• Title/Summary/Keyword: Time-frequency symbol

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Relay Selection Schemes Using STBC Technique in OFDM-Based Cooperative Wireless Communications (OFDM 기반의 무선 협력 통신에서 STBC 기술을 적용한 선택적 릴레이 통신 기법)

  • Lee, Je-Yeon;Yang, Mo-Chan;Yoo, Sung-Cheol;Shin, Yo-An
    • The Journal of Korean Institute of Communications and Information Sciences
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    • v.36 no.7A
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    • pp.640-648
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    • 2011
  • We propose relay selection schemes using STBC (Space Time Block Coding) technique in OFDM (Orthogonal Frequency Division Multiplexing)-based wireless systems. The proposed schemes select the optimum relay having the maximum instantaneous equivalent channel gain among multiple candidate relays. Also, in order to reduce the system overhead, a symbol grouping method which groups some amount of symbols before selecting the optimum relay is proposed. The simulation results show that the proposed relay selection schemes can obtain more selection diversity gain as the number of selectable relay candidates increases. Furthermore, the proposed scheme with the symbol grouping can reduce system overhead without any degradation of the performance in fading channels with low frequency selectivity.

Inter-carrier Interference of Multicarrier-CDMA System in Time Selective Fading Channel (시간선택적 페이딩 환경에서 멀티캐리어-CDMA 시스템의 서브캐리어간 간섭)

  • 김명진;김성필;하경민
    • Proceedings of the IEEK Conference
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    • 2000.06a
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    • pp.53-56
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    • 2000
  • In MC-CDMA systems effects of delay spread of the channel are reduced with increased symbol duration by simultaneously transmitting data symbols on the parallel subcarriers. However, the increased symbol duration causes the system to be more vulnerable to time selective fading. In other words, although MC-CDMA systems are robust against frequency selective fading in a multipath environment, they are sensitive to Doppler spread and hence inter-carrier interference is increased. In this paper, we investigate the effects of time selective fading characteristics of the mobile channel from the viewpoint of desired signal power to inter-carrier interference power ratio at the combiner output of the MC-CDMA receiver.

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Performance Analysis of Frequency Diversity Scheme for OFDM Systems Using Sub-channel Correlation Characteristics (부채별 상관 특성을 이용한 OFDM 시스템의 주파수 다이버시티 기법 성능 분석)

  • 이종식;김장욱;오창헌;조성준
    • The Journal of Korean Institute of Communications and Information Sciences
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    • v.29 no.6A
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    • pp.614-622
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    • 2004
  • In this paper, we propose the frequency diversity scheme for performance improvement of a OFDM system without decreasing the spectral efficiency. In the proposed scheme, information bit is encoded to symbol by a simple procedure, and the encoded symbol is transmitted through the two lowest correlated sub-channels with the particular phase difference. At the receiver, a frequency diversity gain is obtained by a simple signal processing. We also suggest optimum phase difference value to minimize the performance degradation which resulted from a phase difference estimation error and bit/symbol mapping method to minimize BER. As results, at the point of performance improvement, the proposed scheme is excellent even though it requires a little increase of system complexity because of an additional encoding and decoding. In particular, we confirmed through computer simulation that on the same channel environment and bandwidth efficiency, the 27x/1Rx STBC-OFDM system adopting the proposed frequency diversity scheme outperforms the conventional 27x/1Rx STBC-OFDM system performance

Symbol Frame Synchronization Technique for OFDM Burst Mode Transmission

  • Kim, Ki-Yun;Lee, Jun-Yeob;Choi, Hyung-Jin;Kim, Ho
    • Proceedings of the IEEK Conference
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    • 2002.07c
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    • pp.1697-1700
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    • 2002
  • In this paper, we propose a new frame structure and frame synchronization algorithm for OFDM burst mode transmission. On the contrary to conventional OFDM symbol based methods, the proposed preamble for symbol frame synchronization is designed independently in time domain in arbitrary length and is placed at the beginning of the frame. The proposed frame synchronization algorithm by using the preamble is working independent of frequency offset and robust to serious channel environment.

