• Journal of Communications and Networks
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• v.9 no.2
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• pp.128-135
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• 2007

A combined opportunistic multiple beamforming method is presented that the first beam is designed with an aid of channel state information using uplink mid-amble and other beams are orthogonally generated to the first beam sequentially. The power allocation with SINR feedback increases overall throughput for the decentralized systems. The advantages of the proposed scheme is that the first beam is not interfered by other beams guaranteeing quality of service (QoS) and other orthogonal beams are operated opportunistically to obtain multi user diversity. The computer simulation demonstrates that the proposed scheme is effective at a small number of users, which is common in cellular systems, and outperforms conventional spatial division multiple access (SDMA) opportunistic beamforming methods.

• Journal of information and communication convergence engineering
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• v.8 no.4
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• pp.393-398
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• 2010

For multi-cell environments, coordinated random beamforming technique in multiuser MIMO(multiple-input multiple-output) broadcast channel is considered. In order to mitigate severe interference at receivers, the multi-cell environments might require complex transmitter and receiver design because the scheduler decision based on full channel state information (CSI) in one cell must be intertwined with decision made by other cells' CSI. With limited CSI, however, this paper considers a scheme of randomizing transmitters' beamforming but being coordinated with other cell transmitters. The transmitters in each cell share random beamforming patterns and schedule data transmission within coherent scheduling period. The corandomized beams allow the users to be selected with the highest SINRs even in multi-cell environments. We analyze the performance of the proposed scheme. And numerical results show that the scheme achieves better performance than the conventional random beamforming when applying to multi-cell environments.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.2A
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• pp.130-138
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• 2008

In this paper, we propose a user selection method to maximize the sum-rate of downlink over opportunistic beamforming. The throughput of an opportunistic beamforming with non-uniformly distributed or a small number of users can decrease. In order to improve the throughput, we propose a scheduling method that does not use SINR or SNR but uses the effective channel gain of each user obtained from the SINR or SNR feedback. The proposed method makes it possible to select users flexibly according to the distribution of users. In numerical results, we show that the proposed methods improve the average sum-rate about 60% when users are distributed non uniformly.

• Proceedings of the IEEK Conference
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• 2008.06a
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• pp.289-290
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• 2008

We consider cooperative opportunistic beamforming that can mitigate the other cell interference (OCI) in correlated multi-user multiple-input single-output (MISO) cellular environments. By only exploiting the spatial channel information of adjacent cells, the proposed scheme generates the cooperative random beam that statistically avoids the OCI from adjacent cells. Each cell selects a user in an opportunistic manner. Thus, the proposed scheme can simultaneously achieve the multi-user diversity (MUD) gain and the OCI avoidance gain.

• IEIE Transactions on Smart Processing and Computing
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• v.3 no.2
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• pp.83-87
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• 2014

This paper introduces a refined opportunistic interference alignment (OIA) technique that uses minimum mean square error (MMSE) detection at the receivers in multiple-input multiple-output multi-cell uplink networks. In the OIA scheme under consideration, each user performs the optimal transmit beamforming and power control to minimize the level of interference generated to the other-cell base stations, as in the conventional energy-efficient OIA. The result showed that owing to the enhanced receiver structure, the OIA scheme shows much higher sum-rates than those of the conventional OIA with zero-forcing detection for all signal-to-noise ratio regions.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.23 no.12
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• pp.1638-1643
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• 2019

In this paper, we propose an adaptive-perturbation-aided opportunistic precoding (APOP) scheme for multiple-input multiple-output (MIMO) systems. To update a precoding matrix in MIMO systems, the proposed algorithm produces a random perturbation in each time slot. Then the additional adaptive perturbation is also applied, which depends on the reports of achievable data-rates from users. If the prior random perturbation increased the data rate, the adaptive perturbation is set to be the same as the prior random perturbation, otherwise the negative value of the prior random perturbation is applied for adaptive perturbation. Furthermore, to enhance the achievable data rates, the information on the stored precoding matrices in the memory as well as the currently generated precoding matrix is used for scheduling. Simulation results show that compared to conventional opportunistic precoding schemes, higher data rates are achieved by the proposed APOP scheme, especially when there are a relatively small number of users.

• Journal of Communications and Networks
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• v.18 no.6
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• pp.892-901
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• 2016

In the multi-user multiple-input multiple-output (MIMO) system, orthogonal random beamforming (ORBF) scheme is proposed to serve multiple users simultaneously in order to achieve the multi-user diversity gain. The opportunistic space-division multiple access system (OSDMA-S) scheme performs multiple weighting matrices during the training phase and chooses the best weighting matrix to be used to broadcast data during the transmitting phase. The OSDMA-S scheme works better than the original ORBF by decreasing the inter-user interference during the transmitting phase. To save more time in the training phase, a partly random multiple weighting matrices selection scheme is proposed in this paper. In our proposed scheme, the Base Station does not need to use several unitary matrices to broadcast pilot symbol. Actually, only one broadcasting operation is needed. Each subscriber generates several virtual equivalent channels with a set of pre-saved unitary matrices and the channel status information gained from the broadcasting operation. The signal-to-interference and noise ratio (SINR) of each beam in each virtual equivalent channel is calculated and fed back to the base station for the weighting matrix selection and multi-user scheduling. According to the theoretical analysis, the proposed scheme relatively expands the transmitting phase and reduces the interactive complexity between the Base Station and subscribers. The asymptotic analysis and the simulation results show that the proposed scheme improves the throughput performance of the multi-user MIMO system.

• 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.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.11A
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• pp.1013-1020
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• 2008

Cognitive radio(CR) techniques have been considered as a strong solution of frequency scarcity due to the limitation of frequency resource, In this paper, an orthogonal beamforming method is proposed that enables cognitive radio systems to coexist with a primary users' system in the same spectrum and region without causing interference to primary systems. The orthogonal multiple beams for multi-users of CR systems are obtained through the generation of orthogonal subspaces from primary users' channel state information. To increase the sum-rate of the CR systems, the proposed scheme adopts opportunistic radio resource allocation techniques. Simulation results provides that the proposed scheme achieves 2.72bps/Hz, the sum-rate capacity at 20 cognitive users in an area with two antennas at the cognitive radio basestation.