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2. Underwater Acoustic Communications
From the past 4 decades, underwater acoustic communications received more attention for its vast applications in commercial utilization of technology other than military applications. Acoustic communications are normally differs from the other communication systems due to its complexity in signal transition, degradation due to multipath propagation, channel conditions ,salinity and spatial variability conditions.
The field of underwater acoustics is developing rapidly in the last four decades. This is in response to meet the needs that are originating within the sonar and seismic communities. Many mathematical models are being developed to analyze the data collected during field experiments. These models can be used for the prediction of acoustic conditions that can be applied to many problems which include planning of improved at-sea experiments and the designing of optimized sonar systems.
Environmental models can be used as integral parts of acoustic models to generate input parameters and also predict intermediate quantities. The environmental models that are currently in use include sound-speed absorption coefficients, surface reflection losses, bottom reflection losses, surface backscattering strengths, bottom backscattering strengths, ambient noise and surface duct propagation loss.
The general sound wave propagation in the sea or ocean is depends on 3 aspects: salinity, pressure and temperature. We can see this through the graphical representations.
From the above graph, we can understand the sound speed depends on increase and decrease of water depth vs other parameter 
The difference between underwater acoustic communications and terrestrial can be listed as follows: Deployment. The deployment is deemed to be sparser in underwater communications. Cost Underwater receivers are more expensive devices than terrestrial receivers.â€ Power. Huge amount of power is needed in underwater communications for higher distances & advanced complex acoustic digital signal processing in the receivers. While designing an underwater acoustic receiver, some of the major challenges are: To deal with the battery power which is limited, characteristics of the propagation channel delays, power efficiency during stand-by mode, prone to environmental disturbances.
2.2 Previous work in this field and advancements
The underwater acoustic communications are developed extremely from the mid 1960's in response to the increasing needs that are originating inside the seismic & sonar communications. Since that period, several field experiments have been done and were compared with the theory results to analyses & estimate the behavior of the acoustic signals under various conditions. The experimental models are used to predict the conditions and given development, progress was not rapid enough even with the computing operations. This is mainly because of the limitations in the technology during that period. Several models and theory papers are been proposed in the field of underwater acoustic communications.
From the IEEE paper Underwater Acoustic Communications by Azizul H. Quazi and William L. Konrad written in 1982.This paper mainly describes about the solving the problems which are limiting the range and the data rate. Here it's given about the initial development and the 1st practical applications, the telephonic service developed at naval underwater sound laboratory(now NUSC),which is utilized to communicate with submarines using 8.3KHz sideband surpassed carrier, this is done using lower frequencies because to achieve the transmission for longer distances.
We can see the graphical representation of the sound absorption inside the sea water comparing with the fresh water in terms of frequency to attenuation co-efficient. The sea water is a conductive medium, where the electromagnetic energy absorption is extremely high which is about 45âˆšfdB per km, where frequency (f) is measured in Hertz. Some of the experiments and discussions that are present in this paper are about the multipath effects, which have several solutions at the same time having their own limitations. In this order, best possible solution is to eliminate the multiple arrivals by joining the directional beam transducer arrays & careful signal design.
The experiments were conducted on the voice & picture transmission which can be understand using the tables which gives information in data rate capabilities ,error rate using the fixed terminals and also the representation of internal block diagram of the transmitter and receiver .
The summary of this paper is generally observed that an underwater channel in MFSK AND MPSK provides sufficient coherence to support data rates. The results of the picture ,data & voice transmission gives indication that that data rate is excess of kilo-bit p/s is possible over short ranges like less than 10 km .with the increase in the data rate ,requirements of bandwidth will increase. Therefore in-between the communications range and the data rate transmission, trade-of is necessary. It was given that in the future research should consider the trade-off to achieve the realistic multipath environments over different ranges.
In 1995,Adam Zielinski, Senior Member, IEEE, Young-Hoon Yoon, and Lixue Wu have written a paper on Performance Analysis of Digital Acoustic Communications in a Shallow Water Channel. The summary of the paper was a simple model developed to describe the high data rate acoustic signal propagation in shallow water which is initially performed a computer analysis. This model could suggest that transmission of the higher data rate over a shallow channel using the digital Differential phase modulation. The constructive interference between the direct and the multi-path signals are possible using such transmission. Depending upon the sea or the ocean water state, transmission robustness improves with rough conditions of sea. Large number of the phases are required to modulate signal is achieved by transmitting at higher rates.
The channel model approximates with both amplitude & multi-path delay signals of an actual channel will randomly fluctuates. But it was given that this variation was negligible within duration of the signaling element. Therefore the effects of phase detector can be eliminated by using the differential phase modulations .here the ambient noise has been neglected, but its method is to generalize to incorporate it. 
