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FEC (Forward Error Control) is Not Overhead

Learn about forward error control in communication systems and its impact on data transmission. Explore examples of overhead, redundancy, and the design principles of FEC. Discover how FEC helps with error correction and signal robustness in communication channels.

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FEC (Forward Error Control) is Not Overhead

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  1. FEC(Forward Error Control)is Not Overhead Chris Heegard&Matthew B. Shoemake Alantro Communications

  2. Overhead • Subtracts from the primary goal of transmission: • reliable conveyance of user information (messages from transmitter to receiver) Data Data Data Chris Heegard & Matthew Shoemake, Alantro Communications

  3. Examples of Overhead • a frame synchronization in a packet system • a packet preamble • a pilot tone • These functions help with higher level objectives or for estimation and tracking of channel parameters Chris Heegard & Matthew Shoemake, Alantro Communications

  4. Redundancy • Adds to the primary goal of transmission • reliable conveyance of user information Data Data Data Chris Heegard & Matthew Shoemake, Alantro Communications

  5. The Distinguishing Feature • Overhead is independent of the data • It helps with auxiliary functions, yet subtracts from resources • Power • Bandwidth • Data Rate • Redundancy is a function of the data • It help with the primary function, it enhances the utilization of resources Chris Heegard & Matthew Shoemake, Alantro Communications

  6. FEC is considered as “overhead” when applied as a “Band-Aid” • A communications system is designed without coding, it is not robust enough • An ECC (error control code) is used to “fix” the system • improvement of reliability • loss in rate (“overhead”) • This approach is a mistake in the design of a fresh system! Chris Heegard & Matthew Shoemake, Alantro Communications

  7. How to Correctly Design an FEC • Fix a data rate (user rate) R • Define a large signal set that generates more than 2R*T signals in T seconds • Judiciously select a subset of 2R*T signals from the large set • Maximize noise immunity • Keep distinct signal well separated (Energy) Chris Heegard & Matthew Shoemake, Alantro Communications

  8. The “Overhead” of QPSK as a Coding of QAM • Rate = 2 bits/symbol • Overhead = 2 bits/symbol ??? Chris Heegard & Matthew Shoemake, Alantro Communications

  9. Uncoded Modulation • Break data stream into small pieces • map onto independent dimensions • Noise occasionally causes symbol error ==> data error Data: Symbols: Chris Heegard & Matthew Shoemake, Alantro Communications

  10. Coded Modulation • Have each bit of data affect many symbols • Average out the noise with the decoding Data: Symbols: Chris Heegard & Matthew Shoemake, Alantro Communications

  11. Making Signals Robust • Maximize energy that differentiates distinct signals • AWGN (Additive White Gaussian Noise) Chris Heegard & Matthew Shoemake, Alantro Communications

  12. Robust Signals • Multipath • It is intuitively correct that signals that have large energy separation at the input of a filter are better distinguishable at the output of the filter. Filter Chris Heegard & Matthew Shoemake, Alantro Communications

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