Jia-Chin Lin

Least-Squares Channel Estimation for Mobile OFDM Communication on Time-Varying Frequency-Selective Fading Channels

A least-squares (LS) channel estimation (CE) technique for mobile orthogonal frequency-division multiplexing (OFDM) communications over a rapidly time-varying frequency-selective fading channel is investigated in this paper. The proposed technique keeping the comb-type pilot arrangement can achieve a low error probability by accurately estimating channel impulse response (CIR) and effectively tracking rapid CIR time variations. The LS CE technique proposed here is conducted in the time domain (TD). Meanwhile, a generic estimator is serially performed block by block without assistance from a priori channel information and without increasing computational complexity. By taking advantage of linearly frequency-modulated (LFM) or pseudorandom signals transceived for jointly sounding pilot subchannels, the proposed LS CE can inherently perform pseudonoise (PN) matched filtering (MF) to suppress multipath interference (MPI) caused by frequency-selective fading and intercarrier interference (ICI) resulting from data subchannels. The optimality of the proposed technique is verified by taking Cramer-Rao lower bounds (CRLBs) into comparison both on noise- and interference-dominant signal-to-noise ratio (SNR) conditions. In addition, the dual optimality of the LFM and PN pilot symbols is verified for both TD and frequency-domain (FD) CEs. Furthermore, the proposed technique also exhibits good resistance against residual timing errors occurring with the discrete Fourier transform (DFT) demodulation. Extensive computer simulations in conjunction with statistical derivations show the superiority of the proposed technique.

Noncoherent sequential PN code acquisition using sliding correlation for chip-asynchronous direct-sequence spread-spectrum communications

Noncoherent sequential pseudonoise (PN) code acquisition using sliding correlation is proposed in this paper. Noncoherent detection and chip asynchronization should be taken care of to handle a severely noisy environment, while frequency offset and data modulation effects can be simultaneously dealt with in the proposed technique. To realize sequential detection, the cross-correlation sequences at the output end of the integrate/dump (LID) filter under out-of-lock conditions have to be modeled as either a Gaussian random sequence for chip-asynchronous applications or as their upper bound for chip-synchronous applications in order to avoid significantly high probabilities of false alarm caused by the conventional zero-sequence model. Meanwhile, the in-lock sequence also has to be modified by taking the frequency offset and chip-asynchronization effects into account in order to avoid the occurrence of high probabilities of miss. Extensive computer simulation results indicate that the proposed technique can achieve low probabilities of false alarm and miss and can outperform its fixed-sample-size (FSS) counterparts by roughly 2/spl sim/4 dB. This superiority, furthermore, increases with decreasing SNRs and/or decreasing desired error probabilities.

Maximum-likelihood frame timing instant and frequency offset estimation for OFDM communication over a fast Rayleigh-fading channel

A novel maximum-likelihood (ML) technique for frame timing instant and frequency offset estimation in an orthogonal frequency-division multiplexing system is investigated. By taking advantage of the cyclic prefix within the guard interval, the proposed technique can accurately estimate the frame timing instant and frequency offset with no assistance from pilot symbols. The proposed technique also takes fast Rayleigh fading into consideration for the purpose of wireless applications. Furthermore, several combining techniques are also investigated in order to improve the performance of the proposed estimators. Extensive simulation experiments show that the proposed techniques can effectively achieve lower estimation errors in frame timing and frequency offset estimation.

A modified PN code tracking loop for direct-sequence spread-spectrum communication over arbitrarily correlated multipath fading channels

A modified fully digital pseudonoise code tracking loop is proposed for direct-sequence spread-spectrum communication. By taking advantage of the inherent diversity, a modified code tracking loop is embedded into a RAKE receiver in order to avoid problems caused by unstable locked points of error signals. Such unsteadiness of locked points often occurs with a conventional code tracking loop because the error signals may be randomly biased by multipath fading. Thus, a robust pull-in capability can be provided over a time-variant fading channel where multiple propagation paths are arbitrarily correlated. Furthermore, an effective multipath interference regeneration and cancellation technique is also proposed to improve the error characteristics of the proposed technique. Analytical expressions of the error characteristics and error signals are derived and then confirmed by means of extensive computer simulation results. In addition, several simulation results for the timing jitter and the mean time to lose lock are also presented. The very attractive behavior obtained using the proposed technique is verified.

