What is OFDM and why is it used? Essay

INTRODUCTION
The OFDM is a special case of multicarrier modulation in which serial stream of data is divided in parallel and modulated by orthogonal subcarriers with partial overlapping frequency bands. As compare to the single carrier modulation with a narrow bandwidth the OFDM symbols have relatively long time duration that results in less complex equalizer which helps to perform the channel equalization in frequency domain. Due to the features of OFDM like transmission rate, bandwidth efficiency, less complex equalizer OFDM adopted as a major data transmission technique by many wireless communication standards. OFDM is developed to meet demands for higher data rate in communication.
The OFDM concept is based on spreading the data to be transmitted over a large number of carriers, each being modulated at a low rate. OFDM transmission system offers possibilities for alleviating many of the problems encountered with single carrier systems. In digital communications, information is expressed in the form of bits. The term symbol refers to a collection, in various sizes, of bits [1].OFDM data are generated by taking symbols in the spectral space using MPSK, QAM, etc, and convert the spectra to time domain by taking the Inverse Discrete Fourier Transform (IDFT). Because of Inverse Fast Fourier Transform (IFFT) is more cost effective to implement, it is usually used [2].OFDM is able to provide high data rate, allowable Bit Error Rate and maximum delay. OFDM is used to split the total transmission bandwidth into number of orthogonal subcarriers in order to transmit the symbol.
In OFDM Each carrier is modulated using BPSK / QPSK / Mary QAM, Frequency response for each carrier is a Sinc(X), Overlap of frequency response is possible as against FDM where intercarrier spacing is a must , frequency responses of the carriers overlap at zero crossings avoiding Inter Carrier Interference
In Contrast to FDM,the Spectral overlapping among subcarrier aloe in OFDM. It has the advantage of spreading out a frequency selective fade over a symbol. In OFDM equalization is simpler than a single carrier frequency and in differential encoding implement equalization may be avoided. In OFDM some processing is done on the source data such as coding for correcting data. By using IFFT in OFDM symbols are modulated on orthogonal sub carriers. By using FFT demodulation is to be done. In OFDM cyclic prefix is used to find the start of each frame.
Advantages of OFDM
Allows carriers to overlap (no guard band), resulting in lesser wasted bandwidth without any Inter Carrier Interference (ICI)
High data rate distributed over multiple carriers resulting in lower symbol rate (more immune to ISI)
Permits higher data rate as compared to FDM
Increased security and bandwidth efficiency possible using CDMA OFDM (MCCDMA)
Simple guard intervals make the system more robust to multipath effects
OFDM has multipath delay spread tolerance
Bitrates Transmissions are high.
Disadvantages of OFDM
OFDM is more complex than single carrier modulation
It requires more linear power amplifier
Peak to average power ratio is high in OFDM
Because of guard interval power Loss.