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A report on activities on and around OFDM

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

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.

[...] 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. [...]

[...] Heiskala, OFDM Wireless LANs: A Theoretical and Practical Guide Indianapolis, IN: Sams Armstrong, J (1999) ?Analysis of new and existing methods of inter-carrier interference due to carrier frequency offset in IEEE transactions on communications, Vol 47, No.3, March, pp 365- M. Speth, S. A. Fechtel, G. Fock, and H. Meyr, ?Optimum receiver design for wireless broad-band systems using OFDM\?Part IEEE Trans. Commun., vol no pp. 1668-1677, Nov Costello, D.J., Costello Lin., Error Control Coding. Englewood Cliffs, NJ: Prentice-Hall Agrawal, D., Tarokh V., Naguip A., Seshadri N., Space-time coded OFDM for high data-rate wireless communication over wideband channels. [...]

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