• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.9 no.5
• /
• pp.965-972
• /
• 2005

We propose a new resource management scheme, Distributed Channel Allocation Protocol (DCAP), for multimedia Ad Hoc Wireless LANs (AWLANS). This scheme implements a Qualify-of-Service (QoS) providing distributed resource management on the Orthogonal Frequency Division Multiplexing-Code Division Multiple Access (OFDM-CDMA) channel architecture. According to the performance evaluation results for MPEG traffic sources, DCAP can be a good choice of resource management scheme for AWLANs supporting multimedia services on the Broadband Wireless Access(BWA)-type physical layer.

• Journal of Communications and Networks
• /
• v.13 no.6
• /
• pp.655-663
• /
• 2011

Diffuse optical wireless (DOW) systems have the advantage that they do not require point-to-point siting so one transmitter can communicate with several receivers. In this paper, we investigate multiple access scheduling methods for downlink orthogonal frequency division multiplexing (OFDM) in diffuse optical wireless networks. Unlike the radio frequency (RF) channel, the DOW channel has low-pass filter characteristics and so requires different scheduling methods than those developed for the RF channel. Multi-user diversity orthogonal frequency division multiple access (OFDMA) systems nominate a cluster of subcarriers with the largest signal-to-noise-ratio for transmission. However, in a DOW channel, most users would choose the lowest frequency clusters of subcarriers. To remedy this problem, we make two proposals. The first is to use a variable cluster size across the subcarriers; the lower frequency clusters will have fewer subcarriers while the higher frequency clusters will have more subcarriers. This will equalize the capacity of the clusters. The second proposal is to randomize a user's cluster selection from a group of clusters satisfying a minimum threshold. Through simulation it is shown that combining these strategies can increase the throughput while ensuring a fair distribution of the available spectrum.

• The Journal of The Korea Institute of Intelligent Transport Systems
• /
• v.17 no.5
• /
• pp.135-142
• /
• 2018

This paper compares and analyzes advantages and disadvantages of CDMA(code division multiple access) and OFDM(orthogonal frequency division multiplexing) transmission techniques for underwater acoustic channel environments. Computer simulations were carried out in various underwater acoustic channels with varying r.m.s.(root mean square) delay, doppler frequency and the number of multipaths. When r.m.s. delay and doppler frequency are within the tolerance of designed transmission schemes, the computer simulations show that CDMA has better BER performance than that of the OFDM. However, when the doppler frequency exceeds the tolerance, BER performance of the CDMA decreases.

• ETRI Journal
• /
• v.37 no.4
• /
• pp.677-684
• /
• 2015

Recently, orthogonal frequency-division multiplexing (OFDM) was applied to VLC systems owing to its high rate capability. On the other hand, a real-valued unipolar OFDM signal for VLC significantly reduces bandwidth efficiency. For practical implementation, channel estimation is required for data demodulation, which causes a further decrease in spectral efficiency. In addition, the large fluctuation of an OFDM signal results in poor illumination quality, such as chromaticity changes. This paper proposes a spectrally efficient method based on a hidden-pilot-aided precoding technology for VLC with less flickering than a conventional OFDM-based method. This approach can obtain channel information without any loss of bandwidth efficiency while ensuring illumination quality by reducing the flickering effect of an OFDM-based VLC. The simulation results show that the proposed method provides a 6.4% gain in bandwidth efficiency with a 4% reduction in flicker compared to a conventional OFDM-based method.

• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.18 no.5
• /
• pp.1051-1060
• /
• 2014

In this paper, performance of an orthogonal frequency division multiplexing (OFDM) system in an impulsive noise environment is analyzed. Here, it is verified that performance of the OFDM system with window nonlinearity, which is one of the impulsive noise mitigation algorithms, is dependent on the over-sampling rate and the size of window. With respect to the variation of those parameters, we also provide an appropriate region of threshold values that control amplitudes of the sampled waveform of received signals. As a result, a new combination of parameters which improves error performance of OFDM system in an impulsive noise environment as compared to the previously reported parameters is presented.

