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In scheduled-based protocols, also known as fixed reservation–based schemes, a fixed duration of time, frequency, or other domain is scheduled and assigned to nodes for network resources access. The scheduling assignment algorithm is conducted by a centralized base station and aims at avoiding channel collisions. In addition to a collision-free schedule, a device is simply set to sleep when it is not using its time slot to prevent idle listening and message overhearing [190]. This scheme is more suitable for networks that deploy low-mobility nodes and require infrequent topology changes and scheduling adjustment. It also tends to be more predictable and offers deterministic E2E delay. However, in dense networks, nodes should wait to gain access to the wireless medium, and additional queuing delay shall be incurred. Synchronization is an important issue in this approach leading to higher complexity and additional traffic due to additional control packets. The following multiple access schemes are utilized in typical multi-user wireless communication systems.

 TDMA: Time is divided among nodes for a given and identical frequency channel. Therefore, a fixed portion of time is assigned to every node to transmit data. For successful TDMA slot assignment and collision-free communication, tight clock synchronization should be established between nodes. GinMAC [191] and wireless arbitration (WirArb) [192] are some collision-free TDMA-based MAC protocols in time-sensitive IIoT. For instance, GinMAC provides reliable data delivery as well as deterministic time delay for industrial process automation such as closed-loop control systems.17 Given that only one node is allowed to transmit data during the scheduled time slots, TDMA suffers from relatively high delay.

 FDMA: As the name infers, accessible frequency bandwidth is partitioned into non-overlapped sub-channels, where each individual sub-channel is adequate to accommodate transmission of a signal spectrum. Ideally, through proper frequency assignment algorithms, a unique physical frequency is dedicated to every node to offer a collision-free protocol. The FDMA-based protocols support multiple frequencies and require more costly hardware. They generally are not useful for IoT systems because of a high level of power consumption and more complicated design [193].

 CDMA: A MAC channel access method that enables transmission of multiple signals in a single transmission channel. A combination of special encoding scheme and spreading spectrum technology is exploited to send multiple signals through a single channel. The basic principle is that users have access to the whole bandwidth for the entire duration, but they utilize different CDMA codes; this assists the receiver to distinguish among different users. Given that the entire bandwidth is allocated to a CDMA channel, this scheme suffers from limited flexibility in adapting bandwidth, particularly for M2M communication in IIoT systems.

 OFDMA: A multiple access scheme that divides the entire channel resources into small time-frequency resource units. Since the available bandwidth is divided into multiple mutually orthogonal narrowband sub-carriers, several users could share these sub-carriers and simultaneously transmit data. In other words, the signal is first split into multiple smaller sub-signals, and resource units are allocated to them. Then, each data stream is modulated and transmitted through the assigned resource units. OFDMA allows several users with various bandwidth requirements simultaneously to transmit data at different (orthogonal) frequencies. Therefore, channel resources can be assigned with much more flexibility for different types of traffic. In addition to high spectral efficiency, OFDMA can effectively overcome interference and frequency- selective fading caused by multipath. OFDMA is a promising multiple access scheme adopted for wide range of mobile broadband wireless networks such as LTE, Wi-Fi6, and 5G [194–196].