Ibm Essay

Realization of OFDM based Underwater Acoustic Communication

VIJAY A R.SAHADEVAN SHELMA JACOB.K

PROFESSOR PROFESSOR M.TECH SCHOLAR [e-mail] [e-mail] [e-mail] Abstract: Nowadays underwater communication plays a vital role in applications from commercial extends to military purposes. It is necessary for the communication between UUVs,AUVs,divers etc. Present underwater communication systems involve the transmission of information in the form of sound, electromagnetic (EM), or optical waves.All these techniques has their own benefits and limitations.Acoustic communication is the most versatile and widely used technique in underwater environments due to the low attenuation (signal reduction) of sound in water.Acoustic

Mainly it uses the frequency range of 10Hz-1MHz.But as frequency increases the loss rate also increases. A sound wave propagating underwater consists of alternating compressions and rarefactions of the water. These compressions and rarefactions are detected by a receiver, such as the human ear or a hydrophone, as changes in pressure.

We have to take care about the underwater characteristics before designing an UWA system. They are heterogeneous characteristics of underwater, attenuation due to absorption, multipath fading due to reflection and refraction, loss due to Doppler effect and noises in underwater. Because of the heterogeneous characteristics the speed of sound in water is about 1500m/s . Soundspeed in water increases with increasing pressure, temperature and salinity. The maximum speed in pure water under atmospheric pressure is attained at about 74°C; sound travels slower in hotter water after that point; the maximum increases with pressure. Though some sounds can travel remarkable distance through the water, higher frequency sound is absorbed much faster than low frequency. The main cause of sound absorption is the presence magnesium sulphate and boric acid, and geometrical

The effect of noise on the signal is dependent on the signal to noise ratio (SNR), which can be formulated as.

SNR(d,f)=P/(A(d,f)N(f)∆f)

Where d is distance, f is the signal frequency and ΔF is the receiver noise bandwidth.The value of s denotes the shipping activity factor which normally ranges from 0 to1.Similarly windspeed is denoted by w. Since underwater is a frequency selective fading channel, multipath in underwater can be modeled as Rayleigh fading whose coefficients are random.

B OFDM in underwater

Single carrier transmission means one radio frequency carrier is used to carry the information. Hence information in the form of bits carried by one single RF carrier. In OFDM,also known as multicarrier modulation, uses multiple carriers sending some bits on each channel. In OFDM[8,17], all of the sub channels are dedicated to a data source. Multicarrier modulation especially OFDM is suitable for multipath environment as its carriers are orthogonal. As symbol period increases, the error rate is small compared with single carrier. Similarly the data rate is more for multicarrier modulation. But the problem of peak to average ratio (PAPR) arises in case of