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An Efficient Symbol Timing Synchronization Scheme for IEEE 802.11n MIMO-OFDM based WLAN Systems (IEEE 802.11n MIMO-OFDM 기반 무선 LAN 시스템을 위한 효율적인 심볼 동기 방법)

  • Cho, Mi-Suk;Jung, Yun-Ho;Kim, Jae-Seok
    • Journal of the Institute of Electronics Engineers of Korea TC
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    • v.46 no.5
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    • pp.95-103
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    • 2009
  • An efficient symbol time synchronization scheme for IEEE 802.11n MIMO-OFDM based WLAN systems using cyclic shift diversity (CSD) preamble is proposed. CSD is used to prevent unintentional beamforming when the same preamble signal is transmitted through transmit antennas. However, it is difficult to find a proper starting-point of the OFDM symbol with the conventional algorithms because of time offset by multi-peaks which are result from cross-correlation of received CSD preamble with a known short training symbol. In addition, the performance of symbol time sync. is affected by AGC and packet detection position. In this paper, an optimal symbol time synch. algorithm which is composed of the boundary detection scheme between LTS and OFDM symbols, the verification scheme for enhancement of boundary detection accuracy, and the SNR-varying threshold estimation scheme is proposed. Simulation result show that the proposed algorithm has performance gains of 4.3dB in SNR compared to the conventional algorithms at the rate of 1% sync. failure probability for $2{\times}2$ MIMO-OFDM system and 18dB at 0.1% when maximum frequency offset exists. It also can be applied to $4{\times}4$ MIMO-OFDM system without any modification. Hence, it is very suitable for MIMO-OFDM WLAN systems using CSD preamble.

Pilot Symbol Assisted Channel Estimation and Equalization for OFDM Systems in Doubly Selective Channels (주파수 선택적 시변 채널 OFDM 시스템에서의 파일럿 심볼을 이용한 채널 예측 및 등화)

  • Lim, Dong-Min
    • The Journal of Korean Institute of Electromagnetic Engineering and Science
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    • v.18 no.12
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    • pp.1408-1418
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    • 2007
  • In this paper, we analyze the performance of pilot symbol assisted channel estimation and equalization schemes for OFDM systems over frequency-selective time-varying channels and propose methods to improve the system performance. In the least square(LS) and linear minimum mean square error(MMSE) channel estimation, time domain windowing is introduced for banding the frequency domain channel matrix. The linear MMSE and decision feedback equalization schemes are employed with the pilot symbols for channel estimation taken into account in the equalization process. To reduce computational complexity, the band LU matrix factorization algorithm is introduced in solving the linear systems involved in the equalization, and the performances are compared with the known previous results by computer simulations. When time domain windowing is employed in the decision feedback equalization, the matrix related with the decision feedback process is shown to be unhanded and the resultant performance degradation is analyzed.

An Adaptive Path Selection Technique Considering Time Difference of Arrival in Multi-hop Relay Systems (다중 홉 릴레이 시스템에서 전파 시간 차이가 고려된 적응적 경로 선택 기법)

  • Woo, Kyung-Soo;Park, Chang-Hwan;Yoo, Hyun-Il;Kim, Jae-Kwon;Han, Seung-Hee;Cho, Yong-Soo
    • The Journal of Korean Institute of Communications and Information Sciences
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    • v.34 no.4A
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    • pp.291-301
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    • 2009
  • In this paper, the effect of ISI(Inter-Symbol Interference) and ICI(Inter-Carrier Interference) due to time difference of arrival on OFDMA-based mobile multi-hop relay (MMR) systems is analyzed. Analyses are performed for the ISI caused by the previous OFDMA symbol transmitted from neighboring macro or relay cell as well as the ISI caused by the next OFDMA symbol transmitted from neighboring macro or relay cell. Then, an effective SINR(Signal to Interference plus Noise Ratio) estimation method and a path selection method considering time difference of arrival are proposed to minimize the effect of ISI and ICI. It is shown by simulation that the performance degradation caused by time difference of arrival can be significantly mitigated when the proposed path selection method is applied to the uplink of OFDMA-based MMR systems.

