  Fifth Generation (5G) New Radio (NR) does not support data transmission
during random access (RA) procedures, which results in unnecessary control
signalling overhead and power consumption, especially for small data
transmission (SDT). Motivated by this, 3GPP has proposed 4/2-step SDT RA
schemes based on the existing grant-based (4-step) and grant-free (2-step) RA
schemes, with the aim to enable data transmission during RA procedures in Radio
Resource Control (RRC) Inactive state. To compare the 4/2-step SDT RA schemes
with the benchmark 4/2-step RA schemes, we provide a spatio-temporal analytical
framework to evaluate the RA schemes, which jointly models the preamble
detection, Physical Uplink Shared Channel (PUSCH) decoding, and data
transmission procedures. Based on this analytical model, we derive the
analytical expressions for the overall packet transmission success probability
and average throughput in each RACH attempt. We also derive the average energy
consumption in each RACH attempt. Our results show that 2-step SDT RA scheme
provides the highest overall packet transmission success probability, and the
lowest average energy consumption, but the performance gain decreases with the
increase of device intensity.
