**3. Wireless Communication Networks**

We define a wireless communication network (WCN) as the association of wireless elements (WE) to share information with a specific task. The nature of WE defines the association schemes and the network features. A WE can be defined as an electronic artifact that possess sensing, acting, signal processing, and/or communication abilities to perform a specific work [25].

Technology integration in WE allows exploitation of different platforms for establishing WCNs. The network features are defined by the nature, status (Active/No Active), capacities, and relationship of WE considering their physical and technological limitations such as size, power transmission, energy consumption, computing capacity, frequency of operation, sensibility of interference, etc.

The networks are based on schemes, models, and protocols that establish the technology, transmission agreements, architecture, and management. This way, we can describe the association schemes according their functions. Two classes of schemes can be distinguished.

• *Collaborative scheme.* It consists of sharing resources and functions only if the WE has availability and does not jeopardize its operation, i.e., WE priorities are over Wireless Network (WN) priorities. There are no strong commitments among their elements [26].

• *Cooperative scheme.* Its scheme is defined to share assigned resources and perform specific functions for all the elements in the networks. All the tasks must be performed through strict rules or protocols, i.e., WN priorities are over WE priorities. In this case there are strong commitments among their members [27].

WN can be classified as a WCN considering different aspects such as nature, management strategy, topology, coexistence scheme, and type of WE. Figure 1 shows a chart with basic concepts and aspects about WCN.

**Figure 1.** Concepts and aspects of Wireless Communication Networks.

The design of a WCN is motivated or influenced by some of the following requirements: Redundancy of information: the high density of devices makes the data obtained in one of them redundant with respect to other nodes of its environment. Limitation of resources: design and implementation of wireless networks must take into account resource limitations: energy, power, memory, and bandwidth. In addition to being generally limited in size, the devices will depend on their batteries and the power they can extract from the environment for their operation. Topology and dynamic environments. The conditions in which a sensor network is deployed is not fixed, but there may be node movement and even Disappearance or addiction of others. Wireless networks must be able to reconfigure themselves autonomously. Unreliable transmission medium: The use of wireless communications has a considerably higher error rate than the wired communications. Security and privacy: these factors are especially important in military and surveillance applications. Therefore, denial of service, intrusion, or data manipulation attacks on these applications must be foreseen.

Networking is the mechanism to construct networks, where each node obtains information about their neighbors and the rules of communication that define the schemes, strategies, and protocols used to establish communication model among their members. Thus, we can describe, in wide sense, the networking process in following phases:


Presently, dynamic and intermittent behavior of WE represent relevant challenges for networking, so the new requirements of WCN such as flexibility in architecture, mobility, and high traffic play a very important role for modern networks. Reconfigurable ability is considered a prominent network paradigm for future WCN [28]. Besides, high density of WE also represents additional challenges due to closeness of WE and the phenomena that it involves, e.g., multiuser channel access problem, multiuser, and multiple access interferences.

Reconfigurable wireless networks (RWN) are dynamic networks suitable for the frequent changes in topology, connectivity, routing, WE status, and wireless channel conditions. These networks have the ability of modifying their configuration, with or without infrastructure, that establish the allocation decisions; self-organization networks imply an *intelligence* process, which can have centralized or no centralized management, that make them attractive to networks as sensors and ad-hoc.

The access to resources and multiple transmissions in WCN face important challenges such as:


Communication systems used for RWN use orthogonal or semi-orthogonal multiple access channel techniques to avoid multiple users channel problems. They consider temporal, spectral, and spatial dimensions to separate users. However, their implementation requires to control the access to resources using elaborated schemes and protocols to ensure the communication.

These devices have limited resources such as low power, computing capacity, narrow bandwidth transmission, so on, high sensibility to interference, and autonomous behavior. New trends incorporate a communication system that allows the formation of temporal wireless reconfigurable networks (WRN), i.e., sensors and ad hoc networks, that share information by establishing communication links through technologies and schemes that ensure their operation in free bands or unlicensed, e.g., BLE, ZigBee, and WiFi in the bands 2.4 and 5 GHz.

The high density of WDs leads the multiuser channel problem, which occurs when multiple users share spectrum resources. The multiuser channel is divided in two types, the first is the broadcast channel where a single WD transmits signals to many WD receivers e.g., the relationship between the base station (BS) and user equipment (UEs) in cellular networks. Secondary, the multiple access channel uses schemes that distribute the resources and permit the transmission among many transmitters with one receiver [30,31].

The access to the multiuser channel employs schemes to allocate users with conditions to communicate. The schemes based on random allocation is known as random multiple access (RMA) scheme that is widely used e.g., in satellite networks and large wireless networks without signaling overheads [32]. On the other hand, the schemes based on multiple access allocation assign dedicate channels to users using orthogonal or semi-orthogonal division of resources [31,33]. This way, some techniques based on multiple access schemes have been proposed to provide uniformly the resources considering the temporal, frequency and spatial parameters or combinations to separate users [34,35].

The presence of multiple WDs in a given area is widely studied because it can change the transmission considerations in the wireless channel. Multiple access schemes are useful techniques that require previous agreements. This way, the schemes can be described in temporal, spectral, spatial or combinations.

