• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.8A
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• pp.775-783
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• 2006

In this paper, the new technique combining beamforming with space-time coding is proposed for an orthogonal frequency division multiplexing(OFDM) system with multi-input multi-output(MIMO). When MIMO-OFDM system is employing Nt(the number of transmitterantenna) beamfomers and one S-T decoder at Nr receiver antennas, Nt signals removed CCI are outputted at the beamformer and then diversity gain can be got through space-time decoding. As the proposed technique can reduce cochannel interference and get diversity gain in the multi-user environment, the performance of MIMO-OFDM system is very improved. BER performance improvement and convergence behavior of the proposed approach are investigated through computer simulation by applying it to MIMO-OFDM system in the multi-user environment.

• 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 information and communication convergence engineering
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• v.2 no.2
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• pp.71-75
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• 2004

In non-selective frequency environment, it is difficult to take the advantage of path diversity. In the literature, some methods have been proposed to solve the issue. This paper presents a new method to obtain the resolvable path in Indoor Multi-carrier Code Division Multiple Access (MC-CDMA) system by using downlink beam forming. With the aid of downlink beamforming, the most reliable path is found and chosen for the communication link. The new approach is evaluated in term of bit error rate (BER) and power consumption. The simulation results show that the new approach can get better BER performance. However, the cost of BER improvement is a small degradation in power reservation.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.41 no.10
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• pp.17-26
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• 2004

This paper proposes JBSTBC (Joint Beamforming Space-Time Block Coding) scheme for MIMO (Multi-Input Multi-Output) communication systems. To enhance the order of spatial diversity in presence of deteriorative fading correlations as well as inter-substream interferences, the proposed JBSTBC method employs joint eigen-beamforming technique together with the BOLD (block-ordered layered detector) for MIMO-STBC. In order to confirm superiority of the proposed JBSTBC method, the computer simulations are conducted in highly correlated fading situations with providing detailed mathematical derivations for clarifying functionality of the proposed scheme.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.44 no.9
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• pp.1-8
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• 2007

In this paper, the new beamforming is proposed for an orthogonal frequency division multiplexing(OFDM) system with multi-input multi-output(MIMO). Through the proposed Pre-FFT beamforming technique for MIMO-OFDM, the multibeams are formed toward each multi-transmitter antenna of the desired user. The proposed beamforming for MIMO-OFDM can reduce cochannel interference and get diversity gain in the multi-user environment. Therefore, the performance of MIMO-OFDM system is very improved. BER performance improvement of the proposed approach is investigated through computer simulation by applying it to MIMO-OFDM system in the multi-user environment.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.12A
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• pp.991-997
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• 2009

Transmit beamforming is simple method to achieve the full diversity gain that is available in multiple antenna(MIMO) wireless systems. Unfortunately, the prior condition to achieve this gain requires perfect channel knowledge at both transmitter and receiver, which is impractical on account of limited feedback link. Therefore, for the practical system, codebook based feedback scheme is often employed, where the beamforming vector is selected from the codebook to maximize the output signal-to-noise ratio (SNR) at receiver, and the receiver only sends back the index of the best beamforming vector to the transmitter. In this paper we derive analytical expression of average bit error rate (BER) for the codebook based transmit beamforming MISO system over the feedback error channel. Using this analytical result, we present optimum codebook indexing scheme to improve the performance of this system. From some selected numerical examples we show that our proposed codebook indexing scheme can provide nonnegligible performance improvements in terms of average BER over the severe feedback error channel.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.5A
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• pp.347-354
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• 2009

In this paper, we propose a new transmit diversity technique for multiple-input multiple-output (MIMO) systems to improve the link level performance of open-loop systems. By cyclically applying a predetermined set of preceding weight vectors, artificially induced fluctuation is created to achieve additional diversity gain in flat fading channels. To design the set of the precoding vectors, we exploit the knowledge on the distribution of near optimum Preceding vectors observed in a beamforming scheme based on the rotation transformations. Simulation results demonstrate that the proposed open-loop diversity scheme with an arbitrary number of transmit antennas achieves a full diversity gain with computational complexity comparable to a single-input single-output (SISO) system.

• Journal of Communications and Networks
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• v.9 no.2
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• pp.112-117
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• 2007

In this paper, we focus on the total transmission power minimization problem for downlink beamforming multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems while ensuring each user's QoS requirement. Although the linear integer programming (LIP) solution we formulate provides the performance upper bound of the margin adaptive (MA) optimization problem, it is hard to be implemented in practice due to its high computational complexity. By regarding each user's equivalent channel gain as approximate independent values and using iterative descent method, we present a heuristic MA resource allocation algorithm. Simulation results show that the proposed algorithm efficiently converges to the local optimum, which is very close to the performance of the optimal LIP solution. Compared with existing space division multiple access (SDMA) OFDM systems with or without adaptive resource allocation, the proposed algorithm achieves significant performance improvement by exploiting the frequency diversity and multi-user diversity in downlink multiple-input single-output (MISO) OFDM systems.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37A no.12
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• pp.1065-1075
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• 2012

For the purpose of providing high data rate real-time services, radio transmission technologies (RTT) for ship ad-hoc network (SANET) based on the Recommendation ITU-R 1842-1 are designed. Physical layer parameters of SANET are contrived to meet the requirements of the specification. In order to improve the link reliability for SANET, in this paper, we investigate the performance of the SANET with the multiple antennas, where receive combining (RC), transmit diversity (TD), and beamforming (BF) are employed, respectively. Based on the analysis of the packet error rate (PER) under the highly correlated maritime wireless channel model, we select the efficient multiple antenna schemes for SANET to improve the link reliability. In addition, the optimal MCS levels for the single-carrier (SC) SANET with the bandwidth of 25 kHz, and the multi-carrier (MC) SANET with the bandwidth of 50 kHz and 100 kHz are finalized.

• Journal of the Institute of Convergence Signal Processing
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• v.8 no.4
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• pp.266-271
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• 2007

In mobile channel environments, the effect of the multi-path fading reduces the performance of the mobile communication severely. Previous studies on the effect of the multi-path fading show that, only the channel between fixed base stations and mobile terminals are considered. However, the future mobile communication service such as telematics may use the communication between moving mobile terminals, so called mobile ad-hoc communication, the studies on the channel is required for such situations. In this paper we have measured the mobile channel and analyzed the fading characteristics when using the linear array antennas. In the data measurements, we sent the QPSK modulated signal with carrier frequency of 2GHz band and received the signal using the array antennas equipped on moving vehicles. We have analyzed the fading characteristics and the effects of the fading reduction using the CDF of fading depth using data collected from two moving vehicles. The simulation results show that the beamforming technique adds 6 or 7dB to the fading margin than that of the selection diversity depending on channel environments.