• Journal of the Institute of Electronics Engineers of Korea TC
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• v.46 no.1
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• pp.24-31
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• 2009

In this paper, the new technique combing power control with Pre-FFT beamforming is proposed for MIMO(multi-input multi-output)-OFDM(orthogonal frequency division multiplexing) system. As combining the proposed power control with beamforming, we can iteratively control the transmittingpower and update the weight of beamformer together. And then, the beam is formed toward the desired direction and SNIR of each subcarrier is converged to target SNIR. Therefore, the performance of MIMO-OFDM system is very improved. BER performance improvement of the proposed approach is investigated through computer simulation by combining power allocation algorithm with MIMO-OFDM system using Pre-FFT beamformer

• Journal of Advanced Navigation Technology
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• v.14 no.4
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• pp.521-529
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• 2010

In this paper, we propose a new scheduling algorithms in OFDM systems to reduce inter-cell interference. The proposed algorithm applies different subcarrier allocation sequences for different cells which is optimized through integer programming to minimize inter-cell interference. System level simulation is executed to derive the performance of the proposed algorithm. Simulation results show that the proposed scheduling algorithm improves user fairness as well as throughput compared with previous algorithms and therefore improves support of user QoS.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.1A
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• pp.44-50
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• 2007

In typical MC-CDMA systems, different frequency-domain scrambling codes are randomly chosen from a given code set and then assigned to different subscriber stations using the same subcarrier set. When orthogonal codes are employed for the frequency-domain scrambling, the choice of the scrambling code does not affect the detection performance if the timing and frequency synchronizations are perfect and the channel characteristic is flat. However, in practical cases, inter-code interference often appears due to the broken code orthogonality. In this paper, the optimal order of allocating the orthogonal frequency-domain scrambling codes to new subscriber stations is derived under non-negligible timing offsets in MC-CDMA system.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.11A
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• pp.1182-1189
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• 2007

Frequency reuse in a cellular wireless communication environment gives rise to a phenomenon of cochannel interference. This paper introduces a power control strategy for OFDM based communication systems operating in such an environment. Among the existing power control schemes, IWF(iterative waterfilling) is known to exhibit relatively good performance. However, it requires feedback of power level and bit allocation information for each subcarrier from a receiver to its associated transmitter, which can lead to a considerable overhead, especially for the case of employing large number of subcarriers. Motivated by this, we present a simplified power control scheme with reduced overhead feedback, which allocates some nonzero identical power to the subcarriers of which channel conditions are above a certain threshold and zero power to the other ones. Computer simulations show that the proposed strategy produces a good approximation to the performance of the IWF in terms of the transmission power level while it requires less overhead feedback.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.48 no.4
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• pp.24-31
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• 2011

A high data rate Medical Implant Communications Service (MICS) transmitter for implantable medical devices (IMD) is proposed. An orthogonal frequency division multiplexing (OFDM)-based multicarrier scheme is used to overcome the data rate limitation caused by the narrow bandwidth of 300 kHz. The proposed transmitter utilizes multiple MICS channels simultaneously, supporting increased data rate. To satisfy the MICS regulation, various schemes are applied including optimized subcarrier allocation and inverse fast Fourier transform (IFFT) architecture, and additional sidelobe suppression technique. Simulation results show that the proposed transmitter can support a maximum data rate of 4.86 Mbps, which is more than ten times faster than the previous systems.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.10A
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• pp.1042-1049
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• 2007

Frequency reuse in a cellular wireless communication environment gives rise to a phenomenon of cochannel interference. This paper introduces a power control strategy for OFDM based communication systems operating in such an environment. Among the existing power control schemes, IWF(iterative waterfilling) is known to exhibit relatively good performance. However, it requires feedback of power level and bit allocation information for each subcarrier from a receiver to its associated transmitter, which can lead to a considerable overhead, especially for the case of employing large number of subcarriers. Motivated by this, we present a simplified power control scheme with reduced overhead feedback, which allocates some nonzero identical power to the subcarriers of which channel conditions are above a certain threshold and zero power to the other ones. Computer simulations show that the proposed strategy produces a good approximation to the performance of the IWF in terms of the transmission power level while it requires less overhead feedback.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.3A
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• pp.267-273
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• 2006

This paper improves the effects of clustered OFCDM scheme considering the frequency diversity effect over a frequency selective fading channel. In OFCDM with frequency domain spreading compared to OFDM, we can increase uncorrelated symbols by frequency allocation method of correspondent symbols over the same antenna and different antenna after spreading. The simulation results show that the performance of proposed system is improved by approximately 4 dB in ${\sigma}=0.02{\mu}sec$, the performance is improved by approximately 2.5dB in large delay spread in a 12-path Rayleigh fading channel with overall the root mean squared delay spread and the maximum Doppler frequency of 20 Hz. Also, the required average received Eb/No at the average BER of $10^{-3}$ by optimum method is improved by approximately 2.0 dB, compared to that of STA-OFCDM with frequency rearrange. The new method does not require any bandwidth expansion any feedback from the receiver to the transmitter and its computation complexity is similar to clustered OFCDM.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.1A
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• pp.24-33
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• 2007

In this paper, two different dynamic cell coordination strategies for frequency flat and selective fading are proposed for efficient subcarrier allocation in the joint consideration of adaptive modulation and variable frequency reuse in the channel-aware OFDMA downlink multicellular environment. Compared to a conventional OFDMA system without cell coordination, where system throughput may become degraded due to the persistent interference from other cells, the proposed system dynamically allows RNC to apply different reuse factors on each subchannel and scheduling in consideration of channel and interference conditions of individual users so as to increase the system throughput and guarantee QoS of each user. In a frequency flat fading, the dynamic scheme with the proposed scheduling achieves on average three times larger throughput than the conventional dynamic scheme [8]. In a selective fading channel, the proposed schemes showed 2.6 times as large throughput as that of a single reuse factor of one for all subchannels.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.43 no.3 s.345
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• pp.39-48
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• 2006

In this paper, we analyze the spectral efficiency of multicarrier-code division multiple access (MC-CDMA) scheme. First, we derive a generalized formula for the spectral efficiency according to the number of subcarriers involved in, code division multiplexing and the number of codes used (i.e., loading factor), under a given set of channel coefficients. Also, we derive a generalized formula for spectral efficiency of various reduced-complexity systems that divide the full sets of subcarriers into several groups of subcarriers for code division multiplexing. Then, through these derivations, we establish an inter-relationship between the frequency selectivity and diversity order according to the number of multipaths. From the results, we choose the smallest code length while maximizing the diversity effect, provide an optimum subcarrier allocation strategy, and finally suggest a system structure for capacity-maximizing under the smallest code length. Through numerical analyses under simulated environments, we analyze the properties of spectral efficiency of various systems with reduced complexity and choose a major contributing factors to system design and a better system design methodology. Finally, we compare the spectral efficiency of the MC-CDMA scheme and orthogonal frequency division multiplexing (OFDM) scheme to make a relationship between both schemes.