• Journal of Positioning, Navigation, and Timing
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• 제2권1호
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• pp.59-65
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• 2013

The signal broadcast from a GPS satellite experiences code delay and carrier phase advance while passing through the ionosphere, which causes a signal error. Many ionosphere models have been studied to correct this ionospheric delay error. In this paper, the ionosphere modeling for the Korean Peninsula was carried out using a spherical harmonics based model. In contrast to the previous studies, we considered a real-time ionospheric delay correction model using fewer number of basis functions. The modeling performance was evaluated by comparing with a grid model. Total number of basis functions was set to be identical to the number of grid points in the grid model. The performance test was conducted using the GPS measurements collected from 5 reference stations during 24 hours. In the test result, the modeling residual error was smaller than that of the existing grid model. However, when the number of measurements was small and the measurements were not evenly distributed, the overall trend was found to be problematic. For improving this problem, we implemented the modeling with additional virtual measurements.

• 정보학연구
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• 제1권2호
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• pp.39-53
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• 1998

CMA나 MCMA 적응 블라인드 등화 알고리즘은 특별한 형태의 신호점을 원하는 신호점로 간주하므로써 완전한 등화에 도달시에도 원래의 신호점과의 불일치로 인한 오차가 불가피하다. 본 논문에서는 축소 신호점 대신에 결정된 원래의 신호점을 이용한 새로운 오차함수를 도입하여 빠른 수렴 속도로 정상상태에 도달하며, 정상상태에서의 오차를 크게 개선한 새로운 적응 블라인드 등화 알고리즘을 제안한다. 컴퓨터 모의실험을 통하여 제안한 알고리즘이 반송파 위상 복원을 가지는 CMA 및 MCMA등과 같은 CMA 계열의 적응 블라인드 등화 알고리즘들에 비해 잔류 심볼간 간섭과 수렴 속도면에서 향상된 성능을 가짐을 확인 하였다.

• 한국측량학회지
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• 제32권3호
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• pp.233-244
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• 2014

Precise Point Positioning (PPP) is an increasingly recognized precisely the GPS/GNSS positioning technique. In order to improve the accuracy of PPP, the error sources in PPP measurements should be reduced as much as possible and the ambiguities should be correctly resolved. The correct ambiguity resolution requires a careful control of residual errors that are normally categorized into random and systematic errors. To understand effects from two categorized errors on the PPP ambiguity resolution, those two GPS datasets are simulated by generating in locations in South Korea (denoted as SUWN) and Hong Kong (PolyU). Both simulation cases are studied for each dataset; the first case is that all the satellites are affected by systematic and random errors, and the second case is that only a few satellites are affected. In the first case with random errors only, when the magnitude of random errors is increased, L1 ambiguities have a much higher chance to be incorrectly fixed. However, the size of ambiguity error is not exactly proportional to the magnitude of random error. Satellite geometry has more impacts on the L1 ambiguity resolution than the magnitude of random errors. In the first case when all the satellites have both random and systematic errors, the accuracy of fixed ambiguities is considerably affected by the systematic error. A pseudorange systematic error of 5 cm is the much more detrimental to ambiguity resolutions than carrier phase systematic error of 2 mm. In the $2^{nd}$ case when only a portion of satellites have systematic and random errors, the L1 ambiguity resolution in PPP can be still corrected. The number of allowable satellites varies from stations to stations, depending on the geometry of satellites. Through extensive simulation tests under different schemes, this paper sheds light on how the PPP ambiguity resolution (more precisely L1 ambiguity resolution) is affected by the characteristics of the residual errors in PPP observations. The numerical examples recall the PPP data analysts that how accurate the error correction models must achieve in order to get all the ambiguities resolved correctly.

• Journal of Communications and Networks
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• 제10권2호
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• pp.148-155
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• 2008

In wireless data communication systems, retransmission of an erroneous packet is inevitable due to the harsh communication environment. In this paper, an efficient retransmission scheme using cooperation from neighboring nodes is investigated. In the cooperative retransmission scheme, an erroneous packet is transmitted to the destination by cooperative nodes which have favorable channels. This cooperative retransmission scheme requires no a priori information of neighboring nodes and has no limitation on the number of cooperating nodes. Distributed beamforming is used to accommodate multiple cooperating nodes. Phase and frequency offsets of cooperating signals are extracted from the NACK message and used to co-phase retransmitted data packets. The outage probability of the cooperative retransmission scheme is analyzed for the case of perfect synchronization and when the offsets are estimated. To reduce the impact of the residual phase and frequency offsets in cooperating signals, a low-rate feedback scheme is also investigated. It is shown that improved outage probability and reduced packet error rate (PER) performance can be achieved even for long data packets. The proposed cooperative retransmission scheme is found to outperform simple retransmission by the source as well as decode-and-forward cooperation.

• 한국음향학회지
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• 제25권3호
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• pp.113-120
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• 2006

본 논문은 slow 페이딩 채널에서 block type 파일럿 배치에 기반한 OFDM (Orthogonal Frequency Division Multiplexing) 시스템의 등화 기법을 다루고 있다. 수중 채널에서 얻을 수 있는 비트 속도는 셀룰러 폰이나 실내 무선 시스템과 같은 여타 통신 채널에 비해 상대적으로 낮은 편이며, 따라서 채널 추적시 도플러 효과가 중요한 파라미터가 된다. Coherent 복조 방식의 경우 도플러 주파수에 의한 잔여 평균위상에러는 시스템 성능에 치명적으로 작용할 수 있으며, 등화기만으로는 평균 도플러 쉬프트 효과에 대처하지 못할 수 있다. 공통 도플러 효과에 대처하기 위해 주파수 등화기와 더불어 위상에러 추적회로를 사용하여 회전 에러를 배제할 수 있다. 아울러 성능 저하를 최소화하는 수준에서 추적회로의 연산 부담을 줄일 수 있음을 시뮬레이션을 통해 증명하였다.

