# R&D of an Innovative OFDM Communication Payload for Small-Size AUV
## 1 Introduction  
With the dramatically increasing requirement for ocean information perception and acquisition, autonomous underwater vehicle (AUV) play an increasingly important role in not only the traditional field of marine resource exploration, hydrological environment investigation, seabed topographic mapping or reconstruction [1, 2], but also emerging personal applications such as underwater sightseeing, swimming aid and entertainment purposes [3]. In particular, as an important branch of AUV development, in recent years, small-size AUV platforms have drawn extensive attention due to their low cost, small size and high flexibility characteristics. Consider the serious limitation of load capacity, energy supply and cost posed by a small-size AUV platform, it has become an urgent problem to establish as efficient underwater wireless communication link between small-size AUV platforms and the control mother ship. With its easy availability of existing commercial products, many small-size AUVs employ radio or optical communication modules for achieving information transmission near the water surface. However, this type of small-size AUV is extremely limited by the essential constraints in the aspect of communication distance and stability. Underwater acoustic (UWA) communication technology offers a feasible approach to accomplish the underwater requirements of small-size AUV in terms of underwater telemetry, remote control and data transmission [4]. However, the intertwined properties of marine environment contained tide, wave, current and interfacial effect generate the adverse UWA channel characteristics such as complicated multipath, dynamic Doppler, fast time-varying, severe fading and background noise, which can pose a significant challenge to the UWA communication system onboard small-size AUV [5; 6]. Moreover, the incoherent communication technology cannot meet the demands of high-speed data rate for the multitudinous applications of small-size AUV in marine data acquisition and transmission with its constraints of low bandwidth utilization in the severely constrained UWA channels [7]. Different to the incoherent mode, coherent communication provides an effective approach to clearly improve the data transmission rate. However, the phase variations of modulated UWA signals caused by the complicated time-space-frequency characteristics of UWA channel lead to a serious impact on the communication performance. Among the efforts for this purpose, the technology of orthogonal frequency division multiplexing (OFDM) has gained significant interest because of its promising features of a high data transmission rate, high spectrum efficiency and strong anti-multipath [8; 9]. Nonetheless, the performance of OFDM is sensitive to Doppler effect generated by the time-varying UWA channel or moving AUV [10], which generates the accumulation of symbol synchronization error and the destruction of orthogonality and seriously deteriorates the demodulation performance of the receiver. Thus, Doppler estimation and compensation are essential steps in the research and development (R&D) of UWA OFDM communication for realizing stable UWA OFDM communication performance. Numerous Doppler estimation and compensation approaches have been extensively reported. A typical strategy is to estimate Doppler by measuring the frequency offset of the single-frequency signal with known frequency between the received packets and the transmitted signal [11]. Nevertheless, the estimation accuracy of such algorithms with the classic fast Fourier transform (FFT) is insufficient due to its spectrum leakage of windowed signal after FFT calculation. With the purpose of improving the estimation accuracy, some novel approaches such as fractional Fourier transform (FRFT) [12], partial fast Fourier transform (PFFT) [13] and partially shifted fast Fourier transform [14] are introduced to replace the classic FFT with respect to the based frequency measurement Doppler estimation methods.