Kun-Chien Hung

Pilot-Based LMMSE Channel Estimation for OFDM Systems With Power–Delay Profile Approximation

In linear-minimum-mean-square-error (LMMSE) channel estimation for multicarrier systems, one needs to know the channel correlation function. This poses a problem for systems with a small number of pilots operating in time-varying channels. We propose to approximate the channel power-delay profile (PDP) with a shape that can completely be described in two parameters, namely, the mean delay and the root-mean-square (RMS) delay spread. Furthermore, we develop a simple technique to estimate these delay parameters. The approximate PDP is then used to generate the LMMSE filter coefficients for data-subcarrier channel estimation. Mathematical expressions are derived that can be used to predict the accuracy of the various estimates, and they are verified by simulation. The proposed technique is applicable to both point-to-point communication and multiaccess communication where different users may experience different channel conditions. As a practical application, we also specialize the proposed technique to Mobile WiMAX signals and investigate the resulting performance.

Joint Detection of Integral Carrier Frequency Offset and Preamble Index in OFDMA WiMAX Downlink Synchronization

Initial downlink synchronization in orthogonal frequency-division multiple access (OFDMA)-based WiMAX transmission involves carrier frequency synchronization, timing synchronization, and preamble index identification. Due to the structure of the preamble signal, it is reasonable to take a two-stage approach in which one first estimates and compensates the timing offset and the so-called fractional carrier frequency offset (CFO) and then deals with the integral CFO and the preamble index. This paper considers joint detection of integral CFO and preamble index, under the assumption that timing and fractional CFO have been acquired to reasonable accuracy. Based on an optimization formulation, we derive a number of detection methods of different complexity and suboptimality. The methods exploit the quasi-orthogonality among the OFDMA WiMAX preamble sequences as well as the organization of the nonzero subcarriers in the preambles. Simulation results are presented to illustrate the performance of the methods.

Joint carrier recovery and multimodulus blind decision-feedback equalization under high-order QAM

The multimodulus algorithm (MMA) for blind equalization has good performance, but can only tolerate small carrier frequency offsets under high-order QAM. We propose a structure for joint carrier recovery (CR) and blind decision-feedback equalization (DFE), where the DFE is MMA-based. It turns out that a key point is the design and analysis of reduced-constellation phase detectors (PD) that can facilitate a wide frequency lock range and make the CR loop converge in severe intersymbol interference and noise. Based on this, we design a multistage startup procedure for joint blind DFE and CR. Simulations show that the procedure can yield fast convergence in large carrier frequency errors under high-order QAM.

A hybrid variable step-size adaptive blind equalization algorithm for QAM signals

We develop an adaptive decision-feedback equalization algorithm that combines blind adaptation and decision-directed LMS in a dynamic manner according to the amount of equalizer output error. By observing how the mean-square blind equalization error depends on the adaptation step size, we obtain a way of continuously varying the adaptation speed of the overall algorithm with the equalizer output error as well as a way to shift the relative emphasis between blind and decision-directed LMS operation. We also describe the way to estimate the amount of equalizer output error in the algorithm. Simulation results show that the proposed algorithm can achieve faster convergence at a lower complexity than some recently proposed hybrid adaptation algorithms

Optimal Delay Estimation for Phase-Rotated Linearly Interpolative Channel Estimation in OFDM and OFDMA Systems

We consider the phase-rotated linearly interpolative channel estimation technique for multicarrier transmission. The technique models the channel frequency response between two nearby subcarriers as the product of a linear function and a linear-phase factor, where the linear-phase factor may be equivalently modeled in the time domain as a reference delay dubbed the anchor delay in this work. We show that the performance of the technique is a fourth-order function of the channel path delays and the anchor delay. We derive a method to estimate the optimal anchor delay. Analysis and simulation in a context of Mobile WiMAX downlink transmission show that, with the proposed anchor delay estimate, we can attain better performance in channel estimation than conventional linear interpolation and a previously proposed method of phase-compensated linear interpolation.

Variable-step-size multimodulus blind decision-feedback equalization for high-order QAM based on boundary MSE estimation

We consider blind decision-feedback equalization (DFE) under high-order modulation, which presents more difficult operating requirements than lower-order modulation. We base our design on the multimodulus algorithm (MMA). To attain fast convergence speed and low steady-state mean-square error (MSE), we consider varying the adaptation step size according to the presently achieved MSE. For this we investigate the properties of the MSE under blind MMA-based DFE and, based on the results, propose a method to estimate its value. The estimate is obtained by analyzing those equalizer filter outputs whose values fall outside the boundary of the modulations constellation. Simulation results demonstrate the effectiveness of the proposed scheme.

Theory and design of near-optimal MIMO OFDM transmission system for correlated multipath Rayleigh fading channels

We consider channel-coded multi-input multi-output (MIMO) orthogonal frequency-division multiplexing (OFDM) transmission and obtain a condition on its signal for it to attain the maximum diversity and coding gain. As this condition may not be realizable, we propose a suboptimal design that employs an orthogonal transform and a space-frequency interleaver between the channel coder and the multi-antenna OFDM transmitter. We propose a corresponding receiving method based on block turbo equalization. Attention is paid to some detailed design of the transmitter and the receiver to curtail the computational complexity and yet deliver good performance. Simulation results demonstrate that the proposed transmission technique can outperform the conventional coded MIMO OFDM and the MIMO block single-carrier trans- mission with cyclic prefixing.

Jointly iterative channel estimation and data detection for cyclic-prefixed block transmission over time-varying channels

Various forms of cyclic-prefixed block transmission have been considered for signal transmission over multipath fading channels. Among them the most well-known is orthogonal frequency-division multiplexing, or OFDM. Whatever the form, such systems usually transmit many known pilots to assist channel estimation and data detection. We consider receiver techniques that can help improve the bandwidth efficiency by reducing the needed amount of pilots. In particular, we consider using only a block of pilots as the preamble. Then the receiver tracks the channel variation via jointly iterative channel estimation and data detection. It uses a frequency-domain equalizer to reduce the complexity. Simulations show that the tracking ability of the proposed method is good.

Pilot-Aided Multicarrier Wireless Channel Estimation via MMSE Polynomial Interpolation

Orthogonal frequency division multiplexing (OFDM) and multiple access (OFDMA) signal receivers often employ pilot-aided channel estimation. For it, an often considered technique is frequency-domain polynomial interpolation, due to its simplicity. However, the performance of polynomial interpolators suffers in channels with large delay spreads due to modeling error. The problem can be remedied by including a linear phase to the interpolator. In this paper, we derive a method to estimate the optimal phase shift that minimizes the mean-square channel estimation error. We further consider adaptive selection of the interpolation order for best performance. As a practical application, we adapt the proposed channel estimation technique to mobile WiMAX downlink transmission and examine the resulting performance.

LMMSE Channel Estimation for OFDM Systems with Low Number of Distributed Pilots

In linear minimum mean-square error (LMMSE) channel estimation for multicarrier transmission, how to estimate the needed channel correlation function remains a problem for systems with small number of pilots operating in time- varying channels. We consider approximating the channel power- delay profile by a shape characterized fully by the mean delay and the root-mean-square (RMS) delay spread. We present a novel, simple way to estimate these parameters in systems with distributed pilots. The ensuing channel estimation method is also applicable to multi-access communication. As a practical example, we also specialize the proposed technique to Mobile WiMAX signals and investigate the resulting performance. I. INTRODUCTION