  Major goal of ultra reliable and low latency communication (URLLC) is to
reduce the latency down to a millisecond (ms) level while ensuring reliability
of the transmission. Since the current uplink transmission scheme requires a
complicated handshaking procedure to initiate the transmission, to meet this
stringent latency requirement is a challenge in wireless system design. In
particular, in the time division duplexing (TDD) systems, supporting the URLLC
is difficult since the mobile device has to wait until the transmit direction
is switched to the uplink. In this paper, we propose a new approach to support
a low latency access in TDD systems, called channel aware sparse transmission
(CAST). Key idea of the proposed scheme is to encode a grant signal in a form
of sparse vector. This together with the fact that the sensing mechanism
preserves the energy of the sparse vector allows us to use the compressed
sensing (CS) technique in CAST decoding. From the performance analysis and
numerical evaluations, we demonstrate that the proposed CAST scheme achieves a
significant reduction in access latency over the 4G LTE-TDD and 5G NR-TDD
systems.
