  Grant-free access (GFA) has been envisioned to play an active role in massive
Machine Type Communication (mMTC) under 5G and Beyond mobile systems, which
targets at achieving significant reduction of signaling overhead and access
latency in the presence of sporadic traffic and small-size data. The paper
focuses on a novel K-repetition GFA (K-GFA) scheme by incorporating
Reed-Solomon (RS) code with the contention resolution diversity slotted ALOHA
(CRDSA), aiming to achieve high-reliability and low-latency access in the
presence of massive uncoordinated MTC devices (MTCDs). We firstly defines a MAC
layer transmission structure at each MTCD for supporting message-level RS
coding on a data message of $Q$ packets, where a RS code of $KQ$ packets is
generated and sent in a super time frame (STF) that is composed of $Q$ time
frames. The access point (AP) can recover the original $Q$ packets of the data
message if at least $Q$ out of the $KQ$ packets of the RS code are successfully
received. The AP buffers the received MTCD signals of each resource block (RB)
within an STF and exercises the CRDSA based multi-user detection (MUD) by
exploring signal-level inter-RB correlation via iterative interference
cancellation (IIC). With the proposed CRDSA based K-GFA scheme, we provide the
complexity analysis, and derive a closed-form analytical model on the access
probability for each MTCD as well as its simplified approximate form. Extensive
numerical experiments are conducted to validate its effectiveness on the
proposed CRDSA based K-GFA scheme and gain deep understanding on its
performance regarding various key operational parameters.
