(6G) will use millimeter wave, THz (terahertz) frequency band, "optical wireless" communication, but the big challenge for these three is "very high frequency selective path loss" - in LOS (line of sight communication) In the case of a distance of a few meters, the signal power loss easily exceeds 100 dB; under NLoS (non-line-of-sight communication) conditions, the situation will get worse. Thus, it is necessary to use a high gain directional panel antenna
to complete the communication.
Similarly, in a mobile communication system using a lower frequency, an antenna array can be utilized to implement MMO communication, which can extend the communication distance by beam forming, or achieve a larger data transmission rate by spatial multiplexing. In the past few years, the concept of Massive MIMO has been introduced and has been extensively studied as a potential 5G core technology. In such an approach, a very large antenna array with tens to hundreds of antenna elements is utilized to increase spectral efficiency to enable large distance communication. This method has been proven to be "very useful for millimeter-wave 5G communication systems."
When the future 6G mobile communication system uses a higher frequency terahertz band, the single antenna unit will become smaller, so that more antenna units can be embedded in the same area of the bottom plate.
However, linearly increasing the number of antenna elements is not sufficient to overcome the "larger path loss" challenge in the terahertz band. In this context, the industry has recently proposed the concept of "Ultra-Massive MIMO" from very dense plasma nano-antenna arrays.
Plasma nano-antennas can be fabricated using nano materials and meta materials rather than relying on traditional metals, which are much smaller than the wavelengths corresponding to their operating frequencies
This feature allows terahertz antenna elements to be integrated into very dense arrays with "innovative architectures". For example, even if the area of the antenna array is limited to 1 mm X 1 mm (1 mm 2 ), a total of 1024 plasmon nano-antennas designed to operate in the 1 THz band can be packaged within 1 mm square - where the spacing between the antenna elements It is a half plasma wavelength. This plasmonic nano-antenna array can be used for 6G signal transmitters and 6G signal receivers (1024 x 1024) to simultaneously overcome the "diffusion loss" problem (by concentrating the energy of the transmitted signal within extremely limited space) and " Molecular absorption loss "problem (by concentrating the spectrum of the transmitted signal in a "no absorption" window).
Different modes of operation can be adaptively generated by appropriately feeding the antenna elements in the plasmonic nano-antenna array. In nano-MMO (Ultra-Massive MIMO) beam forming all nano-antennas feed the same plasma signal - just like traditional beam forming. This mode can effectively overcome the extremely high attenuation of the nanometer wave signal, the terahertz signal, and the optical wireless signal. In addition, beam forming has the advantage of "avoiding co-channel interference" while utilizing the "angle diversity" effect by dynamically directing narrow beams to the target angular direction. In UItra-Massive MIMO, several antenna elements that can be assigned physical packets or virtual packets are used by a single 6G user. This mode "use multiple streams on a single carrier" to increase the capacity of each user and can be most effective when the wireless communication link is operating in a high SNR (Signal to Noise Ratio) state and bandwidth is limited. It is considered that the Ultra-Massive MIMO channel matrix is well adjusted or equivalently provides sufficient diversity and rank. This mode improves network throughput by spatial multiplexing. Clearly, Ultra-Massive MIMO beam forming is possible with any combination of spatial multiplexing.
In addition, in order to enable the 6G system to make maximum use of millimeter waves, the terahertz channel(s) and provide access at the TbPs level.
In this direction, multi-band Ultra-Massive MIMO enables simultaneous aggregation of unusable frequency bands by utilizing the "electrically tunable frequency response" of graphene-based plasma nano-antennas. By sub-array tuning of the award group (virtually) to different frequencies, a single Ultra-Massive MIMO system can simultaneously cover multiple transmission windows. One of the key advantages is that the “multi-band approach” allows for processing information on a smaller bandwidth, which reduces overall design complexity and increases flexibility in spectrum usage.
In addition to the challenges associated with plasmonic nano-antenna array technology, Ultra-Massive MIMO
The implementation of communication requires the development of novel and accurate channel models to capture the effects of plasmonic nano-antenna arrays on 6G signal transmission and reception, as well as the behavior of extremely large numbers of Hertzian waves propagating in parallel in a wireless environment. Existing MIMO or Massive MIMO channel models for lower frequencies will no longer be applicable because they do not capture the characteristics of the terahertz band radio channel. Includes frequency selective absorption loss or very high reflection loss. Similarly, the few large-scale spatial multiplexing Massive MIMO channel models developed so far have not been considered. (sub-wavelength size separation) and the opportunities it brings.