It is important to highlight that, due to the operational differences mentioned above for both standards at the protocol level, a detailed study of all aspects that impact on the quality perceived by the users is necessary. Since an implementation of a coordination mechanism it is not foreseeable, possible ways to improve coexistence become a priority. Therefore, it is not only a question of comparing the access methods being mutually respectful (LBT in 3 GPP standards and CSMA/CA in WiFi) but also to determine how the channel sharing affects the operation of the upper layers (e.g., the MAC) and describe the interaction with themselves and  
Figure 1: 3 GPP unlicensed standards evolution. each other as well as the impact on the application layer. In this regard, some works can be found in the literature specially focused on mathematical modeling. In [10; 11], the impact of priorities is assessed with a standardized throughput indicator and assuming ideal propagation conditions. However, the problem of heterogeneity of coexisting systems is identified, while the proposed scenario is far from dense. On the other hand, with respect to simulation works, several outstanding papers have appeared lately, among which it is worth mentioning the one by [PERSON] et al. [12] where, despite the use of ns3 tool, a simplified scenario that does not reflect any of the problems posed by coexistence in the foreseeable dense indoor environments is proposed. It also focus on a single priority configuration with a fixed traffic load. Lastly, the exhaustive study made by the ns3-based LAA model creators in [13] presents a setting where only FTP traffic is simulated, and the impact of each relevant random access parameter is studied. However, neither video traffic nor the priorities proposed by the 3 GPP are included. In this field, the contribution of the present work is on the end-to-end assessment of the LBT channel access priorities configurations in order to provide QoS and Quality of Experience (QoE). To this end, two types of services were simulated in an indoor dense environment, one of them being real-time (video streaming) and the other non-real-time (FTP) for different levels of traffic intensity.