# Channel Access in Wireless Smart Grid Networks Operating under ETSI Frame-Based Equipment Rules
## 1 Introduction  
Smart grid operators are seeking to extend their wireless network capacity to accommodate the traffic requirements of modern industrial networks [1; 2]. This capacity can be extended through licensed bands (with cognitive radio) [3] or in unlicensed (shared) bands [4]. The latter is usually more cost-effective and the recently released 6 GHz band provides new spectrum resources [5]. However, operation in unlicensed bands requires appropriate channel sharing methods. The European Telecommunications Standards Institute (ETSI) specification [6] defines such rules, collected under the umbrella term Listen Before Talk (LBT). Two separate LBT variants are defined in the specification, for load-based equipment (LBE) and frame-based equipment (FBE). The former implements the random-based channel access mechanism of the IEEE 802.11 standard. The latter, however, follows a scheduled-access approach and is designed for industrial and smart grid networks, where nodes transmit in duty cycles [7]. A good candidate for implementing FBE channel access in shared bands is found in the fifth generation of wireless cellular networks in new radio unlicensed (NR-U). For coexistence with other radio technologies, NR-U already implements LBT as the channel access protocol [5] and NR-U studies usually assume the LBE variant [8; 9; 10; 11; 12; 13; 14; 15]. However, as a cellular radio access technology, NR-U inherits a scheduled access approach in which nodes have to start transmissions at predefined slot boundaries. This requirement is not a good fit for the random access nature of LBE, because the end of an LBE channel access procedure does not have to match the closest slot boundary. Additional measures are required to guarantee an exact match of the start of a transmission and the synchronization slot boundary, e.g., transmitting a reservation signal [16]. Conversely, the fixed frame structureof FBE seems to be an exact, out of the box solution for synchronizing NR-U transmissions with slot boundaries, which can potentially bring better utilization of channel resources [17]. Therefore, the significance of researching FBE can be summarized as follows: it has support from the industry (since it has been included as part of an ETSI specification); its design is tailored to the periodic traffic requirements of industrial networks; it follows simpler design principles than LBE, making it easier for implementation; and its design conforms with the requirements of cellular networks operating in unlicensed bands. However, the main challenge of implementing FBE is that it lacks a contention resolution mechanism [17], which can lead to unwanted behavior when multiple FBE nodes contend for the channel. In this paper, we focus on industrial network settings, i.e., controlled, greenfield environments, in which FBE devices can be assured exclusive channel access but must nevertheless compete among themselves [18]. Therefore, the case where multiple FBE nodes contend for the channel is an interesting aspect to study. Furthermore, some FBE variants have not yet been evaluated in terms of performance. Therefore, our goal is to evaluate the performance of FBE-based channel access mechanisms. In this paper, we bring the following contributions:  
* a performance evaluation of individual FBE variants in homogeneous networks, i.e., with all nodes operating under a single variant, to determine their contention resolution capabilities;
* a performance evaluation of FBE variants in heterogeneous networks, i.e., with all nodes using one of two competing FBE variants, to determine their coexistence capabilities;
* a set of conclusions regarding FBE configuration, performance, and coexistence as well as the identification of open research areas in this field;
* an FBE channel access simulator, released as open source for the research community, that is well suited for discovering issues of low or unfair channel access probabilities;
* an overview and classification of FBE variants proposed in the literature, including two of our own proposals.