# 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
#### 2.2.2 Harmonic Patterns  
As already pointed out, the SR is not necessarily a damaging phenomenon and can be profitably exploited to improve the performance of a TMA. As a matter of fact, the exploitation of the harmonic patterns endows TMA with smart-antenna capabilities. In this sense, the seminal work by [PERSON] [40] theoretically introduced the beam-scanning capabilities of TMA and, as an example, such features have been efficiently applied to an experimental prototype in [41].  
Adaptive beamforming was originally studied from a practical and simplified point of view by [PERSON] et al. [42], focusing on the harmonic patterns synthesis in [6], and even taking advantage of time redundancy in [43]. Another strategy for beamforming through pulse splitting techniques was investigated in [44]. In such a technique, based on particle swarm optimization, two patterns were simultaneously generated: the first one at the central frequency and the second one at a preselected harmonic with an arbitrary order, keeping the SR below a given level. An example of how harmonic beamforming with TMA can be profitably applied, in this case to blanking in spectrometry and radar, was proposed in [45].  
Beam steering with TMA was also introduced by [PERSON] et al. [46], and a more elaborated version, with control of the side-lobe level, was addressed in [47]. Applications of beam steering like direction finding or direction of arrival (DOA) estimation were analyzed in [48; 49; 50].  
Reflector arrays were also considered for being implemented using time modulation. [PERSON] et al. introduced the concept of time-modulated reflector array (TMRA) in [51], which was analyzed more deeply in [52]. A TMRA makes use of the topology of a conventional reflector array, i.e., consists of a matrix of scattering elements which are illuminated by a feed horn but, instead of phase shifters, it uses discrete time switching to achieve beam steering or beamforming functions. As a result, the radiation pattern is controlled by adjusting the time-domain scattering. It was also proven that, by employing double-layer designs [53], it is possible to increase the energy efficiency of TMRA.  
Another interesting field for TMA applications is wireless power transmission, introduced by [PERSON] et al. in [54]. In this case, the real-time beamforming features of TMA are properly exploited following a two-step procedure: firstly, to precisely localize the tag to be powered and, subsequently, to perform the directive wireless power transmission.  
In recent investigations, the TMA philosophy was involved in more complex hybrid solutions contributing to improve the performance of the overall antenna system. In this way, time modulation was applied --together with in-phase and quadrature (I/Q) modulation-- to phased arrays in order to generate a scanning beam at the single positive sideband [55]. Time modulation was considered, jointly with _retrodirective_ techniques, to synthesize _coaperture_ antenna arrays [56] and also to obtain time-invariant spatial-fine-focusing beampatterns in frequency diverse arrays [57].