  This letter introduces a novel channel coding design framework for
short-length codewords that permits balancing the tradeoff between the bit
error rate floor and waterfall region by modifying a single real-valued
parameter. The proposed approach is based on combining convolutional coding
with a $q$-ary linear combination and unequal energy allocation, the latter
being controlled by the aforementioned parameter. EXIT charts are used to shed
light on the convergence characteristics of the associated iterative decoder,
which is described in terms of factor graphs. Simulation results show that the
proposed scheme is able to adjust its end-to-end error rate performance
efficiently and easily, on the contrary to previous approaches that require a
full code redesign when the error rate requirements of the application change.
Simulations also show that, at mid-range bit-error rates, there is a small
performance penalty with respect to the previous approaches. However, the EXIT
chart analysis and the simulation results suggest that for very low bit-error
rates the proposed system will exhibit lower error floors than previous
approaches.
