Won Mee Jang

Transmitter precoding in synchronous multiuser communications

A synchronous multiuser system operating in an additive white Gaussian noise channel, with or without multipath fading, is considered. It is shown that when either a conventional single user receiver or the RAKE receiver is employed, both multiple access and intersymbol interference can be eliminated by means of a suitable transmitter precoding scheme. Transmitter precoding represents a linear transformation of transmitted signals, such that the mean squared errors at all receivers are minimized. Precoding, with both conventional single user receiver and with the RAKE receiver, results in near-far resistant performance and outperforms considerably the respective schemes without precoding. The crucial assumption, in the multipath case, is that the transmitter knows the multipath characteristics of all channels and that channel dynamics are sufficiently slow so that multipath profiles remain essentially constant over the block of precoded bits.

Joint transmitter-receiver optimization in synchronous multiuser communications over multipath channels

Transmitter optimization, in addition to receiver optimization, contributes significantly to efficient interference suppression in multiple access and multipath channels. The system design is based on the joint optimization of the transmitter and the receiver in a synchronous multiuser channel characterized by multipath propagation. Joint optimization is represented by a linear transformation of the transmitted signals at the transmitter and a linear transformation of received signals at each receiving site that minimize the effect of multiple access and multipath interference. The minimum mean squared error between the true bit value and its estimate at the output of the receiver is taken as the cost function, subject to average and peak transmit power constraints. It is shown that joint transmitter-receiver optimization outperforms significantly either transmitter optimization or receiver-based techniques. The crucial assumption, in the case of multipath channels, is that the transmitter knows the multipath characteristics of all channels and that the channel dynamics are sufficiently slow so that multipath profiles remain essentially constant over a block of preceded bits. The practical applications can be found in indoor and cellular communications, satellite communications, or military communications where nonorthogonal signature waveforms are employed.

MAI and ICI of synchronous downlink MC-CDMA with frequency offset

We analyze the performance of code division multiple access (CDMA) systems with multicarrier (MC) that employ random spreading sequences in downlink frequency selective fading channels. We obtain the probability density function (pdf) of the multiple access interference (MAI) of CDMA systems and extend the results to MC-CDMA systems to determine the pdf of MAI, inter-carrier interference (ICI) and noise in terms of the number of users, the spreading factor, the number of sub-carriers and frequency offset. The results show that the effects of frequency offset vary with system loading. We consider the synchronous downlink of cellular MC-CDMA and derive a Gaussian approximation of the MAI and ICI to reduce the computational complexity in calculating the bit error rate (BER). The accuracy of the Gaussian approximation is examined by computer simulations.

Chip-interleaved self-encoded multiple access with iterative detection in fading channels

We propose to apply chip interleaving and iterative detection to self-encoded multiple access (SEMA) communications. In SEMA, the spreading code is obtained from user bit information itself without using a pseudo noise code generator. The proposed scheme exploits the inherent diversity in self encoded spread spectrum signals. Chip interleaving not only increases the diversity gain, but also enhances the performance of iterative detection. We employ user-mask and interference cancellation to decouple self-encoded multiuser signals. This paper describes the proposed scheme and analyzes its performance. The analytical and simulation results show that the proposed system can achieve a 3 dB power gain and possess a diversity gain that can yield a significant performance improvement in both Rayleigh and multipath fading channels.

Blind Cyclostationary Spectrum Sensing in Cognitive Radios

We propose a blind spectrum sensing method using signal cyclostationarity. Often, signal characteristics of the primary user (PU), such as carrier frequency, data rate, modulation and coding may not be known to cognitive users. This uncertainty introduces difficulties in searching for spectrum holes in cognitive radios. At a low signal-to-noise ratio, it has been understood that monitoring the presence of the PUs signal is hardly possible without knowing its cycle frequencies. The proposed sensing method makes it possible to detect the PUs signal without the relevant information of the signal attributes. Blind spectrum sensing is also much simpler than the conventional cyclostationary spectral correlation detection.

Automatic Modulation Recognition of Digital Signals using Wavelet Features and SVM

This paper presents modulation classification method capable of classifying incident digital signals without a priori information using WT key features and SVM. These key features for modulation classification should have good properties of sensitive with modulation types and insensitive with SNR variation. In this paper, the 4 key features using WT coefficients, which have the property of insensitive to the changing of noise, are selected. The numerical simulations using these features are performed. We investigate the performance of the SVM-DDAG classifier for classifying 8 digitally modulated signals using only 4 WT key features (i.e., 4 level scale), and compare with that of decision tree classifier to adapt the modulation classification module in software radio. Results indicated an overall success rate of 95% at the SNR of 10dB in SVM-DDAG classifier on an AWGN channel.

Self-encoded spread spectrum modulation with differential encoding

In this paper we analyze self-encoded spread spectrum (SESS) modulation with differential encoding and decoding. We show that this structure eliminates the BER effect of error propagation. We also investigate maximum likelihood sequence detection (MLSD) of SESS sequence using the Viterbi algorithm. The results show that MLSD outperforms the feedback detector in terms of the BER performance.

A Simple Upper Bound of the Gaussian Q-Function with Closed-Form Error Bound

We present a simple upper bound of the Gaussian Q-function with its closed-form error bound. The proposed approximation can be applied to the evaluation of the average error probability of digital modulations with its error bound. Our bit error rate (BER) bound can be easily applied to fading channels.

Quantifying Performance in Fading Channels Using the Sampling Property of a Delta Function

We apply the sampling property of a delta function to obtain the probability of error in fading channels. Our approach reduces the integration to a sampling. The sampling point is obtained in terms of fading parameters and the average signal-to-noise ratio (SNR) to provide the closed form solution of the performance.

Joint transmitter/receiver optimization in synchronous multiuser communications over multipath channels

Increasingly evident in recent work, it appears that transmitter optimization in addition to receiver optimization contributes significantly to efficient interference suppression in multiple access and multipath channels. The system design in this paper is based on the joint optimization of the transmitter and the receiver in a synchronous multiuser channel characterized with multipath propagation. Joint optimization is represented by a linear transformation of transmitted signals in the transmitter and a linear transformation of received signals at each receiving site that minimize the effect of multiple access and multipath interference. The minimum mean squared error between the true bit value and its estimate at the output of the receiver is taken as the cost function, subject to the average transmit power constraint. It is shown that joint transmitter-receiver optimization outperforms significantly transmitter optimization and conventional receiver based techniques, matched filter and RAKE receivers. The crucial assumption is that the transmitter knows multipath characteristics of all channels and that channel dynamics are sufficiently slow so that multipath profiles remain essentially constant over a block of preceded bits.