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A report on activities on and around OFDM

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  1. Introduction
  2. Multi-path effect on high speed serial communication
  3. Multi carrier transmission
  4. OFDM implementation using DFT
  5. Guard interval concept
  6. Quality improvement techniques
  7. Conclusion
  8. References

Multi-carrier communications are currently in a state of very rapid development, both in the introduction of new techniques and in 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.

[...] Use of data-aided technique is one applicable to many OFDM digital communication systems such as HDTV and Wireless LAN. Data aided techniques use a known bit pattern or pilot signal to estimate the timing or frequency offset. In short, all these practical issues need be properly handled in implementation of OFDM systems, which is crucial for the performance of a real system In almost all applications of multi-carrier modulation, satisfactory performance cannot be achieved without the addition of some form of coding. [...]

[...] 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. [...]

[...] 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 2? 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 j2? nk / N N N n=0 m ] e N j2? n k N X b [ n ] = X b ( nTs / N ) = Now, K a [ k j 2? nk / N N X b [ = a [ k ] e j 2? nk / N = IDFT [ k k N N a N N $ a ] = a N m=0 $ a ] = a ] $ a ] e j2? n k N n=0 ] N ? 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. [...]

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