• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.3A
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• pp.324-330
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• 2004

In this paper, we propose the TCM(Trellis coded modulation) decoding scheme that reduces the number of operations in branch metric with STBC(space time block codes) channel information and present the implementation results. The proposed TCM decoding scheme needs only 1 signal point in each TCM subset. Using bias point scheme, It detects the minimum distance symbol. The proposed TCM decoding scheme can reduce the branch metric calculations. In case of 16QAM 8 subset, the reduction ratio is about 50％ and for 64QAM 8 subset, about 80％ reduction can be obtained. The results of logic synthesis for the TCM and STBC decoder with the proposed scheme are 57.6K gate count.

• International Journal of Computer Science & Network Security
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• v.23 no.6
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• pp.27-34
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• 2023

This paper proposed an Inter-Carrier-Interference (ICI) Canceling Orthogonal Frequency Division Multiplexing (OFDM) receiver for 5G mobile system to support 500 km/h linear motor high speed terrestrial transportation service. A receiver in such high-speed train sees the transmission channel which is composed of multiple Doppler-shifted propagation paths. Then, a loss of sub-carrier orthogonality due to Doppler-spread channels causes ICI. The ICI Canceler is realized by the following three steps. First, using the Demodulation Reference Symbol (DMRS) pilot signals, it analyzes three parameters such as attenuation, relative delay, and Doppler-shift of each multi-path component. Secondly, based on the sets of three parameters, Channel Transfer Function (CTF) of sender sub-carrier number 𝒏 to receiver sub-carrier number 𝒍 is generated. In case of 𝒏≠𝒍, the CTF corresponds to ICI factor. Thirdly, since ICI factor is obtained, by applying ICI reverse operation by Multi-Tap Equalizer, ICI canceling can be realized. ICI canceling performance has been simulated assuming severe channel condition such as 500 km/h, 2 path reverse Doppler Shift for QPSK, 16QAM, 64QAM and 256QAM modulations. In particular, for modulation schemes below 16QAM, we confirmed that the difference between BER in a 2 path reverse Doppler shift environment and stationary environment at a moving speed of 500 km/h was very small when the number of taps in the multi-tap equalizer was set to 31 taps or more. We also confirmed that the BER performance in high-speed mobile communications for multi-level modulation schemes above 64QAM is dramatically improved by the use of a multi-tap equalizer.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 1998.06a
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• pp.171-175
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• 1998

본 논문에서는 COFDM(Coded Orthogonal Frequency Division Multiplexing) 시스템에서 각 부채널이 갖는 신호대 잡음의 양을 수신단이 측정할 수 있는 특징을 이용하여 콘볼루셔널 디코더 메트릭 계산에 반영함으로서 콘볼루셔널 디코더의 성능이 향상될 수 있는 방안을 제안한다. 이를 위하여 먼저 OFDM 시스템에서 각 부채널이 갖는 신호대 잡음의 양은 부채널에서의 수신신호 전력에 비례함을 보인다. 또한, 콘볼루셔널 디코더의 메트릭 계산방법으로는 연판정 방법을 사용한다. 이때 부채널에서 수신된 신호의 신뢰도로 하여 신뢰도가 떨어지는 수신신호에 대하여는 메트릭 계산 과정에서 영향을 감쇄시키고 신뢰도가 높은 수신신호는 영향을 증대시키도록 한다. 제안된 방법을 경수로 20이고 직접파가 없는 다중경로채널에 적용하여 모의실험한 결과 coherent QPSK 매핑방법을 사용한 COFDM 시스템에서 10-4 BER을 기준으로 할 때 약 1.1dB의 SNR 성능향상을 얻었으며 16-QAM 매핑방법에서는 약 3.1dB 그리고 64-QAM 매핑방법을 사용한 경우 약 6.0dB의 성능향상을 얻었다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.26 no.6A
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• pp.965-979
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• 2001

