Precoding by Pairing Subchannels to Increase MIMO Capacity With Discrete Input Alphabets

• Published in: IEEE transactions on information theory Vol. 57; no. 7; pp. 4156 - 4169
• Main Authors: Mohammed, S K, Viterbo, E, Yi Hong, Chockalingam, A
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01.07.2011; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Applied sciences; Codes; Coding, codes; Communication channels; Complexity theory; Condition number; Encoding; Exact sciences and technology; Information theory; Information, signal and communications theory; Joints; Matrix; MIMO; multiple-input multiple-output (MIMO); Multiplexing; Mutual information; OFDM; Optimization algorithms; orthogonal frequency division multiplexing (OFDM); precoding; Resource management; Signal and communications theory; singular value decomposition (SVD); TECHNOLOGY; TEKNIKVETENSKAP; Telecommunications and information theory; Alphabet; MIMO system; multiple-input multiple-output (MIMO); precoding; Power allocation; Optimization; Coding; Orthogonal frequency division multiplexing; Condition number; Pretreatment; orthogonal frequency division multiplexing (OFDM); Rotation angle; Mutual information; Singular value decomposition; singular value decomposition (SVD)
• ISSN: 0018-9448; 1557-9654; 1557-9654
• DOI: 10.1109/TIT.2011.2146050

We consider Gaussian multiple-input multiple-output (MIMO) channels with discrete input alphabets. We propose a non diagonal precoder based on the X-Codes in to increase the mutual information. The MIMO channel is transformed into a set of parallel subchannels using singular value decomposition (SVD) and X-Codes are then used to pair the subchannels. X-Codes are fully characterized by the pairings and a 2 × 2 real rotation matrix for each pair (parameterized with a single angle). This precoding structure enables us to express the total mutual information as a sum of the mutual information of all the pairs. The problem of finding the optimal precoder with the above structure, which maximizes the total mutual information, is solved by: i) optimizing the rotation angle and the power allocation within each pair and ii) finding the optimal pairing and power allocation among the pairs. It is shown that the mutual information achieved with the proposed pairing scheme is very close to that achieved with the optimal pre coder by Cruz et al., and is significantly better than Mercury/waterfllling strategy by Lozano et al. Our approach greatly simplifies both the precoder optimization and the detection complexity, making it suitable for practical applications.