• Journal of Korea Society of Industrial Information Systems
• /
• v.25 no.5
• /
• pp.13-21
• /
• 2020

This paper proposes a sign reversal channel switching method in space-frequency block code for orthogonal frequency division multiplexing system. In case of sending source data on other antenna, it is necessary for the receiver to change combining method according to the channel variation. If one does not know the predefined channel switching sequence, it is not possible to decode the received data precisely. In transmit data symbols' exchanges for a channel switching, data symbols are exchanged according to a format of space-frequency block code. In this paper, we proposes a simple sign reversal method except exchanging data symbols between transmit antennas. It is shown that this method occurs another combining method for a simple encryption in the receiver.

• Journal of Broadcast Engineering
• /
• v.16 no.2
• /
• pp.226-236
• /
• 2011

In this paper, we propose the channel estimation for the space time coded-orthogonal frequency division multiplexing (ST-OFDM) muti-input multi-output (MIMO) system with comb type pilot arrangements. In the conventional method, specially constructed pilots are inserted in the OFDM symbols at all transmit antennas separately. The proposed algorithm multiplies the orthogonal code to be transmitted OFDM symbols to estimate channels at the receiver. This algorithm can estimate a long channel delay with high accuracy. It is proved by computer simulations.

• Proceedings of the KIEE Conference
• /
• 2005.10a
• /
• pp.173-175
• /
• 2005

Two-dimensional orthogonal variable spreading codes are presented for multiplexing of forward link in direct sequence code division multiple access (DS-CDMA) multiple antennas system And the results of code generation and simulation of 2 dimensional orthogonal variable spreading factors on Jacket matrices are also be investigated. The bit error rate(BER) performance under a multi-user environment for the additive white Gaussian noise (AWGN) channel demonstrated that the proposed scheme could provide flexible rates and lower correlation values

• The Journal of the Acoustical Society of Korea
• /
• v.32 no.5
• /
• pp.377-384
• /
• 2013

In this paper, performance of COFDM (Coded Orthogonal Frequency Division Multiplexing) which is OFDM with a forward error correction code, is studied in frequency selective fading underwater acoustic communication channel. The OFDM is a multiplexing technique resistant to frequency selective multipath channel. In OFDM, a broadband information signal is transformed into several narrow band signals and transmits narrow band signals whose bandwidths are less than the channel coherence bandwidth. However, its performance is degraded in a specific narrow band signal due to its deep fading by multipath. To mitigate this degradation, COFDM which is OFDM with convolution code as a forward error correction code, is evaluated. The performance of COFDM is found to be better than that of OFDM in multipath channel.

• Journal of Broadcast Engineering
• /
• v.16 no.6
• /
• pp.927-940
• /
• 2011

In this paper, we improve a time-domain channel estimation algorithm with multi-input multi-output (MIMO) systems for the next-generation digital television (DTV). The conventional algorithm use orthogonal codes for separating channels from the time-domain orthogonal frequency division multiplexing (OFDM) symbols. However. it has the disadvantage of reduced data-rate because of many pilots. The improved algorithm shows better performance than the conventional one even with reduced number of pilots. The improved algorithm is evaluated by computer simulations.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.31 no.5A
• /
• pp.501-508
• /
• 2006

Orthogonal frequency division multiplexing-code division multiplexing access(OFDM-CDMA) systems without guard interval can enhance the bandwidth performance more efficiently than the systems with guard interval. In this paper, a new technique for an OFDM-CDMA system without guard interval, which can eliminate inter-symbol interference(ISI) and inter-channel interference(ICI), is presented. The proposed algorithm is a new demodulation technique, based on the Pseudo-Decorrelator, and it is named PD algorithm. Simulation results show that the PD algorithm can combat ISI and ICI efficiently for an OFDM-CDMA system without guard interval. The BER performance of the PD algorithm is much better than that of a conventional OFDM-CDMA system without guard interval.

• Journal of the Institute of Electronics and Information Engineers
• /
• v.50 no.1
• /
• pp.21-26
• /
• 2013

In this paper we propose a system where we divide the group with 2 symbols. The two added symbols are separated by multiplexing and later added using the DE-multiplexing technique. In our proposed system a simple Partial Interference Cancelation (PIC) group decoding scheme is used for Double-ABBA(D-ABBA) Quasi-Orthogonal Space Time Code, which reduces the decoding complexity for the higher order Multiple Input Multiple Output (MIMO) space time block coding. Finally we compare the proposed scheme performance using the different modulation schemes.

• Journal of Communications and Networks
• /
• v.15 no.1
• /
• pp.8-14
• /
• 2013

For alleviating the low spectrum efficiency problem of orthogonal frequency division multiplexing (OFDM), due to the strong inter-cell-interference (ICI) at cell's edge, we introduce comb-spectrum code division multiple access (CS-CDMA) into broadband OFDM system at downlink channel for enabling the use of entire spectrum for seamless coverage. In addition, we develop a new method, called orthogonal cell code (OCC) scheme, to assist CS-CDMA for nullifying the ICI from contiguous cells. In system operation, each of the conventional cells is divided into an outer cell and an inner cell, and a mobile station (MS) should access to the CS-CDMA when it is in the outer cell and access to OFDM when it is in the inner cell. This study investigates the spectrum efficiency of using CS-CDMA and makes a comparison with that of long term evolution (LTE) in the following cases; (1) under an assumption of perfect channel state information and (2) based on channel estimates at a MS station. The results show the great advantage of utilizing the proposed system.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.30 no.4C
• /
• pp.200-206
• /
• 2005

Adaptive transmission techniques can improve the performance of wireless communication system by adaptively changing the transmission parameter such as modulation, code-rate, and power according to the channel state. For orthogonal frequency division multiplexing (OFDM) system, the adaptive transmission techniques can be applied to each subcarrier unit. In this paper, we consider the adaptive code-rate OFDM system in which optimal code-rate is applied to each subcarrier according to the subchannel state. Performance analysis show that $3\sim6$dB gain of SNR or up to $30\sim50\%$ increase of data rate are achieved in the condition of bit error rate $10^{-6}$.

• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.5 no.7
• /
• pp.1250-1256
• /
• 2001

A resource management scheme for ad hoc wireless local area networks (AWLAN's) is presented. This scheme combines distributed resource management scheme and orthogonal frequency division multiplexing-code division multiple access(OFDM-CDMA) technique to support multimedia services with QoS guarantees. The performance evaluation has done in terms of blocking rates and QoS loss probability.