• 한국인터넷방송통신학회논문지
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• 제12권2호
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• pp.153-158
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• 2012

본 논문에서는 통신 채널에서 발생되는 찌그러짐과 잡음의 영향을 최소화하기 위하여 사용되는 적응 등화 알고리즘인 CMA 와 MMA 의 성능을 비교하였다. 다른 방식에비해 비교적 간단한 연산으로 고속 등화가 가능한 알고리즘인 CMA (Constant Modulus Algorithm)와 이의 오차 함수를 변형한 MMA (Multi-Modulus Algorithm)의 성능을 분석하여 점대점 또는 점대다점의 디지털 전송에서 적용 가능성을 확인하였다. CMA 알고리즘은 수신 신호의 진폭만을 보상하며 위상은 보상하지 못하므로 별도의 PLL을 사용하여야 하지만, MMA 알고리즘은 진폭과 위상을 동시에 보상할 수 있음을 컴퓨터 시뮬레이션을 통하여 확인하였으며 이의 성능 비교를 위해서 적응 등화기에 필수적인 수렴 특성과 잔류 isi양을 사용하였고, 그 결과 수렴 특성에서는 MMA 방식이, 진폭 보상을 나타내는 잔류 isi 양에서는 CMA 방식이 우월함을 알 수 있었다.

• 제어로봇시스템학회논문지
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• 제20권7호
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• pp.767-773
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• 2014

The carrier-to-noise power ratio is a key parameter for determining the reliability of PVT (Position, Velocity, and Time) solutions which are obtained by a GNSS (Global Navigation Satellite System) receiver. It is also used for locking a tracking loop, deciding the re-acquisition process, and processing advanced navigation in the receiver subsystem. The representative carrier-to-noise power ratio estimation schemes are the narrowband-wideband power ratio method (NW), the MM (Moment Method), and Beaulieu's method (BL). The NW scheme is the most classical one for commercial GNSS receivers. It is often used as an authoritative benchmark for assessing carrier-to-noise power estimation schemes. The MM scheme is the least biased solution among them, and the BL scheme is a simpler scheme than the MM scheme. This paper focuses on the less biased estimation with low complexity when the residual phase noise remains, then proposes a novel carrier-to-noise power ratio estimation scheme with low complexity for GNSS receivers. The asymptotic bias of the proposed scheme is derived and compared with others, and the simulation results demonstrate that the complexity of the proposed scheme is lowest among them, while the estimation performance of the proposed scheme is similar to those of the BL and MM schemes in normal and high gained reception environments.

• 대한전기학회논문지:시스템및제어부문D
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• 제51권2호
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• pp.78-85
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• 2002

We introduce a rotational decision-directed joint algorithm of blind equalization coupled with carrier recovery for 32-QAM demodulation with high symbol rate. The proposed carrier recovery, which we call a rotational decision-directed carrier recovery(RDDCR), removes the residual phase difference by rotating the decision boundary for the kth received symbol by the frequency detector output of the (k-1)th received symbol. Since the RDDCR includes the function of PLL loop filter by rotating the decision boundary, it gives a simpler demodulator structure. The rotational decision-directed blind equalization(RDDBE) with the rotated decision boundary based on the Stop-and-Go Algorithm(SGA) operated during tracking the frequency offset by the RDDCR and removes intersymbol interference due to multipaths and channel noise. Test results show that symbol error rate of $10^{-3}$ is obtained before the forward error correction when SNR equals 15dB with 150KHz of carrier frequency offset and two multipaths, which is the channel condition for 32-QAM receiver.

• 전자공학회논문지S
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• 제34S권11호
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• pp.16-24
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• 1997

In order to prevent the presence of the residual phase difference at the discriminator output by the existing AFC techniques, we propose a new automatic frequency control(AFC) tracking algorithm for QPSK demodulation at the digital direct broadcasting satellite(DBS) receiver, which we call a rotational decision-directed AFC(RDDAFC). The RDDAFC rotates the decision boundary for the kth received symbol by the frequency deterctor output of the (k-1)th received symbol. Tracking performances of carrier frequency offset by the proposed RDDAFC algorithm are evaluated through computer simulations under the practical DBS channel conditions with a carrier frequency offset of 2.3MHz when S/N equals 2dB. Test results show that the total pull-in time of the RDDAFC is 1.697msec for 10$^{-3}$ SER before forwared error correction at the receiver.

• 한국항해항만학회:학술대회논문집
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• 한국항해항만학회 2004년도 Asia Navigation Conference
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• pp.29-36
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• 2004

The paper describes recent experimental results on the development of Digital Magnetic Compass (DMC), which can provide smart automatic correction functions to the magnetic interferences. The design methodology of magnetic sensing circuit with ring-core fluxgate sensor is represented. The performance results of the sensing circuits are discussed with error analysis by polynomial regressions. As test results, the sensing circuit filtered only the second harmonic signal that is proportional to the direction of earth's magnetic field, and it leads to the obtainment of bearing information. In addition, the total residual errors of DMC can be analyzed by the adoption of polynomial regressions. It shown that the possibility of high precise DMC, in the future.