본 논문에서는 OFDM 시스템의 송신기에서 사용되는 선형 필터와 고출력 증폭기의 결합에 의한 \"기억성을 갖는 비선형 왜곡\"을 실제 하드웨어 구현에 적합하게 적은 계산량으로 보상할 수 있는 참조 테이블(look up table; LUT) 기반의 사전왜곡기를 제안한다. LUT를 이용하는 사전왜곡기들은 이미 많이 연구되었으나, 본 논문에서는 다중 입력/단일 출력 형태의 기억성을 갖는 비선형 왜곡을 작은 크기의 LUT를 이용하여 효율적으로 보상한다는 점에서 큰 차이가 있다. 제안 방법에서는 먼저 기억성이 없는 고출력 증폭기만이 존재할 때의 사전왜곡기를 우리가 이미 제안한 고정점 반복 사전왜곡기 알고리즘에 기반하여 LUT로 구현하고, 여기에 선형 역 시스템을 결합하여 기억성을 갖는 비선형 왜곡을 보상한다. 실제 구현 가능성 검토를 위해 제안된 사전왜곡기와 16-QAM, 256-IFFT/FFT OFDM 시스템 기저대역 모뎀을 TMS320C30 DSP를 이용하여 구현하였으며, 실험 결과 작은 크기의 LUT를 이용하여도 원래의 고정점 반복 사전왜곡기에 비해 매우 적은 계산량을 요구하면서 동일한 성능을 얻을 수 있음을 확인하였다.있음을 확인하였다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.38A no.12
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• pp.1030-1038
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• 2013

The proposed Alamouti coded OFDM effectively cancels Inter Carrier Interference (ICI) due to frequency offset between distributed antennas. The conventional Alamouti coded OFDM schemes to mitigate ICI utilize N-point Inverse Fast Fourier Transform/Fast Fourier Transform (IFFT/FFT) operations for OFDM modulation and demodulation processes with total N subcarriers. However, the performance degrades because ICI is also repeated in N periods due to the property of N-point IFFT/FFT operation. In order to avoid this problem, null data are used at the subcarriers with large ICI and thus, data rate decreases. The proposed scheme employs 2N-point IFFT/FFT instead of N-point IFFT/FFT in order to increase sampling rate. By increasing sampling rate, the amount of interference significantly decreases because the period of ICI also increases. The proposed scheme increases the data rate and improves the performance by reducing amount of ICI and the number of null-data. Furthermore, the gain of the performance and data rate of the proposed scheme is significant with higher modulation such as 16-Quadarature Amplitude Modulation (QAM) or 64-QAM.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• v.9 no.1
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• pp.149-152
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• 2005

본 논문은 디지털 마이크로파 다수 반송파 시스템에 적용되는 고안정 국부발진기의 위상잡음에 대해서 분석하였다. 위상잡음의 영향을 분석하기 위해 Lorantizan모델을 사용하여 위상잡음 분산값을 구하였으며, 기저대역 I, Q채널에서 BER과의 특성을 모의실험 하였다. 시스템에 미치는 영향을 알아보기 위해서 BER특성을 분석한 결과, 위상잡음에 따라 영향을 많이 받으며, 특히 고차원 변조방식을 가질수록 위상잡음에 더 영향을 많이 받음을 확인하였다. 모의실험결과 BER을 10$^{-5}$을 기준으로 했을 경우, QPSK 변조방식은 분산값에 따라서 대략 4dB, 16-QAM 변조방식은 대략 9dB 정도의 성능차이를 나타내었다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.9A
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• pp.835-843
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• 2010

Energy spreading transform (EST) based equalization is a very effective technique to remove inter symbol interference (ISI) in frequency selective channel. EST based system uses cyclic prefix (CP) similar to orthogonal frequency division multiplexing (OFDM) system. Since CP is a redundancy, it degrades the data transmission rate. RISIC is an algorithm that removes an inter block interference (IBI) caused by insufficient CP length and reconstructs CP. In this paper, we propose a system that combines the existing EST system with quadrature phase shift keying (QPSK) modulation and RISIC algorithm to enhance the efficiency of the transmission. Also we extend the proposed system to 16 quadrature amplitude modulation (QAM) modulation. The proposed system is shown to performance close to matched filter bound (MFB) even with insufficient CP.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.12 no.1
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• pp.178-203
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• 2018

