## 3 Challenges and Related Recommendation
### Fundamental Theories Challenges
The continuous-space EM field distribution can be derived from the EM theory, which is based on uninterrupted space and time. It is not possible to determine the channel capability only from electromagnetic theory. The channel capability in finite space can be determined using information theory; nevertheless, in an ongoing space, it is not possible. Transmitter theory and wireless transmission channel modelling theory link the electromagnetic theory and information theory. Specifically, the wireless transmission channel connects electromagnetic theory and information theory; it starts at antennas and conveys data via electromagnetic pulses. Keep in mind that as 6G core innovations advance, there will be issues that none of the aforementioned theories can fully address, creating an opportunity and a threat for a joint application of the four theories. The quantity of users, base stations, relays, and/or RISs, for instance, keeps going up as 6G wireless communication networks grow from covering only a local area on land to covering the entire planet through space, air, ground, and sea interconnected networks. In mobile networks like low earth orbit (LEO) satellite, UAV, and vehicle networks, the locations of both base stations and users are always shifting. In order to aid in system design, 6G wireless communication networks are evolving from discontinuous to ongoing spaces, necessitating the collecting of channel strength information (CStrI) and its calculation regardless of location in the continuous area. The aforementioned four theories will inevitably be integrated in this scenario. Furthermore, as the dimensions and elements of transmitters in ultra-massive MIMO expand, the Tx/Rx transmitters become increasingly interconnected with their surroundings. Therefore, it is essential to integrate the theories of transmitters and wireless transmission channels in order to describe Tx/Rx antennas and the underlying wireless transmission channel. There are additional requirements for channel characterization, antenna architecture, and continuous-space channel potential computation due to the transformation of antenna grids from isolated to ongoing windows and the decreasing antenna unit separation in holographic MIMO. Once again, all four of these theories must be considered together. In conclusion, 6G wireless networks offer fresh uses and practical specifications that surpass the reach of any one theory when compared to 5G. Traditional theoretical frameworks have hit roadblocks in their advancement. To lay the theoretical groundwork for 6G wireless communication networks and facilitate fresh findings, it is crucial to investigate the merging of electromagnetic theory and information theory, namely, EM information theory [60; 61]. In terms of baseband processing, two distinct varieties can be described: unified and iterative. In communication systems, channel coding has always been crucial for the unifying thanks to its error-correcting potential. Sixth-generation communication systems must include unified implementations of improved coding schemes and simplified decoding algorithms to accommodate dynamic and varied circumstances. In order to satisfy the strict criteria of 6G, it is often necessary to look at the architecture and execution of the entire baseband processor, rather than just refining one channel coding unit. In contrast, iterative receptions have a longer history of being considered to have better system throughput and connection dependability than the current state of distinct baseband processing techniques [62]. The channel estimation, MIMO detection system, network decoder, NOMA detector, and origin decoder perform recurrent soft information swaps with each other [63; 64; 65; 66]. The heightened utilization of every module due to repetitions may impose significant delay and computational overhead on the system.