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An AWGN Multiaccess Channel*

An AWGN Multiaccess Channel*. Ballard Blair 6.441 - Transmission of Information Massachusetts Institute of Technology May 10, 2007. *Paper: Gallager, R, A Perspective on Multiaccess Channels , IEEE transactions on Information Theory, March 1985. T 1. T 2. Channel. R. T 3. T N.

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An AWGN Multiaccess Channel*

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  1. An AWGN Multiaccess Channel* Ballard Blair 6.441 - Transmission of Information Massachusetts Institute of Technology May 10, 2007 *Paper: Gallager, R, A Perspective on Multiaccess Channels, IEEE transactions on Information Theory, March 1985

  2. T1 T2 Channel R T3 TN Multiaccess Network Motivation • Generalization of point to point • Transmitter not always ready • Applicable to many situations • Cell phones • Ethernet • Encoding Easy (Random), Decoding Hard • Conclusion: Need better models and approaches AWGN Multiaccess channel - Ballard Blair

  3. x T1 Coding R y=w+x+z w T2 Coding z Gaussian Multiaccess Setup AWGN Channel Setup Types of Errors • Only X decoded incorrectly (type 1) • Only Y decoded incorrectly (type 2) • X and Y decoded incorrectly (type 3) Capacity of AWGN Channel: • Energy Constraint for each transmitter (S1 and S2) AWGN Multiaccess channel - Ballard Blair

  4. R2 I(W;Y|X) I(W;Y) I(X;Y) I(X;Y|W) R1 Achievable Rates Constraints 2-Parallel AWGN Channels AWGN Multiaccess channel - Ballard Blair

  5. Higher Dimensionality R3 R1 R2 3 Transmitters (7 constraints) Constraints quickly get out of hand (2n-1 constraints for n-dim) AWGN Multiaccess channel - Ballard Blair

  6. Conclusions • AWGN Multiaccess • Interesting, not practical • Better Models needed (other than power lim) • Modern Approaches • CDMA • OFDM AWGN Multiaccess channel - Ballard Blair

  7. Questions? http://www.ukuleleman.net/2005_08_01_ukemanspeaks_archive.html AWGN Multiaccess channel - Ballard Blair

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