  Machine-type communication requires rethinking of the structure of short
packets due to the coding limitations and the significant role of the control
information. In ultra-reliable low-latency communication (URLLC), it is crucial
to optimally use the limited degrees of freedom (DoFs) to send data and control
information. We consider a URLLC model for short packet transmission with
acknowledgement (ACK). We compare the detection/decoding performance of two
short packet structures: (1) time-multiplexed detection sequence and data; and
(2) structure in which both packet detection and data decoding use all DoFs.
Specifically, as an instance of the second structure we use superimposed
sequences for detection and data. We derive the probabilities of false alarm
and misdetection for an AWGN channel and numerically minimize the packet error
probability (PER), showing that for delay-constrained data and ACK exchange,
there is a tradeoff between the resources spent for detection and decoding. We
show that the optimal PER for the superimposed structure is achieved for higher
detection overhead. For this reason, the PER is also higher than in the
preamble case. However, the superimposed structure is advantageous due to its
flexibility to achieve optimal operation without the need to use multiple
codebooks.
