Char-Dir Chung

Spectrally Precoded OFDM

By bilinearly transforming the rectangularly pulsed multicarrier basis signals, new basis sets containing full-response continuous basis signals are developed for constructing spectrally efficient orthogonal frequency-division multiplexing (OFDM) signals with or without cyclic prefix (CP) or zero padding (ZP). Both multiple-product and linear-combination formats are presented for these basis sets. In the multiple-product format, the developed full-response basis signals are shown to be zero-valued at interval edges, and thus can be used to construct continuous-phase OFDM signals. It is analytically shown that the constructed continuous-phase OFDM signals exhibit relatively small power spectral sidelobes which fall off as f-4, and can thus provide much higher spectral efficiency than the traditional rectangularly pulsed OFDM signal. In the linear-combination format, the proposed basis signals are shown to be the linear combinations of rectangularly pulsed multicarrier basis signals, and thereby allow for the efficient realization of fast Fourier transform (FFT). Specifically, a spectrally precoded signaling structure is developed in conjunction with inverse FFT and guard insertion to construct continuous-phase OFDM signals with or without CP or ZP. The corresponding spectral decoders are also developed to enable zero-forcing (ZF) demodulation for spectrally precoded OFDM. With the zero-forcing demodulation, the spectrally precoded OFDM schemes are shown to outperform in average error-rate characteristics the conventional rectangularly pulsed and correlatively coded OFDM schemes on various delay-spread channels

Reversible data hiding and lossless reconstruction of binary images using pair-wise logical computation mechanism

The purpose of data hiding is to embed relating textural description into an image. The textural description and original host image can be extracted and reconstructed from the stego-image in the data extraction process. However, the reconstructed host image will more or less be damaged or distorted by using traditional data hiding methods. In this paper, we propose a novel data hiding mechanism by hiding data based on pair-wise logical computation (PWLC). The proposed mechanism can achieve the benefits of reversible hidden data extraction and lossless reconstruction of host image without utilizing any information from the original host image. Furthermore, the task of tampering detection can also be accomplished in the proposed mechanism. Experimental results demonstrate the feasibility and validity of our proposed mechanism in the data hiding of binary images.

Spectral precoding for rectangularly pulsed OFDM

Spectrally precoded orthogonal frequency-division multiplexing (OFDM) is a promising rectangularly pulsed OFDM signaling format which can provide very small power spectral sidelobes, while allowing for efficient implementation by fast Fourier transform and guard interval insertion. In this paper, general constraints on spectral precoding are developed for OFDM signals with zero padding (ZP-OFDM) or cyclic prefix (CP-OFDM) to warrant the desirable spectral property that the power spectral sidelobes decay asymptotically as f-2K-2, where K is a preassigned positive integer. In accordance with the constraints, block partitioning is adopted to construct a general signaling format to facilitate the precoder design of the spectrally precoded CP-OFDM that can provide fast decaying sidelobes. New correlative and orthogonal precoders are also devised so that the desirable spectral property of fast sidelobe decaying is achieved with spectrally precoded ZP-OFDM and CP-OFDM signals.

Differentially coherent detection technique for direct-sequence code acquisition in a Rayleigh fading mobile channel

A novel differentially coherent (DC) detection scheme for rapid code acquisition is proposed for a direct sequence (DS) spread-spectrum system being used to communicate over a fast Rayleigh fading mobile radio channel. The scheme processes the received signal at baseband using a complex differential detector with one-chip time delay and then performs a coherent partial correlation of the detector output with the product of a local DS code and its one-chip delayed phase to form a test sample. While still preserving the pseudo-noise attribute at its output, the differential detector is able to effectively suppress slowly fluctuating phase components due to fading and carrier frequency offset, thereby enhancing the in-sync correlation at the following correlator. The performance of this detection scheme is analysed based on a central-limit-theorem argument, and verified by simulation. Both serial and parallel acquisition systems employing the proposed DC detector are compared with the conventional parallel I-Q system by mean acquisition time performance. It is shown that the serial DC acquisition system significantly outperforms the parallel I-Q system in very fast Rayleigh fading environments. >

Correlatively coded OFDM

A class of correlative codes is proposed to significantly improve the spectral performance of the rectangularly pulsed orthogonal frequency-division multiplexing (OFDM) signal with or without cyclic prefix or zero padding. It is analytically shown that the correlatively coded OFDM signal can achieve very high spectral efficiency while yielding an extremely small fractional out-of-band power

Spectrally Precoded OFDM and OFDMA with Cyclic Prefix and Unconstrained Guard Ratios

