  This paper presents the optimal power control policies maximizing the
effective capacity achieved with arbitrary input distributions subject to an
average power constraint and quality of service (QoS) requirements. The
analysis leads to simplified expressions for the optimal power control
strategies in the low power regime and two limiting cases, i.e., extremely
stringent QoS constraints and vanishing QoS constraints. In the low power
regime, the energy efficiency (EE) performance with the constant-power scheme
is also determined by characterizing both the minimum energy per bit and
wideband slope for arbitrary input signaling and general fading distributions.
Subsequently, the results are specialized to Nakagami-m and Rician fading
channels. Also, tradeoff between the effective capacity and EE is studied by
determining the optimal power control scheme that maximizes the effective
capacity subject to constraints on the minimum required EE and average
transmission power. Circuit power consumption is explicitly considered in the
EE formulation. Through numerical results, the performance comparison between
constant-power and optimal power control schemes for different signal
constellations and Gaussian signals is carried out. The impact of QoS
constraints, input distributions, fading severity, and average transmit power
level on the proposed power control schemes, maximum achievable effective
capacity and EE is evaluated.
