Communication systems get updated in a ten-year cycle through envisioning, research, design, validation, and standardization, which is also known as a generation or simply ‘G’. As the well-known work titled ‘What Will 5G Be’ by Andrews et al. pointing out the research directions for 5G communications, we are now providing our 6G perspective in our latest vision paper ‘What Should 6G Be’ published by Nature Electronics before 6G communications and the relevant technologies become the protagonists in academia.

Enlightened by the controversial article ‘Is the PHY Layer Dead?’ by Dohler et al., we noticed that there had been a large number of promising communication technologies and conceptions in the past that are not put into practice. What would be the reasons? First, As long as the communication media are restricted to the electromagnetic wave, all communication systems will be upper bounded by Maxwell's equations and Shannon's theorem without any exception. Given the hard limits to transmission delay and channel capacity, other derivative performance metrics can only be improved on a marginal scale by physical layer (PHY) research. As a result, the improvements of communication services resort to a higher spectrum, a denser network, and a larger number of antennas equipped at the communication devices. Researchers in the communication community use their bet endeavor to improve communication services following these directions in recent decades. However, the technological breakthroughs following these three directions might not lead to real improvement without considering the all-important factor: Human. 

In light of this, when envisioning 6G communications and designing the research roadmap from a macroscopic perspective, we emphasize that human-centric nature should be the core of 6G communications and relevant research activities in our vision paper. Specifically, we want to convey the following key messages to the researchers who are investigating 6G communications:
1. Human-centric mobile communications will still be the most important application of 6G and high security, secrecy, and privacy should be key features of 6G, which are worth paying particular attention by the wireless research community.
2. Artificial intelligence and machine learning could bring unprecedented communication services and experience to users, but we should always be aware of the accompanying challenge of privacy protection, as well as a horrific world watched and controlled by a technocratic ‘big brother.’
3. Due to the human-centric nature, 6G communications should not only serve users living in dense and urban areas, but also bring adequate and fair communication services and connectivity to those living in developing areas, so as to form the worldwide connectivity. 
4. Most importantly, we must admit that the advancement of wireless communications is highly restricted by basic sciences, especially mathematics and physics, as well as information theory and electronics. Without breakthroughs in these subjects, the essential improvement in the capacity and delay related performance metrics of communication systems would be impossible. 

By Shuping Dang, Osama Amin, Basem Shihada, and Mohamed-Slim Alouini
January 2020