Research On Peak-to-average Power Ratio Reduction Technology For MIMO-OFDM Underwater Acoustic Communication System

Low carrier frequency, Strong multi-path interferences, high noise level make the neritic underwater acoustic channel the most difficult wireless communication channel. The frequency bandwidth in underwater acoustic communication is limited. Therefore, researchers concern on looking for a method which owns the ability of anti-multipath, while enhances the channel capacity. Multi-input multi-output (MIMO) technology can improve the channel capacity significantly and essentially without extra energy consumption. Orthogonal frequency division multiplexing (OFDM) has high spectrum efficiency and good performance in anti-multipath and impulse noise. Therefore, combining OFDM with MIMO becomes one of the research hot spots in underwater acoustic communication.Due to the multicarrier modulation, the major drawback of MIMO-OFDM system is the high peak-to-average power ratio. High PAPR requires large liner region of the amplifier, or it will result in distortion. The system cannot make full use of the transmit power and cost much more than ever. So it is necessary to reduce the peak level of the signal before transmitted into the amplifier.Based on the review of the existing PAPR reduction methods in MIMO-OFDM system,paper focus on the selected mapping (SLM) technique. The main work done as follows:1. Simulations and comparisons of different types of PAPR reduction methods are provided. Analysis of the feasibility,advantages and disadvantages of these methods which lay the foundation for later study.2. Two novel peak-to-average-power ratio reduction schemes that require no side information (SI) in underwater acoustic MIMO-OFDM system are proposed. Phase sequence detector based on time-domain interpolation or compressed sensing is designed to distinguish the number of the selected phase sequences. Therefore, the proposed schemes do not need reserve bits for transmitting side information, resulting in an increasing of data rate.Simulation results show that the PAPR reduction performance is not reduced when compared with the traditional selected mapping, and the BER performance is approximately the same as those selected mapping scheme with perfect SI. Experiment carried in the pool also demonstrates the proposed scheme can differentiate between phase sequences, and significantly enhance the quality of the underwater acoustic MIMO-OFDM communication system.3. An improved selected mapping (SLM) method is proposed to overcome the disadvantage of the conventional SLM algorithm, which needs transmitting the serial number of selected scrambling phase sequence as side information. A phase sequence detector based on the decision feedback is designed to find out the serial number of selected scrambling phase sequence at the receiver. Simulations show that the proposed method could offer better performance of bit error rate whether in white Gaussian noise environment or in burst noise environment. So the improved SLM method has no dependence on the side information. It also increases the data rate and enhances the quality of the communication system. Perfect experimental results also prove that the improved selected mapping method can complete reliable underwater acoustic communication.