Underwater acoustic communication

In general the modulation methods developed for radio communications can be adapted for underwater acoustic communications (UAC). However some of the modulation schemes are more suited to the unique underwater acoustic communication channel than others. Some of the modulation methods used for UAC are as follows: • Frequency-shift keying (FSK) • Phase-shift keying (PSK) • Frequency-hopping spread spectrum (FHSS) • Direct-sequence spread spectrum (DSSS) • Frequency and pulse-position modulation (FPPM and PPM) • Multiple frequency-shift keying (MFSK) • Orthogonal frequency-division multiplexing (OFDM) • Continuous Phase Modulation (CPM) The following is a discussion on the different types of modulation and their utility to UAC. Frequency-shift keying FSK is the earliest form of modulation used for acoustic modems. FSK usually employs two distinct frequencies to modulate data; for example, frequency F1 to indicate bit 0 and frequency F2 to indicate bit 1. Hence a binary string can be transmitted by alternating these two frequencies depending on whether it is a 0 or 1. The receiver can be as simple as having analogue matched filters to the two frequencies and a level detector to decide if a 1 or 0 was received. This is a relatively easy form of modulation and therefore used in the earliest acoustic modems. However more sophisticated demodulator using digital signal processors (DSP) can be used in the present day. The biggest challenge FSK faces in the UAC is multi-path reflections. With multi-path (particularly in UAC) several strong reflections can be present at the receiving hydrophone and the threshold detectors become confused, thus severely limiting the use of this type of UAC to vertical channels. Adaptive equalization methods have been tried with limited success. Adaptive equalization tries to model the highly reflective UAC channel and subtract the effects from the received signal. The success has been limited due to the rapidly varying conditions and the difficulty to adapt in time. Phase-shift keying Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing (modulating) the phase of a reference signal (the carrier wave). The signal is impressed into the magnetic field x,y area by varying the sine and cosine inputs at a precise time. It is widely used for wireless LANs , RFID and Bluetooth communication. Orthogonal frequency-division multiplexing Orthogonal frequency-division multiplexing (OFDM) is a digital multi-carrier modulation scheme. OFDM conveys data on several parallel data channels by incorporating closely spaced orthogonal sub-carrier signals. OFDM is a favorable communication scheme in underwater acoustic communications thanks to its resilience against frequency selective channels with long delay spreads. Continuous phase modulation Continuous phase modulation (CPM) is a modulation technique, which is a continuous phase shift, where the phase of the carrier signal varies over time and avoids abrupt changes between successive symbols. This smooth phase trajectory reduces spectral side lobes. Reducing spectral side lobes increases the spectral efficiency of CPM and enables it to transmit data within a narrower bandwidth. Notable variants of CPM include minimum shift keying (MSK) and Gaussian minimum shift keying (GMSK), which uses a Gaussian filter to smooth out phase shifts. Since the underwater environment is highly scattered, it can cause multipath propagation and signal degradation. The CPM's continuous phase feature mitigates these effects and maintains signal integrity. Besides its high spectral efficiency helps make optimal use of limited bandwidth underwater. == Use of vector sensors ==