# Technology Trends for Massive MIMO towards 6G
## 4 Ultra-Massive MIMO at THz Frequencies  
Having explored the transformative potential of IOS/IRS systems for localization and sensing, we now turn our attention to the possibilities and challenges of ultra-massive MIMO at THz frequencies. The ITU-R defines THz frequencies as those in the range 0.1-10 THz, a spectrum that offers immense potential for high-capacity communications and ultra-high resolution environmental sensing. However, operating in this spectrum introduces unique challenges that demand innovative solutions, particularly in the design of transceiver architectures and network strategies. According to ITU-R (International Telecommunication Union Radiocommunication Sector), THz frequencies are those in the range 0.1-10 THz. The lowest frequency region between 0.1 THz and 0.3 THz with the highest potential is usually called the sub-THz regime. THz and sub-THz signals serve as a bridge between radio and optical frequencies. Their wavelengths in the millimeter and sub-millimeter region make them excellent candidates to fulfill the 6G promise of extremely high-capacity communications, good situational awareness, and ultra-high resolution environmental sensing. Such small wavelengths come at the price of high uncertainty in the channel characteristics, leading to unreliable, intermittent radio links that suffer from one or several of the following impairments:  
* **High path losses, molecular absorption, and blockage**: The high free-space path loss motivated by the small antenna aperture areas at these frequencies, together with the molecular absorption, blockage, diffuse scattering, and extra attenuation caused by rain, snow, or fog, lead to highly intermittent links. Link reliability must therefore be improved with the use of ultra-narrow beamforming. * **Low energy efficiency**: RF output power degrades 20 dB per decade for a given power amplifier (PA) technology. This compromises the link budget and reinforces the need of large-scale transceivers with high numbers of antennas. * **Large-scale transceivers**: The high beamforming gain needed to improve link reliability demands large-scale transceivers with a high number of antennas (usually, more than 1024). The sharpened, ultra-narrow beams that they produce pose significant challenges to mobility and beam tracking. * **Phase noise**: At sub-THz/THz frequencies, CP-OFDM performance can be severely degraded by the inter-carrier interference (ICI) resulting from phase noise. Increasing the subcarrier spacing can mitigate its impact, but the correspondingly shorter symbol duration introduces a penalty in coverage and impairs the ability to mitigate large delay spreads. * **Channel sparsity**: Ultra-narrow beams, together with ray-like wave propagation, lead to channels that exhibit small numbers of spatial degrees of freedom and ranks limited to one LoS component and a few multipath components, which challenges MIMO operation. * **Spherical wave and near-field effects**: Large-scale transceivers exhibit significant spherical wave and near-field effects from the electrically large antenna structures that they equip, which introduces complexity to MIMO precoding strategies. * **Beam squint**: The narrowband response of phase shifters in planar arrays introduces a frequency-dependent beam misalignment called beam squint. Losses from beam misalignments can be alleviated by using beam broadening techniques, at the cost of reduced coverage; and avoided with true time delay (TTD) units, at the cost of complexity. There is abundant research on transceiver architectures and network solutions aimed to ameliorate some of the above issues, especially those motivated by the propagation challenges at sub-THz/THz. Among the network solutions, the aforementioned IOS/IRS/RIS equipped with very large numbers of small antenna elements are receiving considerable attention, because of their ability to tailor the characteristics of the reflected and refracted beams [4; 19].