• Journal of the Institute of Electronics and Information Engineers
• /
• v.50 no.7
• /
• pp.34-42
• /
• 2013

In this paper, we compare the performance of wavelet OFDM (Orthogonal Frequency Division Multiplexing) and FD-OFDM(Frequency diversity OFDM) system with conventional OFDM system. Wavelet OFDM system uses wavelet transform rather than Fourier transform and contains intermediate characteristics of CDM (Code Division Multiplexing) and OFDM. In wavelet OFDM system, inter-symbol interference (ISI) can be suppressed effectively and adjacent channel interference can be also minimized well. In FD-OFDM system, each parallel branch symbol is multiplied by the orthogonal sequence and distributed into all sub-carriers. Then, each sub-carrier transmits information composed of the symbol components of all parallel branches in the given frame. FD-OFDM contains the frequency diversity characteristic and, therefore, FD-OFDM can be robust to the narrowband interference. For the comparison among different systems, BER (Bit-Error Rate) performances are evaluated in the presence of narrow-band interference and a harmonic noise channel. From the evaluation results, compared to the conventional OFDM, wavelet OFDM and FD-OFDM shows better robustness against the interference and, especially, wavelet OFDM is the most robust in harmonic noise channel.

• 한국정보통신설비학회:학술대회논문집
• /
• 2008.08a
• /
• pp.525-530
• /
• 2008

There are a few core technologies to enable high-performance $4^{th}$ generation (4G) broadband wireless communication system. A multiple input multiple output (MIMO) provides high-rate transmission through expended channels by multiple array antennas in both sender and receiver side. Also orthogonal frequency division multiplexing (OFDM) is well-known as the most appropriate technique for high data rate transmission such as Mobile WiMAX and WLAN. Efficient decrease of inter-carrier interference (ISI) and inter-carrier interference (ICI) are the reasons for why OFDM is suitable for high-performance transmission, 4G mobile communication. In this paper, we mainly focus on two of objects, combination between MIMO and OFDM, and OFDM channel simulator using Ray-tracing algorithm. The results of this paper can be used implementation of a Wireless Software Platform for 4G Mobile Communication Systems.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.33 no.6A
• /
• pp.592-598
• /
• 2008

In this paper, a novel pilot-aided integer frequency offset estimation scheme is proposed for orthogonal Sequency division multiplexing (OFDM)-based digital video broadcasting (DVB) systems. The conventional pilot-aided integer frequency offset estimation scheme proposed for OFDM-based DVB systems exploits the partial information of combinations provided by pilots. Thus, in this paper, we propose a novel integer frequency offset estimation scheme exploiting all information of pilot combinations. To compare the performance of the proposed scheme with that of the conventional scheme, we conduct a simulation in additive white Gaussian noise (AWGN) and multipath Rayleigh channels, which shows that the proposed scheme outperforms the conventional scheme in terms of the probability of integer frequency offset estimation failure.

• Journal of the Korean Institute of Intelligent Systems
• /
• v.19 no.1
• /
• pp.145-151
• /
• 2009

Orthogonal Frequency Division Multiplexing (OFDM) has been focused on as 4th generation communication method for realization of Ubiquitous Network in land mobile communications services, and has been a standard technology of Wireless Local Area Network (WLAN) for a High Date Rate communication. And in maritime data communication using high frequency (HF) band, 32-point FFT OFDM system is recommended by International Telecommunication Union (ITU). Maritime communication should be kept on connecting when maritime accident or the maritime disaster happen. Therefore, main device FFT should be operated with low power consumption. In this paper we propose a low power 32-point FFT algorithm using radix-2 and radix-4 for low power operation. The proposed algorithm was designed using VHSIC hardware description language (VHDL), and it was confirmed that the output value of Spartan-3 field-programmable gate array (FPGA) board corresponded to the output value calculated using Matlab. The proposed 32-point FFT algorithm will be useful as a leading technology in a HF maritime data communication.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.66 no.6
• /
• pp.949-954
• /
• 2017

Multiple input, multiple output orthogonal frequency division multiplexing (MIMO OFDM) systems are the candidate for the future wireless communications. However, the main drawback of MIMO OFDM systems is their sensitivity to carrier frequency offset (CFO) similar to the single input, single output OFDM (SISO OFDM) systems. The demodulation of a signal with CFO causes large bit error rate and degrade the performance of a symbol synchronizer. It is important to estimate the frequency offset and minimize or eliminate its impact. In this paper, we propose a technique based on observation training symbols for estimating CFO by employing block-by-block estimation for SISO OFDM systems. The technique of SISO OFDM is extended to the MIMO OFDM systems. Simulation results show that the proposed techniques have a superior performance and better accuracy compared to the conventional techniques in the sense of mean square error.