Research On Spread-Spectrum-Based Multiple Access Techniques For Underwater Acoustic Communication

Underwater communication faces the challenge of prolonged propagation,narrow bandwidth,obvious multipath,large signal attenuation,Doppler frequency shift,etc.These factors determine the characteristics of the space-time-frequency variation of the underwater acoustic channel.In the underwater acoustic communication environment with prolonged propagation and narrow available bandwidth,the application of TDMA and FDMA is greatly limited,and the existing land-based MAC protocol cannot be directly applied to a hydroacoustic sensor network.Because of the robustness of CDMA to frequency-selective fading and Doppler effects,it is regarded as one of the more promising multiple access technologies in hydroacoustic sensor networks.CDMA allows multiple users to communicate simultaneously,but as the number of users increases,the attendant near-far effect problem also becomes apparent.This paper proposes a CDMA-MAC-PC protocol based on CDMA power control for underwater acoustic sensor networks.When packets are received in the wrong order,it will feed back a NACK packet instead of the traditional ACK packets after correctly received data packets each time.The CDMA-MAC-PC protocol uses a closed-loop power control algorithm based on signal to interference plus noise ratio(SINR)balance,which let the sink node periodically instructs the sensor node to regulate the optimal transmit power to overcome the near-far effect.The cost and benefits of power control signaling are discussed and evaluated,and simulation results and analysis are given.Because the number of mutually orthogonal spreading codes is generally small,the number of spreading codes with good orthogonality is not sufficient to support sufficient user capacity,and ISI is added which would disrupt the orthogonality of spreading codes,so MAI and ISI have become the main problems that limit the CDMA system.In order to solve this problem,this paper researches the IDMA technology applicable to the underwater acoustic sensor network.On the basis of analyzing the traditional IDMA receiver,two kinds of receivers' improvement schemes are also presented,which are the IDMA receiver based on PIC detection and the IDMA receiver based on DA-BTEQ equalizer.By repeatedly applying equalizers and decoders,their out-of-output information is used as a priori input to each other after deinterleaving/interleaving modules,and it generates gain through iteration to reduce the bit error rate of the received signal.At the same time,a detailed theoretical derivation of the downlink power allocation problem in underwater acoustic environment is made.Finally,the performance simulation and analysis of the three IDMA receivers under the actual underwater acoustic channel are given,and the influence of the interleaving depth on the performance of the independent random interleaver is briefly analyzed.