In electronics and telecommunication , the process of varying periodic wave forms called carrier signal with a signal which contains information to be transmitted called modulating signal is known as modulation. Generally it is the process of delivering the message signal. In modulation process, amplitude, frequency or phase of the carrier wave is varied in accordance with the baseband signal. Then it is transmitted by using a transmitter.
Modulation can be applied to electromagnetic signals such as RADAR, LASER, optics and computer networks and as morse code telegraphy and also as power line networking.
1.2 NEED FOR MODULATION
During base band transmission, it has many limitations like attenuation. In order to overcome the effect, modulation is needed. In this process, the base band signal is shifted from low to high frequency. It is proportional to the carrier frequency.
1.3ADVANTAGES OF MODULATION:
1.31. Antenna’s height will be reduced.
1.32. Mixing of any signals can be avoided.
1.33. The range of communication can be increased.
1.34. Possibility of multiplexing can be more.
1.35. Quality of reception can be improved.
1.31. Antenna’s height:
For radio signal transmission the antenna height should be the multiple of one fourth of wavelength.
1.32.Mixing of signals:
In case of without modulation, then all the baseband signals may have same range and all the signals get mixed together. In receiver end, it is difficult to separate from one another. But with the help of modulation, by using different carrier frequency it could occupy different slots in different channel. It helps in avoiding the mixing of signals.
1.33.Range of communication:
Usually, baseband signals have low frequency while transmission, it gets attenuated. But with the help of modulation the range of communication gets increased.
Two or more signals can be transmitted over the same channel this process is called multiplexing. With the help of modulation mixing of signals can be avoided, different signals can be transmitted simultaneously over the same channel.
1.35.Quality of reception:
With the help of modulation, noise effect can be reduced and the quality of reception can be improved.
1.4 INTRODUCTION TO OFDM
The new emerging and upcoming technology in the wireless communication is the OFDM. This is used mainly used for high data rates. The wireless communication such as wi- max, LTE , DVB etc have adopted the OFDM technique to obtain high data rate.
At present this technique has become favourable and most popular choice for wireless LAN’s. Cellular technologies.OFDM has been proved to be a good and well organized modulation technique for land base communicatio. Many efforts are put forth to make this OFDM technique a popular one in under water communication.
The under water channels are time varying multipath channels that causes intersymbol interference, inter carrier interference and fading. These are the hindrances that occur in the under water communication. The effect of time and frequency spreading poses a challenge in achieving high data rate in the under water. The reason for frequency spreading is the dynamic motion of the medium and the movement between the transmitter and receiver.
OFDM is a multi carrier modulation technique and has its application in the area of highly dispersive channel and this is highly preferable for high data rate in under water channels. The wide band channel is transformed into narrow band channels leading to an ever green communication in large delay speed channels. This technique allows the overlapping of subcarriers and hence the limited channel bandwidth can be efficiently utiliaed. Researches have revealed the application of OFDM for under water communication.
The paper “Orthogonal frequency division multiplexing” by M. A. Atmanand, V. Gowtham and so on explains the technique that can
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