• Journal of Korea Society of Industrial Information Systems
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• v.20 no.6
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• pp.1-10
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• 2015

In this paper, we propose a spectral efficient orthogonal frequency division multiple access(OFDMA) transmission system using multiple relays for a shadow area. The performance of the proposed OFDMA system in a shadow area can be improved by applying space-frequency(SF) block code over relaying multi-path channels. A simple channel cyclicity restoration method is also added in the proposed OFDMA system without cyclic prefix(CP) to recover the destruction of channel cyclicity due to the lack of CP. Simulation results show that the proposed OFDMA system without CP is almost same as that of the conventional OFDMA system with sufficient CP, which increases the spectral efficiency of the system.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.8 no.6
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• pp.1914-1925
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• 2014

Inter-symbol interference (ISI) problem is inevitable when the guard interval (GI) is shorter than the delay spread (DS) for an orthogonal frequency division multiplexing (OFDM) system. Iterative techniques have been proposed to overcome such a problem. However, most of existing algorithms are not efficient for an OFDM system with a small GI working under the channel with a large DS. Especially in the case of the DS spans a longer time than the half of the OFDM symbol duration. On the other hand, conventional algorithms, which can reduce the effects of the severe ISI, often employ several impractical assumptions to support the conclusions. In this paper, we present a robust decision feedback equalizer (DFE) for the OFDM system to overcome the severe ISI problem. The proposed DFE removes the ISI in a same manner as the residual inter-symbol interference cancellation (RISIC) algorithm. However, the inter-carrier interference (ICI) is reduced via cyclicity removal instead of the cyclicity restoration used in the conventional algorithms. The link-level simulation (LLS) results indicate that our proposed DFE scheme can dramatically improve the BER performance when the DS spans longer than the half of ODFM symbol duration.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37A no.12
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• pp.1106-1114
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• 2012

In order to provide high data rate and real time services under maritime environment, link-level performance of ship ad-hoc network (SANET) based on 3GPP LTE and IEEE 802.11p (WAVE) specifications are investigated and discussed in this paper. The measured maritime channel, whose delay spread is longer than the length of guard interval (GI) of both 3GPP LTE and IEEE 802.11p specifications, is adopted for the link-level simulations. For the purpose of eliminating inter-symbol interference (ISI) due to insufficient GI length, double antenna pattern (DAP) scheme and advanced time-domain decision-feedback equalizer (DFE) are proposed for LTE and WAVE systems, respectively. The proposed DFE removes the ISI in a same manner as the residual inter-symbol interference cancellation (RISIC) algorithm, but the inter-carrier interference (ICI) is reduced via cyclicity removal instead of cyclicity restoration used in the RISIC algorithm. Compared with existing schemes, our proposed DFE is a robust technique to overcome the severe ISI channel which has a comparatively large delay spread. Based on simulation results, not only comparisons between systems are discussed, but also some reformative suggestions are given.

• Journal of Communications and Networks
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• v.10 no.3
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• pp.239-252
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• 2008

For orthogonal frequency division multiplexing (OFDM), cyclic prefix (CP) should be longer than the length of channel impulse response, resulting in a loss of bandwidth efficiency. In this paper, we describe a new technique to restore the cyclicity of the received signal when the CP is not sufficient for OFDM systems. The proposed technique efficiently restores the cyclicity of the current received symbol by adding the weighted next received symbol to the current received symbol. Iterative CP reconstruction (CPR) procedure, based on the residual intersymbol interference cancellation (RISIC) algorithm, is analyzed and compared to the RISIC. In addition, we apply the CPR method to Alamouti space-time block coded (STBC) OFDM system. It is shown that in the STBC OFDM, tail cancellation as well as cyclic reconstruction of the CPR procedure should be repeated. The computational complexities of the RISIC, the proposed CPR, the RISIC with STBC, and the proposed CPR with STBC are analyzed and their performances are evaluated in multipath fading environments. We also propose an iterative channel estimation (CE) method for OFDM with insufficient CP. Further, we discuss the CE method for the STBC OFDM system with the CPR. It is shown that the CPR technique with the proposed CE method minimizes the loss of bandwidth efficiency due to the use of CP, without sacrificing the diversity gain of the STBC OFDM system.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.11
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• pp.7456-7462
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• 2015

This paper proposes a spectral efficient space-frequency(SF) block coded single carrier-frequency division multiple access(SC-FDMA) transmission system in a relaying multi-path shadow area. The cyclic prefix(CP) is not used in the proposed system in order to prevent a loss of spectral efficiency due to the use of CP. The destruction of channel cyclicity due to the lack of CP is recovered at the receivers of relays and a destination station. Simulation results show that the performance of the proposed system without CP is almost same as that of the conventional system with full CP, thus increasing the spectral efficiency.