My recent research interests include Noncoherent communication, Channel-coded Communication Systems, Cooperative Communication, Large-scale MIMO System, Non-orthogonal Multiple Access, and Biometrics.
The detailed publication list can be found in the following page : http://mx.nthu.edu.tw/~ymchen/publications.htm


The main contributions in some of the topics are described in the following.

1. Noncoherent communication:

In wireless links, the variation of channel environments can be explicitly modeled by time-varying amplitudes and carrier phase rotations of channel coefficients. For systems with unknown channel coefficients to both the transmitter and the receiver, non-coherent detection is an attractive technique, since it does not require the channel estimation procedures. However, the noncoherent detection methods provided in literature usually suffer from high computational complexities, most of which are exponentially proportional to the number of conveyed information bits. To overcome this major drawback of noncoherent detection, we firstly focused on the design of low-complexity noncoherent detectors. Noncoherent detectors with very low detection complexities were proposed respectively for both the single-input single-output (SISO) and the multiple-input multiple-output (MIMO) noncoherent communication schemes, with only limited losses in bit-error-rate performances.

Secondly, coded centralized/distributed MIMO schemes for non-coherent fast fading channels, where neither the transmitter nor the receiver knows the channel state information, were proposed. The coded bits are transmitted through either unitary space-time modulation (USTM) or spatial multiplexing (SM). Since this approach effectively increases the cardinality of the set of possible transmit (space-time) signal matrices, a large rate gain can be obtained without increasing the modulation order, as shown through achievable-rate analysis. At the receiver, an iterative detection-decoding algorithm is executed cooperatively among the decoder, the coherent detector used for SM, and the non-coherent detector used for USTM. Compared to the scheme described in the literature, the proposed scheme, in some cases, can provide better error performance with lower complexity.

Lastly, in the recently published research results, we used high-order modulation signals to construct coded non-coherent transmission schemes. Considering amplitude/phase modulated signals, the theoretical analyses of bit error rates were investigated, and the design criteria for the noncoherent signal constellations as well as the bit labeling rules were derived for both the Rayleigh block fading channels and the correlated Rayleigh fading channels, respectively. We also optimized the system parameters such that near-capacity performances can be achieved for both channels.

2. Channel-coded communication systems:

In the research field of channel-coded communication systems, we firstly proposed a method that can be used to approach the capacity limit for channels with fast phase variation by using a serial concatenation scheme with an outer channel encoder and an inner block modulator. A distinct feature of the proposed transmission scheme is that coded symbols from adjacent codewords are interleaved into the same input vectors of the block modulator. It was shown that near-capacity performances can be achieved using the proposed concatenated schemes. On the other hand, when considering convolutional codes as the outer channel codes, design criteria for the generator polynomials of the convolutional codes were also derived through the bit-error-rate analysis of the entire coded system.

Secondly, it is known that low-density parity-check (LDPC) codes are widely used to provide capacity-approaching performances in various transmission systems. For the design of LDPC-coded transmission schemes, the extrinsic information transfer (EXIT) chart is a commonly used tool for the performance optimization. However, the Gaussian assumption for soft messages in conventional EXIT-chart-based optimization algorithms result in prediction gaps in the decoding threshold for certain transmission systems. Consequently, we propose an EXIT-chart-based optimization method, in which all the Gaussian assumptions for soft messages can be avoided. Compared to the conventional design methods, the proposed method provides a more accurate threshold prediction and can be used to designed LDPC-coded schemes with better error performances.

Lastly, efficient scheduling methods for the decoding algorithm of LDPC-like channel codes were also proposed to improve the converged decoding threshold and enhance the throughput performance of the coded schemes. Moreover, considering noncoherent distributed space-time modulations, code design algorithms for the outer Raptor codes were investigated in the recent research results.

3. Cooperative communication and large-scale MIMO system:

Firstly, several full-duplex cooperative coherent communication systems for Rayleigh fading channels using rateless codes were investigated based on the decode-and-forward (DF), the compress-and-forward (CF), and the adaptive CF/DF (ACDF) relaying protocols. Secondly, we also proposed two Raptor-coded noncoherent cooperative schemes based on distributed unitary space-time modulation schemes. Considering a wireless relay network with one source node, one destination node, and multiple relay nodes, not only the noncoherent distributed space-time signal sets were designed, but also the relaying protocols were investigated. In addition, aiming at enhancing the convergence speed and the data throughput, a joint code search and signal design algorithm is investigated for the proposed noncoherent cooperative schemes. The related research works are supported by the Ministry of Science and Technology of Taiwan under Grants MOST 104-2218-E-110-012-MY2

Secondly, we have proposed a coded low-complexity massive MIMO scheme considering the generalized spatial modulation in large-scale MIMO systems. Reductions in detection complexity of 99.46% can be achieved when compared to the scheme which uses the ML detector, when considering the 16QAM signal. Moreover, an LDPC-coded GSM scheme that uses the designed CASD detector has been constructed. The variable-node degree distributions of suitable LDPC codes have been designed in order to achieve an error performance which is very close to that of the scheme using the conventional MAP detector, but with a much lower complexity level.

Lastly, a cooperative single-carrier transmission scheme, which considers a distributed quasi-orthogonal space-frequency block code (QOSFBC) that provides improved peak-to-average power ratio (PAPR) values, is constructed for uplink communications. Aiming at reducing the size and computational complexity, the source node is equipped with two transmit antennas, while only one antenna is equipped at the relay node to collaboratively generate a distributed QOSFBC signal with four transmit antennas. To achieve full diversity, a phase rotation strategy is adopted at the relay node. In addition, since single-carrier schemes which consider SFBCs re-permute the spectral elements of the transmitted signals and induce high PAPR values, a modified SFBC coding strategy is proposed to mitigate the high-PAPR problem. Consequently, the constructed distributed single-carrier frequency-division multiple access (SC-FDMA) QOSFBC scheme is capable of providing full transmit diversity over frequency-selective fading channels, with a much lower PAPR value on mobile devices when compared to a conventional SC-FDMA QOSFBC scheme.