  The vision of connecting billions of battery operated devices to be used for
diverse emerging applications calls for a wireless communication system that
can support stringent reliability and latency requirements. Both reliability
and energy efficiency are critical for many of these applications that involve
communication with short packets which undermine the coding gain achievable
from large packets. In this paper, we study a cross-layer approach to optimize
the performance of low-power wireless links. At first, we derive a simple and
accurate packet error rate (PER) expression for uncoded schemes in block fading
channels based on a new proposition that shows that the waterfall threshold in
the PER upper bound in Nakagami-m fading channels is tightly approximated by
the m-th moment of an asymptotic distribution of PER in AWGN channel. The
proposed PER approximation establishes an explicit connection between the
physical and link layers parameters, and the packet error rate. We exploit this
connection for cross-layer design and optimization of communication links. To
this end, we propose a semi-analytic framework to jointly optimize
signal-to-noise ratio (SNR) and modulation order at physical layer, and the
packet length and number of retransmissions at link layer with respect to
distance under the prescribed delay and reliability constraints.