In this paper, authors have been evaluated the Frame Error Rate (FER) performance of IEEE 802.11 a/g/p standard 5 GHz frequency band WLAN over Rayleigh and Rician distributed fading channels in presence of Additive White Gaussian Noise (AWGN). Orthogonal Frequency Division Multiplexing (OFDM) based transceiver is implemented by using real-time signal processing frameworks (IEEE 802.11 Blocks) in GNU Radio Companion (GRC) and Ettus USRP N200 is used to process the symbol over the wireless radio channel. The FER is calculated for each sub-carrier conventional modulation schemes used by OFDM such as BPSK, QPSK, 16, 64-QAM with different punctuated coding rates. More precise SNR is computed by modifying the SNR calculation process of YANS and NIST error rate model to estimate more accurate FER. Here, real-time signal constellations, OFDM signal spectrums etc. are also observed to find the effect of multipath propagation of signals through flat and frequency selective fading channels. To reduce the error rate due to the multipath fading effect and Doppler shifting, channel estimation (CE) and equalization techniques such as Least Square (LS) and training based adaptive Least Mean Square (LMS) algorithm are applied in the receiver. The simulation work is practically verified at GRC by turning into a pair of Software Define Radio (SDR) as a simultaneous transceiver.

• International Journal of Computer Science & Network Security
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• v.23 no.7
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• pp.1-8
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• 2023

An Orthogonal Frequency Division Multiplexing (OFDM) based wireless communication system has drawn wide attention for its high transmission rate and high spectrum efficiency in not only radio but also Underwater Acoustic (UWA) applications. Because of the narrow sub-carrier spacing of OFDM, orthogonality between sub-carriers is easily affected by Doppler effect caused by the movement of transmitter or receiver. Previously, Doppler compensation signal processing algorithm for Desired propagation path was proposed. However, other Doppler shifts caused by delayed Undesired signal arriving from different directions cannot be perfectly compensated. Then Receiver Bit Error Rate (BER) is degraded by Inter-Carrier-Interference (ICI) caused in the case of Multi-path Doppler channel. To mitigate the ICI effect, a modified Delay and Doppler Profiler (mDDP), which estimates not only attenuation, relative delay and Doppler shift but also sampling clock shift of each multi-path component, is proposed. Based on the outputs of mDDP, an ICI canceling multi-tap equalizer is also proposed. Computer simulated performances of one-tap equalizer with the conventional Time domain linear interpolated Channel Transfer Function (CTF) estimator, multi-tap equalizer based on mDDP are compared. According to the simulation results, BER improvement has been observed. Especially, in the condition of 16QAM modulation, transmitting vessel speed of 6m/s, two-path multipath channel with direct path and ocean surface reflection path; more than one order of magnitude BER reduction has been observed at CNR=30dB.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.44 no.10
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• pp.156-161
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• 2007

In a TDD system, the length of slots can be unequal, and the number of downlink (DL) and uplink (UL) slots per frame can be different as well. The advantage of using TDD is the capability to accommodate asymmetric high-bit-rate services for the DL and It, which will be one of the prominent features in 4G systems. This paper analyzes the performance of TDD system on mobile channel environments like indoor pedestrian and vehicular channel, and proposes optimum modulation/demodulation method in TDD system. A rectangular QAM (RQAM) used in various communication systems has good BER performance but the much more signal amplitudes also have become one of the barriers to implement receiver. While PSK receiver is implemented easily because it has a constant amplitude, but it's BER performance is worse than RQAM. APSK proposed in this paper integrates merits of RQAM and PSK, and minimizes demerits of then And a simple method is also proposed to demodulate the soft symbol. The results indicate that the proposed APSK has a little worse performance than RQAM but the dynamic range of APSK is about 4 dB, 8 dB better than RQAM at 16-ary, 64-ary modulation/demodulation respectively.