Low Complexity Discrete Hartley Transform Precoded OFDM System over Frequency-Selective Fading Channel

  • Ouyang, Xing;Jin, Jiyu;Jin, Guiyue;Li, Peng
    • ETRI Journal
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    • v.37 no.1
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    • pp.32-42
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    • 2015
  • Orthogonal frequency-division multiplexing (OFDM) suffers from spectral nulls of frequency-selective fading channels. Linear precoded (LP-) OFDM is an effective method that guarantees symbol detectability by spreading the frequency-domain symbols over the whole spectrum. This paper proposes a computationally efficient and low-cost implementation for discrete Hartley transform (DHT) precoded OFDM systems. Compared to conventional DHT-OFDM systems, at the transmitter, both the DHT and the inverse discrete Fourier transform are replaced by a one-level butterfly structure that involves only one addition per symbol to generate the time-domain DHT-OFDM signal. At the receiver, only the DHT is required to recover the distorted signal with a single-tap equalizer in contrast to both the DHT and the DFT in the conventional DHT-OFDM. Theoretical analysis of DHT-OFDM with linear equalizers is presented and confirmed by numerical simulation. It is shown that the proposed DHT-OFDM system achieves similar performance when compared to other LP-OFDMs but exhibits a lower implementation complexity and peak-to-average power ratio.

Performance Evaluation of Bandwidth Efficient Adaptive QAM Schemes in Flat and Frquency Selective Fading Channels (균일 및 주파수 선택적 페이딩에서 대역폭 효율의 적응 QAM 성능분석)

  • 정연호
    • The Journal of Korean Institute of Communications and Information Sciences
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    • v.25 no.10A
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    • pp.1473-1479
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    • 2000
  • This paper presents the performance evaluation of an adaptive QAM scheme under flat and frequency selective fading channels for indoor wireless communication systems. The QAM modulation is combined with differential encoding and the demodulation process is carried out noncoherently. The adaptation is performed by varying the modulation level of QAM, depending upon received signal strength. The adaptation mechanism allows a 2- or 3-bit increase or decrease at a time, if the channel condition is considered to be significantly good or bad. Simulation results show that the average number of bits per symbol (ABPS) for each symbol block transmitted over a flat fading channel is higher than 5.0 and the BER performance is better than 10^-4 for a SNR value higher than 30 dB. For frequency selective fading channels, an oversampling technique in the receiver was employed. The BER performance obtained for frequency selective fading channels is better than 10^-4 with a SNR value of 40 dB and ABPS is found to be approximately 5.5. Therefore, this scheme is very useful in that it provides both very high bandwidth efficiency and acceptable performance with moderate SNR values over flat and frequency selective fading channels. In addition, this scheme provides reduced receiver complexity by way of noncoherent detection.

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A Study on the Development of SSB Modem (디지털 SSB 모뎀 개발에 관한 연구)

  • Jin, Heung-Du;Choi, Jo-Cheon
    • Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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    • 2007.10a
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    • pp.693-697
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    • 2007
  • The SSB modem performs the modulation process which converts the digital voltage level to the audible frequency band signal and the demodulation process which converts reversely the audible frequency signal to the digital voltage level. The modulator and the demodulator are implemented with a single DSP chip. Because of the SSB specific character, the distortion occurs when the frequency is changed. This distortion has no effect on voice communication, but it has an significant effect on data communication. In other words, it is impossible to send data stream with adjacent 2 periods. Therefore, in case of using 2-tone FSK, it is needed to send at least 3 periods to transmit 1 bit. Therefore we implemented the modem using modified phase-delay shift keying to transmit 1 tone signal for high speed transmission. In the 1200[bps] mode, it generates 0, $187{\mu}s$ delay time at 1.3kHz symbol frequency, and in the 2400[bps] mode, 0, $70{\mu}s$, $130{\mu}s$, $200{\mu}s$ delay time at 1.5kHz symbol frequency. Finally, in the maximum 3600[bps] mode, it generates 0, $100{\mu}s$, $160{\mu}s$, $250{\mu}s$ delay time at 2.0kHz symbol frequency. The measured results of the implemented SSB modem shows a good transfer functional characteristic by spectrum analyzer, almost same bandwidth in pass band and 20dB higher SNR comparing the German PACTOR and American CLOVER and in the experimental transmitting test, we verified the transmitted data is received correctly in platform.

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