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M. Singh et al.
1 Introduction
The modern world’s reliance on digital technologies and the use of high-speed
internet in exchanging information, images, photographs, and videos has put a
burden on the present microwave electromagnetic spectrum. New frequency bands
are needed to meet customer requirements. It is continuously observed from the last
three decades that wireless traffic is projected to be double every eighteen months
[1]. The fifth-generation (5G) network is already moved to the millimeter waves
to fulfill the demands of high-speed networks. According to the Federal Communications Commission (FCC), the millimeter-wave from 24 to 100 GHz has been
allotted for 5G millimeter-wave-based communications. Recently, it is significantly
noted that in developed countries like the USA, China, UK, Canada, and South
Korea 5G networks based upon millimeter wave are already been established and
under trials [2]. Following this trend, it is anticipated that sometime in the next five
to ten years, sixth-generation (6G) wireless communications having data speed of
Terabit-per-second (Tbps) connectivity will become a reality. So, to accommodate
the upcoming demands of higher transmission speeds, FCC has already unlocked
the spectrum around 95 to 3,000 GHz for innovative usage and unlicensed applications to encourage the advancement of new wireless communication systems [3].
There are some hurdles like atmospheric loss and molecular absorption loss at the
THz regime for wireless communications. But for short-range or indoor wireless
communications, the molecular absorption losses for distances far below 1 m seem
to be almost negligible, and thus the THz band performs as a kind of 3 THz wide
transmission window. Moreover, many resonances become significant for transmission distances of over 10 m, and the transmission windows become shorter. From
the past experiments, it is observed that some low loss transmission windows like
w1 = [0.38–0.44 THz], w2 = [0.45–0.52 THz], w3 = [0.62–0.72 THz], and w4 =
[0.77–0.92 THz] can be a good option for future high-speed short-range wireless
communication in the terahertz band. The influence of molecular absorption loss
inside each transmission window is marginal, well under 10 dB/km [4, 5]. Nonetheless, the total path loss is very strong due to the scattering loss, which encourages the
utilization of highly efficient antennas as well as advanced MIMO antenna system
technologies. The significant disadvantage for the terahertz communications network
is the atmospheric loss as already mentioned. As reported, to resolve this challenge,
the high-power transmitters and efficient detectors need to be developed [5–7].
The planar antennas can play a revolutionary role to realize the terahertz shortrange wireless communications due to its significant properties like compact size,
ease of fabrications, and low cost. Despite several advantages, it has some disadvantages like narrow bandwidth and low gain which can’t be considered in proper
utilization of terahertz band for communications. But one can increase the gain, bandwidth, and efficiency of the antenna by deploying some techniques like slot inside
patch, defected ground surface, use of multiple layers of substrate, etc. Also, the
on-chip deployment of MIMO antenna array is only possible by utilizing the planar
M. Singh et al.
1 Introduction
The modern world’s reliance on digital technologies and the use of high-speed
internet in exchanging information, images, photographs, and videos has put a
burden on the present microwave electromagnetic spectrum. New frequency bands
are needed to meet customer requirements. It is continuously observed from the last
three decades that wireless traffic is projected to be double every eighteen months
[1]. The fifth-generation (5G) network is already moved to the millimeter waves
to fulfill the demands of high-speed networks. According to the Federal Communications Commission (FCC), the millimeter-wave from 24 to 100 GHz has been
allotted for 5G millimeter-wave-based communications. Recently, it is significantly
noted that in developed countries like the USA, China, UK, Canada, and South
Korea 5G networks based upon millimeter wave are already been established and
under trials [2]. Following this trend, it is anticipated that sometime in the next five
to ten years, sixth-generation (6G) wireless communications having data speed of
Terabit-per-second (Tbps) connectivity will become a reality. So, to accommodate
the upcoming demands of higher transmission speeds, FCC has already unlocked
the spectrum around 95 to 3,000 GHz for innovative usage and unlicensed applications to encourage the advancement of new wireless communication systems [3].
There are some hurdles like atmospheric loss and molecular absorption loss at the
THz regime for wireless communications. But for short-range or indoor wireless
communications, the molecular absorption losses for distances far below 1 m seem
to be almost negligible, and thus the THz band performs as a kind of 3 THz wide
transmission window. Moreover, many resonances become significant for transmission distances of over 10 m, and the transmission windows become shorter. From
the past experiments, it is observed that some low loss transmission windows like
w1 = [0.38–0.44 THz], w2 = [0.45–0.52 THz], w3 = [0.62–0.72 THz], and w4 =
[0.77–0.92 THz] can be a good option for future high-speed short-range wireless
communication in the terahertz band. The influence of molecular absorption loss
inside each transmission window is marginal, well under 10 dB/km [4, 5]. Nonetheless, the total path loss is very strong due to the scattering loss, which encourages the
utilization of highly efficient antennas as well as advanced MIMO antenna system
technologies. The significant disadvantage for the terahertz communications network
is the atmospheric loss as already mentioned. As reported, to resolve this challenge,
the high-power transmitters and efficient detectors need to be developed [5–7].
The planar antennas can play a revolutionary role to realize the terahertz shortrange wireless communications due to its significant properties like compact size,
ease of fabrications, and low cost. Despite several advantages, it has some disadvantages like narrow bandwidth and low gain which can’t be considered in proper
utilization of terahertz band for communications. But one can increase the gain, bandwidth, and efficiency of the antenna by deploying some techniques like slot inside
patch, defected ground surface, use of multiple layers of substrate, etc. Also, the
on-chip deployment of MIMO antenna array is only possible by utilizing the planar