Jooshik Lee

Low-complexity estimation of 2D DOA for coherently distributed sources

We consider the estimation of two-dimensional (azimuth and elevation) direction-of-arrival (DOA) using a pair of uniform circular arrays under a coherently distributed source model. Since the coherently distributed source is characterized by four parameters, the nominal azimuth DOA, angular extension of the azimuth DOA, nominal elevation DOA, and angular extension of the elevation DOA, the computational complexity of the parameter estimation is normally highly demanding. We propose a low-complexity estimation algorithm, called the sequential one-dimensional searching algorithm by concentrating only on the estimation of DOAs. The SOS algorithm has a basis on the eigenstructure between the steering matrix and signal subspace, and utilizes preliminary estimates obtained at a pre-processing stage. The SOS algorithm estimates the DOAs, but not the angular extensions: although the SOS algorithm does not provide estimates of angular extensions, it is useful when the angular extensions are small. Specifically, it is shown from simulation results that the SOS algorithm exhibits as good an estimation performance as the maximum likelihoood method for coherently distributed sources.

A Method for the Direction-of-Arrival Estimation of Incoherently Distributed Sources

We consider the problem of estimating the nominal direction of arrival (DOA) of an incoherently distributed source. This problem is encountered due to the presence of local scatterers in the vicinity of a transmitter or due to signals propagating through a random inhomogeneous medium. Since the spatial covariance matrix has full rank for an incoherently distributed source, the performance of most high-resolution DOA estimation algorithms conceived under coherently distributed sources, as well as point source models, degrades when scattering is present. In addition, several DOA estimation techniques devised under a distributed source model require a high-dimensional nonlinear optimization problem. In this paper, we propose a novel method based on the conventional beamforming approach, which estimates the nominal DOA from a spatial maximum peak of the output power. The proposed method is computationally more attractive than the maximum likelihood (ML) estimator, although the performance degrades in comparison with the ML estimator, whose asymptotic performance is equivalent to the Cramer-Rao bound (CRB). We derive and compare the asymptotic performances of the proposed method and the redundancy-averaged covariance matching (RACM) method in the single-source case. The simulation results illustrate that the asymptotic performance of the proposed method is better than that of the RACM method.

A new polyphase sequence with perfect even and good odd cross-correlation functions for DS/CDMA systems

A class of polyphase signature sequences for direct-sequence code-division multiple-access (DS/CDMA) systems is proposed. The proposed class has zero periodic (=even) cross-correlation (CC) function and approximate maximum magnitude N//spl pi/ of odd cross-correlation (OCC), where N is the length of sequences. Although the maximum magnitude is relatively large, it is observed that the maximum magnitude has little effect on the performance of the DS/CDMA system, since its frequency is very low. The performance of the proposed sequence in DS/CDMA systems is investigated and shown to be better than that of other sequences for an asynchronous additive white Gaussian noise channel environment with and without Rayleigh multipath fading.

Analysis of quasi-ML multiuser detection of DS/CDMA systems in asynchronous channels

We consider the quasi-maximum-likelihood (quasi-ML) detectors of the reverse link system that uses antenna arrays in asynchronous channels when the channel vector is time-invariant and time-varying. A channel vector estimation method based on eigendecomposition for time-invariant channels and its adaptive version suitable for the time-varying channels are also proposed. It is shown that the proposed quasi-ML detector can be regarded as a beamformer followed by a decorrelating filter and that the proposed system performs better than the conventional decorrelator scheme. It is also observed that the performance gain of the proposed scheme over the conventional decorrelator system increases as the numbers of active users and antenna arrays increase.

Design and performance analysis of a convolutionally coded overlapping multicarrier DS/CDMA system

We consider convolutionally coded multicarrier direct-sequence code-division multiple access (DS/CDMA) systems using overlapping subchannels in time-limited and band-limited channels. A tight hound on the bit error probability of the decoder output is obtained in Rayleigh fading channels when the channel state information is ideally available. It is shown that the proposed system, the convolutionally coded overlapping multicarrier DS/CDMA system with a symbol mapping method utilizing the diversity, outperforms the conventional single carrier and nonoverlapping multicarrier systems using the same convolutional code. It is also shown that the proposed system can provide a larger system capacity and more effectively combat the partial band interference than the conventional ones.

On reducing computational complexity of codebook search in CELP coding

A computationally efficient codebook search method in code-excited linear prediction is proposed. The method can reduce the computational complexity by almost one half compared to the frequency-domain codebook search method that is currently regarded as the fastest search method, while giving almost the same quality of speech. This reduction is possible as a result of the simultaneous use of frequency-domain search and code vector sparsity. >

Uniform circular array in the parameter estimation of coherently distributed sources

We consider the problem of two-dimensional (azimuth and elevation) direction-of-arrival estimation for coherently distributed sources using a pair of uniform circular arrays. We propose a low-complexity estimation algorithm, the sequential one-dimensional searching algorithm. The proposed algorithm has a basis on the eigenstructure between the steering matrix and signal subspace, and utilizes a preliminary estimate obtained by the total least square estimation of signal parameters via the rotational invariance technique as a pre-processing stage. The proposed algorithm exhibits as good estimation performance as the maximum likelihood method for coherently distributed sources. Some simulation results are included to show the performance of the proposed method.

A rate adaptive convolutional coding method for multicarrier DS/CDMA systems

In this paper, a rate adaptive convolutional coding method for multicarrier direct sequence code division multiple access (DS/CDMA) systems is considered In order to accommodate a number of coding rates easily and make the encoder and decoder structure simple, we use the rate compatible punctured convolutional (RCPC) code. We obtain data throughputs at several coding rates by choosing the coding rate which has the highest data throughput in the SINR sense. To achieve maximum data throughput, a rate adaptive system based on the channel state information (the SINR estimate) is proposed. The SINR estimate is obtained by the soft decision Viterbi decoding metric. We show that the proposed rate adaptive convolutionally coded multicarrier DS/CDMA system can enhance spectral efficiency and provide frequency diversity.

Suboptimum multiuser detection of the QS-CDMA systems using antenna array

Multiuser detection has been an interesting topic because of its capability of eliminating multiuser interference and resistance to the near-far problem. The decorrelator approach has been investigated as a suboptimum multiuser detector for a low complexity receiver at the expense of the enhancement and correlation of noise. We consider a quasi-maximum likelihood (quasi-ML) detector in the reverse link system which uses antenna arrays in quasi-synchronous channels. It is shown that the performance of the system is better than that of the conventional decorrelator and that the performance gain over the conventional decorrelator system increases as the number of active users and number of antenna arrays increase.

A code acquisition scheme for DS/CDMA systems

When the code phase offset between two sequences is within one chip duration, the sum of two successive matched filter outputs has a constant value regardless of the residual code phase offset if the noise is absent. Based on this observation, we propose a new code acquisition scheme, which is robust to the variation of the residual code phase offset and outperforms the conventional scheme. Numerical results are also given to show that the proposed scheme is more robust to the variation of the residual code phase offset and has better performance than the conventional scheme on average.