  Due to the limited number of traditional orthogonal pilots, pilot collision
will severely degrade the performance of contention-based grant-free
transmission. To alleviate the pilot collision and exploit the spatial degree
of freedom as much as possible, an extremely sparse orthogonal pilot scheme is
proposed for uplink grant-free transmission. The proposed sparse pilot is used
to perform active user detection and estimate the spatial channel. Then,
inter-user interference suppression is performed by spatially combining the
received data symbols using the estimated spatial channel. After that, the
estimation and compensation of wireless channel and time/frequency offset are
performed utilizing the geometric characteristics of combined data symbols. The
task of pilot is much lightened, so that the extremely sparse orthogonal pilot
can occupy minimized resources, and the number of orthogonal pilots can be
increased significantly, which greatly reduces the probability of pilot
collision. The numerical results show that the proposed extremely sparse
orthogonal pilot scheme significantly improves the performance in
high-overloading grant-free scenario.
