# A Weighted Turbo Equalized Multi-Band Underwater Wireless Acoustic Communications
## 1 Introduction  
In the past, underwater acoustic was utilized for military purpose such as submarine communications and for low-probability detection by third parties. Nowadays, underwater acoustic is considered to be one of the most challenging environments for data communications, such as control of autonomous underwater vehicles (AUV), and undersea command and control. The performance of UWAS (Underwater Wireless Acoustic Sensor) communication depends on factors such as propagation loss (according to the distance of acoustic signals), interference signal (caused by multi-path propagation), background noise, and Doppler effects (which are related to moving sound sources or sea surface roughness) [1; 2; 3]. Additionally, the increase in propagation distance decreases the bandwidth, which results in lower efficiency of data propagation. In the UWAS communication environment, multi-path propagation characteristics, or the Doppler-spread, affect the communication performances according to the spatiotemporal changes, including seabed, sea level, and water depth. Accordingly, channel coding and modem technologies are essential in designing a system that can overcome this challenge [4; 5]. As a UWAS channel has time-varying characteristics, the multi-band communication technology [6; 7; 8], which allocates the same data to different frequency bands, has been recently highlighted because itovercomes the frequency-selective fading that occurs due to the underwater multi-path and the Doppler spread [9; 10; 11]. In other words, the multi-band communication technology overcomes various problems of underwater channel environments and extends the propagation distances, thereby, improving both the performance and the propagation efficiency. However, as long-distance communications use a narrow frequency band, the multi-band communication is based on a limited range of available frequencies. For military submarine applications, UWAS systems are required to remain undetected, be highly reliable, and require long-range communication. The goal of the multi-band underwater communication is to successfully transmit data in various underwater channel environments and extended propagation distances. The research project of EUROPA \"UUV Covert Acoustic Communications\" tested, at a sea trial, using eight sub-bands with turbo coding techniques, at a data rate of 75 bps, had a range of up to 50 km [12]. In the multi-band transmission and reception structure of the reference [12], the turbo coded 5670 bits of each band and the preamble 2880 bits were divided into 45 and then the divided preamble bits and coded bits were sequentially arranged to form a single packet, which was iteratively propagated to each band. However, the multi-band configuration may have worse performance than the single-band one. It is because the performance degradation in a particular band affects the output from the entire bands, which is input into an encoder, thereby decreasing the overall performance. This problem can be solved through a receiving end that analyzes error rates of each band, sets threshold values and allocates lower weights to inferior bands. There are many methods of setting threshold values. This paper proposed to set threshold values by utilizing the preamble error rates, which are known data to be transmitted and received. In the receiver sides, after compensating frequency and phase offset in each band, iterative turbo equalizer is used in the system. The equalizer and turbo decoder are connected through the interleaving functions and de-interleaving that update each other's information recursively. The interleaved output is canceled a posteriori from the proceeding received signal. Interleaving helps receiver convergence. This paper analyzed the efficiency of multi-band acoustic propagation in UWAS communication. For the analysis, an underwater experiment was conducted by using a turbo pi code with 1/3 coding rate [13; 14; 15] and applying up to four bands.