  Most of the recent advances in the design of high-speed wireless systems are
based on information-theoretic principles that demonstrate how to efficiently
transmit long data packets. However, the upcoming wireless systems, notably the
5G system, will need to support novel traffic types that use short packets. For
example, short packets represent the most common form of traffic generated by
sensors and other devices involved in Machine-to-Machine (M2M) communications.
Furthermore, there are emerging applications in which small packets are
expected to carry critical information that should be received with low latency
and ultra-high reliability.
  Current wireless systems are not designed to support short-packet
transmissions. For example, the design of current systems relies on the
assumption that the metadata (control information) is of negligible size
compared to the actual information payload. Hence, transmitting metadata using
heuristic methods does not affect the overall system performance. However, when
the packets are short, metadata may be of the same size as the payload, and the
conventional methods to transmit it may be highly suboptimal.
  In this article, we review recent advances in information theory, which
provide the theoretical principles that govern the transmission of short
packets. We then apply these principles to three exemplary scenarios (the
two-way channel, the downlink broadcast channel, and the uplink random access
channel), thereby illustrating how the transmission of control information can
be optimized when the packets are short. The insights brought by these examples
suggest that new principles are needed for the design of wireless protocols
supporting short packets. These principles will have a direct impact on the
system design.
