## 1 Introduction
### Motivation and Goals of the Work  
As previously stated, both OFDM and single carrier modulations require channel knowledge for transmitter processing (e.g., bit-loading, pre-equalization) or receiver processing (e.g., equalization), aimed at counterbalancing the propagation impairments. Most of the proposed solutions assume perfect channel knowledge at the receiver and/or transmitter side, so the achieved performance cannot match that expected in a real scenario. Actually, the accuracy of CSI unavoidably impacts on the communication performance in terms of trade off between reliability and data rate. However, to the best of our knowledge, only a few works in the literature address such an issue. Furthermore, despite the availability of several channel estimation and prediction techniques, we highlight the absence of a real protocol that drives the joint use of these approaches in an adaptive fashion, so as to maximize the performance in terms of bit error rate (BER), while minimizing the rate reduction caused by the overhead signaling. Finally, it is worth noting that CSI acquisition and transmission optimization are typically performed under the _optimistic_ assumption of absence of interference. The presence of superposing external acoustic sources (mammals' communication, vessels' engines and so forth) may not be negligible when dealing with channel estimation and signal detection [39], and, hence, it should be conveniently taken into account.  
Aimed at the above considerations, in this work we propose an adaptive OFDM scheme able to manage the cyclic prefix length adaptation by resorting to channel knowledge acquired thanks to both estimation and prediction techniques, with the goal of reducing ISI. The employment of estimation, followed by a channel predictive step, is peculiarly tailored to the underwater environment, since it allows reduction in overhead signaling due to training/pilot transmission. Such a saving is not negligible in an UWACs, characterized by very scarce resources. Summarizing, the main contributions of this work are the following:  
* an adaptive mechanism to tune the OFDM cyclic prefix, based on the channel estimation phase;
* a channel tracking mechanism, based on channel estimation and a prediction stage, by resorting to Kalman filtering;
* a _two-side contemporary processing_, that is, transmitter and receiver signal processing is operated independently, thus avoiding the need for a feedback link to continuously communicate measures operated at the receiver side;
* a mechanism to decide when re-estimation is needed, based on channel behavior;
* a frame structure/protocol supporting the adaptivity of the whole system and allowing a practical implementation of both the channel estimation and prediction.  
The approach herein presented differs from the literature as it effectively considers practical situations related both to interference and channel. In fact, these two impairments are generally considered but their temporal features are assumed as static or quasi-static. The proposed study investigates how to handle interference variations and channel variability, in terms of delay spread and amplitude coefficients. The resulting link optimization can also be seen as an implementation guideline for UWACs.  
About the manuscript's organization, in Section 2 we introduce the system model, including the initial interference statistics acquisition, channel statistics estimation procedures, and the CP adaptation mechanism. Section 3 describes the core of the proposed communication protocol, including the two-side contemporary processing based on Kalman filtering prediction and data transmission phase. In Section 4 numerical results and performance comparisons are presented and discussed. Finally, Section 5 concludes the paper.