**2. Smart antenna systems**

As mentioned, SAS are intelligent systems that allow a good SDMA processing [13, 14]; examples of SAS are: digital beamforming systems, adaptive antenna systems, phased array, and others. Smart antennas are customarily categorized, however, as either switched beam or adaptive array systems. There could be a distinction between two major categories of smart antennas in terms of the operation mode:


*Array Pattern Optimization*

both source and destination.

systems, because of the high burden of power of network nodes [2] that may result in destructive phenomena such as low battery depletion and interference. A single antenna may also be built to have certain fixed preferential transmission and reception directions to maximize its energy consumption in a specific direction conserving power in other directions [3]. Using directional antenna could lead to several advantages, in terms of reduction of packet delay or improvement of the overall routing process [4]. In wireless communications, when a single antenna is utilized both to the transmitter and receiver we talk about single input, single output (SISO) [5] systems. Therefore, nowadays, with regard to the latest antenna technologies, the concept of smart antenna systems has spread. SAS are intelligent systems equipped with high efficiency data processing unit. This sort of systems can boost the coverage area and the capability of a radio communication system. The coverage area is simply the area where the communication link between a mobile and the base station can be performed. The capability is a way of measuring the amount of users a system can support in certain area. A smart antenna system generally combines an antenna array with a digital signal processing capacity to transmit and receive in an adaptive, spatial manner. Quite simply, such a system can quickly change the directionality of its radiation patterns in response to its environment. This may considerably increase the performance characteristics (such as capacity) of a wireless system. The employment of SAS in wireless mobile environments allows a much more reliable medium utilization with regards to the classical omnidirectional strategy. For instance, spatial division multiple access (SDMA) attempts to raise the capacity of a system. Generally, smart antennas get into three major categories: single input, multiple output (SIMO), multiple input, single output (MISO), and multiple input, multiple output (MIMO). In SIMO technology, one antenna is used at the source, and two or more antennas are used at the destination. In MISO technology, several antennas are used at the transmitter, and one antenna is utilized at the destination. In MIMO technology, multiple antennas are used at

**Figure 1** illustrates an example of SISO and MIMO systems. In the SISO case, either the transmitter or the receiver uses a single antenna for the communication process; while in the MIMO, an antenna array is employed. In literature, it has been demonstrated how the use of directional antennas and the most recent smart antenna systems (SAS) technology is capable of significantly allowing high quality of service (QoS) requirements in spite of the omnidirectional systems that foresee

**88**

**Figure 1.**

*SISO and MIMO structure example.*

**Figure 2.** *SAS basic operation principle.*

Switched beam antenna systems form multiple fixed beams with high sensitivity in particular directions. These antenna systems detect signal strength, choose from one of several predetermined, fixed beams, and switch from one beam to another as the mobile moves throughout an area. So, they produce a static fixed beam that could be electronically controlled. Adaptive antenna technology, instead, uses adaptive algorithm because of its ability to effectively locate and track various types of signals to dynamically minimize interference and maximize the intended signal reception. In this case, produced beam is variable and adapts itself depending on transmission channel conditions and a weight array that dynamically varies in time. In this context, the spatial structure is used to estimate the direction of arrive (DOA) or angle of arrive (AOA) by nodes. However, both systems attempt to increase gain according to the location of the user. The basic SAS operation principle can be summarized by the following figure.

In **Figure 2**, inputs *<sup>x</sup>*1(*t*),.…,*xM*(*t*) are multiplied by elements of a weight vector *<sup>W</sup>*¯ <sup>=</sup> [*w*1,*w*2,.…*wM*] that varies according to an adaptive algorithm (used only in the adaptive array version); *y*(*t*) is the output, while *e*(*t*) denotes the error; all terms are defined in functions of the discrete time *t*. Instead, when a switched beam approach is employed, because any adaptive algorithm is executed, the weight array can be considered missing or simply as a constant. Based on the kind of produced geometry pattern, SAS can be categorized into different ways. The most common categories include, rectangular, hexagonal, and the circular arrays. However, in 5G technology, the antenna arrays should be adaptive, and it is required that they have an adaptive capability to point the main beam toward the desired direction and steer the nulls toward the undesired interfering directions. In all cases, this adaptive mechanism should be optimized to get best performance or maximum signal to interference plus noise ratio (SINR) at the system's output [15–17].
