LDPC for MIMO Systems July 8, 2004 Jianuxan Du, du@merl.com doc.: IEEE 802.11-02/xxxr0 LDPC for MIMO Systems July 8, 2004 Jianuxan Du, du@merl.com Daqing Gu, dgu@merl.com Jinyun Zhang, jzhang@merl.com Mitsubishi Electric Research Lab, Cambridge, MA

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

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.

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.

Parity Check Structure of QBD-LDPC

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.

Decoding of QBD-LDPC

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.

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

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

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

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