Wireless communication performance based on IEEE 802.11p R2V field trials

Next-generation telematics solutions are being driven by the maturation of recently deployed intelligent transportation systems, assisted by the integration of and rapid collaboration with information communication technology markets and the automotive industry. Examples of this trend can be found in DSRC contexts in which Wi-Fi and WiMAX have become very relevant in vehicular environments. Comprehensive field trials were conducted and relevant measurements were collected in physical environments to investigate the possible environmental factors that occur in roadside-unit-to-vehicle communication scenarios. In these carefully controlled automobile experiments, the most representative parameters are measured and analyzed for DSRC in dynamic environments (e.g., packet loss, latency, and delay spread). Using a systematic testing procedure and through measurement analysis, the communication performance and the operating reliability of wireless communications can be effectively verified; insightful improvements and helpful suggestions and contributions may thus be achieved for telematics system designs. Furthermore, a practical, reasonable, and standardized testing procedure is proposed, and it can be utilized in equipment verification and product certification.

Differentially coherent PN code acquisition with full-period correlation in chip-synchronous DS/SS receivers

Differentially coherent detection techniques for PN code acquisition in direct-sequence spread-spectrum (DS/SS) receivers are designed and analyzed in this letter. The full-period cross correlation (FPC) can be modeled as deterministic for certain types of PN sequences whose autocorrelation functions have ideal shapes the same as those of maximum length shift registration (MLSR) sequences. This model is exploited here to simplify the design and performance evaluation of the proposed detection techniques. Such techniques can be confidently employed as alternatives to noncoherent detectors because of their attractive improvements in the signal-to-noise ratio (SNR) and, thus, are very suitable for commercial code-division multiple-access (CDMA) systems, which often operate in severely noisy environments. The results of statistical analyses show that the proposed techniques can provide rapid acquisition and improved performance in SNR.

Novel Channel Estimation Techniques in IEEE 802.11p Environments

The primary challenge in the current IEEE 802.11p draft standard is the weak performance in multipath environment. Hence a time-domain (TD) and frequency-domain (FD) channel estimation techniques that can enhance the performance of channel estimation are proposed in this paper. The TD technique makes a little modify in current standard to use the symbol prefix to assist channel estimation and obtain remarkable improvement. In addition, the FD pseudo-pilot technique change nothing in the standard and can obtain adequate improvement. Furthermore, this paper also discuss how to employ the preamble to improve the FD channel estimation performance. The channel models adopted for simulation are referencing HIPERLAN/2 and a modified exponential model.

A Fuzzy-Model-Based Chaotic Synchronization and Its Implementation on a Secure Communication System

In this paper, a synchronization mechanism of a fuzzy-model-based chaotic system is proposed. It is then implemented on a secure communication system (SCS) for verifying its functions. The fuzzy controller is designed to guarantee master-slave synchronization to be achieved in a pre-specified convergence time. A hardware implementation with the synchronous control for the SCS is realized with a field-programmable gate array chip and a personal computer. The SCS consists of two main parts: 1) a transmitter, which contains the master system (i.e., chaotic carrier) with the input messages and 2) a receiver, which contains the slave system with the output messages. Based on the proposed synchronization method, the SCS can decrypt the output messages rapidly and correctly in the receiver in which the encrypted signals are generated by a combination of the input messages and the master systems states. It should be emphasized that since we can assign the convergence time in advance so that the user can know the suitable time for the master system to mix the input messages. Finally, the performance and merit of the proposed control scheme is exemplified by practical experiments.

Novel Channel Estimation Techniques on SC-FDMA Uplink Transmission

This paper proposes a novel channel estimation technique for Single Carrier Frequency Division Multiple Access (SC-FDMA) communications operating on the Long Term Evolution (LTE) uplink over time-varying multipath channels. The proposed method keeping the block-type pilot arrangement (BTPA) can improve the performance of the conventional channel estimation (CE) technique with no assistance from apriori knowledge of the channel statistics. In addition, the proposed method based on a sliding windowing can mitigate the interferences effectively with low computational complexity. Extensive computer simulations verify the superiority and improvements achieved by the proposed technique.

Performance evaluations of channel estimations in IEEE 802.11p environments

To solve the defect in IEEE 802.11p standard that coherence bandwidth is insufficient to estimate the channel state in rich scattering environment, hance a time domain channel estimation technique is proposed in this paper. This technique is based on least-square (LS) algorithm, and assisted by Zadoff-Chu sequence in symbol prefix and preamble. The channel models in the simulation are referencing HIPERLAN/2.