Virasit Imtawil

Approaching capacity of large MIMO systems by non-binary LDPC codes and MMSE detection

In this paper, we have investigated the application of non-binary LDPC codes to spatial multiplexing MIMO systems with a large number of low power antennas. We demonstrate that such large MIMO systems incorporating with low-complexity MMSE detector and non-binary LDPC codes can achieve low probability of bit error at near MIMO capacity. The new proposed non-binary LDPC coded system also performs better than other coded large MIMO systems known in the present literature. For instance, non-binary LDPC coded BPSK-MIMO system with 600 transmit/receive antennas performs within 3.4 dB from the capacity while the best known turbo coded system operates about 9.4 dB away from the capacity. Based on the simulation results provided in this paper, the proposed non-binary LDPC coded large MIMO system is capable of supporting ultra high spectral efficiency at low bit error rate.

Achieving Near Capacity of Non-Binary LDPC Coded Large MIMO Systems with a Novel Ultra Low-Complexity Soft-Output Detector

Recently, it has been proved that both MMSE and MF detectors are near optimal detection for large scale MIMO systems, e.g., MIMO systems with hundreds of antennas. In order to attain near capacity region with reasonable complexity, low-complexity detector with soft-output generation is necessary for coded large MIMO systems. We show in this paper that the non-binary LDPC codes and well-known soft-output MMSE detector can be utilized to significantly reduce the gap to capacity. We also propose a novel soft-output MF-based detector for the non-binary LDPC coded large MIMO systems. With this proposed detector, capacity approaching performance, i.e., the gap to capacity of 1.6 dB, can be achieved with ultra low-complexity detection, e.g., just 0.28% of MMSE detection. Moreover, use of the proposed scheme in large MIMO systems is found to be robust to the presence of imperfect channel estimation and spatial fading correlation which are both the realistic scenarios for large MIMO systems.

Ultra low complexity soft output detector for non-binary LDPC coded large MIMO systems

The theoretic results of MIMO capacity tell us that the higher the number of antennas are employed, the higher the transmission rate is. This makes MIMO systems with hundreds of antennas very attractive but one of the major problems that obstructs such large dimensional MIMO systems from the practical realization is a high complexity of the MIMO detector. We present in this paper the new soft output MIMO detector based on matched filtering that can be applied to the large MIMO systems which are coded by the powerful non-binary LDPC codes. The per-bit complexity of the proposed detector is just 0.28% to that of low complexity soft output MMSE detector and scales only linearly with a number of antennas. Furthermore, the coded performances with small information length 800 bits are within 4.2 dB from the associated MIMO capacity.

A novel structure of high rate (3,L) regular QC-LDPC codes for magnetic recording channels

The novel structure of high rate (3,L) regular QC-LDPC codes based on circulant permutation matrices are presented in this paper. The parity-check matrix of the proposed code consists of permutation matrices, their transpose matrices and identity matrices. With this new method, the proposed code is free from girth 4, girth 6, and sometimes girth 8. The construction of parity-check matrix of the proposed codes easily is obtained comparing to those of previously found (3,L) regular QC-LDPC codes. Simulation results on magnetic recording channels show that the BER performance of the proposed codes is very closed to that of other (3,L) regular QC-LDPC codes.

Design of high-rate LDGM codes

Low density generator matrix to construct low-density parity check (LDPC) codes is designed in this paper. The proposed structure significantly reduces the encoding complexity of LDPC codes. Codes in this class can be easily constructed by concatenating the parity-check matrix of array low-density parity-check (ALDPC) codes with the identity matrix and also contains no short cycle of length four. The results shown in this paper indicate that these codes can perform very well at high code rate (R ≥ 0.85) under iterative decoding with no serial or parallel concatenation of two codes.

A detection technique for high order QAM in the presence of transmitter angular skew

Angular skew arising from imperfect biasing of the transmitter can cause a significant performance degradation in high speed optical QAM systems. A simple approach to detect QAM signal under the effect of angular skew is Gram-Schmidt orthogonalization procedure (GSOP). However, this approach has poor performance when the size of QAM constellation exceeds 256. To solve this problem, a new detection algorithm is proposed. From the results, the proposed technique outperforms the GSOP. It provides the skew tolerance of up to 25-degrees at BER of 10-5 for the expense of 1 dB Eb/No penalty.

LDPC Coded 2m-ary QAM for Ultra High Speed Communications

Modern communication systems that employ LDPC codes and QAM modulation can achieve high speed and reliable transmission without bandwidth expansion. In this paper, the performances of LDPC codes with 2m- ary QAM at m>10, which can provide enormous bandwidth efficiency, are studied. Bit error rate performances of various combinations of LDPC codes and order of QAM are provided. From the results, the performance of LDPC coded ultra-high order QAM is excellent making this system very attractive for ultra-high speed communications.

Low-complexity high-rate irregular QC-LDPC codes with applications to PR2 and EPR2 channels

In this paper, we propose a modified high rate 3,L quasi-cyclic low-density parity-check QC-LDPC code based on circulant permutation matrices for efficient encoder implementation. The structure of the code is modified from a 3,L regular QC-LDPC code to be an approximate lower triangular matrix and a linear independent form. We present two novel efficient encoding techniques for generating redundant parity bits. The complexity of the encoder implementation depends on the number of redundant parity bits of the code for one-stage encoding and the length of the code for two-stage encoding. The advantage of both encoding techniques is that few XOR gates are used in encoder implementation. The simulation results show that the proposed codes at code rate 0.884 and 0.953 have good bit error rate performance compared with other LDPC codes over PR2 and EPR2 channels. European Transactions on Telecommunications. Copyright

Code design for very noisy relay channels

From an information-theoretic point of view, it is well known that the capacity of relay channel comprising of three terminals is much more than that of two terminal direct channel especially for low SNRs. Previously invented relay coding strategies have not been designed to achieve this enormous capacity occurring in the low SNR region. In this paper, we propose a simple coding strategy for a relay channel with low SNRs or, equivalently, for a very noisy relay channel. The multiplicative repetition is utilized to design this simple coding strategy. We claim that the proposed strategy is simple since the destination and the relay can decode with almost the same computational complexity by sharing the same structure of decoder. An appropriate static power allocation which yields the maximum throughput close to the optimal one in low SNRs is suggested. Under practical constraints such as equal time-sharing etc., the asymptotic performance of this simple strategy is within 0.5 dB from the achievable rate of relay channel. Furthermore, the performance at short code lengths enjoys a relaying gain by approximately 1.4 dB.

Construction of irregular structured LDPC codes with low encoding complexity for high-rate applications

This paper presents a method to construct irregular structured Low-Density Parity-Check (LDPC) codes based on choosing different identity matrices. The parity-check matrix of the proposed code can be easily generated with the flexible parameters such as code rate, block length or sparsity. The proposed code is well structured which can provide a low-complexity implementation. The encoder of the proposed code has linear encoding complexity making it suitable for hardware implementation. When iteratively decoded with the sum-product algorithm (SPA), the constructed irregular LDPC codes exhibit a very good performance especially for high rate codes.