Orthogonal Frequency Division Multiplexing
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1. INTRODUCTION
1.1. Introduction:-
Orthogonal Frequency Division Multiplexing (OFDM) is one of the latest modulation techniques used in order to combat the frequency-selectivity of the transmission channels, achieving high data rate without inter–symbol interference. The basic principle of OFDM is gaining a wide spread popularity within the wireless transmission community. Furthermore,
OFDM is one of the main techniques proposed to be employed in 4th Generation Wireless Systems. Therefore, it is crucial to understand the concepts behind OFDM. In this paper it is given an overview of the basic principles on which this modulation scheme is based.
Due to the spectacular growth of the wireless services and demands during the last years, the need of a modulation technique that could transmit high data rates at high bandwidth efficiency strongly imposed. The problem of the inter–symbol interference (ISI) introduced by the frequency selectivity of the channel became even more imperative once the desired transmission rates dramatically grew up. Using adaptive equalization techniques at the receiver in order to combat the ISI effects could be the solution, but there are practical difficulties in operating this equalization in real-time conditions at several Mb/s with compact, low-cost hardware. OFDM is a promising candidate that eliminates the need of very complex equalization.
In a conventional serial data system, the symbols are transmitted sequentially, one by one, with the frequency spectrum of each data symbol allowed to occupy the entire available bandwidth. A high rate data transmission supposes a very short symbol duration, conducing at a large spectrum of the modulation symbol. There are good chances that the frequency selective channel response affects in a very distinctive manner the different spectral components of the data symbol, hence introducing the ISI [1]. The same phenomenon, regarded in the time domain consists in smearing and spreading of information symbols such, the energy from one symbol interfering with the energy of the next ones, in such a way that the received signal has a high probability of being incorrectly interpreted.
Intuitively, one can assume that the frequency selectivity of the channel can be mitigated if, instead of transmitting a single high rate data stream, we transmit the data simultaneously, on several narrow-band subchannels (with a different carrier corresponding to each subchannel), on which the frequency response of the channel looks “flat”. Hence, for a given overall data rate, increasing the number of carriers reduces the data rate that each individual carrier must convey, therefore lengthening the symbol duration on each subcarrier. Slow data rate (and long symbol
Sub channel index frequency. The frequency selective channel response duration) on each sub channel merely means that the effects of ISI are severely reduced.

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This is in fact the basic idea that lies behind OFDM. Transmitting the data among a large number of closely spaced subcarriers accounts for the “frequency division multiplexing” part of the name. Unlike the classical frequency division multiplexing technique, OFDM will provide much higher bandwidth efficiency. This is due to the fact that in OFDM the spectra of individual subcarriers are allowed to overlap. In fact, the carriers are carefully chosen to be orthogonal one another. As it is well known, the orthogonal signals do not interfere, and they can be separated at the receiver by correlation techniques. The orthogonality of the subcarriers accounts for the first part of the OFDM name.

OFDM Principles
In this section, the key points of OFDM are presented: the principles of a multicarrier (parallel) transmission, the usage of FFT and the cyclic prefix „trick”. We will also discuss the main challenges that this technique must deal with.
• The concept of multicarrier (parallel) transmission
In a mobile radio environment, the signal is carried by a large number of paths with different strength and delays. Such multipath dispersion of the signal is commonly referred as„ channel-induced ISI”and yields the same kind of ISI distortion caused by an electronic filter [2].
In fact, the multipath dispresion leads to an upper limitation of the transmission rate in order to avoid the frequency selectivity of the channel or the need of a complex adaptive equalization in the receiver. In order to mitigate the time-dispersive nature of the channel, the finding of the multicarrier technique was to replace a single-carrier serial transmission at a high data rate with a number of slower parallel data streams. Each parallel stream will be then used to sequentially modulate a different carrier. By creating N parallel substreams, we will be able to decrease the bandwidth of the modulation symbol by the factor of N, or, in other words, the duration of a modulation symbol is increased by the same factor. The summation of all of the individual sub channel data rates will result in total desired symbol rate, with the drastic reduction of the ISI distortion. The price to pay is of course very important, since the multicarrier transmission seems to act as a frequency multiplexation, which will generate problems in terms of bandwidth efficiency usage. The things go however better than seemed, because in OFDM the carriers are orthogonal to each-other and they are separated by a frequency interval of Δf=1/T. The frequency spectrum of the adjacent sub channels will overlap one another, but the carrier’s orthogonality will eliminate in principle the interchannel interference that we feared of.

• OFDM is a promising candidate for achieving high data rate transmission in mobile environment. The application of OFDM to high data rate mobile communication system is being investigated by many researchers.
• Cimini (1985) proposed a cellular mobile radio system based on OFDM used with pilot based correction. It was shown to provide large improvements in BER performance in a Rayleigh Fading Environment. Introduction of OFDM into cellular world has been driven by two main benefits
• Flexibility: each transceiver has access to all subcarriers within a cell layer.
• Easy equalization: OFDM symbols are longer than the maximum delay spread resulting in flat fading channel which can be easily equalized.
• OFDM benefited from considerable research interest from the military applications and it had been used in several high frequency military systems such as KINEPLEX, ANDEFT and KATHRYN (Zou and Wu, 1995).
• Also the introduction of Digital Audio Broadcasting (DAB) based on OFDM, successful test on OFDM for Digital Television Terrestrial Broadcasting (dTTb) and research on OFDM for HIPERLAN type II and Wireless ATM projects have increased the interested towards OFDM.
• For IMT-2000/UMTS, two OFDM air interface concepts, Band Division Multiple Access (BDMA) and OFDM by Telia, were presented to Study Committee (Ojanperä, 1998).
• Digital Satellite services are becoming popular and application of OFDM to the Satellite Mobile Channel was proposed by Fernando and Rajatheva (1998).
• OFDM used in DAB standard for CD- quality digital audio broadcast.
• One benefit of OFDM in a wireless communications system is that the receiver does not need to constantly adapt an equalizer as a single carrier system would. OFDM system shows much favorable properties such as high spectral efficiency, robustness to channel fading, immunity to impulse interference, capability of handling very strong echoes (multipath fading).
• The mobile radio channel suffers from multipath propagation and the channel is time variant, depending on the speed of the mobile station. Thus, Forward Error Correcting schemes should be used in OFDM system to improve the performance better. Application of suitable coding scheme to OFDM provides a diversity effect through exploitation of the multipath nature of the fading channel.
1.2. Neccesity of PAPR:
• There are some obstacles in using OFDM in transmission system in contrast to its advantages. A major obstacle is that the OFDM signal exhibits a very high Peak to Average Power Ratio (PAPR).
• Therefore, RF power amplifiers should be operated in a very large linear region. Otherwise, the signal peaks get into non-linear region of the power amplifier causing signal distortion. This signal distortion introduces intermodulation among the subcarriers and out of band radiation. Thus, the power amplifiers should be operated with large power back-offs. On the other hand, this leads to very inefficient amplification and expensive transmitters. Thus, it is highly desirable to reduce the PAPR.
• The other limitation of OFDM in many applications is that it is very sensitive to frequency errors caused by frequency differences between the local oscillators in the transmitter and the receiver.
• Carrier frequency offset causes a number of impairments including attenuation and rotation of each of the subcarriers and intercarrier interference (ICI) between subcarriers.

In the mobile radio environment, the relative movement between transmitter and receiver causes Doppler frequency shifts, in addition, the carriers can never be perfectly synchronized. These random frequency errors in OFDM system distort orthogonality between subcarriers and thus intercarrier interference (ICI) occurs. A Number of methods have been developed to reduce this sensitivity to frequency offset.