# Design and Evaluation of an Acoustic Modern
## 1 Introduction  
With the rapid development in underwater information acquisition and transmission, the small underwater autonomous mobile platform is drawing more and more attention from the fields of marine environmental monitoring, ocean sampling, and oceanic engineering because of its low-cost, convenience, and capability for dense deployment. Underwater acoustic communication is urgently required for many autonomous underwater vehicle (AUV) applications to allow transmission of data and control among each other [1]. However, it has been well recognized that multipath expansion, time-varied fading, and Doppler spreading in both time and frequency domains are challenging topics for underwater acoustic communication. For the specific application in small AUVs, because of the relatively short distance as well as low data rate requirement, the problems of low signal noise ratio (SNR) or channel multipath can be mitigated by adopting a suitable modulation manner such as spread spectrum. Thus, in many cases the Doppler spread generated by AUV motion becomes the most severe issue [2] to be addressed by modem design. A significant amount of research has been conducted on Doppler estimation and correction. [PERSON] et al. [3] proposed to calculate the cross-correlation between a branch of local Doppler reference signals and the received signal, the highest correlation among which corresponds to the real Doppler. To reduce the number of correlation calculations needed for a Doppler search, [PERSON] et al. [4] adoptedtwo LFM (linear frequency modulation) signals spaced with a fixed period to produce two correlation peaks, the interval of which offers an indicator for the bulk Doppler. The proposed Doppler estimator proved to be robust despite the complex channel responses. [PERSON] et al. [5] designed a multiple-resampling (MR) scheme, where each MR branch matched the Doppler factor of a particular propagation path. Among other areas of progress, [PERSON] et al. [6] proposed an accurate Doppler shift estimation method, which automatically matched underwater acoustic multipath channels. This Doppler shift estimation scheme also featured high channel adaptability. [PERSON] et al. [7] proposed a Doppler estimation and compensation method for fast-moving transceivers by incorporating the Doppler invariance of hyperbolic frequency modulation. [PERSON] et al. [8] presented a Doppler estimation method using a cyclic prefix to achieve higher accuracy. Compared with extensive and active research in different Doppler estimation strategies, there is a lack of investigation on the theoretical principle of Doppler correction, as it is straightforward that Doppler compensation can be effectively achieved by resampling after estimating Doppler with various approaches. While it is easy and convenient to conduct offline resampling in software, from the viewpoint of hardware implementation, real-time resampling still poses significant difficulties to the engineering application of an acoustic modem, mainly because of the computational complexity involved. For small AUVs where the power supply and processor calculation capability are strictly limited, reconstructing and resampling the frame signal, thus, poses challenges to the onboard electronics. In this paper, a hardware Doppler mitigation approach is proposed in the modem design. By directly tuning the analog-to-digital (AD) sampling rate according to the Doppler estimate, our approach significantly reduces the computational load for its implementation on small AUVs. The proposed modem also utilizes a spread spectrum modulation scheme to enable robust, low data rate communication. Each signal frame begins with a synchronization signal that also serves as the Doppler probe. Once the synchronization is in lock, Doppler can be estimated to calculate the corresponding sampling rate to correct the Doppler spread. Then, the AD sampling rate is adjusted accordingly to directly compensate for the Doppler effects.