This paper focuses on communications within a high-mobility context with OFDM as the modulation of choice. We first show that the Air-to-Ground (AtG) channel is the most difficult to adapt using standard channel equalization methods. We then derive a new channel model (AtGMM) for the AtG case and use it as the basis for analysis and for simulating the effects within the AtG environment. Next, we derive the optimal channel-estimation-based equalizer for the AtG channel; showing performance equivalent to a comparable time-invariant channel. We then analyze the performance of conventional OFDM-based systems using the AtGMM channel model and provide guidelines to maximizing the throughput within differing mobility contexts.