1. An AWGN Multiaccess Channel*
Ballard Blair
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. AWGN Multiaccess channel - Ballard Blair
Motivation
Channel R T1 T2 T3 TN
Multiaccess Network
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
5/10/2007
AWGN Multiaccess channel - Ballard Blair

3. AWGN Multiaccess channel - Ballard Blair
AWGN Channel Setup x R T1 T2
Gaussian Multiaccess Setup
Coding z w
y=w+x+z
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)
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AWGN Multiaccess channel - Ballard Blair

4. AWGN Multiaccess channel - Ballard Blair
Achievable Rates R1 R2
I(X;Y)
I(W;Y)
I(X;Y|W)
I(W;Y|X)
Constraints
2-Parallel AWGN Channels
5/10/2007
AWGN Multiaccess channel - Ballard Blair

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

6. AWGN Multiaccess channel - Ballard Blair
Conclusions
AWGN Multiaccess
Interesting, not practical
Better Models needed (other than power lim)
Modern Approaches
CDMA
OFDM
5/10/2007
AWGN Multiaccess channel - Ballard Blair

7. AWGN Multiaccess channel - Ballard Blair
Questions?
5/10/2007
AWGN Multiaccess channel - Ballard Blair