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1. INTRODUCTION
As more and more attention is paid to the ocean, the requirement of exploration and exploitation to it grows increasingly. In order to realize some tasks in the ocean such as collection of the information, monitoring of the environment, exploration of energy sources, forecast of natural disaster, assistant navigation, underwater communication technique has become a very important scientific research field. Because acoustic wave is adopted to actualize communication in the water instead of electromagnetic wave, it is called underwater acoustic communication. However, the complex environment in the ocean makes it rather difficult to communicate normally, such as the problems of high noise, multipath effect and large delay. Diversity reception technique and coding technique (compression coding and error correction coding) are also used. As the tasks we require underwater vehicle to undertake grow more and more complicated, precise and various, only an underwater vehicle can hardly satisfy all the requirements. Multiple underwater vehicles system is one of the most important developmental directions. Our goal is to set up a communication network to ensure the vehicles to work under control normally. Thus this paper proposes an underwater acoustic communication system based on DS-CDMA for multiple underwater vehicles to communicate simultaneously to the console on the water surface. Spread spectrum, channel coding and Rake Receiver are adopted to improve the communication reliability. Especially, the concept of multiuser will be helpful in the construction of underwater acoustic network.
2. CHARACTERISTICS OF UNDERWATER ACOUSTIC CHANNEL
Underwater acoustic channel is the most complex communication channel that we have known about. It brings many difficulties to underwater communication. In the following we analyze the factors that influence acoustic communications in order to state the challenges posed by the underwater channels for underwater communication. They include,
2.1 Path loss:
2.1.1 Attenuation:
This is mainly provoked by absorption due to conversion of acoustic energy into heat, which increases with distance and frequency. It is also caused by scattering and reverberation (on rough ocean surface and bottom), refraction, and dispersion (due to the displacement of the reflection point caused by wind on the surface).
2.1.2 Geometric Spreading:
This refers to the spreading of sound energy as a result of the expansion of the wavefronts. It increases with the propagation distance and is independent of frequency.
2.2 Noise:
2.2.1 Man made noise:
This is mainly caused by machinery noise (pumps, reduction gears, power plants, etc.), and shipping activity (hull fouling, animal life on hull, cavitation).
2.2.2 Ambient Noise:
This is related to hydrodynamics (movement of water including tides, currents, storms, wind, rain, etc.), seismic and biological phenomena.
2.3 Multi-path:
Multi-path propagation may be responsible for severe degradation of the acoustic communication signal, since it generates Inter-Symbol Interference (ISI).The multi-path geometry depends on the page link configuration. Vertical channels are characterized by little time dispersion, whereas horizontal channels may have extremely long multi-path spreads, whose value depend on the water depth.
2.4 High delay and delay variance:
The propagation speed in the UW-A channel is five orders of magnitude lower than in the radio channel. This large propagation delay (0.67 s=km) can reduce the throughput of the system considerably.
2.5 Doppler spread:
The Doppler frequency spread can be significant in UWA channels causing degradation in the performance of digital communications: transmissions at a high data rate cause many adjacent symbols to interfere at the receiver.
3. THE MODEL OF UNDERWATER ACOUSTIC CHANNEL
Generally speaking, underwater acoustic channel is a kind of slow time-varying coherent multipath channel. In the length of coherent time, it can be simplified as a coherent multipath channel, which only has multipath effect. When the input signal is s(t), the output signal is,
Where L is the number of multipath; is the time varying attenuation factor in the kth transmission path; is the delay; n(t) is additive white noise.
4. COMMUNICATION APPROACHES
4.1 Spread spectrum communication.
Code division multiple access (CDMA) is based on the use of a modulation technique known as spread spectrum. In CDMA scheme all users transmit signals simultaneously on the multiple access channel. Each user employs a spread spectrum modulation. Spread spectrum communication is a communication method that spreads the signal which is to be sent to a wide frequency band. The frequency band is much wider than the bandwidth of the original signal. This is accomplished by multiplying the message signal with a noise-like or pseudo-random spreading signal. To demodulate the signal the receiver cross-correlates the received signal with an exact replica of the spreading function. The cross-correlation produces a maximum only when the codes are matched. Otherwise the result produces a very low value.
Attractive features are,
a. CDMA is highly resistant to interference.
b. Spread spectrum systems are resistant to multipath noises.
c. Highly secure form of communication.
The theoretical basis of spread spectrum communication is Shannon Formula,
C = B log2 [1+ S / N]
Where,
C = channel capacity, bit/s;
B = bandwidth of the signal, Hz;
S = average power of the signal, W;
N =power of the noise, W.
From Shannon Formula, it is obtained that if channel capacity is a constant, we can reduce the requirement of SNR (Signal Noise Ratio) by way of increasing the bandwidth; when SNR is very low (in the ocean for example), it is feasible to extend the bandwidth of the signal by many times to ensure the reliability of communication. And the resistance capability against the noise of the system is greatly improved. Meanwhile, the spread spectrum technique has the characteristic of resistance to multipath, which is also very helpful in underwater acoustic communication.
4.2 DS-CDMA SYSTEM
There are three basic spreading techniques: DS (Direct Sequence), FH (Frequency Hopping) and TH (Time Hopping) spread spectrum. Here direct sequence CDMA system in which original signal is linearly modulated using wide-band PN code is employed. A basic configuration of DS-CDMA system is shown below,

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