# Time Modulated Arrays: From their Origin to Their Utilization in Wireless Communication Systems
## 2 State of the Art
### TMA Design under an (Exclusive) Antenna Perspective
#### 2.2.1 Fundamental Mode Pattern  
Although it represents a natural and immediate area worth of investigation, the development of methods to minimize the SR did not start until the early 2000s when the first optimization algorithms emerged. The reason is that the sophisticated methods necessary to carry out this kind of optimization arrived simultaneously with the development of more powerful computers. In this sense, [PERSON] et al. introduced in 2002 the TMA synthesis supported by systematic algorithms, capable of optimizing the SLL and the SR simultaneously by using differential evolution algorithms [10]. In subsequent works, and with the same optimization objectives, other algorithms were considered: genetic algorithms [11], simulating annealing [12; 13], particle swarm optimization [14], and artificial bee colony [15]. Further works involved additional parameters in the multi-objective optimization, like the half power beam width [16]. Other innovative --and structurally different-- techniques have been recently proposed to minimize the SR based on modulating the RF switches with non-uniform [17] or multiple [18] time modulation periods. Later on, an additional degree of freedom was introduced in the synthesis process by considering the switch-on instants of the periodic rectangular pulses --in addition to the pulse durations-- appliedto the antenna excitations [7; 19]. In [20], the time modulation period was split into several time steps with variable lengths and, for each time step, the switch-on and switch-off times were optimized via the differential evolution algorithm to improve the SR radiation. In a further development, the impact of the periodic pulses time shift on the SR was theoretically analyzed [8]. Likewise, hardware solutions based on sub-arraying techniques [21] or the use of fixed bandwidth elements [22], as well as the impact of pulse shaping [23], were also investigated to reduce the SR. In addition to linear geometrical configurations, planar and even conformal TMA were also analyzed and optimized in terms of SLL and SR [19; 24; 25; 26], as well as planar adaptive absorbers for radar based on phase-switched screen, whose design was modeled by means of the time-switched array theory [26]. Another key aspect in the TMA technique is the antenna gain and how it is affected by both the SR and the switching network. In [27], [PERSON] et al. characterized the TMA gain and quantified the TMA efficiency. More specifically, they separated the switching network efficiency (which accounts for the time fraction that the switches are off) and the TMA power efficiency (the ratio between the fundamental mode mean radiated power and the total mean radiated power, i.e., the useful radiated power divided by total radiated power) being the total efficiency of the TMA the product of both. In order to improve the switching efficiency. In [28], the utilization of single-pole double-throw (SPDT) switches was proposed in such a way that a single RF switch governs two adjacent elements of the array, so that, whenever complementary time sequences are synthesized, the off time is reduced to zero, and consequently the switching efficiency is set to an almost ideal value. More sophisticated structures based on the use of a reconfigurable power divider/combiner have been evaluated in [29] in order to improve the switching network efficiency of the array. On the other hand, since the fluctuations of the antenna gain --due to the application of a periodic on-off modulation to the static excitations-- may severely affect the behavior of a received signal, some research works focused on the switching sequence, primarily to keep the instantaneous directivity of the main beam constant [30; 31]. TMA were also analyzed --exclusively from the radiation properties point of view-- so as to be exploited in different applications, e.g., for the suppression of interferences and undesired signals impinging on the antenna aperture.