  Doubly-selective channels, such as those that occur when the transmitter and
the receiver move relative to each other at high speeds, are a key scenario for
fifth generation (5G) cellular systems, which are mostly based in the use of
the orthogonal frequency-division multiplexing (OFDM) modulation. In this
paper, we consider an OFDM system using quadrature amplitude modulation (QAM)
symbols and we show that, when transmitting over deterministic doubly-selective
channels, the inter-carrier interference (ICI) affecting a symbol can be well
approximated by a complex-valued normal distribution. Based on this, we derive
a lower bound for the link capacity using the Shannon-Hartley theorem. Finally,
we provide an approximation of the bit error probability (BEP) using the
well-known BEP expressions for Gray-coded QAM constellations over additive
white Gaussian noise (AWGN) channels, and show numerical results that confirm
that the proposed BEP expression approximates accurately the bit error ratio
(BER) of the OFDM system for standardized channel models. The proposed
closed-form analytical expressions for the capacity and the BEP do not only
allow for discarding the need of computationally-costly Monte-Carlo system
simulations, but also provide a theoretical framework to optimize the system
parameters directly impacting on the achievable performance.
