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Wireless solutions in IIoT should specify the required resources to create connectivity within the systems. To identify the level of network resource utilization in wireless systems, a set of metrics are introduced as resource indicators: spectrum, time, power, and network computing [55].

1 Spectrum: In the frequency domain, the wireless channels are identified by the channel bandwidth. Given that the data rate of each wireless channel is related to its bandwidth [184], different services in IIoT environment are highly dependent on spectrum resources to accommodate the desired data rate. For instance, in mission-critical communications such as safety systems, the radio spectrum resource should be assigned properly for effective deployment (i.e., high data rate and low latency) [185]. Given that spectrum resources are shared in wireless domain for a large variety of applications, wireless networks suffer from spectrum scarcity. This highlights the need for communication protocol interoperation within different frequency bands and channels [186].

2 Time: Wireless spectrum resource usage could be defined in terms of the transmission time and length. Time manages the temporal operation of wireless resources based on the desired performance such as QoS [187]. In this context, the access time of channel spectrum could be considered a resource indicator to be optimized for wireless networks.

3 Energy consumption: Power consumption of wireless nodes at transmission side is closely dependent on transmission range, quality of transmission channel, and transmission power at RF front ends. The aforementioned resources are essential in the design of a wireless system. Various energy-efficient resource allocation techniques (with given QoS constraints) are proposed to assist in design of wireless systems for IIoT.

4 Computing resources: Network resources are in both hardware and software types (physical and virtual). Basically, computing resources include all network resources tied up to the aforementioned resources (i.e., spectrum, time, and energy consumption). For instance, in TDMA-based transmission, different communication links are assigned to users at different time slots and through computing resources.

To achieve the diversity gain for different applications in IIoT, the resource space is used in multiple dimensions. For example, a wireless solution could exploit a TDMA scheme in the multi-antenna wireless systems, while both power and spectrum are considered in the system design.