Multicarrier Spread-spectrum Underwater Acoustic Communication Research

The most important difference between underwater acoustic (UWA) communication and mobile wireless communication is the specific characteristics of UWA channels.the specific characteristics of UWA channels includes quite limited available bandwidth,serious multipath spread,random time varying,frequency varying and Doppler.UWA spread spectrum communication telemetry exchanges reliability with efficiency, which has strong data confidentiality and anti-interference and performs good and reliable over time varying and intense fading UWA channels. Spread spectrum communication also provides efficient multiple access ability for UWA communication networks. But limited by low data rate, the conventional UWA spread spectrum communication cannot meet the need of future UWA communication.Orthogonal Frequency Devision Multiplexing (OFDM) is a technology which uses multiple orthogonal sub-carriers to transmit data in parallel,which provides high data rate,combats multipath efficiently,use the frequency band fully,is suitable for future UWA communication.Multi-carrier spread spectrum communication,which combines Spread spectrum and OFDM,can combine their advantages .suit the UWA channel better.and have a promising future in UWA communication.This paper mainly focuses on the channel estimation and diversity combining methods of MC-CDMA in UWA communication.All the works are listed as follows:1. Introduce three different schemes of combining OFDM and CDMA,contrast and analysis their performances and select MC-CDMA as our study target.Introduce basic principles and the structures of transmitter and receiver.Then predigested MC-CDMA system in UWA communication is established. Some difficulties and key technologies when MC-CDMA system is used in UWA communication is briefly introduced.2. The channel estimation of MC-CDMA is researched.The multipath acoustic channel is modeled based on its physical properties and the ray theory.The channel estimation based on pilot symbol assisted modulation is mainly focused in this paper. Block and comb pilot pattern is researched and stimulated in the UMA channel,the performance is contrasted and researched.Different estimation algorithms are stimulated,the performance is contrasted and researched.The kinds of schemes based on pilot in comb pilot pattern are proposed.The low-rank approximation MMSE estimation and an improved estimation based on correlation bandwidth is researched to improve MMSE estimation.The results shows that both the estimation algorithms can improve the MMSE estimation effectively.The improved estimation based on correlation bandwidth is better than the low-rank approximation MMSE estimation.3. The diversity combining methods of MC-CDMA is researched.According to the the structures of transmitter and receiver of MC-CDMA system in UWA channels,diversity combining methods is the necessity in MC-CDMA system.Several common diversity combining methods are introduced,.Stimulate and contrast the performance. For proposing the best diversity combining method of MC-CDMA system,the diversity combining parameter k is defined.Then propose the changing rule of the performance of corresponding diversity combining method which is caused by the changing of parameter k in the condition of diverse number of users.