• Proceedings of the Korean Institute of Communication Sciences Conference
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• 2004.11a
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• pp.334-334
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• 2004

• The Journal of Korean Institute of Communications and Information Sciences
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• v.25 no.3B
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• pp.472-486
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• 2000

This paper analysis the performance of joint receiving structure consisting of the decorrelating multiuser detection and beamfromenr-RAKE receive for DS-CDMA communication systems. In asynchronous transmission as the number of simultaneous users increase. the capacity of CDMA system becomes severly reduced due to the nonideal orthogonality between user-assigned PN sequences and improper power control. Accordingly, the CDMA receiving system becomes vulnerable to the multiple access interferences and the near-far problem under multipath fading channel environment. To withstand these undesired performance degradations, this paper proposes the new type of multiuser detection which has a form of the hybrid structure of concatenating beamformer-RAKE receiver and decorrelating multiuser detection. the beam former-RAKE receiver performs temporal and spatial diversity combining with alleviating fading effect and suppressing undesired interferences, and the multiuser detection plays a role of making the receiver robust to the near-far problem. Regarding the individual merit on the usage of either multiuser detection or beamformer-RAKE receiver, the hybrid one is expected to produce the enhanced performance in multipath fading CDMA channel. However major drawback of using decorrelating multiuser detection for practical deployment is arised from its computational complexity , which is exponentially increased as more number of users and transmitted symbols involve. To diminish the computational complexity, this paper exploits an efficient block Toeplitz inversion technique using matrix Levinson polynomial will be introduced. And this paper provided the mathematical analysis to show the efficiency of the proposed joint structure under the multipath propagation environment. And results of a series of exhaustive computer simulations are presented in order to demonstrate the overall performance of the proposed hybrid structure in multipath fading CDMA channel.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.8A
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• pp.592-599
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• 2009

In this paper, we consider multiuser multi-input/multi-output antenna systems with zero-forcing receivers in downlink. In this case, to exploit multiuser diversity, spatial-division multiple access (SDMA) system allows to assign different users to a part of transmit antennas at the base station whereas spatial-division multiplexing (SDM) system assigns all antennas to single user's data stream. In this paper, we present analytical frameworks to evaluate performance of these systems. We first analyze the performance of these two systems by deriving closed-form expressions of achievable throughput. Numerical results show that the derived expressions are very tight. In addition, we approximate the capacity expression of SDM and SDMA systems and compare the SDM with the optimal case.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.7B
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• pp.583-591
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• 2008

This paper proposes a resource allocation algorithm for minimizing the overall distortion of multiple users in orthogonal frequency division multiplex (OFDM). The proposed algorithm exploits the diversity of multiuser and the rate-distortion function using packet distortion model in a system with limited resources. We first induce a rate-distortion function considering error concealment and error propagation properties of H.264 video structures. Then we perform adaptive resource allocation utilizing multiuser diversity for minimizing the overall video quality degradation. We also consider the proportional rate which is pre-determined for each user. Simulation results show that compared to the previous time division multiple access method and the resource allocation method maximizing data rate, the proposed rate allocation algorithm substantially improves the received video quality.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.27 no.7A
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• pp.641-650
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• 2002

Space-Time Block Code(STBC) provides full diversity gains with simple linear processing at the receiver. Interference Cancellation(IC) techniques in system using STBC improve the capacity and performance of wireless systems with co-channel users. Various IC techniques, Minimum Mean-Squared Error(MMSE) and Zero-Forcing(ZF) algorithms in system with STBC were proposed in the literatures in multiuser environment. The performance of these IC techniques were simulated by assuming perfect channel state information(CSI) of multiuser at the receiver. However, in practice it is difficult to know perfect CSI of multiuser at the receiver. Thus, channel estimation scheme is essential at the receiver. Also SNR estimation scheme is required to operate the MMSE IC algorithm. In this paper, we present estimation schemes of CSI and SNR using training sequences. Through extensive computer simulation, we compare and evaluate the performance of IC techniques using the proposed CSI and SNR estimation techniques.