3. Spread Spectrum Modulation
It is defined as “a spread spectrum signal is a signal that is consists of the extra modulations which expands the signal bandwidth beyond what is required by the underlying data modulation” (principles of the spread spectrum communications systems by DONTORRIERI, CHAPTER 2, 2.1PAGE 55).Spread-spectrum modulation is particular signaling methods where do trade of among the bandwidth and performance. It is basically utilized in military guidance and communications systems (one of its main techniques used to achieve the jam-resistant communication systems). Communication systems using spread spectrum signals are helpful in making the channel interception difficult, accommodating the fading channels and multiple channels, suppressing the interference and also capable of providing the multiple access .
In spread spectrum communications, the most dominant and practical approach are the digital communications frequency hopping and the direct sequence modulations. A Direct sequence signal is a spread spectrum signal generated by the direct mixing of the data with a spreading waveform before the final carier modulation. Basically the direct sequence signal with BINARY PHASE SHIFT KEYING (PSK) can be represented using following equation (1).
Where A - is the signal amplitude(t)is the data modulation, p(t) is the spreading wave form, fc- is the carrier frequency ,and Ñ² is the phase at t=0,When ever we consider the phase shift of data communications ,where the amplitude is in+1 and -1 then, we can write the above equation as
The above equation explicitly exhibits the phase shift keying of data communications. Examples of the data modulation and the spreading waveform are given in picture format
The spreading waveform equation is given as
P (t)= .
One of the benefits from spread spectrum is ability to discriminate against multipath, narrowband; multi-access and other structured interference which are arise in RF communications channels. In addition to this spread spectrum signals are difficult to demodulate or to detect by the receivers unauthorized .
A spread spectrum is a signal modulation where the signal frequency is spread over a wide bandwidth; the data is independent of code. The receiver at which the code is synchronized is used for the subsequent recovery of the data. Spread spectrum diagrammatic representation is shown below.
The spread spectrum otherwise known as noise modulation is normally represents like a noise signal to the normal AM/FM receivers due to its spreading over large bandwidth. It was basically used only for the military communications because of its inherent encryption coding. Basically the spread spectrum communications are very widely useful applications with high security. In the present trend, we use them for the military, industrial, avionics, scientific, civil and medical purposes. One of the best features and applications are Jam-resistant communication systems, CDMA radios, High Resolution Ranging in which we can use it for GPS(global positioning system),Wireless Local area Network ,long range wireless phones, cellular base station interconnections.
3.1 Direct Sequence (DS) SS Systems
The spreading of Bandwidth using direct modulation of the signals in the wideband spread signal is called as direct sequence spread spectrum. It is modulated by the carrier before the transmission of the signal. The base band signal and the code bits are usually called as bits and chips. Here the general concept is chip rate is greater than then bit rate. The spreading signal sequence must be same at both the transmitter and the receiver where the signal is decoded .
3.11. DSSS Transmitter:
In the figure shown above, c(t) is the code bits; d(t) -input data bits; x(t) -frequency converted signal.
in the direct sequence spread spectrum, pseudo random generator will generates the code whose length is 1024 chips which will repeat periodically.XOR GATE is used for spreading of the data bits where the converted code, input and the output are shown.
3.2 Previous work in this field and advancements
An IEEE paper “Performance Analysis of a Spread Spectrum Acquisition Algorithm for Satellite Mobile Radio” written by PAUL G. FLIKKEMA and LEE D. DAVISSON in 1992 describes about the parallel acquisition of signal to noise ratio direct sequence spread spectrum signals of frequency & the chip clock PN epoch which is uncertainly due to the Doppler. The outcome of this paper is explained as there is a large increment in the decision space, the performance will be degraded which would be more relative in the case where Doppler is not considered. We can see the tables where they give some specifications, where the performance can be improved with the increment in M. the practical application performance by selecting various parameters can be optimized. In this present mode, the technique adopted by the block-parallel approach which is generalized is a method in a special case, which implicitly uses the digital technology. They also hinted that this approach used for the space technology can be used in the other technologies which also include charge-coupled devices or the surface acoustic waves .
In the recent times, in an IEEE paper it's discussed about the time varying multi-path with modulation scheme of the spread spectrum pulse position in the point to point acoustic communications, “The underwater acoustic channel is a complicated and time-varying multipath channel, and many equalization algorithms have been researched and developed to overcome the difficulties for underwater acoustic communication. Unfortunately, many algorithms are computational intensive and prone to lose convergence due to their sensitiveness to different channel configurations. In this paper, a pulse position modulation (PPM) scheme is proposed, and it uses two M-sequences of low cross-correlation to transfer information, which are modulated on two orthogonal carriers. One is used as a reference sequence, and the other is shifted relative to the reference. Information is carried by the starting time difference between the two sequences in each symbol. Comparing with conventional direct-sequence spread spectrum technique, the proposed scheme is more spectral efficient. Two receiver designs are given, one takes advantages of M-sequences' auto-correlation properties, and the other is motivated by passive phase conjugation (PPC) to take advantages of the channel. Combined with M-sequence, PPC performance is augmented without a receiving array to cover the water column, and it is far less complex than adaptive equalizers for receivers. Results from lake field trials are analyzed, and they verify potential applications of this PPM scheme” .