Spectrally precoded orthogonal frequency-division multiplexing with cyclic prefix (SP-CP-OFDM) and SP-CP-OFDM multiple access (SP-CP-OFDMA) are investigated in this paper. A general constraint on spectral precoding is developed for both SP-CP-OFDM and SP-CP-OFDMA signal formats with unconstrained guard ratios and arbitrary input data statistics to ensure very small power spectral sidelobes decaying asymptotically as f-2J-2, with J a predesigned integer-valued parameter. In the SP-CP-OFDM signaling format, new spectral precoders are devised to meet the constraint. It is shown that the proposed SP-CP-OFDM signals can provide much smaller spectral sidelobes than the previously designed SP-CP-OFDM signals with block partitioning and constrained guard ratios. In the SP-CP-OFDMA signaling format, a general procedure is proposed to facilitate the precoder design for SP-CP-OFDMA signals with arbitrary subcarrier allocation for each user and some spectral precoders are accordingly devised. It is shown that the proposed SP-CP-OFDMA signals can suppress sidelobe powers effectively and thus achieve higher spectral compactness than nonprecoded rectangularly-pulsed CP-OFDMA.

Spectral precoding for constant-envelope OFDM

Spectrally precoded orthogonal frequency-division multiplexing (OFDM) and constant-envelope OFDM (CE-OFDM) are two modified OFDM formats that enhance the signaling features of OFDM, while allowing for the efficient implementation by discrete Fourier transform and the insertion of guard intervals to counteract channel dispersion. The spectrally precoded OFDM signal provides very small power spectral sidelobes decaying asymptotically as f-2J-2 with J integer-valued design parameter, thereby suppressing sidelobe powers effectively and achieving higher spectral efficiency than nonprecoded rectangularly-pulsed OFDM which has power spectral sidelobes decaying asymptotically as f-2. The CE-OFDM signal exhibits a unity peak-to-average power ratio, and thus enables the efficient use of transmitters power amplifier. In this paper, a specific CE-OFDM block signaling format is proposed to incorporate spectral precoding with an aim to suppressing sidelobe powers more effectively than conventional nonprecoded CE-OFDM. The general constraint on spectral precoding is developed to ensure that the power spectral sidelobes of the spectrally precoded CE-OFDM block signal with arbitrary data statistics decay asymptotically as f-2J-2. Some spectral codes previously found for spectrally precoded OFDM are shown to meet the constraint and thus suited for spectrally precoded CE-OFDM. Specifically, spectral performance is analyzed for the spectrally precoded CE-OFDM block signal with independent and identically distributed data to illustrate the prevailing spectral advantage. The corresponding receiver is also developed to demodulate the spectrally precoded CE-OFDM block signal over dispersive channels.

Differentially amplitude and phase-encoded QAM for the correlated Rayleigh-fading channel with diversity reception

This paper studies the differentially amplitude and phase-encoded (DAPE) quadrature amplitude modulation (QAM) transmission over correlated Rayleigh channels with diversity reception. Operating over two successive received symbols, the optimum and an asymptotic maximum-likelihood (AML) differentially coherent receiver are developed and compared with a conventional switched diversity combining (SDC) grid receiver. It is shown that the AML and SDC grid receivers are much simpler in complexity than the optimum receiver in that no channel side information is required in their realization. An exact expression of the bit-error probability (BEP) is obtained for the SDC grid receiver. Based on a union bound argument, a BEP upper bound for the AML receiver is also derived and verified by simulation. Numerical results on 16- and 64-point constellations show that the AML receiver exhibits an almost optimum performance and the SDC grid receiver with a small level of diversity is nearly optimum. It is also shown by simulation that the conventional equal-gain diversity receiver is almost optimal for demodulating a 16-point DAPE QAM signal.

Constant-envelope orthogonal frequency division multiplexing modulation

A constant-envelope signaling approach for orthogonal frequency division multiplexing (OFDM) modulation is proposed. By splitting the standard OFDM complex signals into real and imaginary sections, the proposed approach employs phase modulation in conjunction with signal clipping to endow the transmitted signal with a constant envelope, without destroying the required transmission characteristic associated with OFDM. The resultant constant-envelope signal enables the operation of the transmitters power amplifier at the saturation point and thus maximizes the achievable power efficiency. Despite the constant-envelope advantage, half the bandwidth efficiency is traded off by the approach. It is shown by simulation that the bit error performance of the proposed system is only slightly degraded from the conventional OFDM system.

Parameter estimation of random FH signals using autocorrelation techniques

Multiple-hop autocorrelation (MHAC) processing is considered as a means of estimating the parameters of frequency-hopping signals in the presence of broadband thermal noise. It is shown that power sampling in the MHAC domain suppresses the dependence on the hopping frequency, carrier phase, and hop timing, while maintaining the hop-rate information. Moreover, when the signals hop-rate is known a priori or can be estimated reliably, the multiple-hop combining of single-hop autocorrelation (SHAC) estimates can be employed to extract hop-timing information. Taking advantage of these features, we propose and analyze a hop-rate estimator and an epoch estimator, based on maximum-likelihood arguments. The conditional mean and variance of both estimators are derived analytically and confirmed by extensive simulation. >