Underwater acoustic communication is a technique of sending and receiving messages under water. There are several ways to use such communication but the most common is to use hydrophones. Underwater communication is difficult because of factors such as multi-path propagation, channel time variation, small available bandwidth and strong signal attenuation, especially over long ranges. Compared with terrestrial communication, underwater communication has a low data rate because it uses acoustic waves instead of electromagnetic waves.
At the beginning of the 20th century, some vessels were communicated by underwater bells, systems that competed with primitive Maritime radio navigation services at the time. The Fessenden oscillator then allows communication with the submarine.
Video Underwater acoustic communication
Type of modulation used for underwater acoustic communication
Generally, the modulation method developed for radio communications can be adapted for underwater acoustic communication (UAC). However some modulation schemes are more suitable for unique underwater acoustic communication channels than others. Some of the modulation methods used for UAC are as follows:
- Lock frequency shift (FSK)
- Lock phase shift (PSK)
- Frequency spread spread spectrum (FHSS)
- Spread direct spread spread spread (DSSS)
- Pulse-frequency and modulation (FPPM and PPM)
- Multiple frequency shifts (MFSK)
- Orthogonal-frequency multiplex separation (OFDM)
The following is a discussion of the different types of modulation and utility to UAC.
Locking frequency shift
FSK is the earliest form of modulation used for acoustic modems. UAC before the modem is by percussion of different objects under water. This method is also used to measure the speed of sound in water.
FSK usually uses two different frequencies to modulate data; for example, Frequency F1 to show bit 0 and frequency F2 to show bit 1. Then a binary string can be transmitted by alternating these two frequencies depending on whether it is 0 or 1. The receiver can be as simple as having an analog matching filter to two frequencies and a level detector for decide whether 1 or 0 is accepted. This is a relatively easy form of modulation and is therefore used in the earliest acoustic modems. But a more sophisticated demodulator using Digital Signal Processors (DSP) can be used in the present.
The biggest challenge facing FSK at UAC is the multi-lane reflection. With multi-lanes (especially in UAC), some strong reflections can be present at the receiving hydrophone and the threshold detector becomes confused, thus severely limiting the use of this UAC type to a vertical channel. Adaptive equity method has been tried with limited success. Adaptive rendering tries to model a highly reflective UAC channel and reduce the effects of received signals. Success has been limited due to rapidly changing conditions and difficulty adapting in time.
Lock phase shift
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by altering (modulating) the phase of the reference signal (carrier wave). This signal is impressed into the magnetic field of the x, y area by varying the sine and cosine input at the appropriate time. It is widely used for wireless LAN, RFID and Bluetooth communication.
Each digital modulation scheme uses a limited number of signals to represent digital data. PSKs use a limited number of phases, each assigned a unique binary number pattern. Typically, each phase encodes the same number of bits. Each bit pattern forms a symbol represented by a particular phase. The demodulator, specially designed for a series of symbols used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus restoring the original data. This requires the receiver to be able to compare the phase of the received signal with a reference signal - such a system is called coherent (and referred to as CPSK).
Alternatively, instead of operating with regard to constant reference waves, the broadcast can operate with attention to itself. Single wave broadcast phase changes can be considered as significant items. In this system, the demodulator determines the change in the received signal phase rather than phase (relative to the reference wave) itself. Since this scheme depends on the difference between successive phases, this is called differential phase-shift switching (DPSK). DPSK can be significantly simpler to implement than ordinary PSK because there is no need for a demodulator to have a copy of the reference signal to determine the exact phase of the received signal (it is a non-coherent scheme). Instead, it produces a more erroneous demodulation.
orthogonal frequency multiplex separation
Orthogonal frequency-division multiplexing (OFDM) is a digital multi-carrier modulation scheme. OFDM delivers data across multiple parallel data channels by combining near-orthogonal sub-carrier signals
OFDM is a profitable communication scheme in underwater acoustic communication thanks to its resistance to frequency selective channels with long delay delay.
Maps Underwater acoustic communication
Use of vector sensor receiver
The vector sensor is capable of measuring important non-scalar components of the acoustic field such as wave velocity, which can not be obtained by a single scalar pressure sensor.
In the last few decades, extensive research has been done on the theory and design of vector sensors. Many vector sensor signal processing algorithms have been designed. They are mainly used for underwater target targeting and sonar applications.
The previous underwater acoustic communications system relied on scalar sensors only, which measured acoustic field pressure. Vector sensors measure scalar components and acoustic field vectors in a single point in space, therefore they can function as multi-channel recipients. This differs from existing multichannel underwater receivers, which consist of spatially separated separated pressure sensors, which can produce large arrays.
In general, there are two types of vector sensors: inertia and gradients. The inertial sensor actually measures the speed or acceleration by responding to the acoustic medium movement, while the gradient sensor uses a different approach to estimate the acoustic field gradients such as speed and acceleration.
In the communication example of the sensor vector shown, there is one transmitter pressure transducer, indicated by the black dot, while for the reception of the vector sensor is used, shown by the black box, which measures the pressure and y and z speed components. This is a single, double-input (output) 1-3-SIM system. With more pressure transmitters, one can have multiple-input multiple-output (MIMO) systems as well.
See also
- Acoustic underwater
- Acoustic release
References
External links
- Paper on denoising underwater signals
- DSPComm - manufacturer of subsea acoustic modems
Source of the article : Wikipedia
0 Comments