3.3 Receivers and DSP Implementation
Underwater receivers are much more complex, costly and difficult to construct, but most important part of the communications. The receiver is the place where we construct the signals from the received coded sources. The direct sequence spread spectrum acquisition mode of signal is given to our receivers which consist of different blocks, where the decoding of the signal is occurred. The receivers in general consume more power due to high power instruments.
Using the DSP implementations
Some of the methods implemented in receivers.
1. Cell scattering models
Â· In this the scatters are uniformly distributed, oceans is divided into nuber of cells . each cell containing large number of scatterers , target strenght per unit area or volume is calculated by strenght of back scattering .
2. Point scattering models
* It is an statistical approach where the scatteres are randomly distributed, reverberations are computed by summing the echoes received from each individual scatterers .
Sonar equation .
Noice background .
Reverberation background .
* Passive sonar
Battery utilization is the most crucial point of the entire review, as the receiver is entirely depends on the battery power supply energy consumption should be utilized very significantly .the project will mainly depends upon how the power consumption is down at various stages of the receiver, to minimize the wastage of the energy source and one of the important situation is while the receiver is in stand-by mode ,the design should be made in such a way that power supply is almost low, only to make some reorganization of input signals.
DSP implementation can be understood form the following example taken from the internet. The success of multicarrier modulation in the form of OFDM in radio channels illuminates a path one could take towards high-rate underwater acoustic communications, and recently there are intensive investigations on underwater OFDM. In this workshop, we would like to demonstrate two projects that have been undertaken at University of Connecticut on the implementations of an OFDM acoustic modem, whose receiver algorithms are developed â€¢ PC-based implementation as described in . This implementation is based on Mat lab programming on two laptops, as shown in Fig. 1. Two laptops can communicate with each other via two-way acoustic links. â€¢ DSP-based implementation as described. This implementation is based on a TMS320C6713 DSP board. For an OFDM block duration of 230 ms, the demodulation-plus-decoding time at the receiver is about 200 ms, and hence a real-time one-way communication is accomplished. The bandwidth is 5.5 kHz, and the overall data rate is 3.1 kbps after rate 1/2 convolution coding. These prototypes work well for in-air acoustic channels and are expected to work well for underwater acoustic channels with stationary transceivers. The re sampling operation for fast-varying channels due to mobile transceivers has not been implemented .
The underwater acoustic communication systems have given a very wide scope of development using different technologies where the battery powered instruments are implemented. The receiver systems present inside the ocean or the sea are entirely depended on battery power supply and have no other power sources. The receiver systems are constructed using direct sequence spread spectrum acquisition techniques, which generally consumes high power, to make this system more efficient and to improve the receiver performance for more time period; the power efficient receivers must be considered where the system will also be dealing with computational functions. The system when in standby mode will have to be only be activated when it receives the signal or code and then it will be changing its mode from stand-by to present state ,in order to achieve this ,we use DSP coding techniques and also check the simulation and synthesis results.
.Azizul H. Quazi and William L. Konrad (1982) .Underwater Acoustic Communications.IEEE Trans
 Adam Zielinski, Young-Hoon Yoon, and Lixue Wu (1995). Performance Analysis of Digital Acoustic Communications in a Shallow Water Channel . IEEE Tran
. Guosong Zhang a,*, Jens M. Hovem b, Hefeng Dong a, Shihong Zhou c, Shuanping Duc . An efficient spread spectrum pulse position modulation scheme for point-to-point underwater acoustic communication. IEEE Tran, 19 August 2009 Elsevier Ltd, Applied Acoustics 71 (2010) 11-16.Journal homepage: www.elsevier.com/locate/apacoust [accessed on 30/11/2009]
. M'.Stojanovic, J. G. Proakis, J. A. Rice and M. D. Green(1998) .Spread Spectrum Underwater Acoustic Telemetry. IEEE Tran.
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. The Discovery of Sound in the Sea web site,Â© 2002-2008, University of Rhode Island, Office of Marine Programs. Available at: http://www.dosits.org/siteinfo/info1.htm [accessed on 20/11/2009].
. Paul C.Etter. Underwater Acoustic Modeling And Simulation, Applied Technology Institutes (ATI), Available At : www.aticourses.com_underwater_acoustic_modeling.htm. Accessed on (6th December 2009)
. Sean Mason, Hai Yan, Shengli Zhou, Zhijie Jerry Shi, and Baosheng Li. Demonstration of PC-based and DSP-based Implementations of an OFDM Acoustic Modem.Underwater Sensor Network Laboratory, University of Connecticut(august 2007).Accesed on 8th December 2009