  Grant-free multiple-input multiple-output (MIMO) usually employs
non-orthogonal pilots for joint user detection and channel estimation. However,
existing methods are too complex for massive grant-free access in massive MIMO.
This letter proposes pilot detection reference signal (PDRS) to greatly reduce
the complexity. In PDRS scheme, no iteration is required. Direct weight
estimation is also proposed to calculate combining weights without channel
estimation. After combining, PDRS recovery errors are used to decide the pilot
activity. The simulation results show that the proposed grant-free scheme
performs good with a complexity reduced by orders of magnitude.
