  With the rapid development of satellite communication technologies, the
space-based access network has been envisioned as a promising complementary
part of the future 6G network. Aside from terrestrial base stations, satellite
nodes, especially the low-earth-orbit (LEO) satellites, can also serve as base
stations for Internet access, and constitute the LEO-satellite-based access
network (LEO-SAN). LEO-SAN is expected to provide seamless massive access and
extended coverage with high signal quality. However, its practical
implementation still faces significant technical challenges, e.g., high
mobility and limited budget for communication payloads of LEO satellite nodes.
This paper aims at revealing the main technical issues that have not been fully
addressed by the existing LEO-SAN designs, from three aspects namely random
access, beam management and Doppler-resistant transmission technologies. More
specifically, the critical issues of random access in LEO-SAN are discussed
regarding low flexibility, long transmission delay, and inefficient handshakes.
Then the beam management for LEO-SAN is investigated in complex propagation
environments under the constraints of high mobility and limited payload budget.
Furthermore, the influence of Doppler shifts on LEO-SAN is explored.
Correspondingly, promising technologies to address these challenges are also
discussed, respectively. Finally, the future research directions are
envisioned.
