1. LDPC for MIMO Systems July 8, 2004 Jianuxan Du, du@merl.com
doc.: IEEE /xxxr0
LDPC for MIMO Systems
July 8, 2004
Jianuxan Du,
Daqing Gu,
Jinyun Zhang,
Mitsubishi Electric Research Lab, Cambridge, MA

2. Outline
Introduction
Quasi-block diagonal LDPC for MIMO systems with layered structure
Simulation comparison with convolutional codes
Simulation comparison with V-BLAST

3. Introduction Advantages of LDPC Capacity approaching performance
Parallelizability of decoding, suitable for high speed implementation
Flexibility: LDPC is simply a kind of linear block code and its rate can be adjusted by puncturing, shortening, etc.

4. Quasi-Block Diagonal LDPC Space-time Coding for Layered Systems
Feature: The encoding of different layers are correlated as compared with conventional V-BLAST.
Advantage: The space-time LDPC is designed such that the code can be decoded partially, but with the help of other layers (undecoded part) by the introduction of correlation between different layers.

5. Parity Check Structure of QBD-LDPC

6. Encoding of QBD-LDPC Encoding of QBD-LDPC
Qn-1 Hn= [Pn I] by Gaussian elimination.
The parity check bits for subcode n are given by Pnvn+ Qn-1vn-1 , where is vn the input information bit vector for subcode n, and vn-1 is derived from the subcode n-1.
With the given structure, the information about subcode n-1 is also contained in subcode n. Therefore, information from subcode n can help decoding subcode n-1.

7. Decoding of QBD-LDPC

8. Decoding of QBD-LDPC (Cont’)
The decoding is based on zero-forcing and interference cancellation, which is made possible by the lower-triangular structure of the parity check matrix.
The LLR’s of bits in successfully decoded subcodes are set to maximum or minimum value, depending on the output, to avoid ambiguity caused by the introduction of connection matrices.

9. Performance comparison with convolutional coding
172Mbps
64QAM
Code rate 0.6
Channel Model ‘F’

10. Performance Comparison with Conventional V-BLAST
72Mbps
64QAM
Code rate 0.5
Channel model ‘F’

11. Conclusion LDPC outperforms convolutional coding by about 2dB.
The simulated QBD-LDPC system outperforms conventional LDPC-coded V-BLAST by about 0.5dB.

12. References
[1] R. G. Gallager, Low-Density Parity-Check Codes. Cambridge, MA: MIT Press, 1963.
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[3] G. J. Foschini, “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas,” Bell Labs Technical Journal, pp , Aug
[4] S. Y. Chung, T. J. Richardson, and R. L. Urbanke, “Analysis of sum-product decoding of low-density parity-check codes using a Gaussian approximation,” IEEE Trans. Inform. Theory, vol. 47, pp , Feb
[5] P. Meshkat and H. Jafarkhani, “Space-time low-density parity-check codes,” Signals, systems and Computers, Conference Record of the Thirty-Sixth Asilomar Conference on, vol. 2, pp , Nov