Hung-Chin Wang

Optimized Joint Fine Timing Synchronization and Channel Estimation for MIMO Systems

This paper develops a joint fine timing synchronization and channel estimation scheme for multiple-input multiple-output (MIMO) systems based on a well-designed training sequence arrangement. The proposed approach generalizes a first channel tap selection scheme and presents a majority vote refinement (MVR) of the timing estimates for different transmit-receive links. The MVR fully exploits the features of the training sequence arrangement and the MIMO diversity to improve timing synchronization as well as channel estimation. In addition, an optimal threshold for the proposed first channel tap selection scheme is derived based on a Rayleigh channel assumption. The optimal solution clearly reveals how the threshold should react to the channel statistics, the signal-to-noise ratio, the number of active transmit-antennas, and other design parameters. Based on this, a practical suboptimal threshold that requires neither pre-simulation nor prior information on the channel statistics is then proposed. Also, the performance of the proposed approach is analyzed with respect to threshold selection and MVR, and the theoretical results agree well with the simulation results in several channel models.

On joint fine time adjustment and channel estimation for OFDM systems

This paper addresses training-sequence-based joint fine time adjustment and channel estimation for orthogonal frequency-division multiplexing systems. The proposed approach first derives an optimized threshold that adapts to the channel statistic and the signal-to-noise ratio. Then a practical suboptimal threshold is presented to avoid the estimation of the channel statistic. The suboptimal threshold requires no pre-simulations and performs as well as the optimal one. For performance analysis, the probability of perfect fine time adjustment given a particular threshold is also derived. The theoretical results agree well with the simulation results in both Rayleigh and Ricean channels.

A Compact Preamble Design for Synchronization in Distributed MIMO OFDM Systems

In distributed multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems, signals arrive at the receiver on different timing and are characterized by distinct carrier frequency offsets (CFOs), which makes synchronization rather challenging than that associated with centralized MIMO systems. Current solutions to this problem are mainly based on special preamble designs, where different training sequences are cascaded and then separately used to assist timing synchronization and CFO estimation. Such preamble designs not only increase system overhead but also burden the receivers with independent algorithms for timing synchronization and CFO estimation. In this paper, we propose a low-overhead (compact) preamble having the same length as one OFDM symbol, along with a unified algorithm for both timing synchronization and CFO estimation. Furthermore, the CFO estimation range can be flexibly extended to cope with larger CFOs in the proposed approach. Under the same training overhead and power consumption, simulation results indicate that the proposed approach outperforms a timing synchronization scheme that based on unequal period synchronization patterns.

A Low-Complexity Joint Time Synchronization and Channel Estimation Scheme for Orthogonal Frequency Division Multiplexing Systems

In this paper, a novel training sequence structure along with a joint time synchronization and channel estimation scheme for orthogonal frequency division multiplexing (OFDM) systems is proposed. The proposed training sequence structure is composed of an orthogonal sequence attached by cyclic prefix and cyclic postfix. Both the lengths of the cyclic extensions attached are chosen to be longer than the maximum delay spread of the wireless channel. Based on the unique structure of the training sequence, we present a low-complexity joint time synchronization and channel estimation scheme. Unlike conventional methods, which require either discrete Fourier transform (DFT) operations or matrix inversions to acquire channel impulse response (CIR) information, the proposed scheme requires only a correlator and a comparator. At the expense of the same training sequence overheads, computer simulation results show that the proposed joint scheme outperforms conventional time synchronization methods by orders. Furthermore, the overall bit error rate (BER) performance of the proposed joint scheme is shown to be close to the ultimate bound characterized by perfect synchronization and channel estimation. The proposed joint scheme also provides significant BER performance gain as compared to a scheme that properly combines a conventional synchronization method with a DFT-based channel estimation method.

