Yan Wu

A novel iterative receiver for coded MIMO OFDM systems

We propose a novel iterative receiver for coded multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. Starting from the derivation of the optimal minimum mean squared error (MMSE) estimate of the transmitted data, we show that for BPSK and QPSK modulated signals, the soft estimate is a hyperbolic tangent function of the extrinsic information from the soft-input soft-output (SISO) decoder and the decision statistic from the detector. Compared with the conventional iterative receiver which computes the data estimate using only the decoder output, the proposed structure has significant performance gain, for example, it is 2.0 dB better for groupwise space time block coded (GSTBC) MIMO OFDM system at BER of 10/sup -6/, and 1.5 dB better for vertical Bell Lab layered space-time (VBLAST) OFDM system at BER of 10/sup -5/.

Precoding for Asymmetric MIMO-OFDM Channels

We study space-time precoding techniques for asymmetric multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) channels with more transmit than receive antennas which is a typical downlink setup. Under the condition of perfect channel state information (CSI) at the receiver but no CSI at the transmitter, we consider linear dispersion coding (LDC), groupwise space-time block coding (GSTBC), quasi-orthogonal space-time block coding (QSTBC), and spatial spreading (SS) and study their capacity and diversity performances. Our results show that for both ergodic and outage capacities, SS is worse than LDC, GSTBC and QSTBC, while the latter three have very close/same performance. We also show that by adjusting the phase shift values in the SS matrices, its outage capacities can be improved. In terms of diversity, LDC, GSTBC and QSTBC have both transmit and receive spatial diversity, and LDCs diversity order is slightly higher than the latter two. SS, on the other hand, has not only receive spatial diversity, but also introduces additional frequency diversity. This contributes to their very close bit error rate (BER) performance when turbo processing is deployed at the receiver. Considering the implementation flexibility and complexity, we therefore recommend SS and GSTBC as the promising precoding schemes for asymmetric MIMO-OFDM channels.

Performance analysis of MIMO system with serial concatenated bit-interleaved coded modulation and linear dispersion code

The upper bound on bit error rate (BER) of a multiple-input multiple-output (MIMO) system with the serial concatenated bit-interleaved coded modulation (BICM) and linear dispersion code (LDC) is derived in this paper. This bound is evaluated based on the expurgated bound of BICM and the approach of moment generation function (MGF). Compared with simulated performance of the 16QAM Alamouti code (AC) and optimal QPSK LDC, this bound is tight in the range of medium to high signal-to-noise ratio (SNR). By utilizing the bound, we obtain the optimal choice of code rate and modulation size for fixed transmission rates in BICM-LDC MIMO system.

Application of iterative detector and decoder in serial concatenated coded and pre-transformed systems

The iterative detector and decoder (IDD) in the serial concatenated coded and pre-transformed system is studied in this paper. The performance of the IDD in the single-carrier system over flat fading channel and the orthogonal frequency division multiplexing (OFDM) system over frequency-selective fading channel are thoroughly evaluated. Numerical results show that in the single-carrier system, IDD with 2 to 3 iterations over flat fading channel achieves performance only about 1dB away from that over the additive white Gaussian noise (AWGN) channel. Our study also finds that for OFDM system, the IDD provides considerable performance improvement compared to the coded or pre-coded systems, even when the multipath diversity is low. IN the meanwhile, the excellent performance of IDD does not rely on the optimal design of pre-transformation matrix.

A Bayesian MMSE Turbo Receiver for Coded MIMO Systems

In conventional interference cancellation (IC)-based turbo receivers, extrinsic information (EXT) from the soft-input-soft-output (SISO) decoders is used to compute the statistical mean (SM) of the interfering signals. In this paper, we present a class of Bayesian minimum mean-square-error (MMSE) turbo receivers for coded multiple-input-multiple-output (MIMO) systems. Instead of using the EXT to estimate the prior SM, we use it in the posterior Bayesian MMSE estimation of the interfering signals. The estimation accuracy is, therefore, improved, leading to better bit-error-rate (BER) performance from our proposed Bayesian receivers. For the cases that we have studied, the proposed receiver with two iterations has a lower BER than the conventional one with five iterations, thus significantly reducing the receiver processing delay and the implementation complexity. Its superior performance is also demonstrated by the higher detector output mutual information and fewer iterations to achieve convergence, which is obtained from the extrinsic information transfer (EXIT) chart analysis.

A Two-Dimensional Linear Pre-Transformed (2DLPT) MIMO-OFDM System

We propose an open-loop two-dimensional linear pre-transformed (2DLPT) multiple-input multiple- output (MIMO) orthogonal frequency division multiplexing (OFDM) system. The <i>n</i> _T <i>N</i> times <i>n</i> _T <i>N</i> 2DPLT, with <i>n</i> _T the number of transmit antennas and N the FFT size in the OFDM modulation, is applied in both spatial and frequency domains and is unitary in individual as well as both domains. While the spatial domain transform converts the transmit diversity from the other transmit antennas to frequency diversity, the frequency domain transform fully exploits the frequency domain diversity made available. More over, the unitary transform guarantees the same ergodic capacity as the non-transformed spatial division multiplexing MIMO-OFDM channel. Therefore, the proposed 2DLPT MIMO-OFDM system achieves full diversity of <i>n</i> _T <i>n</i> _R <i>n</i> _L with <i>n</i> _R and <i>n</i> _L representing the number of receive antennas and the number of multipaths in each transmit-receive antenna pair channel, and full capacity.

EXIT chart analysis of bayesian MMSE turbo receiver for coded MIMO systems

The Bayesian minimum mean squared error (MMSE) turbo receiver was proposed in (1) for coded multiple input multiple output (MIMO) systems to exploit both the soft- input soft-output (SISO) decoders extrinsic information and the detector decision statistic in calculating the Bayesian MMSE estimate of the interference, and simulation results showed about 1.5 dB improvement over the conventional turbo receivers at bit error rate (BER) of 10 �5 (1). In this paper, we present the extrinsic information transfer (EXIT) chart analysis of Bayesian MMSE turbo receiver and show that it has much higher output mutual information than the conventional turbo receivers. The EXIT chart also shows that significantly less number of iterations are required to achieve convergence when compared with the conventional receiver. I. INTRODUCTION Multiple-input multiple-output (MIMO) wireless communi- cation systems have attracted a lot of interest in both research community and industry due to its potentials in delivering high spectral efficiency and robust performance against fading. In order to fully exploit the capacity and diversity advantages of MIMO channels, advanced signal processing techniques have to be applied at the transmitter and receiver. In particular, a turbo receiver which exchanges soft information between the