  We consider the multi-user detection (MUD) problem in uplink grant-free
non-orthogonal multiple access (NOMA), where the access point has to identify
the total number and correct identity of the active Internet of Things (IoT)
devices and decode their transmitted data. We assume that IoT devices use
complex spreading sequences and transmit information in a random-access manner
following the burst-sparsity model, where some IoT devices transmit their data
in multiple adjacent time slots with a high probability, while others transmit
only once during a frame. Exploiting the temporal correlation, we propose an
attention-based bidirectional long short-term memory (BiLSTM) network to solve
the MUD problem. The BiLSTM network creates a pattern of the device activation
history using forward and reverse pass LSTMs, whereas the attention mechanism
provides essential context to the device activation points. By doing so, a
hierarchical pathway is followed for detecting active devices in a grant-free
scenario. Then, by utilising the complex spreading sequences, blind data
detection for the estimated active devices is performed. The proposed framework
does not require prior knowledge of device sparsity levels and channels for
performing MUD. The results show that the proposed network achieves better
performance compared to existing benchmark schemes.