A New Synchronization Scheme for OFDM-Based Cooperative Relay Systems

In cooperative relay systems, relayed signals propagate through different channels and are received at the destination node with distinct timing and carrier frequency offsets (CFOs). This feature makes synchronization a rather challenging task as compared with centralized multiple-input multiple-output systems. Unlike conventional preamble designs with good correlation properties only in time domain or frequency domain, a new preamble that has good correlation properties in both time domain and frequency domain is proposed for synchronization in orthogonal frequency-division multiplexing (OFDM) based cooperative relay systems. According to the proposed preamble, a synchronization scheme is developed and a practical threshold is derived that jointly considers the statistic distribution of the correlation function output and the inevitable interference originated from residual CFOs to assist fine timing synchronization. Simulation results verify that the proposed scheme provides notable performance improvement as compared with a previous related work.

An Optimized Joint Time Synchronization and Channel Estimation Scheme for OFDM Systems

This paper proposes a new joint time synchronization and channel estimation scheme based on a conventional training sequence. The training sequence is composed of a maximal length sequence attached by both cyclic prefix and postfix. By analyzing the probability density functions of the corresponding correlator outputs at different time indexes, an optimal threshold is derived to distinguish the correct time index from its preceding ones. Based on the optimal threshold, the correct time index in conjunction with the channel impulse response can be estimated jointly. The proposed joint scheme involves low complexity and shows superiority over a lately-proposed one.

Suboptimal Power Allocation for a Two-Path Successive Relay System with Full Interference Cancellation

Although power allocation considerably affects the symbol error rate (SER) performance of a two-path successive relay system (TPSRS), it is not yet studied in a full interference cancellation (FIC) scenario. In this paper, we develop a suboptimal power allocation scheme for the TPSRS-FIC through minimizing an SER-based cost function. To obtain a closed-form power allocation solution, the cost function is defined as a part of the Taylor series expansion of the theoretical SER. Simulation results show that the proposed scheme can provide almost the lowest SER under limited power resources.

Low-Complexity Joint Timing Synchronization and Channel Estimation for MIMO OFDM Systems

This paper proposes low-complexity joint timing synchronization and channel estimation for multiple-input multiple-output orthogonal frequency-division multiplexing systems based on maximal length sequences. Under a Rayleigh channel assumption, an optimal threshold for the proposed joint scheme is derived, and a practical suboptimal threshold that requires neither pre-simulation nor prior information on the channel statistics is then presented. Furthermore, the performance of the proposed joint scheme is analyzed with respect to threshold selection, and the theoretical results match well the simulation results. It is shown that the proposed joint scheme enjoys much less computational complexity and better performance than a combination of typical synchronization and channel estimation schemes.

WLC43-3: A New Joint Time Synchronization and Channel Estimation Scheme for MIMO-OFDM Systems

In this paper, we extend a training sequence structure, which was originally designed for orthogonal frequency division multiplexing (OFDM) systems, into multiple-input multiple-output (MIMO) scenario. Unlike conventional training sequence structures for MIMO-OFDM systems, the proposed modification eliminates the interference among training sequences transmitted by different antennas, and can be constructed from the same orthogonal sequence. Furthermore, the unique structure enables a low-complexity joint time synchronization and channel estimation scheme that requires only a correlator and a comparator. With the same training sequence length, computer simulation results show that the proposed joint scheme outperforms conventional synchronization and channel estimation methods by orders in MIMO-OFDM systems.

Capacity analysis and power allocation under imperfect channel estimation for AF-based cooperative relay systems

In this paper, we investigate the effect of channel estimation errors on the capacity of amplify-and-forward cooperative relay systems. Since an exact capacity expression in the presence of channel estimation errors is difficult to be found, we turn to derive its lower bound instead. Accordingly, by maximizing the derived lower bound, we propose power allocation schemes for both single-relay and multi-relay systems. For single-relay systems, an analytical solution for inequality constrained problem is too complicated to be obtained. Thus, we propose a gradient descent algorithm with a log-barrier function to solve the inequality constrained problem. For multi-relay systems, in terms of the channel estimates and the corresponding mean-squared errors, we provide a water-filling solution for the optimal power allocation among the relays. Simulation results show that the proposed power allocation schemes yield capacity gain compared with equal power allocation at low to medium signal-to-noise ratios.