• Journal of Communications and Networks
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• v.4 no.3
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• pp.189-198
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• 2002

Precoder has been shown to be able to provide source diversity and design flexibility. In this paper we employ precoding techniques for block transmission based on a multirate filterbank structure. To accommodate multiuser communication with variable data rates, different precoders with corresponding coefficients and up/down sampling rates are used. However, due to unknown multipath distortion, different interferences may exist in the received data, such as multiuser interference, intersymbol interference and interblock interference. To estimate channel parameters for a desired user, we employ all structured signature waveforms associated with different symbols of that user and apply subspace techniques. Therefore better performance of channel estimator can be achieved than the conventional subspace method based only on the signature of the current symbol. The delay for that user can also be jointly estimated. Channel identifiability conditions and asymptotic channel estimation error are investigated in detail. Numerical examples are provided to justify the proposed method. gest either multicode (MC) or multiple processing gain (MPG) mechanism [2], while requiring data rates to be integral multiples of some basic low-rate. In order to support variable rate transmission however, a comprehensive scheme needs to be investigated.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.6A
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• pp.494-499
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• 2009

In this paper, we provide an efficient method of user selection for achieving the maximum system throughput in downlink multiuser Multiple-Input Multiple-Output (MIMO) systems. A proposed method is for selecting a fine user set only with powers of each user channel and angles between them. This algorithm is simpler than SUS because there is no considering about the optimal value of correlation. The proposed method finds the user set toward maximizing system throughput, so it has high performance.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.46 no.7
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• pp.1-5
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• 2009

Random beanforming exploits multiuser diversity gain in static channels. Since the gain is restricted by the user population, some extended works have been proposed. Among them, a codebook-based opportunistic beamforming technique forms multiple random beams with small pilots. The technique however has difficulty in designing beams flexibly by the channel statistics. In this paper, we propose a technique forming the multiple random beams by rotating codebooks. The proposed technique enables the flexible design of beams so that multiuser diversity and beam selection diversity are exploited simultaneously with small pilots robust to the channel statistics.

• Journal of Communications and Networks
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• v.17 no.3
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• pp.267-274
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• 2015

Due to the difficulty of coordination in the cellular uplink, it is a practical challenge how to achieve the optimal throughput scaling with distributed scheduling. In this paper, we propose a distributed and opportunistic user scheduling (DOUS) that achieves the optimal throughput scaling in a single-input multiple-output interfering multiple-access channel, i.e., a multi-cell uplink network, with M antennas at each base station (BS) and N users in a cell. In a distributed fashion, each BS adopts M random receive beamforming vectors and then selects M users such that both sufficiently large desired signal power and sufficiently small generating interference are guaranteed. As a main result, it is proved that full multiuser diversity gain can be achieved in each cell when a sufficiently large number of users exist. Numerical evaluation confirms that in a practical setting of the multi-cell network, the proposed DOUS outperforms the existing distributed user scheduling algorithms in terms of sum-rate.

• Journal of Communications and Networks
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• v.5 no.4
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• pp.354-364
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• 2003

A general method for the evaluation of the symbol error probability (SER) of ultra wideband (UWB) systems with various kind of modulation schemes (N-PAM, M-PPM, Bi-Orthogonal), in presence of multipath channel, multiuser and strong narrowband interference, is presented. This method is shown to be able to include all the principal multiaccess techniques proposed so far for UWB, time hopping (TH), direct sequence (DS) and optical orthogonal codes (OOC). A comparison between the performance of these multiple access and modulation techniques is given, for both ideal Rake receiver and minimum mean square error (MMSE) equalizer. It is shown that for all the analyzed multiple access schemes, a Rake receiver exhibits a high error floor in presence of narrowband interference (NBI) and that the value of the error floor is in-fluenced by the spectral characteristics of the spreading code. As expected, an MMSE receiver offers better performance, representing a promising candidate for UWB systems. When the multiuser interference is dominant, all multiple access techniques exhibit similar performance under high-load conditions. If the number of users is significantly lower than the spreading factor, then DS outperforms both TH and OOC. Finally 2PPM is shown to offer better performance than the other modulation schemes in presence of multiuser interference; increasing the spreading factor is proposed as a more effective strategy for SER reduction than the use of time diversity.