# Research on Co-Channel Interference Cancellation for Underwater Acoustic MIMO Communications
###### Abstract  
Multiple-input-multiple-output (MIMO) communication systems utilize multiple transmitters to send different pieces of information in parallel. This offers a promising way to communicate at a high data rate over bandwidth-limited underwater acoustic channels. However, underwater acoustic MIMO communication not only suffers from serious inter-symbol interference, but also critical co-channel interference (CoI), both of which degrade the communication performance. In this paper, we propose a new framework for underwater acoustic MIMO communications. The proposed framework consists of a Col-cancellation-based channel estimation method and channel-estimation-based decision feedback equalizer (CE-DFE) with CoI cancellation functionalities for underwater acoustic MIMO communication. We introduce a new channel estimation model that projects the received signal to a specific subspace where the interference is free; therefore, the CoI is cancelled. We also introduce a CE-DFE with CoI cancellation by appending some filters from traditional CE-DFE. In addition, the traditional direct adaptive decision feedback equalization (DA-DFE) method and the proposed method are compared in terms of communication performance and computational complexity. Finally, the sea trial experiment demonstrates the effectiveness and merits of the proposed method. The proposed method achieves a more than 1 dB of output SNR over traditional DA-DFE, and is less sensitive to parameters. The proposed method provides a new approach to the design of robust underwater acoustic MODEM. underwater acoustic MIMO communication; co-channel interference cancellation; channel-estimation-based decision feedback equalizer; channel estimation; signal projection +
Footnote †: journal:  
0000-0001-9500-4999-38  
(CP) or zero padding (ZP), and the frequency-domain equalization (FDE) reduces the computational complexity. However, the usage of CP or ZP degrades the bandwidth efficiency. Moreover, the frequency-domain equalizer performs a block-wise process, the duration of a block is typically large, and it is assumed that, within a block, the channel is time-invariant; therefore, it is not applicable in fast-varying channels. In addition, the OFDM systems suffer from a high peak-to-average power ratio, which leads to difficulties in designing power amplifiers. However, single carrier modulation has the advantage of a high spectral efficiency and a fast channel-tracking ability. Under limited-bandwidth acoustic channels, one promising approach to increase the data rate is to use multiple transmitters; this system is referred to as a multiple-input-multiple-output (MIMO) system. Unfortunately, despite suffering from ISI caused by the multipath, the MIMO system also suffers from serious co-channel interference (CoI), which introduces a lot of noise in the per-channel channel decoding process. In the last two decades, underwater acoustic MIMO communications have been investigated in [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18] to fundamentally increase the achievable data rate. Due to interference among concurrent transmission streams, powerful detection algorithms are highly in demand. The most effective way to deal with ISI and CoI is channel equalization for underwater acoustic MIMO systems. The channel equalization for single-carrier MIMO communication includes frequency-domain equalizers [6; 7; 8] and time-domain equalizers [9; 10; 11; 12; 13; 14; 15; 16; 17]. In [6], a single-carrier receiver scheme with bandwidth-efficient FDE was proposed. The proposed algorithm implemented an overlapped-window FDE by partitioning a large block into small subblocks, and a decision-direct channel estimation algorithm was incorporated into the overlapped-window FDE to track channel variations and improve error performance. The proposed FDE reduced the average bit error rate compared to traditional single-carrier equalization by 74.4% and 84.6% for the 400 m and 1000 m range systems.