A report on activities on and around OFDM

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A report on activities on and around OFDM

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Abstract

Multi-carrier communications are currently in a state of very rapid development, both in the introduction of new techniques and in its application to a wide variety of channels and services. Orthogonal Frequency-Division Multiplexing (OFDM) is a bandwidth-efficient signaling scheme for wideband digital communications. With the development of modern digital signal processing technology, OFDM has become practical to implement and has been proposed as an efficient modulation scheme for applications ranging from modems, digital audio broadcast, to next-generation high-speed wireless data communications-WLAN. OFDM supports high data rate transmission which is otherwise limited by phenomena called inter-symbol interference (ISI) in the communication medium between transmitter and the receiver, The main aim of the paper is to discuss valuable components of the orthogonal frequency division multiplexing. A part of it is, use of guard insertion which further reduces effect of multipath, is described in this paper.

Abstract
Introduction
Multi path effect on high speed serial communication
OFDM implementation description using DFT
1. Transmitter implementation
2. Orthogonality concept
3. Receiver implementation
4. Block diagram of OFDM system
5. Guard interval concept
Quality improvement techniques
Conclusion and future work
References

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[...] One way to prevent ISI is to create a cyclically extended guard interval, where each OFDM symbol is preceded by a periodic extension of the signal itself as shown in fig The total symbol duration is Ttotal = Tg + T , where Tg is the guard interval and T is the useful symbol duration. When the guard interval is longer than the channel impulse response, or the multi-path delay, the ISI can be eliminated. However, the ICI, or in-band fading, still exists. [...]

[...] In multi-carrier transmission, the data-stream to be transmitted is split into multiple parallel datastreams of them is transmitted on a separate frequency (or sub-carrier). Each sub-carrier is modulated at a rate so low equivalently, has a symbol period so long) that dispersion does not cause a problem. More realistically, there will be reduced ISI on each sub-channel, which can be either tolerated or easily corrected. Thus, Multi-carrier transmission permits high data rates while maintaining symbol durations much longer than the channels' memory. [...]

[...] Bonek, “Irreducible error performance of a digital portable communication system in a controlled time-dispersion indoor channel,” IEEE J. Select. Areas Commun., vol pp. 1024- Leonard J. Cimini Jr., Larry J. Greenstein, AT&T Labsresearch Middletown, New Jersey Ahmad R.S.Bahai, Burton R. Saltzberg, Mustafa Ergen., “Multi-carrier Digital Communications -Theory and Applications of Second Edition, pages eBook ISBN: 0-387-22576- Weinstein S.B. and Ebert, P.M (1997), “Data transmission by frequency division multiplexing using the discrete Fourier transform”, IEEE transactions on communication technology, Vol No pp. 628- J. A. C. [...]

[...] orthogonal) if the carrier spacing is a multiple of 1 Ts Receiver Implementation The receiver can be implemented using DFT or FFT $ a [ k ] = D F T { X b [ n X b ( t ) = a [ k ] e j 2π k 0 t Ts k N If sampled at rate of Ts / N 1 N 1 $ a ] = N $ a N X n=0 b ] e a j2π nk / N N N n=0 m ] e N j2π n k N X b [ n ] = X b ( nTs / N ) = Now, K a [ k j 2π nk / N N X b [ = a [ k ] e j 2π nk / N = IDFT [ k k N N a N N $ a ] = a N m=0 $ a ] = a ] $ a ] e j2π n k N n=0 ] N δ k ] Equation 1 and 2 are equivalent if fT s = 1 This is the condition required for orthogonality and allows the use of the discrete Fourier transform (DFT) at both ends and thus the use of very efficient digital signal processing Orthogonality Concept The “orthogonal” part of the OFDM name indicates that there is a precise mathematical relationship between the frequencies of the carriers in the system and symbol period. [...]

[...] High rate data transmission and reception can be successfully implemented if the high rate signal to be split into multiple parallel low rate signals and each of them is transmitted on a separate frequency (or sub-carrier) To facilitate separation of the signals at the receiver, the carrier frequencies were spaced sufficiently far apart so that the signal spectra did not overlap. Empty spectral regions between the signals assured that they could be separated with readily realizable filters. The resulting spectral efficiency was therefore quite low. [...]

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