  Massive machine type communication (mMTC) is one of the three fifth
generation mobile networking (5G) key usage scenarios, which is characterized
by a very large number of connected devices typically transmitting a relatively
low volume of non-delay sensitive data. To support the mMTC communication, an
uplink (UL) grant-free sparse code multiple access (SCMA) system has been
proposed. In this system, the knowledge of user equipments' (UEs') status
should be obtained before decoding the data by a message passing algorithm
(MPA). An existing solution is to use the compressive sensing (CS) theory to
detect active UEs under the assumed condition of flat fading channel. But the
assumed condition is not suitable for the frequency selective channel and will
decrease the accuracy of active UEs detection. This paper proposes a new simple
module named refined active UE detector (RAUD), which is based on frequency
selective channel gain analyzing. By making full use of the channel gain and
analyzing the difference between characteristic values of the two status of
UEs, RAUD module can enhance the active UEs detection accuracy. Meanwhile, the
addition of the proposed module has a negligible effect on the complexity of UL
grant-free SCMA receiver.
